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 pi-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 pi-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.

Molecular photoswitches that absorb sunlight and store it in the form of chemical energy are attractive for applications in molecular solar thermal energy storage (MOST) systems. Typically, these systems utilize the absorbed energy to photoisomerize into a metastable form, which acts as an energy reservoir. Diarylethenes featuring aromatic ethene pi-linkers have garnered research interest in recent years as a promising class of T-type photoswitches, which undergo photocyclization from an aromatic ring-open form into a less aromatic or non-aromatic ring-closed form. Based on several recent computational and experimental studies, this perspective analyzes the potential of these switches for MOST applications. Specifically, we discuss how they can be made to simultaneously achieve high energy-storage densities, long energy-storage times, and high photocyclization quantum yields by tuning the aromatic character of the ethene pi-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 remote single-bond axis is a formidable challenge in the field of artificial molecular machines. Here, we present a proof-of-principle design of such photogears that is based on the possibility of using stereogenic substituents to control both the relative stabilities of two helical forms of the photogear and the double-bond photoisomerization reaction that connects them. The potential of the design was verified by quantum-chemical modeling through which photogearing was 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. A stereochemical approach to transmitting the directional double-bond rotary motion of light-driven molecular motors through space onto a remote single-bond axis is put forth and successfully tested by means of quantum-chemical modeling. A key result in the assessment of the approach is that the desired photogearing process is favorable compared to the undesired, free-standing single-bond rotation process ("slippage") with which it competes.**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. Quantum-chemical calculations combined with molecular-dynamics simulations reveal mechanisms for improving the quantum yields by which aromatic compounds form their non-aromatic Dewar isomers, with potential implications in solar-energy storage.

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

Using molecular dynamics analysis and a two-component diffusion model that accounts for the time-dependent crystal surface chemical reaction, we show by extensive numerical simulations that the recently observed prismatic facet growth suppression in single-crystal fibers is the combined action of self-shielding by crystal surface selectivity and self-channeling arising from a point effect due to fibers small diameters and large aspect ratios. We further show that the self-channeling leads to a pyramidal-face-aiming solute flow, resulting in accelerated single-crystal fiber growth. This mesoscopic stimulated matter growth acceleration theory can satisfactorily explain all experimental results reported to date. This new crystal fiber growth technology opens a realm of application possibilities for single-crystal fiber architectures in chip-size photonics.

A popular approach to the calculation of molecular excitation energies is to consider only equilibrium geometries and neglect the effects of thermal motion. Although this static approach is sensible for molecules with distinct potential-energy minima, its adequacy relative to dynamical approaches appears not to have been thoroughly tested. Here, we report a case study investigating how thermal motion accounted for by molecular dynamics simulations influences the optically bright state of astaxanthin, a carotenoid of broad photobiological interest that features 13 conjugated double bonds. Employing several different density functional methods, it is shown that thermal fluctuations in the conjugation result in the Boltzmann-weighted average excitation energies for this state being shifted by up to 0.05 eV relative to those obtained from purely static calculations. Accordingly, it is concluded that the effects of thermal motion on excitation energies of conjugated systems can be quite large even for molecules with distinct potential-energy minima.

The common approach to investigate the impact of aromaticity on excited-state proton transfer by probing the (anti)aromatic character of reactants and products alone is scrutinized by modelling such reactions involving 2-pyridone. Thereby, it is found that energy barriers can be strongly influenced by transient changes in aromaticity unaccounted for by this approach, particularly when the photoexcited state interacts with a second excited state. Overall, the modelling identifies a pronounced effect overlooked by most studies on this topic.

Bacteriophytochromes are photoreceptor proteins of widespread use as templates for the engineering of fluorescent proteins with emission maxima in the near-infrared regime beyond 650 nm ideally suited for deep-tissue imaging of living cells. The main challenge for such engineering is that native bacteriophytochromes have very low fluorescence quantum yields because of competing excited-state deactivation processes, which include both the well-known photoisomerization reaction of their linear tetrapyrrole chromophore and excited-state proton transfer reactions from the chromophore to the surrounding protein. Here, we describe how hybrid quantum mechanics/molecular mechanics modeling of the photochemistry of these proteins has provided valuable guidelines for strengthening the fluorescence through inhibition of the competing non-radiative processes. Specifically, based on the results of such modeling, we present a strategy to inhibit the photoisomerization on steric grounds and identify the most probable proton transfer reaction to exert a negative influence on the fluorescence quantum yields. It is our hope that these results will help stimulate further contributions from quantum chemistry toward realizing the potential for entirely new bioimaging applications commonly attributed to brightly near-infrared fluorescent bacteriophytochromes.

In this work, we take a different angle to the benchmarking of time-dependent density functional theory (TD-DFT) for the calculation of excited-state geometries by extensively assessing how accurate such geometries are compared to ground-state geometries calculated with ordinary DFT. To this end, we consider 20 medium-sized aromatic organic compounds whose lowest singlet excited states are ideally suited for TD-DFT modeling and are very well described by the approximate coupled-cluster singles and doubles (CC2) method, and then use this method and six different density functionals (BP86, B3LYP, PBE0, M06-2X, CAM-B3LYP, and omega B97XD) to optimize the corresponding ground- and excited-state geometries. The results show that although each hybrid functional reproduces the CC2 excited-state bond lengths very satisfactorily, achieving an overall root mean square error of 0.011 angstrom for all 336 bonds in the 20 molecules, these errors are distinctly larger than those of only 0.004-0.006 angstrom with which the hybrid functionals reproduce the CC2 ground-state bond lengths. Furthermore, for each functional employed, the variation in the error relative to CC2 between different molecules is found to be much larger (by at least a factor of 3) for the excited-state geometries than for the ground-state geometries, despite the fact that the molecules/states under investigation have rather uniform chemical and spectroscopic character. Overall, the study finds that even in favorable circumstances, TD-DFT excited-state geometries appear intrinsically and comparatively less accurate than DFT ground-state ones.