Impact of H/D isotopic effects on the physical properties of materials

Abstract

H/D isotope substitution deepens our understanding of molecular interactions, hydrogen bond characteristics, and quantum effects, while also advancing the design of advanced materials, biological research, and energy applications, thereby having a profound impact in interdisciplinary fields. In materials science, deuteration can influence the structure and physical properties of hydrogen-containing solids. A notable example is the discovery in 1942 that deuteration on the strong hydrogen-bonded ferroelectric potassium dihydrogen phosphate (KDP) increases the Curie temperature (T_c) by approximately 107 K, attributed to the geometric isotope effects (GIE) induced by deuteration on hydrogen bonds. Additionally, deuteration can introduce chemical pressure within the lattice, influencing the magnetic and conductive properties of solid materials. Moreover, due to their lower vibrational modes, D atoms can reduce non-radiative transitions, thereby enhancing the optical physical properties of materials. This highlights deuteration as a viable chemical strategy for modulating the physical properties of materials. This review aims to provide a concise overview of the effects of H/D isotope substitution on material structures and physical properties, offering new insights into the regulation of material properties through recent advancements in deuteration.

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Chengdong Liu

Chengdong Liu was born in Hubei Province of China. He completed his Bachelor's degree at Lanzhou University in 2017. Subsequently, under the mentorship of Professor Wei-Sheng Liu, he obtained his Master's degree from the same institution in 2020. Presently, Liu is engaged in the pursuit of a PhD in inorganic chemistry at the Beijing Institute of Technology. His academic journey is being guided by Associate Professor Zi-Shuo Yao and Professor Jun Tao. His research endeavors are centered on the synthesis and performance evaluation of multifunctional magnetoelectric materials. Additionally, he is exploring the intriguing effects of isotope effects on these advanced materials.

Zi-Shuo Yao

Zi-Shuo Yao earned his Ph.D. in 2015 from Kyushu University, under the expert guidance of Professor Osamu Sato. Following his graduation, he continued his academic journey at the Institute for Materials Chemistry and Engineering (IMCE) of Kyushu University, where he served as a postdoctoral fellow. In 2016, Yao transitioned to the School of Chemistry and Chemical Engineering at Beijing Institute of Technology (BIT), taking on the role of an associate professor. His research is dedicated to the exploration of functional molecular single crystals that exhibit dynamic structures. This includes an in-depth study of shape-transformable single crystals, materials with magnetic switchability, and ferroelectric materials. Yao's work aims to unlock the potential of these advanced materials for various applications, contributing significantly to the field of materials science.

Jun Tao

Jun Tao completed his PhD in 2001 under the mentorship of Professor Xiao-Ming Chen at Sun Yat-Sen (Zhongshan) University. Upon graduation, he joined Xiamen University, where he began his academic career. Between 2004 and 2006, Tao broadened his research horizons by engaging in postdoctoral researches with Professor Osamu Sato at the Kanagawa Academy of Science and Technology (KAST) and Kyushu University in Japan, supported by the Japan Society for the Promotion of Science (JSPS). Tao was promoted to full professor in 2008 at the College of Chemistry and Chemical Engineering, Xiamen University. In 2016, he moved to the Beijing Institute of Technology (BIT). Now, he holds the esteemed position of a full Professor at the School of Chemistry and Chemical Engineering. Professor Tao's research interests are centered on the synthesis of bistable molecule-based magnetic materials, his work is pivotal in advancing the understanding and application of these materials in various scientific and technological domains.

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It is a great honor to contribute to the 10th-anniversary collection of this esteemed journal. Over the past decade, the journal has established itself as a leading platform for cutting-edge research in inorganic chemistry, fostering global collaboration and innovation. I deeply value its commitment to excellence, rigorous peer review, and promotion of impactful discoveries. The journal has been instrumental in advancing the field of inorganic chemistry, not only in China and Asia but also internationally, by bridging diverse perspectives and encouraging groundbreaking research. Having published three articles in this journal (Inorg. Chem. Front. 2020, 7, 1196–1204; 2023, 10, 692–698; 2023, 10, 7251–7264), I am proud to be part of this special issue. Congratulations on this remarkable milestone, and I eagerly anticipate the journal's continued contributions to the advancement of inorganic chemistry worldwide.

1. Introduction

In 1931, Harold Urey, while conducting experiments on hydrogen separation and spectroscopic analysis at Columbia University, made a discovery of an isotope with a mass approximately twice that of ordinary hydrogen (protium, H).¹ This breakthrough eventually earned him a Nobel Prize.^2–4 Subsequently, Urey and his colleagues introduced the term “deuterium” (D), derived from the Greek word “deuteros” meaning “second”, to reflect its increased mass relative to hydrogen, attributed to the presence of an additional neutron (Scheme 1).^5–7 In nature, deuterium is primarily found in the form of HDO or D₂O within oceans, with an abundance of approximately 0.0115%.^3,8 This concentration ensures that deuterium is sufficiently available on Earth, making it a valuable resource for various scientific investigations and technological applications.^9–15

Scheme 1 Comparison of physical properties of hydrogen and deuterium.

The quality of D at 2.014 atomic mass units (u) is twice that of H at 1.008 u, which results in a lower vibrational frequency for the carbon–deuterium (C–D) bond compared to the carbon–hydrogen (C–H) bond. This difference leads to a lower zero-point energy (ZPE) of bonds involving a deuterium atom, typically ranging from 1.2 to 1.5 kcal mol⁻¹.⁶ Consequently, the breakage of the X–D bond (where X can be carbon, nitrogen, oxygen, etc.) requires more energy than breaking the X–H bond, which also results in a slightly shorter X–D bond length compared to the X–H bond length.¹⁶ These variations allow deuteration to significantly influence material properties and reveal novel quantum phenomena.^9,17–19 Specifically, deuteration, which involves substituting hydrogen with deuterium in hydrogen-containing materials, affects the structure and vibrational modes of hydrogen bonds. This alteration can further tune and optimize the electrical,^20–25 magnetic,^26–31 and optical^10,32–36 properties of the materials. For example, in ferroelectric materials, deuteration can increase the phase transition temperature, thereby stabilizing the ferroelectric phase.^21,37–41 In luminescent materials, it can reduce non-radiative decay processes, leading to enhanced optical performance.^{11,17,25,42–45} Additionally, in semiconductors and superconductors, deuteration can improve carrier mobility and lifetime, thereby enhancing the overall performance of the materials.^{12,23,46–49}

Previous reviews on the effects of deuteration on physical properties have been limited to focusing on deuterated fluorescent materials and strong hydrogen-bonded ferroelectrics (d_D–H⋯A ≤ 2.65 Å, D: donor; A: acceptor),^50–52 while neglecting the influence of deuteration on weak hydrogen bonds (d_D–H⋯A > 2.90 Å), and other hydrogen-containing ferroelectrics, magnetic, and conductive materials. Therefore, in this review, we primarily examine the impact of H/D isotope substitution on the structure and physical properties of hydrogen-containing compounds. Traditionally, significant deuteration effects have been predominantly observed in systems characterized by strong hydrogen bonds (d_D–H⋯A ≤ 2.65 Å, D = C, N, O, S).^21,53 often involving proton transfer, which was considered a crucial factor driving these effects. However, recent advancements in deuteration research have underscored the importance of deuteration in systems with weak hydrogen bonds (d_D–H⋯A > 2.90 Å) and other hydrogen-containing compounds. Through targeted molecular design, it is possible to achieve substantial deuteration effects even in systems lacking strong hydrogen bonds. Our detailed analysis of deuteration's influence on both structure and physical properties aims to provide researchers with valuable insights for advancing studies in H/D isotope substitution.

