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  • Title: Efficient intramolecular charge transfer in oligoyne-linked donor-pi-acceptor molecules.
    Author: Pålsson LO, Wang C, Batsanov AS, King SM, Beeby A, Monkman AP, Bryce MR.
    Journal: Chemistry; 2010 Feb 01; 16(5):1470-9. PubMed ID: 20024989.
    Abstract:
    Studies are reported on a series of triphenylamine-(C[triple bond]C)(n)-2,5-diphenyl-1,3,4-oxadiazole dyad molecules (n=1-4, 1, 2, 3 and 4, respectively) and the related triphenylamine-C(6)H(4)-(C[triple bond]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 extended 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 solvatochromism 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.
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