title: 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 C=C bond with mixing of S(2) and S(1) states. From point 2 it is concluded that internal conversion of S(1)→S(0) does not require torsional motion to the fully perpendicular state. The S(1) 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 S(1)-S(0) population transfer.
authors.affiliation: Department of Chemistry, Lomonosov Moscow State University, Russian Federation.
authors.forename: Ilya
authors.initials: I
authors.lastname: Ioffe
authors.forename: Alexander L
authors.initials: AL
authors.lastname: Dobryakov
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Granovsky
authors.forename: Nikolaus P
authors.initials: NP
authors.lastname: Ernsting
authors.forename: J Luis Pérez
authors.initials: JL
authors.lastname: Lustres