Effect of bridging aza-substitution on electronic structure and spectral-luminescent properties of isobacteriochlorin NH-isomers

Abstract

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 its reduced derivatives upon picosecond excitation

“…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.…”

“…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].…”

Spectral luminescence properties of hydroporphyrazine metallo complexes

“…This proves that the assignment of the Q 2 (0-0) band is correct (see above and Table 1). Note two points: First of all, we see a similarity to the free base H 2 TAiBC tt , the NH isomer a, for which the Q 2 (0-0) band also undergoes a short-wavelength shift at 77 K [6]. Such sensitivity of the spectral bands to the effect of external factors (the properties of the medium also change on freezing down to 77 K) is not typical for porphyrins.…”

“…The combination of the two indicated structural factors can make it possible to control the physicochemical properties of molecular systems based on porphyrazines. A theoretical study of molecules with hydrogenated pyrrole rings and aza bridges should facilitate a deeper understanding of the electronic structure and physicochemical properties of molecules of this type.Recently a number of papers have been published on study of the photophysics of porphyrazine molecules with hydrogenated pyrrole rings [2][3][4][5][6][7][8][9]. However, mainly free bases have been studied, and only in [2] were some of the objects of investigation a zinc complex of substituted dihydroporphyrazine (tetraazachlorin).…”

“…The structural formula for H2TAiBC tt is shown in [6]; the structure of ZnTAiBC tt is distinguished by replacement of H2 by Zn. Results and Discussion.…”