**4.2 Supramolecules involving solvatochromic entities**

When conducting a deep literature search, it is easily made obvious that there are not many examples of solvent-switchable supramolecular structures such as rotaxanes and catenanes exhibiting also solvatochromism. As mentioned above, solvatochromic supramolecular assemblies exhibit a strong change in position and sometimes the intensity of their absorption spectra, which is achieved by changing

#### **Figure 8.**

*Various pyridinium-based solvatochromic D-π-Α dyes. I, Kosower salt [28]; II, Reichardt's betaine [28]; III, betaine of N-aryl and N′-phenacetyl-4,4′-bipyridines [35, 36]; and IV, pentacyanoferrate(II) complexes with N-aryl4,4′-bipyridines (monoquats) ligands (R = OMe, Me, H, Cl, Br, CN) [37, 38].*

**215**

**Figure 10.**

*from Günbaş et al. [39].*

*Solvent Effects in Supramolecular Systems DOI: http://dx.doi.org/10.5772/intechopen.86981*

in electron acceptor-donor systems.

*solvatochromism (G corresponds to ground and E to excited state).*

**Figure 9.**

linear molecule and the macrocycle molecule.

the polarity of the solvent (solvatochromic effect). This phenomenon is pronounced

In 2007, Toma and his scientific team published a paper in which they disclosed the solvatochromic properties of a [2]rotaxane, which involved a β-cyclodextrin

*(A) Shuttle movement in a [2]rotaxane and (B) solvatochromic shifts observed. Reprinted with permission* 

An interesting example combining solvent-controlled shuttle movement in a rotaxane (see previous section) and solvatochromic behavior was reported by Günbaş et al. in 2011 (**Figure 10**) [39]. The solvatochromic behavior of their [2] rotaxane and its dumbbell-like precursor molecule was investigated in a variety of solvents of different polarities. It was observed that both compounds exhibited solvatochromic shifts in their absorption spectra when increasing the polarity of the solvent. Spectroscopic data showed a wavelength shift of 575 nm in toluene to 621 nm in DMSO for the molecule which corresponds to a positive solvatochromic shift. The observed values were attributed to the pyrrolidine group. For the [2] rotaxane, however, the solvatochromic changes were smaller. The absorbance was shifted from 608 to 621 nm when the solvent was changed from the nonpolar toluene to the highly polar DMSO. In general, both in the case of the [2]rotaxane and the dumbbell molecule, solvatochromic shifts were observed, indicating that polar solvent interacts stronger with the molecules and also stabilizes the excited state. Of course, this effect is stronger for the dumbbell molecule than for [2]rotaxane, an effect which could be attributed to interactions developed among the chromophore

*Plots of CT energy as a function of solvent polarity corresponding to (A) positive and (B) negative* 

*Solvent Effects in Supramolecular Systems DOI: http://dx.doi.org/10.5772/intechopen.86981*

#### **Figure 9.**

*Solvents, Ionic Liquids and Solvent Effects*

supramolecular systems.

**4.2 Supramolecules involving solvatochromic entities**

When conducting a deep literature search, it is easily made obvious that there are not many examples of solvent-switchable supramolecular structures such as rotaxanes and catenanes exhibiting also solvatochromism. As mentioned above, solvatochromic supramolecular assemblies exhibit a strong change in position and sometimes the intensity of their absorption spectra, which is achieved by changing

*Various pyridinium-based solvatochromic D-π-Α dyes. I, Kosower salt [28]; II, Reichardt's betaine [28]; III, betaine of N-aryl and N′-phenacetyl-4,4′-bipyridines [35, 36]; and IV, pentacyanoferrate(II) complexes with* 

*N-aryl4,4′-bipyridines (monoquats) ligands (R = OMe, Me, H, Cl, Br, CN) [37, 38].*

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

**214**

**Figure 8.**

*Plots of CT energy as a function of solvent polarity corresponding to (A) positive and (B) negative solvatochromism (G corresponds to ground and E to excited state).*

the polarity of the solvent (solvatochromic effect). This phenomenon is pronounced in electron acceptor-donor systems.

