Photoactive Moieties Research Articles

Plasmon-Enhanced Multiphoton Polymer Crosslinking for Selective Modification of Plasmonic Hotspots.

A novel approach to selectively modify narrow subareas of metallic nanostructures adjacent to plasmonic hotspots, where strong electromagnetic field amplification occurs upon localized surface plasmon (LSP) excitation, is reported. In contrast to surface plasmon-triggered polymerization, it relies on plasmonically enhanced multiphoton crosslinking (MPC) of polymer chains carrying photoactive moieties. When they are contacted with metallic nanostructures and irradiated with a femtosecond near-infrared beam resonantly coupled with LSPs, the enhanced field intensity locally exceeds the threshold and initiates MPC only at plasmonic hotspots. This concept is demonstrated by using gold nanoparticle arrays coated with two specifically designed polymers. Local MPC of a poly(N,N-dimethylacrylamide)-based copolymer with an anthraquinone crosslinker is shown via atomic force microscopy. Additionally, MPC is tested with a thermoresponsive poly(N-isopropylacrylamide)-based terpolymer. The reversible thermally induced collapse and swelling of the MPC-formed hydrogel at specific nanoparticle locations are confirmed by polarization-resolved localized surface plasmon resonance (LSPR) spectroscopy. These hybrid metallic/hydrogel materials can be further postmodified, offering attractive characteristics for future spectroscopic/bioanalytical applications.

On-Surface Photopolymerization - What Does the Surface Do?

We attained the synthesis of mesoscale-ordered 2D polymers by the topochemical on-surface photopolymerization of fluorinated anthracene triptycene (fantrip) monomers. The underlying protocol is two-staged: (1) Self-assembly of the monomers into a photopolymerizable monolayer structure, where the photoactive anthracene moieties are face-to-face stacked; (2) cross-linking of the self-assembled monolayer into a covalent 2D polymer by photochemically excited [4+4] cycloadditions between the antiparallel aligned anthracene blades. Thereby, the long-range order attained in step (1) is transferred into the covalent state.Yet, the topochemical approach depends crucially on achieving the reactive monomer packing with the appropriate mutual alignment of the anthracene blades. For this initial self-assembly, the underlying surface plays a decisive role, which also extends to the photopolymerization.We show that the photoactive monolayer packing of fantrip monomers can be achieved on both alkane-passivated graphite and iodine-passivated Au(111) or Ag(111). Although the fantrip self-assembly appears essentially similar on these substrates, there are distinct differences in the subsequent photopolymerization. These can be rationalized by the photoexcitation of the substrate and the subsequent transfer of hot carriers to the molecular networks.Nat. Chem. 13, 730-736 (2021)Eur. J. Org. Chem. 2021, 5478–5490 (2021)Trends Chem. 4, 471-474 (2022)Angew. Chem. Int. Ed. 61, e202201044 (2022)

Introduction of photochromic properties to UiO-66-NH2 via “click”-modification by spiropyran molecule

We present a study on incorporating spiropyran photoactive molecules into the UiO-66-NH2 scaffold. Initially, we modified spiropyran molecules by introducing functional groups to facilitate covalent bonding with the MOF structure. Spiropyran molecules with carboxylic groups demonstrated the ability to coordinate zirconium in defect pores of the MOF. Alternatively, the aldehyde group showed potential for forming C–N bonds with amino groups of BDC-NH2 linkers. To validate the formation of C–N bonds within the MOF scaffold, we synthesized a complex salt of spiropyran and individual BDC-NH2 linkers. DFT calculations support our conclusions. We observed that upon introducing the photoactive moiety, the UiO-66-NH2 framework exhibited photoresponse, as demonstrated by FTIR experiments. Based on experimental data and computational results, we hypothesize that both incorporation mechanisms are viable in the functionalization process. However, steric hindrances may impede the incorporation of spiropyran into the pores, leading to surface modification instead. The elucidated mechanisms hold promise for the development of photoresponsive MOF-based smart materials.

