Synthesis and UV/Vis Spectra of J‐Aggregating 5,5′,6,6′‐Tetrachlorobenzimidacarbocyanine Dyes for Artificial Light‐Harvesting Systems and for Asymmetrical Generation of Supramolecular Helices

Abstract

AbstractA new class of dyes, in which the self‐assembling property of surfactants is combined with the capability for light energy propagation over long distances in dye J‐aggregates, is described. This has been achieved by the syntheses of achiral 5,5′,6,6′‐tetrachlorobenzimidacarbocyanine dyes 1, possessing systematically varied hydrophobic and hydrophilic substituents at their nitrogen atoms. These substituents are introduced into the 5,6‐dichlorobenzimidazole precursor 3 by substitution, either firstly by nucleophilic reaction with ψ‐bromoalkylnitriles and secondly by quaternization with alkyl bromides (route II) or firstly by nucleophilic reaction with alkyl bromides and secondly by quaternization with ψ‐bromoalkylnitriles or ψ‐bromoalkyl esters (route III) and subsequent saponification. The UV/Vis spectra of 20 differently substituted dyes containing the same chromophore 1 have been investigated. The spectra of the dye monomers in dimethyl sulfoxide are nearly identical, with no signs of optically activity, whereas in aqueous alkaline solutions quite different spectra are obtained for the dyes, indicating the formation of different aggregates depending on the nitrogen substituents. One of these types of J‐aggregate is optically inactive and displays a single red‐shifted (with respect to the dye monomers) absorption band, resembling the behaviour of J‐aggregates of common cyanine dyes. In the cases of strongly amphiphilic 5,5′,6,6′‐benzimidacarbocyanines 1 with 1,1′‐dialkyl substituents longer than hexyl and 3,3′‐bis(2‐carboxyethyl), 3,3′‐bis(3‐carboxypropyl) or 3,3′‐bis(3‐sulfopropyl) substituents, a new type of J‐aggregate is formed, and is distinguished by a doubly or even triply split J‐absorption band that displays optical activity. A third type of aggregate showing different spectral behaviour occurs when the dyes contain very short 3,3′‐bis(carboxymethyl) substituents or strongly hydrophobic fluorinated octyl groups, or when all four nitrogen atoms are identically substituted by hydrophilic 3‐carboxypropyl groups. The various types of dye aggregates have been characterized through UV/Vis spectroscopic parameters such as the positions and widths of the absorption and fluorescence bands, the Stokes′ shifts, the coupling constants, and the strength of the J‐band splitting. The results provide new prospects for the development of new artificial light‐harvesting systems as well as for the understanding of the evolution of asymmetry in the biosphere. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)