Femtosecond electronic relaxation and real-time vibrational dynamics in 2 '-hydroxychalcone

Abstract

Femtosecond ultrafast electronic relaxation and vibrational dynamics in 2'-hydroxychalcone after deep ultraviolet (DUV) excitation were observed by two types of pump-probe spectroscopy experiments, i. e., DUV-pump pulse and visible-broadband-probe pulse (DUV/Vis) experiments and DUV-pump and DUV-probe (DUV/DUV) pulse experiments. Time-dependent density functional theory (TDDFT) calculations were performed to elucidate relaxation dynamics from the third singlet electronic excited state S-3. The DUV/Vis experiments and TDDFT calculations have disclosed the ultrafast dynamics of internal conversion from the initial S-3 state (tau(1) approximate to 35 fs) to the S-1 state via a rapid process through the S-3/S-2 conical intersection and proton transfer [OH: tau(2)(H) approximate to 93 and OD: tau(2)(D) approximate to 164 fs] before deactivation through the S-1/S-0 conical intersection (tau(3) approximate to 690 fs). Thanks to the ultrashort pump and probe pulses, real-time observation of vibrational modes coupled to the electronic excitation was realized providing both amplitudes and phases. Spectrogram analyses were performed based on the real-time spectra obtained by the DUV/Vis experiments, in which instantaneous vibrational frequencies reflecting molecular structural change after the impulsive excitation were visualized. The vibrational frequency of central C=C bond stretch decreases from similar to 1600 cm(-1) to similar to 1560 cm(-1) in about 200-500 fs and recovers in similar to 550 fs. Normal mode analyses along the decay path support the observed variation of the C=C stretching frequency. The temporal weakening of the central C=C bond is connected with the angle of the two aromatic rings which flip back to the initial conformation.