Catalyst-free 2+2 Photodimerization of 1,4-Bis[2-(4-pyridyl)ethenyl]-benzene in Solution Under Low Power UV Irradiation

Two different kinds of configurations of 1,4-bis[2-(4-pyridyl)ethenyl]-benzene(trans-bpeb and cis-bpeb) were achieved, and a bpeb dimer was synthesized in dimethyl sulfoxide(DMSO). Compared with the previous work that synthesized the bpeb dimer or polymer in crystal with a template agent needed, the reaction occurred in a solution phase in the present method. A hand-held ultraviolet lamp(365 nm) with the power of 12 W and the Watt density of 0.35 mW/cm2 can realize the photodimerization of bpeb, instead of the high-power mercury lamp in most previous studies. Unlike other 2+2 cycloaddition in liquid state using catalysts even noble metals, no catalysts were required here, which is cost-saving. Only the trans-pbeb can start the cycloaddition and the formation of the close J-aggregations of trans-pbeb in DMSO is a precondition for explaining the 2+2 photodimerization. The productivity for the 2+2 cycloaddition product was achieved as 55.6%.