Effect of the electron-accepting centre and solubilising substituents on the redox, spectroscopic and electroluminescent properties of four oxadiazoles and a triazole disubstituted with bithiophene

Five and six heterocycle ring assemblies, comprising triazole or oxadiazole acceptor (A) units and thiophene or alkylthiophene donor (D) units in a DDADD and DDAADD arrangement, have been synthesised and characterised by spectroscopic and electrochemical means. It is demonstrated that the replacement of a weaker acceptor (triazole) by a stronger one (oxadiazole) in DDADD compounds results in lowering of the frontier molecular energy gap by 70 meV from 2.94 to 2.87 eV. For oxadiazole-centred compounds these gaps can be further lowered by increasing the number of A units and alkyl solubilising substituents, down to 2.76 eV for the DDAADD compound containing four alkyl pendants. The photoluminescence quantum yield exceeds 50 % for DDADD compounds and reaches 78 % for DDAADD ones. All compounds, when dispersed in a blend consisting of 70 % poly(vinylcarbazole) (PVK) and 30 % of 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD) (host–guest configuration), demonstrate light-emitting diode behaviour, affording blue, tuneable emission with luminances reaching 120 cd m⁻² and luminous efficiencies of 0.12 cd A⁻¹ for non-optimised devices.