Putting Photomechanical Switches to Work: An Ab Initio Multiple Spawning Study of Donor-Acceptor Stenhouse Adducts

Initial Conditions for Excited-State Dynamics in Solvated Systems: A Case Study

Simulating excited-state dynamics or computing spectra for molecules in condensed phases requires sampling the ground state to generate initial conditions. Initial conditions (or snapshots for spectra) are typically produced by QM/MM Boltzmann sampling following MM equilibration or optimization. Given the switch from a MM to a QM/MM potential energy surface, one should discard a set period of time (which we call the "healing time") from the beginning of the QM/MM trajectory. Ideally, the healing time is as short as possible (to avoid unnecessary computational effort), but long enough to equilibrate to the QM/MM ground state distribution. Healing times in previous studies range from tens of femtoseconds to tens of picoseconds, suggesting the need for guidelines to choose a healing time. We examine the effect of healing time on the nonadiabatic dynamics and spectrum of a first-generation Donor-Acceptor Stenhouse Adduct in chloroform. Insufficient healing times skew the branching ratio of ground state products and alter the relaxation time for one pathway. The influence of the healing time on the absorption spectrum is less pronounced, warning that the spectrum is not a sensitive indicator for the quality of a set of initial conditions for dynamics. We demonstrate that a reasonable estimate for the healing time can be obtained by monitoring the solute temperature during the healing trajectory. We suggest that this procedure should become standard practice for determining healing times to generate initial conditions for nonadiabatic QM/MM simulations in large molecules and condensed phases.

Complete kinetic and photochemical characterization of the multi-step photochromic reaction of donor-acceptor Stenhouse adducts

Donor-acceptor Stenhouse adducts (DASA) are negative photochromic compounds exhibiting a multi-step photoisomerization mechanism. Previous studies have described a first photoactivated step, followed by a thermally controlled one. This study emphasizes the key role of the intermediate species, using high-rate acquisition photokinetic absorption spectroscopy. We have investigated the multi-step processes at different temperatures and irradiation power values. For the first time, we have combined our experimental setup with a three-species photokinetic model to determine the kinetic constants and quantum yields of each step of a DASA compound. Finally, we have identified the key role of the intermediate species, showing that double irradiation experiments allow the tuning of the photochromic reaction.

Compartmentalizing Donor-Acceptor Stenhouse Adducts for Structure-Property Relationship Analysis

The development of photoswitches that absorb low energy light is of notable interest due to the growing demand for smart materials and therapeutics necessitating benign stimuli. Donor-acceptor Stenhouse adducts (DASAs) are molecular photoswitches that respond to light in the visible to near-infrared spectrum. As a result of their modular assembly, DASAs can be modified at the donor, acceptor, triene, and backbone heteroatom molecular compartments for the tuning of optical and photoswitching properties. This Perspective focuses on the electronic and steric contributions at each compartment and how they influence photophysical properties through the adjustment of the isomerization energetic landscape. An emphasis on current synthetic strategies and their limitations highlights opportunities for DASA architecture, and thus photophysical property expansion.

Solvent effects on the second harmonic responses of donor-acceptor Stenhouse adducts: from implicit to hybrid solvation models

The effect of conformational dynamics and solvent interactions on the second-order nonlinear optical (NLO) responses of the open and closed forms of a donor-acceptor Stenhouse adduct (DASA) are investigated by a mixed quantum/classical computational approach, which couples molecular dynamics (MD) simulations and time-dependent density functional theory (TD-DFT) calculations. The latter are further combined with various solvation schemes, including polarizable continuum models, hybrid QM/MM approaches using either non polarizable or polarizable electrostatic embedding, and QM/QM' schemes with explicit treatment of a few molecules of the first solvation shell. The performances of the different solvation models are discussed in the context of comparisons with experimental data obtained from hyper-Rayleigh scattering measurements.

