Contents 1. Introduction 561 2. Biomimetic Analogues of Hemoglobin and Myoglobin 563 2.1. The Proteins 563 2.2. Synthetic Analogues of Mb 565 2.2.1. The Molecular Origin of CO vs O 2 Discrimination by Mb and Hb 565 2.2.2. Electrostatic and H-Bonding Effects on Heme's Affinity for Small Molecules 570 2.2.3. Reversible Oxygen Carriers in Protic Media 572 2.3. Reversible Cooperative O 2 Carriers: Biomimetic Analogues of Hb 573 3. Functional Analogues of the Heme/Cu B Site of Cytochrome c Oxidase 574 3.1. The Enzyme 574 3.2. Methodology of Electrocatalytic Studies of Heme/Cu Analogues 576 3.3. General Considerations for the Design of Biomimetic Heme/Cu Analogues for Electrocatalytic Studies 577 3.4. Electrocatalytic O 2 Reduction by Simple Fe Porphyrins 578 3.5. Biomimetic Electrocatalytic Studies Prior to 2000 582 3.6. Role(s) of Cu B Based on Biomimetic Electrocatalytic Studies 583 4. Conclusions 585 5. Acknowledgments 586 6. Supporting Information Available 586 7. References 586

This review analyzes the inherent scientific challenges of realizing the potential of storing solar energy by photochemical generation of high-energy metastable compounds whose subsequent thermal isomerization releases large amounts of low-temperature (<500 K) heat. Such compounds may be stored at room temperature for days or months, regenerated using sunlight, and may be cycled many times without significant degradation. After highlighting some of the general challenges of solar energy conversion and storage, we discuss how recent advances in understanding the effect of molecular strain on the thermal and photochemical reactivity of small molecules offers new opportunities for a systematic approach to the molecular design of solar thermal fuels, defining the molecular properties which determine the fundamental limits of such a material's performance characteristics.

Force probes allow reaction rates to be measured as a function of the restoring force in a molecule that has been stretched or compressed. Unlike strain energy, approaches based on restoring force allow quantitative molecular understanding of phenomena as diverse as translation of microscopic objects by reacting molecules, crack propagation and mechanosensing. Conceptually, localized reactions offer the best opportunity to gain fundamental insights into how rates vary with restoring forces, but such reactions are particularly difficult to study systematically using microscopic force probes. Here, we show how a molecular force probe, stiff stilbene, simplifies force spectroscopy of localized reactions. We illustrate the capabilities of our approach by validating the central postulate of chemomechanical kinetics--force lowers the activation barrier proportionally to the difference in a single internuclear distance between the ground and transition states projected on the force vector--on a paradigmatic unimolecular reaction: concerted dissociation of the C-C bond.

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Incorporation of small reactive moieties, the reactivity of which depends on externally imposed load (so-called mechanophores) into polymer chains offers access to a broad range of stress-responsive materials. Here, we report that polymers incorporating spirothiopyran (STP) manifest both green mechanochromism and load-induced addition reactions in solution and solid. Stretching a macromolecule containing colorless STP converts it into green thiomerocyanine (TMC), the mechanically activated thiolate moiety of which undergoes rapid thiol-ene click reactions with certain reactive C=C bonds to form a graft or a cross-link. The unique dual mechanochemical response of STP makes it of potentially great utility both for the design of new stress-responsive materials and for fundamental studies in polymer physics, for example, the dynamics of physical and mechanochemical remodeling of loaded materials.