An interesting example combining solvent-controlled shuttle movement in a rotaxane (see previous section) and solvatochromic behavior was reported by Günbaş et al. in 2011 (**Figure 10**) [39]. The solvatochromic behavior of their [2] rotaxane and its dumbbell-like precursor molecule was investigated in a variety of solvents of different polarities. It was observed that both compounds exhibited solvatochromic shifts in their absorption spectra when increasing the polarity of the solvent. Spectroscopic data showed a wavelength shift of 575 nm in toluene to 621 nm in DMSO for the molecule which corresponds to a positive solvatochromic shift. The observed values were attributed to the pyrrolidine group. For the [2] rotaxane, however, the solvatochromic changes were smaller. The absorbance was shifted from 608 to 621 nm when the solvent was changed from the nonpolar toluene to the highly polar DMSO. In general, both in the case of the [2]rotaxane and the dumbbell molecule, solvatochromic shifts were observed, indicating that polar solvent interacts stronger with the molecules and also stabilizes the excited state. Of course, this effect is stronger for the dumbbell molecule than for [2]rotaxane, an effect which could be attributed to interactions developed among the chromophore linear molecule and the macrocycle molecule.

In 2007, Toma and his scientific team published a paper in which they disclosed the solvatochromic properties of a [2]rotaxane, which involved a β-cyclodextrin

#### **Figure 10.**

*(A) Shuttle movement in a [2]rotaxane and (B) solvatochromic shifts observed. Reprinted with permission from Günbaş et al. [39].*

(β-CD) macrocycle [40]. The linear molecule of their rotaxane consisted of trans-1,4-di-[(4-pyridyl) ethylene] benzene (**Figure 11**) and trans ferrocyanide(II) anions ligated by the pyridyl groups of the linear molecule which act as stoppering groups. The UV-Vis absorption spectral analysis of {[FeII(CN)]2(BPEB)} (dumbbell) and the corresponding β-CD-involving rotaxane indicated that the dumbbell molecule exhibited two absorption bands, one around 352 nm and the other at 454 nm.

During the addition of β-CD to the linear molecule solution, a wavelength shift was observed denoting the formation of the [2] rotaxane, which was attributable to the metal-to-ligand charge transfer (MLCT). The formation of [2]rotaxane resulted in a decrease in the energy of MLCT, i.e., a bathochromic shift from 454 to 479 nm. Commonly, the reaction of the iron complex (II) with N-heterocyclic substituents results in deep chromatic shifts of MLCT when the final products are dissolved in less polar solvents. In these systems the hydrophobic forces increase the solvatochromic effect. Thus, for the [2]rotaxane, the low-wavelength shift values are attributed to the inclusion of the β-CD cavity and to the selective solubilization of the rotaxane. This behavior could be related to the stabilization of the energy levels of the complex between β-CD and the ligand leading to a decrease in the energy of MLCT [41].

Various other similar examples of solvation effects on rotaxane have been reported in the literature; one of these examples is the [2]rotaxane reported by Baer and Macartney in 2000 [41]. Cyclodextrin (CD) inclusion complexes with linear guest parts have been studied extensively using a variety of spectroscopic techniques. There have also been several reports of cyclodextrin (α- or β-CD)-based rotaxanes, polyrotaxanes, and catenanes using linear parts (L) containing biphenyl, stilbene, and azobenzene dyes. These [2]rotaxanes can be formed rapidly by the addition of [FeII(CN)5] <sup>3</sup><sup>−</sup> stoppering units, and a cyclodextrin macrocyclic unit can be threaded by the linear Skeleton. It has been proven that such [2]rotaxanes exhibit intense metal-to-ligand charge transfer (MLCT) transition bands in the visible spectrum. This transition as mentioned above is prone to changes in energy and intensity induced by solvents (solvatochromism). Different ligands acting as the axial parts for series of rotaxanes have been exploited so far such as *trans*-1,2-bis(4-pyridyl) ethylene ligand (BPE, **Figure 12**) which can nicely provide a conjugated bridging of inner sphere and intervalence electron transfer between transition metal centers [42]. The 4,4′-azopyridine ligand (AZP, **Figure 12**) has been employed as a bridging ligand as well mainly for ruthenium amines and porphyrins [43, 44]. The APA and PCA, i.e., 4-acetylpyridine azine and 4-pyridinecarboxaldehyde azine, respectively, are interesting examples of azines which have also been reported as building blocks of [2],rotaxanes of this class [45] (structures depicted in **Figure 12** correspond to some prominent examples of such ligands/linear skeletons). Spectroscopic studies conducted in order to validate the maximum wavelength difference for CD-free