Visible-Light-Driven Rapid 3D Printing of Photoresponsive Resins for Optically Clear Multifunctional 3D Objects.

Light-driven 3D printing is gaining significant attention for its unparalleled build speed and high-resolution in additive manufacturing. However, extending vat photopolymerization to multifunctional, photoresponsive materials poses challenges, such as light attenuation and interference between the photocatalysts (PCs) and photoactive moieties. This study introduces novel visible-light-driven acrylic resins that enable rapid, high-resolution photoactive 3D printing. The synergistic combination of a cyanine-based PC, borate, and iodonium coinitiators (HNu 254) achieves an excellent printing rate and feature resolution under low-intensity, red light exposure. The incorporation of novel hexaarylbiimidazole (HABI) crosslinkers allows for spatially-resolved photoactivation upon exposure to violet/blue light. Furthermore, a photobleaching mechanism inhibited by HNu 254 during the photopolymerization process results in the production of optically-clear 3D printed objects. Real-time Fourier transform infrared spectroscopy validates the rapid photopolymerization of the HABI-containing acrylic resin, whereas mechanistic evaluations reveal the underlying dynamics that are responsible for the rapid photopolymerization rate, wavelength-orthogonal photoactivation, and observed photobleaching phenomenon. Ultimately, this visible-light-based printing method demonstrates: (i) rapid printing rate of 22.5mm h-1, (ii) excellent feature resolution (≈20µm), and (iii) production of optically clear object with self-healing capability and spatially controlled cleavage. This study serves as a roadmap for developing next-generation "smart" 3D printing technologies.

Photoresponsive metal-organic framework materials for advance applications.

The interaction between light and materials produces a range of phenomena within molecular systems, leading to advanced applications in the field of materials science. In this regard, metal-organic framework (MOF) materials have become superior candidates to others because of their easy tailor-made synthetic methods via incorporation of photoactive moieties into their structural assembly. Photoresponsive MOFs exhibit a massive variety of exciting properties, including photochromism, photomagnetism, photoluminescence, photon up or down conversion, photoconductivity, nonlinear optical properties, photosalient effects and photoinduced switching of conformations. These photoresponsive properties of MOFs regulate different potential applications, such as on-demand gas sorption and separation, optical sensing, fabrication of photoactuators and photosensing electronic devices, dye degradation, catalysis, cargo delivery, ink-free erasable printing, bio-imaging and drug delivery in biological systems. Therefore, judicious crystal engineering along with an understanding of their structure-property relationship will lead to the fabrication of desired photosensitive MOFs. Herein, we attempted to incorporate categorical descriptions based on advanced applications of photoresponsive MOFs considering a wide range of recent publications.

Molecular Switches of the LD-CISSS Type Based on Ni(II) Azomethine Bis-Chelate Complexes. Quantum Chemical Modeling

(DFT/B3LYP/6-311++G(d,p)) calculations were performed to study Ni(II) azomethine bis-chelates with photoactive moieties (imidazole and benzimidazole derivatives of azo compounds, azomethines, and stilbenes) exhibiting the behavior of molecular magnetic switches by the light-driven coordination-induced spin state switching (LD-CISSS) mechanism. The structural and energy characteristics of the complexes favorable to or restricting the applicability of these complexes as molecular switches were determined.

Viologen-Based Covalent Organic Frameworks toward Metal-Free Highly Efficient Photocatalytic Hydrogen Evolution

