Coherent wavepacket motions of 5,12-bis(phenylethynyl)-naphthacene (BPEN) in an inert solvent, 1-chloronaphthalene (1-CN), and in an electron donating solvent, N,N-dimethylaniline (DMA) were compared by femtosecond white-light supercontinuum transient absorption (WC-TA) spectroscopy. In the inert solvent, 1-CN, both the intensity and the wavelength of the stimulated emission (SE) was modulated by the wavepacket motion, i.e., the maximum of the SE spectrum exhibited a repetitive movement between longer and shorter wavelengths (frequency-modulation) and the SE intensity was stronger at shorter wavelengths (amplitude-modulation). While in the electron donating solvent, DMA, the intensity was quickly diminished and only the frequency-modulation remained. The ultrafast electron transfer (ET) occurring in a few picosecond range rapidly diminishes the population in the vicinity of the Franck-Condon region and the wavepacket motion was no longer capable of recovering the SE intensity.

Femtosecond degenerate four-wave-mixing (DFWM) spectroscopy was carried out to investigate the behavior of coherent wavepacket motion in an ultrafast intermolecular electron transfer (ET) system which consists of a dye molecule, oxazine 1 (Ox1), in an electron donating solvent, N,N-dimethylaniline (DMA). Due to the ultrafast ET in DMA with time constant of ca. 59-81 fs, acceleration of the vibrational dephasing for the excited state mode at 562 cm-1 were observed by the DFWM measurement and confirmed by pump-probe (PP) spectroscopy. Interestingly, the dephasing time of the excited state mode in DMA is in the order of 160-240 fs which is significantly longer than the time constant of ET which indicates that the oscillation is not diminished instantaneously by the ET but somewhat persists into the product state.

Coherent nuclear wavepacket motions were monitored by three types of femtosecond time-resolved spectroscopy, namely, transient absorption measurement utilizing white-light supercontinuum (WC-TA), degenerate four-wave-mixing (DFWM), and pump-probe (PP) measurements, for an ultrafast intermolecular electron transfer (ET) system with a dye molecule, oxazine 1 (Ox1), dissolved in an electron donating solvent, N,N-dimethylaniline (DMA). Vibrational frequencies of the wavepacket motion in the excited and in the ground states were 560-562 and 567-569 cm-1 , respectively, with only a few frequency difference of 5-9 cm-1 , which were clearly distinguishable by the highly accurate measurements. In DMA, the excited state wavepacket motion declined with time constant of 160-240 fs which is somewhat longer than that of the ultrafast ET; 60-80 fs.

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