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

The axial ligand photodissociation processes of halochromium(III) tetraphenylporphyrin ((XCrTPP)-T-III, 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 nu(2) and nu(4) hands and the disappearance of Cr-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 (XCrTPP)-T-III as an axial ligand to form (XCrTPP)-T-III(THF), the transient spectroscopic data indicate that the axial halogen ligand atoms photodissociate to form the Five-coordinate (CrTPP)-T-III(THF) on photoexcitation. The temporal evolutions of photoinduced absorption and bleaching signals of (XCrTPP)-T-III 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 (CrTPP)-T-III* species, and the relatively slow decay component seems to be the recombination process returning to the original five-coordinate (XCrTPP)-T-III species. On the other hand, a significant reduction in the lifetime of photoexcited (ClCrTPP)-T-III in THF was observed as compared with that in benzene. This behavior seems to be caused by the excited five-coordinate (CrTPP)-T-III(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 (pi,pi*) states. The electronic nature of the lowest excited state of the five-coordinate (CrTPP)-T-III(THF)* species is suggested to possess (pi,d(pi)) 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 sigma-donor axial ligands.