**217**

water, aqueous EG mixtures, and neat EG (see **Figure 13**).

*Solvent Effects in Supramolecular Systems DOI: http://dx.doi.org/10.5772/intechopen.86981*

*cyclodextrin-involving [2]rotaxanes (right).*

**Figure 12.**

dumbbells and CD-involving rotaxanes resulted in the following **λmax(nm) {Fe(CN)5L3−/ (Fe(CN)5{L'CD}3−)}**: (*i*) BPE, 460/496 (α-CD) and 478 (β-CD); (*ii*) AZP, 596/698 (α-CD) and 644 (β-CD); (*iii*) PCA 508/536 (α-CD) and 518 (β-CD); and (*iv*) APA 448/456 (α-CD) and 458 (β-CD). It is evident in all cases that the spectra are affected by the presence of α- or β-CD, resulting in bathochromic shifts in the MLCT band in the visible spectrum. This effect constitutes an important response (of high sensitivity) to the polarity of their environment. This effect is much connected to solvatochromism. Indeed the aptitude of both non-rotaxanated dumbbells and the corresponding rotaxanes to yield solvatochromic shifts is very large [41]. In 2012 Deligkiozi et al. reported the synthesis of a [2]rotaxane consisting of a fully conjugated arylazo-based linear part entrapped in α-cyclodextrin and stoppered by bulky dinitrophenyl end groups [46]. Recording the UV-Vis spectra of both compounds, a broadband in the region 300–400 nm was observed, which was attributed to the π-π\* transition of the group (▬N〓N▬). Comparing the spectra of the [2]rotaxane and the dumbbell precursor, a bathochromic shift was observed. Specifically, the maximum wavelength of the dumbbell precursor was positioned at 337 nm, whereas that of the [2]rotaxane was centered at 351 nm. This shift is attributed to the interaction of the α-CD cavity and the (▬N〓N▬) group of the dumbbell-like compound, which causes a decrease in the energy difference between the ground and excited states of the azo-compound leading to bathochromism. Both compounds were found to undergo *E*-*Z* reversible isomerizations, and in the case of the [2]rotaxane, light-induced shuttle movement was reported [46]. Interestingly the same complexes were found to be photoconductive in the solid state, and this was accredited to the extended π-conjugation in these molecules [47, 48]. These findings enforced the same group to develop a system involving the same π-conjugated backbone, however involving strong electron-donating groups (pentacyanoferrate(II) stoppering groups). This led to the formation of strong **D-π-A** systems (**Figure 13**) [49]. More recently, Papadakis et al. exploited the solvatochromism of two CD-containing [2]rotaxanes and their CD-free linear precursor in order to investigate preferential solvation (PS) effects in water/ethylene glycol (EG) mixtures [50]. Pentacyanoferrate(II) groups which served as strong electron donors facilitated charge transfer to the viologen electron-deficient parts. It was proven that the pentacyanoferrate(II) units were able to trigger an intense solvatochromic behavior in such systems in neat solvents, solvent mixtures, and other types of aqueous media. In order to study the solvatochromic behaviors, aqueous/ethylene glycol (EG) mixtures were used as the media, and alteration of the polarity was achieved through changing solvent/cosolvent mole ratio. The medium-responsive behavior of these compounds (**Figure 13**) is mainly pronounced in very polar media such as