2. Influence of the deuteration effect on crystal structure

The concept of “Isotopic polymorphism” was introduced to describe how isotopic substitution affects the molecular arrangement in the solid state. The primary mechanism by which H/D isotope substitution induces isotopic isomerism is through alterations in intermolecular forces caused by deuteration. These changes are more easily achieved in systems characterized by strong hydrogen bonds. Conversely, in systems lacking hydrogen bond, predicting isotope polymorphism due to deuteration is challenging, particularly in small molecular compounds. In such compounds, the solid-state arrangement is highly sensitive to minor changes in substitution patterns.⁵⁴ For instance, deuteration of the classic aromatic compound benzene results in slight molecular modifications without significantly altering the overall lattice structure.^55,56 However, when deuteration is applied to another aromatic compound, pyridine, isotopic polymorphism is observed, likely due to differences between C–H⋯N and C–D⋯N intermolecular interactions.⁵⁷

In contrast, deuteration generally does not induce isotope polymorphism in compounds that lack strong specific intermolecular interactions, such as polymer molecules. Instead, it leads to lattice contraction, which generates internal chemical pressure and influences melting points. For example, in poly(ε-caprolactone) (PCL), the gradual substitution of C–H with C–D results in a reduction in molecular volume and lattice dimensions due to the shorter C–D bonds. This substitution can lead to a maximum unit cell contraction of up to 1.6%, accompanied by a decrease in the melting point by 4.9 K.⁵⁸

Isotopic polymorphism predominantly arises in systems with hydrogen bonds, where deuteration can significantly alter intermolecular forces.^54,59 This phenomenon, known as the geometric isotope effect (GIE) of hydrogen bonds, underscores the substantial structural impact of H/D isotope substitution.^60–65 It is crucial to examine the causes of the GIE in hydrogen bonds in detail. As shown in Scheme 2, the two distances, r₁ and r₂, can represent the two bonds of a hydrogen-bonded system. Alternatively, the coordinates q₁ = (r₂ − r₁)/2 and q₂ = r₁ + r₂ can also describe the system. Generally, when the D–H⋯A bond is linear, q₁ characterizes the deviation of protons from the hydrogen bond center, while q₂ describes the deviation of D⋯A. Upon H/D substitution, q₁ exhibits a notable change, termed the primary geometric H/D isotope effect. In contrast, significant variations in q₂ are often referred to as secondary GIE effects or Ubbelohde effects.^59,66 In weak hydrogen-bonded systems (d_D–H⋯A > 2.90 Å),⁶⁷ the primary GIE effect tends to dominate, resulting in a positive chemical internal pressure that typically shortens the lattice volume along the hydrogen-bonding axis.^16,68,69 Conversely, in strongly hydrogen-bonded systems (d_D–H⋯A ≤ 2.65 Å),^66,70–73 the secondary GIE effect often prevails, significantly lengthening the hydrogen-bond distance (approximately 0.02 to 0.03 Å)^{63,65,72,74–77} and generating a negative chemical pressure that stretches the lattice along the hydrogen-bonding direction.^21,62,78 Both isotopic effects are empirically related to the geometry of hydrogen bonds and isotopic fractionation factors, collectively determining the deformation of hydrogen bonds in the lattice and potentially leading to isotopic polymorphism.^60,79

Scheme 2 Schematic diagram of hydrogen bonding (above), and the changes in hydrogen bonding patterns upon deuteration (below).

Notably, the deuteration on quantum tunneling effect in hydrogen bonds is primarily due to differences in zero-point energies, which may also contribute to the GIE. Deuterium, compared to hydrogen, has a lower zero-point energy and greater mass, reducing the tunneling ability. From the standpoint of energy variation, in hydrogen bonds, the hydrogen atom almost exclusively occupies the lower energy state of two distinct minima, separated by an energy barrier. Upon deuteration, the energy is lowered, changing the energy difference and making it less favorable for quantum tunneling to occur (Scheme 3).⁸⁰ The effect of deuteration on quantum tunneling can be utilized in proton transfer-based ferroelectric materials to enhance the phase transition temperature (mentioned in the chapter 3.1).

Scheme 3 Schematic of deuteration effect on the hydrogen bond potential energy curve. (D: donor, A: acceptor).

In certain specific cases, the influence of deuteration on material structures can lead to changes in physical properties that are opposite to the typical isotope effects. This phenomenon is referred to as the reverse isotope effect (RIE).⁸¹ Generally, isotope effects are characterized by an increase in phase transition temperature, but in some materials, deuteration may suppress the increase in phase transition temperature, leading to reversed changes in physical properties. For example, in the classic ferroelectric material Rochelle salt, deuteration raises the upper Curie point of the ferroelectric phase transition, but has little effect on the lower Curie point, suggesting that deuteration stabilizes the ferroelectric phase by altering the geometry of hydrogen bonds.⁸² The origin of this effect is closely related to the geometry of hydrogen bonds and internal molecular twisting, with its impact on physical properties primarily mediated by changes in molecular dynamics and hydrogen bond structures.⁴¹ Therefore, the influence of the reverse isotope effect on physical properties requires a thorough analysis of the specific structural changes in the material's crystal lattice upon deuteration.

In addition, the substitution of hydrogen with deuterium (H/D isotope substitution) can introduce stress within the unit cell, leading to chemical pressure effects. This deuteration effect, when combined with the vibrational influences of H/D atoms, can significantly alter physical properties of materials, including dielectric and ferroelectric behaviors, magnetism, fluorescence, and conductivity. Consequently, deuteration serves as a strategic chemical approach for modifying material properties, thereby greatly enhancing their application prospects.^9,83–85

3. Effect of deuteration on physical properties

3.1 Deuteration effect on ferroelectric materials

Ferroelectric materials, characterized by their ability to have their polarization direction completely reversed between two opposite states when subjected to an external electric field below the Curie temperature (T_c), have been extensively utilized in sensors, information processing and electromechanical conversion.^86–88 To enhance the ferroelectricity of these materials, such as increasing the T_c for improved applications, deuteration of hydrogen-bonded ferroelectrics has emerged as an effective strategy. However, understanding the impact of H/D isotopic effects on dielectric and ferroelectric properties has been a long-standing challenge for scientists.

The exploration of deuteration's effect on ferroelectric materials began as early as 1942 when Bantle and his colleagues first deuterated the classic ferroelectric compound KH₂PO₄ (KDP), which underwent an H-ordering phase transition. They observed a significant increase in the T_c of the deuteration sample, as evidenced by changes in its specific heat capacity and dielectric constants.⁸⁹ Thirty years later, researchers confirmed through the P–E hysteresis loop that deuteration could increase the T_c of KDP (approximately 107 K) and its derivatives, such as PbHPO₄ (approximately 141 K), while also causing a slight increase in their spontaneous polarization.⁹⁰

The reasons behind the significant increase in the ferroelectric phase transition temperature of KDP-type compounds due to H/D isotope substitution have been debated for decades. Two primary explanations have emerged: the quantum-tunneling model⁹¹ and geometric isotope effect (GIE).^92–95 The quantum-tunneling model attributes the T_c increase mainly to transitions within the system of interacting H atoms, where localization of H (or D) atoms to one of their two potential minima causes heavy atom displacement. Quantum tunnelling, which is the opposite of localization, results in a lower T_c for H-type KDP due to the higher tunneling frequency of H. On the other hand, the GIE suggests that the structural changes induced by deuteration are the dominant factor leading to an increase in T_c. In 1990, McMahon et al. used neutron diffraction to analyze the single-crystal structure of deuterated samples under hydrostatic pressure.⁹⁴ They found that d_D−H⋯A distance in deuterated samples increased significantly, similar to the non-deuterated structure under pressure, confirming that the GIE is the dominant factor in the increase of T_c.

Subsequent researches revealed that deuterating other hydrogen-containing ferroelectrics, such as RbH₂PO₄, NH₄H₂AsO₄, and RbHSO₄ also enhanced their T_c.^96,97 This discovery has revitalized the field of ferroelectrics, positioning deuteration as a promising chemical strategy. Consequently, scientists have conducted detailed studies on deuteration in hydrogen-bonded ferroelectric materials. In 2005, Tokura et al. discovered that deuteration increased the T_c of the organic two-component eutectic ferroelectric Phz-H₂a (Phz: phenazine, H₂a: chloranilic acid) by approximately 51 K (Fig. 1).⁹⁸ To further investigate the reasons behind the T_c increase due to deuterium substitution, halogen substitution experiments were conducted using Phz-H₂b (H₂b: bromanilic acid). As shown in Fig. 1a, D substitution, Cl/Br substitution, and hydrostatic pressure application followed different T_c versus crystal structure relationships, indicated by the orange arrows. Additionally, the GIE of hydrogen bonding in this system was examined, revealing that the O⋯N distance extended after deuteration and contracted after halogen substitution, corresponding to the increase of T_c (Fig. 1b and c). This indicates that the GIE caused by deuteration is not the primary reason for the substantial change in T_c in the system. Furthermore, KDP-type ferroelectrics involve proton transfer between double potential minima, whereas hydrogen bonding in Phz-cocrystals does not involve proton transfer in the form of OH⋯N⁻ to O⁻⋯NH⁺. This suggests that the quantum tunneling model cannot adequately explain the significant T_c increase caused by deuteration in this context.

