Solid State Phosphorescence Enhancement of Pt II -Based Emi�ers Via Combination of π-Hole(Isocyano Group) ⋅⋅⋅ d z2 [Pt II ] and I···Cl Halogen-Bonding Interactions

: The Pt II isocyanide complex [Pt(ppy)Cl(CNC 6 H 4 -C≡C-Ph)] ( 1 , Hppy=2-phenylpyridine) was co-crystallized with 1,4-diiodotetraﬂuorobenzene (1,4-DITFB), yielding 1 ·½(1,4-DITFB) adduct. The I···Cl halogen-bonding and π-π-stacking interactions combined with the rare π-hole(isocyano group) ⋅⋅⋅ d z2 [Pt II ] interactions were identiﬁed via analysis of X-ray diﬀraction data of the co-crystals. These two types of structure-determining interactions supplemented each other, and the system of I ⋯ Cl and π-hole(isocyano group) ⋅⋅⋅ d z2 [Pt II ] contacts achieved a 1D extended ladder-type architecture. The density functional theory calculations, employing a set of computational tools, veriﬁed the role of I ⋯ Cl and π-hole(isocyano group) ⋅⋅⋅ d z2 [Pt II ] noncovalent bonds in the spectrum of noncovalent forces. The solid-state photophysical study revealed an ampliﬁcation of luminescence intensity in the co-crystals, which is a�ributed to the suppression of the nonradiative relaxation pathways due to an increase in the rigidity of the chromophore


Introduction
Luminescent materials based on complexes of transition metals are a subject of rapidly growing interest due to their applications in the design and fabrication of lighting devices [1][2][3][4][5][6][7], chemosensors [8][9][10], and photocatalysts [11][12][13][14].The strong spin-orbit coupling induced by the heavy atom effect facilitates both fast intersystem crossing and formal spin-forbidden triplet radiative decay [15][16][17], which conventionally explains the efficient transition metal species phosphorescence at room temperature (RT).The emission color and efficiency of these species are primarily determined by the choice of metal center and organic ligands (photophysical properties of platinum(II) phosphors have been analyzed in reviews [15,[17][18][19][20][21]); in the solid state, photophysical properties are also intimately associated with their molecular conformations and crystal packing, which is determined through their intermolecular interactions [22][23][24].In comparison with the covalent bonds, the intermolecular noncovalent interactions are in general locally weak; however, despite their low energy, in many cases these interactions can collectively play a dominant role in the formation of crystal packing.In contrast to octahedral d 6 -Ru II and d 6 -Ir III complexes, the open axial coordination sites of the square planar d 8 -Pt II -based species mitigate noncovalent binding to Pt II centers.These noncovalent linkages can significantly modify both ground-and excited-state properties of Pt II -based systems and, hence, the photophysical features [18,[25][26][27][28][29].Thus, the change in the nature of the excited state from MLCT to MMLCT [30][31][32][33][34] occurring with the formation of the metallophilic interactions leads to emission profile alterations and red-shifting radiation.At the same time, aggregation due to other contacts, e.g., hydrogen, halogen, chalcogen bonding, etc., generally leads to no change in emission color, but it can be followed by a quantum yield enhancement in luminescence [35][36][37][38][39].
One of the successful approaches to obtaining improved crystalline forms of luminescent complexes is the preparation of co-crystals [40].In co-crystal, molecules of diverse types alternate in such a way that the overall structure is described by a unit cell, in which strictly definite positions occupy certain types of molecules.Representative examples of such targeted use of noncovalent interactions to create phosphors with improved emission characteristics based on co-crystals of cyclometallated platinum(II) complexes are shown in Figure 1.

