2-NO 2 -5,10,15,20-tetraphenylporphyrin (H 2 TPP-NO 2 ) is shown to exist in solution as an equilibrium mixture of two NH tautomers with different spectral and photophysical properties. At 77 K in a rigid glass solution the fluorescence spectra of the tautomers contain two well-resolved narrow bands that are slightly (∼300 cm -1 ) Stokes-shifted with respect to the corresponding Q X00 absorption bands, with the spectrum of the more stable tautomer being ∼500 cm -1 red-shifted as compared to the spectrum of the less stable tautomer. At room temperature, the tautomers show almost identical broad and structureless fluorescence spectra, markedly red-shifted relative to the longest wavelength absorption band. The Stokes shift increases with an increase of solvent polarity, being as large as ∼2000 cm -1 in N,N-dimethylformamide. Fluorescence lifetimes of the tautomers are found to be markedly (about a factor of 2) different, with both decreasing with an increase of solvent polarity. The reasons for these peculiar fluorescence properties can be understood on the basis of semiempirical quantum chemical calculations. Charge-transfer (CT) states are found to be located between the porphyrinic Q and B states for both tautomers, and a different energy of the CT states gives rise to different fluorescence lifetimes of the tautomers. The calculations also predict a lowering of the Q X state energy and an increase of its dipole moment µ with a decrease of the angle θ between the plane of the NO 2 group and the porphyrin plane (relative to the ground-state equilibrium geometry with θ ) 80°). We suggest that, in solution, such an increase in the µ value should result in an increase of the stabilization interaction between the polar porphyrin and solvent molecules and, consequently, in flattening of the Q X state energy curve as a function of θ and a shift of the minimum of this curve to θ values less than 80°. Both quantum chemical calculations and picosecond transient absorption measurements of the S 1 f S n absorption show that the fluorescent Q X state of H 2 TPP-NO 2 has mainly 1 (π,π*) character with a relatively small charge-transfer admixture.

On the basis or experimems Ihe oscillalor model has been cslnblished Tar individual NH vu~omers or porphyrins with asymmetrical subslirution. CNDO/Z calculations rxplain the inversion of' Q,,(O.O) and Q_,.(O.O) electronic transition intensities in NH !auIomers as a consequence 01 Ihe inversion or LUMO coefficicnu c, and c2 [or fixed x and _v molecular oscillators.

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 ΔE V(T1−S0) down to 0.45 eV, in contrast to ΔE V(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 → 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 T1 → 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.

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