Controlling the conformational changes in donor-acceptor [4]-dendralenes through intramolecular charge-transfer processes

Kanibolotsky, A.L. and Forgie, J.C. and McEntee, G.J. and Talpur, M.M.A. and Skabara, P.J. and Westgate, T.D.J. and McDouall, J.J.W. and Auinger, M. and Coles, Simon J. and Hursthouse, Michael B. (2009) Controlling the conformational changes in donor-acceptor [4]-dendralenes through intramolecular charge-transfer processes. Chemistry - A European Journal, 15 (43). pp. 11581-11593. ISSN 0947-6539

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The synthesis of two [4]-dendralene compounds incorporating thiophene-(p-nitrophenyl) donor-acceptor units is presented. The dendralenes adopt two different conformers in solution and solid state and the transformation between the structures can be controlled by light and heat. The electron-donating components of the dendralenes are represented by bromothienyl (in 13) and ethylenedioxythiophene(EDOT)-thienyl (in 15) end-groups. The most facile transformation involves the isomerisation of donor-acceptor conjugated systems (a conformers) into structures in which only the thiophenes are conjugated (b conformers), and this process is driven by ambient light. The structures of the two conformers of compound 13 are confirmed by single-crystal X-ray diffraction studies and the structural changes in both compounds have been monitored by 1H NMR spectroscopy and absorption studies. The transformations were found to be first-order processes with rate constants of k=0.0027 s-1 and k=0.00022 s-1 for 13 and 15, respectively. Density functional theory calculations at the B3LYP/6-31G* level give credence to the proposed mechanism for the ab conversion, which involves photoinduced intramolecular charge transfer (ICT) as the key step. The EDOT derivative (15) can be polymerised by electrochemical oxidation and a combination of cyclic voltammetry and UV/Vis spectroelectrochemical experiments indicate that the a conformer can be trapped and stabilised in the solid state.