Nature and characterization of the singlet and triplet excited states of the dibenzylideneacetone and di-3-(N-ethylcarbazoylidene)acetone bridging ligands

Abstract

The lowest energy singlet state in di-3-(N-ethylcarbazoylidene)acetone (dNECa), a luminescent model compound at room temperature for dibenzylideneacetone (dba), has been assigned to a charge transfer (CT) state using the medium polarity effect on λmax(F) and the fluorescence polarized spectra of dNECa at 77 K. The dNECa fluorescence quantum yields [Formula: see text] and lifetimes (τF) are solvent sensitive where both [Formula: see text] and τF tend to increase with polarity. The very weak and moderately structured phosphorences have been located for the first time for both dba and dNECa in the 550–800 nm range and are also assigned to a CT state. In this case the CT interactions are less pronounced than λ(0–0)P and τP are much less solvent sensitive. Finally, EHMO type calculations confirm that the CT transition is the lowest energy transition in dba and di-3-(N-methylindoylidene)acetone (a model compound for dNECa), but also suggest that the nπ* state must lie near the CT state in dba.