Figure 1.
Examples of the directed use of nonvalent interactions for the creation of metal phosphors with improved emission characteristics based on adducts of cyclometallated platinum(II) complexes [35,37,41].
Among the wide variety of luminescent Pt II -based systems, square planar cyclometallated complexes [Pt(C^N)XY] containing isocyanide ligands are of particular interest [42][43][44][45][46][47].On the one hand, these ligands are strong field ligands that enhance luminescence efficiency at room temperature due to the destabilization of unoccupied d orbitals and the corresponding 3 MC states increasing a nonradiative pathway of excitedstate relaxation [42].Their luminescence has been a ributed to the phosphorescence from the lowest triplet ligand-centered ( 3 LC) states perturbed by higher-lying singlet and triplet metal-to-ligand charge transfer ( 1,3 MLCT) states.On the other hand, the structure of isocyanide ligands can be modified with various substituents with different electron donor and steric properties, which influence the crystal packing on the complexes due to short-and long-range intermolecular interactions, as well as their photophysical properties [48].In the context of this work, recent examples of photophysical modulations in cyclometallated Pt II complexes with isocyanide ligands deserve separate consideration.Sicilia et al. reported a series of chloride/isocyanide [49] and alkynyl/isocyanide [50] cyclometallated Pt II complexes in which the different electronic and steric effects of the CNR groups affected the crystal packing through short-and long-range Pt•••Pt and/or π•••π interactions and the photophysical properties.It was also recently reported that (isocyano group)•••Pt II interactions can influence photophysical characteristics [29].We previously reported that the co-crystallization of [Pt(ppy)Cl(CNR)]-type complexes with X-F Arenes can leads to an increase in the quantum yield of luminescence with retention of the emission wavelengths [37].

Reagents, Instrumentation, and Methods
Solvents K2[PtCl4] and 2-phenylpyridine (ppyH) were obtained from commercial sources and used as received.[{Pt(ppy)(µ-Cl)}2] was prepared from K2[PtCl4] and ppyH following the procedure in the literature [59].Complex 1 was synthesized via the procedure in the literature [60].UV-VIS spectra were measured with a Shimadzu UV-2550 spectrophotometer.The excitation and emission spectra in the solid state were recorded with a HORIBA FluoroMax-4 spectrofluorometer.The excited-state lifetimes and absolute photoluminescence quantum yields in the solid phase were measured with a HORIBA Scientific FluoroLog-3 spectrofluorometer using a HORIBA Quanta-phi integration sphere.The uncertainty of the quantum yield determinations was in the range of ±5% (an average of three replications, each of which was carried out with different orientations of the sample).

X-Ray Diffraction Study
Crystals of 1•½(1,4-DITFB) were obtained by slow evaporation of an MeCN solution of a mixture of the corresponding isocyanide complex and 1,4-DITFB taken in a 1:2 molar ratio at 20-25° C. A single-crystal X-ray diffraction experiment was carried out on a SuperNova, single source at offset/far, HyPix3000 diffractometer with monochromated CuKα radiation.The crystal was kept at 100(2) K during data collection.The structures were resolved by ShelXT [61] (structure solution program using intrinsic phasing) and refined by means of the ShelXL [62] program incorporated into the OLEX2 program package [63].The crystallographic details are summarized in Table S1.Empirical absorption correction was applied in the CrysAlisPro (Agilent Technologies, Yarnton, UK, 2012) program complex using the spherical harmonics implemented in the SCALE3 ABSPACK scaling algorithm.CCCDC number 2,290,328 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (accessed on 23 August 2023).

Computational Details
Calculations based on the experimental X-ray geometries of dimeric clusters were carried out at the DFT level [64][65][66] of theory using the dispersion-corrected PBE-D3 functional [67][68][69] (basis set def2-TZVP [70,71]) with the help of the Gaussian-09 program package [72].The MEP surfaces of the clusters were presented using 0.001 a.u.isosurfaces [73,74].The color scheme was a blue-green-red scale, with red for ρ+ cut (repulsive) and blue for ρ− cut (a ractive).Green isosurfaces corresponded to weakly repulsive and a ractive interactions.The 3D surfaces and 3D visualizations of the NCI plot [75] with an isovalue = 0.35-0.4and color scale data range of [-0.01; 0.01] were visualized using the VMD 1.9.3 program [76].ELF projections [77] with the QTAIM method developed by Bader [78][79][80][81] were performed and presented in the Multiwfn 3.8 software [73].The 1D profiles of the ED/ESP functions [82,83] were calculated in the Multiwfn program.Views of dimeric clusters were created using Chemcraft [84].The BSSE-corrected interaction energies were calculated by subtracting the sum of the energies of the monomeric species from the total energy of the assembly.

