Subphthalocyanine-type dye with enhanced electron affinity: Effect of combined azasubstitution and peripheral chlorination

Abstract

Novel subphthalocyanine-type dye with enhanced electron-affinity was prepared by trimerization of 5,6-dichloropyrazine-2,3-dicarbonitrile in the presence of BCl3 in p-xylene. The obtained perchlorinated pyrazine fused subporphyrazine [Cl6Pyz3sPA] was characterized by mass-spectrometry, 13C and 11B NMR, IR and UV-VIS spectroscopy, cyclovoltammetry. Its molecular structure was confirmed by single crystal X-ray diffraction. To elucidate the combined effect of hexaazasubstitution and peripheral chlorination in subphthalocyanines on their electronic structure and spectral features, DFT and TD DFT calculations (B3LYP/pcseg-2 basis set) were used. Fusion of the pyrazine fragments instead of benzene rings strongly enhances the electron affinity of the macrocycle due to stabilization of the LUMO. Thus, the first reduction potential is observed at −0.20 V vs Ag/AgCl, that is macrocycle in [Cl6Pyz3sPA] is 0.5 V more easily reduced than in the peripherally hexachlorinated subphthalocyanine [Cl6sPc], the most popular subphthalocyanine-type acceptor for organic photovoltaics. The HOMO-LUMO gap is increased, so that the maximum of the Q band corresponding to the lowest ππ* transition 2a2→1e* shifts hypsochromically by 36 nm and appears at 535 nm. A distinct feature of the peripherally chlorinated macrocycles is appearance of two bands in the UV-region (B1 at ~300 nm and B2 at ~340 nm) corresponding to the electronic transitions 1a1→1e* and 2a1→1e*, respectively. Azasubstitution decreases fluorescence quantum yield, ΦF = 0.20 for [Cl6Pyz3sPA] and 0.37 for [Cl6sPc] in CH2Cl2. Spectrophotometric titrations indicate that unlike [Cl6sPc], that undergoes consecutive protonation of three meso-nitrogens in CH2Cl2-CF3COOH-H2SO4 medium, the presence of pyrazine rings in [Cl6Pyz3sPA] and their involvement in the acid-base interaction decrease the basicity of nitrogens in the meso-positions. The obtained data along with the photoelectric tests on the sublimed thin films point out at the prospects of [Cl6Pyz3sPA] for the photovoltaic applications.