Synthesis and optical properties of heteroaromatic small molecules and oligomers for light-emitting devices

Abstract

New Л-electron deficient heteroaromatic oligomers 175 and 225 have been synthesised by Suzuki cross-coupling methodology and incorporated into OLEDs. Using 175 as the emissive layer in the device configuration lTO/PEDOT/175/Ca/Al: blue-green light ƛmax 500 nm, most likely emanating primarily from excimer states is emitted at a high turn-on vohage. Blue electroluminescence, ƛmax 444 nm, (onset 6.5V) is observed for the device structure lTO/PEDOT/225/Ca/ with no long-wavelength emission from Л -aggregates or exciton states. New 2,5-diphenyl-1,3,4-oxadiazoles and 2-phenyl-5-(2-thienyl)-l,3,4-oxadiazoles heteroaryl- functionalised derivatives have been prepared under standard Sonogashira cross-coupling conditions. UV-Vis absorption and photoluminescence spectra establish that the substitution of a phenyl ring in the 2,5-diphenyl-1,3,4-oxadiazole derivative 232d Vax (PL) 380 nm, by a thienyl ring 239d ƛmax (PL) 415 nm, leads to a red shift in the lowest energy band. The absorption and emission spectra of 240 ƛmax (PL) 480 nm, is further red-shifted compared to 239f ƛmax (PL) 425 nm, which is consistent with the extended ir-conjugation of the central bis(ethynylthiophene) unit of 240.