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Photobleaching reactions of Pyronin B perchlorate in relation to luminescent solar concentrators
Large scale application of solar energy via photovoltaic solar cells could be realized if the present photovoltaic module price is significantly reduced. One approach is to use fewer solar cells by concentrating the incident light. A luminescent solar concentrator (LSC) consisting basically of a transparent plate appropriately doped with luminescent dye molecules is described. The successful use of fluorescent organic dyes in an LSC requires that ways be found to protect the dyes from even very low quantum efficiency photodecomposition reactions. Using the model compound Pyronin B, some methods for studying low quantum yield reactions are elucidated and some photodecomposition pathways for xanthene dyes are proposed. Both the rate and pathway of Pyronin 8 photodecomposition vary with solvent, pH and dye concentration. Values of photobleaching quantum yield varying from 1.4 x 10-5 to 2.5 x 10-8 have been measured. Solvents of high polarity, low pH, low dye concentration, and addition of a low evergy triplet quencher such as cyclooctatetraene all lead to decreasing photobleaching quantum yield. Photoproducts of Pyronin B perchlorate identified under neutral solvent conditions correspond to attack by a nucleophilic reagent at the 9 position and dye products resulting from dealkylation of the Pyronin B amino functions. Solvent and concentration effects are consistent with two mechanisms: a mechanism involving electron transfer quenching of the dye excited state by ground state dye molecules and/or photo decomposition resulting from an increased susceptibility of the dye in its excited state to nucleophilic attack at the 9 position.