jz502644h_si_002.mpg (1.24 MB)
Directionality of Double-Bond Photoisomerization Dynamics Induced by a Single Stereogenic Center
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posted on 2015-12-17, 07:21 authored by Gabriel Marchand, Julien Eng, Igor Schapiro, Alessio Valentini, Luis Manuel Frutos, Elisa Pieri, Massimo Olivucci, Jérémie Léonard, Etienne GindenspergerIn light-driven single-molecule rotary
motors, the photoisomerization
of a double bond converts light energy into the rotation of a moiety
(the rotor) with respect to another (the stator). However, at the
level of a molecular population, an effective rotary motion can only
be achieved if a large majority of the rotors rotate in the same,
specific direction. Here we present a quantitative investigation of
the directionality (clockwise vs counterclockwise) induced by a single
stereogenic center placed in allylic position with respect to the
reactive double bond of a model of the biomimetic indanylidene-pyrrolinium
framework. By computing ensembles of nonadiabatic trajectories at
300 K, we predict that the photoisomerization is >70% unidirectional
for the Z → E and E → Z conversions. Most importantly,
we show that such directionality, resulting from the asymmetry of
the excited state force field, can still be observed in the presence
of a small (ca. 2°) pretwist or helicity of the reactive double
bond. This questions the validity of the conjecture that a significant
double-bond pretwist (e.g., >10°) in the ground state equilibrium
structure of synthetic or natural rotary motors would be required
for unidirectional motion.