Light-driven Molecular Switches Research Articles

What Happens to a Pyrrole Hemithioindigo Photoswitch Trapped in a Fluorescent Protein?

Artificial molecular photoswitches that can be reversibly controlled into different configurations by external light stimulation have broad application prospects in various fields, such as materials and chemical biology. Among them, the pyrrole hemithioindigo (PHT) photoswitch has a geometric structure similar to that of the fluorescent protein chromophore. What happens when the chromophore is replaced by PHT, and does it achieve similar functions to the original one? To answer these questions, we carried out ONIOM(QM/MM) and classical molecular dynamics studies on the photoisomerization mechanism and spectroscopic properties of PHT in the fluorescent protein. The results showed that in the protein environment, the fate of excited PHT is governed by the competition between fluorescence emission and hula-twist isomerization. Due to the strong steric hindrance effects caused by the interlacing residues in the protein that restrict the PHT conformation transformation, the cis-to-trans isomerization process of PHT needs to overcome a barrier of at least 4.9 kcal/mol; thus, fluorescence emission is more dominant in competition. It is also found that the intermolecular interaction between LYS67 and the carbonyl oxygen of PHT has a significant effect on the fluorescence emission. These results revealed the photochemical reaction mechanism of a light-driven molecular switch in the fluorescent protein and provided further theoretical support for the field of chemical biology.

Electronic transport on the two state “ON–OFF” of 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran as a light-driven molecular optical switch: A first-principle study

Here, the electronic transport characteristics of the 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran were studied by using NEGF and DFT methods. By being exposed to UV or visible light, this molecule can change from its MC to SP states. The titled structure was optimized at B3LYP/6–311++G(d,p) level of theory. To obtain conductivity results, the electrons of metal and organic atoms were approached by the single-plus polarization (SZP) for single-zeta and double-plus polarization (DZP) for double zeta basis sets, respectively. We examined some factors in this study, including various molecule geometries, the type of surface materials (Platinum, gold, and silver), the switching ratio, the gap between HOMO and LUMO levels, and the transmission spectra, at different bias voltages. The outcomes show that conductivity transforms from an off-state (high resistance) to an on-state as the molecule switches from MC to SP form (low resistance).

Frontispiece: Visible Light‐Driven Molecular Switches and Motors: Recent Developments and Applications

Visible Light-Driven Molecular Switches and Motors have achieved remarkable advancement and found wide applications in bioscience, materials science, photopharmacology, and beyond. For more information, see the Review by Li et al. (DOI: 10.1002/chem.202103906)

Visible Light-Driven Molecular Switches and Motors: Recent Developments and Applications.

Inspired by human vision, a diverse range of light-driven molecular switches and motors have been developed for fundamental understanding and application in material science and biology. Recently, the design and synthesis of visible light-driven molecular switches and motors have been actively pursued. This emerging trend is partly motivated to avoid the harmful effects of ultraviolet light, which was necessary to drive the classical molecular switches and motors at least in one direction, impeding their employment in biomedical and photopharmacology applications. Moreover, visible light-driven molecular switches and motors are demonstrated to enable benign optical materials for advanced photonic devices. Therefore, during the past several years, visible light-driven molecular switches based on azobenzene derivatives, diarylethenes, 1,2-dicyanodithienylethenes, hemithioindigo derivatives, iminothioindoxyls, donor-acceptor Stenhouse adducts, and overcrowded alkene based molecular motors have been judiciously designed, synthesized, and used in the development of functional materials and systems for a wide range of applications. In this Review, we present the recent developments toward the design of visible light-driven molecular switches and motors, with their applications in the fabrication of functional materials and systems in material science, bioscience, pharmacology, etc. The visible light-driven molecular switches and motors realized so far undoubtedly widen the scope of these interesting compounds for technological and biological applications. We hope this Review article could provide additional impetus and inspire further research interests for future exploration of visible light-driven advanced materials, systems, and devices.

