ADVANCES IN PHOTOCHEMISTRY - download.e-bookshelf.de · ADVANCES IN PHOTOCHEMISTRY Volume 21 Editors DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University,

ADVANCES IN PHOTOCHEMISTRY

Volume 21

Editors

DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

DAVID H. VOLMAN Department of Chemistry, University of California, Davis, California

GUNTHER VON BUNAU Physikalische Chemie, Universitat Siegen, Germany

A WILEY-INTERSCIENCE PUBLICATION

JOHN WILEY & SONS, INC. New York Chichester Brisbane Toronto Singapore


Volume 21


Volume 21

Editors

DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

DAVID H. VOLMAN Department of Chemistry, University of California, Davis, California

GUNTHER VON BUNAU Physikalische Chemie, Universitat Siegen, Germany

A WILEY-INTERSCIENCE PUBLICATION

JOHN WILEY & SONS, INC. New York Chichester Brisbane Toronto Singapore

This text is printed on acid-free paper.

Copyright 0 1 9 9 6 by John Wiley & Sons, Inc.

All rights reserved. Published simultaneously in Canada.

Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permission Department, John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 10158-0012.

Library of Congress Cataloging in Publication Data:

Library of Congress Catalog Card Number: 63-13592 ISBN 0-471-14332-4

Printed in the United States of America

1 0 9 8 7 6 5 4 3 2 1

CONTRIBUTORS

Michael N. R. Ashfold School of Chemistry, University of

Bristol, Bristol BS8 lTS, United Kingdom

Pietro Bortolus Instituto di Fotochimica e

Radiazioni d'Alta Energia, C.N.R. -Area della Ricerca, Via Piero Gobetti 101, 1-40129 Bologna, Italy

Sandra Monti Instituto di Fotochimica e

Radiazioni d'Alta Energia, C.N.R. - Area della Ricerca, Via Piero Gobetti 101, 1-40129 Bologna, Italy

David H. Mordaunt School of Chemistry, University of


Jean-Pierre Pete URA 459, Rkarrangements

Thermiques et Photochimiques, UFR Sciences Exactes et Naturelles, Moulin de la Housse, BP 1039/51687 Reims Cedex 2, France

Steven H. S. Wilson School of Chemistry, University of


PREFACE

Volume 1 of Advances in Photochemistry appeared in 1963. The stated purpose of the series was to explore the frontiers of photochemistry through the medium of chapters written by pioneers who are experts. As editors we have solicited articles from scientists who have strong personal points of view, while encouraging critical discussions and evaluations of existing data. In no sense have the articles been simply literature surveys, although in some cases they may have also fulfilled that purpose.

In the introduction to Volume 1 of the series, the editors noted developments in a brief span of prior years which were important for progress in photochemistry: flash photolysis, nuclear magnetic resonance, and electron spin resonance. Since then two developments have been of prime signifi- cance: the emergence of the laser from an esoteric possibility to an important light source and the evolution of computers to microcomputers in common laboratory use for data acquisition. These developments have strongly influenced research on the dynamic behavior of excited states and other transients.

With an increased sophistication in experiment and interpretation, photochemists have made substantial progress in achieving the fundamental objective of photochemistry: Elucidation of the detailed history of a molecule that absorbs radiation. The scope of this objective is so broad and the systems to be studied are so many that there is little danger of exhausting the subject. We hope that the series will reflect the frontiers of photochemistry as they develop in the future.

DOUGLAS C . NECKERS Bowling Green, Ohio February 1995

vii

CONTENTS

Photochemistry in Cyclodextrin Cavities PIETRO BORTOLUS AND SANDRA MONTI

1

Asymmetric Photoreactions of Conjugated Enones and Esters 135 JEAN-PIERRE PETE

Photodissociation Dynamics of Hydride Molecules: H Atom Photofragment Translational Spectroscopy 217

MICHAEL N. R. ASHFOLD, DAVID H. MORDAUNT, AND

STEVEN H. S. WILSON

Index 297

Cumulative Index, Volumes 1-21 305

ix

PHOTOCHEMISTRY IN CYCLODEXTRIN CAVITIES

Pietro Bortolus and Sandra Monti Istituto di Fotochimica e Radiazioni d’Alta Energia, C.N.R. - Area della

Ricerca, Via Piero Gobetti 101, 1-40129 Bologna, Italy

CONTENTS

I. Introduction 11. Inclusion Complexes

A. Structure of the cyclodextrins B. Formation of complexes C. Characterization of the complexes D. The cyclodextrin reaction media

A. Fluorescence modifications B. Excimer and exciplex emission C. TICT emission D. Proton transfer E. Phosphorescence F. Functionalized cyclodextrins G. Miscellaneous

IV. Photochemical processes A. Unimolecular reactions

111. Photophysical processes

1. Photorearrangements 2. Intramolecular photoreactions of carbonyl compounds

and related processes 3. Photoisomerization reactions

Adoances in Photochemistry, Volume 21, Edited by Douglas C. Neckers, David H. Volman, and Giinther von Biinau ISBN 0-471-14332-4 0 1996 by John Wiley & Sons, Inc.

