Lowest excited state of oxovanadyl(IV) tetraphenylporphyrin

The excited-state relaxation dynamics and pathways of oxovanadyl(IV) tetraphenylporphyrin (OV^IVTPP) have been investigated by nanosecond time-resolved and steady-state emission, transient absorption, and transient resonance Raman spectroscopies. At room temperature, the emission 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. Furthermore, 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 temperature. The energy separation between the tripdoublet state (²T(π,π*)) and the tripquartet one (⁴T(π,π*)) under the proposed relaxation scheme of photoexcited OV^IVTPP has been estimated to be ca. 500 cm^-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 tripquartet state (⁴T(π,π*)) at ambient temperature. The decay times for both photoinduced absorption and bleaching are similar and also in good accordance with the emission decay times as long as temperature and solvent are the same. The time-resolved emission spectra and wavelength dependent decay kinetics at 77 K reveal that the observed emission spectra apparently originate from three different emitting species. These observations are discussed in terms of postulating a quenching state (Q), which is thermally accessible from the tripdoublet state (²T(π,π*)) of OV^IVTPP at room temperature.