Y. Okamoto
Polymer Research Institute, Polytechnic University, 6 Metrotech Center, Brooklyn, NY 11201 (yokamoto@poly.edu)
Nanocrystals of semiconductor materials, such as cadmium selenide, can be tailored to bulk light emitting materials, to absorb and emit different colors. What sets them impart is the ability to fine tune this optical behavior by modifying their size. On the other hand, we have found a full color display with its spectrum covering the entire visible range by incorporating a polymer into silica glass.
The polymers used were poly(quinoline)s, poly(2,6-[4-phenylquinoline]) (PPQ) and poly(2,6-[P-phenylene]-4-phenylquinoline) (PPPQ). The sol-gel technique was employed to prepare a composite of various concentrations of poly(quinoline) and silica. The gelation of the solutions occurred rapidly (~15 min). Then, the gels were heated at 40-50oC to complete the polymerization. When the composite materials were excited by ultraviolet radiation, colored emission spanning nearly the entire visible range with high photoluminescence intensities were observed.
This result is caused by the combined effects of excimer formation and the gelation of inorganic silica. Excimer formation among small molecules is often reported as the interaction of an excited chromophore A* with an unexited chromophore A, A*+ A® (AA)*. Such an excited couple is stabilized due to a resonance contribution, resulting in the shifting of the emission peak toward the red region. π-Conjugated polymers such as poly(quinoline)s are generally stiff chain molecules with relatively planar geometries and very strong intermolecular interactions. It is therefore reasonable to expect that excimers efficiently form among quinoline polymer chains. The possibility may also exist that multiple excimers, (Rn+2)*, form among the trapped polymers in silica channels.
The interchain interaction among these chains results in shift of the emission pesk progressively to longer wavelength with increasing polymer concentration. When the amount of the polymers in the sol-gel is small, <10-3 wt%, the majority of the chains in solution are in the "isolated" state. The isolated polymer chains may be trapped individually in channels of silica domains upon gel formation, resulting in the blue color (~ 440 nm) upon UV radiation. At a higher concentration (~ 5 wt%), the emission peak is around 600 nm and the red emitting color is indicative of possibly a high degree of cofacial chain interaction and stacking of the chromophores. For concentrations in between, less extensive cofacial chain interaction and fewer multiple excimers trapped inside the silica domains than those at 5 wt% lead to the emission of green, yellow and orange colors.