Photochemistry of benzo[2,3-g]quinoxaline derivatives

Since its discovery in 1867, the photodimerisation of anthracene has been one of the most exploited reactions for reversibly connecting materials at a molecular level. The cumbersome synthesis of many substituted anthracene derivatives combined with their tendency to form endoperodixes when irradiated in presence of oxygen and the short wavelengths needed to convert the dimer to its monomer prohibit their use in practical applications. In an attempt to circumvent some of these problems, we undertook the synthesis and the investigation of the photochemistry of benzo[2,3-g]quinoxalines, which are structurally similar to anthracenes. Surprisingly, the photochemical behaviour of these molecules has not been previously investigated. Upon irradiation, 2,3-diphenylbenzo[2,3-g]quinoxaline (DPBQ) and 2,3-di(2-dipyridyl)benzo[2,3-g]quinoxaline (DPyBQ) both underwent photodimerisation to yield di-DPBQ and di DPyBQ respectively. The head-to-tail structure of di-DPBQ was confirmed by X-ray crystallography. Preliminary studies suggested that the presence of ZnCl2 and Zn(OAc)2 reduced the yield of dimerisation of DPyBQ.