Annihilation Limit of a Visible-to-UV Photon Upconversion Composition Ascertained from Transient Absorption Kinetics

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 mu W/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 x 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 (lambda(ex) = 547 nm, delayed signal) and with direct excitation (lambda(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.