Transient absorption and resonance raman investigations on the axial ligand photodissociation of halochromium(III) tetraphenylporphyrin

The axial ligand photodissociation processes of halochromium(III) tetraphenylporphyrin (XCr^IIITPP, X = Cl, Br) have been investigated in noncoordinating and coordinating solvents by transient Raman and absorption spectroscopic techniques. In noncoordinating solvents such as benzene, the upshift of the v-i and vi, bands and the disappearance of Ci-X stretching mode in the transient Raman spectra demonstrate the core size reduction of the porphyrin macrocycle accompanied by the photodissociation of axial halogen ligand atoms in the excited state. In coordinating solvents such as tetrahydrofuran (THF), where the solvent molecule is already attached to XCr^IIITPP as an axial ligand to form XCr^IIITPP(THF), the transient spectroscopic data indicate that the axial halogen ligand atoms photodissociate to form the five-coordinate Cr^IIITPP(THF) on photoexcitation. The temporal evolutions of photoinduced absorption and bleaching signals of XCr^IIITPP in benzene exhibit biphasic decay profiles with time constants of 1 and 20 ms. The shorter decay is likely due to the four-coordinate photoexcited Cr^IIITPP* species, and the relatively slow decay component seems to be the recombination process returning to the original five-coordinate XCr'TPP species. On the other hand, a significant reduction in the lifetime of photoexcited ClCr^IIITPP in THF was observed as compared with that in benzene. This behavior seems to be caused by the excited five-coordinate Cr^IIITPP(THF)* species, which decays rapidly with a time constant of 632 ps due to the participation of low-energy states in the deactivation process below the normally emissive tripmultiplet (π,π*) states. The electronic nature of the lowest excited state of the five-coordinate Cr^IIITPP(THF)* species is suggested to possess (π,d_π) charge transfer character based on the comparison of transient Raman and absorption spectral features with those of other paramagnetic metalloporphyrins. We explain the axial ligand photodissociation processes in terms of the electron density change in metal d Orbitals, which is particularly sensitive to the interaction with σ-donor axial ligands.