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

Anion binding and ion pairing of dipyrrolyldiketone PtII complexes as anion-responsive π-electronic molecules resulted in photophysical modulations, as observed in solid-state phosphorescence properties. Modifications to arylpyridine ligands in the PtII complexes significantly impacted the assembling behaviour and photophysical properties of anion-free and anion-binding (ion-pairing) forms. The PtII 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 PtII 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 PtII 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 PtII complexes.


Introduction
p-Electronic molecules in ordered arrangements demonstrate fascinating electronic and electrooptical properties that are not observed in single molecules. 1Introducing multiple building units in assemblies induces properties that can be modulated by constituent species.Among organic materials, solid-state luminescent materials have received signicant attention owing to their applications in light-emitting diodes, sensors and photonics. 2In particular, crystals of square-planar organoplatinum(II) complexes exhibit fascinating photoluminescence properties, 3,4 such as triplet energy transfer and phosphorescence anisotropy amplication.4h,j The solid-state luminescence of p-electronic species is frequently quenched or weakened by self-association (Fig. 1a le), primarily because of exciton coupling between neighbouring molecules. 5,6To prevent this, introducing bulky groups to interfere with the molecular contact is an effective strategy for enhancing solidstate luminescence by electronic decoupling of organoplatinum p-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.
Ion pairs, which comprise complementarily associated positively and negatively charged species, can be employed for an isolation strategy. 9,10Isolation and electronic decoupling of luminescent cations have been achieved by introducing bulky counteranions. 11Furthermore, ion complexation by ionresponsive p-electronic molecules (receptors) followed by pairing with counterions is appropriate for preparing p-electronic ion pairs.This method can be used to create pseudo-p-electronic anions in the form of receptor-anion complexes.Spatially and electronically isolated charged uorophores (cationic dyes) exhibit uorescence emission in the solid state when combined with well-designed counter species (anion complexes). 12In contrast, including photo-functional anionresponsive p-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-p-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 p-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 (l max and l em , respectively) at 410 and 510 nm, respectively, with an emission quantum yield (F em ) of 42% for 2a in CH 2 Cl 2 . 13Planar geometries around Pt II with a ppy ligand are suitable for stacking structures by themselves and in the form of ion-pairing assemblies. 14The 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.

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 [(PtMe 2 ) 2 (SMe 2 ) 2 ] 15 at r.t. in the presence of triuoromethanesulfonic acid (TfOH) (1 equiv.)and K 2 CO 3 (1.5 equiv.)(Fig. 2).The obtained Pt II complexes were characterized by 1 H and 13 C 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

Solution-state properties
The UV/vis absorption spectra of 2b-e in CH 2 Cl 2 exhibited the l max at ∼310-450 nm, and the spectral features, l max values and absorbance intensities, were complicated depending on the introduced ligand moieties (Fig. 3 and Table 1).For example, 2b displayed the l max at 368, 390 and 412 nm, and similar spectral features were observed in 2c,d, whereas 2e exhibited a distinctive spectrum with the l 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 signicant 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. 16Furthermore, characteristic absorption bands of 2e at 318 and 455 nm can be attributed to the p-p* transition of the benzothienyl-pyridine ligand and intraligand charge transfer from the p orbitals of the benzo unit to the p* orbitals of the pyridyl group, respectively. 17 The phosphorescence spectra of 2b-e in deoxygenated CH 2 Cl 2 exhibited broad emission bands with a l em of 542, 512, 508 and 610/662(sh) nm, respectively (Fig. 3 and Table 1), suggesting the red-shied emissions for 2b,e compared to 2a.The F 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 (s) of 4.6, 9.6, 4.2 and 6.8 ms, respectively, similar to that of 2a (5.1 ms). 13TD-DFT calculations for the optimized T 1 structures at the PCM-M06-2X level in CH 2 Cl 2 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 1 H NMR spectral changes in CD 2 Cl 2 (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 9.35/9.31and 6.41 ppm, respectively, at −50 °C disappeared, whereas the corresponding new signals appeared at 12.55/12.49and 7.42 ppm, respectively.The downeld shis were caused by hydrogen bonding with Cl − , implying the formation of [1 + 1]type Cl − complexes, as demonstrated by theoretical studies (Fig. S39-S42 †). 16he 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 CH 3 CO 2

−
) as TBA salts in CH 2 Cl 2 (Table 2 and Fig. S70-S73 †).In all the derivatives discussed in this study, the K a values were in the order of CH 3 CO 2 − > Cl − > Br − correlating with the basicity.
Cl − complexation produced phosphorescence emissions (l em at 542, 505, 511 and 610/660 nm for 2b-e, respectively) with F em (54%, 32%, 41% and 14%, respectively) and s (4.3-14.2ms), which were comparable to those of 2a$Cl − (l em : 490/520 nm, F em : 48%, s: 2.4 ms) (Table 3).It should be noted that anion complexation of the Pt II complexes in solution did not signicantly affect the emission properties.Pt II complexes with such emission properties can be used as building blocks for solidstate materials whose packing structures and resulting luminescent properties are modulated by anion complexation and ion pairing with countercations.