1

2 PHOTOCHEMISTRY IN CYCLODEXTRIN CAVITIES

B. Bimolecular reactions 1. Photosubstitutions 2. Photocycloadditions 3. Intermolecular H-abstraction 4. Photoinduced electron transfer and photoionization 5. Photocatalytic systems

C. Miscellaneous photoreactions D. Photoactive cyclodextrins V. Final remarks

Acknowledgments References

I. INTRODUCTION

Biological systems, like enzymes, genes, antibodies, and ionophores, are characterized by receptor sites able to selectively bond suitable molecules, giving rise to highly specialized molecular machines that conform to a “logic” based on weak, noncovalent interactions.

Modern trends in chemistry have been inspired by biological examples. Large macrocyclic receptors have been used to associate smaller molecules, and by means of these supramolecular host -guest structures, a novel chemistry developed to control molecular properties and orient reactivity [l].

In the last decade, increasing interest has been devoted to cyclodextrins (CD) as hosts because of the ability of their central cavity to bind a variety of organic and inorganic substrates and to modify their ground- and excited-state behavior [2-51. In addition, these systems provide suitable models for the study of weak interactions, which are difficult to investigate with large proteins or nucleic acids [6,7]. The ability of CDs to form inclusion complexes has led to their widespread utilization in the pharma- ceutical, food, chemical, and plant-protection industries [8]. In the pharma- ceutical industry, CDs have been used either to improve the physical and/or chemical properties of drugs themselves or to enhance the bioavailability of poorly soluble drugs [8,9]. In the food, cosmetics, and tobacco industries, CDs have been used to reduce flavor loss due to air and moisture or to eliminate undesired tastes or microbial contamination.

This chapter is concerned specifically with the excited-state processes in cyclodextrin inclusion complexes. Photophysical and photochemical deactivation pathways are discussed, with particular emphasis on solution

INCLUSION COMPLEXES 3

studies and mechanistic investigations. Literature published during the last two decades, up to and including the first six months of 1994, is reviewed.

11. INCLUSION COMPLEXES

A. Structure of the Cyclodextrins

Cyclodextrins, also called Schardinger dextrins, cycloamyloses, or cyclo- glucans, are a family of cyclic oligosaccharides obtained from starch and related compounds by the action of the amylase of Bacillus macerans. They were discovered by Villiers in 1891 [lo] and the first detailed description of their properties and isolation was made by Schardinger in 1903 [11-131.

The most common CDs are constituted by six (a-), seven (/I-) and eight (y-) units of D( + )glucopyranose units (these compounds are commercially available). Higher CDs, consisting of up to 13 glucose units, have been identified by column chromatography. All the glucose units are in C1 chair conformation and are connected by 1,4-glycosidic bonds. It is known from crystal structure that CDs have a truncated cone shape with a height of approximately 88, and an inner cavity diameter varying between - 5 (a-) and -8 .58, (y-CD). As a consequence of the chair conformation of the

0

CD n.units cavity(A)

6 4.7 - 5.2

6.0 - 6.4 8 7.5 - 8.3

P 7 8

Figure 1. Structure of the CDs and schematic representation of the cavity shape. Diameters from ref. 14.


glucopyranose units, the secondary hydroxyl groups (on C-2 and C-3 atoms of the glucose) are located on the larger rim of the truncated cone, whereas the primary hydroxyl groups are located on the opposite side (Fig. 1). The interior is lined by C-H groups and glycosidic oxygen bridges. The non- bonding electrons of the glycosidic oxygen are directed toward the inside of the cavity, producing a high electron density and lending it some Lewis base character. As a result of this arrangement of the functional groups, the cavity of the CD is relatively hydrophobic while the outside is hydrophilic [14]. High energy water molecules are contained in the CD cavities: an average of 2, 6.5 and 12 molecules occupy the a-, 1- and y-CD cavities, respectively. The presence of water causes a collapsed strained conformation in the a-CD macrocycle [l8]. The solubility of a- and y-CD in water is lo-' m ~ l d m - ~ and that of p-CD is < 2 x 10-*m0ldm-~ [2,3].

B. Formation of Complexes

The complexing ability of CDs was discovered by Pringsheim in 1930 [ 19-21]. Therefore, CDs constitute the first important example of relatively simple organic compounds that exhibit complex formation with other organic molecules. Moreover, because CDs are water-soluble host cavities capable of binding substrates quickly, selectively, and reversibly, and act catalytically in a variety of chemical reactions, they are considered good model enzymes [22-251.

Several intermolecular interactions have been proposed for the formation of CD inclusion complexes in aqueous solution [3, 14, 24-28]. They are:

1. Hydrophobic interactions 2. Hydrogen bonding 3. van der Waals interaction 4. Relief of high-energy water from the CD cavity 5. Release of the strain energy of the CD ring upon substrate inclusion

Regardless of the interaction involved in formation, geometric factors are crucial for determining the stability of the inclusion complex. Guests that are too small easily pass in and out the cavity and so give little bonding. Guest molecules significantly larger than the cavity may associate in such a way that there is only a partial penetration of the molecule in the cyclodextrin cavity. Moreover, the stability of the complex is determined by the hydrophobic character of the guest molecule.