General Description of the X-Ray Structures
The crystallization of [Pt(ppy)Cl(CNC6H4-C≡C-Ph)] and 1,4-DITFB from MeCN solutions at RT was found to lead to 1•½(1,4-DITFB) adduct (Figure 1).Only one type of crystal was obtained despite significant variation in the crystallization conditions (e.g., temperature, rate of evaporation).The peak position of the experimental PXRD pa ern from solids obtained by solvent evaporation of a solution of a mixture of 1 and 1,4-DITFB taken in a 1:2 molar ratio matched well with the simulated PXRD pa ern obtained from the single-crystal X-ray structure 1•½(1,4-DITFB), confirming the homogeneity of the sample and ruling out the presence of another phase (Figure S1.4,see Supplementary Materials).
The co-crystal exhibited C-I•••Cl-Pt short contacts between an I-substituent of 1,4-DITFB and the coordinated chloride of 1; the C-I•••Cl-Pt short contact comprised 86% of the sum of the Bondi vdW radii [86] and the corresponding angle around the I center was close to 180° (C-I•••Cl = 174.6(4)°;Table 1).These geometrical parameters led to the determination of this short contact as halogen bonding (abbreviated as HaB).According to the IUPAC criteria for Type II halogen interactions [87], appropriate theoretical calculations are required to prove the HaB character of the bonding (see later).The I atom from 1,4-DITFB acted as the σ-hole donor and interacted with a nucleophilic lump of Cl atom from 1.
Other infrequent contacts in the crystal structure of 1•½(1,4-DITFB) were NCN⋯Pt (3.536 Å, Figure 3).For the NCN⋯Pt separation, the N⋯Pt distance (3.536(10) Å) was larger than Bondi vdWN + Pt 3.27 Å [88] but shorter than the sum of Batsanov vdWN + Pt 3.65 Å [88] and Alvarez vdWN + Pt 3.95 Å [89].The NCN⋯Pt contact exhibited a comparable length to those in the previous structures of cyclometallated Pt II complexes (Table S1.3, see Supplementary Materials) and even in other (RNC)M species (see CSD search with brief analysis for NCN⋯M contacts in Ref. [29]).In addition, the second determined key factor was the ungular parameter ∠(CN⋯Pt) of this contact.These angles should have occurred near 90°, and a minimal deviation close to 0° was found [29,57].Based on the XRD data, we observed that the ∠(CN⋯Pt) angle with 86.7(4)° satisfied the a ributed ungular parameters.The data-based geometry considerations collected in Table 1 indicate that the Pt nucleophilic site directed to the nitrogen of the CN group.The presence of the πhole(CN)⋯dz 2 (Pt) interaction was verified with a theoretical study (see later).[88], and c Alvarez [89] van der Waals radii were used.

