Abstract
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 a→b 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.
Highlights
Cross-conjugated molecules have been defined as containing three or more unsaturated groups, two of which are conjugated to the third but not directly to each other.[1]
Dendralene 13 was synthesised by a two-fold
Two new [4]-dendralenes bearing thiophene–nitrostyryl donor–acceptor units have been prepared as mixtures of conformers
Summary
Cross-conjugated molecules have been defined as containing three or more unsaturated groups, two of which are conjugated to the third but not directly to each other.[1]. Analogous to the CDCl3 experiment, crystals from a batch of conformer 13 a were dissolved in [D6]DMSO and the proton NMR was recorded taking the same precautions to exclude light from the solution as much as possible (see Figure S1 a in the Supporting Information).
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