Tetraimidazophthalocyanines: influence of protonation and aggregation on spectroscopic observations

The synthesis and spectroscopic characterization of tetraimidazophthalocyanines are presented. This kind of molecules have not been intensively studied due to difficulty in (i) obtaining an easily obtainable synthon: a benzimidazole ring with a variety of substituent, and (ii) achieving the Rosenmund-Von Braun reaction with good yields. The synthesis of novel 2-substituted-5,6-dicyanobenzimidazoles (4a-e) achieved from 4,5-dibromo-N,N'-diacetylated-1,2-phenylene diamine (2a-e) or 2-substituted-5,6-dibromobenzimidazole (3a-e) were optimized. Depending on the starting compound, the reaction could be done according to a two-step pathway (cyclization followed by nitrilation) or a one-pot procedure using microwave irradiation. The target compounds, tetraimidazophthalocyanines (ImiPc(M)) (5a-e; 6a-d), were prepared. The effect of the four imidazoles introduced in the phthalocyanines core were compared and discussed.