Visible light, temperature, and electric field-driven rotation of diffraction gratings enabled by an axially chiral molecular switch

A visible light-driven chiral molecular switch shows reversibletrans–cisphotoisomerization, good compatibility, and high helical twisting power in achiral liquid crystals. Multi-stimuli responsive rotatable in-plane beam steering and chromatic dispersion are accomplished.

Inside Back Cover: Irradiation‐Wavelength Directing Circularly Polarized Luminescence in Self‐Organized Helical Superstructures Enabled by Hydrogen‐Bonded Chiral Fluorescent Molecular Switches (Angew. Chem. Int. Ed. 52/2021)

Irradiation-wavelength directing circularly polarized luminescence in emissive self-organized helical superstructures is demonstrated by Jinbao Guo, Quan Li, and co-workers in their Research Article on page 27158. Phototuning of the turning-off and the polarization inversion of circularly polarized luminescence signals was achieved in the corresponding photostationary state of the same helical superstructure constructed from the light-driven hydrogen-bonded chiral fluorescent molecular switch.

Synthesis of fluorescent molecular switches based on porphyrinoids covalently linked with redox active ligands

5,15-bis(1-hydoxy-2,6-di-tert-butyl-phenyl)10,20-bis-(pentaphenyl-phenyl)phenyl porphyrin and its Nickel complex were synthesized. Oxidation with NOBF4 gave 5,15bis(1-oxo-2,6-ditertairy-butyl-phenyl)10,20-bis(pentaphenyl-phenyl)phenyl-porphodimethene and Ni bis(1-oxo-2,6-ditertairy-butyl-phenyl)10,20-bis-(pentaphenylphenyl)phenyl porphodimethene. The porphyrin conjugates were characterized by NMR, mass, and UV–visible spectroscopy for structural elucidation. The density functional theory studies revealed a decrease in the highest occupied and the lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps in the oxo-porphodimethenes. The photochromism and fluorescence characteristics were also studied. The porphyrin moieties were found to be desirable for the reversible light-driven fluorescent molecular switches.

Free Energy Computation for an Isomerizing Chromophore in a Molecular Cavity via the Average Solvent Electrostatic Configuration Model: Applications in Rhodopsin and Rhodopsin-Mimicking Systems.

We present a novel technique for computing the free energy differences between two chromophore "isomers" hosted in a molecular environment (a generalized solvent). Such an environment may range from a relatively rigid protein cavity to a flexible solvent environment. The technique is characterized by the application of the previously reported "average electrostatic solvent configuration" method, and it is based on the idea of using the free energy perturbation theory along with a chromophore annihilation procedure in thermodynamic cycle calculations. The method is benchmarked by computing the ground-state room-temperature relative stabilities between (i) the cis and trans isomers of prototypal animal and microbial rhodopsins and (ii) the analogue isomers of a rhodopsin-like light-driven molecular switch in methanol. Furthermore, we show that the same technology can be used to estimate the activation free energy for the thermal isomerization of systems i-ii by replacing one isomer with a transition state. The results show that the computed relative stability and isomerization barrier magnitudes for the selected systems are in line with the available experimental observation in spite of their widely diverse complexity.

On the Transition from a Biomimetic Molecular Switch to a Rotary Molecular Motor.

The experimental investigation of the unidirectional motion characterizing the photoisomerization of single-molecule rotary motors requires accessible lab prototypes featuring an electronic circular dichroism (ECD) signal that is sensitive to the geometrical and electronic changes occurring during an ultrafast reactive process. Here we report a combined experimental/computational study of a candidate obtained via the asymmetrization of a light-driven biomimetic molecular switch. We show that the achieved motor has an ECD band that is remarkably sensitive to the isomerization motion, and it is therefore suitable for time-resolved ECD studies. However, we also find that, unexpectedly, the synthesized motor isomerizes on a time scale longer than the subpicosecond time measured for the achiral parent, a result that points to alternative candidates conserving a high reaction speed.