Fig. 1 (a) Temperature dependence of the dielectric constants of Phz-H₂ca and Phz-D₂ca (x = 0.89) (up) and Phz-H₂ba and Phz-D₂ba (x = 0.80) (down) at a frequency of 1 MHz (b and c) correlation between the ferroelectric phase transition temperature Tc and structural changes induced by hydrostatic pressure and deuteration: Filled squares represent Phz-H₂ca, Phz-H₂ba, and the ternary Phz-(H₂ba)_1−y(H₂ca)_y (y = 0.55) cocrystals, with orange arrows showing the H/D isotope effects. Open squares indicate the relationship between T_c and the lattice constant under hydrostatic pressure, derived from the c-axis compressibility of Phz-H₂ca and Phz-H₂ba cocrystals determined experimentally. Reprinted with permission from ref. 98. Copyright (2005) American Chemical Society.

To further elucidate the impact of deuteration on the Phz-cocrystals system, the Tokura research group conducted an in-depth investigation into the mechanism underlying the shift in the Curie temperature (T_c) in these compounds (Fig. 2).^40,99 They measured the dielectric constants of non-deuterated samples under hydrostatic pressure and assessed the dielectric changes of deuterated samples under normal pressure. For the Phz-Hca and Phz-Hba complexes, the trend of T_c shifting towards higher temperature was remarkably consistent when either hydrostatic pressure (up to 0.7 GPa) was applied or the sample was deuterated (Fig. 2c–e). Through high-pressure X-ray diffraction tests, they determined that the change in T_c is not significantly correlated with the O⋯N distance. The effects of deuteration were found to be somewhat analogue to those of applying pressure. However, hydrostatic pressure tends to accelerate proton transfer reactions, leading to ferroelectric stable multi-ion ground states, which contrasts with the effects of deuteration. In this ionic cocrystal system, the ferroelectric phase transition can be attributed to the competition between ionization energy and the increased electrostatic gain of the ionized lattice.⁹⁹ Under hydrostatic pressure, increased ionicity is achieved by enhancing the attractive interactions between acid–base molecules in the ac plane (Fig. 2f and g), whereas deuteration slows down this process. Consequently, deuteration significantly influences the T_c, akin to the effect observed in deuterated KDP-type ferroelectrics, where the GIE is predominant. Notably, in these ferroelectric materials formed by acid–base eutectic pairs, H/D isotope substitution results in a shift towards higher temperatures (ΔT_c = 30 to 70 K) without affecting the ferroelectric spontaneous polarization (P_s) or causing significant changes in dielectric curvature.

Fig. 2 (a) Chemical structures of Phz and H₂xa, (b) temperature-dependent dielectric constant of Phz-H₂xa (X = Cl, Br) and their deuterated cocrystals, (c–e) temperature variation of relative dielectric constant of Phz-H₂ca, Phz-H₂ba, Phz-D₂ba under various pressures, (f) view of the molecular packing along the b-axis (g) observation of the neutral and ionic segregation model on the (001) plane. Reprinted with permission from ref. 99. Copyright (2013) American Chemical Society.

The deuteration effect is also observed in molecular “stator-rotor” systems, such as those composed of Dabco (1,4-diazabicyclo-[2.2.2]octane), where protons can transfer within strong hydrogen bonds. This effect is likely influenced by the impact of H/D substitution on quantum tunnelling, particularly in systems where proton transfer occurs. A notable example is the compound [H₂dabco]·[2CB]₂ ([H₂dabco]²⁺ = diprotonated Dabco, 2CB⁻ = 2-chlorobenzoate),²⁴ which features a structurally asymmetric unit containing a complete trimeric unit. In this structure, a Dabco molecule bridges two acid molecules, with N–H⋯O hydrogen bond lengths (d_N⋯O) of 2.555(2) and 2.593(2) Å, respectively (Fig. 3a). Variable temperature dielectric and second harmonic generation (SHG) tests indicate that this compound exhibits ferroelectricity below 323 K. Given that [H₂dabco]²⁺ and CBA have nearly identical acid dissociation constants (pK_a values, Fig. 3b), it is implied that proton transfer may occur in [H₂dabco]²⁺, accompanied by an ordered-disordered variation (Fig. 3c). Upon H/D isotope substitution, the compound displays a significant deuteration effect, akin to other strong hydrogen-bonded ferroelectric compounds, with a transition temperature (T_c) increase of approximately 12 K (Fig. 3d).

Fig. 3 (a) Crystal structure of the high-temperature and low-temperature phases of the compound [H₂dabco]·[2CB]₂, (b) schematic diagram of acid dissociation for the relevant compounds, (c) schematic diagram of spontaneous polarization for the compound, (d) differential scanning calorimetry plot of the non-deuteration and deuteration sample. Reprinted with permission from ref. 24. Copyright (2016) American Chemical Society.

In a related study, Xiong et al. synthesized a new compound [HDabco][TFSA] (TFSA = bis(trifluoromethylsulfonyl) ammonium),²² whose ferroelectricity is solely induced by proton ordering, thereby significantly enhancing the deuteration effect (Fig. 4). Single-crystal X-ray diffraction (SC-XRD) analysis revealed that [HDabco]⁺ cations linked via N–H⋯N hydrogen bonds, forming a one-dimensional hydrogen bond chain that extends head-to-tail along two directions parallel to the ab plane, resulting in spontaneous polarization (Fig. 4a). The anions are situated between these hydrogen-bonded chains, creating a zero-dimensional structure. Unlike other Dabco-type compounds, which exhibit a certain order–disorder phase transition of the Dabco cation, this compound does not undergo such a transition. Specifically, the d_N⋯N distance is 2.728 Å in the ferroelectric phase and increases to 2.746 Å in the paraelectric phase, accompanied by thermal fluctuations of protons between the two double well potentials. The ferroelectricity of this compound arises from the stabilization of proton thermal fluctuations, which are fixed in one of the minimum positions, leading to an ordered dipole arrangement and polarization due to the infinite one-dimensional [HDABCO]⁺ hydrogen bonding chain extension. As depicted in Fig. 4b and c, this complex exhibits a large deuteration effect (ΔT ≈ 53 K), confirming its classification as a hydrogen-bonded ferroelectric and extending the operational temperature range of ferroelectricity from low temperatures (274.3 K) to above room temperature (327.4 K).

Fig. 4 (a) Schematic diagram of double-well proton coordinates along the one-dimensional hydrogen-bonded chain, (b) DSC of [HDABCO][TFSA] and [DDABCO]-[TFSA] (c) temperature dependent SHG response signals of [HDABCO][TFSA] and[DDABCO][TFSA]. Reprinted with permission from ref. 22. Copyright (2020) American Chemical Society.

In ferroelectric materials characterized by strong hydrogen bonds, deuteration generally leads to an increase in the T_c value, although the mechanisms responsible these changes can vary. The primary factors influencing H/D isotope substitution in such systems include structural changes, effects on quantum tunneling, and internal chemical pressure. However, when multiple factors are at play, deuteration can also result in a decrease in T_c. For example, in the hydrogen-bonded complex (Im)(TPA) (Im = imidazolium cation, TPA = hydrogen terephthalate monoanion),⁶² the single-crystal structure features a short hydrogen-bonded chain of COO–H⋯OOC within an anion channel-like host formed by TPA (Fig. 5). This structure contains two types of hydrogen bonds: strong O–H⋯O interactions, with d_O⋯O of 2.47–2.59 Å, and weaker N–H⋯O interactions, with d_N⋯O of 2.67–3.00 Å. Upon deuteration, the strong hydrogen bond of the carboxyl–carboxylate pair (COO–H⋯OOC) undergoes significant geometric deformation, with an increase in bond length of approximately 0.02 Å. This alteration affects the dynamics and arrangement of the Im guest molecules within the host channel (Fig. 5e). Conversely, deuteration of the weaker N–H⋯O interactions does not result in substantial changes in the molecular state. Overall, deuteration induces structural changes that create atypical chemical pressure within the molecule. This pressure is transmitted to the guest molecule through intermolecular interactions, leading to a decrease in T_c (ΔT_c ≈ 33 K) during the dielectric anomalous phase transition and even resulting in unpredictable isotopic polymorphism (Fig. 5b–d).

