Upconversion photochemistry occurring between palladium(II) octaethylporphyrin (PdOEP, 1) and 9,10-diphenylanthracene (DPA, 2) in toluene successfully sensitizes nanostructured WO(3) photoanodes (E(g) = 2.7 eV) to sub-bandgap non-coherent green photons at low power density.

No abstract

Noncoherent sensitized green-to-near-visible upconversion has been achieved utilizing palladium(II) octaethylporphyrin (PdOEP) as the triplet sensitizer and anthracene as the energy acceptor/annihilator in vacuum degassed toluene. Selective 547 nm excitation of PdOEP with incident irradiance as low as 600 μW/cm(2) results in the observation of anthryl fluorescence at higher energy. Stern-Volmer analysis of the dynamic phosphorescence quenching of PdOEP by anthracene possesses an extremely large K(SV) of 810,000 M(-1), yielding a triplet-triplet energy transfer quenching constant of 3.3 × 10(9) M(-1) s(-1). Clear evidence for the subsequent triplet-triplet annihilation (TTA) of anthracene was afforded by numerous experiments, one of the most compelling was an excitation scan illustrating that the Q-band absorption features of PdOEP are solely responsible for sensitizing the anti-Stokes fluorescence. The upconverted emission intensity with respect to the excitation power was shown to vary between quadratic and linear using either coherent or noncoherent light sources, illustrating the expected kinetic limits for the light producing photochemistry under continuous wave illumination. Time-resolved experiments directly comparing the total integrated anthracene intensity/time fluorescence data produced through upconversion (λ(ex) = 547 nm, delayed signal) and with direct excitation (λ(ex) = 355 nm, prompt signal) under conditions where the laser pulse is completely absorbed by the sample reveal annihilation efficiencies of approximately 40%. Similarly, the delayed fluorescence kinetic analysis reported by Schmidt and co-workers (J. Phys. Chem. Lett. 2010, 1, 1795-1799) was used to reveal the maximum possible efficiency from a model red-to-yellow upconverting composition and this treatment was applied to the anthryl triplet absorption decay transients of anthracene measured for the PdOEP/anthracene composition at 430 nm. From this analysis approximately 50% of the anthryl triplets that decay by TTA produce singlet fluorescence, consistent with the notion that annihilation spin statistics does not impose efficiency limits on upconversion photochemistry.

We present a red-to-blue upconversion system based on triplet–triplet annihilation in a solid-state film configuration that significantly enhances the photocurrent of a model solar cell device. The film is robust against oxygen quenching and can be readily tailored to existing solar cell architectures. The photovoltaic performance of upconversion-assisted dye-sensitized photoelectrochemical cells was measured under both high-power coherent laser and low-power incoherent light irradiation (light-emitting diode and simulated AM1.5G sunlight). By utilizing low-energy photons that would otherwise be wasted, the photocurrent is enhanced by as much as 35% under one-sun light intensity when a model solar cell device is coupled with a TTA film and a reflector.

The phosphorescent metalloporphyrin sensitizer PtTPTNP (TPTNP = tetraphenyltetranaphtho[2,3]porphyrin) has been successfully coupled with perylenediimide (PDI) or rubrene utilized as triplet acceptors/annihilators to upconvert 690 nm incident photons into yellow fluorescence through sensitized triplet-triplet annihilation at overall efficiencies in the 6-7% range while exhibiting exceptional photostability.

Microcapsules that achieve multicolor triplet–triplet annihilation (TTA)-based upconversion (UC) in both aqueous and dry phases without deoxygenation are presented for the first time. Platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP) was used as a sensitizer and perylene, 9,10-bis(phenylethynyl)anthracene (BPEA), and a boron dipyrromethene derivative (BD-2) were employed as acceptors for red to blue, cyan, and green UC, respectively. Additional color tuning was introduced into microcapsules by embedding rose bengal onto the microcapsule shell, resulting in UC-mediated excitation of a distal fluorophore through a trivial process. Microcapsules were further modified to host superparamagnetic nanoparticles for magnetic-induced collection, which permitted sorting and color separation potentially instrumental for various photonics-based applications.

Photochemical upconversion (UC) of low-energy photons that would otherwise be wasted could drastically improve the efficiency of solar technologies by allowing them to harness a greater fraction of the solar spectrum. Although UC through the triplet–triplet annihilation (TTA) mechanism operates efficiently under low-power irradiation such as sunlight, its ability to improve solar device efficiencies is limited by the narrow light absorption bands of its sensitizer chromophores. This bottleneck on UC performance can be overcome by employing multiple sensitizers in tandem, but such an approach has thus far been studied exclusively in solution-based TTA-UC systems requiring intensive deoxygenation and sealing procedures. This study presents the first dual-sensitizer TTA-UC system in a solid-state host suitable for practical applications. We fabricate thin polyurethane films containing two benchmark TTA-UC sensitizers in a range of different concentrations and characterize their red-to-blue and green-to-blue UC performance as a function of excitation intensity. The broadband absorption of the dual-sensitizer films significantly enhances their performance under simultaneous low-intensity excitation of the two sensitizers, giving rise to anti-Stokes fluorescence surpassing the combined anti-Stokes fluorescence of the films’ single-sensitizer analogues. We circumvent trade-offs between light absorption and TTA-UC performance at high sensitizer concentrations by harnessing the films’ unique versatility to produce an alternative “multijunction” TTA-UC system comprising overlaid single-sensitizer films, thereby achieving strong broadband light absorption and superior TTA-UC performance.

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.