Photophysics of Arylene and Heteroaryleneethinylenes

The absorption spectra and the stationary and time-resolved emissions of a series of the title compounds are investigated. Various combinations of ethinylene, p-phenylene, biphenyl-4,4‘-diyl, fluorenyl-2,7-diyl, 3- or 4-pyridyl, phenanthroline-3,8-diyl, 2,2‘- bipyridine-5,5‘-diyl, and 2,2‘- bipyridine-4,4‘-diyl molecular units allow a systematic structure variation, e.g., size of π system, type of aza-substitution, linear or angular chains. Most compounds are highly fluorescent. Radiationless deactivation via internal conversion and to a lesser extend intersystem crossing become efficient if forbidden states exist close to the strongly allowed ππ* states (proximity effect) which can be traced back to a smaller size of the π system or reduced conjugation due to meta-linkages of heteroaromatic rings. Aza-substitution may change the deactivation behavior but it does insignificantly influence the absorption and fluorescence spectra. Replacing phenanthrene for 2,2‘-bipyridine in the larger compounds does not alter the spectroscopic and the deactivation behaviors.