**4. Solvatochromic supramolecular systems**

#### **4.1 Generalities**

Solvatochromism is a well-studied phenomenon occurring in many diverse systems. It is described as the change in color (*χρώμα*, Greek word for color) induced by solvents. In a broader context, the term solvatochromism covers changes in the electronic (UV-Vis), FTIR, Raman, or EPR spectra induced by solvents [28]. A

**Figure 7.** *Solvent-induced shuttle movement in a [2]rotaxane. Reprinted with permission from Cai and coworkers [26].*

vast number of solvatochromic compounds have been reported to date exhibiting large structural diversity [29]. The most frequently studied class of solvatochromic dyes involves dyes bearing a **D-π-Α** structure where **A** is an electron-withdrawing moiety, **D** is an electron-donating group, and **π** is a conjugated system (often aromatic), separating **A** and **D**. **D-π-Α** dyes have recently received much attention as they can be used in hi-tech applications including materials with nonlinear optical (NLO) properties [30, 31], chromotropic sensors and molecular switches [32, 33]. They serve also in many cases as multifunctional building blocks for supramolecular architectures, e.g., in rotaxanes [34]. Some examples of such dyes are depicted in **Figure 8**. The common characteristic of the compounds **I–IV** is that their **D** part (an iodine anion in **I**, a phenolate in **II**, a carbanion in **III**, and an iron(II) cation in **IV**) is capable of transferring an electron pair (in **II** and **III**) or a single electron (in **I** and **IV**) to the electron-deficient positively charged pyridinium ring. The π-system through which the charge transfer occurs is either the aromatic backbone of pyridine itself (the cases of dyes **I**, **III**, and **IV**) or another π-system in conjugation with the pyridine ring (the case of dye **II**). This charge transfer (CT) is induced by light. The required energy of light for the CT transition depends strongly on solvent polarity [28]. Noteworthy, solvent polarity can affect CT energy in various ways. When the increase of medium polarity leads to a drop of the CT energy of a dye, the corresponding effect is called positive solvatochromism. In those cases bathochromic shifts in the electronic spectra of the compound are induced by an increase in solvent polarity (**Figure 9A**). When the opposite effect is observed, the observed phenomenon is called negative solvatochromism (**Figure 9B**). The main focus of this section is the solvatochromism in supramolecular systems.