Fig. 5 (a) Chemical structure of (Im)(TPA) (1) and deuterated sample, (b) phase diagram of 1-d₀ and 1-d₅ as a function of temperature, (c and d) phase and dielectric transitions of 1 and deuterated sample (e) local H-bonded configurations of 1-d₀ and 1-d₅. Reprinted with permission from ref. 62. Copyright (2018) Springer Nature.

The impact of deuteration on the ferroelectric properties is generally negligible when the hydrogen bond is weak. For instance, in Dabco-type molecular ferroelectrics, the compound [(H₂dbco)Cu(H₂O)₆(SeO₄)₂] exhibits ferroelectricity below a critical temperature (T_c) of approximately 133 K.¹⁰⁰ The polarity of the ferroelectric phase in this compound arises from the ordered-disordered motion of the Dabco molecule. Upon deuteration, SC-XRD analysis reveals no significant change in the hydrogen bond length (d_D–H⋯A), and the T_c is altered by only 1.6 K. Similarly, in weak hydrogen bonding systems such as Im-ClO₄ and Im-IO₄,^101,102 with hydrogen bond lengths of 3.297 Å and 3.247 Å, respectively, both undergo an ordered-disordered ferroelectric phase transition. Deuterium substitution in these systems result in minimal changes to the geometry of the hydrogen bonds and negligible effect on T_c. This phenomenon is also observed in metal–organic-frameworks-type (MOFs-type) ferroelectrics, such as (DMA)Co(HCOO)₃ (DMA = Dimethylammonium), which undergoes an order–disorder phase transition¹⁰³ and has a hydrogen bond length of 2.903 Å. After replacing all H atoms with D in the MOFs, only a slight change in ferroelectricity is observed, with an increase in T_c by 4 K.

From the literature, it is evident that significant deuteration effects in ferroelectric materials typically occur in systems with strong hydrogen bonds, where proton transfer is involved.⁴¹ In contrast, systems with weak hydrogen bonds generally exhibit minimal impact on ferroelectric properties upon H/D isotope substitution.¹⁰⁴ Further, the pronounced deuteration effect is closely linked to the origins of ferroelectricity; when ferroelectricity is associated with the non-directional motion of protons, deuteration can significantly influence the T_c. However, there is a scarcity of literature addressing substantial effects of deuteration on spontaneous polarization (P_s), with most studies indicating negligible or no significant changes in P_s upon deuteration.

We have proposed a novel investigation approach that leverages the anisotropic characteristics of single crystal lattices, by applying anisotropic chemical pressure through selective deuteration.²¹ By focusing on ferroelectric compounds crystallized in the trigonal system, we have achieved a substantial deuteration effect on materials characterized by weak hydrogen bonds (Fig. 6). In the compound [Co^II(en)₃]SO₄, the SO₄²⁻ anion undergoes an order–disorder variation, which is related to its antiferroelectric-paraelectric-ferroelectric phase transition (Fig. 6a). This transition is associated with a 22 K intermediate phase ferroelectric window (P_s = 0.71 μC cm⁻²).¹⁰⁵ A distinctive feature of this compound's structure is the uniaxial alignment of both anions and cations along the polar c-axis, interconnected through weak N–H⋯O hydrogen bonds (d_N⋯O = 2.967 Å to 3.031 Å) and further linked by electrostatic interactions in both parallel and perpendicular directions. Despite the phase transition in the single-crystal structure, the SO₄²⁻ dianions, which are closely tied to the polarity of the single crystal, remain aligned along the crystallographic C₃ axis. This alignment facilitates the rapid propagation of minor changes in bond lengths throughout the crystal lattice.

Fig. 6 (a) Structural changes of the compound during the temperature-dependent process, (b) schematic diagram of the anisotropic deuteration, (c) schematic diagram of unit cell changes upon deuteration, (d) phase diagram of the deuteration extent of 1-N-D_x as a function of the T_c (e) effects of anisotropic deuteration on antiferroelectric (AFE)-ferroelectric (FE)-paraelectric (PE) phase transitions in compounds with different deuteration. Reprinted with permission from ref. 21. Copyright (2023) Elsevier Inc.

The symmetry of the molecule, with both anion and cation possessing D₃ symmetry and crystallized in trigonal lattice with anisotropic structural characteristics, results in different physical phenomena upon C–H/N–H deuterium substitution presents (Fig. 6b). The C–H/N–H bond is shortened due to the decrease of ZPE after deuteration, subjecting the lattice to anisotropic chemical pressure. Specifically, when the C–H atoms are deuterated, the chemical pressure is applied in the direction of the perpendicular polar axis (c-axis), leading to its elongation due to the Poisson effect. Conversely, when N–H atoms are deuterated, the chemical pressure is mainly applied in the direction of the parallel c-axis, resulting in its shortening (Fig. 6c). In systems with weak hydrogen bonds, the bond length change caused by deuterium substitution is minimal, and the molecular structure change is not significant, but it can affect lattice deformation (∼0.3%).

When C–H atoms are deuterated, the ferroelectric window temperature increases from 22 K to 56 K (Fig. 6e), and the spontaneous polarization increases by about four times (P_s = 2.80 μC cm⁻²). In contrast, when the N–H atom is deuterated, as the deuterium ratio increases, the ferroelectric window gradually narrows, and the spontaneous polarization value gradually decreases (Fig. 6d). When the deuterium substitution ratio of N–H exceeds 50% (N–D_x > 50%), the ferroelectric properties completely disappear, marking the first observation of ferroelectric isomerization caused by the deuteration effect. In the fully deuterated sample (N–D + C–D), the lattice exhibits an overall contraction accompanied by a small increase in its ferroelectric temperature window, an effect similar to that observed in other order–disorder ferroelectrics such as NaNO₂ and triglycerides (TGS), where T_c can be increased by the hydrostatic pressure induced by lattice contraction.¹⁰⁶

In the [Co^II(en)₃]SO₄ system, the substitution of deuterium for hydrogen in different molecular sites has distinct effects on P_s. specifically, C–D substitution leads to an increase in P_s, whereas N–D substitution results in a gradual decrease. This behavior can be understood through the ferroelectric mechanism of this compound, where the ferroelectric switching is attributed to the displacement of SO₄²⁻ dianions along the crystallographic c-axis. This displacement involves the reorientation of S–O bonds induced by an electric field. C–H deuteration effectively elongates the polar axis, thereby expanding the movement space available for the SO₄²⁻ dianions. In contrast, N–H deuteration shortens the polar axis, compressing the movement space of the anions and leading to a gradual reduction in spontaneous polarization in the N–D_x samples. This unconventional method of modulating P_s in ferroelectrics suggests that deuteration can introduce molecular chemical pressure within the anisotropic lattice. Consequently, this provids a novel strategy for tuning the properties of hydrogen-containing ferroelectric materials.

The sensitivity of ferroelectric properties to lattice distortions highlights the importance of considering the chemical pressure induced by deuteration as a critical factor. In systems characterized by strong hydrogen bonds, deuteration leads to structural elongation, which facilitates the adjustment of electrical properties through the application of positive chemical pressure, thereby influencing T_c. Conversely, in systems with weaker hydrogen bonds, the introduction of negative chemical pressure through design modifications involving trigonal lattices offers a versatile approach to property tuning. Furthermore, the impact of deuteration on thermal vibrations in ferroelectrics is significant, as it may induce energy changes that affect the T_c of the ferroelectric phase transition. Thus, the strategy utilization of deuteration provides a valuable tool for modifying ferroelectric properties across different systems.

