Structural Insights into the Vapochromic Behavior of Pt- and Pd-Based Compounds

Abstract

Anionic complexes having vapochromic behavior are investigated: [K­(H2O)]­[M­(ppy)­(CN)2], [K­(H2O)]­[M­(bzq)­(CN)2], and [Li­(H2O) n ]­[Pt­(bzq)­(CN)2], where ppy = 2-phenylpyridinate, bzq = 7,8-benzoquinolate, and M = Pt­(II) or Pd­(II). These hydrated potassium/lithium salts exhibit a change in color upon being heated to 380 K, and they transform back into the original color upon absorption of water molecules from the environment. The challenging characterization of their structure in the vapochromic transition has been carried out by combining several experimental techniques, despite the availability of partially ordered and/or impure crystalline material. Room-temperature single-crystal and powder X-ray diffraction investigation revealed that [K­(H2O)]­[Pt­(ppy)­(CN)2] crystallizes in the Pbca space group and is isostructural to [K­(H2O)]­[Pd­(ppy)­(CN)2]. Variable-temperature powder X-ray diffraction allowed the color transition to be related to changes in the diffraction pattern and the decrease in sample crystallinity. Water loss, monitored by thermogravimetric analysis, occurs in two stages, well separated for potassium Pt compounds and strongly overlapped for potassium Pd compounds. The local structure of potassium compounds was monitored by in situ pair distribution function (PDF) measurements, which highlighted changes in the intermolecular distances due to a rearrangement of the crystal packing upon vapochromic transition. A reaction coordinate describing the structural changes was extracted for each compound by multivariate analysis applied to PDF data. It contributed to the study of the kinetics of the structural changes related to the vapochromic transition, revealing its dependence on the transition metal ion. Instead, the ligand influences the critical temperature, higher for ppy than for bzq, and the inclination of the molecular planes with respect to the unit cell planes, higher for bzq than for ppy. The first stage of water loss triggers a unit cell contraction, determined by the increase in the b axis length and the decrease in the a (for ppy) or c (for bzq) axis lengths. Consequent interplane distance variations and in-plane roto-translations weaken the π-stacking of the room-temperature structure and modify the distances and angles of Pt­(II)/Pd­(II) chains. The curve describing the intermolecular Pt­(II)/Pd­(II) distances as a function of temperature, validated by X-ray absorption spectroscopy, was found to reproduce the coordinate reaction determined by the model-free analysis.

Photophysical Properties of Cyclometalated Platinum(II) Diphosphine Compounds in the Solid State and in PMMA Films

“…Reduced pair distribution functions, G ( r ), of L1-DPPM , L1-DPPE , and L1-DPPP were recorded in the solid state and showed a highly disordered molecular solid. As illustrated in Figure , which shows L1-DPPM , the observed G ( r ) is consistent with prior measurements of molecular solids showing sharp well-defined intramolecular correlations at low r (below r ∼3.7 Å), poorly defined broad density oscillations representing disordered intermolecular correlations at higher r (at above r ∼8.0 Å), and an intermediate region containing both inter- and intramolecular correlations (Figures and ). − Significantly, there is little evidence of a well-defined atomic order past 1 nm in these samples, suggesting a highly disordered emissive solid state.…”

Coordination-based vapochromic behavior of a luminescent Pt(ii) complex with potassium ions

“…On the other hand, detailed in situ X-ray measurements for [K(H 2 O)][Pt(CN) 2 (bzq)] and [K(H 2 O)][Pt(CN) 2 (ppy)], reported by Caliandro et al , suggested that intermolecular metallophilic and π–π interactions are crucial to the vapochromic process in the desorption of H 2 O molecules. 40 Our results suggest that the interaction between guest molecules and coordination sites is also important for vapochromic behavior (desorption of the guest molecules). After DMA vapor exposure, 1 exhibited emission centered at 523 nm, which was tentatively assigned to the mixed 3 MLCT/ 3 LC emission from the Cl 2 ppy ligand.…”

“…Further, the H 2 O vapor induced vapochromic behaviors of [K(H 2 O)][M(CN) 2 (ppy)] and [K(H 2 O)][M(CN) 2 (bzq)] (M = Pt or Pd) were further investigated using several experimental techniques. 40 Single crystal X-ray structural analysis of [K(H 2 O)][Pt(CN) 2 (ppy)] revealed that potassium ions were coordinated by cyanide ligands and water molecules. Furthermore, X-ray diffraction and TG measurements indicated that the H 2 O molecule showed coordination(adsorption)- and dissociation-based vapochromic behavior.…”

Multistimuli-Responsive Properties of Aggregated Isocyanide Cycloplatinated(II) Complexes

“…Among chromic Pt II systems, a number of cyclometalated Pt II complexes have been developed as a means to achieve bright, colorful luminescent materials. ,,,,, On the other hand, isocyanides are versatile synthons for photoluminescent platinum complexes, which show stimuli response properties, and on some occasions, the luminescence can be modulated by noncovalent interactions . In this field, some chromic cycloplatinated complexes containing the strong field ligand isocyanides have been documented, , and it has been demonstrated that their photophysical properties can be modulated effectively by adjusting steric effects, the strength of the intermolecular interactions, or the medium.…”