Photoisomerization around a Fulvene Double Bond: Coherent Population Transfer to the Electronic Ground State?

Abstract

Photoisomerization around a central fulvene-type double bond is known to proceed through a conical intersection at the perpendicular geometry. The process is studied with an indenylidene–dihydropyridine model compound, allowing the use of visible excitation pulses. Transient absorption shows that 1) stimulated emission shifts to the red and loses oscillator strength on a 50 fs timescale, and 2) bleach recovery is highly nonexponential and not affected by solvent viscosity or methyl substitution at the dihydropyridine ring. Quantum-chemical calculations are used to explain point 1 as a result of initial elongation of the central CC bond with mixing of S₂ and S₁ states. From point 2 it is concluded that internal conversion of S₁→S₀ does not require torsional motion to the fully perpendicular state. The S₁ population appears to encounter a sink on the torsional coordinate before the conical intersection is reached. Rate equations cannot model the observed ground-state recovery adequately. Instead the dynamics are best described with a strongly damped oscillatory contribution, which could indicate coherent S₁–S₀ population transfer.