Photoswitches are organic or organometallic chromophores that undergo a reversible chemical transformation upon absorption of light. Among the most commonly studied photoswitches are stilbenes and azobenzenes, capable of efficient interconversion between cis and trans isomers. When one isomer is significantly less thermodynamically stable than the other, photoisomerization of the stable to the metastable isomer converts a fraction of the absorbed photon energy into excess free energy (chemical potential). If the metastable isomer is sufficiently inert at room temperature, its photoconversion provides a means of storing solar energy, which is recovered by triggering heat‐releasing thermal conversion of the metastable to the stable isomer. In other words, such a photoswitch acts as a battery that captures solar energy, stores it as chemical potential and releases it on demand as heat. This process is known as molecular solar thermal energy storage or a molecular solar thermal battery. Unlike the more established conventional solar thermal storage, which uses sunlight to heat, melt or vaporize material, molecular solar thermal energy storage does not require thermal insulation to prevent discharge but relies on the kinetic activation barrier separating the two isomers. Unlike solar‐to‐chemical energy conversion by photosplitting of H2O or photoreduction of CO2, which comprise open‐system cycles, photoswitches are thermodynamically closed storage media. Successful deployment of molecular solar thermal energy storage requires new photoswitches that combine a seemingly contradictory set of molecular parameters: a large difference in the free energies of the two isomers separated by a large kinetic barrier; a high quantum yield of photogeneration of the metastable isomer that itself is either photochemically inactive or transparent to sunlight; highly selective isomerizations that allow many charge/discharge cycles without accumulation of side‐products even at high discharge temperatures. While the optimal photoswitch for molecular solar thermal energy storage remains to be invented, a large body of empirical observations acquired in the past decade provides several potentially valuable starting points for such a search.

Incorporating hidden length into polymer chains can improve their mechanical properties, because release of the hidden length under mechanical loads enables localized strain relief without chain fracture. To date, the design of hidden length has focused primarily on the choice of the sacrificial bonds holding the hidden length together. Here we demonstrate the advantages of adding mechanochemical reactivity to hidden length itself, using a new mechanophore that integrates ( Z )-2,3-diphenylcyclobutene-1,4-dicarboxylate, with hitherto unknown mechanochemistry, into macrocyclic cinnamate dimers. Stretching a polymer of this mechanophore more than doubles the chain contour length without fracture. DFT calculations indicate that the sequential dissociation of the dimer, followed by cyclobutene isomerization at higher forces yields a chain fracture energy 11 times that of a simple polyester of the same initial contour length and preserves high energy-dissipating capacity up to ∼3 nN. In sonicated solutions cyclobutene isomerizes to two distinct products by competing reaction paths, validating the computed mechanochemical mechanism and suggesting an experimental approach to quantifying the distribution of single-chain forces under diverse loading scenarios.

We study the role of the dimer structure of light-harvesting complex II (LH2) in excitation transfer from the LH2 [without a reaction center (RC)] to the LH1 (surrounding the RC) or from the LH2 to another LH2. The excited and unexcited states of a bacteriochlorophyll (BChl) are modeled by a quasispin. In the framework of quantum open system theory, we represent the excitation transfer as the total leakage of the LH2 system and then calculate the transfer efficiency and average transfer time. For different initial states with various quantum superposition properties, we study how the dimerization of the B850 BChl ring can enhance the transfer efficiency and shorten the average transfer time.