Covalent organic frameworks (COFs) have shown promise in the field of photocatalysts for hydrogen evolution. Many studies have been carried out using various electroactive and photoactive moieties such as triazine, imide, and porphyrin to produce COFs with different geometric structures and units. Electron transfer mediators like viologen and their derivatives can accelerate the transfer of electrons from photosensitizers to active sites. Herein, the combination of a biphenyl-bridged dicarbazole electroactive donor skeleton with a viologen acceptor structure is reported for the photocatalytic hydrogen evolution of novel COF structures with various alkyl linkers {TPCBP X-COF [X = ethyl (E), butyl (B), and hexyl (H)]}. The structures became more flexible and exhibited less crystal behavior as the length of the alkyl chain increased according to scanning and transmission electron microscopy images, X-ray diffraction analyses, and theoretical three-dimensional geometric optimization. In comparison, the H2 evolution rate of the TPCBP B-COF (12.276 mmol g-1) is 2.15 and 2.38 times higher than those of the TPCBP H-COF (5.697 mmol h-1) and TPCBP E-COF (5.165 mmol h-1), respectively, under visible light illumination for 8 h. The TPCBP B-COF structure is one of the best-performing catalysts for the corresponding photocatalytic hydrogen evolution in the literature, producing 1.029 mmol g-1 h-1 with a high apparent quantum efficiency of 79.69% at 470 nm. Our strategy provides new aspects for the design of novel COFs with respect to future metal-free hydrogen evolution by using solar energy conversion.

Impact of Photoactive Monomer Location in Photoresponsive Block Copolymer/Ionic Liquid Solutions

By incorporating a photoactive moiety into a block polymer, the phase behavior can be controlled with light. Previous studies have looked at situating azobenzene, a common photoresponsive molecule, along the backbone of the polymer as a pendant group, and even as part of the solvent rather than as a comonomer. Here, we study the effect of the positioning of pendant azobenzene groups along the polymer backbone on the lower critical disorder–order transition of poly(methyl methacrylate)-block-poly(benzyl methacrylate) (PMMA-b-PBnMA, or MB) in the ionic liquid 1,3-dimethyl imidazolium bis(trifluoromethylsulfonyl)imide. Using small-angle X-ray scattering and ultraviolet (UV)-irradiated small-amplitude oscillatory shear rheology, the placement of azobenzene statistically along the benzyl methacrylate backbone (MBsA) is compared to locating it as a midblock between the PMMA and the PBnMA (MAB) or as an end block after the PBnMA (MBA). Two concentrations of polymer in the ionic solvent were studied, 35 and 50 wt %. At 35 wt %, MBsA microphase-separated at 60 °C, MBA at 100 °C, and followed by MAB at 120 °C, a trend that was repeated at 50 wt %. MBsA was the only polymer to order onto a lattice at 35 wt %, forming hexagonally close-packed spheres. Both MBsA and MBA formed hexagonally packed cylinders at 50 wt %. MBsA consistently ordered onto a lattice over the temperature range of interest, while MBA only did so at 50 wt %, and MAB remained disordered at both concentrations. MBsA was also the only sample of the three to successfully transition reversibly between order and disorder with light. Therefore, adjusting the location of the azobenzene units within the thermo- and photoresponsive polymer solution significantly changes the overall behavior of the solutions and the ability to control that behavior with light and temperature.

Xanthate-supported photo-iniferter (XPI)-RAFT polymerization: facile and rapid access to complex macromolecules.

Xanthate-supported photo-iniferter (XPI)-reversible addition-fragmentation chain-transfer (RAFT) polymerization is introduced as a fast and versatile photo-polymerization strategy. Small amounts of xanthate are added to conventional RAFT polymerizations to act as a photo-iniferter under light irradiation. Radical exchange is facilitated by the main CTA ensuring control over the molecular weight distribution, while xanthate enables an efficient photo-(re)activation. The photo-active moiety is thus introduced into the polymer as an end group, which makes chain extension of the produced polymers possible directly by irradiation. This is in sharp contrast to conventional photo-initiators, or photo electron transfer (PET)-RAFT polymerizations, where radical generation depends on the added small molecules. In contrast to regular photo-iniferter-RAFT polymerization, photo-activation is decoupled from polymerization control, rendering XPI-RAFT an elegant tool for the fabrication of defined and complex macromolecules. The method is oxygen tolerant and robust and was used to perform screenings in a well-plate format, and it was even possible to produce multiblock copolymers in a coffee mug under open-to-air conditions. XPI-RAFT does not rely on highly specialized equipment and qualifies as a universal tool for the straightforward synthesis of complex macromolecules. The method is user-friendly and broadens the scope of what can be achieved with photo-polymerization techniques.