INCLUSION COMPLEXES 5

As a result of the complexation, the properties of the included substance, such as solubility, chemical reactivity [2, 3, 28-31], pK, value [32,33], diffusion [3,34], and spectral properties, will be changed.

C. Characterization of the Complexes

The formation of complexes in solution was studied using mainly UV-VIS spectroscopy and conductometric, chromatographic, and calorimetric methods [ 14,28 and references therein]. These techniques allow determination of the thermodynamic parameters of complexation but do not give information on the geometry or the dynamic properties of the complex.

'H NMR spectroscopy was successfully applied [35] to derive geometri- cal and kinetic data. If inclusion does occur, protons located within the cavity (i.e., H-3, H-5) are shielded and their resonances are shifted upfield. Alternatively, if association takes place at the exterior of the torus, protons H-1, H-2, and H-4 are more strongly affected. I3CNMR chemical shifts and perturbation on the guest protons can provide further information. More refined experiments, like those exploiting ' H nuclear Overhauser enhancement, can afford the precise location of the molecule in the cavity.

Light emission spectroscopy was also widely used to study complexation. Owing to the sensitivity to local interactions and the possibility of time resolution, this technique proved to be useful for extracting both structural and kinetic properties (see Section 111) [36].

Determination of the complex geometry can be accomplished using X-ray diffractometry. However, this method was applied in a limited number of cases because of the intrinsic difficulty of obtaining single crystals of inclusion complexes, and induced circular dichroism (ICD) was used instead in most instances. Interpretation of the ICD spectra was performed by following a general rule derived by Harata and Uedaira [37,38]. They used the exciton coupling model of Tinoco [39] to calculate, with the prerequisite of axial or equatorial inclusion geometry in the CD cavity, the rotational strength of a series of substituted naphthalenes. They derived a positive sign for the signal relative to electronic transitions polarized along the axes of the CD and a negative sign for transitions polarized normal to it. This approach has been used to assign polarization direction to the electronic bands of a large number of molecules or, alternatively, to derive the inclusion geometry from the ICD sign of known transitions [40 and references therein]. However, Kodaka recently showed that if the molecule is placed outside the cavity, an opposite sign results, that is, transitions polarized along the CD axis give negative bands and those perpendicular to it exhibit positive contributions [41,42]. This result proves that the rule of


the signs should be applied with much caution because the presence of an ICD signal is not necessarily an indication of inclusion, and different complex geometries, with the guest associated inside and outside the cavity, can induce cancellation effects in the global ICD signal. Finally, the presence of different equilibrium conformations in loosely bound inclusion complexes plays an important role in determining the ICD pattern. The rotational strength of a series of substituted methylphenols included in the /?-CD cavity was calculated using an implemented version of the Kirkwood-Tinoco model. Starting from geometries optimized by molecular mechanics calculations and from electronic transitions obtained by a quantum mechanical semiempirical method, the calculation showed that symmetry breaking and structural flexibility contribute essentially to the shape of ICD spectra [43].

D. The Cyclodextrin Reaction Media

The result of a photochemical experiment with a C D system may strongly be affected by the physical status of the medium. As a result of the complexation equilibrium, the light excitation in solution necessarily in- volves a fraction of uncomplexed molecules which react in the solvent. Furthermore, a continuous exchange of guest location between the cavity and the bulk solvent phase occurs. Entry rates may vary over many orders of magnitudes, depending on the size of the guest and on the presence of charged substituents. For example, for some substituted azo-dye salts, the reaction time for entry in the a-CD cavity ranges from a few milliseconds to several minutes in pseudo first-order conditions [44-461. In some cases a two-step mechanism with a fast first binding and a slow subsequent intramolecular structural transformation was observed [46]. Exit rates also vary greatly, depending on the structural properties of the complex [47]. This fact can be of major importance in the interpretation of results pertaining to excited states that may relocate during their lifetime. Another important factor is the inclusion complex geometry in solution. A single time-averaged structure is often considered. However, the presence of noninterconverting or slowly interconverting structures has been proposed for the 1: l trans-stilbene-/?-CD complex [48] and for the 1:l diphenyl- methyl-tert-butyl-nitroxide-/?-CD [49, SO]. Recently, a study of the fluorescence decay of a number of anilinonaphthalene sulfonates in the presence of /?-CD supported the view that a 1: 1 complex can be present in several slightly different conformations [Sl, 521. Furthermore, the possibility of simultaneous formation of complexes with different stoichiometries has to be considered. Four types of CD:guest complexes with 1: 1,1:2,2: 1, and 2:2 stoichiometry have been observed depending on the size and structural features of the guest with respect to the host cavity (e.g., ref. 53).

PHOTOPHYSICAL PROCESSES 7

a b Figure 2. Examples of spatial organization of solid CD complexes: (a) channel type; (b) cage type.

