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Diketopyrrolopyrrole derivatives demonstrating moderate NIR-to-Vis TTA-mediated photon upconversion quantum yield (up to 3.8% out of 50%) and statistical probability f (up to 15.6%) are promoted as stable alternatives to a rubrene emitter.

Simplified thermal evaporation of rubrene in an inert atmosphere is demonstrated to alter the morphology of photon upconverting (UC) films, and subsequently ramp their NIR-to-vis UC quantum yield up to a record value of 1.2% (out of maximum 50%).

Rubrene is the most widely used triplet–triplet annihilation (TTA) emitter for NIR-to-vis photon upconversion (UC), however, strong singlet fission (SF) in the solid films quenches its emission and hampers practical TTA-UC applications. Herein, the issue was addressed by decorating the rubrene with sterically demanding 3,5-di-tert-butylphenyl side-moieties at the periphery and the core to result in 40-fold enhancement of the emission quantum yield. Nevertheless, the sterically crowded rubrenes were found to exhibit lower sensitized UC performance compared to the conventional rubrene, which was ascribed to inefficient triplet energy transfer from a sensitizer and poor TTA (for the core-modified rubrene only). By exploiting the distinct feature of rubrenes to simultaneously express both SF and TTA in the solid films, their TTA efficiency was assessed independently from TET in the sensitizer-free films. The results implied a trade-off between suppressed SF and enhanced TTA in the rubrene emitters, which could be addressed via careful selection of the degree of sterical hindrance and linking position of the side-moieties. Thorough analysis of the prompt and delayed fluorescence revealed that the bulky side-moieties at the periphery do not impede TTA, i.e., it is as efficient as that of unsubstituted rubrene, whereas these moieties linked directly to the core suppress TTA dramatically. The current study unveils an advantage of the peripheral linking vs. core linking pattern of rubrene emitters, thereby providing valuable insights for their rational modification towards improved NIR-to-vis UC efficiency in the solid state.

Low-power near-infrared (NIR)-to-visible (vis) photon upconversion (UC) systems are in demand for biomedical, photovoltaic and photocatalytic applications; however, the practical utilization is still hampered by low UC efficiency. Aiming to identify efficiency-limiting factors, particularly in metallophthalocyanine-sensitized rubrene systems operational in the NIR-vis range, we thoroughly assessed subsequent energy transfer steps in the TTA-mediated UC scheme. A key limiting factor in the optimized UC systems was found to be rubrene's low statistical probability (f = 15.5 ± 3%) to obtain a singlet from two triplets via TTA. The f estimated under the dominance of TTA attained by continuous-wave excitation, i.e. the regime frequently encountered (or desired) in practical applications, was determined to be 4 times lower as compared to that obtained under femtosecond pulsed-laser excitation conditions. The results also demonstrate that the benefit of achieving larger emitter concentrations by introducing solubility increasing alkyl groups into the emitter in anticipation of enhanced triplet energy transfer cannot outcompete the severely reduced statistical probability factor (f = 5.3 ± 1%) of t-butyl-substituted rubrene. The maximum UC quantum yield (ΦUC = 5.6 ± 1.2%) estimated and verified by two independent methods in the optimized Pd-phthalocyanine–rubrene system is among the largest reported for NIR-to-vis UC systems absorbing at >730 nm. ΦUC is defined here as the number of UC photons emitted per number of absorbed ones, implying a theoretical limit of 50% for TTA-mediated UC.