Coupled Rocking Motion in a Light-Driven Rotary Molecular Motor

Abstract

Coupled motion is ubiquitous in Nature as it forms the base for the direction, amplification, propagation, and synchronization of movement. Herein, we present experimental proof for the coupling of the rocking motion of a dihydroanthracene stator moiety with the light-induced rotational movement of an overcrowded alkene-based molecular motor. The motor was desymmetrized, introducing two different alkyl substituents to the stator part of the molecular scaffold, resulting in the formation of two diastereomers with opposite axial chirality. The structure of the two isomers is determined with nuclear Overhauser effect spectroscopy NMR and single-crystal X-ray analysis. The desymmetrization enables the study of the coupled motion, that is, rotation and oscillation, by ¹H NMR, findings that are further supported by density functional theory calculations. A new handle to regulate the rotational speed of the motor through functionalization in the bottom half was also introduced, as the thermal barrier for thermal helix inversion is found to be largely dependent on the alkyl substituents and its orientation toward the upper half of the motor scaffold. In addition to the commonly observed successive photochemical and thermal steps driving the rotation of the motor, we find that the motor undergoes photochemically driven rotation in three of the four steps of the rotation cycle. Hence, this result extends the scope of molecular motors capable of photon-only rotary behavior.

Figure 1

(a) Conformations of DHA; (b) rotational cycle of motor 1; and (c) desymmetrization of motor 1 to afford motor 2, with the DFT-simulated structure of (S)-(P)-(R_a)-2_S [B3LYP/6-31G(d,p)].

Figure 2

(a) Synthetic procedure for motor 2; (b) relative configuration assignment of (R_a)-2_S by ¹H NOESY NMR; and (c) ORTEP image (ellipsoid probability at 50%) of the X-ray crystal structure of (R_a)-2_S. Protons are omitted for clarity.

Figure 3

(a) Structure of 2 with indication of hydrogen atoms followed in ¹H NMR; (b) schematic representation of the rotational cycle with indication of the color coding used for the ¹H NMR spectra; and (c) selected region of the ¹H NMR spectrum of (S_a)-2_S in tetrachloroethane-d₂ at −35 °C (top). (S_a)-2_M forms upon irradiation at 365 nm, and a PSS of 68:32 ((S_a)-2_M: (S_a)-2_S) establishes after 1 h of irradiation (middle) and after THI after keeping in the dark at room temperature for 24 h and formation of (R_a)-2_S (bottom). (d) Selected region of the ¹H NMR spectrum of (R_a)-2_S in tetrachloroethane-d₂ at −35 °C (top), after irradiation at 365 nm for 30 min inducing formation of (R_a)-2_M (middle), and after THI during keeping in the dark at 75 °C overnight, forming (S_a)-2_S (bottom). Hydrogen atoms are assigned in Figure 3a.

Figure 4

(a) Photokinetic profile of the irradiation of (R_a)-2_S at 365 nm in tetrachloroethane-d₂ at −35 °C observed by ¹H NMR spectroscopy. Intensities are given relative to the normalized integration of the NMR signal. (b) Representative selected regions of ¹H- NMR spectra recorded during the prolonged irradiation. Time stamps are indicated in the photokinetic profile on top.

Figure 5

(a) UV–vis spectra of (R_a)-2 in dichloroethane (10^–5m) initially (purple), after irradiation at 365 nm for 12 min at 10 °C (orange), and after keeping in the dark for 50 min at 10 °C (green); (b) UV–vis spectra of (S_a)-2 in dichloroethane (10^–5m) initially (purple), after irradiation at 365 nm for 2 min at 10 °C (orange), and after keeping in the dark for 25 min at 20 °C (green); and (c) CD spectra of the first eluted fraction (F1) of (S_a)-2_S and the second eluted fraction (F2) of (S_a)-2_S in dichloroethane at room temperature, after irradiation at 365 nm, and after THI after keeping in the dark at room temperature.

Figure 6

Thermal conversion from metastable (S_a)-2_M to stable (R_a)-2_S via pathways A (top, purple) and B (bottom, orange). For all intermediates, a schematic top view of the molecule along the central double bond axis of the motor is shown. For all transition states, the energy barriers are quoted in kJ mol^–1. The rate-determining ring flip in the upper half is indicated with rds.

Figure 7

Proposed mechanism for the rotational cycle of 2 with a coupled rocking motion.