*Pyridine-involving ligands BPE, PCA, AZP, and APA (left) and the corresponding dumbbells and* 

**Figure 11.** *Pioneering solvatochromic [2]rotaxane synthesis by Toma and coworkers [40].*

*Solvent Effects in Supramolecular Systems DOI: http://dx.doi.org/10.5772/intechopen.86981*

**Figure 12.**

*Solvents, Ionic Liquids and Solvent Effects*

MLCT [41].

addition of [FeII(CN)5]

(β-CD) macrocycle [40]. The linear molecule of their rotaxane consisted of trans-1,4-di-[(4-pyridyl) ethylene] benzene (**Figure 11**) and trans ferrocyanide(II) anions ligated by the pyridyl groups of the linear molecule which act as stoppering groups. The UV-Vis absorption spectral analysis of {[FeII(CN)]2(BPEB)} (dumbbell) and the corresponding β-CD-involving rotaxane indicated that the dumbbell molecule exhibited two absorption bands, one around 352 nm and the other at 454 nm.

During the addition of β-CD to the linear molecule solution, a wavelength shift was observed denoting the formation of the [2] rotaxane, which was attributable to the metal-to-ligand charge transfer (MLCT). The formation of [2]rotaxane resulted in a decrease in the energy of MLCT, i.e., a bathochromic shift from 454 to 479 nm. Commonly, the reaction of the iron complex (II) with N-heterocyclic substituents results in deep chromatic shifts of MLCT when the final products are dissolved in less polar solvents. In these systems the hydrophobic forces increase the solvatochromic effect. Thus, for the [2]rotaxane, the low-wavelength shift values are attributed to the inclusion of the β-CD cavity and to the selective solubilization of the rotaxane. This behavior could be related to the stabilization of the energy levels of the complex between β-CD and the ligand leading to a decrease in the energy of

Various other similar examples of solvation effects on rotaxane have been reported in the literature; one of these examples is the [2]rotaxane reported by Baer and Macartney in 2000 [41]. Cyclodextrin (CD) inclusion complexes with linear guest parts have been studied extensively using a variety of spectroscopic techniques. There have also been several reports of cyclodextrin (α- or β-CD)-based rotaxanes, polyrotaxanes, and catenanes using linear parts (L) containing biphenyl, stilbene, and azobenzene dyes. These [2]rotaxanes can be formed rapidly by the

be threaded by the linear Skeleton. It has been proven that such [2]rotaxanes exhibit intense metal-to-ligand charge transfer (MLCT) transition bands in the visible spectrum. This transition as mentioned above is prone to changes in energy and intensity induced by solvents (solvatochromism). Different ligands acting as the axial parts for series of rotaxanes have been exploited so far such as *trans*-1,2-bis(4-pyridyl) ethylene ligand (BPE, **Figure 12**) which can nicely provide a conjugated bridging of inner sphere and intervalence electron transfer between transition metal centers [42]. The 4,4′-azopyridine ligand (AZP, **Figure 12**) has been employed as a bridging ligand as well mainly for ruthenium amines and porphyrins [43, 44]. The APA and PCA, i.e., 4-acetylpyridine azine and 4-pyridinecarboxaldehyde azine, respectively, are interesting examples of azines which have also been reported as building blocks of [2],rotaxanes of this class [45] (structures depicted in **Figure 12** correspond to some prominent examples of such ligands/linear skeletons). Spectroscopic studies conducted in order to validate the maximum wavelength difference for CD-free

<sup>3</sup><sup>−</sup> stoppering units, and a cyclodextrin macrocyclic unit can

