The present study theoretically investigated structures and dynamics of 5-dibenzosuberene (DBCH), 5-dibenzosuberenol (DBCH-5-ol), and 5-dibenzosuberenone (DBCH-5-one) in the excited states, in order to clarify specific behavior of transient species of DBCH-5-one in picosecond time region. The theoretical results confirm that the transient species observed in picosecond time-resolved absorption spectra of DBCH and DBCH-5-ol correspond to the S1 state and those in nanosecond ones to the T1 state. On the other hand, both picosecond and nanosecond transient species of DBCH-5-one are shown to be the T1 state, which means that the lifetime of S1 transient species is shorter than the picosecond order. Furthermore, the short lifetime of the S1 transition species is found to be due to large transition probability of S1→T1 intersystem crossing. Since the S1 state has n-π* character and the T1 state π-π* one, the spin-orbit coupling and, therefore, the transition probability between the S1 and T1 states are large, which corresponds to El-Sayed rule.
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