Phototriggered Complex Motion by Programmable Construction of Light-Driven Molecular Motors in Liquid Crystal Networks

Abstract

Recent developments in artificial molecular machines have enabled precisely controlled molecular motion, which allows several distinct mechanical operations at the nanoscale. However, harnessing and amplifying molecular motion along multiple length scales to induce macroscopic motion are still major challenges and comprise an important next step toward future actuators and soft robotics. The key to addressing this challenge relies on effective integration of synthetic molecular machines in a hierarchically aligned structure so numerous individual molecular motions can be collected in a cooperative way and amplified to higher length scales and eventually lead to macroscopic motion. Here, we report the complex motion of liquid crystal networks embedded with molecular motors triggered by single-wavelength illumination. By design, both racemic and enantiomerically pure molecular motors are programmably integrated into liquid crystal networks with a defined orientation. The motors have multiple functions acting as cross-linkers, actuators, and chiral dopants inside the network. The collective rotary motion of motors resulted in multiple types of motion of the polymeric film, including bending, wavy motion, fast unidirectional movement on surfaces, and synchronized helical motion with different handedness, paving the way for the future design of responsive materials with enhanced complex functions.

Figure 1

Representation of programmable construction of polymer films, molecular motors in liquid crystal networks by photolithography. Light-driven rotary motors are organized in liquid crystal networks with the designed alignment. The polymeric liquid crystal films containing racemic motors are able to perform fast wavy motion and can move on surfaces, while films with enantiomerically pure motors show complex helical motion upon being exposed to UV-light irradiation.

Figure 2

Light-driven rotation of molecular motor M1. (A) Rotary cycle of M1 (only one enantiomer is shown here). Steps 1 and 3: photoisomerization; steps 2 and 4: thermal helix inversion. (B) Partial ¹H NMR of E-M1 (CD₂Cl₂, −40 °C): (red) stable-E-M1, before irradiation (λ ≥ 365 nm); (green) after irradiation; and (blue) after standing at room temperature in the dark for 2 h. (C) Partial ¹H NMR of Z-M1 (CD₂Cl₂, −40 °C): (red) stable-Z-M1, before irradiation (λ ≥ 365 nm); (green) after irradiation; and (blue) after standing at room temperature in the dark for 2 h.

Figure 3

(A) Chemical structures of M1, liquid crystal mixtures (RM 82, RM 105, RM 23), and the photoinitiator (IRG 819). (B) Two-step procedure for the preparation of the alignment layers. The black arrows indicate the polarization direction of the UV light. Before the second exposure step, the sample is rotated at 90°. (C) Phototriggered wavy motion of the polymeric liquid crystal film. (D) Phototriggered translational movement of a piece of the polymeric LC film on a rough surface. The UV-light intensity is 100 mw/cm².

Figure 4

(A) Chemical structures of (R) and (S)-M1. (B) Phototriggered twisting of polymeric ribbons. When the ribbon contains the (S)-motor, the ribbon shows right-handed twisting, and vice versa. (C) Two-step procedure for the preparation of alignment layers. The black arrows indicate the polarization direction of the UV light. Before the second exposure step, the sample is rotated at 90°. (D) Phototriggered helical motion of the polymeric films with different shapes. The UV-light intensity is 230 mw/cm².

Figure 5

(A) Representative scheme for the preparation of the polymeric LC film with different handedness. (B) Preparation of the polymeric film with four stripes (a)–(d). Liquid crystal mixtures with the (R) or (S) motor (0.03 wt %) were filled alternatingly from the same side of a liquid crystal cell with the planar alignment at 80 °C. (C–F) Transmittance spectra of different stripes of the polymeric film. Stripes (a) and (c) showed 100% transmittance of the left-handed CPL, while stripes (b) and (d) showed 100% transmittance of the left-handed CPL. (G) Synchronized left–right- left-right–left-, right–left-right-, and left-right–left-right-handed motion in one single ribbon by single-wavelength irradiation. (H) Two-step procedure for the preparation of the alignment layers. The black arrows indicate the polarization direction of the UV light. (I) The resulting “V”-shaped film showed different helical motion upon light irradiation. The UV-light intensity is 230 mw/cm².