Relaxation pathways of photoexcited non-steroidal anti-inflammatory drugs: flufenamic and mefenamic acids

The photophysical behavior of flufenamic (FLF) and mefenamic (MEF) acids as a function of solvent is investigated by using steady-state and time-resolved measurements. The results suggest that there is a deactivation pathway that involves a change in molecular conformation and the nature of which is strongly dependent on the solvent polarity. In non-polar solvent the deactivation of the excited singlet states proceeds via excited state intramolecular proton transfer (ESIPT). In polar aprotic solvents the results indicate that the new emission band that appears in the long wavelength region is assigned to a twisting intramolecular charge-transfer (TICT) state. In agreement with the experimental results, MO calculations suggest that the degree of twisting of substituted aniline moiety around C-N bond is higher for FLF than for MEF. Very efficient pathway for deactivation of the singlet excited states has been observed in polar protic solvents, indicating that the excited state deactivates partly via intermolecular hydrogen-bonding, which competes with the TICT