Dynamic effects on the nonlinear optical properties of donor acceptor stenhouse adducts: insights from combined MD + QM simulations

The second-order nonlinear optical (NLO) responses of a donor-acceptor stenhouse adduct (DASA) are investigated by using a computational approach combining molecular dynamics simulations and density functional theory (DFT) calculations. Specific force fields for the open and closed photoswitching forms are first parameterized and validated according to the Joyce protocol, in order to finely reproduce the geometrical features and potential energy surfaces of both isomers in chloroform solution. Then, DFT calculations are performed on structural snapshots extracted at regular time steps of the MD trajectories to address the influence of the thermalized conformational dynamics on the NLO responses related to hyper-Rayleigh scattering (HRS) experiments. We show that accounting for the structural dynamics largely enhances the HRS hyperpolarizability (βHRS) compared to DFT calculations considering solely equilibrium geometries, and greatly improves the agreement with experimental measurements. Furthermore, we show that the NLO responses of the NLO-active open form are correlated with the bond order alternation along the triene bridge connecting the donor and acceptor moieties, which is rationalized using simple essential state models.

Robust, Ultrafast and Reversible Photoswitching in Bulk Polymers Enabled by Octupolar Molecule Design

Improving the photoswitching rate and robustness of photochromic molecules in bulk solids is paramount for practical applications but remains an on-going challenge. Here, we introduce an octupolar design paradigm to develop a new family of visible light organic photoswitches, namely multi-branched octupolar Stenhouse Adducts (MOPSAs) featuring a C3-symmetrical A3-(D-core) architecture with a dipolar donor-acceptor (D-A) photochrome in each branch. Our design couples multi-dimensional geometric and electronic effects of MOPSAs to enable robust ultrafast reversible photoswitching in bulk polymers. Specifically, the optimal MOPSA (4 wt %) in commercial polyurethane films accomplishes nearly 100 % discoloration in 6 s under visible light with ∼ 100 % thermal-recovery in 17.4 s at 60 °C, while the acquired kinetics constants are 3∼7 times that of dipolar DASA counterpart and 1∼2 orders of magnitude higher than those of reported DASAs in polymers. Importantly, the MOPSA-doped polymer films sustain 500 discoloration/recovery cycles with slow degradation, superior to the existing DASAs in polymers (≤30 cycles). We discover that multi-dipolar coupling in MOPSA enables enhanced polarization and electron delocalization, promoting the rate-determining thermal cyclization, while the branched and non-planar geometry of MOPSA induces large free volume to facilitate the isomerization. This design can be extended to develop spiropyran or azobenzene-based ultrafast photochromic films. The superior photoswitching performance of MOPSAs together with their high-yield and scalable synthesis and facile film processing inspires us to explore their versatile uses as smart inks or labels for time-temperature indicators, optical logic encryption and multi-levelled data encryption.

Dynamical Effects of Solvation on Norbornadiene/Quadricyclane Systems

Molecules that can undergo reversible chemical transformations following the absorption of light, the so-called molecular photoswitches, have attracted increasing attention in technologies, such as solar energy storage. Here, the optical and thermochemical properties of the photoswitch are central to its applicability, and these properties are influenced significantly by solvation. We investigate the effects of solvation on two norbornadiene/quadricyclane photoswitches. Emphasis is put on the energy difference between the two isomers and the optical absorption as these are central to the application of the systems in solar energy storage. Using a combined classical molecular dynamics and quantum mechanical/molecular mechanical computational scheme, we showcase that the dynamic effects of solvation are important. In particular, it is found that standard implicit solvation models generally underestimate the energy difference between the two isomers and overestimate the strength of the absorption, while the explicit solvation spectra are also less red-shifted than those obtained using implicit solvation models. We also find that the absorption spectra of the two systems are strongly correlated with specific dihedral angles. Altogether, this highlights the importance of including the dynamic effects of solvation.

Visible light-responsive materials: the (photo)chemistry and applications of donor-acceptor Stenhouse adducts in polymer science

