Transient Absorption and Resonance Raman Investigations on the Axial Ligand Photodissociation of Halochromium(III) Tetraphenylporphyrin

Abstract

The axial ligand photodissociation processes of halochromium(III) tetraphenylporphyrin (XCrIIITPP, 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 ν2 and ν4 bands 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 XCrIIITPP as an axial ligand to form XCrIIITPP(THF), the transient spectroscopic data indicate that the axial halogen ligand atoms photodissociate to form the five-coordinate CrIIITPP(THF) on photoexcitation. The temporal evolutions of photoinduced absorption and bleaching signals of XCrIIITPP 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 CrIIITPP* species, and the relatively slow decay component seems to be the recombination process returning to the original five-coordinate XCrIIITPP species. On the other hand, a significant reduction in the lifetime of photoexcited ClCrIIITPP in THF was observed as compared with that in benzene. This behavior seems to be caused by the excited five-coordinate CrIIITPP(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 CrIIITPP(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.

Inorganic–organic hybrid photochromic materials

“…This intermediate possesses (p,d p ) charge transfer character based on theoretical calculations. 71 A similar result has also been observed in a Ti(III)-porphyrin complex in solution. 72 Linkage isomerization.…”

Ligand Recruitment and Spin Transitions in the Solid-State Photochemistry of Fe(III)TPPCl

“…The Raman spectroscopy and photochemistry of metalloporphyrins have been studied extensively over the years. ,,,,,,,− The vibrational bands exhibiting substantive changes in the photoproduct spectrum reported here are well-known to correlate with the oxidation state, spin state, and ligation of the iron atom in the porphyrin compound. In response to changes in the population of the different d-orbitals the metal atom may move in or out of the plane of the porphyrin ring, and the porphyrin core may expand or contract around the metal atom.…”

“…Reduction of the iron atom to Fe (II) causes the iron atom to increase in size and shift more into the plane of the porphyrin ring. ,,,,,,, The expansion of the ring shifts the characteristic ν 2 , ν 3 , ν 4 and ν 8 vibrations to lower energy. Five-coordinate high-spin Fe (II) compounds can be formed with 2-methylimidazole or 1,2-dimethylimidazole ligands.…”

Photoinduced Reduction of Manganese(III) meso-Tetrakis(1-methylpyridinium-4-yl)porphyrin at AT and GC Base Pairs

“…The population of the d x 2 −y 2 orbital, essentially by antibonding electrons, causes large out-of-plane displacement of the Mn II ion. 38,39 The core size of the macrocycle of Mn II (TMPyP) 4+ is increased, which does not favor charge-transfer transitions. As a consequence, Mn III (TMPyP) 4+ , in which charge-transfer reaction occurs easily, exhibited the Soret absorption band near ∼460 nm which is in contrast with that of 435 nm for Mn II (TMPyP) 4+ .…”