We investigate the second-harmonic generation induced by performing transient all-optical poling in a solution of the cyanine dye 3,3'-diethyloxadicarbocyanine iodide (DODCI). Second-harmonic generation demonstrates that the DODCI dye associated with its iodide counter ion is noncentrosymmetric. Polarization analysis of the process reveals that the iodide counter ion may be located at one of the extremities of the polymethine molecule, along its main axis, Interestingly, we show that the noncentrosymmetric supramolecular assembly is favored in low polarity solvents. (c) 2005 Elsevier B.V. All rights reserved.