The transient absorption of 1,3,5-tri-tert-butyl-pentalene

The transient absorption of 1,3,5-tri-tert-butyl-pentalene in cyclohexane solution has been measured at room temperature exciting at 340 nm with femtosecond pulses. Two relaxation processes have been determined, the first, very rapid, with time constant 650 fs and the second, occurring on a picosecond timescale, with estimated decay time 13 ps. The former is due to the vibronic relaxation from upper S levels to the minimum, the 3 S3 latter to the decay from S to This interpretation is in good agreement with results of ab initio calculations 3 S0 . on pentalene and its alkyl derivatives, 1,3,5-tri-ethyl, 1,3,5-tri-sec-propyl and 1,3,5-tri-tert-butyl. MCSCF/CAS calculations, perturbatively corrected within the scheme of quasi-degenerate perturbation theory (QDPT), of vertical S0-Sn excitation energies allow us to study the dependence of excitation energies on substitution. Calculated S3-Sn and S4-Sn absorption spectra of pentalene at the S0, S3 and S4equilibrium geometries indicate that vibronic levels of S3 rather than S4 of are excited by the strong pump pulse with subsequent decay within this state.