Cyclodextrins as Supramolecular Hosts for Dye Molecules

Abstract

Cyclodextrins possess a hydrophobic cavity due to which they are suitable for inclusion of various organic dyes. The complex formation between CDs and dyes has been employed to affect the photophysical and photochemical characteristics of organic dyes such as fluorescence enhancement, charge and proton transfer, energy transfer, photochromic transformations and intramolecular excimer/exciplex formation. This fundamental approach has also potential nanotechnological application in creation of optoelectronic devices. Thus, the fluorescent hybrid nanomaterials consisting of supramolecular assemblies of cyclodextrins with fluorescent dyes can be considered as multivalent scaffolds for the construction of various devices applicable in science and technology, in fluorescence spectroscopic and microscopic techniques providing high sensitivity and imaging of cells with high resolutions. In some cases, fluorescent hybrid materials composed of CD-dye complexes have been successfully used instead of the fluorescent organic molecules in sensing and bioimaging studies.

1. Introduction

Cyclodextrins (CDs) present oligosaccharides (between 973 and 2163 Da) composed from D-glucopyranose units (Figure 1) [1]. CDs are commonly classified as α-, β- and γ- CDs containing six, seven and eight glucopyranose units, respectively [2, 3, 4]. Cyclodextrins may include molecules whose size and polarity are compatible with their lipophilic interior cavity. The interaction forces participated in complexation are dipole-dipole interaction, electrostatic interaction, van der Waals interaction, dispersion forces, hydrophobic interaction, and conformational strain reduction [5, 6].

Figure 1.

Structures of α-, β- and γ- CDs.

Complexation reactions involving cyclodextrins are important for two purposes: research investigations of the molecule interactions with cyclodextrin, and applied technologies (pharmaceutical chemistry, food, cosmetic, chemical synthesis and catalysis) [7, 8, 9]. It is important to note that the ability of cyclodextrins to bind guest molecules in their cavities has been used to affect the photochemical and photophysical properties of organic dyes, such as enhancement of fluorescence and phosphorescence, intramolecular charge transfer, intermolecular hydrogen bonding, intramolecular excited proton transfer, intermolecular excimer/exciplex formation. [10, 11, 12, 13, 14, 15, 16].

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-Dimethyl-aminophenylmethylene-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.

Figure 2.

Structure of dye DPO-β-CD 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.

Figure 3.

Structure of dye CSB-β-CD complex.

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.

Figure 4.

Structures of dyes 1, 2.

The complex formation of 4-amino-2,5-dimethoxybenzanilide (Blue RR (FBRR)) and 4-amino-5-methoxy-2-methylbenzanilide (violet B, FVB) with hydroxypropyl-α-cyclodextrin (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.

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 (Q_fl) 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 Q_fl 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.

Figure 5.

Structures of complex between dye 3 and α-, β-, γ-CDs.

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).

Figure 6.

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

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 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.

Figure 7.

Structures of complexes between DMABN and α-CD.

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 α- and 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 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.

Figure 8.

Structures of complexes between MHDMAB and β-CD.

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.

Figure 9.

Complex CB[7]@4@HP-β-CD.

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.

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, 33, 34]. To extend the range of commercial applications, photochromes are typically introduced in different materials, including cyclodextrins (CDs) [35, 36, 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.

Figure 10.

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

The same styryl dyes 5, 6 have been exploited as photoactive guests in three-component 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.

Figure 11.

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

The α-cyclodextrin [2]-rotaxanes have been obtained with alkane-, stilbene- (Figure 12) and azobenzene-based axles with different substituents [40, 41, 42, 43]. The rotaxanes based on azobenzene and stilbene derivatives were found to demonstrate 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 α-cyclodextrin [2]-rotaxanes behave as a molecular shuttle. This type of light-powered rotaxane exhibits favorable repeatability and presents a novel light-driven molecular machine.

Figure 12.

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

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).

Figure 13.

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

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, 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 k_CDPM/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.

Figure 14.

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

4. CD-dye complexes in biology and medicine

CD systems have been extensively studied in biology and medicine, because CD counterpart acts like a comparable protein structure, providing the proper environment and arrangement of the substrates. In some cases, the processes taking place in these systems mimic those occurring in living organisms.

A novel approach towards controlled ligand–DNA interactions has been developed based on supramolecular complex of dye 9 and HP-β-CD [48]. Dye 9 is not able to coordinate with DNA (Figure 15). The irradiation of encapsulated 9 caused the electrocyclic transformation to product 10 which could not be able to be bound with HP-β-CD but easily interacts with DNA. Thus, the process begins with photoinduced in situ generations of a DNA ligand from the encapsulated styryl heterocycle, continues with the association of the ligand with the nucleic acid, and ends with the removal of the bound ligand from the DNA binding site using CB[7]. Despite the simultaneous presence of several host molecules, HP-β-CD, CB[7] and DNA, each step of the transformation cascade is not affected by the presence of other components. It is important to note that the phototransformation of precursor 9 into DNA intercalator 10 inside the cyclodextrin cavity significantly increases the biocompatibility of the method.

Figure 15.

Association and redistribution equilibria of ligands 9 and 10 in the presence of hosts.

Molecules like cyclodextrins can be applied to solve both the solubility and the toxicity of the fluorescent dyes using in fluorescence imaging techniques to visualize and monitor specific biological targets or processes in living systems. Thus, Alexandru Rotaru and co-authors demonstrated a low level of fluorescent dye 11 toxicity (Figure 16) by the formation of cyclodextrin inclusion complex resulting in the successful application in cell staining [49]. Applications of this type of compound are limited due to high toxicity and water solubility problems. The addition of β-CD to dye 11 solutions in ratio of 3:1 results in dye being soluble in water. Also, fluorescent indolizinyl-pyridinium salt/β-cyclodextrin inclusion complexes demonstrated absence of cytotoxicity. Due to found in experiments cellular permeability, long-lived intracellular fluorescence and selective accumulation within acidic organelles, the dye 11 can be identified as remarkable candidate for intracellular labelling of acidic organelles (lysosomes or mitochondria).

