The urgent need for sustainable energy solutions has propelled research into efficient energy storage methods. Solar thermal energy storage, which converts solar energy into heat and stores it for later use, offers a promising avenue for addressing intermittent solar power. However, current technologies often suffer from limitations in energy density, storage efficiency, and long-term stability. This research addresses these challenges by proposing a novel approach to solar thermal energy storage that leverages the unique properties of thermally activated delayed fluorescence (TADF) molecules. TADF materials exhibit high efficiency in converting light into triplet excitons, which can then be used to drive chemical reactions for energy storage. The core idea presented in this paper is to use a TADF molecule not just as a photosensitizer but also as an integral part of the energy storage unit and a signal transducer. This integrated approach offers the potential for significant improvements in efficiency and stability compared to conventional solar thermal energy storage systems. The study aims to demonstrate the feasibility of this concept by designing, synthesizing, and characterizing novel TADF-based molecular composites for solar thermal energy storage. The ultimate goal is to develop a system with high energy storage capacity, efficient energy conversion, and long-term cyclability for practical applications.

Existing solar thermal energy storage technologies encompass a range of approaches, each with its own set of advantages and disadvantages. Sensible heat storage, involving storing thermal energy in a material's temperature increase, is simple but suffers from low energy density. Latent heat storage, utilizing phase-change materials to store energy during phase transitions, offers higher energy density but can face challenges with slow heat transfer and material degradation. Thermochemical storage methods, based on reversible chemical reactions, often boast higher energy densities and can store energy at higher temperatures, but typically involve complex reaction processes and may lack reversibility. The literature reveals a need for materials with improved energy storage capacity, fast charging and discharging rates, and prolonged operational lifespan. TADF molecules, with their ability to efficiently harvest triplet excitons, emerge as promising candidates for enhancing the performance of thermochemical energy storage systems. Prior studies have explored TADF materials in various applications such as organic light-emitting diodes (OLEDs), but their potential in solar thermal energy storage has yet to be fully explored. This research builds upon existing knowledge of TADF and thermochemical storage to develop a novel, integrated system.

The researchers synthesized molecular composites based on the TADF core phenoxazine-triphenyltriazine (PXZ-TRZ) and norbornadiene (NBD). Three compounds were created: PZDN (two NBD units), PZTN (four NBD units), and PZQN (a variation with altered substitution). Visible light excitation was used to induce energy transfer to the triplet state of NBD, resulting in the isomerization of NBD to quadricyclane (QC). The efficiency of this photoisomerization was assessed using steady-state and time-resolved spectroscopy, including monitoring changes in fluorescence and phosphorescence. The triplet state energy levels of the TADF molecules were analyzed to optimize energy transfer to NBD. Proton NMR spectroscopy was employed to provide direct evidence of the NBD → QC photoisomerization by monitoring characteristic peaks associated with NBD and QC. Kinetic modeling was performed to quantify the photoconversion efficiency (Q.Y.eff). The reverse reaction, QC → NBD, was studied at room temperature and at elevated temperatures, with and without a cobalt tetraphenylporphyrin (CoTPP) catalyst. The kinetics of the reverse reaction were determined using temperature-dependent NMR spectroscopy, applying the Arrhenius equation and transition state theory. Differential scanning calorimetry (DSC) measurements were used to determine the enthalpy change (ΔH) during the QC → NBD isomerization. A durability test was conducted on PZDN to assess the reversibility and cycle life of the energy storage-release process.

The study successfully demonstrated the feasibility of using TADF molecules for solar thermal energy storage. The synthesized PZDN and PZTN composites showed efficient NBD → QC photoisomerization upon visible light excitation, with photoconversion efficiencies of 59.4% and 14.3%, respectively. This efficiency was linked to the proximity of the PXZ-TRZ core and the NBD moiety, with closer proximity leading to more efficient energy transfer. In contrast, PZQN, with its lower triplet state energy, exhibited significantly reduced photoisomerization efficiency (3.7%). Proton NMR studies provided clear evidence of the NBD → QC conversion and its reversibility. The reverse QC → NBD isomerization was achieved at room temperature using a CoTPP catalyst, indicating the potential for efficient energy release at ambient conditions. Kinetic analysis revealed a high activation energy (23.59 ± 0.5 kcal/mol) for the thermal reverse isomerization of PZDQC. DSC measurements confirmed the highly exothermic nature of the QC → NBD reaction, with a ΔH of –161.8 kJ/mol for PZDQC. The durability test showed excellent fatigue resistance for PZDN over five reaction cycles, highlighting the potential for long-term stability. Notably, the triplet state energy of the TADF molecules was shown to play a crucial role in determining the energy transfer efficiency to NBD. By tuning the triplet state energy to be slightly higher than that of NBD, efficient energy transfer and subsequent photoisomerization were achieved.

The findings address the need for improved solar thermal energy storage by demonstrating a novel approach using TADF molecules. The high photoconversion efficiency of PZDN, combined with the efficient and reversible thermal isomerization (with catalysis), showcases the potential for practical applications. The significant impact of the triplet state energy on the energy transfer efficiency highlights the importance of precise molecular design in optimizing these systems. The use of a catalyst for the reverse reaction mitigates the high activation energy barrier, leading to efficient energy release at room temperature. This study contributes significantly to the field by integrating TADF molecules into a thermochemical storage system, overcoming limitations faced by conventional technologies. The ability to tune the energy levels of TADF molecules and achieve high reversibility under ambient conditions represents a significant advancement in solar thermal energy storage.

This research successfully demonstrated a new approach to solar thermal energy storage using TADF molecules. The high photoconversion efficiency, coupled with efficient and reversible energy release, underscores the potential of TADF-based systems. The findings open avenues for further research into optimizing molecular design, exploring different TADF cores and NBD analogs, and investigating alternative catalytic strategies. The development of such efficient and durable solar thermal energy storage systems could contribute significantly to the global transition towards sustainable energy.

The study focused on a specific set of TADF molecules and NBD derivatives. Further research is needed to explore the generality of the approach with other TADF molecules and energy storage moieties. While the durability test showed good reversibility, long-term stability over thousands of cycles needs further investigation. The cost-effectiveness of large-scale synthesis and the potential environmental impact of the materials used require further assessment for practical implementation.