CN-Tuning: A Pathway to Suppress Singlet Fission and Amplify Triplet-Triplet Annihilation Upconversion in Rubrene

Incoherent photon upconversion (UC) mediated by triplet-triplet annihilation (TTA) offers multiple implementation pathways, from bio-related applications to energy harvesting in photovoltaics. Despite its potential, TTA-UC efficiency in the near-infrared (NIR) range is notably low, particularly in solid-state systems. Rubrene (Rub), a key annihilator in this spectral range, faces significant concentration quenching, primarily due to singlet fission (SF). To mitigate this issue, herein, the cyano (CN) functionalization of Rub is explored to reduce SF and boost solid-state UC performance. By systematically increasing the number of CN groups, the modified Rub is examined in both solution and solid-state environments, benchmarking it against unmodified Rub and assessing each energy transfer step. The findings reveal that CN modification, while detrimental to UC efficiency in solution, markedly improves solid-state UC performance. Specifically, Rub functionalized with two peripheral CN groups achieves a UC quantum yield (Phi UC) of 3.0% (out of a theoretical maximum of 50%) in the solid-state-nearly tripling the maximum yield achieved with unmodified Rub. This advancement not only presents a more efficient and stable alternative to conventional Rub annihilators but also highlights the potential of CN-tuning for enhancing other NIR annihilators in the solid state.