A Photochemical Method to Evidence Directional Molecular Motions

Abstract

Light driven synthetic molecular motors represent crucial building blocks for advanced molecular machines and their applications. A standing challenge is the development of very fast molecular motors able to perform rotations with kHz, MHz or even faster frequencies. Central to this challenge is the direct experimental evidence of directionality because analytical methods able to follow very fast motions rarely deliver precise geometrical insights. Here, a general photochemical method for elucidation of directional motions is presented. In a macrocyclization approach the molecular motor rotations are restricted and forced to proceed in two separate ~180° rotation-photoequilibria. Therefore, all four possible photoinduced rotation steps (clockwise and counterclockwise directions) can be quantified. Comparison of the corresponding quantum yields to the unrestricted motor delivers direct evidence for unidirectionality. This method can be used for any ultrafast molecular motor even in cases where no high energy intermediates are present during the rotation cycle.

Directionality Reversal and Shift of Rotational Axis in a Hemithioindigo Macrocyclic Molecular Motor

“…60 The utility of macrocyclization was further employed by our group to evidence unidirectionality in ultra-fast molecular rotary motors in a photochemical way. 61 In larger macrocyclic setups it even becomes possible to control multiple types of motions, such as 360° motor rotation, 180° biaryl rotations, and structural reconfigurations, within the same molecular machine with very high precision. 62 However, in virtually all of these motor systems, a defined chemical bond (double or single bond) represents the actual rotational axis.…”