Donor-acceptor Stenhouse adduct (DASA) photoswitches have gained a lot of attention since their discovery in 2014. Their negative photochromism, visible light absorbance, synthetic tunability, and the large property changes between their photoisomers make them attractive candidates over other commonly used photoswitches for use in materials with responsive or adaptive properties. The development of such materials and their translation into advanced technologies continues to widely impact forefront materials research, and DASAs have thus attracted considerable interest in the field of visible-light responsive molecular switches and dynamic materials. Despite this interest, there have been challenges in understanding their complex behavior in the context of both small molecule studies and materials. Moreover, incorporation of DASAs into polymers can be challenging due to their incompatibility with the conditions for most common polymerization techniques. In this review, therefore, we examine and critically discuss the recent developments and challenges in the field of DASA-containing polymers, aiming at providing a better understanding of the interplay between the properties of both constituents (matrix and photoswitch). The first part summarizes current understanding of DASA design and switching properties. The second section discusses strategies of incorporation of DASAs into polymers, properties of DASA-containing materials, and methods for studying switching of DASAs in materials. We also discuss emerging applications for DASA photoswitches in polymeric materials, ranging from light-responsive drug delivery systems, to photothermal actuators, sensors and photoswitchable surfaces. Last, we summarize the current challenges in the field and venture on the steps required to explore novel systems and expand both the functional properties and the application opportunities of DASA-containing polymers.

Nonlinear Optical Responses of Photoswitchable Donor-Acceptor Stenhouse Adducts

This work combines hyper-Rayleigh scattering (HRS) experiments performed in the NIR range (1.30 and 1.60 μm) and quantum chemical calculations to provide a comprehensive description of the second harmonic generation (SHG) responses of donor-acceptor Stenhouse adducts (DASAs). Representative derivatives of the three generations of DASAs, which differ by the nature of their electron-donating and withdrawing moieties and also include clickable species, have been synthesized and their photoswitching behavior fully characterized. The HRS measurements allow us to establish relationships between the magnitude of the SHG response of open forms and the nature of the donor and acceptor groups. The largest SHG responses are obtained for derivatives incorporating either a barbituric acid or an indanedione acceptor unit, while N-methylaniline appears as the most efficient donor group. The calculations support well the experimental data and show that high hyperpolarizabilities are associated to low excitation energies and large extent of the photoinduced intramolecular charge transfer, which enhances the dipole moment variation between the ground and first dipole-allowed electronic excited state. In addition, a complete investigation of the photoswitching kinetics of DASAs in chloroform solution shows important differences, highlighting in particular the role of the donor group on the photoswitching efficiency.

Revisiting photocyclization of the donor-acceptor stenhouse adduct: missing pieces in the mechanistic jigsaw discovered

Donor-acceptor Stenhouse adducts (DASA) have recently emerged as a class of visible-light-induced photochromic molecular switches, but their photocyclization mechanism remains puzzling and incomplete. In this work, we carried out MS-CASPT2//SA-CASSCF calculations to reveal the complete mechanism of the dominant channels and possible side reactions. We found that a new thermal-then-photo isomerization channel, i.e., EEZ → EZZ → EZE, other than the commonly accepted EEZ → EEE → EZE channel, is dominant in the initial step. Besides, our calculations rationalized why the expected byproducts ZEZ and ZEE are unobserved and proposed a competitive stepwise channel for the final ring-closure step. The findings here redraw the mechanistic picture of the DASA reaction by better accounting for experimental observations, and more importantly, provide critical physical insight in understanding the interplay between thermal- and photo-induced processes widely present in photochemical synthesis and reactions.

Enhanced Sampling Aided Design of Molecular Photoswitches

Advances in the evolving field of atomistic simulations promise important insights for the design and fundamental understanding of novel molecular photoswitches. Here, we use state-of-the-art enhanced simulation techniques to unravel the complex, multistep chemistry of donor-acceptor Stenhouse adducts (DASAs). Our reaction discovery workflow consists of enhanced sampling for efficient chemical space exploration, refinement of newly observed pathways with more accurate ab initio electronic structure calculations, and structural modifications to introduce design principles within future generations of DASAs. We showcase our discovery workflow by not only recovering the full photoswitching mechanism of DASA but also predicting a plethora of new plausible thermal pathways and suggesting a way for their experimental validation. Furthermore, we illustrate the tunability of these newly discovered reactions, leading to a potential avenue for controlling DASA dynamics through multiple external stimuli. Overall, these insights could offer alternative routes to increase the efficiency and control of DASA's photoswitching mechanism, providing new elements to design more complex light-responsive materials.

