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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.

Coherent wavepacket oscillation accompanying the ultrafast photoexcited intramolecular charge separation (CS) of 9,9′-bianthryl (BA) and 10-cyano-9,9′-bianthryl (CBA) in a room temperature ionic liquid, N,N-diethyl-N-methyl-N-(methoxyethyl)ammonium tetrafluoroborate (DemeBF4), was investigated by femtosecond time-resolved transient absorption spectroscopy. The frequency of the coherent oscillation observed for CBA in nonpolar n-hexane solution (Hex) was 377 cm−1, while this oscillation was undetectable in DemeBF4. For BA in DemeBF4, coherent oscillation with a frequency of 394 cm−1 was observed, which is similar to that for CBA in Hex. CS of CBA occurs in the ultrashort time range of ≤100 fs, while that of BA occurs in a few picosecond range [E. Takeuchi et al., J. Phys. Chem. C 120, 14502–14512 (2016)]. Hence, the oscillation of CBA in Hex and that of BA in DemeBF4 are assigned to the molecular vibration in the locally excited state, while this oscillation dephases instantaneously for CBA in DemeBF4 due to the ultrafast CS and no oscillation was generated in the CS state. This result suggests that the CS reaction is not mediated by a specific intramolecular vibration in the CS state but occurs incoherently through higher levels of multiple vibrational modes.

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.

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 was 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.

Femtosecond transient absorption spectroscopy revealed that initial ultrafast charge transfer process of asymmetric 10-cyano-9,9’-bianthryl in ionic liquid occurs in the sub-100 femtosecond domain which is much faster than that of symmetric 9,9'-bianthryl.