3.2 Deuteration effect on magnetic materials

In magnetically ordered materials, the super-exchange interaction is described by the Heisenberg-Dirac Hamiltonian: , where S_i and S_j are total spin operators associated with magnetic ions M_i and M_j, and J is the exchange integral related to these ions, arising from the paramagnetic ion interactions.¹⁰⁷ For ferromagnetic interaction, J > 0, while for antiferromagnetic interaction, J < 0. Magnetic exchange is typically measured at critical temperatures, such as the Curie temperature (T_c) and Néel temperature (T_N). Deuteration generally has little effect on the total spin operators of magnetic ions M_i and M_j.²⁹ However, in most cases, H/D substitute can influence the ordering temperature and the position of magnetically centered ions, which in turn affects J and slightly alters the magnetic ordering process.^108–110

Early research on H/D substitution focused on magnetically ordered hydrated crystals at low temperatures, where super-exchange interactions are mediated by crystalline water molecules. Studies comparing hydrated and deuterated samples of ferromagnetic Cu(NH₄)₂Br₄·2H₂O and antiferromagnetic MnCl₂·4H₂O revealed that deuteration of crystallization water modified the super-exchange parameter J, altering T_c/T_N by a few percentage points, but had minimal impact on molar susceptibility (χ).²⁹ A different scenario is observed in lower-dimensional chain hydrates such as MnCl₂·H₂O and CoCl₂·H₂O.¹⁰⁹ In this structure, the d_O–H⋯Cl distances exceed 3.0 Å in the interchain, resulting in negligible changes in crystal structure and lattice parameters upon deuteration, typically less than 0.1%. Consequently, cation spacing changes minimally, preserving magnetic interactions between cations. For CoCl₂·H₂O, no difference in T_N (∼16.2 K) is observed upon deuteration, while χ_max changes from 0.604 to 0.615 emu mol⁻¹. In MnCl₂·H₂O, T_N decreases by 0.5 K after deuteration, and χ_max increases from 0.304 to 0.336 emu mol⁻¹. Similar conclusions were drawn from the deuteration study of NiCl₂·H₂O,¹¹⁰ where deuteration had no significant effect on structure or the χ_M ∼ T curve. The most notable change in magnetism occurs between NiBr₂·2H₂O and NiBr₂·2D₂O. Here, deuteration increases the unit cell size (∼0.3%), affecting the magnetic interactions (J) between chains, which impacts T_N (approximately −0.9 K) and χ_max (approximately 0.03 emu mol⁻¹). The larger χ_max and smaller T_N align with weaker interchain antiferromagnetic interactions due to unit cell expansion from deuteration.

Changes in magnetic ordering temperature are linked to the magnetic ordering of center ions and exchange interactions between magnetic centers, primarily driven by structural modifications from deuteration. Additionally, H/D substitution can induce lattice internal forces, or chemical pressure, impacting magnetic ordering. In the magnetoelectric compound [Co^II(en)₃]SO₄, as illustrated in Fig. 6, the non-deuterated compound exhibits two-step magnetic switching due to an ordered-disordered phase transition of the SO₄²⁻ dianion, driving a structural transition of [Co^II(en)₃]²⁺ that affects the angular momentum of Co(II) unquenched orbitals.¹⁰⁵ Consequently, the window for two-step magnetic switching is altered (Fig. 7), with significant shifts in the C–H deuteration sample and gradual changes in the N–H deuteration sample. Notably, C–H deuteration stretches the polar axis (c-axis), not only altering the phase transition but also eliminating magnetic hysteresis.²¹

Fig. 7 (a) The temperature dependence curve of magnetic anisotropy for non-deuteration [Co^II(en)₃]SO₄, (b) the temperature-dependent magnetic properties along the single-crystal c-axis for N–H bond deuterated samples; (c) the temperature-dependent magnetic properties along the single-crystal c-axis for C–H bond deuterated samples; (d) the temperature-dependent magnetic properties along the single-crystal c-axis for perdeuterated samples. Reprinted with permission from ref. 21. Copyright (2023) Elsevier.

Spin-crossover (SCO) materials, recognized for their magnetic bistability, can alter the spin state of centre metal ions (3d⁴–3d⁷) in response to external stimuli.^111–114 When H/D substitution occurs in these materials, it can significantly affect the T_1/2 point of the spin transition and the χ_MT value, particularly when proton transfer is involved. For instance, in the typical SCO co-crystal [Fe(bpp)₂](L)₂(HL)·5H₂O (HL = isonicotinic acid N-oxide),³¹ where the shortest hydrogen bond measures 2.425 Å and is associated with proton transfer, deuteration results in a decrease of approximately 6 K in the critical temperature of phase transition (Fig. 8). Conversely, in systems without proton transfer, such as [FeL₁(HIm)_x] (x = 1.8–2.2) and [FeL₂(MeOH)]₂·azpy (L₁ = {diethyl-(E,E)2,2-[1,2-phenylbis(iminomethylidyne)]bis[3-oxobutanoate](2⁻)-N,N′,O3,O3′}, L₂ = {2,2-[1,2-phenylbis(iminomethylidyne)]bis[1-phenylbutane-1,3-dione](2⁻)-N,N′,O3,O3′}), deuteration of hydrogen bonds shows negligible impact on the magnetic transition, with a T_1/2 shift of about 1 K, regardless of whether deuteration occurs in imidazole C–H or the guest molecule methanol.¹¹⁵

Fig. 8 (a) The chemical formula of the ligand (b) schematic diagram of hydrogen bonds in the crystal at 240 K (c) DSC curves of the complex, solid circles represent deuterated samples, while hollow circles represent non-deuterated samples, (d) the temperature dependence of the magnetic susceptibility, black represent non-deuterated samples, while orange represents deuterated samples. Reprinted with permission from ref. 31. Copyright (2021) Royal Society of Chemistry.

In certain systems, the influence of H/D substitution on SCO behavior is linked to molecular kinetic factors, such as molecular mass and zero-point energy (ZPE). For example, in the Hofmann-type two-dimensional (2D) SCO compound {[Fe^II(pyridine)₂][Au^I(CN)₂]₂}_n and its deuteration analogue {[Fe^II(pyridine-D₅)₂][Au^I(CN)₂]₂}_n, the χ_MT ∼ T curve remains largely unchanged, indicating minimal impact from H/D substitute.¹¹⁶ However, in the [Fe(pyridine)₂Ni(CN)₄], [Fe(pyridine-D₅)₂Ni(CN)₄], and [Fe(pyridine-¹⁵N)₂Ni(CN)₄] complexes,²⁸ the H/D and N/¹⁵N isotope effects on thermal SCO behavior are similar, with the spin transition temperature decreasing by 8 K for H/D substitute and 7 K for N/¹⁵N substitute (Fig. 9a and b). These isotope effects, driven by kinetic factors, significantly reduce the enthalpy change of the SCO process by approximately 13%. Similarly, in the SCO compound [FeL₂][BF₄]₂ (L = 2,6-di(pyrazol-1-yl)pyridine),²⁷ C–H deuteration lowers the T_1/2 by about 5 K (Fig. 9c). Comparing the entropy change measurements during the SCO of deuterated and non-deuterated samples, researchers observed a significant reduction in entropy change – by 10% or more − in partially or fully deuterated compounds, akin to the effects of applied pressure, where the entropy variation during the transition from low spin to high spin is also reduced.

Fig. 9 (a) The structure of Fe(pyridine)₂[Ni(CN)₄], (b) thermal spin-crossover in Fe(pyridine)₂[Ni(CN)₄] and its isotope substitute sample, Reprinted with permission from ref. 28. Copyright (1999) Royal Society of Chemistry. (c) Thermal spin crossover in [FeL₂][BF₄]₂ (hollow square) and deuterated sample (solid circle). Reprinted with permission from ref. 27. (2019) WILEY-VHC.

Recently, Mason et al. proposed manipulating the spin transition through H/D isotope substitution of organic ligands, building on previous research. This approach aims to regulate the spin transition temperature by inducing subtle yet direct perturbations in bonding vibration properties, electronic properties, and intermolecular interactions due to different H/D masses.³⁰ They synthesized the complex [Fe{HB(tz)₃}₂] (HB(tz)³⁻ = bis[hydrotris(1,2,4-triazol-1-yl)borate]) in a proton-transfer-free system to assess the effect of deuteration on the thermodynamic and pressure dependence of the SCO transition (Fig. 10). As depicted in Fig. 10a and b, the transition of the central ion from a low-spin to a high-spin state results in an increase in the Fe–N bond length from 1.988 Å to 2.144 Å, accompanied by a 3.1% unit cell volumetric expansion and an entropy change of 99 J kg⁻¹ K⁻¹. Notably, isobaric high-pressure DSC experiments on samples [Fe{HB(tz)₃}₂], [Fe{DB(tz)₃}₂], and [Fe{DB(tz-d₂)₃}₂] with varying degrees of deuterium substitution revealed that increased deuteration gradually reduces the SCO transition temperature, with full deuteration lowering T_1/2 by approximately 4 K and simultaneously reducing the entropy change during the SCO process (Fig. 10c and d). These findings suggest that H/D isotope substitute and isotropic hydrostatic pressure exert similar effects, implying that deuteration can serve as an intramolecular chemical pressure to manipulate the thermodynamic and T_1/2 of SCO materials.

