**3. Effect of CD-dye complex formation on the dye photochromism**

Photochromism is the reversible photoinduced transformation of molecules. Photochromic molecules have been extensively applied as components of nonlinear devices, optical memories, and optical switches [32–34]. To extend the range of commercial applications, photochromes are typically introduced in different materials, including cyclodextrins (CDs) [35–37].

The aim of the work [38] was to study the complex formation of crown-containing styrylheterocycles **5**, **6** with modified cyclodextrin HP-β-CD in aqueous solutions. The investigation of the photochemical reactions of these compounds proceeding in the cavity of cyclodextrin has been carried out (**Figure 10**). It was shown that the process of complex formation causes a significant increase in solubility and results in an intensive luminescent response of styrylheterocycle molecules. In such complexes, the reversible *E–Z* photoisomerization is taking place in aqueous media. The photoisomerization does not cause the destruction of 1:1 complexes, staying guest molecules encapsulated. In opposite, encapsulated 1:2 complex was not found in *Z*-form.

The same styryl dyes **5**, **6** have been exploited as photoactive guests in threecomponent systems containing both HP-β-CD and cucurbit[7]uril (CB[7]) host molecules [39]. The formation of complexes *E*-**5** and *E*-**6** with HP-β-CD occurs with constants logK11 = 3.58 and logK11 = 3.04 correspondently (**Figure 11**). Starting from the *E*-**5**, *E*-**6**, the phototransformation under light (≥ 320 nm) is observed and includes two consecutive photochemical reactions, an *E-Z* isomerization reaction and a 1-aza-1,3,5-hexatrienic electrocyclic reaction in which the formation of C-N bond was observed. The cyclic product gives stable heteroaromatic cations as a result of elimination of the hydride with atmospheric oxygen (products **7**, **8** (**Figure 11**). The physicochemical analysis of the phototransformation showed that the formation of Z-isomer can occur in HP-β-CD, whereas the formation of heteroaromatic cations **7**, **8** leads to the destruction of HP-β-CD complex (**Figure 11**). The presence of CB[7] in the **5** or **6** solution causes the formation of novel complexes **5**CB[7] or **6**CB[7]. Thus, a three-component system involving styryl dye and both HP-β-CD and CB[7] hosts can be switched on by the synchronous host–guest complexation of dye with HP-β-CD or CB[7] by phototransformation of the dye component.

The α-cyclodextrin [2]-rotaxanes have been obtained with alkane-, stilbene- (**Figure 12**) and azobenzene-based axles with different substituents [40–43]. The rotaxanes based on azobenzene and stilbene derivatives were found to demonstrate

**Figure 10.** *Photoisomerization of complexes 5, 6 with HP-β-CD.*

**Figure 11.**

*Photochemical transformation in three-component systems containing both HP-β-CD, CB[7] and photoresponsive dye 5 or 6.*

#### **Figure 12.**

*Structure of α-cyclodextrin [2]-rotaxanes based on stilbene derivatives.*

photochemically induced reversible mutual conversion between its *trans*- and *cis*-isomers, resulting in the moving of the cyclodextrin back and forth along the axle. The αcyclodextrin [2]-rotaxanes behave as a molecular shuttle. This type of light-powered rotaxane exhibits favorable repeatability and presents a novel light-driven molecular machine.

Chen et al. [44] obtained that phthalocyanine containing azabenzene moiety in *trans* form can easily enter into α-CD, but in *cis* form azobenzene moiety is not planar what prevents host-guest interaction between phthalocyanine and α-CD (**Figure 13**).

Mulder et al. studied dithienylethene-tethered β-CD dimers in which the irradiation with UV light caused photochemical ring-closure reaction [45]. Tetra(*p*sulfophenyl)porphyrin (**TPPS**) was used as a guest for the interaction with dithienylethene-tethered β-CD. It was found that the alternation of UV and visible light irradiations caused a reversible release and uptake of porphyrin.

Synthesis of cyclodextrin polymers using cross-linking agents has been described in literature [46]. Such substituted by CD polymers are called cyclodextrin-based nanosponges [46]. Using this approach, a series of photochromic polymers were prepared by forming various spiropyran (**SP**) inclusion complexes in the CD cavities of the β-CD polymer (**CDP**) (**Figure 14**) [47]. The β-CD is not able to include the **SP**, *Cyclodextrins as Supramolecular Hosts for Dye Molecules DOI: http://dx.doi.org/10.5772/intechopen.107042*

**Figure 13.** *Structure of α-cyclodextrin complex with phthalocyanine containing azabenzene moiety.*

**Figure 14.**

*Photochromic transformation of SP to PM and structure of β-CD polymer (CDP).*

whereas the photomerocyanine form (**PM**) can bind with β-CD. Indeed, the decolouration rate of **PM** forms is decreased in the presence of β-CD. Consistent with experimental UV/VIS spectra, the quantum chemical calculations provided valuable insight into the substituent and CDP effects on the **PM** decolouration process. It was found that kCD**PM**/k**PM** ratio is larger than 20, k is constant of decolouration. Thus, NO2-**SP** and Br-**SP** exhibit slower decolouration rates in β-CD than alone **SP** because the narrow β-CD cavity hinders deep inclusion of the bulky naphthopyryl moiety.
