Relaxation processes in a 4 dimethylaminochalcone molecule after excitation with a light pulse of duration 70 fs were studied. During 0.4-1 ps after excitation, an absorbance of an excited state S 1 with a maximum at ~460 nm is formed in both polar and nonpolar media. Subsequent relaxation processes depend on the polarity of the medium. In nonpolar hexane, the 4 dimethylaminochalcone molecule transits to the triplet state having an absorption maxi mum at 570 nm (lifetime longer than 600 ps) for 20 ps. In polar aprotic acetonitrile, the absorbance at 460 nm decreases slowly (during hundreds of picoseconds), indicating that the molecules return to the ground state. The induced emission from the level S 1 in a region of 520-550 nm and fluorescence from the same level with a maximum at 537 nm are also observed in acetonitrile. Thus, a reason for a sharp decrease in the fluorescence yield on going from polar to nonpolar media was found. The mechanism of fluorescence quenching of 4 dimethylaminochalcone in nonpolar media is confirmed by the data on phosphorescence. The phosphorescence of 4 dimethylaminochalcone is observed at -196 °C in nonpolar sol vents, indicating a triplet excited state, while no phosphorescence is revealed in polar solvents.Key words: fluorescent probe, 4 dimethylaminochalcone, femtosecond dynamics, polarity of medium, triplet state.4 Dimethylaminochalcone (DMAC) as a fluorescent probe is used for studying biological membranes and lipo proteins. 1-6 It is poorly soluble in water and, hence, binds to lipids in the presence of mem branes or lipoproteins. 1 In polar solvents, the quantum yield of fluores cence can reach 0.26; however, the fluorescence of DMAC in water is almost completely quenched due to the forma tion of a hydrogen bond (other protic solvents quench DMAC flluorescence in the same manner). 3,4 When bind ing to proteins or lipids, a DMAC molecule is shielded from water, and the fluorescence yield can increase to 0.1 and higher. However, if the molecule has "drown" in the lipid depth and got into a nonpolar environment, the yield decreases to 0.01 and lower. Thus, it seems clearly why polar protic solvents quench fluorescence, while rea sons for fluorescence quenching in nonpolar media are unknown. In this work, we attempted to elucidate this problem.
Experimental4 Dimethylaminochalcone was synthesized and kindly pre sented by B. M. Krasovitskii (Institute for Single Crystals, Na tional Academy of Sciences, Kharkov, Ukraine). 7 Acetonitrile, 95% EtOH, methylcyclohexane, and hexane (Aldrich) were used.The femtosecond dynamics of transient absorption spectra of DMAC was studied on an experimental system based on a femtosecond CPM dye laser designed at the N. N. Semenov Institute of Chemical Physics of the Russian Academy of Sci ences. 8 The "pump-probe" spectroscopy technique was used. The second harmonic from the main sequence of pulses gener ated by the system was used as pump pulses. The bearing wave length of a pump pulse was 308 nm, and the duration was 70 fs. Probe pulses were carr...
