Enhanced solid-state phosphorescence of organoplatinum π-systems by ion-pairing assembly

Abstract

Anion binding and ion pairing of dipyrrolyldiketone Pt^II complexes as anion-responsive π-electronic molecules resulted in photophysical modulations, as observed in solid-state phosphorescence properties. Modifications to arylpyridine ligands in the Pt^II complexes significantly impacted the assembling behaviour and photophysical properties of anion-free and anion-binding (ion-pairing) forms. The Pt^II complexes, in the presence of guest anions and their countercations, formed various anion-binding modes and ion-pairing assembled structures depending on constituents and forms (solutions and crystals). The Pt^II complexes emitted strong phosphorescence in deoxygenated solutions but showed extremely weak phosphorescence in the solid state owing to self-association. In contrast, the solid-state ion-pairing assemblies with tetraalkylammonium cations exhibited enhanced phosphorescence owing to the formation of hydrogen-bonding 1D-chain Pt^II complexes dispersed by stacking with aliphatic cations. Theoretical studies revealed that the enhanced phosphorescence in the solid-state ion-pairing assembly was attributed to preventing the delocalisation of the electron wavefunction over Pt^II complexes.

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Introduction

π-Electronic molecules in ordered arrangements demonstrate fascinating electronic and electrooptical properties that are not observed in single molecules.¹ Introducing multiple building units in assemblies induces properties that can be modulated by constituent species. Among organic materials, solid-state luminescent materials have received significant attention owing to their applications in light-emitting diodes, sensors and photonics.² In particular, crystals of square-planar organoplatinum(II) complexes exhibit fascinating photoluminescence properties,^3,4 such as triplet energy transfer and phosphorescence anisotropy amplification.^4h,j The solid-state luminescence of π-electronic species is frequently quenched or weakened by self-association (Fig. 1a left), primarily because of exciton coupling between neighbouring molecules.^5,6 To prevent this, introducing bulky groups to interfere with the molecular contact is an effective strategy for enhancing solid-state luminescence by electronic decoupling of organoplatinum π-systems.^4c,e,j,7 Therefore, appropriate isolation arrangements for emissive species have been highly demanded. Furthermore, it is important to maintain rigid structures to decrease the rates of non-radiative decay processes that interfere with phosphorescent emission.^3c,8 Thus, both isolation and robustness in packing structures are required for effective luminescence.

Fig. 1 (a) Conceptual diagram of anion binding and ion pairing for emission enhancement: stacking structure of anion-responsive luminescent molecules (left) and charge-by-charge assembly with hydrogen-bonding 1D-chain structures of receptor–anion complexes as pseudo π-electronic anions (right) and (b) anion complexation of dipyrrolyldiketone Pt^II complex 2a.

Ion pairs, which comprise complementarily associated positively and negatively charged species, can be employed for an isolation strategy.^9,10 Isolation and electronic decoupling of luminescent cations have been achieved by introducing bulky counteranions.¹¹ Furthermore, ion complexation by ion-responsive π-electronic molecules (receptors) followed by pairing with counterions is appropriate for preparing π-electronic ion pairs. This method can be used to create pseudo-π-electronic anions in the form of receptor–anion complexes. Spatially and electronically isolated charged fluorophores (cationic dyes) exhibit fluorescence emission in the solid state when combined with well-designed counter species (anion complexes).¹² In contrast, including photo-functional anion-responsive π-electronic systems would also provide luminescent crystalline states through anion complexation with hydrogen bonding and alternate stacking with countercations (Fig. 1a right). The anion complexation would give rise to rigid structures by hydrogen bonds tightly connecting building units (pseudo-π-electronic anions comprising luminescent receptors) with the support of the charge-by-charge arrangement by electrostatic and dispersion forces. Designing luminescent anion receptors and combining them with countercations for isolation enable the control of the photophysical properties.

As luminescent π-electronic systems, dipyrrolyldiketone Pt^II complexes (e.g., 2a, Fig. 1b), with phenylpyridine (ppy) as a C^N ligand, have been synthesized as phosphorescent anion sensors, exhibiting absorption and emission maxima (λ_max and λ_em, respectively) at 410 and 510 nm, respectively, with an emission quantum yield (Φ_em) of 42% for 2a in CH₂Cl₂.¹³ Planar geometries around Pt^II with a ppy ligand are suitable for stacking structures by themselves and in the form of ion-pairing assemblies.¹⁴ The Pt^II complexes would exhibit luminescent properties according to the introduced C^N ligands in solution and ion-pairing assemblies. Furthermore, the solid-state luminescent properties, whose control by countercations is also challenging, have not been elucidated. In this study, enhanced solid-state phosphorescence was achieved by ion pairing and the isolation of emissive anion complex moieties by countercations, for diverse Pt^II complexes.

