Proton Transfer in 3-Hydroxyflavone Studied by High-Resolution 10 K Laser-Excited Shpol'skii Spectroscopy

Abstract

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.

On the intramolecular proton transfer of 3-hydroxyflavone in the first singlet excited state: A theoretical study

“…This compound shows a dual fluorescence emission. It has been widely proved that the normal form of the excited state N* and its tautomeric form T* are responsible for the blue and green fluorescence bands, respectively (see Scheme 1) [1][2][3][4][5][6][7][8][9]. As the exact wavelength of the fluorescence bands and the rate of the ESIPT are strongly dependent on the interaction with the environment, 3HF (and related systems) are of special interest as they are among the best candidates to be used as fluorescent probes in biological media and as starting materials for designing other molecular probes [10][11][12].…”

“…The adiabatic 0-0 band is located at 365.75 nm [9]. The fluorescence spectrum is characterized by a highly Stokesshifted broad band (485-625 nm) with a maximum at 500-530 nm depending on the solvent [6,8,9,[15][16][17][18]. The blue emission of the N* form cannot be observed in aprotic solvents.…”

“…The reported values for the time constants are 39 and 93 fs [8,9]. The ultrafast nature of the ESIPT reaction does not necessarily mean that the reaction is barrierless.…”

Proton Transfer Reactions in the Excited Electronic State

“…Experiments in various solvents and in jets [10,84,85] showed clearly that the ESIPT reaction in 3HF is extremely fast (high k þ ) and may proceed in the femtosecond time range ($40-100 fs) even in cryogenic conditions and supercritical jets.…”

Fotofísica e propriedades dinâmicas de sistemas moleculares

“…Em particular, as propriedades de fluorescência destes compostos tem sido objetivo de muitos trabalhos experimentais [227][228][229][230][231][232][233][234][235][236][237][238][239][240] , incrementando-se assim, o interesse no estudo teórico de estados excitados destes compostos [241][242][243] . Na literatura são reportadas varias características da fluorescência dos flavonóides.…”