1-Propionyl-5-dimethylaminonaphthalene (8, 1,5-Prodan) and two derivatives where the amino group is constrained in a seven-membered (9) and five-membered (10) ring are prepared. All three exhibit strong fluorescence and similar degrees of solvatochromism. Their fluorescence is strongly quenched in alcohol solvents. Because the amino group in 9 and especially 10 is forced to be coplanar with the naphthalene ring, the similar photophysical behavior of all three suggests that emission arises from a planar excited state (planar intramolecular charge transfer).
Chen, Tao; Lee, Samuel W.; and Abelt, Christopher J., 1,5-Prodan Emits from a Planar Intramolecular Charge-Transfer Excited State (2018). ACS OMEGA, 3(5), 4816-4823.