Reorganization energies of diprotonated and saddle-distorted porphyrins in photoinduced electron-transfer reduction controlled by conformational distortion

Kinetics of photoinduced electron transfer from a series of electron donors to the triplet excited states of a series of nonplanar porphyrins, hydrochloride salts of saddle-distorted dodecaphenylporphyrin ([H ₄DPP]Cl₂), tetrakis(2,4,6-trimethylphyenyl)porphyrin ([H₄TMP]Cl₂), tetraphenylporphyrin ([H₄TPP] Cl₂), and octaphenylporphyrin ([H₄OPP]Cl₂), were investigated in comparison with those of a planar porphyrin, zinc [tetrakis(pentafluorophenyl)]porphyrin [Zn(F₂₀TPP)(CH ³CN)], in deaerated acetonitrile by laser flash photolysis. The resulting data were evaluated in light of the Marcus theory of electron transfer, allowing us to determine reorganization energies of electron transfer to be 1.21 eV for [H₄TMP]Cl₂,1.29 eV for [H ₄TPP]Cl₂, 1.45 eV for [H₄OPP]Cl₂, 1.69 eV for [H₄DPP]Cl₂, and 0.84 eV for [Zn(F ₂₀TPP)(CH₃CN)]. The reorganization energies exhibited a linear correlation relative to the out-of-plane displacements, which represent the degree of nonplanarity. The rate of electron-transfer reduction of diprotonated porphyrins is significantly slowed down by conformational distortions of the porphyrin ring. This indicates that the reorganization energy of electron transfer is governed by structural change, giving a larger contribution of inner-sphere bond reorganization energy rather than outer-sphere solvent reorganization energy.