Photoswitches based on the norbornadiene-quadricyclane (NBD-QC) isomer pair capture solar energy by undergoing the NBD→QC photoisomerization reaction. The energy-storing QC isomer contains two highly strained three-membered rings, which implies that it may be susceptible to photodegradation through exposure to the ultraviolet (UV) component of sunlight. In the present study, we use nonadiabatic molecular dynamics simulations to investigate the UV-induced photochemistry of the QC isomer of a representative NBD-QC photoswitch with a push-pull substitution motif. A pattern recognition algorithm is employed to analyze and systematize the simulation results. We find that the predominant outcome of UV absorption, observed in 75 out of the 100 simulated trajectories, is the opening of the four-membered ring which belongs to the QC moiety, although in about half of the trajectories which follow this relaxation pathway, the ring promptly closes again. However, in a small fraction of trajectories, the QC moiety instead breaks up, leading to the formation of a carbene photoproduct. We hypothesize that that the breakup of the QC moiety is the first step in a reaction pathway which leads to the decomposition of the photoswitch.

Dithienylarene photoswitches are an emerging class of diarylethenes, in which photochemical ring closure is accompanied by pronounced changes in aromaticity. This distinctive feature enables new opportunities for applications, including molecular solar thermal energy (MOST) storage, a promising approach within renewable energy technologies. Molecular switches used as MOST systems are designed to absorb solar energy, store it in chemical bonds, and release it as heat. However, fast and efficient on-demand release of energy poses a significant challenge. Herein, we present the synthesis of a pyridine-appended, biphenylene-bridged dithienylarene switch, BPPyr, and demonstrate that its protonation state exerts a major influence on the kinetics of its thermal ring opening. Specifically, protonation of the pyridine units ([BPPyr-H2]2+ ) drastically reduces the thermal half-life of the ring-closed isomer from 2.9 h to 15 s in EtOH at 25 °C but, crucially, without significantly deteriorating the fatigue resistance upon repeated isomerization cycles. The parent BPPyr form with its original properties could be recovered via deprotonation; furthermore, N-methylation of BPPyr produced a water-soluble, ionic compound ([BPPyr-Me2]2+ ) switchable by visible light. Leveraging its unique properties also in a confined environment possibly akin to that of a future device, [BPPyr-Me2]2+ was incorporated in a gelatin matrix to form a sunlight-responsive hydrogel. Complementing the experimental results, quantum chemical calculations were performed to explain the mechanism by which protonation/methylation of BPPyr lowers the free-energy barrier for thermal ring opening. Altogether, this work advances the understanding of structure-property relationships for diarylethenes and provides a novel design for on-demand energy release by MOST systems.

The chemical basis underlying the striking blue hue of live H. americanus, known as American lobster, are studied in evolutionary biology and in polyene physical chemistry. Carapace colouration is generated by the antioxidant astaxanthin bound within the carotenoprotein crustacyanin complexes. Here, we present the ex vivo structure of the most abundant α-crustacyanin and β-crustacyanin forms, determined respectively by cryo-electron microscopy and X-ray crystallography to a resolution of 2.75 Å. Our structural analysis reveals α-crustacyanin as an elongated arrangement of β-crustacyanin heterodimers tethered by an heptatricopeptide repeat (HPR) protein. In vitro complex formation between the β-crustacyanin unit with a synthetic heptatricopeptide reproduces the observed blue colour of α-crustacyanin, identifying the HPR protein, in concert with crustacyanins, as contributor in tuning carapace colour. Overall, these results explain how nature adjusts the colour across the entire visible spectrum by exploiting the bathochromic shift of astaxanthin from its unbound red form (λmax = 472 nm) firstly to the β-crustacyanin violet bound form (λmax = 591 nm), and then to the α-crustacyanin bound blue form (λmax = 631 nm).

