The Photoinduced E → Z Isomerization of Bisazobenzenes: A Surface Hopping Molecular Dynamics Study

Photoliquefaction and phase transition of m-bisazobenzenes give molecular solar thermal fuels with a high energy density

A series of m-bisazobenzene chromophores modified with various alkoxy substituents (1; methoxy, 2; ethoxy, 3; butoxy, 4; neopentyloxy) were developed for solvent-free molecular solar thermal fuels (STFs). Compounds (E,E)-1-3 in the crystalline thin film state exhibited photoliquefaction, the first example of photo-liquefiable m-bisazobenzenes. Meanwhile, (E,E)-4 did not show photoliquefaction due to the pronounced rigidity of the interdigitated molecular packing indicated by X-ray crystallography. The m-bisazobenzenes 1-4 exhibited twice the Z-to-E isomerization enthalpy compared to monoazobenzene derivatives, and the latent heat associated with the liquid-solid phase change further enhanced their heat storage capacity. To observe both exothermic Z-to-E isomerization and crystallization in a single heat-up process, the temperature increase of differential scanning calorimetry (DSC) must occur at a rate that does not deviate from thermodynamic equilibrium. Bisazobenzene 1 showed an unprecedented gravimetric heat storage capacity of 392 J g-1 that exceeds previous records for well-defined molecular STFs.

Photoresponsive Carbon-Azobenzene Hybrids: A Promising Material for Energy Devices

Advancements in renewable energy technology have been a hot topic in the field of photoresponsive materials for a sustainable community. Organic compounds that function as photoswitches is being researched and developed for use in a variety of energy storage systems. Azobenzene photoswitches can be used to store and release solar energy in solar thermal fuels. This review draws out the significance of azobenzene as photoswitches and its recent advances in solar thermal fuels. The recent developments of nano carbon templated azobenzene, their interactions and the effect of substituents are highlighted. The review also introduces their applications in solar thermal fuels and concludes with the challenges and future scope of the material. The advancements of solar thermal fuels with cost effective and desired optimal properties can be explored by scientists and engineers from different technological backgrounds.

Photoswitches with different numbers of azo chromophores for molecular solar thermal storage

We investigate three azo-chromophore-containing photoswitches (1, 2 and 3) for molecular solar thermal storage (MOST) based on reversible Z-E isomerization. 1, 2 and 3 are photoswitchable compounds that contain one, two and three azo chromophores, respectively. In solution, 1, 2 and 3 were charged via UV-light-induced E-to-Z isomerization. Among these three compounds, 2 exhibited an energy density as high as 272 ± 1.8 J g^-1, which showed the best energy storage performance. This result originated from the low molecular weight, a high degree of photoisomerization, and moderate steric hindrance of 2, which demonstrated the advantages of the meta-bisazobenzene structure for MOST. In addition, we studied the performances of these photoswitches in the solvent-free state. Only 1 showed photoinduced reversible solid-to-liquid transitions, which enabled the charging of 1 in a solvent-free state. The stored energy density for 1 in a solvent-free state was 237 ± 1.5 J g^-1. By contrast, 2 and 3 could not be charged in the solvent-free state due to the lack of solid-state photoisomerization. Our findings provide a better understanding of the structure-performance relationship for azobenzenebased MOST and pave the way for the development of high-density solar thermal fuels.

On the Computational Design of Azobenzene-Based Multi-State Photoswitches

In order to theoretically design multi-state photoswitches with specific properties, an exhaustive computational study is first carried out for an azobenzene dimer that has been recently synthesized and experimentally studied. This study allows for a full comprehension of the factors that govern the photoactivated isomerization processes of these molecules so to provide a conceptual/computational protocol that can be applied to generic multi-state photoswitches. From this knowledge a new dimer with a similar chemical design is designed and also fully characterized. Our theoretical calculations predict that the new dimer proposed is one step further in the quest for a double photoswitch, where the four metastable isomers could be selectively interconverted through the use of different irradiation sequences.