Synthesis and exploring the excited-state PES of photochromic hydrogen bond-assembled [2]rotaxane based on 1,3-Diazabicyclo-[3.1.0]hex-3-enes

Here, the synthesis of photochromic hydrogen bond-assembled [2]rotaxanes using bis-fumarate as a thread for the first time is reported. In fact, photochromic 1,3-diazabicyclo[3.1.0]hex-3-ene moieties were used as stoppers and two-atom spacers managed good binding sites for the tetralactam macrocycles in clipping reactions. Moreover, the yields of photochromic [2]rotaxanes highly depended on the NO2 substituent stoppers. While the thread with a para –NO2 substituent as stopper units was shown to be an excellent template for the synthesis of photochromic [2]rotaxanes. The structures of the [2]rotaxanes are established clearly in solution by chemical shifts of the 1H 13C NMR signals and UV–Vis spectra. A pronounced bathochromic shift was occurred in the excitation wavelength of photochoromic [2]rotaxanes compared with the absorption band of photochromic threads. Therefore, these organizations can be applied in light-driven molecular switches and motors. The reversible transformation of trans and cis geometric photoisomers under UV radiation was identified. In other efforts, the possibility of the process of trans to cis interconversion of the fumarate linker under UV irradiation has been examined computationally and it has appeared that it may cause the transverse of the bis-fumarate linker inside the tetralactam macrocycle to some extent.

Fast T-Type Photochromic Crystals of Diarylbenzene

Photochromic molecules are of great interest as light-driven molecular switches in research fields of material and life sciences. In particular, fast T-type photochromic molecules with half-life times of the thermal back reaction (t1/2) on the order of milliseconds can change not only their color but also their physicochemical properties only under photoirradiation. Herein, we report fast T-type photochromic crystals with the t1/2 on the order of milliseconds composed of a diarylbenzene by utilizing intramolecular CH–N hydrogen bonding interactions. Our results would lead to novel photoresponsive crystalline materials whose color and physicochemical properties can be modulated only under photoirradiation.

Fluorescent molecular photoswitch based on basket-shaped porphyrins

Here we present two novel basket-shaped porphyrins bridged by photochromic diarylethene derivatives, which show significant photoswitchable fluorescent properties. Their synthesis, structures, photochromism and fluorescence characteristics were reported in detail. They are desirable for reversible light-driven fluorescent molecular switches.

Trajectory surface-hopping dynamics of light-driven imine switches

We report the photodynamics of switchable photoisomerization processes of a camphorquinone imine (model-k) and alkene imine (model-a) by trajectory surface-hopping (TSH) molecular dynamics at the SA4-CASSCF/def2-SVP level. In both cases, ultrafast nonadiabatic Z-E photoisomerizations are observed. For model-k, the product yields of Z → E and E → Z photoisomerization are 0.47 and 0.27, respectively; and those for model-a are much higher (0.72 and 0.79). Therefore, the alkene-imine is expected to be a superior candidate for ultrafast light-driven molecular switches.

Current–Voltage Characteristics of the Aziridine-Based Nano-Molecular Wires: a Light-Driven Molecular Switch**Supported by the Damghan University, the Ferdowsi University of Mashhad and the Islamic Azad University of Shahrood.

Using nonequilibrium Green’s function formalism combined first-principles density functional theory, we analyze the transport properties of a 4,4-dimethyl-6-(4-nitrophenyl)-2-phenyl-3,5-diaza-bicyclo[3.1.0]hex-2-ene molecular optical switch. The title molecule can convert between closed and open forms by visible or ultraviolet irradiation. The I–V characteristics, differential conductance, on-off ratio, electronic transmission coefficients, spatial distribution of molecular projected self-consistent Hamiltonian orbitals, HOMO-LUMO gaps, effect of electrode materials Y (111) (Y = Au, Ag and Pt) on electronic transport and different molecular geometries corresponding to the closed and open forms through the molecular device are discussed in detail. Based on the results, as soon as possible the open form translates to the closed form, and there is a switch from the ON state to the OFF state (low resistance switches to high resistance). Theoretical results show that the donor/acceptor substituent plays an important role in the electronic transport of molecular devices. The switching performance can be improved to some extent through suitable donor and acceptor substituents.