Inclusion complexes can be precipitated from aqueous solutions of C D by using an excess of guest. Such precipitates contain complexes ordered in a crystalline network. Two main structures have been determined with channel- and cage-type molecular arrangement [2]. In the channel-type structure, the molecules are piled in a head-to-head (Fig. 2a) or head-to-tail orientation. In the cage-type structure, the cavity of the single CD molecule faces and is blocked by other CD molecules, resulting in isolated cages (Fig. 2b). The mutual orientation of guest molecules in the crystal and the tight molecular packing around them may be crucial to the photochemistry of the system.

111. PHOTOPHYSICAL PROCESSES

A. Fluorescence Modifications

Interest in modifying the photochemical properties of molecules by inclusion in the CD cavity arose from studies to determine the properties of the cavity itself as a model of an enzymatic site. Fluorescent probes, sensitive to the environment, are used to study the binding sites of proteins. Anilinonaph- thalene sulfonates are a class of such probes often utilized. In 1967, Cramer and co-workers reported that the fluorescence quantum yield of l-anilino-8-naphthalene sulfonate 1 was increased tenfold by the addition of 10-2M p- and y-CD and doubled by the addition of 10-’M a-CD [54]. The increase of the emission efficiency was accompanied by a blue shift of the fluorescence maximum. The different efficiency obtained by increasing the emission yield was attributed to a different mode of complexation: a-CD (inner diameter -6w) in only capable of including the aniline residue, whereas 8- and y-CD are big enough to enclose the naphthalene sulfonate residue. The fluorescence increase for CD-complexed fluorophores is a rather general phenomenon resulting from protection of the bound fluorophore from external quenchers (mainly oxygen), decrease of the


H03S 3

H03S

0 0 5 6

intramolecular rotation freedom, and exposure of the bound fluorophore to a less polar environment.

A similar effect of the CDs on the fluorescence of 2-(ptoluidinyl)naphthalene-6-sulfonate (2) was reported by Kondo and co-workers [55], who demonstrated the existence of complexes with different stoichiometry by applying the Benesi-Hildebrand treatment to the fluorescence variations of 2 as a function of CD concentration. For a-CD, the Benesi-Hildebrand plot - reciprocal fluorescence intensity versus reciprocal CD concentration-was a straight line, whereas for /3- and y-CD it deviated from the linearity at high CD concentrations. This indicates that a 1:l complex is formed between a-CD and 2, while p- and y- CD also form 2: 1 (host:guest) complexes. It was suggested that CD complexes 2 by including the toluidinyl moiety of the probe. In complexes with 2:l stoichiometry, the CDs include the naphthalene moiety as well as the toluidinyl part of 2. This mode of complexation for 2 with a- and p-CD was confirmed in reference 56. The fluorescence quantum yields for 1:l and 2: l complexes are reported in Table 1.

The formation of 2: 1 complexes for the interaction between 2 and 8-CD was confirmed by time-resolved fluorescence and conductometric measurements [57]. The fluorescence decay of 2 in the presence of B-CD was fitted


TABLE 1 CDs

Fluorescence Quantum Yields (af) for 2 in Water and in the Presence of

Medium 9, Reference

H2O 9 x 10-4 a-CD (1 1)” 4 x 4.6 x b-CD (1:l)” 2.4 x 5.3 x lo-’

(2: 1)” 1.36 x lo-’; 7.4 x 7-CD (1 : 1)” 4 x 10-3

(2:l)” 7 x 10-2

55 55,56 55,56 55,56 55 148

“Host to guest ratio.

by the sum of three exponentials attributed to a loose encounter complex of 2 with the CD (z = 0.25ns), to a complex in which the probe entered the CD cavity (z = 1.45ns), and to a complex in which each end of 2 is encapsulated by one CD (z = 3.511s). Interestingly, reference 58 suggests that when 2: 1 complexes are formed, the naphthyl group is complexed first and the second complexation occurs on the anilino functionality.

Fluorescence lifetime data of 1, 4, 5, and 6 in presence of lo-* Mp-CD were collected with frequency-domain fluorometry. These probes gave only 1: 1 complexes with p-CD [58] and, given the association constant values, the complex molar fraction was >0.95 for 4 and 5 and 0.1 for 6. The fluorescence decay of all the probes was best described by unimodal Lorentzian lifetime distribution [51,59] rather than by a mono- or biexponential function corresponding to the emission of the complexed and the free probe. This distribution was attributed to the coexistence of molecules included in the cavity to different extents. It was proposed that, in the case of 4, the apolar benzene ring enters the cavity first and penetrates until the whole naphthalene is included. This is the most stable and, hence, the most populated conformation of the complex. The distribution of the lifetimes suggests that at any time there is an ensemble of molecules in different stages of complexation which have slightly different lifetimes.

