Chiral 0D hybrid lead-bromide perovskites with strong nonlinear chiroptical properties

Abstract

Chiral optical materials enable simultaneous linear and nonlinear optical properties and have emerged as a new class of materials desirable for applications in chiroptical information technology. Herein, we developed two pairs of hybrid lead-bromide perovskites (R-/S-APD)PbBr₄ and (1R,2R-/1S,2S-DACH)2PbBr₆·2H₂O, and systematically investigated their linear and nonlinear chiroptical responses. Second-harmonic generation circular dichroism (SHG-CD) measurements reveal a high anisotropy factor (g_SHG-CD) of up to 1.58 for (1S,2S-DACH)₂PbBr₆·2H₂O, which is the highest value among those of the reported chiral perovskites to date. Notably, these perovskites display a high laser damage threshold (LDT) of up to 59.36 GW cm⁻². This study demonstrates that the 0D chiral hybrid lead-bromide perovskite system can simultaneously exhibit both high LDT and g_SHG-CD, thereby opening a new route for the design of high-performance chiral nonlinear optics.

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Introduction

Nonlinear optical (NLO) materials are extensively used in various optical devices and play an indispensable role in laser and information technologies.^1–5 Second harmonic generation (SHG) is a nonlinear process of converting two low-frequency photons into one coherent high-frequency photon within a nonlinear medium. Such frequency-doubling processes are the key basis of many advanced applications, including all-optical switching,⁶ biological microscopic imaging,⁷ optical modulation⁸ and quantum technology.⁹ Noncentrosymmetry in crystal structures is the fundamental prerequisite for designing SHG-active materials. An effective strategy to obtain a noncentrosymmetric crystal structure is through the introduction of chiral or asymmetric organic components.^10,11

Chiral materials are intrinsically noncentrosymmetric, and can interact differently with left- and right-handed circularly polarized light, leading to chiroptical effects such as circular dichroism (CD),¹² circularly polarized luminescence (CPL)^13,14 and SHG circular dichroism (SHG-CD).¹⁵ Recently, great interest has been paid to chiral perovskites, which simultaneously possess inherent noncentrosymmetric structures and intrinsic chirality, and have shown attractive applications in NLO,^16–18 chiroptoelectronics^19–21 and spintronics.^22–25 Several chiral hybrid organic–inorganic perovskites based on different organic components and metals have been developed for SHG effects, including (L-ipp)PbI₃,²⁶ A₂BBiX₆,²⁷ MHyPbBr₃²⁸ (R/S-MBA)₄Bi₂Br₁₀,²⁹ (R/S-3AP)₄AgBiBr₁₂,³⁰ MAGeBr₃,¹⁰ (R/S-MBA)CuBr₂¹⁵ and (R/S-MBA)BiI₄.¹ However, chiral hybrid perovskites with SHG-CD properties were not reported until very recently. The first SHG-CD of hybrid perovskites was reported on (R-MPEA)_1.5PbBr_3.5(DMSO)_0.5 nanowires, exhibiting an effective second-order NLO coefficient of 0.68 pm V⁻¹.³¹ Subsequently, SHG-CD of the bulk single crystals of [(R/S)-3-aminopiperidine]PbI₄ showed an anisotropy factor (g_SHG-CD) of 0.21.³² Higher g_SHG-CD values of 0.41, 0.58 and 0.8 were achieved in copper- and bismuth-perovskite systems.^29,33,34 The first chiral hybrid germanium iodide exhibited a g_SHG-CD of 0.48.³⁵ We proposed a new strategy to construct a chiral mixed halide hybrid perovskite, which showed a high g_SHG-CD of 1.56.³⁶ Recently, the (R-MBACl)₂PbI₄ film was reported to exhibit a high g_SHG-CD value of up to 1.57.³⁷

Over the past years, diverse metal cations, such as Pb²⁺, Ge²⁺, In²⁺, Bi³⁺ and Cu²⁺, have been introduced to construct chiral hybrid perovskites for SHG-CD investigation.^29,31–36 Compared to other ions, Pb²⁺ ions have partially filled d-electron orbitals, which contribute to strong electronic polarizability and electronic transitions. Therefore, lead-containing hybrid perovskites feature strong second-order susceptibility, showing great potential in SHG-CD materials.^28,38–48 However, only two investigations on SHG-CD of lead-bromide hybrid perovskite systems have been performed, and hence the structure–property correlation is not fully understood.^31,34 Besides, lower dimensional perovskites, especially zero-dimensional (0D) hybrid perovskites, generally exhibit more pronounced nonlinear optical responses compared to their three-dimensional (3D) counterparts due to confined electron motion and strong electron–hole interaction.⁴⁹ However, the SHG-CD studies on lead-bromide perovskite systems have only been conducted on two-dimensional (2D) crystals, while that of 0D lead-bromide hybrid perovskites remains unexplored.

