Mono- versus Dinuclear Pt(II) 6-(5-Trifluoromethyl-Pyrazol-3-yl)-2,2′-Bipyridine Complexes: Synthesis, Characterization, and Remarkable Difference in Luminescent Properties

A series of charge-neutral mononuclear Pt(II) complexes Pt(fpbpy)(pz) (3a), Pt(fpbpy)(dmpz) (4a), Pt(fpbpy)(dbpz) (5a), and Pt(fpbpy)(dtfpz) (6a), fpbpyH = 6-(5-trifluoromethyl-pyrazol-3-yl)-2,2'-bipyridine, pzH = pyrazole, dmpzH = 3,5-dimethylpyrazole, dbpzH=3,5-di-tert-butylpyrazole, and dtfpzH=3,5-bis(trifluoromethyl)pyrazole, and the cationic Pt(II) dimer [{Pt(fpbpy)}(2)(mu-pz)](+) (3b), [(Pt(fpbpy)}(2)(mu-dmpz)](+) (4b), and [{Pt(fpbpy)}(2)(mu-dbpz)](+) (5b) were synthesized. Series a mononuclear complexes reveal two distinctive ligand arrangements. As unveiled by X-ray crystallography, 3a exhibits a nearly perfect planar geometry, while structural determination on 6a shows a perpendicular arrangement of dbpz ligand due to steric congestion. In sharp contrast, the dinuclear complexes, exemplified by 4b and 5b, display an intramolecular Pt center dot center dot center dot Pt separation of 3.601 and 3.403 angstrom, respectively. As for photophysical properties, the structural variation leads to a salient difference in emission features between 3a (580 nm) and 6a (510 nm). The results are rationalized by the contribution of ligand-to-ligand charge transfer and intraligand pi-pi* transition for 3a and 6a in the lowest-lying excited state, respectively. On the other hand, dinuclear complexes 3b and 4b reveal dual phosphorescence (denoted as P-1 and P-2 bands), for which the short wavelength emission (the P-1 band) is akin to that observed for the intraligand pi-pi* transition of 6a, while the much red-shifted, broad emission (the P-2 band) is attributed to the formation of intramolecular ligand-metal-to-metal charge transfer excimer transition. Further studies of relaxation dynamics on both 3b and 4b showed fast excited-state equilibrium between the P-1 and P-2 bands. In contrast, only the P-2 emission band was resolved for 5b, indicating its exergonic excimer formation. Supplementary support of the excited-state thermodynamics is also provided by time-dependent density functional theory calculations, incorporating both geometry optimized S-0 and T-1 states.