Tuning the Thermal Isomerization of Phenylazoindole Photoswitches from Days to Nanoseconds.

The growing interest in light-driven molecular switches and optical oscillators led to the development of molecules that are able to interconvert from a stable to a metastable configuration upon photochemical triggering and to return to the thermodynamically stable form as soon as the light stimulus is removed. Controlling a wide range of back-isomerization lifetimes in the dark is a crucial goal for potential application of these compounds such as molecular machines. We herein present a novel class of easily synthesizable azo photoswitches based on the arylazoindole core. Most notably, minimal modifications of the core, such as methylation, dramatically change the Z-to-E thermal isomerization rate from days (Me in position 1) to the nanosecond range (Me in position 2). Moreover, fine-tuning of the Z-to-E lifetimes can be achieved choosing a proper dimethyl sulfoxide-water (or buffered water) solvent mixture. The photochemical and thermal mechanisms have been elucidated by a thorough computational and spectroscopic analysis. This allowed to detect three different pathways of thermal isomerization and to identify the hydrazone tautomer of the phenylazoindole as the major actor in the fast Z-E thermal isomerization of the NH-substituted switch in protic media.

Dynamic control of function by light-driven molecular motors

Light-driven artificial molecular switches and motors afford dynamic control of various molecular systems, ranging from catalytic to biological. This Perspective provides insight into the challenges that must be addressed to transition the field from the proof-of-concept stage to realization of its myriad applications.

Visible Light-Driven Artificial Molecular Switch Actuated by Radical-Radical and Donor-Acceptor Interactions.

We describe a visible light-driven switchable [2]catenane, composed of a Ru(bpy)3(2+) tethered cyclobis(paraquat-p-phenylene) (CBPQT(4+)) ring that is interlocked mechanically with a macrocyclic polyether consisting of electron-rich 1,5-dioxynaphthalene (DNP) and electron-deficient 4,4'-bipyridinium (BIPY(2+)) units. In the oxidized state, the CBPQT(4+) ring encircles the DNP recognition site as a consequence of favorable donor-acceptor interactions. In the presence of an excess of triethanolamine (TEOA), visible light irradiation reduces the BIPY(2+) units to BIPY((•+)) radical cations under the influence of the photosensitizer Ru(bpy)3(2+), resulting in the movement of the CBPQT(2(•+)) ring from the DNP to the BIPY((•+)) recognition site as a consequence of the formation of the more energetically favorable trisradical complex, BIPY((•+)) ⊂ CBPQT(2(•+)). Upon introducing O2 in the dark, the BIPY((•+)) radical cations are oxidized back to BIPY(2+) dications, leading to the reinstatement of the CBPQT(4+) ring encircled around the DNP recognition site. Employing this strategy of redox control, we have demonstrated a prototypical molecular switch that can be manipulated photochemically and chemically by sequential reduction and oxidation.

Fast and stable light-driven molecular switch based on a 5a,13-methanoindolo[2,1-b][1,3]benzoxazepine ring system

We developed a new type of light-driven molecular switch with ultrafast switching speeds and high fatigue resistance. The aforementioned switch is based on a 5a,13-methanoindolo[2,1-b][1,3]benzoxazepine ring system formed by the treatment of 1′,3′-dihydrospiro[chromene-2,2′-indole] derivatives with strong bases. Laser excitation of the obtained uncoloured molecules of trans- and cis-2-nitro-5a,13-methanoindolo[2,1-b][1,3]benzoxazepine induces the cleavage of the C–O bond and the opening of the bicyclic ring system in nanoseconds to form coloured zwitterionic species with a quantum yield of approximately 0.1. The photogenerated species absorb in the visible spectrum with λmax = 450–480 nm and thermally revert back to the ground state in several nanoseconds, making this light-driven switch one of the fastest among organic photoswitches.

