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 pi-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. (C) 2009 Elsevier B.V. All rights reserved.