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 l em at 523/682, 591, 673, 652 and 627 nm with F em of 0.7%, ∼0.1%, 1.8%, ∼0.1% and ∼0.2%, respectively (Fig. S86-S90 †). 18Such red-shied 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 F 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 CH 2 Cl 2 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 (dened by the core atoms without hydrogen atoms) of 0.103-0.174Å.The Pt II complexes 2b-e formed columnar p-p stacking structures with stacking distances of 3.21-3.38Å.Among them, 2b,e showed stackeddimer alignment with Pt/Pt distances of 3.46 and 3.29 Å,  respectively.The p-p stacking structures were identied by Hirshfeld surface analysis (Fig. S33-S37 †). 20There are no notable intermolecular interactions on the lateral side of the columnar p-p stacking structures except for 2c, which showed N-H/p interactions between pyrrole rings.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$Cl − -TPA + (TPA + : tetrapropylammonium) was also prepared as precipitates by adding n-hexane to a mixture of 2a and TPACl in CH 2 Cl 2 . 21Compositions of the Cl − -binding Pt II complex and countercations in all the solid-state samples were fully characterized by using 1 H NMR. In contrast to the anion-free states, solid-state 2a$Cl − -TPA + , 2a$Cl − -TBA + and 2a$Cl − -TPeA + exhibited phosphorescence with the l em at 654, 522 and 509 nm with F 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 l em of 2a$Cl − -TBA + and 2a$Cl − -TPeA + were similar to the solutionstate l 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-shied emission.The emission lifetimes, such as 440 ms for 2a$Cl − -TPeA + , were nearly 200 times longer than those of the monomers in solution (Fig. S82 †).
The enhanced phosphorescence intensities in the ionpairing assemblies were investigated using solid-state packing structures revealed by single-crystal X-ray analysis.The ion pair 2a$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$Cl − and TBA + are alternately arranged on the a-axis to form a charge-by-charge assembly with a Pt/Pt distance of 8.52 Å.In contrast, crystalstate 2a$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 − $$$Cl − distances of 8.90 and 10.72 Å, respectively.Hirshfeld surface analysis of the crystal structures indicated no characteristic close contacts between the Pt II complex and cation, suggesting that both cations exhibited similar interactions.3][24][25] Although the exact assembling structure for the precipitates of 2a$Cl − -TPA + could not be determined by single-crystal X-ray analysis, synchrotron XRD analysis revealed no characteristic diffraction pattern, suggesting the less ordered arrangement of 2a$Cl − and TPA + (Fig. S83 †).The speculated slipped stacking of 2a$Cl − in the solid-state 2a$Cl − -TPA + could be correlated with the red-shied phosphorescence.
In any ion pairs of 2a$Cl − , the isolation of the Pt II complexes by aliphatic cations, required for enhancing phosphorescence intensities, was clearly indicated by the X-ray structures of the ion-pairing assemblies.Furthermore, the rigidication of packing structures by Cl − -bridged 1D-chain structures is vital for enhancing the phosphorescence intensities.The larger F em of 2a$Cl − -TBA + than that of 2a$Cl − -TPeA + can be ascribed to the stabilization of the 1D-chain structure by hydrogen bonding.The electrostatic energy, revealed by EDA, originating mainly from hydrogen-bonding interactions for the 2a/Cl − $$$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 †).
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$Cl − -TPA + , 26 2c$Cl − -TBA + and 2c$Cl − -TPeA + showed the l em at 507,  504 and 515 nm with comparable and enhanced F em of 1.7%, 3.2% and 2.4%, respectively, due to the formation of charge-bycharge assemblies. 27Similar to the XRD pattern of 2a$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 F 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 p-electronic ligands controlled emissive properties, as observed in the red-shied 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 CH 2 Cl 2 /n-hexane exhibited enhanced phosphorescence with a F em of 7.5%, which is 75 times greater than that of 2b (Fig. S87 †). 28The solid-state phosphorescence properties, l em and F em , in ion-pairing assemblies of the anionresponsive Pt II complexes were modulated by the introduced pelectronic C^N ligands and coexisting cations.