Means to control rotary motion at the nanoscale are central to the design and operation of artificial molecular machines powered by such motion. For this task, it is natural to consider molecular gears, which are characterized by their ability to perform coupled rotations around two (or more) chemical bonds. However, most such gears rely on passive, thermal activation, which makes them sensitive to Brownian motion. In this concept, following a brief review of the historic development of molecular gears, we highlight some recent experimental and computational results that have helped show how this problem can now be addressed by means of the type of molecular photogearing achieved when the double-bond rotary motion produced by a light-activated molecular motor is transmitted through space onto a single-bond axis. Furthermore, we discuss the formidable challenge to maintain a preferred direction of rotation during this transmission, which is critical for performing mechanical work. Finally, we point out some research directions suitable for maximizing the future usefulness of molecular gears and photogears.

The possibility to use the reversible cycling of molecular photoswitches between isomeric forms as a means to store and release solar energy has stimulated the development of candidate systems based on several different core structures, such as the dihydroazulene/vinylheptafulvene (DHA/VHF) couple. However, a major challenge in these efforts is to simultaneously realize many of the performance criteria required of the switches for such applications. Here, we take on this challenge by first introducing an all-around performance descriptor that combines three key criteria (related to energy density, storage time and light-absorption characteristics), and by then using density functional theory (DFT) methods to calculate its values for 52 newly designed DHA/VHF switches. Through this approach, we are able to identify several switches with excellent overall properties that contain a structural motif absent in all DHA/VHF compounds considered for solar-energy storage in the existing literature. For some of these switches, we also provide retrosynthetic analyses for their preparation and perform DFT calculations to demonstrate that they form the energy-storing VHF isomer through a facile DHA  VHF photoisomerization reaction. All in all, we conclude that these switches show great promise for further development towards applications in solar-energy storage.

The harmonic oscillator model of aromaticity (HOMA) offers a straightforward route to quantifying aromaticity that requires no other information than the bond lengths of the conjugated ring in question. Given that such information is often readily obtainable from quantum-chemical calculations, it is pertinent to improve this parametrized model as much as possible. Here, a new version of HOMA is presented where, atypically, the corresponding parameters are derived from the actual bond lengths of both aromatic and antiaromatic (rather than nonaromatic) reference compounds, as calculated with a high-level method. The resulting model, which we denote HOMAc, covers CC, CN, NN, and CO bonds and is tested at eight different levels of theory for 45 (single-ring, multi-ring, carbocyclic, N,O-heterocyclic) molecules across the aromatic-antiaromatic spectrum. Thereby, it is found that HOMAc provides a description of aromaticity and antiaromaticity in better accord with magnetic, energetic, and π-delocalization-based reference data than does the standard parametrization of HOMA. Altogether, the results highlight the possibility to realize more reliable geometry-based probing of (anti)aromaticity with the use of HOMAc and with substantial freedom in the choice of quantum-chemical method.

The last decades have seen a wealth of progress in the design and synthesis of molecular motors for converting light energy into directed rotary motion around a double bond. Yet, realizing the full potential of these systems in the field of artificial molecular machines will inevitably require a breakthrough in the formidable challenge to construct molecular photogears for transmitting such motion through space and onto a remote single-bond axis, without losing control of the direction of rotation. Here, we unveil a surprisingly straightfoward mechanism for achieving this goal in a single photochemical step by incorporating a propeller-shaped barrelene motif into the protonated Schiff-base skeleton of a minimal light-driven molecular motor. Corroborating the mechanism by state-of-the-art computational modeling, our study also identifies strategies for optimizing the photogearing efficiency through modulation of steric interactions. Overall, the results of this work help establish a new route for constructing molecular photogears by combining molecular-motor and propeller-shaped structures.

