Real-time observation of dynamic coupling between the stretching and bending modes in a polythiophene

By real-time vibrational spectroscopy using 6.3 fs pulses, both instantaneous frequency and amplitude modulated with frequency of 120–130 cm−1 were observed for the C–C stretching mode with the central frequency of 1343 cm−1 in a poly-(substituted thiophene). This provided the evidence that a stretching mode dynamically couples with another stretching mode mediated by a low-frequency bending mode. Instantaneous bond-order changed during the bending motion due to the change in the distribution of π-electrons. The coupling between the stretching and bending modes was found to be maintained for longer than 1200 fs after the start of the coherent molecular vibration.