**216**

**Figure 11.**

*Pioneering solvatochromic [2]rotaxane synthesis by Toma and coworkers [40].*

*Pyridine-involving ligands BPE, PCA, AZP, and APA (left) and the corresponding dumbbells and cyclodextrin-involving [2]rotaxanes (right).*

dumbbells and CD-involving rotaxanes resulted in the following **λmax(nm) {Fe(CN)5L3−/ (Fe(CN)5{L'CD}3−)}**: (*i*) BPE, 460/496 (α-CD) and 478 (β-CD); (*ii*) AZP, 596/698 (α-CD) and 644 (β-CD); (*iii*) PCA 508/536 (α-CD) and 518 (β-CD); and (*iv*) APA 448/456 (α-CD) and 458 (β-CD). It is evident in all cases that the spectra are affected by the presence of α- or β-CD, resulting in bathochromic shifts in the MLCT band in the visible spectrum. This effect constitutes an important response (of high sensitivity) to the polarity of their environment. This effect is much connected to solvatochromism. Indeed the aptitude of both non-rotaxanated dumbbells and the corresponding rotaxanes to yield solvatochromic shifts is very large [41].

In 2012 Deligkiozi et al. reported the synthesis of a [2]rotaxane consisting of a fully conjugated arylazo-based linear part entrapped in α-cyclodextrin and stoppered by bulky dinitrophenyl end groups [46]. Recording the UV-Vis spectra of both compounds, a broadband in the region 300–400 nm was observed, which was attributed to the π-π\* transition of the group (▬N〓N▬). Comparing the spectra of the [2]rotaxane and the dumbbell precursor, a bathochromic shift was observed. Specifically, the maximum wavelength of the dumbbell precursor was positioned at 337 nm, whereas that of the [2]rotaxane was centered at 351 nm. This shift is attributed to the interaction of the α-CD cavity and the (▬N〓N▬) group of the dumbbell-like compound, which causes a decrease in the energy difference between the ground and excited states of the azo-compound leading to bathochromism. Both compounds were found to undergo *E*-*Z* reversible isomerizations, and in the case of the [2]rotaxane, light-induced shuttle movement was reported [46]. Interestingly the same complexes were found to be photoconductive in the solid state, and this was accredited to the extended π-conjugation in these molecules [47, 48]. These findings enforced the same group to develop a system involving the same π-conjugated backbone, however involving strong electron-donating groups (pentacyanoferrate(II) stoppering groups). This led to the formation of strong **D-π-A** systems (**Figure 13**) [49]. More recently, Papadakis et al. exploited the solvatochromism of two CD-containing [2]rotaxanes and their CD-free linear precursor in order to investigate preferential solvation (PS) effects in water/ethylene glycol (EG) mixtures [50]. Pentacyanoferrate(II) groups which served as strong electron donors facilitated charge transfer to the viologen electron-deficient parts. It was proven that the pentacyanoferrate(II) units were able to trigger an intense solvatochromic behavior in such systems in neat solvents, solvent mixtures, and other types of aqueous media. In order to study the solvatochromic behaviors, aqueous/ethylene glycol (EG) mixtures were used as the media, and alteration of the polarity was achieved through changing solvent/cosolvent mole ratio. The medium-responsive behavior of these compounds (**Figure 13**) is mainly pronounced in very polar media such as water, aqueous EG mixtures, and neat EG (see **Figure 13**).

#### **Figure 13.**

*A) The dumbbell-shaped push-pull backbone developed by Deligkiozi et al. (top) and the corresponding cyclodextrin involving [2]rotaxanes (bottom). B) The bathochromic shifts of the MLCT band of the [2] rotaxane with α-cyclodextrin depicted in A observed in aqueous ethylene glycol mixtures. Reprinted with permission from: [34] and [50].*

In some cases rotaxanes and catenanes can also exhibit solvent-dependent emission of light. A representative example is that by Baggerman et al. involving [2]rotaxanes and [3]rotaxanes bearing a tetraphenoxy perylene diimide core [51]. In their work Baggerman et al. observed the influence of hydrogen bonding developed between the amide and the wheel macrocycle of these rotaxanes on the optical behavior of the chromophore (perylene). Specifically, they showed that both absorption and fluorescence spectra are bathochromically shifted upon rotaxanation. All systems including the wheel-free axle (WFA) exhibited fluorosolvatochromism with red shifts of up to 47 nm (WFA case) and a reduced fluorosolvatochromism when going to the [2] rotaxanes and [3] rotaxanes [51]. On the other hand, Boer et al. very recently exploited the solvent-dependent excimer and exciplex emissive behavior of naphthalene diimide metallomacrocycles and catenanes and thus managed to perform a solution speciation of the metallosupramolecular complexes and their solvent-dependent nature [52].