Theoretical Study
The DFT calculations, MEP analysis [90][91][92], and the quantum theory of atoms in molecules (QTAIM) [80] with a noncovalent interaction plot (NCI plot) [93,94] and the electron localization function (ELF) [95][96][97] were used to reveal the nature of I•••Cl and NCN⋯Pt interactions in real space.The discussion detailed in the following subsections is based on the single-point calculations for the XRD geometries of our systems (i.e., without optimization) due to the nature of the crystal-packing effects of the intermolecular interactions in 1•½(1,4-DITFB), whereas within the studies of interactions, the electrostatics played a secondary role.
First, we computed the MEP surfaces to study the electron-rich and electron-poor parts of 1 and 1,4-DITFB (Figure 5).As expected, the MEP minimum of complex 1 was located at the chloride ligand (−39.5 kcal/mol).At the Pt atom, the MEP was also negative (−23.4 kcal/mol), which was determined by mutual contribution of the anionic ligands and the dz 2 orbitals at the Pt II site.In the isocyano group, the MEP was also negative (−9.8 at C and −2.1 at N, measured perpendicular to the molecular plane) but less negative than the charge of the Pt atom.At the aromatic H-atoms, the MEP maximum, ranging from 13.4 to 25.2 kcal/mol, was large and positive.The MEP maximum of 1,4-DITFB was located at the iodine σ-hole (31.1 kcal/mol).For a more comprehensive understanding of the nature of the noncovalent interactions, we employed DFT calculations.These calculations were performed on two different supramolecular associates, namely, (1•(1,4-DITFB)) and (1)2, denoted as "A" and "B," respectively, which exemplify the two binding modes shown in Figure 4.The QTAIM/NCI plot analyses are given in Figure 6 as a 3D visualization of various noncovalent interactions and in Figure S2.1 (see Supplementary Materials) as a typical NCI plot, i.e., a sca er graph of RDG vs. sign(λ2)ρ, namely, the product of the sign of λ2 (the second largest eigenvalue of the electron density Hessian matrix) and ρ (the electron density).In the supramolecular associate A, the HaB was characterized by a bond critical point (BCP), a bond path, and a disk-shaped reduced density gradient (RDG) isosurface that connects I to Cl atoms.The position of the I atom coincided with the MEP minimum, as shown in Figure 5.We defined energies for this HaB according to the procedures proposed by Espinosa et al. [98], Vener et al. [99], and Tsirelson et al. [100], and it can be stated that the estimated strength was in the range of 3.0-4.6kcal/mol (Table 2).The I atoms were also connected to the aromatic H atoms' adjacent BCPs and bond paths, revealing the presence of ancillary C−H•••I hydrogen bonds.Dimer B was connected by seven (3, −1) bond critical points (BCPs, shown as small blue spheres) and bond paths (black lines).One bond BCP connected N and Pt atoms, disclosing the existence of NCN⋯Pt interactions.Dimer B also had BCPs and bond paths connecting the Pt to a C atom of the aromatic ring, demonstrating the occurrence of Pt⋯π interactions.The additional five BCPs and bond paths connected the C atoms of the aromatic rings and the alkyl group, corresponding to the π⋯ π interactions in dimer B.
The complexation energy for the supramolecular associate A was −8.0 kcal/mol and could be mainly a ributed to the HaB, with small participation of the hydrogen bonds, which is supported by the NCI plot analyses.The dimerization energy for dimer B was ΔE = −23.5 kcal/mol, which combined the intermolecular interactions of Pt1•••N1, Pt1•••C20, and π-π stacking.There are reports indicating a significant contribution of stacking interactions between chelate and aromatic rings and π-π stacking for the formation of the supramolecular architecture of metal-containing particles in the solid state [101,102], so it is likely that in the case of 1•½(1,4-DITFB), these interactions also make an undeniable contribution to the organization and stabilization of the final crystal structure.Nevertheless, π-π stacking interactions belong to nondirectional intermolecular interactions, and among all the noncovalent interactions found in dimer B, only the Pt•••N interaction has a directional character [29,57].Therefore, and also considering the fact that intermolecular (isocyano group)•••Pt II interactions have previously been reconsidered with respect to their influence on photophysical performance [29], the Pt•••N interaction was further investigated theoretically.The energy for dimer 1,4-DITFB•••1,4-DITFB ((1,4-DITFB)2) was −6.0 kcal/mol, which is weaker than the dimerization energy for supramolecular associates A and B.  b Eint = 0.67G(r) (Tsirelson's empirical correlation between the interaction energy and kinetic energy density of electrons at the bond critical points (3, −1) for noncovalent interactions I•••X [100]).c E = −V(r)/2 (Espinosa's empirical correlation for the estimation of the energies of the hydrogen bonds [98]).d E = 0.429G(r) (Vener's empirical correlation for the estimation of the energies of the hydrogen bonds [99]).
Analysis of ELF and electron density/electrostatic potential distribution along the bond path.To determine the philicities of noncovalently interacting atoms, we used ELF projections with critical points and bond paths from QTAIM electron density topology.In the ELF projection for the I•••Cl bond, the path went through the lone-pair orange area of Cl and the blueish area with low ELF values around I, which confirmed the σ-hole of the I atom and the HaB nature of the noncovalent interaction (Figure 7, left) [52,[103][104][105][106] with the nucleophilic chloride ligand and the electrophilic I centers of 1,4-DITFB.
The Pt•••N bond path between the metal center and the N atom passed through the filled dz 2 orbitals of the Pt atom with small values of ELF, indicating the weakly nucleophilic manner of the metal atom in the weakly polar NCN⋯Pt interaction (Figure 7, right).The comparison of the minima of the electron density (ED) and the electrostatic potential (ESP) along the binding path also allowed us to verify the affinities, since, in the case of a polar noncovalent interaction, the ESP minimum shifts towards the nucleophilic atom and the ED minimum shifts towards the electrophilic site [107].The analysis of the 1D profiles of the ED and ESP functions [108]

