LOWEST EXCITED-STATE OF OXOVANADYL(IV) TETRAPHENYLPORPHYRIN

The excited-state relaxation dynamics and pathways of oxovanadyl(IV) t etraphenylporphyrin ((OVTPP)-T-IV) have been investigated by nanosecon d time-resolved and steady-state emission, transient absorption, and t ransient resonance Raman spectroscopies. At room temperature, the emis sion spectrum shows a single broad featureless band centered near 790 nm in noncoordinating solvents such as toluene and benzene and at 814 nm in tetrahydrofuran (THF), whereas at 77 K the emission bands in the same solvents exhibit blue-shifts to 745 and 784 nm, respectively. Fu rthermore, the emission decay time in benzene increases from 40 to 60 ns with a decrease in temperature from 323 to 268 K. In contrast, the decay time in THF is ca. 10 ns, which is almost independent of tempera ture. The energy separation between the tripdoublet state (T-2(pi,pi) ) and the tripquartet one (T-4(pi,pi)) under the proposed relaxation scheme of photoexcited (OVTPP)-T-IV has been estimated to be ca, 500 c m(-1) in benzene and toluene. The transient absorption spectra at room temperature denote a broad featureless absorption centered at around 480 nm with a ground-state bleaching at 555 nm. All the spectroscopic results including the transient Raman spectra led us to conclude that the electronic nature of the lowest excited state should be the tripqu artet state (T-4(pi,pi)) at ambient temperature. The decay times for both photoinduced absorption and bleaching are similar and also in goo d accordance with the emission decay times as long as temperature and solvent are the same. The time-resolved emission spectra and wavelengt h dependent decay kinetics at 77 K reveal that the observed emission s pectra apparently originate from three different emitting species. The se observations are discussed in terms of postulating a quenching stat e (Q), which is thermally accessible from the tripdoublet state (T-2(p i,pi)) of (OVTPP)-T-IV at room temperature.