Highly Effective Generation of Singlet Oxygen by an Imidazole-Linked Robust Photosensitizing Covalent Organic Framework.

Developing effective photosensitizers to initiate the generation of singlet oxygen (1O2) is of great significance in both chemistry and physiology. Herein, linking the photoactive porphyrin moieties by in situ-formed robust imidazole groups, a covalent organic framework (COF), PyPor-COF, was successfully designed and synthesized. Detailed characterizations reveal that it not only possesses high crystallinity, permanent porosity, and robust stability but also shows a semiconductive photoresponse activity. As demonstrated by electron paramagnetic resonance experiments, the COF can initiate the generation of 1O2 efficiently under visible-light irradiation, the efficiency of which is higher than that of the pristine porphyrin-based reactant and even higher than some commonly used commercially available photosensitizing agents. Anticancer experiments prove that it can efficiently trigger the production of 1O2 in a physiological environment. This work demonstrates that the imidazole-linked porphyrin-incorporated COF is a highly promising photosensitizer that can even be applied in photodynamic therapy.

Anthracene / fluorescein based semi-conducting polymer for organic photovoltaics: Synthesis, DFT, optical and electrical properties

A new soluble semi-conducting polymer (AnFL) designed on alternating anthracene and fluorescein photoactive moieties has been synthesized via Williamson polycondensation. Different technics were combined to evidence its structural, optical and electrical properties. NMR and FTIR spectroscopies were used to confirm the macromolecular structure. Optical properties were studied by UV–visible absorption and photoluminescence spectroscopies, evidencing fluorescence resonance energy transfer (FRET) and photoinduced electron transfer (PET) from anthracene core to fluorescein unit which enhance the photovoltaic performance of the polymer. A single-layer [ITO/AnFL/Al] device was elaborated and investigated using current–voltage measurements where a typical diode behavior was observed with a turn-on voltage of 4.4 V. Results show also that the current is well described with space-charge-limited current conduction mechanism, with a charge carrier mobility of 2.38.10−5 cm2V−1s−1. The preliminary photovoltaic characteristics of the nanocomposite AnFL:PCBM were estimated and the blend showed an efficient interfacial charge transfer suggesting good photovoltaic performance for AnFL. All results showed that this functional polymer exhibits motivating photophysical properties and shows promising photovoltaic trends.

Effective Photocatalytic Initiation of Reactive Oxygen Species by a Photoactive Covalent Organic Framework for Oxidation Reactions

Reactive oxygen species (ROS) play a vital role in both chemistry and physiology. Developing effective photocatalysts becomes a feasible approach to trigger the generation of ROS. Herein, a photosensitive covalent organic framework (COF) was rationally designed and constructed by incorporating the photoactive triphenylamine moieties into its skeleton, characterizations of which not only showed the success formation of the highly crystalline and stable framework, but also revealed the photoactive property derived from triphenylamine-based building block. Electron paramagnetic resonance measurements displayed that the COF was an effective photocatalyst for generating ROS with one unpaired electron, that is, superoxide radical anion (O2• –). Subsequently, its high efficiency, selectivity, and reusability in photocatalytic aerobic oxidation of sulfides confirmed that the COF is a promising photocatalyst in the applications of ROS-involved reactions, demonstrating that precisely embedding organic photochromic groups into COFs could be a powerful strategy to fabricate metal-free heterogeneous photocatalysts for targeted reactions.