Results and discussion

Synthesis and characterization of Pt^II complexes

Four different C^N ligands were used to produce Pt^II complexes 2b–e in 7.0–27% yields by treating dipyrrolyldiketone 1 with the mixture of the ligands and [(PtMe₂)₂(SMe₂)₂]¹⁵ at r.t. in the presence of trifluoromethanesulfonic acid (TfOH) (1 equiv.) and K₂CO₃ (1.5 equiv.) (Fig. 2). The obtained Pt^II complexes were characterized by ¹H and ¹³C NMR and ESI-TOF-MS. Conformations without pyrrole inversions (py0i conformations) were suggested by the theoretically optimized structures: doubly pyrrole-inverted (py2i) conformations of 2b–e, suitable for anion binding, were less stable by 6.84, 6.79, 7.10 and 6.15 kcal mol⁻¹, respectively, due to the enhanced molecular electric dipoles.¹⁶

Fig. 2 Synthesis of dipyrrolyldiketone Pt^II complexes 2b–e.

Solution-state properties

The UV/vis absorption spectra of 2b–e in CH₂Cl₂ exhibited the λ_max at ∼310–450 nm, and the spectral features, λ_max values and absorbance intensities, were complicated depending on the introduced ligand moieties (Fig. 3 and Table 1). For example, 2b displayed the λ_max at 368, 390 and 412 nm, and similar spectral features were observed in 2c,d, whereas 2e exhibited a distinctive spectrum with the λ_max at 388 and 409 nm along with those at 318 and 455 nm. Similarly to 2a,¹³ the main absorption bands of 2b–e were assigned as the lowest-lying singlet states, originating primarily from the HOMO-to-LUMO transition with a significant contribution from ligand-to-ligand charge transfer (LLCT) from the dipyrrolyldiketone unit to the arylpyridine ligands and metal-to-ligand charge transfer (MLCT) from Pt^II to arylpyridine ligands; the theoretical study was conducted using time-dependent (TD)-DFT calculations at the CAM-B3LYP level using the 6-31+G(d,p) basis set with the LanL2DZ basis set and associated effective core potentials for Pt, which were used for the calculations in the following parts.¹⁶ Furthermore, characteristic absorption bands of 2e at 318 and 455 nm can be attributed to the π–π* transition of the benzothienyl-pyridine ligand and intraligand charge transfer from the π orbitals of the benzo unit to the π* orbitals of the pyridyl group, respectively.¹⁷

Fig. 3 UV/vis absorption spectra (solid lines, CH₂Cl₂) and normalized emission spectra (dashed lines, deoxygenated CH₂Cl₂) with the excitation at 410, 368, 411, 368 and 388 nm of 2a (blue), 2b (orange), 2c (green), 2d (purple) and 2e (red) (inset: photographs of Pt^II complexes under UV₃₆₅ (0.03 mM)).

Table 1 Summary of the UV/vis absorption (λ_max) and emission (λ_em) maxima with quantum yields (Φ_em) of 2b–e with 2a^a as a reference in CH₂Cl₂

	λ max [nm]	λ em [nm]	Φ em [%]	τ [μs]
a Ref. 13. b Excitation wavelengths for emission spectra are in bold.
2b	368/390/412	542	68	4.6
2c	361/393/411	512	50	9.6
2d	368/390/408	508	50	4.2
2e	388/409	610/662(sh)	16	6.8
2a	364/393/410	510	42	5.1

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The phosphorescence spectra of 2b–e in deoxygenated CH₂Cl₂ exhibited broad emission bands with a λ_em of 542, 512, 508 and 610/662(sh) nm, respectively (Fig. 3 and Table 1), suggesting the red-shifted emissions for 2b,e compared to 2a. The Φ_em of 2b–d were 68%, 50% and 50%, respectively, which were greater than that of 2e (16%). The phosphorescence emissions of 2b–e originating from the triplet states were suggested by the emission lifetimes (τ) of 4.6, 9.6, 4.2 and 6.8 μs, respectively, similar to that of 2a (5.1 μs).¹³ TD-DFT calculations for the optimized T₁ structures at the PCM-M06-2X level in CH₂Cl₂ showed theoretically estimated emission maxima at 499, 524, 495 and 714 nm for 2b–e, respectively, which are close to the observed values (Fig. S56†). The phosphorescence emissions of 2c,e were mainly derived from the LUMO-to-HOMO (84% and 96%, respectively) transitions, whereas those of 2b,d were ascribed to the LUMO-to-HOMO−1 (68%) and LUMO+1-to-HOMO (60%) transitions, respectively.

