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 pi 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 pi pi* states (proximity effect) which can be traced back to a smaller size of the pi 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.