Light-triggered unidirectional molecular rotors: theoretical investigations on conformational dynamics and laser control

Abstract

Two light-triggered molecular motors based on chiral overcrowded alkenes have been studied in the electronic ground state: a second-generation motor (2) and a redesigned motor (3). A semiempirical Monte-Carlo-type of conformational search has been implemented to find local minima in the ground state PESs of 2 and 3, which then have been reoptimized by ab-initio calculations. While in 3 only the four isomers of the rotary cycle are found, new isomers have been found in the case of 2, leading to different reaction pathways for the thermal helix-inversion. TSs for all the possible thermal conversions have been also computed. The obtained E_a values are in excellent agreement with those reported in the literature. The simple model BCH (core unit of many motors) has been studied from a quantum chemical and quantum dynamical point of view. The controversial nature of BCH's electronic transitions has been investigated using high-level ab-initio multiconfigurational and perturbational methods, including the development of a basis set specific to the problem at hand. The first two excited states of Bu-symmetry ((pi,3s)-Rydberg and (pi,pi*), respectively) are resolved at the MS-CASPT2-level of theory, providing vertical transition energies and oscillator strengths matching the experimental values. In addition, the origin of the (p,p*)-band is computed, yielding an energy value well below the FC-value of the (pi,3s_R)-maximum, explaining this band's unexpected intensity. Finally, a one-dimensional PES along BCH's torsional coordinate has been computed at the MS-CASPT2-level of theory, and quantum dynamical simulations have been carried out. These have focused on the obtainment of control laser fields that are able to trigger unidirectionality even in the symmetric PES (as opposed to 2 and 3 system). Optimal control strategies as well as the intuitive IR+UV-scheme both succeeded in achieving sustained, unidirectional torsional motion of BCH in the excited state