Vibrational wave-packet control in cyanine dye molecules with free and restricted conjugated backbones

Quantum control of photochemical processes such as photodissociation or photoisomerization can be realized by controlling wave-packet dynamics towards the reaction coordinate on the excited-state potential surface. We already demonstrated "real-time wave-packet engineering" by combining a highly sensitive wave-packet spectrometer, which enabled us to measure the time-resolved difference transmission spectra at 20 frame-per-second, with a computer-controlled pulse shaper. We were successful in selectively exciting mode-specific motion of a wave-packet related to either the twisting or bending mode individually. In the present paper, we demonstrate wave-packet control in three different thiacarbocyanine dyes, DTTCI, DNTTCI, and SMP-39. They are very similar to each other, but the latter two have a six-membered ring as a bridge in the molecular backbone. DTTCI and SMP-39 are symmetric, whereas DNTTCI is asymmetric. Therefore, the twisting and bending motions are likely restricted and it is interesting to compare the wave-packet generation and control among these molecules. The excitation pulse from a mode-locked Ti:sapphire oscillator was just resonant with the Franck-Condon transition of the three molecules. The excitation pulses were modulated with a sum of quadratic and sinusoidal phase shifts by the pulse-shaper, resulting in arbitrary "chirped pulse sequence" with adjustable second-order dispersion and sub-pulse interval. Oscillatory structure associated with the wave-packet motion was clearly observed, which correspond to around 5-THz and 15-THz vibrational mode frequencies. The lower and higher modes are strongly related to the twisting and bending motions of cyanine molecules, respectively, as suggested by the calculation. Due to the restriction in the backbones, DNTTCI and SMP-39 showed mode splitting at the higher and lower modes, respectively. The chirp dependence seemed to be similar between these split modes. The twisting or bending motion was selectively - excited by tailored pulses. The optimized pulse for the twisting mode was a single positively chirped pulses, while that for the bending mode was a pulse sequence of negatively chirped pulses. The present results suggest that excitation of mode-specific wave-packets using shaped pulses should be a general property on the twisting and bending motions in cyanine dye molecules. In conclusion, selective excitation of either twisting or bending motion was successful by means of phase controlled femtosecond pulses in cyanine dye molecules with both free and restricted backbones.