Dual-function artificial molecular motors performing rotation and photoluminescence

Abstract

Molecular machines have caused one of the greatest paradigm shifts in chemistry, and by powering artificial mechanical molecular systems and enabling autonomous motion, they are expected to be at the heart of exciting new technologies. One of the biggest challenges that still needs to be addressed is designing the involved molecules to combine different orthogonally controllable functions. Here, we present a prototype of artificial molecular motors exhibiting the dual function of rotary motion and photoluminescence. Both properties are controlled by light of different wavelengths or by exploiting motors' outstanding two-photon absorption properties using low-intensity near-infrared light. This provides a noninvasive way to both locate and operate these motors in situ, essential for the application of molecular machines in complex (bio)environments.

Fig. 1.. Concept and compounds used in this study.

(A) Scheme illustrating rotation and PL from a higher-lying excited-state isomer of a second-generation molecular motor. (B) Conceptual potential energy diagram illustrating the mode of operation of the molecular motors described in this study capable of performing rotation and undergoing relaxation via PL. (C) Compounds used in this study.

Fig. 2.. Spectroscopic behavior related to rotation and PL of motors 1 to 3.

(A) Absorption spectra of 1_s (red), 2_s (green), and 3_s (violet) and the mixtures of 1_s:1_m (dark red), 2_s:2_m (dark green), and 3_s:3_m (dark violet) at the PSS after irradiating the samples with LEDs of an appropriate wavelength (1_s: 455-nm LED; 2_s: 420-nm LED; 3_s: 470-nm LED) for 3 min. The insets show ODs, over 10 consecutive cycles of irradiation with identical irradiation time and light intensity, followed by relaxation of the metastable to the according stable isomers in the dark via THI. The shaded contours display steady-state PL spectra of 1_s, 2_s, and 3_s under 400-nm excitation. (B) ΔOD spectra of 1 (red), 2 (green), and 3 (violet) after two-photon irradiation at 800 nm for 15 min. The peak irradiation intensity was set to 1.4 GW cm⁻² for all compounds. The inset shows the normalized ΔOD at 500 nm of 1 over eight consecutive cycles with identical irradiation time at a light intensity of 4.5 GW cm⁻². Before starting a new irradiation cycle, the samples were kept in the dark for 40 to 60 min to ensure complete recovery of the stable isomer. The molar concentration of all compounds was set identical to 1.0 × 10⁻⁵ and 1.0 × 10⁻⁶ M for the measurements of the absorption and PL spectra, respectively, with MeCN as the solvent. arb. u., arbitrary units.

Fig. 3.. Time-resolved PL properties of motor 1.

(A to C) Time-resolved PL maps of 1_s under 1PE at 445 nm (A) and 400 nm (B), as well as 2PE at 800 nm (C). The white lines show the mean wavelengths calculated as ∫λS(λ, t)dλ/ ∫ S(λ, t)dλ of spectral slices S(λ, t) at a particular time t of the PL maps (the region of 10% of maximum PL intensity was selected as at lower intensities the mean wavelength becomes too noisy). The mean wavelengths (energies) and PL maps with a longer time scale as well as those for motors 2 and 3 are available in section S9. (D) PL transients (dots) of 1_s integrated over different wavelength regions under 400 nm (blue) and 445 nm (green) 1PEs as well as 800 nm 2PE (red). The solid lines show the best fits to a single-exponential function convoluted to the apparatus function (black). The molar concentration of 1_s was set to 1.0 × 10⁻⁵ M with MeCN as the solvent. The peak intensities were ~0.5 MW cm⁻² for 400- and 445-nm excitations and ~1 GW cm⁻² for 800-nm excitation.