Photoinduced proton transfer in the excited singlet state of 3,7-dihydroxynaphthoic acid and solvent effect

The photophysics of 3,7-dihydroxynaphthoic acid (DHNA) has been studied in different aprotic and protic solvents by steady-state and nanosecond transient emission spectroscopy. Both the monomer and the dimer of DHNA have been detected in the ground as well as in excited state in dilute solution (∼10−5 mol dm−3). A large Stokes shifted emission (∼6500 cm−1) indicates that DHNA undergoes thermodynamically favourable proton transfer in the excited state. It is proposed that the added base promotes the proton transfer only in polar protic solvents. The mechanism of the static processes has been investigated by making steady-state fluorescence quenching measurements and that of the dynamic processes by nanosecond fluorescence lifetime techniques.