Fig. 10 (a) Thermal spin-crossover in Fe[HB(tz)₃]₂ (b) DSC curves and temperature-dependent unit cell volume changes of Fe[HB(tz)₃]₂ (c) DSC curves of Fe[HB(tz)₃]₂ and various deuterated samples. (d) The effect of pressure on the onset temperatures of heating transitions for non-deuterated samples, measured using high-pressure DSC. Reprinted with permission from ref. 30. Copyright (2022) American Chemical Society.

Predicting the performance of magnetic materials upon deuteriation is challenging. Generally, H/D isotope substitution has a less significant impact on magnetic materials compared to ferroelectric properties, as ferroelectric properties are closely related to the spatial structure of material. In contrast, magnetism arises from interactions among magnetic ions, which are difficult to manipulate, along with contributions from electron spin and orbital motion. Deuteration typically affects the crystal lattice and weakens the vibration mode of chemical bonds, influencing the critical temperature of phase transitions from both kinetics and thermodynamics perspectives, often decreasing the magnetic transition temperature and reducing the entropy change during magnetic variation.

3.3 Deuteration effect on photophysical materials

Deuteration, the substitution of hydrogen with deuterium isotopes within molecular structures, has been extensively documented to significant alter the photophysical characteristics of compounds. This modification has been shown to boost the quantum yields and lifetimes of both fluorescence and phosphorescence in a variety of chemical systems, including aromatic hydrocarbons and aliphatic carbonyl compounds.¹⁷ The extent of these enhancements is contingent upon the specific site of deuteration, and the use of deuterated solvents can further influence the photophysical properties of solutes.¹¹⁷ Given the anisotropic nature and distinct spatial configurations of solid-state crystalline materials, deuteration can modulate their lattice properties, affecting both structural and vibrational aspects. This chapter, therefore, concentrates on the impact of deuteration on luminescence within solid crystalline solutes and its potential to augment photophysical properties.

Crystalline covalent organic frameworks (COFs) are known for their suboptimal photoluminescence performance, which is often hampered by aggregation-caused quenching (ACQ) and thermal losses stemming from intramolecular vibrations. These factors restrict their utility in photophysical applications. However, Wang et al. have demonstrated that deuteration can substantially improve the quantum yields and lifetimes of luminescent COFs.³² They synthesized a novel azine-linked COF, SCU-COF-9a, derived from the condensation of 2,4,6-teiformyl (Tp) and 3,3′-((4,4′′-diamino-[1,1′:4′,1′′-terphenyl]-2′,5′-diyl)bis(oxy))bis(propane-1,2-diol) (Tpa-NH₂). The non-deuterated version of this COF displayed weak fluorescence. Through systematic deuteration at various sites (Fig. 11a–d), they observed that increasing both the number and degree of deuteration sites led to marked enhancement in the fluorescence quantum yield and emission lifetime of the COFs, with improvements reaching up to 19-fold compared to the non-deuterated samples (Fig. 11e–g). Utilizing density functional theory (DFT) calculations to analyze the electronic states and excited-state geometries within the COFs, they concluded that C/N–D and O–D⋯O bond vibrations play a crucial role in suppressing non-radiative decay processes and reducing energy loss, thereby promoting radiative transitions and enhancing luminescence.

Fig. 11 (a) Schematic of H-Tpa deuteration method, (b) SSNMR 1D 13C{'H} cross polarization spectra of SCU-COF-9 and its deuterated analog, (c) synthesis of SCU-COF-9 and its single crystal structure, (d) hydrogen bonds of SCU-COF-9, (e–g) photoluminescence properties of SCU-COF-9 and its deuterated analog. Reprinted with permission from ref. 32. Copyright (2021) WILEY-VHC.

Expanding on their previous work, Wang and colleagues have significantly improved the photoluminescence of COF materials through deuteration (Fig. 12).³⁴ They synthesized a fully deuterated variant of TFPE, termed DD-TFPE, which exhibited a remarkable fluorescence quantum yield of 90%. This was achieved by starting with the aggregation-induced emission (AIE)-active luminescent building block, tetra-(4-formyl-(1,1′-biphenyl))ethylene (HH-TFPE), which itself has a fluorescence quantum yield of 73%. Employing a “bottom-up” deuteration strategy (Fig. 12a and b), They then coupled this compound with piperazine through a condensation reaction to form the crystalline deuterated DD-COF. The solid-state fluorescence quantum yield of DD-COF was found to be 81%, a substantial increase over the non-deuterated HH-COF, which has a yield of 43%. This surpasses the fluorescence quantum yields reported for all luminescence COFs materials to date (Fig. 12c). The enhanced luminescent efficiency is attributed to the effective suppression of non-radiative decay pathways, particularly those involving vibrational quenching, which is facilitated by the use of heavier isotopes with lower zero-point vibrational frequencies, such as deuterium in place of hydrogen. Additionally, deuterated COFs demonstrated superior photostability under UV irradiation compared to their non-deuterated counterparts. This improvement is partly due to the reduced excitation state energy following C–H to C–D substitution, which promotes radiative relaxation and reduces non-radiative decay pathways. It is also related to the lower zero-point vibrational energy of C–D bonds.

Fig. 12 (a) Schematic diagram of deuteration mode, (b) synthesis of HH-COF and perdeuterated DD-COF, (c) photoluminescence properties of HH-COF and perdeuterated DD-COF. Reprinted with permission from ref. 34. Copyright (2024) American Chemical Society.

Deuteration has been observed to have a similar enhancing effect on the Near-Infrared (NIR) emission of certain complexes.⁴⁵ In a recent development, researchers have synthesized a series of Pt(II)-based NIR emitters, including H-3-f, HPh-3-f, and their deuterated counterparts, D-3-f and DPh-3-f (Fig. 13a). They observed that in solid-state films, the deuterated analogs significantly increased the photoluminescence quantum yield and caused a slight shift in the emission peak (Fig. 13b). Specifically, the maximum emission wavelengths for compounds H-3-f and HPh-3-f were at 810 nm, with photoluminescence quantum yields of 29% and 50%, respectively. Upon deuteration, the emission wavelengths shifted to 800 nm and 798 nm, while the quantum yields increased to 50% and 67%, respectively. The authors attributed the observed photophysical differences post-deuteration to variations in C–H versus C–D vibrational modes. The increase in photoluminescence quantum yield was linked to the lower average frequency of C–D vibrations and the strong energy gap dependence arising from the large Huang–Rhys (HR) factors in the strong coupling limit (Fig. 13c),¹¹⁸ which is particularly pronounced in Pt complexes due to their larger Stokes shifts between absorption and emission. The absence of a significant peak shift can be attributed to the relatively small HR factor in the weak coupling region for organic NIR materials, as their π-delocalization enhancement reduces the energy gap in the NIR range. Consequently, the impact of changes in C–H/D stretching frequencies on the Frank–Condon overlap is minimized.

Fig. 13 (a) Scheme of deuteration methods for Pt complexes, (b) UV/Vis absorption and emission spectra of Pt complexes, (c) plot of dominant nonradiative attenuation channel (k_IC) as a function of C–H energy gap. Reprinted with permission from ref. 44. Copyright (2024) WILEY-VHC.

In the realm of materials science, deuteration has emerged as a potent strategy for boosting the luminescent efficiency of materials.^{33,36,43,119,120} Pioneering this field, Wang et al. in 2013 first elucidated the profound influence of deuteration on the longevity of phosphorescent materials.^35,44 Their groundbreaking work revealed a remarkable 20-fold enhancement in the lifetime of a green phosphorescent device, which was doped with Ir(ppy)₃-D₂₄, albeit with a slight reduction in brightness. This significant improvement was ascribed to the “deuteration effect”, which they hypothesized stemmed from the diminished internal energy of the molecule. The rationale behind this lies in the fact that the vibrational energy associated with C–D bonds is inherently lower than that of C–H bonds.

