The development of solar energy can potentially meet the growing requirements for a global energy system beyond fossil fuels, but necessitates new scalable technologies for solar energy storage. One approach is the development of energy storage systems based on molecular photoswitches, so-called molecular solar thermal energy storage (MOST). Here we present a novel norbornadiene derivative for this purpose, with a good solar spectral match, high robustness and an energy density of 0.4 MJ kg⁻¹. By the use of heterogeneous catalyst cobalt phthalocyanine on a carbon support, we demonstrate a record high macroscopic heat release in a flow system using a fixed bed catalytic reactor, leading to a temperature increase of up to 63.4 °C (83.2 °C measured temperature). Successful outdoor testing shows proof of concept and illustrates that future implementation is feasible. The mechanism of the catalytic back reaction is modelled using density functional theory (DFT) calculations rationalizing the experimental observations.

Molecules capable of reversible storage of solar energy have recently attracted increasing interest, and are often referred to as molecular solar thermal energy storage (MOST) systems. Azobenzene derivatives have great potential as an active MOST candidate. However, an operating lab scale experiment including solar energy capture/storage and release has still not been demonstrated. In the present work, a liquid azobenzene derivative is tested comprehensively for this purpose. The system features several attractive properties, such as a long energy storage half-life (40 h) at room temperature, as well as an excellent robustness demonstrated by optically charging and discharging the molecule over 203 cycles without any sign of degradation (total operation time of 23 h). Successful measurements of solar energy storage under simulated sunlight in a microfluidic chip device have been achieved. The identification of two heterogeneous catalyst systems during testing enabled the construction of a fixed bed flow reactor demonstrating catalyzed back-conversion from cis to trans azobenzene at room temperature under flow conditions. The working mechanism of the more suitable catalytic candidate was rationalized by detailed density functional theory (DFT) calculations. Thus, this work provides detailed insights into the azobenzene based MOST candidate and identifies where the system has to be improved for future solar energy storage applications.

This study describes the role of two novel visible-light sensitive mono-allyl (TXA) and di-allyl sulfone (TXo2A) thioxanthone derivatives as photosensitizers and photoinitiators (PIs) for free-radical and cationic polymerizations (FRP and CP) and thiol–ene processes upon visible-light emitting diode exposure (LEDs@405 nm, 455 and 470 nm). The photoactivable groups incorporated into TXA and TXo2A have extended their absorption to the visible region up to 510 nm. TXo2A is the first example of a monomeric photoinitiator having two allyl side-chain groups. These new photoinitiating systems have first demonstrated unambiguously that they can be used as PIs and H donor molecules upon visible LED irradiation for the FRP of acrylates without additives. TXA and TXo2A also show highly efficient initiating properties when associated with judicious co-initiators i.e. an iodonium salt (Iod) as an electron acceptor, a tertiary amine (methyldiethanol amine, MDEA) as an electron donor, or a thiol derivative (trimethylolpropane tris (3-mercaptopropionate, TT) as a H-donor, under air and in laminate, for FRP, CP and thiol–ene reactions. These unprecedented initiating combinations promote high acrylate and epoxy conversions even under air, with a higher efficiency than the well-known CQ (and TX)-based photoinitiating systems used as references. As evidenced by nanosecond laser flash photolysis (LFP), electron paramagnetic resonance spin-trapping (EPR ST) and fluorescence/phosphorescence experiments, TXA (or TXo2A) can play a triple role, first as a proton/proton-coupled electron transfer initiator using MDEA or TT, as an electron donor when associated with Iod, and free-radical PIs following an intermolecular electron/proton transfer reaction. Interestingly, the photoinduced 3D interpenetrated polymer network (IPN) synthesized via the concomitant FRP and CP of an acrylate/epoxide monomer mixture upon LED@405 or 455 nm exposure has shown the capability to support viable cell proliferation without any cytotoxicity.

We report the first example of direct far-red endothermic triplet sensitizedZ-to-Ephotoswitching of azobenzene derivative (Azo1) in a condensed phase of a liquid Azo1 co-assembled within a liquid surfactant-protein bioplastic film in air.

The design of molecular solar fuels is challenging because of the long list of requirements these molecules have to fulfil: storage density, solar harvesting capacity, robustness, and heat release ability. All of these features cause a paradoxical design due to the conflicting effects found when trying to improve any of these properties. In this contribution, we will review different types of compounds previously suggested for this application. Each of them present several advantages and disadvantages, and the scientific community is still struggling to find the ideal candidate suitable for practical applications. The most promising results have been found using norbornadiene-based systems, although the use of other alternatives like azobenzene or dihydroazulene cannot be discarded. In this review, we primarily focus on highlighting the optical and photochemical aspects of these three families, discussing the recently proposed systems and recent advances in the field.