Herein, we report the synthesis of a 0D chiral lead-bromide hybrid perovskite, (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O, as a high-performance SHG-CD material. (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O was synthesized by introducing chiral cations, 1R,2R-/1S,2S-diaminocyclohexane. Meanwhile, 1D (R-/S-APD)PbBr₄ (R-/S-APD = R-/S-3-aminopiperidine) was also synthesized to investigate the influence of organic cations. The circular dichroism (CD) spectra of these perovskites confirmed the successful transfer of chirality from the chiral cations to the inorganic framework. (S-APD)PbBr₄ exhibited a strong SHG response across a wide excitation wavelength range from 820 nm to 1040 nm. The effective second-order nonlinear optical coefficient (d_eff) is up to 0.4 pm V⁻¹ for (S-APD)PbBr₄. Notably, these perovskites display a high LDT of up to 59.36 GW cm⁻². Furthermore, a high g_SHG-CD of 1.58 was achieved in (1S,2S-DACH)₂PbBr₆·2H₂O, which is the largest value among those of the reported chiral perovskites. This study demonstrates that a 0D chiral hybrid lead-bromide perovskite system can simultaneously exhibit both high g_SHG-CD and LDT, thereby expanding the scope of chiral SHG-active materials in the field of chiral NLO.

Results and discussion

Synthesis

Fig. 1a illustrates the molecular structures of two pairs of chiral organic amines (R-/S-APD and 1R,2R-/1S,2S-DACH) utilized to synthesize chiral hybrid perovskites. Single crystals of (R-/S-APD)PbBr₄ and (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O were synthesized according to the cooling crystallization method. Briefly, Pb(OAc)₂·3H₂O or PbO and the corresponding ammonium salt were dissolved into HBr and heated to 120 °C while stirring to afford a faint yellow solution. Slow cooling of the solutions to room temperature resulted in the formation of plate-like crystals and needle-like crystals, respectively. For the large-size crystal preparation, by employing the bottom-seeded solution growth method under an ambient atmosphere, bulk chiral single crystals of several millimeters in size were obtained,⁴³ as shown in Fig. 1b. The detailed synthetic process for crystal growth is described in the experimental section.

Fig. 1 (a) Molecular structures of the R-APD, S-APD, 1R,2R-DACH and 1S,2S-DACH (R-/S-APD = R/S-3-aminopiperidine, 1R,2R-/1S,2S-DACH = 1R,2R-/1S,2S-diaminocyclohexane). (b) The photographs of (R-/S-APD)PbBr₄ and (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O single crystals (the scale bar is 5 mm). (c) Crystal structures of (R-APD)PbBr₄ and (S-APD)PbBr4. (d) Crystal structures of (1R,2R-DACH)₂PbBr₆·2H₂O and (1S,2S-DACH)₂PbBr₆·2H₂O. The crystals were viewed along the a-axis.

The powder X-ray diffraction (PXRD) patterns of (R-/S-APD)PbBr₄ and (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O match well with the corresponding simulated results from the crystal structure, confirming the high phase purity of these single crystals (Fig. S1, ESI†). The PXRD spectra of these materials between those as-prepared and exposed to air for 60 days show no substantial changes, indicating their excellent ambient stability (Fig. S2, ESI†). The thermogravimetric analysis (TGA) curves of four chiral perovskites revealed decomposition temperatures higher than 580 K (Fig. S3, ESI†), suggesting their high thermostability is comparable to that of the chiral lead halide perovskites.^31,50,51 In Fig. S3c and d (ESI†), the first weight loss observed in (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O can be attributed to the loss of coordinated H₂O in the crystals, while the second weight loss corresponds to their decomposition. The remarkable environmental stability and thermostability of these chiral hybrid perovskite crystals make them promising candidates for practical applications in nonlinear optics.

