Proton transfer in 3-hydroxyflavone studied by high-resolution 10 K laser-excited Shpol'skii spectroscopy

High-resolution 10 K Shpol'skii spectra of 3-hydroxyflavone (3HF) and its deuterated analogue (3DF) in n-octane and n-octane/octanol mixtures are presented for the first time. In pure n-octane for both 3HF and 3DF, well-resolved excitation and emission spectra were observed, showing fluorescence shifted from 380-460 to 513-550 nm because of excited-state intramolecular proton/deuteron transfer (ESIPT/ESIDT). Compared to those of 3DF, the 3HF excitation and emission bands are much wider because of lifetime-limited homogeneous broadening. Proton transfer is at least a factor of 4 faster than deuteron transfer. From the homogeneous contribution to the total bandwidth of 3HF, the rate constants of ESIPT and ground-state back proton transfer were estimated to be 39 ± 10 and 210 ± 30 fs, respectively. The effect of four octanol additives was investigated. Only for 2-octanol and-though less favorable-3-octanol, a new site in the emission spectrum was observed, blue-shifted over 7 and 10 nm, respectively, versus the 3HF spectrum in n-octane. The new site is attributed to a 1:1 3HF/octanol complex. Its ground-state vibrational pattern differs from that of free 3HF. For 3DF, no Shpol'skii spectrum of a complex could be obtained. It is suggested that in the complex the proton/deuteron transfer mechanisms differ from those of the free molecules; furthermore, a molecular structure for the tautomeric form of the complex is proposed.