Fluorescence of 8-Acyl-1-Pyrrolidinylnaphthalenes

Four 8-acyl-1-pyrrolidinylnaphalenes are prepared where the acyl group is pivaloyl (6), benzoyl (7), benzyloxycarbonyl (8), and ethyloxycarbonyl (9). Crystal structures for 6–8 show that both the carbonyl and pyrrolidinyl groups are nearly perpendicular to the naphthalene ring. Esters 8 and 9 fluoresce more strongly than ketones 6 and 7. All show some solvatofluoro-chromic emission from a charge-transfer excited state. Calculations suggest that both the acyl and amino groups twist back toward planarity with the naphthalene in the relaxed first singlet excited state. With 8 and 9, co-planarity is within 20°, while with 6 and 7, the carbonyl approaches no closer than 30°. With 6 and 7, the charge-transfer emission is replaced with a shorter wavelength band with more polar solvents. Despite the twisted geometries and steric interference toward planarization, these systems do not show emission from a twisted intramolecular charge-transfer (TICT) state.