Control of Photoconversion Yield in Unidirectional Photomolecular Motors by Push–Pull Substituents

Abstract

Molecular motors based on the overcrowded alkene motif convert light energy into unidirectional mechanical motion through an excited state isomerization reaction. The realization of experimental control over conversion efficiency in these molecular motors is an important goal. Here, we combine the synthesis of a novel “push–pull” overcrowded alkene motor with photophysical characterization by steady state and ultrafast time-resolved electronic spectroscopy. We show that tuning of the charge transfer character in the excited state has a dramatic effect on the photoisomerization yield, enhancing it to near unity in nonpolar solvents while largely suppressing it in polar solvents. This behavior is explained through reference to solvent- and substituent-dependent potential energy surfaces and their effect on conical intersections to the ground state. These observations offer new routes to the fine control of motor efficiency and introduce additional degrees of freedom in the synthesis and exploitation of light-driven molecular motors.

Excited State Dynamics in Unidirectional Photochemical Molecular Motors

“…The exceptional behavior of 1 CNOMe is also apparent in the FlUC data, where the amplitude of the ultrafast to picosecond components (a measure of the FC state to dark state transition moment) is much larger than for any of the other derivatives, and the decay is wavelength independent. 70,71 Further the oscillation in the FlUC data, corresponding to low frequency Raman active modes, which are observed with a variable pattern for all of the symmetric derivatives, are essentially absent for 1 CNOMe , suggesting that its CT excited state configuration has substantially reorganized the excited state PES.…”

“…Ultimately the yield of the product isomer will depend on the location and topology of the CI, and both can be modified by substituent and solvent. 38,80,81 Means by which such substituent and solvent effects can modify the reaction coordinate are illustrated in Figure 12b, originally conceived for 1 and its derivatives 70 but likely applicable to other PMMs. The initial decay of the FC state is dominated by intramolecular forces, the steric repulsion in the excited state.…”

“…(b) Absorption (solid lines) and emission (dashed lines) as a function of solvent for 1 (red) or 1 CNOMe (black). Adapted from ref . Copyright 2023 American Chemical Society.…”

“…The same series was interrogated by FlUC, TA, and FSRS. , The TA data are shown for 1 CNCN in cyclohexane with the corresponding global analysis in both cyclohexane and methanol shown in Figure b,c. These are typical for the symmetrically substituted derivatives.…”

“…An observed solvent polarity effect suggested the imposition by substituents of CT configurations, which was realized. Such substituents both enhance the yield of the metastable product in nonpolar solvents and introduce solvent polarity control of PMM efficiency . Before summarizing PMM excited state dynamics, we will describe an extension of these ultrafast methods to the third generation of PMMs.…”