Photophysics of diimine platinum(II) bis-acetylide complexes

A comprehensive photophysical investigation has been carried out on a series of eight complexes of the type (diimine)Pt(-C≡C-Ar)₂, where diimine is a series of 2,2′-bipyridine (bpy) ligands and -C≡C-Ar is a series of substituted aryl acetylide ligands. In one series of complexes, the energy of the Pt → bpy metal-to-ligand charge transfer (MLCT) excited state is varied by changing the substituents on the 4,4′- and/or the 5,5′-positions of the bpy ligand. In a second series of complexes the electronic demand of the aryl acetylide ligand is varied by changing the para substituent (X) on the aryl ring (X = -CF₃, -CH₃, -OCH₃, and -N(CH₃)₂). The effect of variation of the substituents on the excited states of the complexes has been assessed by examining their UV-visible absorption, variable-temperature photoluminescence, transient absorption, and time-resolved infrared spectroscopy. In addition, the nonradiative decay rates of the series of complexes are subjected to a quantitative energy gap law analysis. The results of this study reveal that in most cases the photophysics of the complexes is dominated by the energetically low lying Pt ← bpy ³MLCT state. Some of the complexes also feature a low-lying intraligand (IL) ³π,π* excited state that is derived from transitions between π- and π*-type orbitals localized largely on the aryl acetylide ligands. The involvement of the IL ³π,π* state in the photophysics of some of the complexes is signaled by unusual features in the transient absorption, time-resolved infrared, and photoluminescence spectra and in the excited-state decay kinetics. The time-resolved infrared difference spectroscopy indicates that Pt ← bpy MLCT excitation induces a +25 to + 35 cm^-1 shift in the frequency of the C≡C stretching band. This is the first study to report the effect of MLCT excitation on the vibrational frequency of an acetylide ligand.