A multi-stage single photochrome system for controlled photoswitching responses

The ability of molecular photoswitches to convert on/off responses into large macroscale property change is fundamental to light-responsive materials. However, moving beyond simple binary responses necessitates the introduction of new elements that control the chemistry of the photoswitching process at the molecular scale. To achieve this goal, we designed, synthesized and developed a single photochrome, based on a modified donor-acceptor Stenhouse adduct (DASA), capable of independently addressing multiple molecular states. The multi-stage photoswitch enables complex switching phenomena. To demonstrate this, we show spatial control of the transformation of a three-stage photoswitch by tuning the population of intermediates along the multi-step reaction pathway of the DASAs without interfering with either the first or final stage. This allows for a photonic three-stage logic gate where the secondary wavelength solely negates the input of the primary wavelength. These results provide a new strategy to move beyond traditional on/off binary photochromic systems and enable the design of future molecular logic systems.

Controlling the Isomerization of Photoresponsive Molecules through a Limiting Tautomerization Strategy

Controlling the multistage photoresponsivity remains a challenge, in part, due to the spontaneous tautomerization between isomers. Herein, we present a strategy to access three independent states (linear, cyclic keto, and cyclic enolate) of crown ether (CE)-substituted donor-acceptor Stenhouse adducts (DASAs) by limiting the tautomerization of the closed isomers. The linear-cyclic keto isomerization is reversibly triggered by treatment with metal ions (Na⁺ or K⁺) and CE, while the linear-cyclic enolate isomerization is induced by green light and heat. Density functional theory and molecular dynamics calculation results suggest that the steric effect and supramolecular interaction between the electron-donating and electron-withdrawing moieties play an important role in hindering the tautomerization between cyclic keto and cyclic enolate DASA-CE. The strategy to influence key steps in the photoswitching process inspires well-controlled multistage isomerization of photoresponsive molecules.

Improving the kinetics and dark equilibrium of donor-acceptor Stenhouse adduct by triene backbone design

DFT calculations were used to find an optimal substitution site on the triene backbone of a donor-acceptor Stenhouse adduct photoswitch to tune the equillibrium and switching kinetics of DASA without modifying the donor and acceptor groups. Using this approach we demonstrate a new means to tuning DASA based photoswitches by increasing the energy of the closed form relative to the open form. To highlight the potential of this approach a new DASA derivative bearing a methyl substituent on the 5-position of the triene was synthesized and the effect of this substitution was studied using ¹H NMR spectroscopy, time-dependent UV-Vis and solvatochromic analysis. The new DASA derivative shows a higher dark equillibrium, favoring the open form, and drastically faster thermal recovery than the unsubstituted derivative with the same donor and acceptor.

In Silico Discovery of Multistep Chemistry Initiated by a Conical Intersection: The Challenging Case of Donor-Acceptor Stenhouse Adducts

Detailed mechanistic understanding of multistep chemical reactions triggered by internal conversion via a conical intersection is a challenging task that emphasizes limitations in theoretical and experimental techniques. We present a discovery-based, hypothesis-free computational approach based on first-principles molecular dynamics to discover and refine the switching mechanism of donor-acceptor Stenhouse adducts (DASAs). We simulate the photochemical experiment in silico, following the "hot" ground state dynamics for 10 ps after photoexcitation. Using state-of-the-art graphical processing units-enabled electronic structure calculations we performed in total ∼2 ns of nonadiabatic ab initio molecular dynamics discovering (a) critical intermediates that are involved in the open-to-closed transformation, (b) several competing pathways which lower the overall switching yield, and (c) key elements for future design strategies. Our dynamics describe the natural evolution of both the nuclear and electronic degrees of freedom that govern the interconversion between DASA ground-state intermediates, exposing significant elements for future design strategies of molecular switches.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Donor-Acceptor Stenhouse Adducts: Exploring the Effects of Ionic Character

The effects of solution-state dielectric and intermolecular interactions on the degree of charge separation provide a route to understanding the switching properties and concentration dependence of donor-acceptor Stenhouse adducts (DASAs). Through solvatochromic analysis of the open-form DASA in conjunction with X-ray diffraction and computational theory, we have analyzed the ionic character of a series of DASAs. First- and third-generation architectures lead to a higher zwitterionic resonance contribution of the open form and a zwitterionic closed form, whereas the second-generation architecture possesses a less charge-separated open form and neutral closed form. This can be correlated with equilibrium control and photoswitching solvent compatibility. As a result of the high contribution of the zwitterionic resonance forms of first- and third-generation DASAs, we were able to control their switching kinetics by means of ion concentration, whereas second-generation DASAs were less affected. Importantly, these results show how the previously reported concentration dependence of DASAs is not universal, and that DASAs with a more hybrid structure in the open form can achieve photoswitching at high concentrations.

