Upconversion enables the energy of two long wavelength photons to be combined, resulting in photoexcitation that would otherwise require a single short wavelength photon. Longer wavelength photons penetrate more deeply and so upconversion is especially useful for phototherapy and applications of photochemical reactions on a large scale. This process is shown on the cover where a green target absorbs two red incident rays and emits a single blue ray. See Huang et al.

The interface between an electrode and electrolyte is crucial for electrochemical devices. Charged and neutral species interact in this region, which can lead to compositional and structural changes of the electrode, in turn altering the interface itself. Further to this, an electrochemical system’s reaction efficiency and selectivity, and overall physical stability can degrade over time. Understanding how electrochemical interfaces behave before, during, and after operation is therefore critical. This Review discusses a range of complementary techniques to study this interface as shown on the cover where incident irradiation of different wavelengths interrogate a golden surface. See Pastor et al.

Solar reforming is the sunlight-driven transformation of waste feedstocks into valuable fuels and chemicals. It encompasses a set of emerging technologies that have the potential to support the energy and chemical industries as they transition towards a sustainable circular economy. This review discusses the concept, configurations and metrics of solar reforming, and proposes future directions. The cover depicts an artistic rendition of a solar reforming reactor where sunlight drives the simultaneous conversion of carbon dioxide to fuels and solid waste to chemicals. See Bhattacharjee et al.

The use of ratchet mechanisms to directionally bias molecular level dynamics is increasingly well understood but their potential for enabling thermodynamically unfavourable chemical transformations is less well recognised. Mechanisms of light-driven endergonic synthesis parallel the ratchet mechanisms that underpin the operation of artificial molecular motors. Advances in molecular biology, heterogeneous catalysis and artificial photosynthesis also reveal fundamentally related phenomena. The cover image highlights how ratchet mechanisms enable energetically uphill processes by providing an alternative pathway (analogous to Maxwell’s demon) for otherwise-forbidden endergonic chemical reactions (illustrated by the apparently unachievable task given to Sisyphus), powered by an orthogonal exergonic procedure, often through catalysis. Mastery of molecular ratchets could be transformative for expanding the chemist’s toolbox, offering new paradigms in reactivity, complexity and control. See Borsley et al.