8392 Chem. Commun., 2010, 46, 8392–8394 This journal is c The Royal Society of Chemistry 2010 Conformational control in a bipyridine linked p-conjugated oligomer: cation mediated helix unfolding and refoldingw Kizhumuri P. Divya, a Sivaramapanicker Sreejith, a Cherumuttathu H. Suresh b and Ayyappanpillai Ajayaghosh* a Received 16th August 2010, Accepted 6th September 2010 DOI: 10.1039/c0cc03287e A chiral p-conjugated oligomer having alternate bipyridine and carbazole moieties connected through acetylinic bonds undergoes helical folding in chloroform–acetonitrile (40/60, v/v) as evident by fluorescence and circular dichroism changes. In the presence of transition metal cations such as Zn 2+ defolding of the helical conformation occurs. Upon decomplexation of the cation with EDTA, the helical conformation is regained. Synthetic oligomers and polymers, that mimic the conformational changes of biopolymers, has attracted much attention in recent years. 1,2 In this context, m-phenyleneethynylene based oligomers have been extensively studied by Moore et al., mainly due to the ease of tracking of the chain folding processes using absorption, emission and circular dichroism (CD) spectroscopic techniques. 3 Folding and unfolding of such structures is usually achieved by a change in polarity of the solvent, 4 temperature of the medium, 5 interaction of guest molecules, 6 or by light. 7 Cation induced folding is a commonly encountered phenomenon in synthetic foldamers. 8 However, a cation assisted unfolding of helically folded oligomer chains in a polar medium is relatively rare. 9 Herein, we report a Zn 2+ /EDTA mediated unfolding and refolding of a helically folded carbazole-bipyridine based foldamer 2 in a polar environment. The oligomer 2 having a chiral side chain at the carbazole moiety was synthesized by the reaction of 4,4 0 -dibromo-2,2 0 - bipyridine and (S)-9-(3,7-dimethyloctyl)-3,6-diethynyl-9H- carbazole using Sonogashira coupling. 10 The oligomer was isolated, purified and characterized by 1 H-NMR, GPC (polystyrenes as standard and THF as eluent) and MALDI-TOF analyses (ESIw). The MALDI-TOF spectral analysis showed peaks corresponding to 4, 5, and 6 repeat units. GPC showed an average molecular weight (M n ) of 3105 g mol À1 for 2 with a polydispersity index of 1.98. A model compound 1 was prepared by the reaction of (S)-9-(3,7-dimethyloctyl)-3- ethynyl-9H-carbazole with the required bipyridine derivative and purified, and characterized by FAB mass and 1 H-NMR spectral analyses. The emission spectrum of the model compound 1 in chloroform (2.5 Â 10 À5 M) exhibited a maximum at 416 nm. In aceto- nitrile (2.5 Â 10 À5 M) the emission maximum is shifted to 474 nm (58 nm red-shift) with ca. 25% quenching of the emission intensity (Fig. 1a). The emission spectrum of the oligomer 2 in chloroform (8 Â 10 À6 M) exhibited a maximum at 416 nm with a shoulder at 464 nm (Fig. 1b). Interestingly, the emission is red-shifted to 472 nm (Dl = 56 nm) with almost complete quenching (ca. 95%) in acetonitrile. Fig. 1 Emission spectral changes of (a) the model compound 1 [2.5 Â 10 À5 M, l ex = 340 nm] and (b) the oligomer 2 [8 Â 10 À6 M, l ex = 350 nm] in chloroform (- - - -) and acetonitrile (—). a Photosciences and Photonics Group, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST, CSIR), Trivandrum, India. E-mail: ajayaghosh62@gmail.com; Fax: (+) 91-471-249-0186; Tel: 04712515306 b Computational Modeling and Simulation Section, National Institute for Interdisciplinary Science and Technology (NIIST), CSIR, Trivandrum, India w Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c0cc03287e COMMUNICATION www.rsc.org/chemcomm | ChemComm Downloaded by Nanyang Technological University on 26 June 2012 Published on 07 October 2010 on http://pubs.rsc.org | doi:10.1039/C0CC03287E View Online / Journal Homepage / Table of Contents for this issue