Towards synthetic unimolecular [Fe2S2]-photocatalysts sensitized by perylene dyes

A new set of perylene-based [Fe2S2(CO)6] photocatalyst dyads was synthesized and characterized. Four different strategies were employed to facilitate photoinduced electron transfer from the chromophore to the catalytically active site, increasing the chance for successful photocatalytic proton reduction. X-ray photoelectron spectroscopy measurements demonstrated the covalent attachment of the catalyst and provided insights on oxidation states of the iron. Visible light absorption maxima of the presented complexes reached up to 550 nm with reduction potentials between −1.14 V and −1.20 V because of increased electron density of the photoactive moiety. Quantum chemical calculations were performed to evaluate the employed design strategies based on the energy levels of the frontier orbitals and allowed to elucidate the electronic states involved in the initial photoactivation as well as to assess the charge transfer phenomena within the Franck-Condon point.

Ionic liquid-based unconventional photoinitiators for aqueous polymerization

A series of as-synthesized phosphonium and imidazolium ionic liquids (ILs) with high water-solubility are utilized as efficient photoinitiators (PIs) for the polymerization of vinyl monomers such as N-isopropylacrylamide (NIPAM) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) efficiently upon UV irradiation (λmax = 310 nm) in aqueous medium at ambient temperature and pressure. The photopolymerization proceeds through free radical pathway as established by EPR, 1H NMR, SEC and MALDI-TOF-MS along with some control experiments. The kinetic study demonstrates that this photopolymerization is fast enough to produce high-molecular-weight polymers with ∼ 70% yield even after 30 mins of UV irradiation. Control experiments show that these IL-based PIs have high water solubility and capable of producing high-molecular-weight polymers compared to those of polymers obtained by other commercially available aqueous PIs. Additionally, poly(NIPAM) and poly(DMAEMA) are grafted from the photoactive pendent IL moiety of the poly(ionic liquid), poly(triphenyl-4-vinylbenzylphosphonium chloride) (P[VBTP][Cl]) backbone by the grafting-from copolymerization. The presence of either poly(NIPAM) or poly(DMAEMA) graft segment induces thermoresponsiveness to their P[VBTP][Cl]-based copolymers.

Photo Click Reaction of Acylsilanes with Indoles.

Light‐mediated coupling of acylsilanes with indoles is reported. This photo click reaction occurs under mild conditions (415 nm) mostly in quantitative yield and provides stable silylated N,O‐acetals via light mediated siloxycarbene generation with subsequent indole‐N‐H insertion. We show that this very efficient and fully atom economic coupling process can be applied to conjugate complex systems, as documented by the clicking of carbohydrates with indole alkaloids. The method is also applicable to the conjugation of polymer chains. The linking acetal moiety can be readily cleaved and it is also shown that wavelength‐selective coupling and cleavage with acyl silanes bearing a second photoactive moiety is possible. This is documented by a successful polymerization/depolymerization sequence and by a polymer folding/unfolding process.

Photo Click Reaction of Acylsilanes with Indoles

AbstractLight‐mediated coupling of acylsilanes with indoles is reported. This photo click reaction occurs under mild conditions (415 nm) mostly in quantitative yield and provides stable silylated N,O‐acetals via light mediated siloxycarbene generation with subsequent indole‐N‐H insertion. We show that this very efficient and fully atom economic coupling process can be applied to conjugate complex systems, as documented by the clicking of carbohydrates with indole alkaloids. The method is also applicable to the conjugation of polymer chains. The linking acetal moiety can be readily cleaved and it is also shown that wavelength‐selective coupling and cleavage with acyl silanes bearing a second photoactive moiety is possible. This is documented by a successful polymerization/depolymerization sequence and by a polymer folding/unfolding process.

