A fast time-resolved infrared spectroscopic investigation into the nature of the lowest excited state and excimer formation in PtII diimine cyanides

Time-resolved infrared spectroscopy (TRIR) has been used to probe the nature of the lowest excited state of [PtII(dpphen)(CN)2] (dpphen = 4,7-diphenyl-1,10-phenanthroline) both in fluid solution at room temperature and in a glass at 77 K. The positions of the ν(CN) bands in [PtII(dpphen)(CN)2] are only slightly (less than 5 cm−1) shifted upon formation of the excited state, thus supporting their assignment as the π–π* intraligand state. At 77 K [PtII(dpphen)(CN)2] has a highly structured luminescence with a lifetime of 170 μs, which is also characteristic of a π–π* excited state. In contrast, the lowest excited state of [Ru(bpy)2(CN)2] has MLCT character in both fluid solution at ambient temperature and at 77 K as shown by a large positive shift of ν(CN) relative to the ground state. Also we have monitored the monomer–excimer equilibrium of [PtII(dpphen)(CN)2] in fluid solution using the ns-TRIR technique.