Anion-binding behaviours were revealed by ¹H NMR spectral changes in CD₂Cl₂ (1.0 mM) (Fig. S74–S77†). Upon the addition of 3.3 equiv. of tetrabutylammonium chloride (TBACl) to, as an example, 2c, the signals of the pyrrole NH and bridging CH at 9.35/9.31 and 6.41 ppm, respectively, at −50 °C disappeared, whereas the corresponding new signals appeared at 12.55/12.49 and 7.42 ppm, respectively. The downfield shifts were caused by hydrogen bonding with Cl⁻, implying the formation of [1 + 1]-type Cl⁻ complexes, as demonstrated by theoretical studies (Fig. S39–S42†).¹⁶

The anion-binding constants (K_a) of 2b–e in a [1 + 1]-binding mode were evaluated by the changes in UV/vis absorption spectra caused by the addition of anions (Cl⁻, Br⁻ and CH₃CO₂⁻) as TBA salts in CH₂Cl₂ (Table 2 and Fig. S70–S73†). In all the derivatives discussed in this study, the K_a values were in the order of CH₃CO₂⁻ > Cl⁻ > Br⁻ correlating with the basicity. Cl⁻ complexation produced phosphorescence emissions (λ_em at 542, 505, 511 and 610/660 nm for 2b–e, respectively) with Φ_em (54%, 32%, 41% and 14%, respectively) and τ (4.3–14.2 μs), which were comparable to those of 2a·Cl⁻ (λ_em: 490/520 nm, Φ_em: 48%, τ: 2.4 μs) (Table 3). It should be noted that anion complexation of the Pt^II complexes in solution did not significantly affect the emission properties. Pt^II complexes with such emission properties can be used as building blocks for solid-state materials whose packing structures and resulting luminescent properties are modulated by anion complexation and ion pairing with countercations.

Table 2 Anion-binding constants (K_a, M⁻¹) of 2b–e with 2a^a as a reference in CH₂Cl₂

	2b	2c	2d	2e	2a
a Ref. 13.
Cl^−	2000	2200	1200	1900	1300
Br^−	320	240	50	310	61
CH3CO2^−	7400	22 000	5400	15 000	15 000

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Table 3 Summary of the UV/vis absorption (λ_max) and emission (λ_em) maxima with quantum yields (Φ_em) of 2b–e with 2a^a as a reference upon the addition of TBACl (2000 equiv. for 2b–e and 3000 equiv. for 2d) in CH₂Cl₂

	λ max [nm]	λ em [nm]	Φ em [%]	τ [μs]
a Ref. 13. b Excitation wavelengths for emission spectra with bold.
2b	372/387/410	542	54	4.3
2c	367/390/409	505	32	6.4
2d	372/411	511	41	14.2
2e	388/409	610/660(sh)	14	6.8
2a	369/389/410	490/520(sh)	48	2.4

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Solid-state luminescence and packing structures

In contrast to the phosphorescence emission in the dispersed solution state, the solid-state Pt^II complexes obtained from the single crystals (vide infra) exhibited extremely weak luminescence: 2a–e exhibited λ_em at 523/682, 591, 673, 652 and 627 nm with Φ_em of 0.7%, ∼0.1%, 1.8%, ∼0.1% and ∼0.2%, respectively (Fig. S86–S90†).¹⁸ Such red-shifted emission with emission quenching can be caused by the excimer formation of excited Pt^II complexes,¹⁹ as theoretically discussed in the following section. Notably, the Φ_em of solid-state 2a–e are lower than those of 1,3-diphenyl-1,3-propanedione Pt^II complexes.^4h The details of the assembled structures were revealed by X-ray analysis for the single crystals of 2b–e obtained by vapour diffusion of n-hexane into CH₂Cl₂ solutions (Fig. 4 and S19–S23†).²⁰2b–e exhibited planar pyrrole-non-inverted (py0i) conformations in stacking structures, similar to 2a,¹³ with mean-plane deviations (defined by the core atoms without hydrogen atoms) of 0.103–0.174 Å. The Pt^II complexes 2b–e formed columnar π–π stacking structures with stacking distances of 3.21–3.38 Å. Among them, 2b,e showed stacked-dimer alignment with Pt⋯Pt distances of 3.46 and 3.29 Å, respectively. The π–π stacking structures were identified by Hirshfeld surface analysis (Fig. S33–S37†).²⁰ There are no notable intermolecular interactions on the lateral side of the columnar π–π stacking structures except for 2c, which showed N–H⋯π interactions between pyrrole rings.