The complexes of 2,3 (sodium salt) and 4 (potassium salt) with 8-CD and (2,3,6- tri-O-methyl)-P-CD were studied using steady-state fluorescence and time-correlated, single-photon counting techniques [52]. The formation of both 1 : l and 2:l complexes between p-CD and 2,3 was confirmed. Trimethyl-p-CD gave evidence only of 1: 1 complexes. The fluorescence decay of systems giving exclusively 1 : 1 complexes was collected at CD concentrations that ensure more than 90% complexation. The analysis performed using a continuous lifetime distribution model


with a logarithmic-normal function gave the best results. The wide lifetime distribution and the red shift (12nm) of the emission maximum when the system was excited in the red edge of the absorption indicate the inhomogeneity of the system. This inhomogeneity was attributed to the inclusion of molecules in the conformation that differed in twist angle between the electron-donating amino group and the electron-withdrawing naphthylsulfonate group rather than to complexes that varied in depth of insertion of the guest in the cavity.

The polyacrylate functionalized 8-CD, 7, is much more efficient than its model compound acryloyl-p-CD or p-CD itself in enhancing the fluorescence of 1 (seven times) and 2 (20 times) [60]. The wavelength of maximum fluorescence of 1 shifts from 515nm (water) to 495nm (p-CD) and 475 nm (7); the fluorescence maximum of 2 shifts from 500 nm (water) to 460 nm (p-CD) and 437 nm (7). The polymer complex showed exclusively 2: 1 stoichiometry; the larger effect on fluorescence was attributed to cooperation by two CDs linked to the polymer backbone in the binding of the probe as shown in Scheme 1.

Coinclusion of the aromatic guest and a small molecule, usually an aliphatic alcohol, in the cavity of a dextrin (i.e., the formation of a three-component complex) often enhances the emission. In this type of complex, the fluorophore is sandwiched between the CD wall and the alcohol, which acts as a spacer. More water molecules are expelled from the cavity and the fluorophore experiences an environment less polar than that experienced in the absence of a spacer. The phenomenon was first reported in reference 61: the enhanced fluorescence of a-naphthylacetic acid observed after addition of y-CD was further increased by addition of cyclohexanol.

2-Anilinonaphthalene-6-sulfonic acid, 4, complexation with B- or y-CD in the presence of alcohols was studied using steady-state and time-resolved fluorescence spectroscopy [62]. The /3-CD-4 fluorescence was decreased by

CD poly-p-cD

7

poly 8-CD-2

Scheme 1


most of the alcohols; an opposite trend was found for y-CD complexes. In the presence of cyclodextrins the fluorescence decay was best described by two lifetimes: the short-lived was discrete in nature and attributed to free 4; the long-lived, described by a continuous Lorentzian distribution, was attributed to the complexed 4.

The addition of alcohol to the y-CD complex increased the.center lifetime of the Lorentzian distribution and decreased the relative distribution width quite regularly with increasing alcohol size. The environment around 4 became more hydrophobic with increasing alcohol concentration, in agreement with the steady-state measurements. The alcohol locks 4 in a few conformations and the lifetime distribution narrows.

For the p-CD complex, alcohol addition increased the distribution width and only slightly changed the center lifetime; this indicates a disruption of the CD-4 complex through competitive binding to the cavity. The average rotational correlation time, recovered from steady-state anisotropy, indicated that global rotation dominated the rotational diffusion of the complexes.

Alkane sulfates (n = 5-8, 10, 12) and alkylsulfates (n = 8, 10, 12, 14, 16, 18) added to 1 and 5 complexed with P-CD had the same disrupting effect exerted by alcohols on the P-CD-4 complex [63].

Removal of 2 from the a- and P-CD cavity was also induced by the presence of urea [56], some inorganic salts, guanidine hydrochloride, and tetra-n-butylammonium bromide [64]. A urea concentration of 7 M decreased the association constant of the complex a-CD-2 from 120M-' to 55M-'. For the double complexation of p-CD with 2 described above, a different behavior of the two arms of the Benesi-Hildebrand plot, reciprocal fluorescence intensity versus l/[CD], was found. The arm corresponding to low CD concentration was strongly affected by urea: the association constant K,,,, calculated from the ratio slope/intercept, decreased from 2200 to 95 M- '. The arm corresponding to high [CD] was scarcely affected: the calculted K,,, decreased from 1.5 x lo5 to 1.4 x 105M-2. Urea competes with 2 for complexation with P-CD in a 1: 1 complex while in the 2: 1 complex 2 is protected from the antagonistic action of urea. Fluorescence intensity and lifetime (q) of a-CD-2 and P-CD-2 was decreased by LiClO,, CsBr, and guanidinium hydrochloride, considerably increased by tetrabutylammonium bromide, and slightly increased by 7 M LiC1. The fluorescence enhancement was accompanied by a blue shift of the fluorescence emission maximum, while the decrease was concomitant with a red shift. The first three salts (salting4 agents) weaken the binding between CD and 2. The reduced polarity experienced by 2, as indicated by the fluorescence enhancement induced by the tetrabutylammonium bromide, was attributed to the wrapping of the butyl groups around the complex.


The fluorescence increase following CD complexation found for anilinonaphthalene-sulfonate derivatives is paradigmatic and is exploited in analytical chemistry. A review on this aspect and on CD applications in analytical chemistry has recently been published [65].

