Supramolecular Chemistry DOI: 10.1002/ange.200703075 Macroscopic Origin of Circular Dichroism Effects by Alignment of Self-Assembled Fibers in Solution** MartinWolffs,SubiJ.George,Z ˇ eljkoTomovic ´,StefanC.J.Meskers,* AlbertusP.H.J.Schenning,*andE.W.Meijer* We report herein an unexpected circular dichroism (CD) effect observed for a dilute solution of an achiral oligo(p-phenylene vinylene) derivative, A-OPV3 (Scheme 1) in dodecane (c = 510 À5 m). This effect likely originates from macroscopic phenomena during the self-assembly process of A-OPV3 into fibers after slow cooling from 363 K to 293 K at a rate of 60 Kh À1 (Figure1a). To our surprise, the CD response increased in intensity after shaking the fully trans- parent, nonviscous solution. To shed light on this observation, we studied the self-assembly of A-OPV3 in more detail. Upon cooling the A-OPV3 solution from363to293K,atypicalblueshiftoftheabsorption maximum to l = 407 nm with a shoulder at l = 475 nm is observed. This finding indicates that at high temper- ature the molecules are molecularly dissolved, while at low temperature they are self-assembled into fibers (Fig- ure1b). [1] However, at low temperatures, a small response in the linear dichroism (LD) spectrum is observed, which increased slightly after shaking (Figure 1c). LD and CD are absent at higher temperatures. The atomic force microscopy (AFM) images of dried drop-cast solutions of these self- assembled systems on mica showed large bundles of long fibers (Figure 1d). The three spectra (UV/Vis, CD, and LD) are almost identical in shape (Figure 1a–c). However, when monitoring the spectra during the temperature-induced self-assembly process (absorption and CD intensity at l = 475 nm and the LD intensity at l = 400 nm), the absorbance changes first, while the LD and CD signals arise together at lower temperatures. [2] Because A-OPV3 molecules form fibrous aggregates with a length that increases upon lowering the temperature,thisbehaviorindicatesthatacertainfiberlength is needed to induce the LD and CD effects. Hence, the increase in the LD is most likely associated with the (partial) alignment of the A-OPV3 fibers in solution arising from convectiveflow,whichiscausedbytemperaturedifferencesin the cuvette. [3] Additional measurements show that the align- ment is less significant when a smaller cuvette (0.1 vs. 1 cm) is used, [2] that is, when the flow pattern is altered. The flow pattern induced by shaking might enhance the alignment Scheme 1. The molecular structures of A-OPV3 and S-OPV4. Figure 1. a) CD spectra at 293 K of an A-OPV3 solution in dodecane after cooling from 363 K at 60 Kh À1 (black) and after shaking the solution (red). [2] b) Absorption spectra at 363 K (black), at 293 K after cooling (red), and after shaking (green; 1 cm cuvette, c = 510 À5 m) c) LD spectra of an A-OPV3 solution after cooling (black) and after shaking (red; c = 510 À5 m, T = 293 K) d) AFM image of A-OPV3 on mica drop cast from a dodecane solution (c = 110 À5 m). [*] M. Wolffs, Dr. S. J. George, Dr. Z ˇ . Tomovic ´, [+] Dr. S. C. J. Meskers, Dr. A. P. H. J. Schenning, Prof. Dr. E. W. Meijer Laboratory for Macromolecular and Organic Chemistry Eindhoven University of Technology PO Box 513, 5600MB Eindhoven (The Netherlands) Fax: (+ 31)40-245-1036 E-mail: s.c.j.meskers@tue.nl a.p.h.j.schenning@tue.nl e.w.meijer@tue.nl [ + ] Current Address: Elastogran, BASF group Global PU Specialties Research E-KUE/KFH-A10 Elastogranstrasse 60, 49448 Lemförde (Germany) [**] We would like to thank Prof.Dr. T. Aida and his group members for stimulating discussions, Dr. P. E. L. G. Leclre for the AFM measurements and the Council for Chemical Sciences of the Netherlands Organization for Scientific Research (NWO-CW) for funding. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie 8351 Angew. Chem. 2007, 119, 8351–8353 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim