Cyclic azacyanines: Experimental and computational studies on spectroscopic properties and unique reactivity

The absorption and fluorescence properties of cyclic azacyanine (CAC) derivatives were examined in several solvents. The presence of electron donating or withdrawing groups on the CAC impacts spectroscopic properties. The general solvent relaxation displayed by azacyanine derivatives is in accordance with Lippert-Mataga's prediction but exception is noted in the case of protic solvent due to specific hydrogen bonding interactions. Fluorescence lifetime decay studies indicate a relaxation time in the nanosecond timescale with mono exponential decay. Donating substituents markedly increase the excited state lifetime, whereas withdrawing groups marginally decrease the excited state lifetime. Quantum chemical computations were used to explore the origins of the reactivity and spectroscopic properties of CACs; results are consistent with a model in which regioselectivity results from differences in mechanistic steps occurring after initial attack by hydroxide on the CAC. [Figure not available: see fulltext.]