Synthesis, Structure, and Spectral and Electrochemical Properties of Meso-Tetraaryl-27-Thiasapphyrins

An alternate synthetic route was developed for tetraaryl 27-thiasapphyrins by condensation of meso-aryl dipyrromethanes with 2,5-bis(p-tolylhydroxymethyl)thiophene under acid-catalyzed conditions. Previously, 27-thiasapphyrins have been prepared by a multistep synthesis using 2,2′-bipyrrole as the key precursor. In our strategy, the pyrrole-pyrrole bond was generated to form 27-thiasapphyrins by oxidative coupling of meso-aryl dipyrromethane and 2,5-bis(p-tolylhydroxymethyl)thiophene, and the desired 27-thiasapphyrins were isolated in 8-12% yields. The crystal structure of one of the 27-thiasapphyrins indicates that the macrocycle is almost planar and the thiophene sulfur atom is in the plane whereas the four pyrrole nitrogen atoms slightly deviate from the plane defined by four meso carbon atoms. Absorption spectroscopy showed four well defined Q bands in 590-770nm and one strong Soret band in 470-480nm region for 27-thiasapphyrins. The 27-thiasapphyrins are stable under redox conditions and undergo two-to-three reversible oxidations and one-to-two reversible reductions. The macrocycles are weakly fluorescent with low quantum yields and singlet state lifetimes.