Sensing different micellar microenvironments with solvatochromic dyes of variable lipophilicity Marcos Caroli Rezende * , Carolina Mascayano, Luis Briones, Carolina Aliaga Facultad de Química y Biología, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile article info Article history: Received 10 August 2010 Received in revised form 24 December 2010 Accepted 5 January 2011 Available online 11 January 2011 Keywords: Lipophilicity Solvatochromic phenolate betaine dyes Micelles Block copolymers Dynamics simulation abstract The UV/Vis spectral behavior of three solvatochromic N-alkyl a-pyridones of varying lipophilicity was investigated in aqueous solutions of a block copolymer at variable temperatures. The solvatochromic responses of the three analogues to the distinct microenvironments were compared and interpreted in terms of their different location in the microheterogeneous system. The interpretation was supported by 1 H NMR measurements and by molecular dynamics simulations that mimicked the interactions of the dyes with the polymeric micelle in water. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Solvatochromic betaine dyes have been employed for some time as polarity sensors in homogeneous and microheterogeneous media [1e4]. The position of their longest-wavelength band in a series of solvents has been used to characterize their polarity. Ideally, the same betaine dye should be employed in all media, so that its spectral responses in different solvents can be compared. However, because of solubility problems, this is not always possible, and betaine dyes with different solubilities are sometimes employed as sensors for different solvents. In such cases, the solvatochromic sensors should differ in their solubility, as a consequence of their different lipophilicity, but not in their spectral response to the medium. This assumption had traditionally been taken for granted, until it was shown that, even for analogous probes in homogeneous media, their lipophilicity is a factor to be considered in characterizing their microenvironment. Analogous compounds with a common donoreacceptor pair but different lipophilicities have shown different spectral responses in pure solvents and in binary solvent mixtures [5e7]. In consequence, in the characterization of micellar microenvironments with the aid of sol- vatochromic dyes, we are faced with an ambiguous situation even if a pair of analogous sensors with different lipophilicities is employed. A different spectral response by the pair may either be caused (a) by their differences in lipophilicity, which affect their intrinsic response to the same environment, or (b) by the fact that they actually sense different microenvironments. In a previous work in which we compared the spectra of the two analogous a-pyridone betaine dyes 1 and 3 in pure solvents and in a binary solvent mixture, we drew attention to the fact that they behaved differently even in homogeneous media [7]. Their different behavior in the presence of a micelle was cautiously interpreted by us as arising from their sensing different microenvironments, a conclusion that was based on the observation that their differences were larger in that medium than in pure solvents. Our caution was motivated by the complex nature of micellar solutions. Conclusions on the polarity of these systems from the position of the solvatochromic band of a dye in their presence may be meaningless. A solvatochromic probe in the presence of a micelle may be partitioned between different microenvironments, yielding a complex response that depends on factors such as relative concentrations and temperature. We have recently examined this situation in a study of the thermochromism of the E T (30) betaine dye 4 in micellar solutions of block polymers (“poloxamers”) [8] (Scheme 2). These copolymers are well known for their tendency to form micelles in water by aggregation of their individual chains (“unimers”) in a temperature range that depends on the polymer composition and its concentration [9e11]. We showed that, as the temperature was raised, the sol- vatochromic dye was partitioned between two microenvironments, the aqueous unimeric solution and the polymeric micelle. Thus, these * Corresponding author. E-mail address: marcos.caroli@usach.cl (M.C. Rezende). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2011.01.001 Dyes and Pigments 90 (2011) 219e224