A popular approach to developing molecular solutions for solar‐energy storage is based on exploiting the reactions of molecular photoswitches. However, given that the reactions in question are usually the reverse of one another, it becomes imperative to handle conflicting performance criteria when optimizing the reactions. Here, studying diarylethene switches operated by electrocyclization (for storing the solar energy) and cycloreversion (for releasing the solar energy) reactions, we show that these processes can be made to simultaneously exhibit the desired characteristics by introducing a tricyclic rather than monocyclic π‐linker as the bridge between the two aryl units. Specifically, we perform quantum chemical calculations to demonstrate that such a scenario is realizable by tailoring, using aromaticity, certain parts of the tricyclic structure for electrocyclization and other parts for cycloreversion. Furthermore, employing this strategy, we identify several diarylethene switches, each with their own unique tricyclic π‐linker, that concurrently meet key performance criteria like large energy‐storage densities and long energy‐storage times. Accordingly, we conclude that there appears to be considerable structural flexibility in implementing the ideas for efficient diarylethene‐based solar‐energy storage put forth in this work.

Realizing the potential of T-type diarylethene photoswitches for solar energy-storage applications requires simultaneously achieving high energy-storage densities and long storage times, which is possible by tuning the aromaticity of the π-linker.

The construction of molecular photogears that can achieve through-space transmission of the unidirectional double-bond rotary motion of light-driven molecular motors onto a single-bond axis is a formidable challenge in the field of artificial molecular machines. Here, we present a new design of such photogears that is based on the possibility to use stereogenic substituents to control both the relative stabilities of the two helical forms of the photogear and the double-bond photoisomerization that connects them. The potential of the design is verified by quantum-chemical modeling through which photogearing is found to be a favorable process compared to free-standing single-bond rotation (“slippage”). Overall, our study unveils a surprisingly simple approach to realizing unidirectional photogearing.

Molecular switches based on the norbornadiene-quadricyclane (NBD-QC) isomer pair are among the most promising candidates for applications in molecular solar thermal energy storage (MOST). In these compounds, solar energy is captured through a photoinduced [2 + 2] cycloaddition reaction whose mechanism is only partially understood. This holds true especially for NBD derivatives containing the type of push-pull substitution pattern that was previously proven necessary to attain reasonable photoisomerization quantum yields. In the present contribution, we report a computational investigation of the photochemistry of NBD-QC switches with precisely such a substitution pattern. Static calculations provide information on the structures of the excited electronic states involved in the photoinduced cycloaddition reaction, and the topographies of the relevant ground- and excited-state potential energy surfaces. Furthermore, nonadiabatic molecular dynamics (NAMD) simulations allow an estimation of the reaction time scale and quantum yield. The simulation results paint a detailed picture of the energy capture process: the photoinduced cycloaddition reaction begins in the spectroscopically bright excited state of the molecular switch. In the model compound for which we performed NAMD simulations, ring closing takes place on a time scale of roughly 150 fs, which makes it one of the fastest known photoisomerization reactions.

Dithienylethene photoswitches with an aromatic pi-linker as the bridge between the two thiophene units are attractive starting materials for developing molecular solar thermal energy (MOST) storage systems, partly because the aromaticity of their ring-open forms is a favorable feature with regard to the energy-storage densities of their ring-closed forms produced by photoinduced electrocyclization (photocyclization) reactions. At the same time, this typically leads to small barriers for their thermal cycloreversion reactions, which are not desirable in this context. Here, we use computational methods to show that this problem can be circumvented with polycyclic heteroaromatic pi-linkers. Specifically, through the tuning of the aromatic character of the individual rings of such a pi-linker (like indole or isoindole), it is shown to be possible to strike a delicate balance between the seemingly contrasting requirements of simultaneously achieving both a high energy-storage density and a large cycloreversion barrier. Furthermore, this design is also found to provide for a quick and efficient photocyclization reaction, owing to the onset of excited-state antiaromaticity in the pi-linker upon light absorption of the ring-open form. Altogether, dithienylethenes with polycyclic heteroaromatic pi-linkers appear to have both thermal and photochemical properties suitable for further development into future MOST systems. Through the incorporation of a polycyclic heteroaromatic pi-linker between their thiophene units, dithienylethene switches are shown computationally to exhibit a photocyclization reaction well exploitable for solar-energy storage, while also occupying a sweet spot for such applications where contrasting requirements on energy-storage densities and thermal cycloreversion barriers can be met. image

