Synthesis and optical properties of phenylene vinylene derivatives with cyano groups

Three different phenylene vinylene derivatives: compounds 1-3, containing one or two -CN groups, were synthesized. The nano-sized (1 and 2) and micro-sized (3) particle suspensions of the compounds were formed via H-aggregation when water was added to their solutions in tetrahydrofuran. The absorption and fluorescence emission spectra of the suspensions were blue- and red-shifted, respectively, compared to those of the respective solutions in THF. The fluorescence intensities observed from the suspensions of compounds 1 and 2 were much greater than those of their respective solutions, but compound 3 exhibited the opposite behavior. According to their energy-minimized molecular conformations, excimer formations of compounds 1 and 2 that are twisted in the isolated state may be less favorable than that of compound 3 that is flat, upon aggregations in the suspensions. Thus, the enhanced fluorescence emission of the suspensions from compounds 1 and 2 was attributed to both their tendencies of intramolecular planarization and less favorable excimer formation in the nanoparticles due to their twisted conformations, while the decreased emission in the suspension of compound 3 was attributed to its favorable excimer formation.