Photophysical Measurements
The photophysical properties of 1•½(1,4-DITFB) co-crystals were studied in the solid state and were compared with the photophysical properties of 1, which were previously described [37].Similarity to 1, the luminescence spectra of 1•½(1,4-DITFB) (Figure 9) were broad, partially structured, and, in accordance with the literature data, could be assigned to a metal-perturbed 3 LC intraligand transition with a small admixture of 3 MLCT [37].The difference between 1 and 1•½(1,4-DITFB) is that the adduct showed an additional longerwavelength emission band in the 550-750 nm region.These findings can be interpreted in terms of the formation of bi-molecular states such as ground-state dimers and/or excited states-excimers [109].Monoexponential transient decays were observed for the emission at 517 nm and at 617 nm.The lifetimes obtained from exponential decay fits were characterized by τobs = 0.03 µs in both cases.In addition, the co-crystallization of 1 with 1,4-DITFB provided to increase the Φem up to 0.02%, whereas the 1 was extremely weakly emissive at RT.These data are coherent with our DFT computational data indicating a significant dimerization energy for supramolecular associates, which is likely to have affected the increase in molecular rigidity.Therefore, our results and other examples of adducts of cyclometallated platinum complexes with perfluoroarenes or haloperfluoroarenes shown in Figure 1 indicate that, in all cases, the formation of adducts led to an increase in the quantum yield of phosphorescence, with insignificant spectral changes.

Figure 4 .
Figure 4. Noncovalent interactions of 2D ladder-type architecture.Purple do ed line indicates the Pt-Cl•••I-C HaBs, dark blue-the NCN⋯Pt contact.

Figure 6 .
Figure 6.QTAIM and NCI plot analyses of the model supramolecular associates (A,B).Only intermolecular interactions are represented.Solid line indicates the Pt-Cl•••I-C HaBs and NCN⋯Pt contact, dashed line indicates the Pt⋯π and π⋯π contacts.

Table 2 .
The BSSE-corrected complexation energies ΔE (kcal/mol) for dimers A and B (Figure 6a,b) and BCP topological analysis for I•••Cl and NCN⋯Pt interactions, where sign(λ2)ρ(r) is the electron density, ∇ 2 ρ(r) is the Laplacian of the electron density, G(r) is the electronic kinetic energy density, V(r) is the electronic potential energy density, and H(r) is the local electronic energy density.0.68(−V(r)) (Tsirelson's empirical correlation between the interaction energy and potential energy density of electrons at the bond critical points (3, −1) for noncovalent interactions I•••X [100]).
along the I•••Cl bond path showed the clear nucleophilic nature of the Cl atom with the shift in the ESP minimum toward it, whereas the ED minimum shifted toward the electrophilic I atom, which also confirmed the HaB nature of these noncovalent C-I•••Cl-Pt interactions (Figure 8, Left).At the same time, for the NCN⋯Pt interaction, the ED and ESP minima almost overlapped with the feeble Ptdirected ESD and C-directed ED minima shifts, meaning that the NCN⋯Pt interaction should be regarded as a π-hole(CN)⋯dz 2 (Pt) interaction, with the N atom as an electrophile and the Pt center as a nucleophile (Figure 8, Right).

Table 1 .
Geometrical parameters of short contacts in the structure of 1