Does Triplet–Triplet Annihilation Favor Excimer Formation? A Case Study of Upconverted Fluorescence Emission with Zinc(II) Octaethylporphyrin-Doped 9,10-Bis[(trialkylsilyl)ethynyl]anthracene Nanoaggregates

An excimer is an excited species formed due to rapid energy (or charge) exchange between excited and ground-state molecules. The compound 9,10-bis[(triisopropylsilyl)ethynyl]anthracene had been previously reported to exhibit strong excimeric emission, along with weaker excitonic emission, in its nanoaggregate form. In this study, upconverted excimer emission was observed from two zinc(II) octaethylporphyrin (ZnOEP)-doped 9,10-bis[(trialkylsilyl)ethynyl]anthracene nanoaggregates upon photoexcitation of ZnOEP. Specifically, excimer emissions were monitored through direct singlet photoexcitation as well as from the triplet–triplet annihilation (TTA)-mediated upconversion process. Notably, the excimer-to-exciton emission ratios were found to be higher for the upconversion pathway in both doped nanoaggregate systems. A possible mechanistic explanation is provided for the enhanced excimer formation via the TTA process as compared to the direct singlet excitation route. Additionally, a correlation is drawn between TTA-mediated excimer formation and the effective triplet yield from singlet fission, offering valuable insights into the design of improved singlet fission materials. The possible consequence of favored excimer formation on TTA-UC and the efficiency of the organic light-emitting diode is also discussed.