Abstract
We propose a new concept exploiting thermally activated delayed fluorescence (TADF) molecules as photosensitizers, storage units and signal transducers to harness solar thermal energy. Molecular composites based on the TADF core phenoxazine–triphenyltriazine (PXZ-TRZ) anchored with norbornadiene (NBD) were synthesized, yielding compounds PZDN and PZTN with two and four NBD units, respectively. Upon visible-light excitation, energy transfer to the triplet state of NBD occurred, followed by NBD → quadricyclane (QC) conversion, which can be monitored by changes in steady-state or time-resolved spectra. The small S1-T1 energy gap was found to be advantageous in optimizing the solar excitation wavelength. Upon tuning the molecule’s triplet state energy lower than that of NBD (61 kcal/mol), as achieved by another composite PZQN, the efficiency of the NBD → QC conversion decreased drastically. Upon catalysis, the reverse QC → NBD reaction occurred at room temperature, converting the stored chemical energy back to heat with excellent reversibility.
Highlights
We propose a new concept exploiting thermally activated delayed fluorescence (TADF) molecules as photosensitizers, storage units and signal transducers to harness solar thermal energy
Treating 2 with n-butyl lithium followed by reaction with cyanuric chloride yielded key intermediate 341, which reacted with carbon or oxygen nucleophiles to afford multiple norbornadiene-tethered species, PZDN, PZTN, and PZQN, with different linkers
PZDN and PZTN maximize the harvest of solar energy to trigger the forward NBD → QC reaction by PZDN → NBD triplet-triplet energy transfer
Summary
We propose a new concept exploiting thermally activated delayed fluorescence (TADF) molecules as photosensitizers, storage units and signal transducers to harness solar thermal energy. The development of photosensitizers to optimize MOST systems has been challenging, as the photosensitizer must have allowed electronic transitions similar to those of solar photons and a triplet state energy higher than that of NBD. In other words, this requirement implies that the energy between the S1 and T1 states is as close as possible for the photosensitizer to optimize energy harvesting. This requirement implies that the energy between the S1 and T1 states is as close as possible for the photosensitizer to optimize energy harvesting This viewpoint inspires the new concept of linking a photosensitizer to materials that exhibit thermally activated delayed fluorescence (TADF)[35–38] to achieve a suitable MOST system (see Fig. 1a). Since Adachi and coworkers[35] reported highly efficient OLEDs by harvesting the triplet state of TADF molecules, the quest to develop TADF molecules, understand their corresponding photophysics and identify applications in OLEDs has been one of the hottest research areas during the past decade
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