Triplet-Triplet Energy Transfer: A Simple Strategy for an Efficient Visible Light-Induced Photoclick Reaction

Photoclick reactions combine the advantages offered by light-driven processes and classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photo-crosslinking, and protein labeling. Despite these advances, the dependency of most of the photoclick reactions on UV light poses a severe obstacle for their general implementation, as this light can be absorbed by other molecules in the system resulting in their degradation or unwanted reactivity. However, the development of a simple and efficient system to achieve bathochromically shifted photoclick transformations remains challenging. Here, we introduce triplet-triplet energy transfer as a fast and selective way to enable visible light-induced photoclick reactions. Specifically, we show that 9,10-phenanthrenequinones (PQs) can efficiently react with electron-rich alkenes (ERAs) in the presence of a catalytic amount (as little as 5 mol %) of photosensitizers. The photocycloaddition reaction can be achieved under green (530 nm) or orange (590 nm) light irradiation, representing a bathochromic shift of over 100 nm as compared to the classical PQ-ERAs system. Furthermore, by combining appropriate reactants, we establish an orthogonal, blue and green light-induced photoclick reaction system in which the product distribution can be precisely controlled by the choice of the color of light.

Diarylethene Isomerization by Using Triplet-Triplet Annihilation Photon Upconversion

Green-to-blue triplet-triplet annihilation photon upconversion with the well-studied upconversion pair 9,10-diphenylanthracene (DPA)/platinum octaethylporphyrin (PtOEP) was used to reversibly drive the photoisomerization of diarylethene (DAE) photoswitches by using visible light. By carefully selecting the kinetic and spectral properties of the molecular system as well as the experimental geometry, a single green light source can be used to selectively trigger both the ring-opening and the ring-closing reactions, whilst also inducing fluorescence from the colored closed isomer that can be used as a readout to monitor the isomerization process in situ. The upconversion solution and the DAE solution are kept physically separated, allowing them to be characterized both concomitantly and individually without further separation processes. The ring-closing reaction using upconverted photons was quantified and compared to the efficiency of direct isomerization with ultraviolet light.

Heterocyclic Hemipiperazines: Water-Compatible Peptide-Derived Photoswitches

Hemipiperazines are a recently discovered class of peptide-derived molecular photoswitches with high biocompatibility and therapeutic potential. Here, for the first time we describe photochromism of heterocyclic hemipiperazines. They demonstrate long thermal lifetimes, and enlarged band separation between photoisomers. Efficient photoisomerization occurs under aqueous conditions, although with a need for organic co-solvent. Bidirectional switching with visible light is observed for an extended aromatic system.

Stepping Out of the Blue: From Visible to Near-IR Triggered Photoswitches

Light offers unique opportunities for controlling the activity of materials and biosystems with high spatiotemporal resolution. Molecular photoswitches are chromophores that undergo reversible isomerization between different states upon irradiation with light, allowing a convenient means to control their influence over the system of interest. However, a significant limitation of classical photoswitches is the requirement to initiate the switching in one or both directions using deleterious UV light with poor tissue penetration. Red-shifted photoswitches are hence in high demand and have attracted keen recent research interest. In this Review, we highlight recent progress towards the development of visible- and NIR-activated photoswitches characterized by distinct photochromic reaction mechanisms. We hope to inspire further endeavors in this field, allowing the full potential of these tools in biotechnology and materials chemistry applications to be realized.

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 has 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.

Cyclization from Higher Excited States of Diarylethenes Having a Substituted Azulene Ring

The cyclization reaction of diarylethenes having an azulene ring occurs only via higher excited states. Novel diarylethenes having an azulene ring with a strong donor or acceptor were synthesized and examined in these reactions. A derivative having an electron-donating 1,3-benzodithiol-2-ylidenemethyl group at the 1-position of the azulene ring showed photochromism, whereas neither a derivative having a π-conjugated electron-donating group at the 3-position of the azulene ring nor derivatives having a π-conjugated electron-withdrawing group at the 1- or 3-position of the azulene ring showed any photochromism. The photoreactivities of these compounds were explained by calculating forces and bond orders on the excited states using density functional theory (DFT) and time-dependent (TD)-DFT.

