Structure-Property Relationships in Chromophore-Organogel Assemblies: Consequences for Triplet-Triplet Annihilation and Singlet Fission
Better utilization of the solar spectrum by absorbing photons that are currently wasted by solar cells, due to the bandgap mismatch with the semiconductor, will help to improve the device efficiency. Two low energy photons can be upconverted to one high energy photon through triplet-triplet annihilation (TTA-UC); one high energy photon can be down converted to form two charge carriers through singlet fission (SF). Here we have used the versatile and easily synthesized oxotriphenylhexanoate (OTHO) gelator as platform for TTA-UC and SF in a self-assembly structure by covalent incorporation of chromophores at well-defined positions in the gelator. The semi-solid matrix was hypothesized to facilitate the interaction between chromophores, such as energy transfer, electronic coupling, and exciton migration. To study the photophysical properties in TTA-UC, we combined the benchmark pair platinum octaethylporphyrin (PtOEP) as sensitizer and 9,10-diphenylanthracene (DPA) as annihilator, where DPA was attached to the OTHO gelator in different aromatic rings. For SF, 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-PC), a known chromophore to perform singlet fission in different environments, was attached to OTHO in different positions. We achieved TTA-UC in the self-assembly gels and the photophysical properties, the triplet energy transfer, and the triplet-triplet annihilation were affected by the position to where DPA is incorporated in OTHO. The different positions to where TIPS-PC was included on the OTHO backbone affected the gelation, the interactions between chromophore moieties, and the photophysical properties of the samples. This study suggests that easily synthesized self-assembly OTHO gels allow for control of chromophore interactions to be used as for up- and down conversion of photons. However, to increase the efficiency it is necessary to achieve a better mechanistic understanding of the photophysical process for TTA-UC and SF in a semi-solid matrix.
triplet-triplet annihilation upconversion