Dipping of the bound fluorophore into a low-polarity environment is probably the prevailing cause of fluorescence enhancement when the fluorescence yield of the guest in a polar solvent is lower than that in apolar solvents. In addition to the example of anilinonaphthalene sulfonates already described, some other examples are now described.

The fluorescence quantum yield for benzene, phenol, ethoxybenzene, aniline, N-methyl-, N-dimethyl-, and N-diethylaniline was enhanced following complexation with /?-CD [66]. All these molecules have fluorescence quantum yields (Qf) in water inferior to those in ethanol. The effect of complexation was particularly striking for aniline derivatives (a sixfold increase was reported for N-methylaniline), which can interact with water through H-bonding. The increase of Of was attributed to a decrease of the radiationless processes due, in part, to a decrease of rotation freedom and, mainly, to the expulsion of water from the cavity.

The increase of Qf from 0.14 to 0.24 reported for phenol in reference 66 was affected by an underestimated association constant (K,, , = 40 M-'). The accepted value of the constant, as redetermined by several authors [67 and references therein], is about 100M-'. On the basis of this value, the increase of Qf for phenol is much smaller. This was confirmed in a detailed study of the photophysical behavior of phenol and methylated derivatives included in the cavity of fl-CD [67]. The emission quantum yields and lifetimes of phenol and p-cresol were scarcely affected by complexation. By contrast, those of di- and trimethylated derivatives were markedly enhanced (see Table 2). A decrease of the Sl-+Sb radiationless deactivation rate constant occured in the complexes and was more marked for heavily substituted derivatives. The deactivation rate constants became similar to those observed in alcoholic media. It was suggested that the phenolic -OH group is located close to the hydroxyl residues of the fl-CD rim and that the molecular degrees of freedom are hindered to different extents, depending on the degree of interaction of the substituent groups with the cavity walls [67,68]. Molecular mechanics calculations of the complexes' structure are in agreement with the geometry suggested above [43].

Other examples of fluorescence enhancement in CD-containing solutions are reported in analytical papers describing studies in which the phenomenon is exploited to lower the detectability limit of a variety of organic compounds, mainly pesticides and drugs [69-80 and references therein].

Separation of chiral compounds by chromatographic techniques in which the CDs are used as functional hosts attached to the stationary phase has


TABLE 2 Fluorescence Quantum Yields and Lifetimes of Phenols (ar, zr) and of their fl-CD Inclusion Complexes (ab, eb) in Water'

Free Molecule Bound Molecule

Guest @f 7f ( 4 @b 7b(ns)

Phenol 0.13 3.2 0.15 3.7 p-Cresol 0.14 3.1 0.17 3.8 2,6-DMPb 0.044 1.25 0.14 5.25

2,4,6-TMPb 0.025 0.6 0.12 3.65 3,5-DMPb 0.090 2.9 0.20 4.7

3,4,5-TMPb 0.036 0.95 0.13 3.75

"Adapted with permission from ref. 67. bDMP, dimethylphenol; TMP, trimethylphenol.

been accomplished [65]. In solution, few examples of fluorescence-detected enantioselective complexation by CD were reported.

Inclusion of (S>-a-( l-naphthyl)ethylamine, (9-NEA, 8, and the corresponding (R) stereoisomer in the three cyclodextrins was investigated by steady-state and time-resolved fluorescence techniques [Sl]. The lifetime of (S)-NEA in pure water, 5.2 ns, was not affected by the addition of c1- CD and increased to 16.611s in the presence of 5 x M /?-CD and to 5.95 ns in the presence of 5 x 10-3M y-CD. The rotational correlation time, calculated only in the presence of B-CD from the fluorescence anisotropy decay, was 697 + 54ps, indicating rotation of the entire host-guest complex rather than tumbling of (S)-8 in the host. No difference in fluorescence lifetimes of (9-8 and (R)-8 was found in water or in the presence of /?-CD. The same lifetime quenching by (S)-a-phenethylamine was found for both stereoisomers included in /?-CD: quenching in the presence of /?-CD was lower than in pure water owing to the protection afforded by the cavity. These data indicate no chiral recognition in a water solution of /?-CD-8


complexes. Diastereomeric discrimination in the excitation and emission spectra as well as in the fluorescence lifetime of 8 was detected in dimethyl- sulphoxide (DMSO)/water 60/40 solution and, particularly, in the presence of 5 x lo-’ M /I-CD.

9 The (S) enantiomer of l,l‘-bi~-Znaphtho1(9) 2,2’-dimethoxy-l,l’-binaph-

thyl, and l,lf-binaphthyl-2,2’-diyl hydrogen phosphate were preferentially included by heptakis(2,3,6 tri-O-methyl)-/I-CD [82]. Addition of CD to the (S)-enantiomer caused fluorescence enhancement higher than that observed for the (R)-enantiomer. Moreover, the complex between the trimethyl-/I-CD and (S)-9 was found to have a stoichiometry of 2: 1 while the complex with (R)-g has 1: 1 stoichiometry. Unsubstituted 8-CD did not give enantioselective complexation.

