Porphyrazines with Annulated Chalcogen-Containing Heterocycle: Study of Spectral-Luminescent Properties and Quantum-Chemical Calculations of the Excited Electronic States

Joint experimental and theoretical study of spectral-luminescent properties and electronic structure of the molecules of new phthalocyanine analogues - derivatives of octaphenylporphyrazine and tetrakis(tert-butyl)phthalocyanine carrying an annulated 1,2,5-thiadiazole or 1,2,5-selenadiazole cycle instead of two phenyl groups or a benzene ring - has been performed. The annulation of a five-membered heterocycle results in bathochromic shifts of the Qx band and Soret band; the Qy - Qx(S2-S1) interval increases and the fluorescence spectra become mirror-symmetrical to the absorption spectra in the region of the G -> Qx transition. Replacement of a benzene ring in the tetrakis(tert-butyl)phthalocyanine molecule by the 1,2,5-selenadiazole cycle lowers the fluorescence quantum yield from 0.77 to 0.14 which may be accounted for by the internal heavy-atom effect as well as by intramolecular charge transfer. The fluorescence quantum yield of octaphenylporphyrazine is very low, but for the derivatives it is comparatively high, phi(F)=0.06, i.e. the fluorescence is not quenched but enhanced when the heterocycle is annulated. The quantum-chemical calculations of the ground and excited states of the original structures of phthalocyanine, octaphenylporphyrazine, and their derivatives with the annulated thiadiazole cycle have been performed. It is shown that the chalcogen substituted compounds exist as isomers with the NH - NH axis passing through the pyrrole rings devoid of the heterocycle. The calculations based on the INDO/Sm method give reasonable interpretation of the experimental absorption spectra. The description of the properties of the lowest excited Q states is sufficiently good for the phenyl substituted compounds. At the same time, for the Soret band region it is rather difficult to achieve agreement with experiment. Specifically, the energies of the G -> B and G -> N transitions and the relation between their intensities strongly depends on the values of the dihedral angle made by phenyl ring and the macrocycle.