Lowest Excited State of Oxovanadyl(IV) Tetraphenylporphyrin

Abstract

The excited-state relaxation dynamics and pathways of oxovanadyl(IV) tetraphenylporphyrin (OV IV TPP) 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 ( 2 T(π,π*)) and the tripquartet one ( 4 T(π,π*)) under the proposed relaxation scheme of photoexcited OV IV TPP 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 ( 4 T(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 ( 2 T(π,π*)) of OV IV TPP at room temperature.

Light-Induced Electron Spin Polarization in Vanadyl Octaethylporphyrin: I. Characterization of the Excited Quartet State

“…The fit yields a value of 47 cm -1 for Δ E DQ , 20 μs for k DG -1 , and 1.06 ms for k QG -1 . These values are in line with what one would expect and are similar to those obtained from other related porphyrins. , The value of 47 cm -1 for Δ E DQ is almost identical to that reported for EtioVO 8,9 (40 cm -1 ) but is considerably smaller than the value ∼500 cm -1 suggested for vanadyl tetraphenylporphyrin (TPPVO) . This result is reasonable because the 3 (π,π*) excitation involved in the lowest trip-doublet and trip-quartet states of OEPVO and EtioVO have the same 3 ( a 1 u e g ) electronic configuration, whereas for TPPVO, the configuration is 3 ( a 2 u e g ). ,

7 Temperature dependence of the spin polarization lifetime of OEPVO in E7.

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“…Generally, when electronic relaxation from the trip-quartet state takes place both as decay from the thermally repopulated trip-doublet state and direct relaxation from the trip-quartet state, the lifetimes of the excited states are determined by the relaxation rates of the two excited states and by the energy gap. ,, Whether the trip-doublet and trip-quartet states are in thermal equilibrium or not depends on temperature region as well as the relaxation rates and the energy gap.…”

“…Time-resolved optical spectroscopy in its various forms provides a good method for monitoring the excited-state kinetics of such complexes . However, it has the disadvantage that it can be difficult to distinguish nearly degenerate states of different multiplicity, which can be crucial for the photochemistry.…”

An Efficient Indirect Mechanism for the Ultrafast Intersystem Crossing in Copper Porphyrins

“…Therefore, only metalloporphyrins with one single unpaired electron in the n d orbital exhibit this mechanism. Indeed for the first row transition metals vanadium IV oxide porphyrins (3d 1 ) also exhibit strong phosphorescence . In turn, for the other first row transition metal porphyrins, the back electron transfer to the porphyrin, leading to the ground state should be preferred.…”

A Novel Demetalation Process for Vanadyl- and Nickelporphyrins from Petroleum Residue by Photochemical Reaction and Liquid−Liquid Extraction

“…Practically no decrease in the VOTPP concentration was observed for irradiation at wavelengths for λ > 400 nm. Although Jeoung et al have reported that VOTPP in solution is photoexcited by the irradiation at wavelengths for λ > 427 nm, this is insufficient to cause photodecomposition of the VOTPP in tetralin. For irradiation at wavelengths for λ < 400 nm, however, the VOTPP concentration was observed to decrease gradually with increasing irradiation time, but the photodecomposition rate was enhanced remarkably for wavelengths of λ < 300 nm.…”