Formation of three-component complexes was observed for several aromatic hydrocarbons.

The addition of ethanol, 1-propanol, 1-butanol, and 1-pentanol to a fluorene (10) solution containing /I-CD, 4.2 x M, shifted to the red the

10 absorption spectrum and increased the fluorescence yield. The formation of 1: 1: 1 complexes, in which fluorene partially extrudes from the cavity, were formed. Their formation constants are higher than that of the 1:l complex /I-CD-10. Similar results were obtained with nitriles [83].

Fluorescence enhancement caused by the formation of a three-component complex was also reported for coronene in the presence of y-CD (adamantane and 1-adamantanol were other components) [84], for perylene in the presence of 1-pentanol and fi- or y-CD [ 8 5 ] , and for azulene in the presence of /I-CD (ethanol, 1-propanol, 1-butanol, and 1-pentanol were the third components). In the last case, viewing the fluorescence variations as a


function of the b-CD and alcohol concentrations, the stoichiometry proposed for the ternary complex was 2b-CD:2 alcohol: 1 azulene; the proposal was supported by ICD and NMR data [86].

The addition of l-pentanol to a p-CD solution of l-cyanonaphthalene, 11, caused a blue shift of the absorption and fluorescence spectra: the fluorescence became structured and its lifetime increased from 8.211s (z of the b-CD-11 complex) to 11 ns. Because these modifications required the simultaneous presence of b-CD and l-pentanol, the formation of an 1:l:l complex was proposed and it was suggested that the hydrophobic aliphatic chain of the alcohol is inserted into the cavity. For this ternary complex, a rate constant for fluorescence quenching by I - of k, = 1.0 x lo9 dm3 mol- ' s- ' was found. This should be compared with k, = 2 x 10" dm3 mol-ls-' for the fluorescence quenching of 11 in water and k , = 4.6 x 109dm3 mol-'s-l for the fluorescence quenching of the complex b-CD-11 [87].

CN

11 Acridine and 2-acet ylnaphthalene are examples of molecules that are

emitting in polar and/or H-bonding solvents but nonemitting in apolar solvents. A decrease of fluorescence emission should be observed for these molecules following their inclusion in the relatively apolar cavity of the dextrins.

Addition of b-CD to an acridine solution caused a decrease of fluorescence emission [88]. A Benesi-Hildebrand treatment of the emission intensity changes indicated 1 : 1 stoichiometry for the complexation with association constant K,,, = 287 M - '. Induced circular dichroism studies [40] and NMR, H-3 and H-5 protons perturbed, indicated that acridine is included in the CD. The interaction of the aromatic nitrogen with the glycosidic oxygens of the cavity is believed to be responsible for the fluorescence quenching. The presence of 1 YO alcohol (v/v) decreased the extent of fluorescence quenching [89]. It is suggested that in the ternary complex, for which NMR evidence was obtained, the acridine occupies a position that is more exposed to the bulk aqueous environment than in the 1 : 1 complex.

Quenching of fluorescence was also observed for 2-acetylnaphthalene in the presence of the three dextrins and 2,6-di-O-methyl- and 2-hydroxy-


propyl-8-CD. Fluorescence lifetime data confirmed that quenching is static and, in the case of the interaction with y-CD, gave some evidence for the formation of two complexes with different stoichiometry [90].

The protection offered by C D against the quenching action of foreign substances depends mainly on the geometry of the complex and information about the degree of the protection is scattered in various papers (vide infra).

12

An example of the quenching of 2,3-dimethylnaphthalene (12) fluorescence by I -, oxygen, and several water-soluble organic quenchers has been reported [91]. The formation of p-CD-12 enhanced the fluorescence of the aromatic compound. The fluorescence lifetime of 12 in air-saturated solutions increased from 40ns in pure water to 70ns in the presence of fi-CD, owing to decreased oxygen quenching. The fluorescence of the complex was quenched by I-, fumaronitrile, acrylonitrile, acrylamide, and 2-hy- droxyethylmethacrylate. The Stern-Volmer plot for fluorescence intensity quenching was linear when I - was the quencher and showed a downward curvature, which was more pronounced with increasing [p-CD], for organic quenchers. The same behavior was shown by z0/z plots, indicating the dynamic nature of the quenching. The curvature was attributed to two deactivating processes involving the free and the CD-bound quencher. The second process is less efficient than the one with free quencher, and its contribution to overall quenching grows' with increased [CD].

Pyrene is a useful probe for studying the polarity of the medium because its fluorescence vibronic structure is very sensitive to the microenvironment [92,93]. In particular, the ratio of the intensities of the first (373 nm) to the third (384 nm) fluorescence vibronic maxima (R = I/III) strongly decreases with the decreasing polarity of the medium. The interaction of pyrene (13) with the three CDs has been thoroughly investigated by photophysical methods. There are considerable discrepancies among the results of the various reports, which are probably related to the very low solubility of 13 in H,O (c 5 x lo-' M). The low solubility imposes the need for special precautions when preparing the samples [94].

