Fluorescence properties of fluor molecules confined within nanoscale pores in a polymer matrix
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Fluorescence properties of fluor molecules confined within nanoscale pores in a polymer matrix Valery N. Bliznyuk and Ayman F. Seliman, Department of Environmental Engineering and Earth Sciences, Clemson University, Clemson, South Carolina 29634, USA Scott M. Husson, Department of Chemical and Biomolecular Engineering, Clemson University, Clemson, South Carolina 29634, USA George Chumanov, Department of Chemistry, Clemson University, Clemson, South Carolina 29634, USA Timothy A. DeVol, Department of Environmental Engineering and Earth Sciences, Clemson University, Clemson, South Carolina 29634, USA Address all correspondence to Valery N. Bliznyuk at [email protected] (Received 9 March 2015; accepted 27 May 2015)
Abstract We demonstrate that fluorescence properties of organic fluors embedded in a porous polystyrene matrix are highly sensitive to the average pore size and pore-size distribution of the matrix. The effect can be understood as two different types of confinement imposed to the fluor molecules by the matrix. First, there is geometrical confinement that restricts the fluor oscillations due to its physical contact with a pore wall. Second, there is an electronic confinement due to a local polarization of the wall material by molecular dipoles. The effects lead to a spectral shift and enhancement of the fluorescence intensity of the material.
Organic fluors (or dyes) are used in applications ranging from painting and labeling to luminescence sensing of ultra-low analyte concentrations, as well as scintillation to detection ionizing radiation.[1–4] The active portion of an organic fluor has a system of pi-conjugation (also known as a chromophore) and is responsible for a specific color of the material. Scintillation occurs when ionizing radiation energy absorbed by the host matrix is transferred to the electrons of the fluor placing it in an excited state, which subsequently relaxes to the ground state resulting in the emission of light.[4–6] Owing to an extended conjugated system (aromatic) fluor molecules have a natural tendency to aggregate even in relatively dilute solutions. Depending on the chemical structure and arrangement of the dye molecules in the aggregate, either red-shifted J-aggregates or blue-shifted H-aggregates can be observed.[5–9] Control over formation of dye aggregates is highly desirable, as isolated single chromophores emit more brightly when a high density of them can be achieved without aggregation. The latter situation may be realized when the dye molecules are trapped within nanoscale pores of nanosilica or zeolite matrices. Previously, variation of optical properties was reported for visible light dyes under geometrical confinement in nanopores of zeolites,[10] silica particles,[11,12] and dendritic polymer molecules.[13] Spectral shifts of 3–300 nm were observed. However, the size of the pores was material specific and was not varied broadly in those experiments. In this paper, we report optical properties of polymer–fluor scintillator systems with fine control and variation of th
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