Photochemical reactions enabling efficient transformation of aromatic systems into energetic but stable non-aromatic isomers have a long history in organic chemistry. One recently discovered reaction in this realm is that where derivatives of 1,2-azaborine, a compound isoelectronic with benzene in which two adjacent C atoms are replaced by B and N atoms, form the non-hexagon Dewar isomer. Here, we report quantum-chemical calculations that explain both why 1,2-azaborine is intrinsically more reactive toward Dewar formation than benzene, and how suitable substitutions at the B and N atoms are able to increase the corresponding quantum yield. We find that Dewar formation from 1,2-azaborine is favored by a pronounced driving force that benzene lacks, and that a large improvement in quantum yield arises when the reaction of substituted 1,2-azaborines proceeds without involvement of an intermediary ground-state species. Overall, we report new insights into making photochemical use of the Dewar isomers of aromatic compounds.

Poly-CPDTBT, as typical low-band gap copolymers, have potential applications in organic bulk heterojunction solar cells. To have a clear picture of its excited-state processes, the first task is to understand their excited states, in particular, electronic character and relevant optical absorption. Herein, the low-lying singlet excited states of Poly-CPDTBT oligomers were investigated via Algebraic Diagrammatic Construction Second Order (ADC(2)) and time-dependent density functional theory (TDDFT) method with several functionals. Six CPDTBTN (N = 1–6) oligomers were taken as prototypes to study their excited states in detail. The results provide interesting clues to extrapolate the photophysical properties of such polymers with potential applications in photovoltaic materials. The result provided by ωB97XD functional gives good agreement with the experiment result. The vertical excitation energies of the four lowest excited states decrease almost linearly with increasing polymerization degree (N) for CPDTBTN (N = 1–6). The transition density analysis indicates that the local excitations (LE) and the short-distance charge transfer (CT) excitations between two adjacent CPDT and BT units are dominant for low-lying excited states for short oligomers. For the long-chain oligomers (trimer to hexamer), the transition density shows a ladder (or zigzag) pattern along the diagonal blocks at the planar geometry. For long oligomers, the whole chain is involved in the transitions, and the CT excitations only exist between two adjacent CPDT and BT units. The present work provides a valuable basis for understanding the excited-state processes of Poly-CPDTBT and other conjugated polymers that conduct solar energy conversions, which has great significance for the development of new solar cells.

The construction of molecular photogears that can achieve through-space transmission of the unidirectional double-bond rotary motion of light-driven molecular motors onto a single-bond axis is a formidable challenge in the field of artificial molecular machines. Here, we present a new design of such photogears that is based on the possibility to use stereogenic substituents to control both the relative stabilities of the two helical forms of the photogear and the double-bond photoisomerization that connects them. The potential of the design is verified by quantum-chemical modeling through which photogearing is found to be a favorable process compared to free-standing single-bond rotation (“slippage”). Overall, our study unveils a surprisingly simple approach to realizing unidirectional photogearing.

Stereochemically defined tetrasubstituted olefins are widespread structural elements of organic molecules and key intermediates in organic synthesis. However, flexible methods enabling stereodivergent access to E and Z isomers of fully substituted alkenes from a common precursor represent a significant challenge and are actively sought after in catalysis, especially those amenable to complex multifunctional molecules. Herein, we demonstrate that iterative dual-metal and energy transfer catalysis constitutes a unique platform for achieving stereodivergence in the difunctionalization of internal alkynes. The utility of this approach is showcased by the stereodivergent synthesis of both stereoisomers of tetrasubstituted β-boryl acrylates from internal alkynoates with excellent stereocontrol via sequential carboboration and photoisomerization. The reluctance of electron-deficient internal alkynes to undergo catalytic carboboration has been overcome through cooperative Cu/Pd-catalysis, whereas an Ir complex was identified as a versatile sensitizer that is able to photoisomerize the resulting sterically crowded alkenes. Mechanistic studies by means of quantum-chemical calculations, quenching experiments, and transient absorption spectroscopy have been applied to unveil the mechanism of both steps.