Metal-free Fenton-like photocatalysts based on covalent organic frameworks

Fenton reaction is one of the most efficient and widely used advanced oxidation processes (AOPs) for remediating the ever-growing water pollution. Metal-free photocatalysts for Fenton-like reactions have gathered enormous interest for their advantages including broad pH operation range and high stability. Herein, we report a metal-free covalent organic framework (COF)-based standalone photocatalyst with superior reactivity for Fenton-like reactions, surpassing well-known metal-free photocatalysts and being comparable to the metal-containing catalysts, as well as the remarkable generality. The outstanding performance can be attributed to large porosity, high density of photoactive triazine moiety, the suitable bandgap that originated from the partly interrupted π-conjugation, efficient charge separation and transfer due to the AA stacking, as well as abundant H2O2 binding sites and reaction centers. This work not only provides an efficient metal-free photocatalyst for Fenton-like reactions, but also paves the way for COFs towards industrial applications, aiming for a greener future.

Photoresponsive aggregation-induced emission polymer film for anti-counterfeiting

The development of solid-state smart materials, in particular those showing photoresponsive luminescence, is highly desirable for their cutting edge applications in displays, sensors, data-storage, and anti-counterfeiting. However, to achieve both excellent photoresponsive performance and bright luminescence in solid state remains challenge. Herein, we integrate a novel photochromic fluorophore YL into flexible polymer chains, thereby enabling the resultant polymer PYL with reversible photoisomerization upon aggregation. Remarkably, the polymer PYL possesses excellent photochromic properties and aggregation-induced emission (AIE) activity, which can be attributed to the photoactive YL moiety. Upon light exposure, its film exhibits reversibly off-to-on fluorescent modulation with quick response, high emission efficiency and signal contrast, sharply different from the weak emission in solution. The novel photoresponsive AIE polymer with invisible/visible color and fluorescence transformation allows for advanced anti-counterfeiting applications. This work provides an efficient platform for constructing solid-state photocontrollable luminescent materials.

2D Porphyrinic Metal-Organic Frameworks Featuring Rod-Shaped Secondary Building Units.

Metal-organic frameworks (MOFs) encompass a rapidly expanding class of materials with diverse potential applications including gas storage, molecular separation, sensing and catalysis. So-called ‘rod MOFs’, which comprise infinitely extended 1D secondary building units (SBUs), represent an underexplored subclass of MOF. Further, porphyrins are considered privileged ligands for MOF synthesis due to their tunable redox and photophysical properties. In this study, the CuII complex of 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H2L-CuII, where H2 refers to the ligand’s carboxyl H atoms) is used to prepare two new 2D porphyrinic rod MOFs PROD-1 and PROD-2. Single-crystal X-ray analysis reveals that these frameworks feature 1D MnII- or CoII-based rod-like SBUs that are coordinated by labile solvent molecules and photoactive porphyrin moieties. Both materials were characterised using infrared (IR) spectroscopy, powder X-ray diffraction (PXRD) spectroscopy and thermogravimetric analysis (TGA). The structural attributes of PROD-1 and PROD-2 render them promising materials for future photocatalytic investigations.

Disentangling Chromophore States in a Reversibly Switchable Green Fluorescent Protein: Mechanistic Insights from NMR Spectroscopy.

The photophysical properties of fluorescent proteins, including phototransformable variants used in advanced microscopy applications, are influenced by the environmental conditions in which they are expressed and used. Rational design of improved fluorescent protein markers requires a better understanding of these environmental effects. We demonstrate here that solution NMR spectroscopy can detect subtle changes in the chemical structure, conformation, and dynamics of the photoactive chromophore moiety with atomic resolution, providing such mechanistic information. Studying rsFolder, a reversibly switchable green fluorescent protein, we have identified four distinct configurations of its p-HBI chromophore, corresponding to the cis and trans isomers, with each one either protonated (neutral) or deprotonated (anionic) at the benzylidene ring. The relative populations and interconversion kinetics of these chromophore species depend on sample pH and buffer composition that alter in a complex way the strength of H-bonds that contribute in stabilizing the chromophore within the protein scaffold. We show in particular the important role of histidine-149 in stabilizing the neutral trans chromophore at intermediate pH values, leading to ground-state cis-trans isomerization with a peculiar pH dependence. We discuss the potential implications of our findings on the pH dependence of the photoswitching contrast, a critical parameter in nanoscopy applications.