Plasmon-enhanced two-photon-induced isomerization for highly-localized light-based actuation of inorganic/organic interfaces

Thermoplasmonic Effect Enables Indirect ON-OFF Control over the Z-E Isomerization of Azobenzene-Based Photoswitch

Proper formulation of systems containing plasmonic and photochromic units, such as gold nanoparticles and azobenzene derivatives, yields materials and interfaces with synergic functionalities. Moreover, gold nanoparticles are known to accelerate the Z-E isomerization of azobenzene molecules in the dark. However, very little is known about the light-driven, plasmon-assisted Z-E isomerization of azobenzene compounds. Additionally, most of the azobenzene-gold hybrids are prepared with nanoparticles of small, isotropic shapes and azobenzene ligands covalently linked to the surface of nanostructures. Herein, a formulation of an innovative system combining azobenzene derivative, gold nanorods, and cellulose nanofibers is proposed. The system's structural integrity relies on electrostatic interactions among components instead of covalent linkage. Cellulose, a robust scaffold, maintains the material's functionality in water and enables monitoring of the material's plasmonic-photochromic properties upon irradiation and at elevated temperatures without gold nanorods aggregation. Experimental evidence supported by statistical analysis suggests that the optical properties of plasmonic nanometal enable indirect control over the Z-E isomerization of the photochromic component with near-infrared irradiation by triggering the thermoplasmonic effect. The proposed hybrid material's dual plasmonic-photochromic functionality, versatility, and ease of processing render a convenient starting point for further advanced azobenzene-related research and 3D printing of macroscopic light-responsive structures.

Disequilibrating azobenzenes by visible-light sensitization under confinement

Photoisomerization of azobenzenes from their stable E isomer to the metastable Z state is the basis of numerous applications of these molecules. However, this reaction typically requires ultraviolet light, which limits applicability. In this study, we introduce disequilibration by sensitization under confinement (DESC), a supramolecular approach to induce the E-to-Z isomerization by using light of a desired color, including red. DESC relies on a combination of a macrocyclic host and a photosensitizer, which act together to selectively bind and sensitize E-azobenzenes for isomerization. The Z isomer lacks strong affinity for and is expelled from the host, which can then convert additional E-azobenzenes to the Z state. In this way, the host-photosensitizer complex converts photon energy into chemical energy in the form of out-of-equilibrium photostationary states, including ones that cannot be accessed through direct photoexcitation.

A Theoretical Investigation about Photoswitching of Azobenzene Adsorbed on Ag Nanoparticles

The optical properties of hybrid systems composed of silver nanoparticles (NPs) and azobenzene molecules were systematically investigated by combining the real-time time-dependent density functional theory (RT-TDDFT) approach with the classical electrodynamics finite difference time domain (FDTD) technique for the solution of Maxwell’s equations. In order to reflect the chemical interaction between azobenzene and metal more exactly, except for adsorbed molecules, a Ag cluster separated from NP was also dealt, using RT-TDDFT. We studied the different factors affecting the surface-enhanced absorption spectra. It was found that the electric field amplified by plasmon resonance of Ag NPs can have an overall enhancement to the molecular light absorption throughout the whole energy range. The resonance between the electron and the plasmon excitation results in a larger percentage of enhancement in the absorption spectrum the closer the resonance peak is. The enhancement ratio of the resonance peak is the largest. The plasmon–exciton coupling and the optical properties of different isolate isomers influence the line shape of the absorption spectra. The dipole interaction and electronic transfer between azobenzene molecules and Ag NPs also change the shape of spectroscopy from the absorption enhancement ratio and the location of the peak. Physical and chemical factors lead to photoswitching in these hybrid systems together.

Two-photon absorption and two-photon-induced isomerization of azobenzene compounds

The process of two-photon-induced isomerization occurring in various organic molecules, among which azobenzene derivatives hold a prominent position, offers a wide range of functionalities, which can be used in both material and life sciences. This review provides a comprehensive description of nonlinear optical (NLO) properties of azobenzene (AB) derivatives whose geometries can be switched through two-photon absorption (TPA). Employing the nonlinear excitation process allows for deeper penetration of light into the tissues and provides opportunities to regulate biological systems in a non-invasive manner. At the same time, the tight focus of the beam needed to induce nonlinear absorption helps to improve the spatial resolution of the photoinduced structures. Since near-infrared (NIR) wavelengths are employed, the lower photon energies compared to usual one-photon excitation (typically, the azobenzene geometry change from trans to cis form requires the use of UV photons) cause less damage to the biological samples. Herein, we present an overview of the strategies for optimizing azobenzene-based photoswitches for efficient two-photon excitation (TPE) and the potential applications of two-photon-induced isomerization of azobenzenes in biological systems: control of ion flow in ion channels or control of drug release, as well as in materials science, to fabricate data storage media, optical filters, diffraction elements etc., based on phenomena like photoinduced anisotropy, mass transport and phase transition. The extant challenges in the field of two-photon switchable azomolecules are discussed.

