Low-power green-to-blue and blue-to-UV upconversion in rigid polymer films

Green light excitation of platinum octaethylporphyrin (PtOEP) in poly(methyl methacrylate) (PMMA) films containing 9,10-diphenylanthracene (DPA) yielded emission of blue light under ambient conditions. This energy upconversion process was easily observed using low-power monochromatic excitation. The blue light emission arose from fluorescence of the lowest excited singlet state of DPA formed by spin-allowed annihilation of two DPA triplets excited by the transfer of the triplet energy from PtOEP to DPA. The upconversion emission intensity was proportional to the square of the excitation intensity, and an upconversion quantum efficiency of ∼0.0002 was measured at an excitation intensity of only 0.9 mW cm⁻². High concentrations of molecularly doped emitter facilitated triplet energy migration in rigid PMMA. Upconverted fluorescence was also observed from other emitters and with other sensitizers, including blue-to-UV upconversion in a PMMA film containing the non-heavy metal sensitizer 2-methoxythioxanthone and the emitter 2,5-diphenyloxazole.