Crystal structures

The crystal structures of (R-/S-APD)PbBr₄ and (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O were analyzed using single-crystal X-ray diffraction. The crystal data and structure refinement are provided in Tables S1–S4 (ESI†).

Fig. 1c illustrates the crystal structures of (R-APD)PbBr₄ and (S-APD)PbBr₄, which feature one-dimensional (1D) structures. This 1D structure that combines edge-sharing chains resulted from the use of the R-/S-APD cation in (R-/S-APD)PbBr₄, which is similar to that found in (2,6-dmpz)₃Pb₂Br₁₀.^52,53 (R-APD)PbBr₄ and (S-APD)PbBr₄ adopt the noncentrosymmetric orthorhombic space group of P2₁2₁2₁ with unit cell parameters a = 8.7802 Å, b = 10.8321 Å, c = 13.9609 Å and a = 8.7810 Å, b = 10.8005 Å, c = 13.7783 Å, respectively. The crystal structure consists of [PbBr₆]_∞⁴⁻ chains sitting in the columnar cages composed of (R-/S-APD)²⁺ cations. The ammonium moieties hydrogen bond to bromide ions (Fig. S4, ESI†) with N⋯Br distances ranging from 2.613 to 2.820 Å. The inorganic chain is based on edge-shared octahedral [PbBr₆]⁴⁻ moieties (Fig. S5a and b, ESI†), which are different from the typical corner-sharing octahedral units found in 2D and 3D hybrid perovskites.^27,54 The inorganic chain is then separated and charge-balanced by (R-/S-APD)²⁺ cations, as shown in Fig. S5c and d (ESI†). Then, inorganic chains and organic components are arranged alternately to form the 1D crystal structure. This structure can be considered as the bulk assembly of 1D core–shell quantum wires, in which the insulating organic shells surround the core copper chloride wires.⁵⁵

An exotic type of 0D structure that contains isolated [PbBr₆]⁴⁻ octahedron resulted from the use of the 1R,2R-/1S,2S-DACH cation in (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O. Fig. 1d shows the crystal structures of (1R,2R-DACH)₂PbBr₆·2H₂O and (1S,2S-DACH)₂PbBr₆·2H₂O, which exhibit a mirror image relationship with each other. The orthorhombic space group P2₁2₁2₁ governs the (1R,2R-DACH)₂PbBr₆·2H₂O and (1S,2S-DACH)₂PbBr₆·2H₂O crystallization, with unit cell parameters of a = 9.8774 Å, b = 10.8539 Å, c = 24.3071 Å and a = 9.9329 Å, b = 10.9337 Å, c = 24.3960 Å, respectively. The adjacent [PbBr₆]⁴⁻ octahedrons are spatially separated by organic amines and H₂O molecules, resulting in a typical 0D organic–inorganic hybrid structure. Each asymmetry unit of (1R,2R-DACH)₂PbBr₆·2H₂O and (1S,2S-DACH)₂PbBr₆·2H₂O contains an inorganic octahedron [PbBr₆]⁴⁻, two organic amine cations (1R,2R-/1S,2S-DACH)²⁺ and two H₂O molecules (Fig. S6a and b, ESI†). The 0D inorganic framework is primarily supported by N–H⋯Br hydrogen bonds and O–H⋯Br hydrogen bonds (Fig. S7, ESI†) between the organic cations, inorganic octahedrons and H₂O molecules. These hydrogen bonds formed by the introduction of chiral amine cations (1R,2R-/1S,2S-DACH)²⁺ could affect the distortion level of inorganic [PbBr₆]⁴⁻ octahedron (as discussed below).⁵⁶

Structural distortion is commonly present in the perovskite structure, as for these two pairs of compounds, which is evaluated by the level of distortion of the [PbBr₆]⁴⁻ octahedron. The distortion level (Δd) of inorganic [PbBr₆]⁴⁻ octahedron can be quantitatively calculated based on the length of Pb–Br using the following eqn (1),^57,58