Multiple Azoarenes Based Systems - Photoswitching, Supramolecular Chemistry and Application Prospects

In the recent decades, the investigations on photoresponsive molecular systems with multiple azoarenes are quite popular in diverse perspectives ranging from fundamental understanding of multiple photoswitches, supramolecular chemistry, and various application prospects. In fact, several insightful and conceptual designs of such systems were investigated with architectural distinctions. In particular, the demonstration of applications such as data storage with the help of multistate or orthogonal photoswitches, light modulation of catalysis via cooperative switching, sensors using supramolecular host-guest interactions, and materials such as liquid crystals, grating, actuators, etc. are some of the milestones in this area. Herein, we cover the recent advancements in the research areas of multiazoarenes containing systems that have been classified into Type-1 {linear, non-linear, and core-based (A)}, Type-2 {tripodal C₃ -symmetric (C3)} and Type-3 {macrocyclic (M)} structural motifs.

(Hetero)arylazo-1,2,3-triazoles: "Clicked" Photoswitches for Versatile Functionalization and Electronic Decoupling

The development of light-responsive chemical systems often relies on the rational design and suitable incorporation of molecular photoswitches such as azobenzenes. Linking a photoswitch core with another π-conjugated molecular entity may give rise to intramolecular electronic coupling, which can dramatically impair the photoswitch function. Decoupling strategies have been developed based on additionally inserting a linker that can disrupt the through-bond electronic communication. Here we show that 1,2,3-triazole-a commonly used decoupling spacer-can be directly merged into the azoswitch core to construct a class of "self-decoupling" azoswitches called (hetero)arylazo-1,2,3-triazoles. Such azotriazole photoswitches are easily accessed and modularly functionalized by click chemistry. Their photoswitch property can be optimized by rational design of the substituent groups or heteroaryl rings, allowing (near-)quantitative E⇆Z photoisomerization yields and tunable Z-isomer thermal half-lives from days to years. Combined experimental and theoretical results demonstrate that the electronic structure of the photoswitch core is not substantially affected by various substituents attached to the 1,2,3-triazole unit, benefiting from its cross-conjugated nature. The combination of clickable synthesis, tunable photoswitch property, and self-decoupling ability makes (hetero)arylazo-1,2,3-triazoles intriguing molecular tools in developing photoresponsive systems with desired performance.

Non-adiabatic Excited-State Molecular Dynamics: Theory and Applications for Modeling Photophysics in Extended Molecular Materials

Optically active molecular materials, such as organic conjugated polymers and biological systems, are characterized by strong coupling between electronic and vibrational degrees of freedom. Typically, simulations must go beyond the Born-Oppenheimer approximation to account for non-adiabatic coupling between excited states. Indeed, non-adiabatic dynamics is commonly associated with exciton dynamics and photophysics involving charge and energy transfer, as well as exciton dissociation and charge recombination. Understanding the photoinduced dynamics in such materials is vital to providing an accurate description of exciton formation, evolution, and decay. This interdisciplinary field has matured significantly over the past decades. Formulation of new theoretical frameworks, development of more efficient and accurate computational algorithms, and evolution of high-performance computer hardware has extended these simulations to very large molecular systems with hundreds of atoms, including numerous studies of organic semiconductors and biomolecules. In this Review, we will describe recent theoretical advances including treatment of electronic decoherence in surface-hopping methods, the role of solvent effects, trivial unavoided crossings, analysis of data based on transition densities, and efficient computational implementations of these numerical methods. We also emphasize newly developed semiclassical approaches, based on the Gaussian approximation, which retain phase and width information to account for significant decoherence and interference effects while maintaining the high efficiency of surface-hopping approaches. The above developments have been employed to successfully describe photophysics in a variety of molecular materials.

Exciton localization in excited-state dynamics of a tetracene trimer: a surface hopping LC-TDDFTB study

Excitons in the molecular aggregates of chromophores are key participants in important processes such as photosynthesis or the functioning of organic photovoltaic devices. Therefore, the exploration of exciton dynamics is crucial. Here we report on exciton localization during excited-state dynamics of the recently synthesized tetracene trimer [Liu et al., Org. Lett., 2017, 19, 580]. We employ the surface hopping approach to nonadiabatic molecular dynamics in conjunction with the long-range corrected time-dependent density functional tight binding (LC-TDDFTB) method [Humeniuk and Mitrić, Comput. Phys. Commun., 2017, 221, 174]. Utilizing a set of descriptors based on the transition density matrix, we perform comprehensive analysis of exciton dynamics. The obtained results reveal an ultrafast exciton localization to a single tetracene unit of the trimer during excited-state dynamics, along with exciton transfer between units.

