Conformation-Dependent Photophysics of Bifunctional Hydrogen Bond Donor/Acceptor Molecules

A series of heteroazaaromatic molecules possessing both a hydrogen bond donor and acceptor groups linked by a single bond was studied using electronic absorption and fluorescence techniques, combined with quantum-chemical calculations. Fluorescence lifetimes and quantum yields strongly depend on the solvent. Double exponential decays are observed in alcohols. This was interpreted in terms of alcohol-induced syn-anti rotamerization in the ground electronic state. The driving force for the conformational change is provided by two factors: (a) solvent polarity increase; (b) the formation of two separate hydrogen bonds in the anti species. In the syn structure, formation of cyclic hydrogen bonds leads to rapid depopulation of the lowest excited singlet state via two competing mechanisms: proton transfer and enhanced internal conversion. The experimental results are rationalized by calculations which include specific and nonspecific solvent effects.