Excitation-dependent fluorescence of triphenylamine-substituted tridentate pyridyl ruthenium complexes

Two polypyridyl ruthenium complexes, bis[4-(N,N'-diphenylamino)phenyl-2,2':6',2''-terpyridine]ruthenium(II) (1) and bis[4'-(4-{2-[4-(N,N'-diphenylamino)phenyl]ethylene}phenyl)-2, 2':6',2''-terpyridine]ruthenium(II) (2), have been synthesized. They possess an extended conjugation and strongly coupled electronic states. The features of these compounds were carefully studied from several respects. Steady-state spectroscopy showed that the two compounds had strong excitation dependent emission behaviors caused by mixing features of different electronic states. Femtosecond fluorescence upconversion spectroscopy was also used to investigate the fluorescence dynamics of the compounds. An ultrafast relaxation time of approximately 100 fs of the (1)MLCT (metal-to-ligand charge-transfer) states, which may originate from an ultrafast intersystem crossing to form (3)MLCT states, was found in both samples. However, thermal populated states and vibration associated excited state interactions were suggested for 1 with excitation at wavelengths below 400 nm, whereas vibrational energy redistribution with a time scale of few picoseconds was observed in the extended conjugated system of 2. These compounds will have potential application in both artificial photosynthesis systems and photovoltaic devices.