where d is the average bond length of the Pb–Br bond and d_i is the bond length of the six individual Pb–Br bonds. The Pb–Br bond length is marked in Fig. S8 (ESI†) and the Δd values are summarized in Table S5 (ESI†). The results suggest that the [PbBr₆]⁴⁻ octahedron is significantly distorted with Pb atoms deviating from the balanced sites. For (R-APD)PbBr₄ and (S-APD)PbBr₄, the Δd is calculated to be 2.05 × 10⁻³ and 1.86 × 10⁻³, respectively. This Δd is smaller than that of previously reported chiral 1D hybrid perovskite, which indicates smaller octahedral distortion.^59,60 The values of Δd are calculated to be 1.4 × 10⁻⁴ for (1R,2R-DACH)₂PbBr₆·2H₂O and 1.34 × 10⁻⁴ for (1S,2S-DACH)₂PbBr₆·2H₂O, respectively, indicating that there is no substantial difference in structural distortion between these two enantiomers. In comparison, the (R-/S-APD)PbBr₄ octahedrons have higher Δd values than (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O, which illustrates the formation of greater distortion in the octahedrons.

Electronic properties

To investigate the linear optical and chiroptical properties of (R-/S-APD)PbBr₄ and (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O crystals, the crystals were thoroughly ground with KBr and pressed into pellets. The UV-vis-NIR absorption and CD spectra of chiral hybrid perovskites are shown in Fig. 2, with peak positions mainly distributed in the ultraviolet region. The solid UV-vis-NIR optical absorption spectra of (R-APD)PbBr₄ and (S-APD)PbBr₄ are presented in Fig. 2a, displaying a similar profile with absorption peaks at 324 nm. The absorption of 1D Pb–Br hybrid perovskite is mainly in the UV region, previously shown in other 1D Pb–Br systems such as C₄N₂H₁₄PbBr₄⁶¹ and [(H₂O)(C₆H₈N₃)₂Pb₂Br₁₀].⁶² For (1R,2R-DACH)₂PbBr₆·2H₂O and (1S,2S-DACH)₂PbBr₆·2H₂O, the absorption peaks were observed at 315 nm, as shown in Fig. 2b. The absorption peak observed at 315 nm was attributed to the ligand-to-metal charge transfer band of [PbBr₆]⁴⁻. The bandgaps were estimated by extrapolating the high-energy slope to the imaginary axis parallel to the x-axis, where the absorption edge is interrupted by the exciton peak.⁶³ The corresponding optical bandgap energies are 3.33 eV, 3.33 eV, 3.61 eV and 3.61 eV for (R-APD)PbBr₄, (S-APD)PbBr₄, (1R,2R-DACH)₂PbBr₆·2H₂O and (1S,2S-DACH)₂PbBr₆·2H₂O, respectively (Fig. S9, ESI†).

Fig. 2 UV-vis-NIR absorption spectra and circular dichroism (CD) spectra of (a) (R-APD)PbBr₄ and (S-APD)PbBr₄, and (b) (1R,2R-DACH)₂PbBr₆·2H₂O and (1S,2S-DACH)₂PbBr₆·2H₂O.

CD signals of (R-APD)PbBr₄ and (S-APD)PbBr₄ appear at the same wavelengths of 287 nm and 347 nm but with opposite signs. The enantiomeric perovskites of (1R,2R-DACH)₂PbBr₆·2H₂O and (1S,2S-DACH)₂PbBr₆·2H₂O also exhibit similar positions but oppositely signed CD patterns at 267 nm and 324 nm. The CD signals of organic ammonium salts R-/S-APD·2HCl and 1R,2R-/1S,2S-DACH·2HCl appear at approximately 261 nm and 264 nm (Fig. S10, ESI†), respectively. The CD signals for these chiral organic ammonium salts are mainly located lower than 300 nm. However, when they are incorporated into the chiral perovskites, the resulting perovskites exhibit opposite CD values in the visible region higher than 300 nm. These observations indicate that chirality is successfully transferred from the chiral organic cations to the perovskites.³³ The CD spectra of the (R-APD)PbBr₄ and (S-APD)PbBr₄ exhibit absorption peaks mainly centered in two regions: one is from 250 nm to 300 nm, and the other is from 300 nm to 380 nm. Similarly, (1R,2R-DACH)₂PbBr₆·2H₂O and (1S,2S-DACH)₂PbBr₆·2H₂O also exhibit two CD signal regions, ranging from 250 nm to 290 nm and from 290 nm to 350 nm, respectively. In each region, bisignate CD signals can be observed from one configuration with two peaks, which is attributed to the splitting of the energy state with the opposite spin states of electrons.^64,65 Such bisignate CD signals correspond to one absorption peak in the related absorption spectra and is usually interpreted as the Cotton effect, meaning that the chiral perovskites exhibit characteristic CD signals at the highest absorption wavelengths since they have different absorption coefficients to left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) lights.^66,67 In addition, the g_CD spectra of these chiral perovskites can be calculated by using the following equation⁶⁸

