On the UV spectroscopy and photodynamics of octatetraene

Abstract

The UV absorption spectrum of all-trans-octatetraene is reinvestigated theoretically, focussing on the strongly dipole-allowed ${1\mathrm{A}}_{\mathrm{g}}$–${1\mathrm{B}}_{\mathrm{u}}$ transition. The dynamical calculations rely on ab initio 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 ab initio approach. The C–C single bond and $\mathrm{C}=\mathrm{C}$ 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 ${1\mathrm{B}}_{\mathrm{u}}$ state to the lower-lying ${2\mathrm{A}}_{\mathrm{g}}$ state proceeding on a time scale of $\sim 50\mathrm{fs}$. 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 ${1\mathrm{B}}_{\mathrm{u}}$ –${2\mathrm{A}}_{\mathrm{g}}$ energy gap on the dynamics.

GRAPHICAL ABSTRACT

Keywords:

• nuclear quantum dynamics
• potential energy surface
• CASPT2
• MS-CASPT2
• spectroscopy

Acknowledgments

This paper is dedicated to Prof. Peter G. Szalay on the occasion of his 60th birthday. We are indebted to Prof. Andreas Dreuw for valuable discussions and a careful reading of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Financial support by the Deutsche Forschungsgemeinschaft (Grant No. Ko945/23-1) is gratefully acknowledged.