Probing Solvation Effects in Binary Solvent Mixtures with the Use of Solvatochromic Dyes

*Ioanna Deligkiozi and Raffaello Papadakis*

### **Abstract**

In this work three molecules exhibiting dual sensing solvatochromic behaviors are examined in the context of solvation in binary solvent mixtures (BSMs). The compounds studied involve two functional groups with high responsiveness to solvent polarity namely pentacyanoferrate(II) (PC) and azo groups. Two of these compounds are [2]rotaxanes involving *alpha-* or *beta-* cyclodextrin (CyD) and the third is their CyD-free precursor. The dual solvatochromic behavior of these compounds is investigated in water/ethlylene glycol (EG) mixtures and their dual solvatochromic responses are assessed in terms of the intensity of solvatochromism and the extent of preferential solvation. To achieve this the linear solvation model by Kamlet, Abboud and Taft [*J. Organomet. Chem.* 1983, *48*, 2877–2887] and the two-phase model of solvation by Bagchi and coworkers [*J. Phys. Chem.* 1991, *95*, 3311–3314] are employed. The influence of the presence or lack of CyD (*alpha-* or *beta-*) on these dual solvatochromic sensors is analyzed.

**Keywords:** solvatochromic dyes, rotaxanes, preferential solvation, (non) specific solute-solvent effects, azo dyes

#### **1. Introduction**

Nowadays, solvatochromic probes (SPs) are regularly utilized in various types of applications which require sensing of environmental/medium effects in either a qualitative or a quantitative manner [1–9]. Today there is a large variety of published solvatochromic dyes corresponding to different media, e.g. organic solvents [8, 10], ionic liquids [10, 11], solvent mixtures [10, 12–15] or solvent comprising polarity modifiers [16]. What often appears to be challenging is the choice of a suitable solvatochromic probe for the description of a physicochemical problem encompassing solvent polarity effects. It has been observed that for the same solvent/cosolvent mixture, different solvatochromic dyes may provide different quantitative results [17, 18]. Indeed in many cases, different spectroscopic techniques applied on the same ternary system solvent/cosolvent/probe(solute) may provide different results. Therefore, probing solvent polarity effects and preferential solvation (PS) phenomena occurring in solvent mixtures of two or more

All compounds involved in this work (**1** and **2a,b**; **Figure 1**) have been reported in earlier publication by the author and coworkers and their synthesis, isolation and spectral analysis have been also thoroughly described [19]. All solvatochromic UV– Vis shifts have been recorded on a Perkin-Elmer Lambda 25 UV/Vis spectrophotometer. The deconvolutions of all UV–Vis spectra were implemented according to

*Probing Solvation Effects in Binary Solvent Mixtures with the Use of Solvatochromic Dyes*

All solvatochromic compounds used in this work are isolated as stable solid compounds of green-blue color. The measurements presented were conducted in fresh solutions of each compound in the desired H2O/EG mixtures (typically prepared 15 min prior to measurement). That time corresponds to the equilibration time (each sample was vigorously stirred after mixing). Directly after this period of time their electronic absorption spectra were recorded. It was observed that in all cases the solutions remained unmodified as concluded through check of the absorbances of the bands maxima which were found to be stable for at least 30 minutes after equilibration. This observation clearly indicates that all three compounds are

In this work a renowned PS model is employed in order to describe PS phenomena occurring in BSMs comprising solvatochromic solutes. The model was introduced by Bagchi and coworkers about thirty years ago and is also known as "the two-phase model of solvation" (TPMS) [32–34]. TPMS considers that solvent molecules in a BSM are distributed between a local phase and a bulk phase according to

In Eq. 1 S1 and S2 symbolize the two mixed solvents in the bulk phase while *S***<sup>1</sup>** and *S***<sup>2</sup>** symbolize the two solvents in the solvation (local) phase. Throughout this work water will be considered as *S1* whereas EG as *S2*. At the equilibrium described by Eq. 1, PS constant (*Kps*) will be related to an expression comprising both solvent-

ð Þ *<sup>N</sup>*<sup>1</sup> � *<sup>N</sup>*<sup>2</sup> *<sup>ϵ</sup>*<sup>12</sup> � *<sup>N</sup>*<sup>0</sup>

<sup>2</sup> � *<sup>ϵ</sup>*<sup>0</sup>

In Eq. 2 *ϵSi* are the interaction energies among the solute and *i*-solvent while *ϵij* corresponds to the interaction energies between solvents *i* and *j*. *N*<sup>i</sup> corresponds to the number of *i*-solvent molecules. The bulk phase in solvent molecule numbers is designated with the superscript 0. It is noteworthy that Eq. 2 involves two terms. The first bracketed term in the right-hand side of the equation corresponds to the contribution of solute-solvent interactions, while the terms in the second bracket

Finally, Eq. 3, provides *Kps* (the preferential solvation constant) related to both bulk (*x*) and local (*y*) solvent mole fractions along with the measured transition

<sup>22</sup> <sup>þ</sup> ð Þ *<sup>ϵ</sup>*<sup>11</sup> � *<sup>ϵ</sup>*<sup>22</sup>

<sup>1</sup> � *<sup>N</sup>*<sup>0</sup> 2 � �*ϵ*<sup>0</sup>

<sup>11</sup> � *<sup>ϵ</sup>*<sup>0</sup> <sup>22</sup> � � 2

*S***<sup>2</sup>** þ *S***<sup>1</sup>** ⇌ *S***<sup>1</sup>** þ *S***<sup>2</sup>** (1)

<sup>12</sup> � *<sup>N</sup>*1*ϵ*<sup>11</sup> <sup>þ</sup> *<sup>N</sup>*<sup>0</sup>

1 *ϵ*0 <sup>11</sup>þ