The maximum g_CD are −1.27 × 10⁻⁴ and 1.34 × 10⁻⁴ at 350 nm for (R-APD)PbBr₄ and (S-APD)PbBr₄, and 6.55 × 10⁻⁴ and −6.75 × 10⁻⁴ at 362 nm for (1R,2R-DACH)₂PbBr₆·2H₂O and (1S,2S-DACH)₂PbBr₆·2H₂O (Fig. S11, ESI†), which are comparable to other lead-based perovskites.^69,70

To further understand the electronic band structures and density of states (DOS) of the chiral hybrid perovskites, first-principles calculations based on density functional theory (DFT) were performed. The bandgaps of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O were determined to be 3.35 eV and 3.65 eV, respectively, as shown in Fig. 3a and b. For (R-APD)PbBr₄ and (1R,2R-DACH)₂PbBr₆·2H₂O, the bandgaps were 3.39 eV and 3.68 eV, as shown in Fig. S12a and S12b (ESI†), respectively. The calculated bandgap is consistent with the observed optical bandgap taken from the absorption spectra (Fig. S9, ESI†). According to the partial DOS of (R-/S-APD)PbBr₄ and (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O, the conduction band minimum (CBM) is derived mainly from the Br and Pb orbitals, whereas the valence band maximum (VBM) comprises Br orbitals, as shown in Fig. 3c and d. The calculated energy band structure reveals that the CBM is localized at the Γ point, while the VBM is localized at the S point, indicating an indirect band gap for (S-APD)PbBr₄. Moreover, both the CBM and VBM are located at Γ points, indicating a direct band gap of (1S,2S-DACH)₂PbBr₆·2H₂O.

Fig. 3 (a) and (b) Calculated electronic band structures of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O. (c) and (d) Calculated partial DOS of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O.

SHG properties

The introduction of chiral organic cations to an inorganic framework disrupts the centrosymmetric structures of the perovskites that endow the perovskites with SHG properties. The wavelength- and powder-dependent SHG measurements were conducted and the influence of different chiral organic amines on the SHG effect was also analyzed. For the SHG measurements, we utilized a homemade microscope equipped with a femtosecond pulsed laser as the pump source. Fig. 4a illustrates the schematic diagram of the SHG measurements, where λ/2 and λ/4 plates are employed to adjust the linearly polarized and circularly polarized angles, respectively. Fig. S13a (ESI†) shows the intensity of SHG mapping for (S-APD)PbBr₄, obtained at 520 nm when pumped at 1040 nm. For (1S,2S-DACH)₂PbBr₆·2H₂O, the mapping detects the SHG signal at 440 nm when pumped at 880 nm (Fig. S13b, ESI†). The mapping image demonstrates the crystal morphology and SHG activity of the single crystal. Fig. 4b and c present the wavelength-dependent SHG spectra collected from the (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O crystals, respectively, when pumped at various wavelengths with the same excitation power at 50 mW. The (S-APD)PbBr₄ crystals exhibit SHG responses across a wide range of wavelengths and exhibit the highest SHG intensity when pumped at 940 nm. The wavelength-dependent SHG spectra of (1S,2S-DACH)₂PbBr₆·2H₂O exhibit distinct SHG signals when the excitation wavelength is tuned between 940 nm and 1040 nm in 20 nm steps. Notably, when pump light is set at 1020 nm, the (1S,2S-DACH)₂PbBr₆·2H₂O single crystal exhibits the strongest SHG signal collected at 510 nm.

Fig. 4 (a) Schematic diagram of the SHG measurements. The λ/2 and λ/4 plates are used for tuning linearly polarized and circularly polarized angles, respectively. (b) and (c) NLO spectra of the (S-APD)PbBr₄ crystals and (1S,2S-DACH)₂PbBr₆·2H₂O crystals pumped at various wavelengths. (d) The logarithmic plot of SHG intensity as a function of the incident power of (S-APD)PbBr₄ crystals pumped at 860 nm. The solid line is a linear fitting with a slope of 1.94. (e) The logarithmic plot of SHG intensity as a function of the incident power of (1S,2S-DACH)₂PbBr₆·2H₂O crystals pumped at 900 nm. The solid line is a linear fitting with a slope of 2.10. (f) A comparison of the SHG signal intensities of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O crystals and Y-cut quartz at 1000 nm under the same test conditions.

