P-LED용 Polymetallole의 Photoluminescence 효율증가에 관한연구

Abstract

Polysiloles have recently received much attention because of their unusual electronic properties. These unusual optical and electrical properties can be useful in electronic devices, such as electron transporting materials, light-emitting diodes (LEDs), and chemical sensors. Poly(2,3,4,5-tetraphenyl)siloles (1, so called polysilole,PTPS) shown in Scheme 1 possess both 2,3,4,5-tetraphenyl-1-silacyclopenta- 2,4-diene and Si-Si backbone but the unsaturated five-membered ring of the silole shifts their optical absorption and emission spectra into the visible spectral region. Polysilanes are well known as thermally stable polymers and they exhibit efficient emission in the UV region, high hole mobility, and high nonlinear optical susceptibility. These novel properties arise from σ-σ* delocalization of electrons along the Si-Si backbones. Siloles (2) has a low reduction potential and a low-lying LUMO due to σ*-π* conjugation arising from the interaction between the σ* orbital of the silicon atom and the π* orbital of the butadiene moiety of the five membered ring. Scheme 1. Molecular structure of polysilole 1 and silole 2 The aggregation of highly emissive organics and polymers into a solid state causes an emission-quenching effect, since the aggregation of molecules forms less emissive species such as eximers. Reduction of emission efficiency in solid state has been a major problem in device applications of light-emitting organic molecules. Many attempts to prevent aggregate formation have been done through chemical, physical, and engineering approaches. In contrast, few results on aggregation-induced emission (AIE) properties have been recently reported. Herein, we report an aggregation-induced emission of polysiloles. Fluorescence emission and excitation spectra were recorded on a Perkin-Elmer Luminescence Spectrometer LS 50B. The solvents were determined to be free of emitting impurities prior to use. 2 emits near 380 nm, however polysilole 1 show emission near 520 nm with 340 nm excitation wavelength. Polysilole aggregate exhibits nearly identical emission band. Polysilole nanoaggregates are stronger fluorescent than polysiloles at the identical concentration. For silole solution in pure THF, the photoluminescence (PL) intensity is very weak and the emission peak near 520 nm was observed. As increasing water fraction, the emission intensity of polysilole aggregates increases dramatically. There is no shift in the emission wavelength. In the range of water fraction between 0% and 40%, the emission intensity of polysilole aggregatesincreases slightly. However, the intensity of emission band increases about 11 times, when the water fraction is 90%. Luo et al. reported similar behavior for the silole aggregates. Stability of polysilole aggregates with 90% water fraction has been investigated. PL intensity polysilole aggregates has not been changed over a month. This indicates that polysilole nanoaggregates show neither further aggregation nor degradation. We have also developed new type of composite nanoaggregates which are synthesized from simple blending of two photoluminescence polymers. These composite nanoaggregates result the energy migration of polymethylphenylsilane (PMPS) to polytetraphenylsilole (PTPS) or polytetraphenylgermole (PTPG). PMPS, PTPS, and PTPG exhibit emission bands at 360, 520, and 510 nm, respectively. Energy migrations from PMPS to PTPS or PTPG result increased pholuminescence quantum efficiency of PTPS or PTPG. However, it is interesting to note that these energy migrations are not observed in organic solutions where the polymers are completely dissolved as polymer molecules. The photoluminescence efficiencies of PTPS or PTPG nanoaggregates increase with increasing of PMPS and exhibit the greatest when ratio of PMPS and PTPS is 10:9 in wt%. In case of PTPG, the ratio of PMPS and PTPG for the greatest PL efficiency is 10:40. PTPS nanoaggragates exhibits 1370% better PL efficiency 90% water fraction. In addition PTPS/PMPS composite nanoaggregates shows about 24.4 times greater PL efficiency compare to PTPS in THF solution