Photophysical and dynamic NMR studies on 4-amino-7-nitrobenz-2-oxa-1,3-diazole derivatives: Elucidation of the nonradiative deactivation pathway

A series of 4-aminonitrobenzoxadiazole (NBD) derivatives in which the amino nitrogen is part of a four- to seven-membered heterocyclic ring and a second series of 4-dialkylamino NBD derivatives with different alkyl chain lengths have been prepared and fully characterized and their photophysical properties investigated in an attempt to find out the nonradiative deactivation pathway of the fluorescent state in these systems. It is observed that the fluorescence properties of the NBD derivatives are highly sensitive to the structure of the amino moiety and the polarity of the surrounding media. It is shown that the nonradiative rate constants for the derivatives with large alkyl groups and large ring systems are considerably higher than those for systems with smaller alkyl groups and smaller ring systems. Dynamic NMR experiments have been carried out to probe the internal motion in the systems. The internal rotation around the bond connecting the amino nitrogen and the NBD ring is found to be rather slow in the ground state. The rate for the internal rotation is found to be the highest for the six-membered ring, and this has been interpreted in terms of the partial double bond character of the C-N bond. The results of the experiments seem to indicate that inversion of the amino nitrogen plays the most important role in determining the fluorescence efficiency of the systems.