Sc. Jeoung et al., Transient absorption and resonance raman investigations on the axial ligand photodissociation of halochromium(III) tetraphenylporphyrin, J PHYS CH A, 104(21), 2000, pp. 4816-4824
The axial ligand photodissociation processes of halochromium(III) tetraphen
ylporphyrin ((XCrTPP)-T-III, X = Cl, Br) have been investigated in noncoord
inating and coordinating solvents by transient Raman and absorption spectro
scopic 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 tetrahy
drofuran (THF), where the solvent molecule is already attached to (XCrTPP)-
T-III as an axial ligand to form (XCrTPP)-T-III(THF), the transient spectro
scopic data indicate that the axial halogen ligand atoms photodissociate to
form the Five-coordinate (CrTPP)-T-III(THF) on photoexcitation. The tempor
al evolutions of photoinduced absorption and bleaching signals of (XCrTPP)-
T-III in benzene exhibit biphasic decay profiles with time constants of 1 a
nd 20 ms. The shorter decay is likely due to the four-coordinate photoexcit
ed (CrTPP)-T-III* species, and the relatively slow decay component seems to
be the recombination process returning to the original five-coordinate (XC
rTPP)-T-III species. On the other hand, a significant reduction in the life
time of photoexcited (ClCrTPP)-T-III in THF was observed as compared with t
hat in benzene. This behavior seems to be caused by the excited five-coordi
nate (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 deactivatio
n process below the normally emissive tripmultiplet (pi,pi*) states. The el
ectronic nature of the lowest excited state of the five-coordinate (CrTPP)-
T-III(THF)* species is suggested to possess (pi,d(pi)) charge transfer char
acter based on the comparison of transient Raman and absorption spectral fe
atures with those of other paramagnetic metalloporphyrins. We explain the a
xial ligand photodissociation processes in terms of the electron density ch
ange in metal d orbitals, which is particularly sensitive to the interactio
n with sigma-donor axial ligands.