Mechanistic Insights into the Triplet Sensitized Photochromism of Diarylethenes

Operating photoswitchable molecules repetitively and reliably is crucial for most of their applications, in particular in (opto)electronic devices, and related to reversibility and fatigue resistance, which both critically depend on the photoisomerization mechanism defined by the substitution pattern. Two diarylethene photoswitches bearing biacetyl triplet sensitizers either at the periphery or at the core were investigated using both stationary as well as transient UV/Vis absorption spectroscopy ranging from the femtosecond to the microsecond time scale. The diarylethene with two biacetyl moieties at the periphery is switching predominantly from the triplet excited state, giving rise to an enhanced fatigue resistance. In contrast, the diarylethene bearing one diketone at the photoreactive inner carbon atom cyclizes from the singlet excited state and shows significantly higher quantum yields for both cyclization and cycloreversion.

Photochromic Diarylethenes Designed for Surface Deposition: From Self-Assembled Monolayers to Single Molecules

The efficient switching that can occur between two stable isomers of diarylethenes makes them particularly promising targets for opto- and molecular electronics. To examine these classes of molecules for electronics applications, they have been subjected to a series of scanning tunneling microscopy (STM) experiments, which are the focus of this Review. A brief introduction to the chemical design of diarylethenes in terms of their switching capabilities along with the basics of STM are presented. Next, initial STM studies on these compounds under ambient conditions are discussed. An overview of how molecular design affects the isomerization and self-assembly of diarylethenes at the solid-liquid interface as investigated by STM is then presented, as well as single-molecule studies under ultrahigh vacuum. The last section presents further prospects for molecular design in the field.

Involving Synergy of Green Light and Acidic Responses in Control of Unimolecular Multicolor Luminescence

Conversion of multicolor luminescence is one of desirable goals in study and development of next-generation molecular emitters, whereas involving visible light into the control of the above-mentioned ability has been poorly addressed due to the need of a relatively complicate molecular design. In this work, we present a novel dyad with a linkage of 4-piperazinyl-1,8-naphthalimide and cyanostyryl-modified azulene moiety, upon which the luminescence signal can be orthogonally controlled by protonation and green light irradiation. The superior features of the protonation induced excited state energy alteration, followed by green light driven photoisomerization led to a progressive luminescent color conversion among blue, yellow and green at the single molecular level. This strategy may bring in novel insights for preparing advanced function-integrated optoelectronic materials.

Nanoscopic Visualization of Soft Matter Using Fluorescent Diarylethene Photoswitches

The in situ imaging of soft matter is of paramount importance for a detailed understanding of functionality on the nanoscopic scale. Although super-resolution fluorescence microscopy methods with their unprecedented imaging capabilities have revolutionized research in the life sciences, this potential has been far less exploited in materials science. One of the main obstacles for a more universal application of super-resolved fluorescence microscopy methods is the limitation of readily available suitable dyes to overcome the diffraction limit. Here, we report a novel diarylethene-based photoswitch with a highly fluorescent closed and a nonfluorescent open form. Its photophysical properties, switching behavior, and high photostability make the dye an ideal candidate for photoactivation localization microscopy (PALM). It is capable of resolving apolar structures with an accuracy far beyond the diffraction limit of optical light in cylindrical micelles formed by amphiphilic block copolymers.

Photoswitchable NIR-Emitting Gold Nanoparticles

Photo-switching of the NIR emission of gold nanoparticles (GNP) upon photo-isomerization of azobenzene ligands, bound to the surface, is demonstrated. Photophysical results confirm the occurrence of an excitation energy transfer process from the ligands to the GNP that produces sensitized NIR emission. Because of this process, the excitation efficiency of the gold core, upon excitation of the ligands, is much higher for the trans form than for the cis one, and t→c photo-isomerization causes a relevant decrease of the GNP NIR emission. As a consequence, photo-isomerization can be monitored by ratiometric detection of the NIR emission upon dual excitation. The photo-isomerization process was followed in real-time through the simultaneous detection of absorbance and luminescence changes using a dedicated setup. Surprisingly, the photo-isomerization rate of the ligands, bound to the GNP surface, was the same as measured for the chromophores in solution. This outcome demonstrated that excitation energy transfer to gold assists photo-isomerization, rather than competing with it. These results pave the road to the development of new, NIR-emitting, stimuli-responsive nanomaterials for theranostics.