Efficient Intramolecular Charge Transfer in Oligoyne-Linked Donor-pi-Acceptor Molecules

Studies are reported on a series of triphenylamine-(C C)(n)-2,5-diphenyl-1,3,4-oxadiazole dyad molecules (n = 1-4, 1, 2, 3 and 4, respectively) and the related triphenylamine-C6H4-(C C)(3)-oxadiazole dyad 5. The oligoyne-linked D-pi-A (D=electron donor, A=electron acceptor) dyad systems have been synthesised by palladium-catalysed cross-coupling of terminal alkynyl and butadiynyl synthons with the corresponding bromoalkynyl moieties. Cyclic voltammetric studies reveal a reduction in the HOMO-LUMO gap in the series of compounds 1-4 as the oligoyne chain length increases, which is consistent with ex-tended conjugation through the elongated bridges. Photophysical studies provide new insights into conjugative effects in oligoyne molecular wires. In non-polar solvents the emission from these dyad systems has two different origins: a locally excited (LE) state, which is responsible for a pi-pi* fluorescence, and an intramolecular charge transfer (ICT) state, which produces charge-transfer emission. In polar solvents the LE state emission vanishes and only ICT emission is observed. This emission displays strong solvato-chromism and analysis according to the Lippert-Mataga-Oshika formalism shows significant ICT for all the luminescent compounds with high efficiency even for the longer more conjugated systems. The excited-state properties of the dyads in non-polar solvents vary with the extent of conjugation. For more conjugated systems a fast non-radiative route dominates the excited-state decay and follows the Engelman-Jortner energy gap law. The data suggest that the non-radiative decay is driven by the weak coupling limit.