On the UV spectroscopy and photodynamics of octatetraene

The UV absorption spectrum of all-<i>trans</i>-octatetraene is reinvestigated theoretically, focussing on the strongly dipole-allowed 1Ag–1Bu transition. The dynamical calculations rely on <i>ab initio</i> multistate CASPT2 (MS-CASPT2) computations of the underlying potential energy surfaces and coupling elements, and the multiconfiguration time-dependent Hartree (MCTDH) method for solving the time-dependent Schrödinger equation for the nuclear motion. The vibronic structure of the absorption band near 2700 Å is well reproduced, better than ever before by a purely <i>ab initio</i> approach. The C–C single bond and C=C double bond stretching modes dominate the vibrational structure of the transition. While the latter resembles nearly harmonic motion on weakly coupled potential energy surfaces, the time-dependent calculations reveal a subpicosecond electronic population transfer from the 1Bu state to the lower-lying 2Ag state proceeding on a time scale of ∼50fs. The details of this internal conversion process and its relation to the conical intersection between the two excited states are discussed, paying particular attention to the impact of the 1Bu –2Ag energy gap on the dynamics.