Stability Tuning of Vapor-Adsorbed State of Vapochromic Pt(II) Complex by Introduction of Chiral Moiety

Abstract

New luminescent Pt(II) complexes with chiral ester chains, [PtCl 2 (R,R-bpybe)] (R-1; bpybe = 2,2′-bipyridine-4,4′dicarboxylic acid dibutyl ester) and its racemic mixture (rac-1) with the chiral isomer, S-1, were synthesized, and their vapochromic behavior was investigated. Single-crystal X-ray structural analysis revealed that the rac-1 crystal was composed of only one crystallographically independent column formed by alternating stacking of R-1 and S-1 by the effective intermolecular Pt•••Pt interaction. In contrast, three types of columnar structures with different Pt•••Pt interactions were found for the R-1 crystal, probably because of the different packing of the chiral ester chains between the columns. Consequently, the estimated molecular volume of R-1 was slightly larger than that in the racemic crystal rac-1, although they have the same chemical formula. The X-ray structure of the toluene-adsorbed rac-1 (rac-1•toluene) also indicated that the intermolecular Pt•••Pt interaction, which was effective for unsolvated rac-1, was completely canceled out by adsorption of toluene vapor. Both the rac-1 and R-1 crystals exhibited similar vapochromism driven by toluene vapor adsorption/desorption that switched the emission origin between the strongly emissive 3 MMLCT (metal− metal-to-ligand charge transfer) to the weakly emissive 3 π−π* phosphorescence. Although both crystals had the same chemical formula, the toluene vapor desorption temperature of R-1•toluene (84 °C) was obviously lower than that of rac-1•toluene (107 °C), suggesting that the binding interaction with toluene molecules was weaker in R-1•toluene than in rac-1•toluene.

Realization of Distinct Mechano- and Piezochromic Behaviors via Alkoxy Chain Length-Modulated Phosphorescent Properties and Multidimensional Self-Assembly Structures of Dinuclear Platinum(II) Complexes

“…Apart from changing the electronic properties, , the denticity and the coordination modes of the ligands that can perturb the HOMO–LUMO energies, − changes in the steric effects and the nature of the side-chains , and functionalities capable of intra- or intermolecular noncovalent bond interactions, such as hydrogen bonding, π–π stacking, hydrophobic interaction, electrostatic interaction, metal–metal interaction, etc., − can lead to the perturbation of intermolecular forces. Given the propensity of platinum­(II) complexes to form supramolecular self-assemblies via metal–metal interactions, together with π–π interactions, a minor perturbation of the molecular structure may induce significant changes in their supramolecular assembly and photophysical properties. , Thus, there has been a growing interest in the exploration of platinum­(II) systems in developing functional stimuli-responsive materials, such as in novel sensors, encryption, and memory devices, when exposed to external stimuli, including heat, vapor, mechanical forces, and pressure. ,− In particular, the effects of mechanical forces and pressure have attracted much attention in the exploration of the mechanochromic and piezochromic properties of various organic dyes, small molecules, , metal–organic frameworks (MOFs), and metallacycles . Mechano- and piezochromic materials based on platinum­(II) complexes have attracted much attention. ,− However, most of the works are focused on organic molecules and metal complexes of d 10 (coinage metal) and d 8 electronic configuration. − ,,− Among the d 8 metal complexes, platinum­(II) complexes have been studied but mostly confined to mononuclear platinum­(II) systems, with dinuclear platinum­(II) complex systems almost unexplored except for the famous discrete Pt 2 (pop) 4 4– lantern-type system. ,− Unlike the study of the mononuclear platinum­(II) system, where only intermolecular metal–metal and π–π interactions can be mod...…”

Chromic Platinum Complexes Containing Multidentate Ligands

“…The family of bipyridine‐based vapochromic complexes increased in number with the incorporation of other ancillary ligands, such as chlorides, [52] thioglucose, [53] acetylides, [54–56] phenylpyridine, [57] and tartrate [58] . All of these species display a chromic behavior originating from a vapor‐induced molecular rearrangement and the disruption of Pt⋅⋅⋅Pt interactions.…”

Chameleonic Photo- and Mechanoluminescence in Pyrazolate-Bridged NHC Cyclometalated Platinum Complexes

“…Cyclometalated complexes of Pt­(II) are characterized by outstanding photoluminescent properties, arising from the radiative deactivation of triplet excited states, which are at the origin of many challenging applications such as optical sensors, biological imaging, or light emitting devices . The planar geometry of the mononuclear complexes allows them to assemble through Pt···Pt and π–π interactions.…”

“…However, many transition metal complexes exhibit aggregation-induced phosphorescent emission (AIPE), − and they can successfully be used to achieve white light , or NIR , organic-light-emitting diodes by adjusting the doping concentration. The emission color strongly depends on the extent of these intermolecular interactions, and in their turn, on environmental factors able to affect them, such as temperature variations, mechanical force, , or volatile solvent molecules embedded into the lattice. , Thus, these compounds become thermo-, mechano-, and/or vapoluminescent complexes, enlarging the technological interest of these smart functional materials.…”