Redesigning donor-acceptor Stenhouse adduct photoswitches through a joint experimental and computational study

Many studies have recently explored a new class of reversible photoswitching compounds named Donor-Acceptor Stenhouse Adducts (DASAs). Upon light irradiation, these systems evolve from a coloured open-chain to a colourless closed-ring form, while the thermal back-reaction occurs at room temperature. In order to fulfill the requirements for different applications, new molecules with specific properties need to be designed. For instance, shifting the activation wavelength towards the red part of the visible spectrum is of relevance to biological applications. By using accurate computational calculations, we have designed new DASAs and predicted some of their photophysical properties. Starting from well-studied donor and acceptor parts, we have shown that small chemical modifications can lead to substantial changes in both photophysical and photoswitching properties of the resulting DASAs. Furthermore, we have also analysed how these substitutions impact the electronic structure of the systems. Finally, some pertinent candidates have been successfully synthesized and their photoswitching properties have been characterized experimentally.

The modulation effect of pi–pi interactions on the electronic and photochromic properties of viologen complexes containing N,N′-bis(carboxyethyl)-4,4′-bipyridinium

Two viologen complexes containing N,N′-bis(carboxyethyl)-4,4′-bipyridinium (BCEbpy) were prepared, namely [Zn(H₂O)₆]·(BCEbpy)·(p-BDC)·3H₂O (1) and [Co(H₂O)₆]·(BCEbpy)·(p-BDC)·3H₂O (2) (p-H₂BDC = 1,4-benzenedicarboxylic acid), and their crystal structures, photochromism, frontier molecular orbitals, Hirshfeld surfaces and 2D fingerprint plots were investigated. The modulation effects of pi–pi interactions were explored on the electronic and photochromic properties of compounds 1 and 2. Due to the existence of photo-response viologen radicals, both complexes 1 and 2 display excellent photo-response properties in the sequence 1 < 2. The results indicate that compound 1 exhibits intramolecular electron transfer; compound 2 exhibits both intramolecular and intermolecular electron transfer, which is mainly due to the change of electronic and steric structures caused by pi–pi interactions with a faster photo-response rate than that of compound 1. The donor–acceptor modes, matching principles and inter/intramolecular atom–atom close contacts were illustrated by the density functional theory (DFT)-B3LYP/6-311(d,p) method.

Two viologen complexes containing BCEbpy were prepared and displayed excellent photo-response properties by the modulation effect of pi–pi interactions.

Photophysical Properties of Donor-Acceptor Stenhouse Adducts and Their Inclusion Complexes with Cyclodextrins and Cucurbit[7]uril

Donor-acceptor Stenhouse adducts (DASAs) are a novel class of solvatochromic photoswitches with increasing importance in photochemistry. Known for their reversibility between open triene and closed cyclized states, these push-pull molecules are applicable in a suite of light-controlled applications. Recent works have sought to understand the DASA photoswitching mechanism and reactive state, as DASAs are vulnerable to irreversible "dark switching" in polar protic solvents. Despite the utility of fluorescence spectroscopy for providing information regarding the electronic structure of organic compounds and gaining mechanistic insight, there have been few studies of DASA fluorescence. Herein, we characterize various photophysical properties of two common DASAs based on Meldrum's acid and dimethylbarbituric acid by fluorescence spectroscopy. This approach is applied in tandem with complexation by cyclodextrins and cucurbiturils to reveal the zwitterionic charge separation of these photoswitches in aqueous solution and the protective nature of supramolecular complexation against degradative dark switching. DASA-M, for example, was found to form a weak host-guest inclusion complex with (2-hydroxypropyl)-γ-cyclodextrin, with a binding constant K = 60 M-1, but a very strong inclusion complex with cucurbit[7]uril, with K = 27,000 M-1. This complexation within the host cavity was found to increase the half-life of both DASAs in aqueous solution, indicating the significant and potentially useful stabilization of these DASAs by host encapsulation.