Influence of Temperature on Low-Power Upconversion in Rubbery Polymer Blends

Abstract

Abstract:The upconverting properties of a dye cocktail composed of palladium(II) octaethylporphyrin (PdOEP, triplet sensitizer) and 9,10-diphenylanthracene (DPA, triplet acceptor/annihilator) were investigated as a function of temperature in several low glass transition temperature (T g ) polymer hosts including an ethyleneoxide-epichlorohydrin copolymer (EO-EPI) and the polyurethanes Texin 270, Texin 285, and Tecoflex EG-80A. Selective excitation of PdOEP at 544 nm in the presence of DPA in these materials resulted in anti-Stokes blue emission from DPA, a consequence of sensitized triplet-triplet annihilation (TTA) photochemistry, confirmed by the quadratic dependence of the upconverted fluorescence intensity with respect to incident light power. The upconversion process was completely suppressed by cooling a PdOEP/DPA blend film to below the T g of the respective polymer. However, the blue emission was clearly visible by the naked eye upon heating these films to room temperature (290 K). Subsequently, the upconverted emission intensity increased with increasing temperature and was found to be completely reversible upon several heating and cooling cycles provided the temperature remained below 400 K. Heating samples above this temperature resulted in unrecoverable failure of the material to produce upconverted photons. The phosphorescence intensity decay of PdOEP in the polymer host, Tecoflex EG-80A, adequately fits to a sum of two exponential functions as well as the Kohlrausch-Williams-Watts (KWW) stretched exponential model. Increasing the temperature of the film increases the complexity and heterogeneity of the system as evidenced by the lower values obtained from the KWW model as the temperature increases.

Photochemical upconversion: present status and prospects for its application to solar energy conversion

“…49 Consequently, most of the reported systems are based on soft 'rubbery' polymers. Up to now, TTA-UC has been demonstrated in several host matrices including, but not limited to, cellulose acetate, 50 polystyrene, 51 a copolymer of ethyleneoxide and epichlorohydrin, 49,52 polyurethane, 53,54 or poly-butylacrylate. 55 It was further shown that upconverting nanoparticles can be formed using the blending approach with different polymeric host media, including polystyrene 56 or poly-butylacrylate.…”

Low-power photon upconversion through triplet–triplet annihilation in polymers

“…Building upon this initial study, the groups of Weder and Castellano conducted a systematic study that explored how the nature of the elastomeric matrix and the temperature influence the upconverting properties. 15 Using the above-described P(EO-EP)-based blend and similar materials based on a series of thermoplastic polyurethane elastomers as matrices (Fig. 7), it was shown that cooling the PdOEP/DPA-containing elastomer films to below the glass transition temperature (T g ) of the respective polymer completely suppresses the upconverted emission.…”

“…7). 14,15 This matrix was selected on the account of its adequate transparency, high dye solubility, and suitable mechanical properties, but most importantly because of its low crystallinity and low glass transition temperature (À37 C), which impart rubbery behavior as well as rotational and possibly translational mobility to the dissolved dye molecules. Indeed, when the PdOEP in the blend film was selectively excited with non-coherent light from a Xe lamp (l ex ¼ 544 AE 18 nm) at various incident power densities between 6 and 27 mW cm À2 , blue upconverted DPA fluorescence was clearly observable.…”

“…Interestingly, for almost half a century, TTA-UC, also referred to as sensitized photon upconversion, had been limited to liquid solutions of sensitizeremitter pairs. The recent discovery that TTA-UC can also be realized in solid polymer matrices, 14,15 which on account of their processability and mechanical characteristics are more suitable for many potential applications than liquid solutions, has further fueled the interest in this process. Several recent reviews provide excellent accounts of the photophysical aspects of TTA-UC, known chromophore systems and principles for their design and selection, as well as general strategies to develop solid-state TTA-UC materials.…”

Transition-Metal Complexes for Triplet–Triplet Annihilation-Based Energy Upconversion

“…And a new Pd(II) complex (Pd-9) was synthesized and can be used as the sensitizer [38]. In addition, TTA upconversion with Pd (II) complex (Pd-1) as sensitizer was observed in polyfluorene [39] and P (EO/EP) matrix [40], and the influence of temperature on upconversion in rubbery polymer blends was also investigated [41]. …”