Toward Fast and Efficient Visible‐Light‐Driven Molecular Motors: A Minimal Design

Abstract

A key goal in the development of light‐driven rotary molecular motors is to facilitate their usage in biology and medicine by shifting the required irradiation wavelengths from the UV regime to the nondestructive visible regime. Although some progress has been made toward this goal, most available visible‐light‐driven motors either have relatively low quantum yields or require that thermal steps follow the photoisomerizations that underlie the rotary motion. Here, a minimal design for visible‐light‐driven motors without these drawbacks is presented and evaluated on the basis of state‐of‐the‐art quantum chemical calculations and molecular dynamics simulations. The design, featuring dihydropyridinium and cyclohexenylidene motifs and comprising only five conjugated double bonds, is found to produce a full 360° rotation through fast photoisomerizations (excited‐state lifetimes of ≈170–250 fs) powered by photons with energies well below 3 eV.

Molecular motors with high quantum efficiency and visible-light responsiveness: Meeting two challenges in one design

“…However, our study [47] did not consider sufficiently many NAMD trajectories that a quantitative assessment could be made as to whether the photoisomerization QYs of 0 are high or even comparable to those previously documented for a much smaller UV-powered Schiff-base motor [38]. Indeed, performing a statistically meaningful number of CASSCF-NAMD simulations is a very demanding task when five conjugated double bonds are present.…”

“…First, despite containing only five conjugated double bonds, 0 can be powered by photons with energies below 3 eV, i.e., well within the visible regime [47]. Second, 0 shares many favorable features previously attributed to UV-powered Schiff-base motors, such as undergoing ultrafast E ® Z and Z ® E photoisomerizations [36,38] and producing 360° unidirectional rotary motion from these processes without intermediary thermal steps [38].…”

“…In our own recent computational work, we have put forth a minimal design of a visiblelight-driven molecular motor, shown as motor 0 in Fig. 1 [47]. Specifically, subjecting this design -a Schiff-base motor comprising a dihydropyridinium motif connected by an olefinic bond to a vinyl-substituted cyclohexenylidene motif -to quantum chemical calculations and non-adiabatic molecular dynamics (NAMD) simulations [48][49][50][51] in the framework of complete active space self-consistent field (CASSCF) theory [52][53][54], two key observations were made.…”

“…Since the purpose of the work is to test the photochemical efficiency of any suitable visible-light-driven Schiff-base motor, rather than to deepen the previous study of 0 [47], a somewhat smaller (than 0) motor of this type was considered, for which the required simulations are more tractable. This motor, hereafter denoted motor 1 and whose ability to be powered by visible light is confirmed in Section 3.1, is pictured in Fig.…”

“…As can be seen, 1 also contains five conjugated double bonds but features a point-chiral cyclopentenylidene motif instead of an axialchiral cyclohexenylidene motif, which may alter the photochemical behavior. Furthermore, 1 lacks the bulky Br substituents of 0, which merely serve to prevent facile isomerization between the axially chiral enantiomers of 0 [47].…”