(2)

**2.2 Examining the stability of compounds 1,2a,b in solution**

stable in solution and therefore suitable for the current investigation.

Eq. 1. The local phase lies in the vicinity of the solvation area.

solvent solute-solvent interaction energies (Eq. 2).

2 *ϵ*0

"

*kTlnKPS* ¼ ½ �þ *ϵ<sup>S</sup>*<sup>2</sup> � *ϵ<sup>S</sup>*<sup>1</sup>

<sup>þ</sup> *<sup>N</sup>*2*ϵ*<sup>22</sup> � *<sup>N</sup>*<sup>0</sup>

describe the solvent non-ideality effects.

**11**

previous work [19].

*DOI: http://dx.doi.org/10.5772/intechopen.98180*

**2.3 Preferential solvation model**

**Figure 1.**

*The three solvatochromic compounds involved in this study.*

solvents are considered as highly difficult tasks [17]. The complexity of those physicochemical problems is high and the interpretation of the solvent-solvent or solute-solvent effects sensed by SPs needs to be carefully undertaken. In this work, the authors examine the solvatochromic responses of two probing groups: the azogroup and the pentacyanoferrate(II) group of three molecules dissolved in binary solvent mixtures (BSMs) involving water and ethylene glycol (EG). From the three molecules employed two are [2]rotaxanes involving *alpha-* or *beta-* cyclodextrin (CyD) (compounds **2a** and **2b** respectively, **Figure 1**) and the third is their precursor lacking a CyD wheel (compound **1**, **Figure 1**; in the text will be called cyclodextrin-free dumbbell-like compound or CFD). All three are recently developed solvatochromic compounds [19] and they involve the same π-conjugated viologen-based linear skeletons bearing an azobenzene bridge and pentacyanoferrate(II) end-groups (**Figure 1**). The aforementioned compounds fall under the umbrella of an important family of multifunctional dyes primarily because of the high technological and industrial importance of azobenzene dyes [20–22] as well as the pronounced chromic and redox behavior [23], photochromism [24], photoconductivity [25] and strong electron withdrawing aptitude of viologens (also known as paraquats) [23, 26]. This strong electron accepting capacity of para- and monoquats is vital for the development of push-pull systems [27–30]. Towards the latter milestone the use of suitable electron donating substituents is vital. Papadakis et al. has shown that pentacyanoferrate(II) units can trigger an intense solvatochromic behavior in such systems in various types of media [14, 16, 30, 31] and more recently Deligkiozi et al. hinted that the *<sup>n</sup>* ! *<sup>π</sup>* <sup>∗</sup> transitions of the azo group in **<sup>1</sup>** and **2a,b** are sensitive to solvent polarity [19]. In this work the dual solvatochromic sensing of these compounds is thoroughly examined in terms of solvent-solute, solvent-solvent and preferential solvation effects. In this work water/ethylene glycol mixtures were chosen as perfect BSM candidate-models of dipolar media with substantial contributions in the development of specific effects (mainly H-bonding).