Fig. 4 Single-crystal X-ray structures, with selected stacking and Pt⋯Pt (italic) distances, of (a) 2a as a reference,¹³ and (b) 2b, (c) 2c, (d) 2d and (e) 2e as packing diagrams. Atom colour codes in Fig. 4 and the following figures: brown, pink, blue, red, yellow green, yellow and grey refer to carbon, hydrogen, nitrogen, oxygen, fluorine, sulfur and platinum, respectively.

The extremely weak crystal-state emissive properties were modulated by isolating the Pt^II complexes without stacking by themselves. Single crystals of the ion pairs 2a·Cl⁻-TBA⁺ and 2a·Cl⁻-TPeA⁺ (TPeA⁺: tetrapentylammonium) were prepared by vapour diffusion of n-hexane into the THF solutions of 2a and corresponding tetraalkylammonium salts (Fig. 5). The ion pair 2a·Cl⁻-TPA⁺ (TPA⁺: tetrapropylammonium) was also prepared as precipitates by adding n-hexane to a mixture of 2a and TPACl in CH₂Cl₂.²¹ Compositions of the Cl⁻-binding Pt^II complex and countercations in all the solid-state samples were fully characterized by using ¹H NMR. In contrast to the anion-free states, solid-state 2a·Cl⁻-TPA⁺, 2a·Cl⁻-TBA⁺ and 2a·Cl⁻-TPeA⁺ exhibited phosphorescence with the λ_em at 654, 522 and 509 nm with Φ_em (relative intensities to the solid-state 2a) of 3.6% (5.1), 6.2% (8.9) and 2.6% (3.7), respectively, indicating enhanced phosphorescence properties (Table 4, Fig. 6a, b and S86†). The λ_em of 2a·Cl⁻-TBA⁺ and 2a·Cl⁻-TPeA⁺ were similar to the solution-state λ_em of 2a·Cl⁻, suggesting the phosphorescence derived from the monomeric Cl⁻ complex as suggested by single-crystal packing structures (vide infra). Notably, 2a·Cl⁻-TPA⁺ exhibited a distinctive red-shifted emission. The emission lifetimes, such as 440 μs for 2a·Cl⁻-TPeA⁺, were nearly 200 times longer than those of the monomers in solution (Fig. S82†).

Fig. 5 Preparation procedures of ion-pairing assemblies comprising Cl⁻-binding Pt^II complexes and countercations.

Table 4 Solid-state properties (emission peaks, emission lifetimes and quantum yields) of 2a and its ion pairs of Cl⁻ complexes with tetraalkylammonium cations

	λ em [nm]	τ 1 [μs]/f1 [%]	τ 2 [μs]/f2 [%]	τ 3 [μs]/f3 [%]	Φ em [%]
2a	523	140/31	550/69	—	0.7
2a·Cl^−-TPA^+	654	110/24	440/76	—	3.6
2a·Cl^−-TBA^+	522	2.9/43	23/25	210/32	6.2
2a·Cl^−-TPeA^+	509	6.3/8	86/24	440/68	2.6

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Fig. 6 (a) (i) Photographs and (ii) quantum yields of solid-state 2a,¹³2a·Cl⁻-TPA⁺, 2a·Cl⁻-TBA⁺ and 2a·Cl⁻-TPeA⁺, (b) solid-state emission spectra of 2a (grey), 2a·Cl⁻-TPA⁺ (red), 2a·Cl⁻-TBA⁺ (green) and 2a·Cl⁻-TPeA⁺ (yellow) and (c) single-crystal X-ray structures of (i) 2a·Cl⁻-TBA⁺ and (ii) 2a·Cl⁻-TPeA⁺ as top 1D chain and side packing structures. The atom colour code in (c): green (spherical) refers to chlorine. Magenta and cyan denote the receptor–Cl⁻ complex 1D-chain and cation parts, respectively. For the crystals of 2a in (a), reflections of excitation light at UV_{365 nm} were observed in partially pink colour.

