Understanding the limitations of NIR-to-visible photon upconversion in phthalocyanine-sensitized rubrene systems†
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.