Hee-Won Shin

Preparation of diarylethene copolymers and their photoinduced refractive index change

Diarylethene copolymers were synthesized from 1-(6 0 -vinyl-2 0 -methylbenzo[b]thiophene-3 0 -yl)-2-(2 00 -methylbenzo[b]thiophene-3 00 -yl)hexafluorocyclopentene (VMBTF6) and 1-[6 0 -(4 000 -vinylbenzoyl)-2 0 -methylbenzo[b]thiophene-3 0 -yl]-2(2 00 -methylbenzo[b]thiophene-3 00 -yl)hexafluorocyclopentene (VBMBTF6) via living free radical techniques using stable TEMPO derivatives. The diarylethene content was controlled by the feed ratio of diarylethene derivatives and styrene. Transparent photochromic polymer films were prepared from the diarylethene copolymers with narrow molecular weight dispersion (PD � 1:3) by spin coating method. Photoinduced refractive index changes (DnTE) of the polymer films, with 25 mol% of diaryethene content, accompanied by the photochromic change were determined as 0.0009 and 0.0030 for poly(styrene-ran-VMBTF6) and poly(styrene-ran-VBMBTF6), respectively. 2002 Published by Elsevier Science B.V.

Photochromism of diarylethene derivatives in rigid polymer matrix: structural dependence, matrix effect, and kinetics

Abstract Photochromic reactions of three diarylethene derivatives embedded in three types of polymer matrices were studied using a variety of spectroscopic techniques and quantum ab initio computations. It was found that the photochromic efficiencies of diarylethene molecules depend on the structural and electronic characteristics of Franck–Condon region in the excited states, and also depend strongly on the sizes of free inner spaces of the matrices. The steady-state and time-resolved fluorescence studies revealed the detailed kinetics of the planar and twisted conformers in the excited states. The observed fluorescence lifetimes of the planar and twisted conformers were ∼2.6 ns and 8∼30 ns.

Directional random lasing in dye-TiO2 doped polymer nanowire array embedded in porous alumina membrane

We have demonstrated the lasing in the porous alumina membrane filled with hybrid polymer nanowires which consisted of poly(N-vinylcarvazole), rhodamine 6G, and TiO2 nanoparticles. The angle-resolved photoluminescence measurement suggested that lasing had a strong directionality along the hybrid polymer nanowires which were embedded within the nanochannels of the membrane. Although wavelengths of the lasing peaks were not affected by excitation and detection angles, lasing behavior strongly depended on the pore diameters of the membranes utilized. It is suggested that the closed loops for lasing are formed via multiple scattering induced by TiO2 nanoparticles embedded in the hybrid polymer nanowires.

Fabrication of Patterned Liquid-Crystalline Nanocomposites and their Novel Characteristics

ABSTRACT Highly ordered nanostructures of liquid-crystal molecules were patterned within porous anodic alumina of different pore diameters. As the utilized pore diameter of the porous membrane decreases, progressively red-shifted emissions are observed for the embedded liquid-crystal nanostructures at room temperature. This unique spectral progression, depending on the pore size, is very similar to the emission trend induced by the thermotropic phase-transitions of the bulk antiferroelectric liquid-crystal. Such characteristics are considered to be due to structural confinement of the antiferroelectric liquid-crystal molecules within the finite size of nanoporous channels.

A Streak Camera Study of Amplified Spontaneous Emission in Polyfluorene Thin Film

ABSTRACT : We report on the photoluminescence (PL) properties of poly[2,7-(9-9-dioctylfluorene)] (PF) thin film under strong optical pumping using a streak camera system. When the excitation energy density increases -above 72 μJ·cm 2 , the emission spectrum becomes narrower and PL decay curve comes to be faster simultaneously. These behaviors are clear evidence of Amplified Spontaneous Emission (ASE) due to a waveguided Stimulated Emission in slab structure of thin film. ASE threshold of 72 μJ·cm -2 is comparable with previous reports and PF is attractive as a gain medium for plastic lasers. Since the first investigation on the electroluminescence (EL) of poly(p-phenylenevinylene), 1 semiconducting π-conjugated polymers have become more attractive materials for applications in optoelectronic devices such as light emitting diodes (LED), plastic lasers, and optical amplifiers. 2-5 Among the π-conjugated polymers, particularly a number of polyfluorene polymers and its copolymers could be promising candidates for realizing these devices due to the unique properties of good thermal and oxidative stability, good solubility in common organic solvents, excellent charge transport ability, and high photoluminescence (PL) quantum efficiency.