The enhanced phosphorescence intensities in the ion-pairing assemblies were investigated using solid-state packing structures revealed by single-crystal X-ray analysis. The ion pair 2a·Cl⁻-TBA⁺ exhibited an anion-binding mode with hydrogen bonding of the singly inverted pyrrole NH, bridging CH and pyrrole CH (Fig. 6c(i), S24† and Table 5). The Cl⁻ also interacted with the pyrrole NH of neighbouring 2a, resulting in a Cl⁻-bridged 1D-chain structure based on the singly pyrrole-inverted (py1i) conformation. Importantly, 2a·Cl⁻ and TBA⁺ are alternately arranged on the a-axis to form a charge-by-charge assembly with a Pt⋯Pt distance of 8.52 Å. In contrast, crystal-state 2a·Cl⁻-TPeA⁺ exhibited a packing structure with a py0i conformation (Fig. 6c(ii) and S25†). The pyrrole NHs of 2a formed hydrogen bonds independently, resulting in a Cl⁻-bridged chain structure. 2a·Cl⁻ and TPeA⁺ were alternately arranged to form a charge-by-charge assembly with a Pt⋯Pt distance of 8.34 Å. The assembling modes, with different numbers of inverted pyrrole rings, are determined by the alkyl chain lengths of the countercations (TBA⁺ and TPeA⁺), resulting in different nearest Cl⁻···Cl⁻ distances of 8.90 and 10.72 Å, respectively. Hirshfeld surface analysis of the crystal structures indicated no characteristic close contacts between the Pt^II complex and cation, suggesting that both cations exhibited similar interactions. This is also supported by the energy decomposition analysis (EDA) in the framework of the fragment molecular orbital method at the FMO2-MP2 using mixed basis sets including NOSeC-V-DZP with MCP with TZP for Pt, demonstrating that the dispersion forces were effective between the Pt^II complex and cations (Fig. S57 and S58†).^22–25 Although the exact assembling structure for the precipitates of 2a·Cl⁻-TPA⁺ could not be determined by single-crystal X-ray analysis, synchrotron XRD analysis revealed no characteristic diffraction pattern, suggesting the less ordered arrangement of 2a·Cl⁻ and TPA⁺ (Fig. S83†). The speculated slipped stacking of 2a·Cl⁻ in the solid-state 2a·Cl⁻-TPA⁺ could be correlated with the red-shifted phosphorescence.

Table 5 Summary of the representative distances in the crystal structures of 2a·Cl⁻-TBA⁺ and 2a·Cl⁻-TPeA⁺

	2a·Cl^−-TBA^+	2a·Cl^−-TPeA^+
a Distances in charge-by-charge assemblies. b Distances in Cl^−-bridged 1D-chain structures.
N(–H)⋯Cl^− [Å]	3.17, 3.18	3.14
Cbridging(–H)⋯Cl^− [Å]	3.78	—
Cβ(–H)⋯Cl^− [Å]	3.63	—
Pt⋯Pt^a [Å]	8.52	8.33
Cl^−···Cl^−^b [Å]	8.90	10.72

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In any ion pairs of 2a·Cl⁻, the isolation of the Pt^II complexes by aliphatic cations, required for enhancing phosphorescence intensities, was clearly indicated by the X-ray structures of the ion-pairing assemblies. Furthermore, the rigidification of packing structures by Cl⁻-bridged 1D-chain structures is vital for enhancing the phosphorescence intensities. The larger Φ_em of 2a·Cl⁻-TBA⁺ than that of 2a·Cl⁻-TPeA⁺ can be ascribed to the stabilization of the 1D-chain structure by hydrogen bonding. The electrostatic energy, revealed by EDA, originating mainly from hydrogen-bonding interactions for the 2a⋯Cl⁻···2a structure in the 1D-chain of 2a·Cl⁻-TBA⁺ possessed a larger absolute value by 10.7 kcal mol⁻¹ than that of 2a·Cl⁻-TPeA⁺ (Fig. S57 and S58†).

Charge-by-charge assemblies were observed in the single-crystal packing structures of 2c,d with tetraalkylammonium cations (Fig. 7a, S28–S31† and Table 6). A crystal of 2c·Cl⁻-TBA⁺, prepared from CH₂ClCH₂Cl/n-hexane, for example, demonstrated a py1i conformation with Cl⁻ binding by pyrrole NH, β-CH and bridging CH. A Cl⁻-bridged chain structure similar to 2a·Cl⁻-TBA⁺ was formed by the interaction between uninverted pyrrole NH and neighbouring Cl⁻. In contrast, a single crystal of 2c·Cl⁻-TPeA⁺, prepared from CH₂ClCH₂Cl/n-hexane, exhibited a py0i conformation and a chain structure (Fig. 7b and S29†) as observed in 2a·Cl⁻-TPeA⁺. Furthermore, crystallization of 2d·Cl⁻-TBA⁺ from CH₂Cl₂/n-hexane (2d·Cl⁻-TBA_M⁺) and CH₂ClCH₂Cl/n-hexane (2d·Cl⁻-TBA_E⁺) produced two pseudo polymorphs, with 2d exhibiting py1i conformations that form Cl⁻-bridged chain structures in both (Fig. 7c and S31†). In all the ion-pairing assemblies revealed by X-ray analysis, Pt^II complexes in Cl⁻-bridged polymers were isolated by aliphatic cations in charge-by-charge arrangements.