Computational design of a molecular triple photoswitch for wavelength-selective control

meta-Trisazobenzenes allow for wavelength-selective photo-switching of the individual azobenzene branches.

A small single molecule with multiple photoswitchable subunits, selectively and independently controllable by light of different wavelengths, is highly attractive for applications in multi-responsive materials and biological sciences. Herein, triple photoswitches are presented consisting of three independent azobenzene (AB) subunits that share a common central phenyl ring: the meta-trisazobenzenes (MTA). It is the unique meta-connectivity pattern leading to decoupling of all azo-subunits although they do overlap spatially. Based on this pattern, we design a triple MTA photoswitch, as proof-of-principle, with three different, electronically independent AB branches on the computer, which can be individually photo-excited to trigger ultra-fast E → Z isomerization at the selected AB branch.

Twist and Return-Induced Ring Strain Triggers Quick Relaxation of a ( Z)-Stabilized Cyclobisazobenzene

Continuous irradiation of the thermodynamically stable ( Z, Z)-cyclobisazobenzene does not lead to accumulation of a ( Z, E) or ( E, E) isomer as one might expect. Our combined experimental and computational investigation reveals that Z → E photoisomerization still takes place on an ultrafast time scale but induces large ring strain in the macrocycle, which leads to a very fast thermal back-isomerization, preventing photostationary accumulation of ( E)-isomers.

Controlling Heat Release from a Close-Packed Bisazobenzene-Reduced-Graphene-Oxide Assembly Film for High-Energy Solid-State Photothermal Fuels

A closed-cycle system for light-harvesting, storage, and heat release is important for utilizing and managing renewable energy. However, combining a high-energy, stable photochromic material with a controllable trigger for solid-state heat release remains a great challenge for developing photothermal fuels (PTFs). This paper presents a uniform PTF film fabricated by the assembly of close-packed bisazobenzene (bisAzo) grafted onto reduced graphene oxide (rGO). The assembled rGO-bisAzo template exhibited a high energy density of 131 Wh kg^-1 and a long half-life of 37 days owing to inter- or intramolecular H-bonding and steric hindrance. The rGO-bisAzo PTF film released and accumulated heat to realize a maximum temperature difference (DT) of 15 °C and a DT of over 10 °C for 30 min when the temperature difference of the environment was greater than100 °C. Controlling heat release in the solid-state assembly paves the way to develop highly efficient and high-energy PTFs for a multitude of applications.

Dynamics of Azobenzene Dimer Photoisomerization: Electronic and Steric Effects

While azobenzenes readily photoswitch in solution, their photoisomerization in densely packed self-assembled monolayers (SAMs) can be suppressed. Reasons for this can be steric hindrance and/or electronic quenching, e.g., by exciton coupling. We address these possibilities by means of nonadiabatic molecular dynamics with trajectory surface hopping calculations, investigating the trans → cis isomerization of azobenzene after excitation into the ππ* absorption band. We consider a free monomer, an isolated dimer and a dimer embedded in a SAM-like environment of additional azobenzene molecules, imitating in this way the gradual transition from an unconstrained over an electronically coupled to an electronically coupled and sterically hindered, molecular switch. Our simulations reveal that in comparison to the single molecule the quantum yield of the trans → cis photoisomerization is similar for the isolated dimer, but greatly reduced in the sterically constrained situation. Other implications of dimerization and steric constraints are also discussed.

Connectivity matters – ultrafast isomerization dynamics of bisazobenzene photoswitches

We have investigated the ultrafast dynamics of o-, m- and p-bisazobenzenes, which represent elementary building blocks for photoswitchable multiazobenzene nanostructures.