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Synthesis, characterization and luminance properties of conjugated phenyleneethynylene systems
Abstract
This work involves syntheses, characterization and electronic spectroscopic study of PPE-related segmented oligomers with and without pendant hydroxyl groups (“sticky” groups). The crystal organization of the molecules was studied and, as much as possible, correlated with their optoelectronic properties. Multi-step syntheses using Sonogashira coupling methodology was used to make all the final molecules. Synthesis, characterization and electronic spectroscopic studies were carried out for the following systems having hydroxyl-pendant sticky groups: 1,4-bis(phenylethynyl)-2,5-bis(2-hydroxyethoxy)benzene, BHE-PE2.5; 2,5-bis(2-hydroxyethoxy)-1,4-bis(3,4,5-trimethoxyphenylethynyl)benzene (B6OMe-PE2.5); 2,5-bis-(2-hydroxyethoxy)-1,4-bis-(4′ -biphenylethynyl)-benzene (BHE-PEBP2.5). A number of model or synthetic intermediate systems were made having the same chromophores as the sticky group systems, but with hydroxyl being protected or replaced with a non-hydrogen-bonding group: 1,4-dimethoxy-2,5-bis(2-phenylethynyl)benzene, BOMe-PE2.5; 2,5-bis(2-acetoxyethoxy)-1,4-bis(phenylethynyl)benzene BAc-PE2.5 (a diacetate analogue of BHE-PE2.5); 2,5-bis-(2-acetoxyethoxy)-1,4-bis-(4 ′-biphenylethynyl)-benzene (BAc-BPE2.5). All the PE oligomers have two UV-vis absorbance bands, one in 304-320 nm and other in the 350–370 nm region. Fluorescence emission is found in the 390–400 nm region upon excitation at various wavelengths. Fluorescence quantum yields in acetonitrile at room temperature range between 0.50-0.92. B6OMe-PE2.5 has the highest quantum efficiency, which may be attributed to the methoxy end groups. BHE-PE2.5 crystallographically packs with its central phenyl rings criss-cross stacked along the π stack axis, potentially yielding crossed-dipole interactions. This type of packing motif is unusual and potentially interesting for electronic purposes. The addition of an extra phenyl ring in BHE-PEBP2.5 disrupts the criss-cross π-stacking motif. The addition of methoxy groups in B6OMe-PE2.5 provided additional hydrogen bonding receptor sites, which also disrupts criss-cross π-stacking. LED devices with configuration ITO/PEDOT-PSS/emitter/Ca(Al) were fabricated by Dr. Ali Cirpan of Karasz group in Polymer Science and Engineering Department using all the oligomers as emissive layers. All except B6OMe-PE2.5 showed electroluminescence. The most promising results by far among these were obtained for BOMe-PE2.5 in terms of luminescence efficiency (0.55 cd/A) and strength, probably because its crystallography does not include π-stacking. Alternating block copolymer analogues of model compound BHE-PE2.5 were synthesized by step growth polymerization with bis acid chlorides. Poly-Terphthaloyl PE, Poly-Isophthaloyl PE, Poly-Oxaloyl PE and Poly-Adipoyl PE degrees of polymerization ranging from 2-10 based on GPC analysis. These systems exhibited electronic spectra similar to their molecular analogues, but were not effective as emissive layers in our typical LED configuration. 2,5-Bis[2-{2-(4-pyrenebutyryloxy)-ethoxy}-ethoxy]-1,4-bis[2-phenylethynyl]benzene PE-Py8 was synthesized from BHE-PE2.5 as an energy transfer multichromophore. This study aimed to provide insights for possible efficient energy harvesting using phenyleneethynylene as an energy acceptor core unit. Its solution photophysical properties showed dominant energy transfer from pyrene antennae to central chromophore on direct excitation of pyrene with an efficiency of ηET=42% in chloroform. 2,7-Bis(3,4,5-trimethoxyphenylethynyl)fluorene OFPE was synthesized and characterized with the objective to achieve blue fluorescence emission and provide extra photostability compared to its 1,4-phenylenevinylene analogue (OFPV) having same end groups but olefin linkers instead of alkynes. OFPE exhibited good photoluminescence quantum yield in solution (0.72 in acetonitrile) with strong blue emission having CIE color coordinates of (0.166, 0.108). Its LEDs with the above-stated configuration had good solid state luminescence at 430 nm with a maximum efficiency of 0.42 at 10 V using an emitter layer made of a 50% (w/w) blend with PMMA.
Subject Area
Organic chemistry
Recommended Citation
Sanyal, Nibedita, "Synthesis, characterization and luminance properties of conjugated phenyleneethynylene systems" (2007). Doctoral Dissertations Available from Proquest. AAI3275766.
https://scholarworks.umass.edu/dissertations/AAI3275766