Triplet-triplet annihilation based light upconversion (UC) into the UV region is a process in which an organic molecular system, composed of a sensitizer and an annihilator, absorbs low-energy visible photons and via triplet annihilation converts them into higher-energy UV photons. Some of the areas where this process can be applied are photocatalysis [1], hydrogen generation by water splitting [2] and germicidal purification of contaminated water [3]. The majority of UC studies are focused on light conversion in solutions, whereas for broader practical applications thin films (solid phase) are required.
The aim of this study is to assess photophysical properties of UV annihilators, choose a suitable sensitizer by considering the energy levels of molecules, and optimize the annihilator concentrations to achieve high UC quantum yield (UCQY). The annihilators researched in this work are 2,5-diphenylfuran (PPF), 2,5-diphenyloxazole (PPO), 2,5-bis(3,5-di-tert- butylphenyl)furan (tB-PPF), and 2-Phenylbenzimidazole (2PB). The quantum yields were determined using an absolute measurement approach with an integrating sphere. The singlet and triplet exciton lifetimes, used to calculate the efficiency of inter- and intra-molecular transitions, were measured by time correlated single photon counting (TCSPC) and time-resolved phosphorescence spectroscopy with time-gated detection.
Thin film samples were prepared with annihilator concentrations ranging from 0.1% to 100% in a poly(methyl methacrylate) (PMMA) matrix. The 2PB and tB-PPF had low fluorescence quantum yield (FLQY), rendering these annihilators unsuitable for UC systems. In contrast, the FLQY of PPF and PPO reached 60% and 74% respectively. At high concentrations PPO exhibited a smaller spectral red shift, making it the most suitable for UC among the four annihilators.
A MR-TADF sensitizer (BNCzPTZ) was utilized for these UC systems, where the best performing film comprising of 25 %wt PPO and 0.1 %wt BNCzPTZ in a PMMA matrix achieved UCQY of 0.7%. Triplet energy transfer reached high efficiency of 97%. However, UCQY was limited by significantly lower FLQY (only 15% in this sample), and intersystem crossing efficiency (73%).
UCQY could be further improved by optimizing the sensitizer concentration. It is expected that the lower BNCzPTZ concentration would result in higher FLQY in the UC films due to reduced Förster resonance energy transfer back to sensitizer.