Low-power light upconversion by triplet-triplet annihilation (TTA-UC) was only recently achieved in glassy materials. Here, a new strategy based on covalent tethering of diphenylanthracene (DPA) emitters to a polymeric backbone is reported. The design aims to optimize the efficiency of this photophysical process in glassy polymeric materials by increasing the emitter content. To that end, DPA molecules were covalently attached to a methacrylate-type monomer and further copolymerized with methylmethacrylate (MMA). Green-to-blue (543 to 440 nm) upconversion was observed at power densities as low as 32 mW/cm 2 in films prepared by solution casting and compression molding (co)polymers containing 8-72 wt% of DPA and palladium octaethyl porphyrin (PdOEP) as a sensitizer (0.03-0.7 wt%). The upconversion intensity was studied as a function of DPA and PdOEP contents and the results suggest that upconversion is optimal for DPA and PdOEP weight fractions of 34 and 0.05 wt% respectively.Several optical upconverting schemes are known to cause an anti-Stokes shift, i.e., the emanating photons have a higher energy than the incident electromagnetic radiation (Fig. 1). [1][2][3] While most upconverting processes require coherent, highpower light produced by lasers (e.g. second-or third-harmonic generation, two-photon absorption emission), upconversion by triplet-triplet annihilation (TTA-UC) is feasible at much lower power densities and with non-coherent, polychromatic excitation. 4,5 The framework is therefore attractive for a range of applications, for example biological imaging and display techniques. [6][7][8][9][10][11][12] Perhaps the biggest promise of TTA-UC is that it can overcome the Shockley-Queisser solar power conversion limit, 13-15 as it permits harnessing photons from the portion of the solar spectrum beyond a solar cell's bandgap. 16,17 The TTA-UC process relies on a series of energy transfer steps between a pair of chromophores, a sensitizer and an emitter ( Fig. 1). 3,[18][19][20][21][22] The sensitizer absorbs incident photons and undergoes intersystem crossing, before a triplet excited state is transferred to the emitter. Two excited emitter molecules then form an encounter complex, and via a triplettriplet annihilation step an emitter singlet excited state is populated. Delayed fluorescence from that singlet manifold ultimately leads to emission of an upconverted photon.Since its discovery in the 1960s, TTA-UC has long been limited to liquid solutions, in which the intermolecular energy transfer steps are facilitated by high degrees of translational and rotational mobility. 5,23 Since solvent-based TTA-UC systems exhibit several drawbacks for practical applications, including limited lifetime 1 and complex implementation, 6,8,[24][25][26][27] the discovery that TTA-UC could be achieved in properly chosen polymers 28 has triggered new interest solid upconverting materials. 7 The first examples of TTA-UC in polymers were based on physical mixtures of sensitizers in conjugated polymers. 3,29 One further improve...
Low‐power light upconversion by triplet–triplet annihilation (TTA‐UC) was only recently demonstrated in glassy polymers and the upconversion efficiency in these materials is typically much lower than in solution. As aggregation of the chromophores was thought to be the culprit, we here report the covalent tethering of a suitable chromophore pair to a polymeric backbone. The new materials were based on the sensitizer‐bearing monomer palladium meso‐phenoxy‐tris(heptyl)porphyrin‐ethylmethacrylate (PdmPH3PMA), which was copolymerized with a diphenylanthrancene methacrylate (DPAMA), as the emitter‐bearing monomer, and methyl methacrylate (MMA) as an optically inert comonomer. The DPA content was kept within a narrow range of 30–37 wt %, while the PdmPH3PMA content was varied between 0.73 and 0.012 wt %. To explore additional compositions, blends of a high‐porphyrin‐content terpolymer with a DPAMA‐MMA copolymer were also prepared. All of the materials studied were processed into thin films by solution‐casting and displayed blue TTA‐UC emission. The UC emission intensity was found to strongly depend on the composition and the underlying effects were investigated through a systematic study. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1629–1639
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