Recently, the molecular infiltration strategy has been ingeniously harnessed to stabilize blue thermally activated delayed fluorescence (TADF) emitters. By employing fully deuterated compound, such as D-5CzBN/D-5tCzBN (Fig. 14a), high-energy vibrations are effectively quelled,³⁶ culminating in heightened efficiencies and a twofold increase in device lifetime when compared to their protonated counterparts. Moreover, this infiltration effect not only yields a narrower and blue-shifted emission spectrum in solid films but also bolsters the transfer of Förster energy to TAFD-sensitized fluorescence light-emitting diodes (OLEDs) in deep blue emitters (Fig. 14b). These deuterated compounds have achieved a maximum external quantum efficiency of 33.1% in devices and have demonstrated a remarkable lifetime of 1365 hours, retaining 80% of their initial luminance–surpassing even the performance of blue phosphorescent OLEDs. DFT calculations have shed light on the mechanism behind this enhancement, indicating that deuteration modulated fluorescence by increasing the overlap integrals between the vibrational wavefunctions of the excited state (v = 0 of S₁) and the ground state (v = n of S₀). This, in turn, augments radiative decay in accordance with the Franck–Condon principle. Furthermore, perdeuterated emitters, with their more compact vibrational energy alignments, exhibit larger Franck–Condon integrals, leading to narrower and blue-shifted emission spectra. These findings underscore the exceptional potential of deuteration as a design strategy in the realm of photophysical physics.

Fig. 14 (a) Structural formula of 5CzBN/5tCzBN and its deuterated analogs, (b) UV/Vis absorption and photoluminescence properties of 5CzBN/5tCzBN and its deuterated analogues, (c) schematic diagram of the vibronic coupling between the ground (S₀) excited (S₁) states of non-deuterated and perdeuterated molecules. Reprinted with permission from ref. 36. Copyright (2024) Springer Nature.

Deuteration, a sophisticated chemical synthesis technique, has not only provided substantial control over the photophysical properties of solid-state materials in research settings but has also translated into significant enhancements in optical performance across commercial applications, including OLEDs and other light-emitting technologies. One of the most compelling benefits of deuteration is its compatibility with a range of other strategies aimed at enhancing luminescence, all without substantially altering the structural integrity of the luminescent molecules themselves. Moreover, deuteration selectively targets the bond vibration frequencies while largely preserving other physical and chemical properties of the molecules, thus maintaining the essential photophysical properties of the monomers. In terms of the photophysical properties, the doubled nuclear-to-mass ratio, reduced ZPE, and attenuated bond vibration frequencies all contribute to a substantial weakening of non-radiative transitions. This makes deuteration an intriguing and promising application in the field of photophysics.

3.4 Deuteration effect on conductivity materials

Conductivity is a measure of material's capacity to conduct electric charge, a property influenced by the mobility of charge carriers, their concentration, and the crystal lattice structure of the material.¹²¹ Additionally, vibrational modes and electron–phonon interactions play a significant role in determing the conductive properties of materials.¹²² The replacement of H with D capitalized on the latter's greater mass and lower vibrational frequency, which can shift the energy landscape of the material. This alteration can mitigate the effects of high-energy vibrational modes, thus potentially enhancing the mobility of charge carriers. Moreover, deuteration can optimize electron–phonon coupling, further augmenting conductivity.^9,25,123 For example, in single crystals of (D_xMA)PbI₃, where x = 0, 0.15, 0.99 and MA = methylammonium, deuteration has been shown to markedly improve both carrier lifetime and mobility.⁸³ Specifically, the samples (D_0.99MA)PbI₃ and (D_0.15MA)PbI₃ exhibit substantial enhancements in carrier lifetime and mobility, with values reaching 328.2 ns and 296.2 cm² V⁻¹ s⁻¹, respectively. These figures are approximately twice and four times greater than those observed in the non-deuterated MAPbI₃. This enhancement is believed to stem from a moderate level of deuteration achieving an optimal balance among the hyperfine interactions of the organic molecules, the phonons within the organometallic material, and electron–phonon coupling. This fine-tuning underscores the strategic potential of deuteration in optimizing the conductivity of materials.

In systems characterized by strong hydrogen bonds, the influence of deuteration on electrical conductivity may stem not only from changes in vibrational modes but also from structural alterations, particularly those affecting the length of hydrogen bonds. In 2014, Mori et al. reported on an organic conductor crystal, κ-H₃(Cat-EDT-TTF)₂ (Fig. 15), which is based on hydrogen-bonded molecular units.²⁶ The single-crystal structure analysis revealed that this material is composed of two crystallographically equivalent catechol-fused ethylenedithiotetrathiafulvalene (Cat-EDT-TTF) frameworks (Fig. 15a), each bearing a +0.5 charge, connected by an anionic [O⋯H⋯O]⁻¹ type strong hydrogen bond (d_O⋯O = 2.453(5) Å at 50 K). Upon deuteration, the O⋯O distance expands to 2.501(3) Å at 50 K. More intriguingly, the GIE of the hydrogen bonds is accompanied by electron movement, leading to a hydrogen-bond-driven switch in electrical conductivity. Specifically, the compound transitions from a paramagnetic semiconductor with a dimer-Mott-type electronic structure at room temperature to a diamagnetic insulator with a charge-ordered (CO) electronic structure. As illustrated in Fig. 15b and c, the sample exhibits typical semiconducting behavior in its resistivity and undergoes a semiconductor-insulator-like phase transition at 182 K upon deuteration, which is markedly different from the continuous monotonic increase observed in the non-deuterated sample. Additionally, the room-temperature electrical conductivity and activation energy of the deuterated sample are 6.2 S cm⁻¹ and 0.08 eV, respectively, slightly higher than the 3.5 S cm⁻¹ and 0.11 eV observed in the non-deuterated sample. This enhancement is likely due to the chemical pressure effect induced by deuteration. This mechanism is believed to arise from the transfer or displacement of deuterium within the [O⋯D⋯O]⁻ hydrogen bonds, coupled with electron transfer between the hydrogen-bonded Cat-EDT-TTF π systems, as a consequence of the cooperative coupling between deuteration dynamics and conductive TTF π electrons. More importantly, theoretical calculations suggest that the fundamental discrepancy in electrical conductivity is intricately linked to the quantum isotope effect associated with differences in ZPE.

Fig. 15 (a) The synthesis scheme of compound κ-H₃(Cat-EDT-TTF)₂ and κ-D₃(Cat-EDT-TTF)₂, (b) temperature dependence of electrical resistivity, (c) temperature dependence of magnetic susceptibility. Reprinted with permission from ref. 26. Copyright (2014) American Chemical Society.

In weak hydrogen-bond systems, subtle structural changes induced by different deuteration methods in the complex 2-(Me₂-DCNQI)₂Cu (DCNQI = N,N′-dicyanoquinonediimine) lead to significant variations in its electrical conductivity (Fig. 16).¹²⁴ Notably, in the non-deuterated sample, no phase transition occurred across the entire temperature range. When a methyl group in the ligand was deuterated, it induced an insulator-to-semiconductor phase transition at 58 K, as clearly observed in the Fig. 16b. With progressive deuteration of the hydrogen-substituted groups on the ligand, the conductive transition temperature T_c of the sample increased from 58 K to 82 K upon perdeuteration. Interestingly, applying pressure to the non-deuterated sample also induced a similar phase transition. Specifically, at 110 bar, a pronounced phase transition occurs at 47 K, and increasing the pressure to 400 bar raised it to 76 K (Fig. 16c). The observed changes in T_c are attributed to two main factors: on one hand, deuteration compresses the lattice, affecting inelastic neutron scattering and inducing chemical pressure within the lattice; on the other hand, the increased mass due to H/D isotope substitution affects atomic vibrations.

Fig. 16 (a) Phase transition temperatures of compound 1 and its deuterated analogues, (b) temperature dependence of the conductivity of 1a, 1b, 1c and 1d, (c) pressure dependence of the conductivity of 1a. Reprinted with permission from ref. 124. Copyright (1992) WILEY-VHC.

Subsequently, scientists continued to investigate the structure of the compound 2-(Me₂-DCNQI)₂Cu and proposed that the structure of the non-deuterated sample is in a critical state,^23,48 with the ligand coordination angle at low temperatures being very closely to the proposed critical angle of 126.4°, which exceeding this angle would induce a phase transition to a strictly one-dimensional semiconductor. By combining conductivity, magnetic susceptibility measurements, electron spin resonance (ESR) spectroscopy, and X-ray diffraction (XRD) studies, and analyzing the relationship between the coordination angle and temperature, a metal–semiconductor phase diagram was constructed. This diagram provides a comprehensive description of the compound's phase transition behavior and is related to several factors involving the combination of spherical substituents in the ligand, including cooling, pressure application, and changes in substituents. It can be inferred that the significant secondary deuterium isotope effect on the physical properties of these compounds is due to structural changes induced by chemical pressure from deuteration, specifically the shortening of the c-axis and the resulting compression of the Cu–N coordination bonds.