#### **4.3 Supramolecular solvatochromism**

Solvatochromism derived by an interaction of solvent molecules with a supramolecular system is characterized by various authors as "supramolecular solvatochromism." In many cases the systems involve transition metals coordinated to ligands forming supramolecular architectures in which solvent molecules can be trapped or simply interact with parts of the system giving rise to different responses, e.g., in their electronic spectra.

A characteristic early such example is that reported by Lee and Kimizuka in 2002 [53]. In their studies they developed a lipid-packaged 1D supramolecular complex bearing platinum. The anionic lipids acted as counter anions of the positively charged Pt complex. The electronic spectra of the as described supramolecular complex were found to be readily influenced by solvent polarity and that the packaging of the complex is vital for the overall medium-responsive properties. Some years later, Kuroiwa et al. reported on the supramolecular solvatochromism of lipid-packaged, mixed-valence linear platinum complexes (**Figure 14**) which were investigated in dispersions employing the solvents CHCl3, chlorocyclohexane, and methylcyclohexane [54]. It was found that solid samples were all indigo-colored but displayed supramolecular thermochromism, attributed to heat-induced dissociation and concomitant recovery of coordination chains. The reassembled supramolecular complexes exhibited color changes depending on the solvent employed, and the CT energy measured was found to decrease as the polarity of the organic medium

**219**

**Figure 15.**

*from: Nikolayenko et al.[56].*

*Solvent Effects in Supramolecular Systems DOI: http://dx.doi.org/10.5772/intechopen.86981*

CHCl3, and 1,2-dichloroethane.

*Structure of the two Pt(IV) complexes of Kuroiwa et al. [54].*

**Figure 14.**

of the variable colors of crystals.

increases following the sequence: methylcyclohexane, benzene, chlorocyclohexane,

Such approaches gain more and more the attention of materials scientist as it could be envisioned that through mixing optical sensors and solvatochromic species or even aggregachromic compounds, the effects of various stimuli on the rheology of viscoelastic gels (VEGs) could be facilitated and this is considered as an important step before the development of new products based on VEGs becomes reality [55]. More recently, Nikolayenko et al. reported the supramolecular solvatochromic behavior of a dinuclear copper(II)-involving metallacycle [56]. The authors exploited the intense solvatochromic behavior of the CuII complex and the capacity of the macrocycle to trap small solvent molecules like tetrahydrofuran, diethyl ether, and pentane at temperatures well above their boiling points. The latter effect is attributed to the suitable guest shape and size which drastically limit lattice diffusion. Solvent exchange was found to induce intense color changes (**Figure 15B**) and sizable shifts in the visible region of the diffuse reflectance spectra (**Figure 15B**). The high intensity of the supramolecular solvatochromic effect is furthermore excellently illustrated through microphotographs

*A) Synthesis of [Cu2Cl4L2].2DMSO (1) from 1,10-[1,4-phenylenebis(methylene)]bis(2-methyl-1Himidazole) and copper(II) chloride dihydrate. B) Visible-region diffuse reflectance spectra (bottom) quantitatively illustrate the broad-spectrum (540–624 nm) solvatochromism of 1–6. Reprinted with permission*  *Solvent Effects in Supramolecular Systems DOI: http://dx.doi.org/10.5772/intechopen.86981*

