**2. Effect of CD-dye complex formation on dye photophysical properties**

For dye possessing high extinction coefficient and large difference in dipole moment between ground and singlet excited state, the photo-induced charge transfer (ICT) can be proposed. For such dyes, fluorescence quantum yield and singlet excited state lifetime are sensitive to the polarity of the solvent. Also dye–solvent interaction such as hydrogen bonding can be realized for polar dyes [17]. 4-(p-N,N-Dimethylaminophenylmethylene-2-phenyl-5-oxazolone (**DPO**) is the polar dye demonstrated ICT upon photoirradiation. The blue shift found in the emission spectrum of dye **DPO** on adding β-CD points that the molecule is buried in the hydrophobic cavity of β-CD (**Figure 2**). In the resulting dye complex, restrictions on molecular movement appear, which causes the processes of nonradiative deactivation. Thus, restrictions on the motion of the dye molecule in the complex with β-СD lead to a noticeable increase in the fluorescence quantum yield. In this work, it was shown that the dye **DPO** in ICT excited state becomes more polar which results in destabilization of CD-**DPO** complex.

Similar effect on fluorescence has been shown upon complex formation of 2 styrylbenzothiazole containing 15-crown-5 ether fragment (**CSB**) with hydroxypropyl-β-CD (HP-β-CD) (**Figure 3**) [18]. The fluorescence quantum yield of **CSB** is enhanced by 5 times in HP-β-CD relative to water. **CSB** in the HP-β-CD cavity takes place very probably along the molecular axis with a nearly anti-parallel dipole-dipole orientation.

*Cyclodextrins as Supramolecular Hosts for Dye Molecules DOI: http://dx.doi.org/10.5772/intechopen.107042*

**Figure 4.** *Structures of dyes 1, 2.*

Optical characteristics of ketocyanine dye molecules **1**, **2** (see **Figure 4**) are dependent on the immediate environment [19]. Thus, in an alcohol solvent, the anisotropy of dye fluorescence is inferred. The dyes are only slightly soluble in water. Increased solubility in β-CD indicates dye–β-CD interaction. As shown by investigation of β-CD - ketocyanine dyes **1**, **2** complexes, the strong dye–β-CD interaction is accompanied by high values of fluorescence anisotropy. The dyes **1**, **2** interact through the carbonyl part with hydroxyl groups in β-CD, such interactions are important for stabilization of complex.

The complex formation of 4-amino-2,5-dimethoxybenzanilide (Blue RR (**FBRR**)) and 4-amino-5-methoxy-2-methylbenzanilide (violet B, **FVB**) with hydroxypropyl-αcyclodextrin (HP-α-CD) and HP-β-CD was studied in [20]. **FVB** and **FBRR** in HP-α-CD demonstrated lack of complexation probably due to smaller cavity size. The deep penetration of the **FVB**/**FBRR** in HP-β-CD than that of HP-α-CD may be due to the difference in size, also the strong hydrogen bonding of the alcoholic OH on the CD ring with the CONH group of the guests was suggested. The dual fluorescence of both dyes was observed through the normal emission around 350 nm and the very low TICT band around 455 nm. Upon addition of HP-α-CD, the emission slightly increased, and the intensity ratio of the TICT band and the normal band *Ia/Ib* was the same. When the concentration of HP-α-CD increased, the *Ia/Ib* ratio decreased. It has been shown that FVB/FBRR TICT radiation significantly affects the formation of inclusion complexes with different geometries. The explanation was done in terms of differences in the internal diameters of both CD cavities, as well as the change in charge distribution in **FVB** and **FBRR** in CD complexes.

#### **Figure 5.** *Structures of complex between dye 3 and α-, β-, γ-CDs.*

1-Methyl-4-(4-aminostyryl) quinolinium iodide **3** forms inclusion complexes with α,β,γ-cyclodextrins in the ground and excited states (**Figure 5**) [21]. The fluorescence quantum yields (Qfl) were 0.043 in water, 0.06 in γ-CD, 0.08 in α-CD, and 0.38 in β-CD. The increase in the fluorescence Qfl indicates better accommodation of the dye in β-CD compared to the other cavities. The fluorescence spectra showed an additional band at longer wavelength in case of γ-CD. This may be attributed to an excimer of two adjacent **3** molecules.

The interesting observations have come out when **Coumarin** dye forms the complex with electron transfer (ET) with N,N-dimethylanyline (**DMA**) in DMF [22]. The ET occurs in a contact ion pair between **Coumarin** and **DMA**. It was also found that the ET rate decreases in a polar solvent medium. This happens because the hydrogen bonds between the **Coumarin** and **DMA** are partially broken due to the presence of solvent molecules between the reactants. **Coumarin** dye in DMF binds to cyclodextrin molecule to form 1:1 and 2:1 complexes through hydrogen bonding. ET process between **Coumarin**-CD complex and **DMA** was not observed (**Figure 6**).

