Microwave accelerated synthesis and evaluation of conjugated oligomers based on 2,5-di-thiophene-[1,3,4]thiadiazole

A series of P-conjugated 2,5-di-thiophene-[1,3,4]thiadiazole based pentamers have been synthesised by microwave promoted palladium catalysed cross-coupling reactions. The aromatic groups that terminate each pentamer have a profound effect on the observed optical, electrochemical and liquid crystalline properties. Substitution of progressively less electron rich aromatic groups at the periphery of the pentamers decreased the energy gap between the ground state and the first excited state and also lowered the energy of the frontier orbitals sufficiently to impede oxidation and facilitate reduction. The thermotropic calamitic mesogen, 2,5-bis-[5-(4-hexyl-phenyl)-thiophen-2-yl]-[1,3,4]thiadiazole, 7 showed non-dispersive ambipolar charge transport in the nematic, smectic and crystalline phases. The charge mobility increased concomitantly with increasing molecular order of the mesophase to a maximum in the highest ordered smectic phase (k(hole) - 4 x 10(-3) cm(2) V(-1) s(-1), k(electron) - 8 x 10(-3) cm(2) V(-1) s(-1)).