5483wileyonlinelibrary.com biocompatibility. Accordingly, there has been a surge in the exploitation of CPNs in numerous exciting studies. [ 9 ] Covalent polymers, however, suffer from the need for tedious chemical modifi cation to tune the emission color of their corresponding nanoparticles.Supramolecular polymers, which are formed from low-molecular-weight monomers by noncovalent interactions, provide a promising scaffold for the fabrication of fl uorescent nanoparticles with well fi ne-tuned emission properties. [ 10 ] On the one hand, the small building blocks of supramolecular polymers provide synthetic accessibility superior to that of covalent polymers. On the other hand, the excitation energy transfer from energy donor to acceptor fl uorophores of the monomers occurs effi ciently in supramolecular copolymers. Such a process can be used to adjust the emission properties of functional materials. [ 11 ] Recently, we have reported the synthesis of water-dispersible nanospheres of hydrogen-bonded supramolecular polymers by the miniemulsion method. [ 12 ] Bis-ureidopyrimidinone (Bis-UPy) monomers polymerized through the quadruple H-bonding of the ureidopyrimidinone (UPy) motif to form supramolecular polymers, [ 13 ] which further aggregated into nanospheres in the emulsifi ed organic droplets. Herein, we prepared FNPs based on fl uorophore-functionalized Bis-UPys by the miniemulsion method ( Scheme 1 ). The emission properties of the nanoparticles could be fi ne-tuned by effi cient excitation energy transfer in co-assemblies of the energy donor and acceptor fl uorophore-based monomers. The photoswitchable FNPs with high on-off fl uorescence contrast (85%) were prepared by copolymerization of Bis-UPys containing fl uorophore and photochromic dithienylethene. The FNPs have been successfully applied in cellular imaging and as fl uorescent inks. Results and Discussion Synthesis, Preparation, and Characterization of FNPsFour fl uorophore-functionalized Bis-UPys were synthesized for the preparation of supramolecular polymer-based fl uorescent nanoparticles: Based on their different emission colors, fl uorophore 9,10-diphenylanthracene, 1,3,5,7-tetramethyl boron dipyrromethene (BODIPY), 3,5-dithiolated BODIPY, and naphthalene bisimide were chosen as the functional groups A facile approach for the preparation of supramolecular polymer-based fl uorescent nanoparticles (FNPs) is reported. FNPs with homogeneous shape and size distribution are fabricated from low-molecular-weight molecules, and thus, different compositional constituents can be effi ciently incorporated via copolymerization. The emission color of the FNPs covers a wide region from blue to near infrared and can be easily tuned using effi cient excitation energy transfer. The photoswitchable fl uorescent nanoparticles with high on-off fl uorescence contrast are also simply prepared by copolymerization of monomers containing a fl uorophore and a photochromic unit. Our FNPs are successfully applied in living cell imaging and as fl uorescent inks.

The influence of mechanical force on the rates of model reductive elimination reactions depends on the structure of the force-transducing ligand and provides a measure of geometry changes upon reaching the transition state.

Artificial light-harvesting nanoparticles were prepared from supramolecular polymers comprised of pillar[5]arene with anthracene-derived donors and acceptors through host-guest interactions. The resulting water-dispersible nanoparticles displayed efficient energy transfer and excellent light harvesting ability in part because the steric bulk of pillar[5]arene suppressed the self-quenching of the chromophores.

Large or repeated mechanical loads usually degrade polymers by accelerating fragmentation of their backbones but rarely, they can cause new backbone bonds to form. When these new bonds form faster than the original bonds break, mechanical degradation may be arrested or reversed in real time. Exploiting such constructive remodeling has proven challenging because we lack an understanding of the competition between bond-forming and bond-breaking reactions in mechanically-stressed polymers. Here we report the molecular mechanism and analysis of constructive remodeling driven by the macroradical products of mechanochemical fragmentation of a hydrocarbon backbone. By studying the changing compositions of a random copolymer of styrene and butadiene sheared at 10 °C in the presence of different additives we developed an approach to characterizing this growth/fracture competition, which is generalizable to other underlying chemistries. Our results demonstrate that constructive remodeling is achievable under practically relevant conditions, requires neither complex chemistries, elaborate macromolecular architectures or free monomers, and is amenable to detailed mechanistic interrogation and simulation. These findings constitute a quantitative framework for systematic studies of polymers capable of autonomously counteracting mechanical degradation at the molecular level.

Pillar[6]arene-containing multilayer films have been fabricated by alternating deposition of a stoichiometric complex consisting of both pillar[6]arene and methyl viologen with a photoreactive polyelectrolyte, diazoresin (DAR). After photoinduced cross-linking of the multilayer films, the guest molecule, methyl viologen, can be removed. Then, multilayer films with artificial binding sites are fabricated. The films show good properties for molecular uptake and release as well as selectivity for molecules with methyl viologen moieties. It is anticipated that this kind of multilayer films have future applications in the fields of enrichment of molecular dyes and purification of methyl viologen-polluted water.