Dual fluorescence (from TICT and plane molecule) of 4-dimethylaminobenzonitrile (**DMABN**) has been studied in α-cyclodextrin (α-CD) complex [23]. **DMABN** molecules are located in two different positions inside the α-CD cavity

**Figure 6.** *Structures of complexes between Cumarin dye, DMA and β-CD.*

*Cyclodextrins as Supramolecular Hosts for Dye Molecules DOI: http://dx.doi.org/10.5772/intechopen.107042*

**Figure 7.** *Structures of complexes between DMABN and α-CD.*

(**Figure 7**). The first position is when the dimethylamino group of the **DMABN** molecule is headed towards the larger rim of the α-CD cavity. In this position, amino group is in a slightly polar and slightly rigid environment. In the second position, the dimethylamino group of the **DMABN** molecule is headed towards the smaller rim of the α-CD cavity. In the second position, amino group is in the least polar and most rigid environment. The intensities of both plane and TICT fluorescence bands are enhanced in both types of complexes with α-CD. However, the fluorescent band of plane molecule is more enhanced upon complex formation with α-CD than those of TICT band.

It has been known that the molecule methyl *o*-hydroxy-p-dimethylaminobenzoate (**MHDMAB**) demonstrates triple fluorescence i.e., the normal-locally excited state emission, IF(LE), the intramolecular proton-transfer tautomer emission, IF(IPT), and twisted intramolecular charge-transfer emission, IF(TICT) [24]. It was found that αand β-cyclodextrins affect both emission modes LE and TICT of the fluorescence spectrum of **MHDMAB** in aqueous solution (**Figure 8**) [24]. This study showed that **MHDMAB** in α-CD and β-CD formed both 1:1 and 1:2 inclusion complexes. The photophysical behavior of **MHDMAB** is modified significantly upon encapsulation of the dye inside β-CD cavities. The short-wavelength emission band of **MHDMAB** in

**Figure 8.** *Structures of complexes between MHDMAB and β-CD.*

water is increased as the concentration of β-CD increases, also new emission bands at 450 nm (IPT) and 525 nm (TICT) appeared. The time-resolved experiments gave the fluorescence decay time of the fast component originating from the emission of the hydrogen-bonded complex (**MHDMAB**-H2O), whereas the decay times of the slow component are related to the fluorescence from IPT and TICT states.

The irradiation of bisstyryl dye **4** with 335 nm light causes the light absorption by the neutral 4-styrylpyridine fragment. The irradiation results in the fluorescence of the positively charged part of the bisstyryl dye in the region of 550 nm (see **Figure 9**). Thus, RET from the neutral to the charged part occurs in dye **1** [25]. The experiments have shown that, in the presence of CB and formation of complex **3**@HP-β-CD, it does not affect the efficiency of the resonant energy transfer process, while binding to CB [7] molecules or simultaneously to HP-β-CD and CB[7] molecules reduce the efficiency of FRET. The observed effect can be explained by the fact that the optical characteristics of styryl fragments also noticeably change during CB[7] encapsulation, and the mutual arrangement of styryl fragments in supramolecular complexes also changes.

Supramolecular systems containing porphyrinoid compounds are of great interest due to such characteristics as high molar absorption in the ultraviolet and visible regions of light, easily changing properties, high chemical stability, and long lifetime of the first excited singlet state [26]. In some porphyrin and phthalocyanine macrocyclic systems, CDs provide the desired supramolecular architecture [27]. Thus, in the porphyrin-CD complexes obtained by Kuroda et al. [28] and Lang et al. [29], electron transfer was discovered. A similar process was found in complexes of adamantaneamine-modified porphyrins with mono-6-*p*-nitrobenzoyl-β-cyclodextrin [30]. Detailed steady-state and time-resolved fluorescence measurements revealed two pathways for electron transfer: dynamic quenching occurring between free donors and free acceptors in solution, and static quenching between donors and acceptors bound in a supramolecular complex.

Other research groups investigated supramolecular assemblies which were composed not only of porphyrins but also of other porphyrinoids, for example, phthalocyanines [31]. One of the most interesting examples was presented as self-assembled complexes containing tetra(*p*-sulfophenyl)porphyrin (**TPPS**) and permethylated-β-CD, conjugated axially to phthalocyanine or subphthalocyanine molecules [31]. The efficient energy transfer from the photoexcited phthalocyanine to a free-base porphyrin occurred comparable to the values found for multiporphyrin arrays linked with covalent bonds.

**Figure 9.** *Complex CB[7]@4@HP-β-CD.*
