Texaphyrin sensitized near-IR-to-visible photon upconversion

Near-IR (NIR) absorption from a Cd(ii) texaphyrin (TXP) has been successfully coupled with rubrene triplet acceptors/annihilators in vacuum degassed dichloromethane to upconvert NIR (670-800 nm) incident photons into yellow fluorescence through sensitized triplet-triplet annihilation. Stern-Volmer analysis of dynamic energy transfer quenching of TXP by rubrene using transient absorption spectroscopy revealed Stern-Volmer and bimolecular quenching constants of 21,000 M(-1) and 5.7 × 10(8) M(-1) s(-1) respectively, for the triplet-triplet energy transfer process. The upconverted emission intensity with respect to the incident excitation power density at 750 nm was shown to vary between quadratic and linear, illustrating the expected kinetic limits for the light producing photochemistry under continuous wave illumination. Furthermore, with increasing TXP sensitizer concentration, the characteristic quadratic-to-linear crossover point shifted to lower incident photon power density. This is consistent with the notion that stronger photon capture in the sensitizer leads to experimental conditions promoting upconversion under milder excitation conditions. The maximum quantum yield of the TXP-sensitized rubrene upconverted fluorescence was 1.54 ± 0.04% under dilute conditions determined relative to [Os(phen)3](PF6)2 under continuous wave excitation conditions. This saturating quantum efficiency was realized when the incident light power dependence reached the quadratic-to-linear crossover point and was constant over the region where the composition displayed linear response to incident light power density. In pulsed laser experiments at higher sensitizer concentrations, the triplet-triplet annihilation quantum yield was determined to saturate at approximately 13%, corresponding to an upconversion yield of ∼10%, suggesting that the dichloromethane solvent either lowers the T2 state of the rubrene acceptor or is somehow attenuating the annihilation reaction between excited rubrene triplets.