Power-dependence SHG intensities of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O are presented in Fig. 4d and e. The intensity of the SHG signals scales quadratically with the power of the pump, which is consistent with previous literature reports.^15,29,51 This observation further confirms the two-photon nature of the second-order NLO response. Remarkably, the synthesized chiral perovskites exhibit a large LDT, and the SHG intensity remains in line with the fitted linear relationship until the pump power reaches approximately 1392 mW for (S-APD)PbBr₄ and 1362 mW for (1S,2S-DACH)₂PbBr₆·2H₂O. This power can be considered as the LDT of chiral perovskites, which corresponds to the excitation power of (S-APD)PbBr₄ at 59.36 GW cm⁻² and (1S,2S-DACH)₂PbBr₆·2H₂O at 54.22 GW cm⁻². The pulse width, repetition rate and laser spot are ∼100 fs, ∼80 MHz and ∼20 μm in diameter, respectively.^36,71 Notably, these LDT values are larger than those of most previously reported chiral metal perovskites (Fig. S14 and Table S6, ESI†). This observation indicates that the synthesized chiral perovskites possess high laser stability, and thus show great promise for investigating the nonlinear chiroptical devices.

The SHG intensity I_2ω can be calculated according to eqn (3):^72,73

where L is the microplate thickness, Δk is the wave-vector difference, d_eff is the second-order NLO coefficient, n_ω and n_2ω indicate the refractive index of waves, and I_ω and I_2ω are the laser irradiance at ω and 2ω, respectively. To evaluate the second-order NLO coefficient (d_eff) of chiral hybrid perovskites, we utilized a commercial Y-cut quartz with a d_eff of 0.3 pm V⁻¹ as the reference. The NLO coefficient is estimated according to eqn (4)^33,35,74where the superscripts “S” and “ref” refer to the S-samples and Y-cut quartz, L is the effective crystal thickness, P_in represents the incident light intensity, and I_out represents the output intensity of SHG. During the SHG measurement, both the S-samples and Y-cut quartz were characterized under the same laser conditions. Based on the test data, the SHG intensity of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O is 1.8 times and 0.2 times that of Y-cut quartz when pumped at 1000 nm, as shown in Fig. 4f. The SHG intensities of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O are 2 times and 0.22 times that of Y-cut quartz (Fig. S15, ESI†), respectively, when pumped at 1020 nm, which are similar with that pumped at 1000 nm. Then, we determined the absolute d_eff of the (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O to be 0.4 pm V⁻¹ and 0.13 pm V⁻¹, respectively. Compared with (1S,2S-DACH)₂PbBr₆·2H₂O, (S-APD)PbBr₄ has a larger d_eff. This increase in SHG intensity could also be attributed to the greater octahedral distortion in (S-APD)PbBr₄ crystals than in (1S,2S-DACH)₂PbBr₆·2H₂O.³⁶

Polarization-dependent SHG responses of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O collected at 450 nm and 520 nm are shown in Fig. 5a and b. The plots exhibit a dipolar profile that fits well to a cos⁴θ function. The corresponding polarization ratio ρ can be calculated by using the following eqn (5):

where I_max and I_min are the maximum and minimum values of SHG intensity.⁵¹ The calculated polarization ratio ρ was determined to be 94.23% and 85.63% for (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O, respectively. The polarization-dependent SHG response of (S-APD)PbBr₄ was also tested at 1040 nm and the corresponding polarization ratio ρ was 90.4% (Fig. S16, ESI†). This result indicates a significant difference in the SHG response between different polarization states for these chiral perovskites. Subsequently, we further studied the NLO chiroptical effects of chiral perovskites by measuring the SHG-CD, which is defined as the normalized difference between SHG intensities obtained with left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light excitations. By replacing the λ/2 plate with a λ/4 plate (Fig. 4a), we tuned the fundamental laser pump from linearly polarized to LCP or RCP. As depicted in Fig. 5c and d, the SHG intensity was changed following the rotation of the λ/4 plate. In particular, the SHG intensity of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O was stronger under RCP excitation compared to LCP excitation. In contrast, the SHG intensity of (R-APD)PbBr₄ and (1R,2R-DACH)₂PbBr₆·2H₂O under a left-handed circularly polarized laser was stronger than that under a right-handed circularly polarized laser (Fig. S17, ESI†). These observations demonstrate the presence of SHG-CD in the chiral perovskites, indicating a strong coupling between the chiral organic cations and the inorganic framework, leading to the emergence of chiroptical properties.

