Ultrafast Excited-State Dynamics in trans-(N-Heterocyclic carbene)platinum(II) Acetylide Complexes

This study probes femto- and picosecond excited-state dynamics of a series of N-heterocyclic carbene (NHC) ligand-containing platinum(II) complexes of the type trans-(NHC)₂Pt^II(CC-Ar)₂, where CC-Ar is an arylacetylide. By using femtosecond transient absorption spectroscopy, two dynamic processes are observed: an ultrafast singlet → triplet intersystem crossing (<0.3 ps), followed by geometric/electronic relaxation that takes place on a 2-10 ps time scale. The geometric/electronic relaxation is attributed to ligand torsional modes, mainly arising from twisting of the aryl units relative to the square-planar PtL₄ unit. The dynamics of this relaxation process depend somewhat on steric constraints induced by substituent groups attached to the (benz)imidazole and phenyl ligands. The geometric relaxation dynamics slow with increasing solvent viscosity. The experimental studies also reveal that the different conformers can be photoselected by varying the excitation at different near-UV wavelengths. To corroborate the experimental findings, density functional theory calculations were conducted to probe the effects of geometry and steric hindrance on the ground-state energy surface. The calculations suggest that the barrier for torsion of the CC-Ar units increases as N-substituents on the NHC ligands increase in the order CH₃ < cyclohexyl < n-butyl and as the CC-Ar units are substituted in the 3 and 5 positions with tert-butyl groups.