We observe the instantaneous frequency and amplitude modulations of the C-C stretching mode with a central frequency of 1343 cm-1 in a poly (substituted thiophene) using 6.3 fs laser pulses by the method of real-time vibrational spectroscopy. Spectrogram analysis shows that both amplitude and instantaneous frequency of the mode with a central frequency of about 1343 cm-1 are modulated. Both modulation frequencies are found to be 120-130 cm-1, which provides evidence of dynamical coupling between a stretching mode and another stretching mode mediated by a low-frequency bending mode. © 2011 Chinese Optics Letters.