Fig. 5 (a) and (b) Polarization dependence of the SHG intensity of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O crystals as a function of the linear polarization angle pumped at 900 nm and 1040 nm, respectively. The purple and green line is the nonlinear fitting of data points. (c) and (d) SHG intensity of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O crystals at various elliptic polarized angles under 1040 nm laser. The laser is left-handed circularly polarized when the rotation angles are 45 and 225°, while it is right-handed circularly polarized at 135 and 315°, as indicated by the blue arrows. (e) Comparison of g_SHG-CD in the literature and in this work.

Then, to quantify the SHG-CD effect of these chiral hybrid lead-bromide perovskites, the g_SHG-CD was calculated based on eqn (6):^29,33,75

where I_LCP and I_RCP denote the SHG intensities excited by left- and right-handed circularly polarized lasers, respectively. The g_SHG-CD values of the chiral hybrid perovskites are summarized in Table S7 (ESI†), and the (1S,2S-DACH)₂PbBr₆·2H₂O possesses a g_SHG-CD value of 1.58, which is higher than that of (S-APD)PbBr₄ with a g_SHG-CD value of 0.6. Combining the CD and SHG-CD measurements, both the g_CD and g_SHG-CD values indicate a consistent trend, that the (1S,2S-DACH)₂PbBr₆·2H₂O exhibit much higher chiral effects than (S-APD)PbBr₄ in either linear or nonlinear optical responses. Fig. 5e illustrates the recent progress in representative chiral perovskite materials and their g_SHG-CD values. Notably, (1S,2S-DACH)₂PbBr₆·2H₂O exhibits the highest g_SHG-CD among reported chiral hybrid perovskites. This high g_SHG-CD-featured chiral hybrid perovskite shows promise for application in circularly polarized light detection.

The different values of g_SHG-CD for (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O indicate that the SHG-CD properties of chiral organic–inorganic hybrid perovskite materials are modulated by the organic cations. In the study of nonlinear optics in a hybrid perovskite system, the dimensionality of the crystal is one important factor in determining the intensity of SHG-CD responses.⁵⁹ For chiral hybrid perovskites, the large cation radii of chiral organic molecules render them devoid of connectivity between the metal-halide octahedra, thus lowering their structural dimensionality.¹ Having two or more chiral centers in one chiral molecule could have a strong impact on many chiral properties, including tunable signal reversal of circular dichroism,⁷⁶ enhanced interactions with polarized light⁷⁷ and dynamic light multiplexing.⁷⁸ In this work, by utilizing large volume double chiral center amines 1R,2R-/1S,2S-DACH, 0D structures of chiral perovskites (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O were obtained.^79,80 1D structures of (R-/S-APD)PbBr₄ were synthesized by employing R-/S-APD. Using double chiral center amines, low-dimensional hybrid perovskites can be obtained. These lower-dimensional perovskites often exhibit stronger SHG-CD effects because of the larger spatial separation between organic and inorganic components, leading to a pronounced dipole moment. Also, such a low-dimensional structure in (1S,2S-DACH)₂PbBr₆·2H₂O gives rise to strong electron–phonon coupling, which enables the possible realization of the enhancement of the SHG-CD effect.¹ To evaluate the electron–phonon coupling of (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O, we calculated the temperature dependence of the band gap renormalization (Fig. S19–S23, ESI†).⁸¹ (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O exhibit phonon-induced zero-point renormalizations reaching 77 meV and 144 meV, respectively. These results indicate (S-APD)PbBr₄ and (1S,2S-DACH)₂PbBr₆·2H₂O have strong electron–phonon coupling.⁸² Additionally, the band gap renormalization is significantly influenced by temperature (Fig. S19–S23, ESI†). Notably, (1S,2S-DACH)₂PbBr₆·2H₂O demonstrates stronger electron–phonon coupling, as evidenced by the higher zero-point renormalization and more pronounced temperature dependence of the band gap renormalization.

