SOLVATOCHROMISM OF FLUOROPHORES WITH AN INTRAMOLECULAR HYDROGEN BOND AND THEIR USE AS PROBES IN BIOMOLECULAR CAVITY SITES

The solvatochromism of nine fluorophores which possess a five- or six-membered-ring intramolecular hydrogen bond is investigated. Upon variation of the polarity/polarizability, acidity and basicity of the solvent used, the observed fluorescence frequency shifts of the molecules studied are shown to correlate linearly with the empirical solvent-scale solvent polarity and polarizability (SPP); additionally, solvent basicity (SB) and solvent acidity (SA) components are found to be needed in appropriate cases. Density functional calculations are carried out for the ground electronic state and configuration interaction calculations for the first excited electronic state, yielding dipole moments and electronic state energies. In those fluorophores in which polarity emerges as the dominant origin of their solvatochromism, an excellent correlation is found with the ground and first excited-state dipole moments. The understanding achieved is especially relevant for fluorophores with an intramolecular hydrogen bond, which have been proposed lately as polarity probes in biochemical environments. According to the direction of the solvatochromism found (i.e., fluorescence shifts to the red, to the blue, or no significant shifts) with increasing the polarity of the medium, three classes of fluorophores are defined and interpreted with the support of theoretically evaluated state dipole moments. Criteria are developed for the structural and photophysical properties that a fluorophore based on proton-transfer-tautomer fluorescence must display to be an outstanding polarity probe. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 421–438, 1999