Fig. 7 Single-crystal X-ray structures of (a) 2c·Cl⁻-TBA⁺, (b) 2c·Cl⁻-TPeA⁺ and (c) 2d·Cl⁻-TBA⁺_E as top 1D-chain and side packing structures.

Table 6 Summary of the representative distances in the crystal structures of 2c·Cl⁻-TBA⁺, 2c·Cl⁻-TPeA⁺ and 2d·Cl⁻-TBA⁺_E

	2c·Cl^−-TBA^+	2c·Cl^−-TPeA^+	2d·Cl^−-TBAE^+
a Distances in charge-by-charge assemblies. b Distances in Cl^−-bridged 1D-chain structures.
N(–H)⋯Cl^− [Å]	3.14, 3.18, 3.19, 3.20	3.14, 3.18	3.10, 3.16
Cbridging(–H)⋯Cl^− [Å]	3.84, 3.85	—	3.82
Cβ(–H)⋯Cl^− [Å]	3.65, 3.72	—	3.73
Pt⋯Pt^a [Å]	8.74	8.33	8.83
Cl^−···Cl^−^b [Å]	9.01	10.94	8.87

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As expected from the crystal packing structures, emission enhancement of the solid-state ion-pairing assemblies of 2c,d was observed, as was a similar tendency for 2a (Table 7, Fig. 8a, b, S88 and S89†). For example, solid-state ion pairs 2c·Cl⁻-TPA⁺,²⁶2c·Cl⁻-TBA⁺ and 2c·Cl⁻-TPeA⁺ showed the λ_em at 507, 504 and 515 nm with comparable and enhanced Φ_em of 1.7%, 3.2% and 2.4%, respectively, due to the formation of charge-by-charge assemblies.²⁷ Similar to the XRD pattern of 2a·Cl⁻-TPA⁺, the synchrotron XRD pattern of 2d·Cl⁻-TPA⁺/TPeA⁺ precipitates exhibited less clear diffraction patterns with broad peaks, suggesting less ordered arrangements of 2d·Cl⁻ and countercations. The Φ_em of 2d·Cl⁻-TPA⁺ was estimated to be 1.7%, which is greater than that of the other 2d·Cl⁻ ion pairs. Substituents at π-electronic ligands controlled emissive properties, as observed in the red-shifted phosphorescence at 606–617 nm with enhanced quantum yields of 1.1–2.7% for the ion pairs of 2e·Cl⁻ with tetraalkylammonium cations as precipitates (Table 7, Fig. 8c and S90†). Furthermore, the 2b·Cl⁻-TPA⁺ precipitate obtained from CH₂Cl₂/n-hexane exhibited enhanced phosphorescence with a Φ_em of 7.5%, which is 75 times greater than that of 2b (Fig. S87†).²⁸ The solid-state phosphorescence properties, λ_em and Φ_em, in ion-pairing assemblies of the anion-responsive Pt^II complexes were modulated by the introduced π-electronic C^N ligands and coexisting cations.

Table 7 Solid-state properties (emission peaks, emission lifetimes and quantum yields) of 2c–e and its ion pairs of Cl⁻ complexes with tetraalkylammonium cations

	λ em [nm]	τ 1 [μs]/f1 [%]	τ 2 [μs]/f2 [%]	τ 3 [μs]/f3 [%]	Φ em [%]
a Accurate values could not be determined due to low Φem.
2c	673	61/90	450/10	—	1.8
2c·Cl^−-TPA^+	507	49/15	390/85	—	1.7
2c·Cl^−-TBA^+	504	6.3/41	80/14	410/44	3.2
2c·Cl^−-TPeA^+	515	7.7/10	57/25	390/65	2.4
2d	652	120/28	500/72	—	∼0.1^a
2d·Cl^−-TPA^+	627	29/36	190/64	—	1.7
2d·Cl^−-TBA^+	563	120/26	490/74	—	0.7
2d·Cl^−-TPeA^+	594	30/17	330/83	—	0.8
2e	627	100/21	440/79	—	∼0.2^a
2e·Cl^−-TPA^+	617	130/28	510/72	—	1.9
2e·Cl^−-TBA^+	606	130/27	510/73	—	1.1
2e·Cl^−-TPeA^+	606	96/18	420/82	—	2.7

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Fig. 8 (i) Photographs and (ii) quantum yields of the Pt^II complexes (a) 2c, (b) 2d and (c) 2e and their Cl⁻ complexes as ion pairs with TPA⁺, TBA⁺ (2d·Cl⁻-TBA_E⁺ for 2d) and TPeA⁺ as solid-state samples.

