EXCITED-STATE PROPERTIES AND DEACTIVATION PATHWAYS OF 7-AMINOCOUMARINS

Solvent polarity effects on the photophysical properties and spectral shifts of some 7-aminocoumarins were investigated in a combined experimental and theoretical study. Solvatochromic shifts were found in accordance with Onsager's continuum model and excited state dipole moments were estimated and compared to semiempirically calculated values. In solvents of a dielectric constant D<10 the fluorescence quantum yield is generally unity with the only exception of the primary 7-aminocoumarins C120 and C151 in alkane solution. Ground and/or excited state complexation with polar solvent molecules enhances fluorescence. The onset of a nonradiative decay pathway for flexible 7-aminocoumarins with an acceptor substituent on the alpha-pyrone ring at a solvent D approximate to 10 is in good agreement with a rotatory decay mechanism to a twisted intramolecular charge transfer (TICT) state. This internal quenching is temperature activated, the activation barrier and pre-exponential factor increasing with polarity. TICT formation is discussed on the basis of dielectric solvation effects on the energy-hypersurface describing the twist of the amino group.