Quantum-chemical investigation of the conformational dynamics of mono-meso-phenyl-substituted octaalkylporphyrins in the triplet excited state

The detailed theoretical analysis (semiempirical PM3, DFT, and TD-DFT calculations) of the nature of nonplanar conformations of sterically encumbered mono-meso-phenyl-substituted octaalkylporphyrins in the excited triplet state has been carried out. It was found that the OEP-mesoPh molecule in the triplet excited state may occur in the highly nonplanar conformation characterized by the out-of-plane displacement of the single C-m1-C-1 bond and the increased overlap of the porphyrin and the meso-phenyl ring. For the sterically strained porphyrins, the transition into this nonplanar conformation is accompanied by the essential decrease of the vertical triplet-singlet energy gap DeltaE(V)(T-1-S-0) down to 0.45 eV, in contrast to DeltaE(V)(T-1-S-0) similar to 1.33 eV for the planar conformation. This leads to the experimentally detected drastic reduction of the triplet state decays for the sterically strained OEP-mesoPh type molecules. The calculated T-1 --> T-n transient absorption spectra for planar and nonplanar conformations have been compared with the corresponding experimental data obtained for the investigated compounds. A remarkable accord between experimental results and theoretical calculations on T-1 --> T-n absorption spectra in the red and near-IR region provides additional support for the conformational reorganization leading to the formation of the highly nonplanar distortion of OEP-mesoPh molecule in the triplet excited state.