A limited number of papers report on the interaction between 13 and a-CD. In 1982 it was found that the addition of a-CD had no effect on the absorption, fluorescence emission, and lifetime of aqueous solutions of


pyrene [95]. The authors worked with a concentration of 13 near to solubility limit. Later it was found that pyrene fluorescence is quenched by the addition of a-CD [96] and an association constant was calculated assuming a 1: 1 stoichiometry for the complex. Recently [97], the interaction was studied by following the variations of fluorescence lifetime and intensity of I and I11 vibronic bands. The intensity of both bands decreases with increasing [a-CD] but their ratio (R) undergoes only a limited decrease from 1.85 to 1.7; fluorescence lifetime was unaffected by the presence of a-CD. The variations in peak intensity with varying [a-CD] were tested for several models of complexation, but the best fit was obtained by a model involving sequential 1 : 1 and 2: 1 complex formation. It was concluded that two molecules of a-CD cap 13 from opposite sides, leaving the central portion of the molecule directly exposed to water.

Many papers were published on the interaction of 13 with fl-CD. In the first study [98], a sequential complexation of pyrene with fl-CD to give a 1:l complex which successively associates to give a 2:2 complex was proposed because of the decrease in excimeric emission of a M pyrene solution due to the presence of clusters of 13. The decrease had a temporal evolution paralleled by the variation of R. It was suggested that 13, suspended in water, was solubilized in a less polar environment, which caused the increase in fluorescence lifetime. In this pioneering paper, it was first reported that the excited state of a CD-complexed molecule is protected against the action of quenching agents. The 8-CD-13 fluorescence quenching rate constant generated by O,, CH,NO,, I-, and T1’ decreased with increasing j3-CD concentration.

Qualitatively similar results were obtained in other studies [95, 99, 1001, which reported that the formation of a 1 : 1 complex causes an increase of the fluorescence yield and a decrease of R. Moreover, in reference 95, increases in absorption and fluorescence lifetime (200 ns in water, 370 ns in lo-, fl-CD) were noticed.

The sequential formation of 1 : 1 and 2: 1 complexes was proposed based on the absorption spectral variations which occurred with an isosbestic point for [fl-CD] < 10-’M; above this concentration a new, red-shifted band appears, indicating the formation of higher complexes [loll. A Benesi-Hildebrand plot of reciprocal R versus reciprocal [ B-CD] is linear for [j-CD] < lo-, M, indicating the formation of a 1: 1 complex in this [fl-CD] range; at [j3- CD] =- 10-,M, the plot is linear versus 1/[j-CDI2, indicating 2: 1 complexation.

In reference 102 it is reported that, by exploiting the R variations with [j-CD], in the whole [p-CD] range explored, l/AR versus l/[CD] has an upward curvature while l/AR versus l/[CD]’ is linear, indicating the


formation of a 2: 1 complex following the process:

2B-CD + 13 G= (fl-CD-13-B-CD) (1)

Sequential complexation was confirmed in reference 97, where it was reported that fl-CD addition decreases the R value and increases the intensity of the two vibronic bands. Solutions of fl-CD containing 13 always exhibited a biexponential decay: the shortest, t1 = 130 ns, has the same lifetime of 13 in water, the largest, t2 = 300ns, indicates that 13 experiences a hydrophobic environment. The ratio of the preexponentials A, /A , grew monotonically with [B-CD]. The data are consistent with a sequential complexation, as in the complexation of 13 with a-CD. In the 1: 1 complex, the included pyrene has the same lifetime as 13 in water because a substantial portion of the molecule is still exposed to the solvent: when 13 is encapsulated by two cyclodextrins, it experiences a low-polarity microenvironment and its lifetime consequently increases. In the same paper, the complexing ability of a polymer-supported fl-CD of the general formula

OH

CD-CHrO (.H2-kH-CH20f CD-CHrOH-

is also reported. The polymeric CDs contain 4-5 CD in the chain and differ by the average number of ether linkages between the CD units. The ratio n = linkers/CD varied over the range 1-8, allowing considerable variation in the flexibility of the chain between the CD units. Polymeric systems with different average spacer lengths exhibited large variations in binding ability: polymers with larger spacers have greater complexation ability, which is attributed to cooperative binding of a second CD to form a 2:l clam shell, as shown in Figure 3. The minimum length of the spacer allowing such complexation is n = 3.

In reference 95, the formation of a 1:2 (host:guest) complex between 13 and y-CD was proposed on the basis of an excimer-like emission, peaked at 470 nm, which accompanied the monomer emission in a solution containing 13 at 5 x lo-’ M and y-CD at M; correspondingly, two lifetimes were found t1 = 7711s (excimer emission) and z2 = 25011s (monomer emission). The broadening of the absorption spectrum of 13 in the presence of y-CD, the dependence on the excitation wavelength of the excimer/monomer emission quantum yields, the dependence of the fluorescence excitation spectrum on the emission wavelength, and the absence of a rise time in the

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