Controlling rotary motion of molecular motors based on oxindole

Advanced theoretical design of light-driven molecular rotary motors: enhancing thermal helix inversion and visible-light activation

In this study, we have advanced the field of light-driven molecular rotary motors (LDMRMs) by achieving two pivotal goals: lowering the thermal helix inversion (THI) barrier and extending the absorption wavelength into the visible spectrum. This study involves the structural reengineering of a second-generation visible LDMRM, resulting in the synthesis of a novel class, specifically, 2-((2S)-5-methoxy-2-methyl-2,3-dihydro-1H-cyclopenta[a]naphthalen-1-yl)-3-oxo-2,3-dihydro-1H-dibenzo[e,g]indole-6,9-dicarbonitrile. This redesigned motor stands out with its two photoisomerization stages and two thermal helix inversions, featuring exceptionally low THI barriers (4.00 and 2.05 kcal mol-1 at the OM2/MRCI level for the EM → EP and ZM → ZP processes, respectively). Moreover, it displays absorption wavelengths in the visible light range (482.98 and 465.76 nm for the EP and ZP isomers, respectively, at the TD-PBE0-D3/6-31G(d,p) level), surpassing its predecessors in efficiency, as indicated by the narrow HOMO-LUMO energy gap. Ultrafast photoisomerization kinetics (approximately 0.8-1.6 ps) and high quantum yields (around 0.3-0.6) were observed through trajectory surface hopping simulations. Additionally, the simulated time-resolved fluorescence emission spectrum indicates a significantly reduced "dark state" duration (0.09-0.26 ps) in these newly designed LDMRMs compared to the original ones, marking a substantial leap forward in the design and efficiency of LDMRMs.

Coupled Rotary Motion in Molecular Motors

Biological molecular machines play a pivotal role in sustaining life by producing a controlled and directional motion. Artificial molecular machines aim to mimic this motion, to exploit and tune the nanoscale produced motion to power dynamic molecular systems. The precise control, transfer, and amplification of the molecular-level motion is crucial to harness the potential of synthetic molecular motors. It is intriguing to establish how directional motor rotation can be utilized to drive secondary motions in other subunits of a multicomponent molecular machine. The challenge to design sophisticated synthetic machines involving multiple motorized elements presents fascinating opportunities for achieving unprecedented functions, but these remain almost unexplored due to their extremely intricate behavior. Here we show intrinsic coupled rotary motion in light-driven overcrowded-alkene based molecular motors. Thus far, molecular motors with two rotors have been understood to undergo independent rotation of each subunit. The new bridged-isoindigo motor design revealed an additional dimension to the motor's unidirectional operation mechanism where communication between the rotors occurs. An unprecedented double metastable state intermediate bridges the rotation cycles of the two rotor subunits. Our findings demonstrate how neighboring motorized subunits can affect each other and thereby drastically change the motor's functioning. Controlling the embedded entanglement of active intramolecular components sets the stage for more advanced artificial molecular machines.

Coupled Rocking Motion in a Light-Driven Rotary Molecular Motor

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.

Highly Efficient Oxindole-Based Molecular Photoswitches

3-Benzylidene-indoline-2-ones play a prominent role in the pharmaceutical industry due to the diverse biomedical applications of oxindole heterocycles. Despite the extensive reports on their biological properties, these compounds have hardly been studied for their photochemical activity. Here, we present 3-benzylidene-indoline-2-ones as a promising class of photoswitches with high yields, robust photochemical switching with quantum yields reaching up to 50 % and potential for biological applications.

A visible-light-driven molecular motor based on barbituric acid

We present a class of visible-light-driven molecular motors based on barbituric acid. Due to a serendipitous reactivity we observed during their synthesis, these motors possess a tertiary stereogenic centre on the upper half, characterised by a hydroxy group. Using a combination of femto- and nanosecond transient absorption spectroscopy, molecular dynamics simulations and low-temperature 1H NMR experiments we found that these motors operate similarly to push-pull second-generation overcrowded alkene-based molecular motors. Interestingly, the hydroxy group at the stereocentre enables a hydrogen bond with the carbonyl groups of the barbituric acid lower half, which drives a sub-picosecond excited-state isomerisation, as observed spectroscopically. Computational simulations predict an excited state "lasso" mechanism where the intramolecular hydrogen bond pulls the molecule towards the formation of the metastable state, with a high predicted quantum yield of isomerisation (68%) in gas phase.

Simultaneously improving the efficiencies of photo- and thermal isomerization of an oxindole-based light-driven molecular rotary motor by a structural redesign

We designed a novel highly efficient light-driven molecular rotary motor theoretically by using electronic structure calculations and nonadiabatic dynamics simulations, and it showed excellent performance for both photo- and thermal isomerization processes simultaneously. By the small structural modification based on 3-(2,7-dimethyl-2,3-dihydro-1H-inden-1-ylidene)-1-methylindolin-2-one (DDIYM) synthesized by Feringa et al. recently, an oxindole-based light-driven molecular rotary motor, 3-(1,5-dimethyl-4,5-dihydrocyclopenta[b]pyrrol-6(1H)-ylidene)-1-methylindolin-2-one (DDPYM), is proposed, which displays a significant electronic push-pull character and weak steric hindrance for double-bond isomerization. The newly designed motor DDPYM shows a remarkable improvement of the quantum yield for both EP → ZM and ZP → EM photoisomerization processes, compared to the original motor DDIYM. Furthermore, the rotary motion in photoisomerization processes of DDPYM behaves more like a pure axial rotational motion approximately, while that of DDIYM is an obvious precessional motion. The weakness of the steric hindrance reduces the energy barriers of the thermal helix EM → EP and ZM → ZP inversion steps, and would accelerate two ground-state isomerization steps significantly. Our results confirm the feasibility of simultaneously improving the efficiencies of photo- and thermal isomerization of oxindole-based light-driven molecular rotary motors and this design idea sheds light on the future development of more efficient molecular motors.

Light-triggered Modulation of Supramolecular Chirality

Dynamically controlling the supramolecular chirality is of great significance in development of functional chiral materials, which is thus essential for the specific function implementation. As an external energy input, light is remote and accurate for modulating chiral assemblies. In non-polarized light control, some photochemically reactive units (e. g., azobenzene, ɑ-cyanostilbene, spiropyran, anthracene) or photo-induced directionally rotating molecular motors were designed to drive chiral transfer or amplification. Besides, photoexcitation induced assembly based physical approach was also explored recently to regulate supramolecular chirality beyond photochemical reactions. In addition, circularly polarized light was applied to induce asymmetric arrangement of organic molecules and asymmetric photochemical synthesis of inorganic metallic nanostructures, in which both wavelength and handedness of circularly polarized light have effects on the induced supramolecular chirality. Although light-triggered chiral assemblies have been widely applied in photoelectric materials, biomedical fields, soft actuator, chiral catalysis and chiral sensing, there is a lack of systematic review on this topic. In this review, we summarized the recent studies and perspectives in the constructions and applications of light-responsive chiral assembled systems, aiming to provide better knowledge for the development of multifunctional chiral nanomaterials.