Coherent Nuclear Wave Packets in Q States by Ultrafast Internal Conversions in Free Base Tetraphenylporphyrin

Persistence of vibrational coherence in electronic transition has been noted especially in biochemical systems. Here, we report the dynamics between electronic excited states in free base tetraphenylporphyrin (H₂TPP) by time-resolved fluorescence with high time resolution. Following the photoexcitation of the B state, ultrafast internal conversion occurs to the Q_<i>x</i> state directly as well as via the Q_<i>y</i> state. Unique and distinct coherent nuclear wave packet motions in the Q_<i>x</i> and Q_<i>y</i> states are observed through the modulation of the fluorescence intensity in time. The instant, serial internal conversions from the B to the Q_<i>y</i> and Q_<i>x</i> states generate the coherent wave packets. Theory and experiment show that the observed vibrational modes involve the out-of-plane vibrations of the porphyrin ring that are strongly coupled to the internal conversion of H₂TPP