Light-directing chiral liquid crystal nanostructures: from 1D to 3D.

Endowing external, remote, and dynamic control to self-organized superstructures with desired functionalities is a principal driving force in the bottom-up nanofabrication of molecular devices. Light-driven chiral molecular switches or motors in liquid crystal (LC) media capable of self-organizing into optically tunable one-dimensional (1D) and three-dimensional (3D) superstructures represent such an elegant system. As a consequence, photoresponsive cholesteric LCs (CLCs), i.e., self-organized 1D helical superstructures, and LC blue phases (BPs), i.e., self-organized 3D periodic cubic lattices, are emerging as a new generation of multifunctional supramolecular 1D and 3D photonic materials in their own right because of their fundamental academic interest and technological significance. These smart stimuli-responsive materials can be facilely fabricated from achiral LC hosts by the addition of a small amount of a light-driven chiral molecular switch or motor. The photoresponsiveness of these materials is a result of both molecular interaction and geometry changes in the chiral molecular switch upon light irradiation. The doped photoresponsive CLCs undergo light-driven pitch modulation and/or helix inversion, which has many applications in color filters, polarizers, all-optical displays, optical lasers, sensors, energy-saving smart devices, and so on. Recently, we have conceptualized and rationally synthesized different light-driven chiral molecular switches that have very high helical twisting powers (HTPs) and exhibit large changes in HTP in different states, thereby enabling wide phototunability of the systems by the addition of very small amounts of the molecular switches into commercially available achiral LCs. The light-driven chiral molecular switches are based on well-recognized azobenzene, dithienylcyclopentene, and spirooxazine derivatives. We have demonstrated high-resolution and lightweight photoaddressable displays without patterned electronics on flexible substrates. The wide tunability of the HTP furnishes reflection colors encompassing the whole visible spectrum and beyond in a reversible manner. Photomodulation of the helical pitch of the CLCs has been achieved by UV, visible, and near-infrared (NIR) light irradiation. NIR-light-induced red, green, and blue (RGB) reflections have been leveraged only by varying the power density of the IR laser. Some chiral switches are found to confer helix inversion to the cholesteric systems, which qualifies the CLCs for applications where circularly polarized light is involved. Dynamic and static primary RGB reflection colors have been achieved in a single film. LC BPs have been fabricated and investigated in the context of self-organized 3D photonic band gap (PBG) materials, and dynamic phototuning of the PBG over the visible region has been achieved. Omnidirectional lasing and tuning of the laser emission wavelength have also been attained in monodisperse photoresponsive CLC microshells fabricated by a capillary-based microfluidic technique. This Account covers the research and development in our laboratory starting from the design concepts and synthesis of photodynamic chiral molecular switches to their applications in the fabrication of photoresponsive CLCs and BPs. Potential and demonstrated practical applications of photoresponsive CLCs, microshells, and BPs are discussed, and the Account concludes with a brief forecast of what lies beyond the horizon in this rapidly expanding and fascinating field.

Conductance switching in single light-sensitive molecular device with carbon nanotube electrodes

By applying nonequilibrium Green's function formalism combined first-principles density functional theory, a new mechanism for optical switch of single molecular device with carbon nanotube electrodes is proposed. The molecule comprises the switch can convert between enol and keto isomers upon photoinduced excited state hydrogen transfer in the molecular bridge. Theoretical results show that these two isomers exhibit very different current-voltage characteristics both in armchair and zigzag junction, which can realize the on and off states of the molecular switch. Meantime, the chirality of the single-walled carbon nanotube (SWCNT) electrodes strongly affects the switching characteristics of the molecular junctions. The maximum value of on-off ratio can reach 72 at 1.6 V for the switch with zigzag SWCNT electrodes, suggesting potential applications of this junction in future design of light-driven molecular switches.