Figure 16.

Structure of fluorescent dye 11.

Nanosponges prepared based on β-cyclodextrin and diphenyl carbonate have the capacity to interact with small molecules in their matrix [50]. Flavonoid quercetin was loaded into such nanosponges (Figure 17) [51]. The dissolution of the quercetin nanosponges was significantly higher compared with the pure drug. The stability of encapsulated quercetin nanosponge was markedly improved. In addition, the antioxidant activity of the quercetin in composition of nanosponges was higher than pure quercetin.

Figure 17.

Structures of quercetin and nanosponge.

Supramolecular ensembles of porphyrinoid-CD are formed both through covalent binding and through the formation of inclusion complexes [27]. Such systems are biomimetics that mimic the natural breakdown of carotenoids [52], cytochrome P450 mediated hydroxylation [53], and oxygen binding by hemoglobin [54].

The system reversibly bound O₂ was proposed by Zhou and Groves [55], it is based on self-assembly of the Fe(II)-tetra(p-sulfophenyl)porphyrin (Fe(II)TPPS) and β-CD derivative having pyridylmethyl moiety and PEG groups. Pyridyl moiety served as a ligand for Fe(II)TPPS and PEG chains spanned over the porphyrin surface, protecting the second binding site. The binding of O₂ and CO was proved by optical method.

Another analytical method has been used by Koji Kano and his coworkers [56]. Their systems contained substituted β-CD dimers and Fe(II)TPPS (Figure 18). 4-Sulfonatophenyl groups of porphyrins were embedded in β-CD moieties of the dimer, whereas the pyridine linker coordinated the Fe(II) central ion. The hydrophobic environment within the Fe(II) ionic centre of the supramolecular complex was crucial for the efficient O₂ binding, this is why the affinity of such complexes to O₂ is high and stabilities of O₂ adducts are significant.

Figure 18.

Complex of substituted β-CD dimer and Fe(II)TPPS.

Porphyrinoids are widely used as photosensitizers in photodynamic therapy (PDT), the PDT method is a promising way to treat cancer. Complexation with CD improves the photosensitizing properties of porphyrinoids since an increase in their quantum yield of singlet oxygen is observed in such complexes. This fact is of great importance for PDT [27]. The complex formation with HP-β-CD improves the efficacy of PDT for the treatment of G361 malign melanoma by using zinc-tetra(p-sulfophenyl)porphyrin ZnTPPS4 [57]. Thus, after 24 h incubation of cell cultures with 10^−l M ZnTPPS4 and 1 mM HP-β-CD, the cells were irradiated for 7.5 min at the total irradiation dose of 12.5 J cm⁻² which gave rise to DNA damage.

Innovative drug delivery system was proposed based on gold nanoparticles covered by cationic poly(cyclodextrin) (P(CD+)) and alginate (alg−) layers [58]. 4-Hydroxy-tamoxifen was placed in the nanocapsules’ shell via inclusion with the cyclodextrin cavities. It was also demonstrated that 4-hydroxy tamoxifen can be efficiently delivered to podocytes in vitro using CD-containing nanocapsules as carriers.

Tetrazines functionalized with adamantane groups and naphthalimide antennas can form supramolecular complex with β-cyclodextrin (β-CD) in aqueous solutions [59]. The organic anchoring groups and the tetrazine itself fit well the requirements for cavity cyclodextrin inclusion. This approach was applied for development of biosensors with electrochemical and fluorescence properties [60]. Tetrazine derivatives were immobilized at the electrogenerated polypyrrole-β-CD film through the host-guest interactions between tetrazine derivatives and β-CD. This new original molecular architecture allows the immobilization of glucose oxidase modified by β-CD (Figure 19). The absorption band at 425 nm in UV–Vis spectra recorded for ITO electrodes belonging to naphthalimide fragment confirmed the formation of assembly. The oxidation of glucose in presence of oxygen with the concomitant production of H₂O₂ was investigated by electrochemistry. The prepared electrodes were thus maintained at 0.7 V in presence of glucose to detect, through its oxidation, the enzymatically generated H₂O₂.

Figure 19.

ITO electrode covered by assembly containing glucose and naphthalimide fragment.

5. Conclusion

The examples of complexes of photoactive compounds with CD container molecules presented in this chapter are hybrid materials created by combining two (or more) different elements of a different nature. In this context, they can be considered as a very large and heterogeneous class of materials. Such materials include molecular and supramolecular assembled materials, polymers, or nanosized objects, nanostructured and hybrid architectures with organic or biological characteristics. Such organic hybrid systems combine particular properties of the components, which explains the wide range of properties exhibited by the systems and the great possibilities in the development of methods for their synthesis.

Organic hybrid materials composed of cyclodextrin receptors and photoactive components, provide a great opportunity for improving of photophysical characteristics, increasing functionality, and extending the field of application. Supramolecular functionalization of photoactive CD molecules, which play the role of platforms for the immobilization of bioelements such as enzymes, antibodies, nucleic acids (DNA, RNA, microRNA), and other functional groups, is of great importance for the manufacture of analytical devices for biosensors. This approach can also be used to obtain novel hybrid organic photoactive materials.

Acknowledgments

This work was supported by the Ministry of Science and Higher Education of the Russian Federation (Contract/agreement No. 075-00697-22-00).