Mechanism for phosphorescence enhancement

Enhancing the phosphorescence in the ion-pairing assembly of Pt^II complexes was elucidated by DFT calculations based on an ONIOM approach^29,30 for the packing structures of 2a and 2a·Cl⁻-TBA⁺. Computational models for solid-state 2a and 2a·Cl⁻-TBA⁺ were constructed by cutting out the 1 × 1 × 2 and 3 × 2 × 2 unit cells (Fig. S61 and S67†), where the centre parts and surroundings were treated as quantum mechanics (QM) regions at the DFT level and molecular mechanics (MM) regions at the UFF level, respectively. The electronic structures of the QM region were calculated at the CAM-B3LYP/6-31+G(d,p) and 3-21G with LanL2DZ for Pt in 2a and 2a·Cl⁻-TBA⁺, respectively. In both cases, the QM regions exhibited C_i site symmetry.

The optimized S₀ geometry of solid-state 2a showed pseudo-generate T₁ (A_g) and T₂ (A_u), where A_g and A_u denote the irreducible representations of the excited electronic states, as well as T₃ (A_u) and T₄ (A_g) states, whose wavefunctions were symmetrically delocalized over the 2a dimer (Fig. S62†). The geometries of T₁ (A_g) and T₄ (A_g) were further optimized owing to the distribution of their wavefunctions at the dipyrrolyldiketone and ppy ligand, respectively. The optimized T₁ and T₄ states with lower symmetries of C₁ owing to a symmetry breaking of pseudo-Jahn–Teller distortion³¹ exhibited the lowest excited states with excitation energies of 2.25 eV/551 nm and 1.95 eV/636 nm, respectively (Fig. S63†). The S₀–T₁ electron density difference at the T₁ optimized structure (Fig. 9a(i)) demonstrated that T₁ was an excited state localized on the single molecule. In contrast, the S₀–T₄ electron density difference at the T₄ optimized structure (Fig. 9a(ii)) indicated that T₄ was an excited state with the wavefunction asymmetrically delocalized over the dimer. The calculated phosphorescence spectrum from T₁ reproduced the sharp shape of the experimental spectrum in the short-wavelength region, whereas that from T₄ reproduced the broad shape in the long-wavelength region (Fig. S64†). The T₄ state mainly comprised the HOMO–LUMO transition (the CI coefficient: 0.599) and HOMO-8–LUMO transition (0.234). The HOMO and HOMO-8 originated from the intermolecular interaction between the Pt d_z² orbital in one molecule and the π orbital of the ppy ligand in the other (Fig. 9b).³² Thus, this intermolecular interaction was responsible for the energetically stable T₄ formation.

Fig. 9 (a) Electron density differences for solid-state 2a (i) between S₀ and T₁ at the T₁ optimized structure and (ii) between S₀ and T₄ at the T₄ optimized structure (isosurface value: 5 × 10⁻⁴ a.u), (b) orbital levels and molecular orbitals at the T₄ optimized structure (isosurface value: 2 × 10⁻² a.u.) and (c) electron density differences for solid-state 2a·Cl⁻-TBA⁺ (i) between S₀ and T₁ at the T₁ optimized structure and (ii) between S₀ and T₆ at the T₆ optimized structure (isosurface value: 5 × 10⁻⁴ a.u.). The red and blue regions are positive and negative in electron density differences, respectively. Only the QM region is shown for simplicity.

The Φ_em values depend on nonradiative rate constants from the excited to the ground states. The rate constant significantly increases with diagonal vibronic coupling constants (VCCs) of the final electronic state because of easier acceptance of electronic excitation energy.³³ For the solid-state 2a, the diagonal VCCs of S₀ at the T₄ optimized structure were larger than those at the T₁-optimized structure (Fig. S65†), suggesting that the nonradiative transition from the T₄ state was fast. Vibronic coupling density (VCD)^33,34 elucidated that the large VCCs arose from the strong coupling between the electronic state and vibrational modes distributed over the ppy ligand (Fig. S66†). Thus, the low Φ_em of the solid-state 2a was attributed to T₄ with large diagonal VCCs.

