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UDC 535.37We have investigated the spectral characteristics, quantum yield ϕ F , lifetime τ F , and polarization of fluorescence of NH-isomers of dibenzobarrelene disubstituted tetraazaisobacteriochlorin at 293 and 77 K. The following information has been obtained from low-temperature measurements: 1) the weak shoulder on the edge of the long-wavelength 0-0 absorption band disappears at 77 K and, therefore, can be assigned to the cis-NH-isomer [the principal component of the tautomeric mixture is the trans-isomer a (b)]; 2) the low value of ϕ F at room temperature is due to temperature-viscosity quenching whereas ϕ F at 77 K is close to 1 and τ F = 5.3 nsec; 3) the energy of the weak electronic transition G → Q 2 and the interval ∆E Q 2 Q 1 ≈ 2750 cm -1 have been determined. Quantum-chemical calculations found that replacing the methine bridges by nitrogen stabilizes the non-aromatic trans-isomer a (b) whereas the non-planarity of the macrocycle and the alternation of bond lengths increase for the cis-isomers. The calculated electronic absorption spectra of the a isomer reproduce rather well the experimental data. It is shown that the four-orbital model is applicable for the lower excited states Q 1 and Q 2 for all NH-isomers.

Transient absorption spectra of tetraazaporphine and substituted derivatives of tetraazachlorin and tetraazabacteriochlorin were obtained upon picosecond excitation with various probe pulse delay times in order to resolve the open question about the reasons for fluorescence quenching in tetraazaporphine hydrogenated derivatives. The quantum yield of triplet state T 1 formation has been estimated. It has been shown that in all investigated cases radiationless de-excitation of the fluorescent level S 1 occurs by two channels: S 1 ‫ב‬ T 1 and S 1 ‫ב‬ S 0 , the latter being predominant. As the S 1 level becomes lower, the transition rate for this channel increases. For tetraazaporphine and its derivatives, a quasi-photochemical mechanism is proposed that accounts for the anomalous efficiency of the S 1 ‫ב‬ S 0 channel in the dissipation of the electronic excitation energy.Introduction. Bridging tetraaza-substitution in the tetrapyrrole macrocycle and hydrogenation of the pyrrole rings is one of the factors capable of causing fundamental changes in the electronic spectra of porphine derivatives. Combination of these factors deserves consideration as a method for creating compounds with controlled properties. Spectral and luminescence characteristics of hydrogenated tetraazaporphine derivatives (also called porphyrazine) have been well studied [1][2][3][4][5][6][7][8].Studies showed that hydrogenation of one or two opposed pyrrole rings in tetraazaporphine (H 2 TAP) causes significant fluorescence quenching [1,3,5,8]. This quenching may be due to an increase of the probabilities for the two radiationless intramolecular processes of intersystem crossing S 1 ‫ב‬ T 1 , k ST , and internal conversion S 1 ‫ב‬ S 0 , i.e., energy exchange of the S 1 electronic state onto vibrations of the molecule in the ground state S 0 , the probability of which increases with lowering of the S 1 level. Preference has been given to the first de-excitation channel [1, 3, 5] on the basis of indirect literature data, in particular, a comparison with fluorescence of bacteriochlorin [9] and 2,3-naphthalocyanine [10] derivatives. In these instances, lowering of the S 1 level was also accompanied by fluorescence quenching although the quenching was weaker. Both pathways were mentioned in another study [8].The kinetics of absorption spectra upon picosecond excitation of H 2 TAP, dibenzobarrelene-substituted tetraazachlorin (H 2 TAC t ), and dibenzobarrelene-substituted tetraazabacteriochlorin (H 2 TABC