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 S 0 geometry of solid-state 2a showed pseudogenerate T 1 (A g ) and T 2 (A u ), where A g and A u denote the   irreducible representations of the excited electronic states, as well as T 3 (A u ) and T 4 (A g ) states, whose wavefunctions were symmetrically delocalized over the 2a dimer (Fig. S62 †).The geometries of T 1 (A g ) and T 4 (A g ) were further optimized owing to the distribution of their wavefunctions at the dipyrrolyldiketone and ppy ligand, respectively.The optimized T 1 and T 4 states with lower symmetries of C 1 owing to a symmetry breaking of pseudo-Jahn-Teller distortion 31 exhibited the lowest excited states with excitation energies of 2.25 eV/551 nm and 1.95 eV/636 nm, respectively (Fig. S63 †).The S 0 -T 1 electron density difference at the T 1 optimized structure (Fig. 9a(i)) demonstrated that T 1 was an excited state localized on the single molecule.In contrast, the S 0 -T 4 electron density difference at the T 4 optimized structure (Fig. 9a(ii)) indicated that T 4 was an excited state with the wavefunction asymmetrically delocalized over the dimer.The calculated phosphorescence spectrum from T 1 reproduced the sharp shape of the experimental spectrum in the short-wavelength region, whereas that from T 4 reproduced the broad shape in the long-wavelength region (Fig. S64 †).The T 4 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 z 2 orbital in one molecule and the p orbital of the ppy ligand in the other (Fig. 9b). 32Thus, this intermolecular interaction was responsible for the energetically stable T 4 formation.
The F em values depend on nonradiative rate constants from the excited to the ground states.The rate constant signicantly increases with diagonal vibronic coupling constants (VCCs) of the nal electronic state because of easier acceptance of electronic excitation energy. 33For the solid-state 2a, the diagonal VCCs of S 0 at the T 4 optimized structure were larger than those at the T 1 -optimized structure (Fig. S65 †), suggesting that the nonradiative transition from the T 4 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 F em of the solid-state 2a was attributed to T 4 with large diagonal VCCs.
The charge-by-charge packing structure signicantly affected the electronic states of 2a.In 2a$Cl − -TBA + , the pseudodegenerate T 1 (A g ) and T 2 (A u ) as well as T 3 (A u ) and T 4 (A g ) wavefunctions at the S 0 optimized structure were distributed over the dipyrrolyldiketone unit, whereas the T 5 (A u ) and T 6 (A g ) wavefunctions were distributed over the ppy ligand (Fig. S68 †).The optimized geometries of T 1 (A g ) and T 6 (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 F 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 solidstate 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 selfassociation, resulting in enhanced phosphorescence emission.Solid-state arrangement of the Pt II complexes and their photophysical properties were inuenced 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 Fig. 9 (a) Electron density differences for solid-state 2a (i) between S 0 and T 1 at the T 1 optimized structure and (ii) between S 0 and T 4 at the T 4 optimized structure (isosurface value: 5 × 10 −4 a.u), (b) orbital levels and molecular orbitals at the T 4 optimized structure (isosurface value: 2 × 10 −2 a.u.) and (c) electron density differences for solid-state 2a$Cl − -TBA + (i) between S 0 and T 1 at the T 1 optimized structure and (ii) between S 0 and T 6 at the T 6 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.
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 solidstate emission properties.Moreover, introduction of p-electronic cations 35 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).

Fig. 1
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 p-electronic anions (right) and (b) anion complexation of dipyrrolyldiketone Pt II complex 2a.

Table 1
Summary of the UV/vis absorption (l max ) and emission (l em ) maxima with quantum yields (F em ) of 2b-e with 2a a as a reference in CH 2 Cl 2 a Ref.13.bExcitation wavelengths for emission spectra are in bold.

Table 2
Anion-binding constants (K a , M −1 ) of 2b-e with 2a a as a reference in CH 2 Cl 2 a Ref.13.

Table 3
Summary of the UV/vis absorption (l max ) and emission (l em ) maxima with quantum yields (F 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 CH 2 Cl 2 a Ref.13.bExcitation wavelengths for emission spectra with bold.

Table 4
Solid-state properties (emission peaks, emission lifetimes and quantum yields) of 2a and its ion pairs of Cl − complexes with tetraalkylammonium 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 a Accurate values could not be determined due to low F em .