Spectral luminescence properties of hydroporphyrazine metallo complexes

Abstract

We have studied the spectral luminescence properties of a zinc complex of dibenzobarreleno-substituted tetraazaisobacteriochlorin (a synthetic tetrapyrrole pigment which is the structural analog of the biologically important chromophore isobacteriochlorin) and a palladium complex of dibenzobarreleno-substituted tetraazachlorin. We have shown that despite the substantial effect of aza-substitution on the electronic structure and the electronic spectra of the tetrapyrrole molecules, the effect of hydrogenation of adjacent pyrrole rings on the spectral luminescence properties is similar in compounds with nitrogen and carbon bridges. As in the free base case, the fluorescence of the studied zinc complex is strongly quenched at 293 K but flares up at 77 K (temperature and viscosity-dependent quenching). To study the substituted Zn-tetraazaisobacteriochlorin, we determined the position of the 0-0 band of the low-intensity transition S 0 → S 2 and showed that the S 2 → S 1 energy difference increases with bridge aza substitution. For the Pd complex of dibenzobarreleno-substituted tetraazachlorin, we could not detect phosphorescence in the near IR region (to 1150 nm); we detected weak fluorescence.Introduction. Hydrogenation of pyrrole rings in a tetrapyrrole macrocycle and bridge aza substitution are two structural factors strongly affecting the spectral characteristics of this biologically important chromophore. As we know, the presence of a hydrogenated pyrrole ring in the chlorophyll molecule (a universal photosynthetic pigment) is responsible for the green color of plants, which is important for efficient light harvesting. In bacterial photosynthesis, the working substance is bacteriochlorophyll, in the molecule of which two opposite pyrrole rings are hydrogenated. It is not as well known that tetrapyrrole pigments with two adjacent hydrogenated rings, for example, siroheme, play an important role in nature [1]. Symmetric tetraaza substitution in the porphyrin macrocycle leads to the tetraazaporphin structure (the unsubstituted tetraazaporphin is also called porphyrazine). Derivatives of porphyrazine (phthalocyanines and their analogs) have become widely used in science and technology. 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). In this paper here, the results obtained for the palladium complex of this chlorin and the zinc complex of substituted tetraazaisobacteriochl...

Spectral and Luminescent Characteristics of a Hexaphenyltetraazachlorin Zinc Complex

“…It is important to note that the Stokes shift was very small, 2 nm or 40 cm -1 . It also decreased but not as much upon freezing other tetraazachlorins [1,[3][4][5][6] whereas it was large for ZnPh 6 TAC tm at 293 K.…”

“…These were tetramethylhexaphenyltetraazachlorin [2] (H 2 Ph 6 TAC tm , where tm = tetramethyl) and N-methylpyrrolidineoctaphenyltetraazachlorin, the spectral properties of which differed slightly. The zinc complexes were studied in parallel with the free bases in a series of studies [3][4][5] in order to expand information on the properties of the macrocycles. Therefore, it seemed advisable to continue the work [1] by studying the photophysical characteristics of the zinc complex ZnPh 6 TAC tm :…”