Reaction dynamics and coherent nuclear motions in the photodissociation of diphenylcyclopropenone (DPCP) were studied in solution by time-resolved absorption spectroscopy. Subpicosecond transient absorption spectra were measured in the visible region with excitation at the second absorption band of DPCP. The obtained spectra showed a new short-lived band around 480 nm immediately after photoexcitation, which is assignable to the initially populated S2 state of DPCP before the dissociation. The dissociation takes place from this excited state (the precursor of the reaction) with a time constant of 0.2 ps, and the excited state of diphenylacetylene (DPA) is generated as the reaction product. The transient absorption after the dissociation decayed with a time constant of 8 ps that is very close to the S2-state lifetime of DPA, but the spectrum of this 8-ps component was different from the S2 absorption observed with direct photoexcitation of DPA. We conclude that the dissociation of DPCP generates the S2 state of DPA that probably has a cis-bent structure. At later delay times (>30 ps), the transient absorption signals are very similar to those obtained by direct photoexcitation of DPA. This confirmed that the electronic relaxation from the S2 state of the product DPA occurs in a similar manner to that of DPA itself, i.e., the internal conversion to the S1 state and subsequent intersystem crossing to the T1 state. In order to examine the coherent nuclear dynamics in this dissociation reaction, we carried out time-resolved absorption measurements for the 480-nm band with 70 fs resolution. It was found that an underdamped oscillatory modulation with a 0.1-ps period is superposed on the decay of the precursor absorption. This indicates that DPCP exhibits a coherent nuclear motion having a ∼330-cm−1 frequency in the dissociative excited state. Based on a comparison with the measured and calculated Raman spectra of ground-state DPCP, we discuss the assignment of the “330-cm−1 vibration” and attribute it to a vibration involving the displacement of the CO group as well as the deformation of the Ph–C=C–Ph skeleton. We consider that this motion is closely related to the reaction coordinate of the photodissociation of DPCP.

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