Young Il Park

Tailored Electronic Structure and Optical Properties of Conjugated Systems through Aggregates and Dipole−Dipole Interactions

A series of PPVO (p-phenylene vinylene oligomer) derivatives with functional groups of varying electronegativity were synthesized via the Horner-Wadsworth-Emmons reaction. Subtle changes in the end group functionality significantly impact the molecular electronic and optical properties of the PPVOs, resulting in broadly tunable and efficient UV absorption and photoluminescence spectra. Of particular interest is the NO2-substituted PPVO which exhibits photoluminescence color ranging from the blue to the red, thus encompassing the entire visible spectrum. Our experimental study and electronic structure calculations suggest that the formation of aggregates and strong dipole-dipole solute-solvent interactions are responsible for the observed strong solvatochromism. Experimental and theoretical results for the NH2-, H-, and NO2-substituted PPVOs suggest that the stabilization of ground or excited state dipoles leads to the blue or red shift of the optical spectra. The electroluminescence (EL) spectra of H-, COOH-, and NO2-PPVO have maxima at 487, 518, and 587 nm, respectively, in the OLED device. This trend in the EL spectra is in excellent agreement with the end group-dependent PL spectra of the PPVO thin-films.

Fluorescence Detection of Microcapsule-Type Self-Healing, Based on Aggregation-Induced Emission

An extrinsic self-healing coating system containing tetraphenylethylene (TPE) in microcapsules was monitored by measuring aggregation-induced emission (AIE). The core healing agent comprised of methacryloxypropyl-terminated polydimethylsiloxane, styrene, benzoin isobutyl ether, and TPE was encapsulated in a urea-formaldehyde shell. The photoluminescence of the healing agent in the microcapsules was measured that the blue emission intensity dramatically increased and the storage modulus also increased up to 105 Pa after the photocuring. These results suggested that this formulation might be useful as a self-healing material and as an indicator of the self-healing process due to the dramatic change in fluorescence during photocuring. To examine the ability of the healing agent to repair damage to a coating, a self-healing coating containing embedded microcapsules was scribed with a razor. As the healing process proceeded, blue light fluorescence emission was observed at the scribed regions. This observation suggested that self-healing could be monitored using the AIE fluorescence.

Highly efficient blue emitting materials based on indenopyrazine derivatives for OLEDs [Invited]

Two indenopyrazine compounds for organic light emitting diodes were synthesized with phenanthrene or pyrene side groups that have high photoluminescence (PL) quantum efficiency (QE); 6,6,12,12-Tetraethyl-2,8-di-phenanthren-9-yl-6,12-dihydro-diindeno[1,2-b;1,2-e]pyrazine (PA-EIP) and 2-(10b,10c-Dihydro-pyren-1-yl)-6,6,12,12-tetraethyl-8-pyren-1-yl-6,12-dihydro-diindeno[1,2-b;1,2-e]pyrazine (PY-EIP). The PL spectra of PA-EIP and PY-EIP in film state were 440 nm and 468nm in the blue region, respectively. Td values for PA-EIP and PY-EIP were very high at 432°C and 441°C. Tm values were 385°C and 390°C for PA-EIP and PY-EIP. When the synthesized compounds were used as emitting layers in non-doped OLED devices, PA-EIP and PY-EIP showed luminance efficiencies of 1.35 and 5.15 cd/A, power efficiencies of 0.69 and 2.81 lm/W, and CIEs of (0.17, 0.15) and (0.19, 0.30), respectively. This result shows that efficiency of final emitter increases with increasing efficiency of side group. Additionally, in the solution-processed white OLED devices using PY-EIP as one of emitting materials, the device showed high luminance efficiency of 3.95 cd/A, power efficiency of 2.45 lm/W and CIE value of (0.269, 0.294) at 6 V.

An efficient lactone-to-lactam conversion for the synthesis of thiophene Pechmann lactam and the characterization of polymers thereof

Despite having unique structural features, e.g., high co-planarity and a strong polar bicyclic lactam structure, thiophene bipyrrolylidene-2,2′(1H,1′H)-dione (TBPD) has been less explored as a dye, mainly due to the quite low yield in its synthesis via lactone-to-lactam conversion. We reported an efficient methodology for synthesizing TBPD in high yield using p-toluenesulfonic acid monohydrate and a catalytic amount of 4-dimethylaminopyridine in the chloroform solvent. From the newly synthesized series of TBPD-based donor–acceptor-type polymers, we fabricated organic field-effect transistors (OFETs), which were subjected to a systematic study on the relationship between film microstructure and charge transport. Among them, the annealed PTBPD-Th film revealed a more ordered lamellar packing with the highest number of interlayers and preferential edge-on orientation, yielding the best hole mobility (up to 0.46 cm2 V−1 s−1). The improved synthesis of TBPD and our findings concerning related polymers could promote further research and development associated with the TBPD unit.