The charge-by-charge packing structure significantly affected the electronic states of 2a. In 2a·Cl⁻-TBA⁺, the pseudo-degenerate T₁ (A_g) and T₂ (A_u) as well as T₃ (A_u) and T₄ (A_g) wavefunctions at the S₀ optimized structure were distributed over the dipyrrolyldiketone unit, whereas the T₅ (A_u) and T₆ (A_g) wavefunctions were distributed over the ppy ligand (Fig. S68†). The optimized geometries of T₁ (A_g) and T₆ (A_g) exhibited the wavefunctions localized on the single molecule by the pseudo-Jahn–Teller distortion (Fig. 9c and S69†). In contrast to the solid-state 2a, the electronic wavefunction was not delocalized over dimer for these states because of the presence of TBA⁺ between 2a·Cl⁻. The TBA⁺ in the charge-by-charge assembly clearly decoupled the electronic interaction, resulting in enhanced phosphorescence derived from the monomeric Pt^II complex. Although the Φ_em values of the ion-pairing assemblies are lower than those in the solution state because of the reduced interactions in the solvated monomeric forms, introduction of aliphatic cations is efficient for inhibiting the self-association and enhancing the phosphorescence properties.

Conclusions

An ion-pairing strategy has been applied for fabricating solid-state phosphorescent materials by isolating dipyrrolyldiketone Pt^II complexes, as emissive anion-responsive molecules, in the form of anion complexes with aliphatic cations. Charge compensation between the anion complexes, in anion-bridged rigid chain structures, and countercations interfered with self-association, resulting in enhanced phosphorescence emission. Solid-state arrangement of the Pt^II complexes and their photophysical properties were influenced by the introduced arylpyridine ligands and coexisting cations. Furthermore, the facile recrystallization procedures in the ion-pairing strategy for preparing luminescent materials can be applied for large-scale production. The ion-pairing strategy used in this study does not require the solution-state anion-binding mode. To the best of our knowledge, such a room-temperature phosphorescence enhancement by anion binding and ion-pairing assembly has not been demonstrated thus far. The mechanism of the phosphorescence intensity modulated by anion binding and ion pairing was clearly revealed by theoretical studies for stacking structures. Assembly systems with multiple components, based on the combination of host systems and cationic species, would provide diverse materials with controllable emission wavelengths, quantum yields and lifetimes. Introducing more bulky and robust countercations would further improve the solid-state emission properties. Moreover, introduction of π-electronic cations³⁵ would also result in the formation of materials with intriguing photophysical properties. Further studies on ion-pairing luminescent materials are currently being conducted by designing and synthesizing charged building units and precursors (ion-responsive molecules).

Data availability

Data supporting the work in this publication are available via the ESI and associated crystallographic data.†

Author contributions

H. M. designed and conducted the project. Y. H., K. K., Y. M. and H. T. carried out the synthesis, characterization and property examinations. H. S., H. I. and N. T. evaluated the solid-state absorption and emission spectra. W. O. and T. S. conducted the theoretical calculations. Y. H. and N. Y. analyzed the single-crystal X-ray structures.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Numbers JP18H01968 and JP22H02067 for Scientific Research (B), JP19K05444 and JP22K05253 for Scientific Research (C), JP20J22745 for JSPS Fellows and JP20H05863 for Transformative Research Areas (A) “Condensed Conjugation” and the Ritsumeikan Global Innovation Research Organization (R-GIRO) project (2017–22 and 2022–27). Theoretical calculations were partially performed using the Research Center for Computational Science, Okazaki, Japan (Projects: 19-IMS-C085, 20-IMS-C079, 21-IMS-C077 and 22-IMS-C077) and Information Initiative Center, Hokkaido University. The synchrotron radiation experiments were performed at BL02B1 (2019B1638), BL40XU (2021B1703) and BL40B2 (2021B1828) with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). We thank Prof. Atsuhiro Osuka, Dr Takayuki Tanaka, Dr Akito Nakai and Dr Koki Kise, Kyoto University, for single-crystal X-ray analysis, Dr Kunihisa Sugimoto, Kindai University, for synchrotron radiation single-crystal X-ray analysis, Dr Noboru Ohta, JASRI/SPring-8, for synchrotron radiation XRD analysis, Prof. Shigeki Kuwata, Ritsumeikan University, for elemental analysis and Prof. Hitoshi Tamiaki, Ritsumeikan University, for various measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, analytical data, computational details and CIF files for the single-crystal X-ray structural analysis. CCDC 2158662–2158674. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc04564a

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