Chiou-Ling Chang

Experimental and theoretical investigations of absorption and emission spectra of the light-emitting polymer MEH-PPV in solution

Abstract We report the absorption and photoluminescence spectra of the MEH-PPV polymer in a chloroform solvent. In the analysis, we assume that the polymer consists of short conjugated segments which are called oligomers with different lengths. The appearance of each chain length is controlled by a distribution function. Based on the conformational disorder caused by torsional motion, we found that the distribution function should take a Gaussian form with its center and width being adjustable parameters. While the energy level of the emissive S1 state of the oligomer and the corresponding oscillator strength are calculated from exciton theory. In addition, the associated vibrational motions are considered in determining the spectral shape in which both displacement and distortion of the potential surfaces are taken into account. With this information, the calculation result can fit the experiment reasonably well.

White-light electrofluorescence switching from electrochemically convertible yellow and blue fluorescent conjugated polymers

We report the study of the fluorescence switching properties of the conjugated copolymers containing triphenylamine, fluorene, benzo[2,1,3]thiadiazole, and cyclic urea moieties. The copolymers show excellent thermal stability and good solubility in polar organic solvents. While the electrofluorescent device (EFD) of P1 emits yellow light under UV excitation, fluorescence intensity is switched off upon electrochemical oxidation. In addition, the fluorescent behavior of the EFD of P1 can be reversibly switched between the non-fluorescent (oxidized) state and the fluorescent (neutral) state with a superb contrast ratio (If/If0) of 21.4. Furthermore, a white-light electrochemical fluorescence switching device is achieved by blending of P1 (yellow) with P2 (blue). Since the fluorescent conjugated polymers of P1 and P2 have their emission simultaneously quenched under the low working potential, the EFD could show a white–dark state of fluorescence with a high contrast ratio (If/If0) of 14.6.

Tunable Electrofluorochromic Device from Electrochemically Controlled Complementary Fluorescent Conjugated Polymer Films

The fluorescent behavior of the electrofluorochromic devices (Type I) of greenish-yellow emitting P1 and blue emitting P2 can be reversibly switched between the nonfluorescent (oxidized) state and the fluorescent (neutral) state with a superb on/off ratio of 23.8 and 21.9, respectively. Moreover, a tunable electrofluorochromic device (Type II) based on two P1 and P2 polymeric layers that are coated individually on two independent ITO electrodes shows switchable blue-white-(greenish-yellow) multifluorescence states.

The thermofluoric behavior of poly(fluorenetolyldiphenylamine)–oxadiazole pair in a polymer matrix

Under UV irradiation, 1,4-bis(5-(4-octan-2-ylphenyl)-1,3,4-oxadiazol-2-yl)naphthalene (NOXD) and poly(fluorenetolyldiphenylamine) (PFT) are blue light emissive in solid film. When NOXD and PFT were blended to form a neat thin-film, yellowish exciplex emission was observed. The emissive properties vary when the materials were blended into different kinds of polymeric hosts; while polystyrene–NOXD–PFT (5 : 1 : 1) and PMMA–NOXD–PFT (5 : 1 : 1) showed blue light emission, poly(acrylonitrile-co-methyl acrylate) (P(AN-co-MA))–NOXD–PFT (5 : 1 : 1) showed blue and yellow dual emissions. When the film was heated at 140 °C for 2 min, the yellow light exciplex emission was enhanced. However, upon cooling to ambient temperature, the exciplex emission intensity gradually dropped back. Similar thermofluoric behaviors were observed when the thermally crosslinked polyepoxy-polymercaptan–NOXD–PFT film (20 : 1 : 1) was heated at 160–170 °C; the blue light emissive film showed yellow light emission, and turned white when cooling down to ambient temperature. This thermofluoric phenomenon is recyclable. We attributed the observation of the yellow emission to the segregation of NOXD from the polymeric host at high temperature that allows NOXD to aggregate with PFT, leading to exciplex emission.