Phosphorescence of palladium and platinum complexes of benzo-fused hydroporphyrazines

Palladium and platinum complexes of benzo-fused hydroporphyrazines, analogs of phthalocyanine in whose molecules one or two isoindole fragments are substituted by hydrogenated pyrrole rings giving macrocycles of the chlorin, bacteriochlorin, and isobacteriochlorin types, have been synthesized. Phosphorescence in the near IR region has been detected at 291 and 77 K for complexes of tribenzotetraazachlorin and dibenzotetraazaisobacteriochlorin. The energy of the lowest triplet level T 1 has been determined from the positions of 0–0-bands in phosphorescence spectra. As compared to complexes of phthalocyanine, hydrogenation of a pyrrole ring has a stronger influence on the energy of the T 1 level (lowering) whereas hydrogenation of two adjacent pyrrole rings results in a larger S 1 –T 1 gap. The phosphorescence quantum yield and lifetime have been measured. Based on the data obtained, the rate constants of the radiative and non-radiative de-excitation of the T 1 level have been calculated. The highest phosphorescence quantum yield is obtained for Pt dibenzotetraazaisobacteriochlorin (1.7% at 77 K). The influence of the nature of the metal atom on the electronic absorption spectra of metal complexes of benzohydroporphyrazines is analyzed taking into account literature data for bacteriochlorophyll a, metal complexes of bacteriopheophytin a, and chlorophyll-like molecules having less complex structures.