Quantum-Chemical Investigation of the Conformational Dynamics of Mono-<i>meso</i>-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 Cm1−C1 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 ΔEV(T1−S0) down to 0.45 eV, in contrast to ΔEV(T1−S0) ∼ 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 T1 → Tn 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 T1 → Tn 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.