Regulation of optical properties for fluorescent triphenylamine-silver hybrid based on SPR effect

In this study, a two photon absorbing (TPA) material consisting silver nanoparticles and triphenylamine-thiol derivative (TBS) has been prepared through interfacial coordination effect according to soft-hard-acid-base principle. The interfacial structure and morphology of the hybrid are researched in detail. Linear and nonlinear optical properties of the hybrid are studied. Upon interfacial coordination, the hybrid shows red-shifted UV-Vis absorption, causing from enhanced electronic drawing strength due to existence of Ag atom. The results also indicate that the surface Plasmon resonance (SPR) effect of Ag nanoparticles (~6 nm) brings about enhancement in single photon fluorescence emission and two photon absorption. Compared with free TBS, Ag-TBS hybrids show higher TPA cross-section (δ), which is 8784 GM for TBS and up to 103876 GM for Ag-TBS hybrid, showing ~12 fold increase. Due to excellent TPA property, the hybrids have good application in the field of optical power limiting and its limiting threshold is 0.49 J/cm^-2. This type of interfacial coordination induced hybrid provides a promising strategy to regulate linear optical properties and optimize nonlinear performance.

Two-photon isomerization triggers two-photon-excited fluorescence of an azobenzene derivative

Photoisomerization provides a general means for photo-controlling molecular structure and function, and two-photon induced isomerization has some advantages over one-photon processes. Here, we report the two-photon absorption (TPA) induced optical properties and characteristics of an azobenzene derivative, namely N-(3,4,5-octanoxyphenyl)-N'-4-[(4-hydroxyphenyl)azophenyl]1,3,4-oxadiazole (AOB-t8). With the activation of red light, the photoproduct of cis-AOB-t8 isomers has been unambiguously identified, and the two-photon induced isomerization process of AOB-t8 in THF is characterized with the relevant rate constants of forward and backward reactions. The TPA coefficients and cross-sections of AOB-t8 at different wavelengths, and nonlinear optical refractive index are determined from femtosecond nonlinear optical measurements. Particularly, the results indicate that the two-photon excited emission can be effectively triggered by the two-photon-induced isomerization from trans- to cis-AOB-t8, demonstrating an on-switch from non-fluorescence to visible blue emission. Consistent with the observations from one-photon excitation, we suggest that the enhanced two-photon excited fluorescence of AOB-t8 solution originates from the combined contribution of cis-AOB-t8 monomer and its aggregates, i.e., the conformation transition from trans- to cis-AOB-t8 opens the emission channel and the self-assembly aggregation of cis-AOB-t8 isomers further enhances the fluorescence upon red light excitation.

Near-Field Optical Drilling of Sub-λ Pits in Thin Polymer Films

Under UV illumination, polymer films can undergo chain scission and contract. Using this effect, tightly focused laser light is shown to develop runaway near-field concentration that drills sub-100 nm pits through a thin film. This subwavelength photolithography can be controlled in real time by monitoring laser scatter from the evolving holes, allowing systematic control of the void diameter. Our model shows how interference between the substrate and film together with near-field focusing by the evolving crevice directs this formation and predicts minimum pit sizes in films of 100 nm thickness on gold substrates. The smallest features so far are 60 nm diameter pits using 447 nm light focused onto polystyrene through a ×100 objective (NA = 0.8). Such arrays of pits can be easily used as masks for fabricating more complex nanostructures, such as plasmonic nanostructures and biomicrofluidic devices. This demonstration shows the potential for harnessing near-field feedback in optical direct-writing for nanofabrication.

Enhanced photocatalytic activity of gold nanoparticles driven by supramolecular host–guest chemistry

Gold nanoparticles coated with cyclodextrins show enhanced plasmon-based photocatalytic activities by promoting catalyst–reactant approximationviasupramolecular host–guest complex formation.