Superconductors are a unique class of conductive materials that, when cooled below a critical temperature, display zero electrical resistance and completely expel magnetic fields. This phenomenon allows electric currents to flow without any energy loss and prevents magnetic fields from penetrating the superconducting material. The impact of deuteration on superconducting materials has been a subject of scientific inquiry since the early 1980s.

A particular focus has been on a family of organic superconductors, (TMTSF)₂X, where TMTSF⁺ stands for tetramethyltetraselenafluvalene and X represents various anions such as PF₆⁻, AsF₆⁻, SbF₆⁻, TaF₆⁻, ReO₄⁻, and ClO₄⁻.¹²³ Among these, compound with X = ClO₄⁻ has garnered significant interest due to its superconductivity at ambient pressure. Deuteration results in a slight decrease in the superconducting transition temperature (T_c) by approximately 0.1 K. The authors attribute this change to the closer packing of organic molecules in adjacent layers due to deuteration, which increases the spring constant and, consequently, the frequency of phonon vibrations. They suggest that the effect of deuteration on the unit cell volume is the primary driver of T_c variation, rather than the mass difference between H and D.

The complex (BEDT-TTF)₂Cu(NCS)₂, with BEDT-TTF being bis(ethylenedithiolo)tetrathiafulvalene, has been extensively studied due to its high superconducting transition temperature, which ranges from 10.2–10.4 K for the protonated salt. Experiments using the perdeuterated donor molecule D-BEDT-TTF have yielded a slightly higher T_c of 11.0 K.^47,125 The research emphasizes the importance of understanding the electron–phonon interaction mechanisms behind the increases in T_c due to deuteration. It also proposes that the subtle changes in the hydrogen bond coupling between BEDT-TTF molecules and [Cu(NCS)₂]⁻ anions, which form linear chains, could contribute to the rise in T_c. Moreover, the most drastic change in the electronic state upon deuteration of BEDT-TTF salts is the superconductor-insulator transition observed in κ-(BEDT-TTF)₂Cu[N(CN)₂]Br.^126–128 Deuteration modifies the carrier properties by precisely tuning the molecular arrangement within the two-dimensional BEDT-TTF sheets, thereby altering the system's electronic structure and inducing a shift in the Mott transition. This demonstrates the utility of H/D isotope substitution as a technique for tuning the electronic transport properties and phase transition behavior of low-dimensional materials.

It is noteworthy that H and D can serve as doping agents to enhance the electrical conductivity of materials. Recently, Kitagawa et al. demonstrated that doping the primary material with H/D can significantly alter the conductivity of ZnO with minimal lattice disruption (Fig. 17).⁴⁶ They investigated the conductivity of pure ZnO thin films under non-equilibrium hydrogen doping conditions at 7 K (Fig. 17a and b). Exposure to H₂⁺ irradiation at 7 K led to a dramatic increase in conductivity by five orders of magnitude, accompanied by approximately 1% extension along the c-axis (Fig. 17c and d). This substantial enhancement is attributed to n-type doping effects from interstitial and/or substitutional hydrogen within the ZnO lattice (Fig. 17e). In contrast, D₂⁺ irradiation at 7 K resulted in a more modest increase in conductivity, on the order of one magnitude. The researchers attribute the significant difference in conductivity to the mass disparity between hydrogen and deuterium, which influences the metastable trapping sites createed by non-equilibrium doping at 7 K and the migration rates of trapped hydrogen (deuterium) atoms at elevated temperatures.

Fig. 17 (a) Schematic diagram of the crystal structure of H_xZnO, (b) the impact of H/D isotope effects on the electrical resistivity of ZnO, (c) the temperature dependence of the electrical resistivity of D₂-irradiated samples before exposure to a 1 kV field, (d) XRD patterns of ZnO before and after D₂ irradiation, (e) schematic diagram of the electronic structure of ZnO at 7 K. Reprinted with permission from ref. 46. Copyright (2021) American Chemical Society.

The influence of deuteration on electrical conductivity is indeed multifaceted, encompassing a range of physical and chemical effects. Deuteration can lead to alterations in the hydrogen bond structures within a material, which in turn can impact its conductivity. Furthermore, it can generate chemical pressure that influences the material's ability to conduct electricity. Moreover, the introduction of H or D isotopes as dopants can substantially enhance the lifetime and mobility of charge carriers in the original material. The larger mass and lower vibrational frequency of deuterium, compared to hydrogen, can lead to an optimization of electron–phonon coupling. This optimization can further improve the material's conductive properties. The heavier deuterium atoms can also affect the material's lattice dynamics, potentially leading to changes in the density of states and thus influencing the overall electric conductivity.

4. Outlook

Deuteration holds substantial promise for enhancing the physical characteristics of a variety of hydrogen-containing materials. By meticulously adjusting the levels of deuteration, it becomes feasible to optimize various properties such as dielectric and ferroelectric properties, electrical conductivity through modifications in hydrogen bond structures, reduction of high-energy vibrational modes, and enhancement of electron–phonon interactions within these materials. This method could pay the way for the creation of advanced materials with specialized functionalities, applicable in sectors like semiconductors, superconductors, and optoelectronics. Additionally, the quantum isotope effects resulting from deuteration could reveal new physical phenomena, furthering our comprehension of quantum materials. The synergy between deuteration and other material modification techniques, such as doping and pressure application, could provide more precise control over material properties and broaden their potential uses.

In addition to the physical properties mentioned above, deuteration can also serve as a doping strategy to mitigate ion migration in perovskite materials, thereby enhancing their X-ray detection performance.¹²⁹ For instance, in the perovskite MAPbI₃ (MA = methylammonium), deuteration reduces the vibration frequency of the inorganic framework, resulting in alterations to the electronic structure and electrical properties of the material. It has been demonstrated that increasing the degree of deuteration of the organic cations leads to a more significant improvement in the performance of MAPbI₃-based X-ray detectors. Furthermore, in other perovskite materials, deuteration can reduce their chemical reactivity by lowering bond vibration frequencies, while also tuning phonon modes and molecular dynamics.^130,131 This leads to significant improvements in thermal conductivity, stability, and optoelectronic properties, thus expanding the practical application potential of these deuteration perovskite materials.

However, despite its considerable potential, the application of deuteration faces several challenges as highlighted by current research:

(1) Although deuterium is relatively abundant in nature, the high cost associated with its separation may limit its extensive application in deuteration processes.¹³² There is an urgent need to explore and develop more efficient methods for H-to-D extraction and separation.

(2) Deuterated materials in most hydrogen-containing systems are prone to H/D exchange with atmospheric water vapor, which can compromise their chemical stability. Moreover, deuteration might introduce new structural defects, potentially affecting the long-term performance and reliability of the materials. Therefore, the stability of materials post-deuteration must be thoroughly assessed when employing deuteration as a synthetic strategy.

(3) Ensuring uniformity and consistency in H/D isotopic substitution in practical applications is a significant technical hurdle. Non-uniformity in deuteration, akin to doping processes, can lead to material property discrepancies, negatively impacting research outcomes and practical applications. Accurate characterization of deuterated samples often requires a combination of analytical techniques, with both neutron diffraction and Raman spectroscopy being essential for precisely evaluating the accuracy and orientation of deuteration within materials.

(4) The influence of deuteration on material properties is multifaceted, affecting structural, vibrational, electronic, and optical characteristics. A thorough understanding of these complex relationships requires detailed theoretical models and extensive experimental data, posing a significant challenge to current research capabilities in materials science.

In summary, these challenges underscore the need for researchers to continuously refine experimental methods for H/D substitution and to develop new theoretical models in the study and application of deuterated materials. This ongoing effort is vital for achieving a comprehensive understanding and effective utilization of the intricate effects of deuteration on material properties.

Author contributions

C. L. performed the literature survey and prepared the initial draft. Z.-S. Y and J. T. revised the draft and wrote the final version.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants 22071009 (Z.-S. Y.), 22371015 and 92061106 (J. T.)) and Xiaomi Young Scholar Program (Z.-S. Y.).

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