Cyclodextrins emission

Of particular interest in the application of cyclodextrins is the enhancement of luminescence from molecules when they are present in a cyclodextrin cavity. Polynuclear aromatic hydrocarbons show virtually no phosphorescence in solution. If, however, these compounds in solution are encapsulated with 1,2-dibromoethane (enhances intersystem crossing by increasing spin-orbit coupling external heavy atom effect) in the cavities of P-cyclodextrin and nitrogen gas passed, intense phosphorescence emission occurs at room temperature. Cyclodextrins form complexes with guest molecules, which fit into the cavity so that the microenvironment around the guest molecule is different from that in... 

Fig. B9.3.1. A absorption spectrum of the B excitation polarization spectra of the model multi-chromophoric cyclodextrin CD7(6) and compound NAEt and CD7(6). Solvent mixture variations in the emission maximum as a func- (9 1 v/v) of propylene glycol and 1,4-dioxane at tion of the excitation wavelength (broken line). 200 K (adapted from Berberan-Santos et al.a)).

Figure 1.8 Use of capillary electrophoresis for separating the diastereomers quinine (QN) and quinidine (QD) (H-QN is hydroquinine, QD-N-OX is quinidine iV-oxide, H-QD is hydroquinidine, 3-OH-QD is 3-hydroxyquinidine, and asterisk is an unidentified impurity). Reprinted from [17], copyright 2001, with permission from Elsevier. (Capillary 47 cm X 75 pm i.d. (40 cm to detector) (Polymicro Technologies) running buffer 50 mM phosphoric acid containing 15 mM /3-cyclodextrin adjusted to pH 2.5 with NaOH voltage 7 kV current 21 pA injection at 0.5 psi for 4 s detector fluorescence (HeCd laser) excitation 325 nm, emission 450 nm.)...

As very different systems, Ueno et al. prepared a-helix peptides bearing y-cyclodextrin and one (16,17) or two (18) naphthalene units in their side chains (Fig. 2) [38], They are alanine-based peptides composed of 17 amino acid residues. The peptides 16 and 17, which have y-CD and one naphthalene unit, display a simple monomer emission and the fluorescence intensity slightly decreases with increasing guest concentration. On the other hand, the peptide 18, which has y-CD and two naphthalene units on both sides of the y-CD unit, exhibits considerable excimer emission in addition to the predominant monomer emission and the excimer emission decreases with increasing guest concentration. This indicates that 18 excludes the two naphthalene units from the y-CD cavity to the opposite directions upon guest accommodation. 

The ESIPT of 2-(2 -hydroxyphenyl)-4-methyloxazole (HPMO) (27) has been explored by Douhal and co-workers [166] for its probe characteristics in a variety of organized media which include cyclodextrin, calixarene, micelle, and HSA. The incorporation of HPMO into hydrophobic cavities in an aqueous medium involves the rupture of its intermolecular hydrogen bond to water and formation of an intramolecular hydrogen bond in the sequestered molecule. Upon excitation (280-330 nm) of this entity, a fast intramolecular proton-transfer reaction of the excited state produces a phototautomer (28), the fluorescence of which (Xm = 450 170 nm) shows a largely Stokes-shifted band. Because of the existence of a twisting motion around the C2—C bond of this phototautomer, the absorption and emission properties of the probe depend on the size of the host cav-... 

As with most discoveries, these ideas then passed through a lull before takeoff. Then the Chicago-New York axis of Yang and Turro in the USA investigated the first example (6) in the host-guest mould [10]. Their focus was on the demonstration of exciplex emission of (6) in aqueous solution due to protection by an encapsulating /J-cyclodextrin. Ever since Weller s experiments, exciplex emissions had been observed to fall off rapidly in intensity as solvent polarity was increased [4], A formally related example due to Tazuke from 1982 must, however, be pointed out, where a hydrophobic polymer microdomain in mixed and neat aqueous solution allows exciplex emission from pyrene and dimethyl-aniline pendants [11], The importance of (6) in the present context stems from the correlation of pH-dependent emissions from the naphthalene moiety and from the exciplex. Co-occurrence of externally switchable photophysics and supramolecular phenomena would later become common in the research literature [12]. 

Inclusion complexes have also been shown to aid excimer formation provided that the cavity dimensions and polarity are appropriate. Thus Ueno et al. (1980) have shown that the lipophilic cavity of y-cyclodextrin can accommodate two molecules of sodium (l-naphthyl)methyl acetate and that this leads to enhanced excimer emission. Excimer formation by diaryl-methylammonium salts is promoted by y-cyclodextrin but not by a- and P-cyclodextrins (Emert et al., 1981a). The best results were obtained with compounds having fairly large aryl groups e.g. 4-biphenylyl and 1-naphthyl. The hydrophobic character of these groups helped to solubilise the compounds in the interior of the cyclodextrin. 

Figure 2.13. Inclusion of a guest inside the cavity of a cyclodextrin indnces light emission...

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