Influencing Equilibria Large Electric-Field Effects on the Dipolar Aggregation of Merocyanine Dyes** Rüdiger Wortmann,* Ulrich Rösch, Mesfin Redi- Abshiro, and Frank Würthner* Dedicated to Professor Wolfgang Liptay on the occasion of his 75th birthday Theeffectsofexternalelectricfieldsonmacroscopicsystems, including phases with chemical equilibria, is of fundamental interest in chemical thermodynamics. [1] General thermody- namic equations for dielectrics of variable composition in external electric fields have been developed by Koenig [2] basedontheworkbyGuggenheim. [3] Inaddition,thedynamic effectsofpulsedelectricfieldswereinvestigatedandapplied in studies of fast reaction kinetics and dielectric relaxation phenomena related to chemical rate processes. [4–6] The influenceofbiologicalmembranepotentialsonthedimeriza- tionofafluorescentdyewasstudiedbyWebbandDragsten. [7] Liptay et al. investigated electric-field effects on the forma- tion of charge transfer (CT) complexes. [8] Herein, we report theobservationofanunusuallystrongelectric-fieldeffecton the dimerization equilibrium of the strongly dipolar mero- cyanine dye 1. The electronic structure of merocyanine dyes, such as 1, withelectron-donatingandelectron-acceptinggroupsisoften described in terms of a two-state valence-bond model. This simple model assumes resonance of neutral and zwitterionic structures and provides a useful qualitative interpretation of phenomena, such as solvatochromism and bond-length alter- nation. [9] The electronic structure of the dyes may vary between the “neutral” form 1 (left) and the “zwitterionic” form(right).Dyeswithequalcontributionsoftheneutraland zwitterionic forms are in the “cyanine-limit”. The two-level valence-bond model has been discussed critically at a higher leveloftheory. [10] Merocyanine dyes have been intensely studied in the quest for promising chromophores for electrooptically active materials, such as electrooptical and photorefractive poly- mers. [11] It has been demonstrated that the photorefractive figure-of-merit of the dyes is maximized close to the cyanine limitandchromophoresoptimizedaccordingtothisprinciple have been presented. [12] As a result of the zwitterionic character of the dyes near to and beyond the cyanine limit theirdipolemomentsareverylargewhichmayleadtodimer formationbydipolaraggregationinthepolymerandinliquid solution. [13] Dimer formation is especially pronounced for dye 1 and can be observed even in dilute dioxane solution. The absorption spectrum, Figure1, shows two concentration- dependent bands with a clear isobestic point over a broad concentration range. The concentration dependence of the bandsindicatesanequilibrium2MÐDbetweentheonlytwo species, monomeric (M) and dimeric (D) dye units. The intensity of the monomer band decreases with increasing concentration while that of the dimer increases. The dimer bandishypsochromicallyshiftedwithrespecttothemonomer band as is typical for H-type aggregates. This effect is attributed to an (almost) antiparallel ordering of the dipolar dyemolecules(Scheme1). [13] A quantitative analysis of the concentration dependence by nonlinear regression allows deconvolution of the optical Figure 1. Optical absorption spectrum (g, “exp”) of a solution of 1 in dioxane (c 0M = 5.1610 6 m, T = 298 K) and derived spectra of pure monomer and dimer (c). Scheme 1. [*] Prof. Dr. R. Wortmann, Dipl.-Chem. U. Rösch, Dipl.-Chem. M. Redi-Abshiro Institut für Physikalische Chemie Technische Universität Kaiserslautern Erwin-Schrödinger-Strasse, 67633 Kaiserslautern (Germany) Fax: (+ 49)631-2052187 E-mail: ruediger.wortmann@chemie.uni-kl.de Prof. Dr. F. Würthner Institut für Organische Chemie Universität Würzburg Am Hubland, 97074 Würzburg (Germany) Fax: (+ 49)931-8884756 E-mail: wuerthner@chemie.uni-wuerzburg.de [**] This work was supported by the Volkswagen Foundation and the Fonds der Chemischen Industrie. We thank our Erasmus students Jordi Bonet (Murcia) and Ewan Bennet (Glasgow) for performing some of the experiments. Communications 2080 # 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200250782 Angew. Chem. Int. Ed. 2003, 42, 2080 – 2083