4-Dimethylaminochalcone as fluorescent probe: effect of the medium polarity on relaxation processes in the excited state

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 S₁ 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 maximum 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 S₁ 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 solvents, indicating a triplet excited state, while no phosphorescence is revealed in polar solvents.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1607–1610, August, 2004.