*Solvents, Ionic Liquids and Solvent Effects*

**Figure 13.**

*permission from: [34] and [50].*

In some cases rotaxanes and catenanes can also exhibit solvent-dependent emission of light. A representative example is that by Baggerman et al. involving [2]rotaxanes and [3]rotaxanes bearing a tetraphenoxy perylene diimide core [51]. In their work Baggerman et al. observed the influence of hydrogen bonding developed between the amide and the wheel macrocycle of these rotaxanes on the optical behavior of the chromophore (perylene). Specifically, they showed that both absorption and fluorescence spectra are bathochromically shifted upon rotaxanation. All systems including the wheel-free axle (WFA) exhibited fluorosolvatochromism with red shifts of up to 47 nm (WFA case) and a reduced fluorosolvatochromism when going to the [2] rotaxanes and [3] rotaxanes [51]. On the other hand, Boer et al. very recently exploited the solvent-dependent excimer and exciplex emissive behavior of naphthalene diimide metallomacrocycles and catenanes and thus managed to perform a solution speciation of the metallosupra-

*A) The dumbbell-shaped push-pull backbone developed by Deligkiozi et al. (top) and the corresponding cyclodextrin involving [2]rotaxanes (bottom). B) The bathochromic shifts of the MLCT band of the [2] rotaxane with α-cyclodextrin depicted in A observed in aqueous ethylene glycol mixtures. Reprinted with* 

molecular complexes and their solvent-dependent nature [52].

Solvatochromism derived by an interaction of solvent molecules with a supramolecular system is characterized by various authors as "supramolecular solvatochromism." In many cases the systems involve transition metals coordinated to ligands forming supramolecular architectures in which solvent molecules can be trapped or simply interact with parts of the system giving rise to different

A characteristic early such example is that reported by Lee and Kimizuka in 2002 [53]. In their studies they developed a lipid-packaged 1D supramolecular complex bearing platinum. The anionic lipids acted as counter anions of the positively charged Pt complex. The electronic spectra of the as described supramolecular complex were found to be readily influenced by solvent polarity and that the packaging of the complex is vital for the overall medium-responsive properties. Some years later, Kuroiwa et al. reported on the supramolecular solvatochromism of lipid-packaged, mixed-valence linear platinum complexes (**Figure 14**) which were investigated in dispersions employing the solvents CHCl3, chlorocyclohexane, and methylcyclohexane [54]. It was found that solid samples were all indigo-colored but displayed supramolecular thermochromism, attributed to heat-induced dissociation and concomitant recovery of coordination chains. The reassembled supramolecular complexes exhibited color changes depending on the solvent employed, and the CT energy measured was found to decrease as the polarity of the organic medium

**4.3 Supramolecular solvatochromism**

responses, e.g., in their electronic spectra.

**218**

**Figure 14.** *Structure of the two Pt(IV) complexes of Kuroiwa et al. [54].*

increases following the sequence: methylcyclohexane, benzene, chlorocyclohexane, CHCl3, and 1,2-dichloroethane.

Such approaches gain more and more the attention of materials scientist as it could be envisioned that through mixing optical sensors and solvatochromic species or even aggregachromic compounds, the effects of various stimuli on the rheology of viscoelastic gels (VEGs) could be facilitated and this is considered as an important step before the development of new products based on VEGs becomes reality [55]. More recently, Nikolayenko et al. reported the supramolecular solvatochromic behavior of a dinuclear copper(II)-involving metallacycle [56]. The authors exploited the intense solvatochromic behavior of the CuII complex and the capacity of the macrocycle to trap small solvent molecules like tetrahydrofuran, diethyl ether, and pentane at temperatures well above their boiling points. The latter effect is attributed to the suitable guest shape and size which drastically limit lattice diffusion. Solvent exchange was found to induce intense color changes (**Figure 15B**) and sizable shifts in the visible region of the diffuse reflectance spectra (**Figure 15B**). The high intensity of the supramolecular solvatochromic effect is furthermore excellently illustrated through microphotographs of the variable colors of crystals.

#### **Figure 15.**

*A) Synthesis of [Cu2Cl4L2].2DMSO (1) from 1,10-[1,4-phenylenebis(methylene)]bis(2-methyl-1Himidazole) and copper(II) chloride dihydrate. B) Visible-region diffuse reflectance spectra (bottom) quantitatively illustrate the broad-spectrum (540–624 nm) solvatochromism of 1–6. Reprinted with permission from: Nikolayenko et al.[56].*