Furthermore, from the crystal cell structure of (1S,2S-DACH)₂PbBr₆·2H₂O, there are 8 chiral cations. However, only 4 chiral cations are contained in (S-APD)PbBr₄ (Fig. S24, ESI†). As the noncentrosymmetric crystal structure is obtained by the introduction of chiral cations, more chiral amines lead to stronger asymmetry in (1S,2S-DACH)₂PbBr₆·2H₂O than (S-APD)PbBr₄. Therefore, such a high value of g_SHG-CD in (1S,2S-DACH)₂PbBr₆·2H₂O primarily originates from the lower dimensional (0D) structure of the Pb-based chiral perovskites, where the chiral ligands per volume will be significantly more than in the 1D counterparts.⁸³ Overall, low-dimensional structures and stronger asymmetry lead to improved SHG-CD performance and thus a higher g_SHG-CD value. This property–structure relationship analysis provides new insights for designing low dimensional and fine-tuning of the SHG-CD properties in chiral hybrid perovskite materials.

Recent investigations have shown that the g_CD of organic–inorganic hybrid perovskites is highly sensitive to the material's dimensionality.^84,85 In 1D hybrid perovskites, the structure is confined along one dimension. The increased confinement and reduced dimensionality promote stronger interactions between the chiral organic cations and excitons, leading to a significant enhancement in the circular dichroism response.^86–88 However, in 0D perovskites, the excitons are completely confined, leading to discrete energy levels. As a consequence, the anisotropy factor in the CD spectra is generally enhanced as the dimensionality is reduced from 1D to 0D.⁸⁹ For the SHG-CD effect, compared to 1D hybrid perovskites, 0D crystals have stronger confinement effects that can enhance the electric dipole interactions, and increase the anisotropy. Additionally, stronger quantum confinement in 0D structures can also lead to an enhanced SHG-CD effect, because of altered electronic band structures and increased light–matter interaction at the interface.⁹⁰

Conclusions

In summary, we have successfully designed and synthesized two pairs of chiral hybrid perovskites, (R-/S-APD)PbBr₄ and (1R,2R-/1S,2S-DACH)₂PbBr₆·2H₂O. Furthermore, (1S,2S-DACH)₂PbBr₆·2H₂O exhibits a remarkable g_SHG-CD of 1.58, which is the highest value among the reported chiral hybrid perovskites. This high g_SHG-CD can be attributed to the material's unique 0D crystal structure, which enhances the asymmetry of (1S,2S-DACH)₂PbBr₆·2H₂O. Additionally, these chiral hybrid lead bromide perovskites exhibited strong nonlinear chiroptical effects, with the effective second-order NLO coefficient reaching up to 0.4 pm V⁻¹ for (S-APD)PbBr₄ and 0.13 pm V⁻¹ for (1S,2S-DACH)₂PbBr₆·2H₂O. The LDT is as high as 59.36 GW cm⁻² for (S-APD)PbBr₄, which is one of the largest values among the chiral hybrid perovskites. Our work demonstrates that the 0D chiral hybrid lead-bromide perovskite system can achieve both high g_SHG-CD and LDT, expanding the range of chiral SHG-active materials towards chiral nonlinear optical application.

Author contributions

H. L. Zhang and B. Sun conceived the idea for the manuscript and designed the experiments. H. Yang synthesized the materials and conducted the SCXRD, UV-vis-NIR absorption spectra and circular dichroism spectra. S. D. Wu contributed to the theoretical calculations. H. Yang, J. Guan and P. Wang contributed to the SHG measurements. H. L. Zhang, J. Xu, B. Sun and Q. Wang discussed the results and commented on the manuscript at all stages. H. L. Zhang led the project.

Data availability

All experimental and characterization data are available in the ESI.†

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC. 92256202, U22A20399, 22221001, 22075117, 22073038, 22005128), the 111 project 2.0 (BP1221004), the Science and Technology Major Program of Gansu Province (22ZD6GD060, 22ZD6FA006) and the Fundamental Research Funds for the Central Universities (lzujbky-2022-kb01, lzujbky-2021-sp59). The authors are thankful for the Supercomputing Center of Lanzhou University for providing the computing time and beam line BL14B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2339434, 2339435, 2339436 and 2339439. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qm00627e

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