Controllable Formation of Pyrene (C₁₆H₁₀) Excimers in Polystyrene/Tetrabutylammonium Hexafluorophosphate Films through Solvent Vapor and Temperature Annealing

Pyrene, because of its wide applications in fluorescence sensors, has been studied extensively. Nevertheless, the formation of pyrene excimers in the solid state has not been well understood compared to that in its solution state. It has been shown that electrospun pyrene/polystyrene/tetrabutylammonium hexafluorophosphate thin films demonstrate high selectivity and sensitivity in detecting nitro-aromatic, nitrate ester, and nitro-amine explosives, while thin films made from the same solution via other procedures (e.g., spinning coating, solution casting, dip coating) do not exhibit satisfactory detecting capability, partially because of the low excimer fluorescence. In this study, three parameters, evaporation rate, temperature, and solvent vapor pressure, that most likely influence the formation of pyrene excimers are examined individually. Experimental results show that high solvent vapor pressure annealing plays the most important role in enhancing the pyrene excimer fluorescence, while high-temperature (around and greater than the glass transition temperature of the polymer) annealing in the absence of the solvent significantly suppresses the excimer fluorescence, indicating that pyrene excimers are metastable in the polymer thin films. Evaporation rate has a marginal effect on the formation of pyrene excimers. Moreover, this study has shown that it is possible to convert a low excimer fluorescence polymer film to a high-fluorescence one through high solvent vapor pressure annealing, which provides a novel strategy for manufacturing high-sensitivity sensing materials