Supramolecular Dynamics Studied Using Photophysics - Langmuir

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Langmuir 2006, 22, 9100-9111

InVited Feature Article Supramolecular Dynamics Studied Using Photophysics Cornelia Bohne* Department of Chemistry, UniVersity of Victoria, PO Box 3065, Victoria, British Columbia, Canada ReceiVed May 2, 2006. In Final Form: July 28, 2006 Dynamics is an essential feature of supramolecular systems, and it’s understanding will be central in achieving new chemical function. The methodology to obtain association and dissociation rate constants for fast binding of guests to host systems in real time is described. Examples are provided for binding of guests to cyclodextrins or bile salt aggregates with an emphasis on the type of information and mechanistic insight that can only be uncovered from kinetic studies and is not apparent in thermodynamic investigations.

Introduction Supramolecular chemistry promises an avenue to chemistry that cannot be accomplished by individual building blocks.1 The field started with the investigation of small host-guest complexes involving a few building blocks and evolved, in parallel, into the synthesis of intricate supermolecules with a variety of different building blocks and into the generation of self-assemblies.1-8 Guiding motivations have been to mimic the complexity and function of biological systems and to provide the concepts and building blocks required for a bottom-up approach to nanoscale systems. Supramolecular systems are maintained by intermolecular interactions where reversibility is a key feature. Reversibility is not only a consequence of more labile interactions when compared to covalent bonds but it is essential for function and formation of large assemblies. For example, transport of solutes between different environments requires the specific binding of the solute in one medium and its ready release into the second medium. Very tight and irreversible binding would lead to sequestration and not transport. Supramolecular assemblies formed by repetitive units require mechanisms for error correction during their formation in which the “wrong” assemblies dissociate back into building blocks. Without error correction, a multitude of entities would be formed instead of discreet self-assemblies. The development of new supramolecular structures is driven by the desire to achieve complex functions, such as selective ion recognition, transport, catalysis, sensing, replication and electric or mechanical work.1,9,10 In parallel, many systems are synthesized to understand fundamental concepts underlying supramolecular * To whom correspondence should be addressed. E-mail: [email protected]. Phone: 250-721-7151. Web site: http://www.foto.chem.uvic.ca/. (1) Lehn, J. M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH: Weinheim, Germany, 1995. (2) Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D.; Vo¨gtle, F.; Lehn, J. M. ComprehensiVe Supramolecular Chemistry; Pergamon: New York, 1996; Vols. 1-11. (3) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 13121319. (4) Whitesides, G. M.; Simanek, E. E.; Mathias, J. P.; Seto, C. T.; Chin, D.; Hammen, M.; Gordon, D. M. Acc. Chem. Res. 1995, 28, 37-44. (5) Fyfe, M. C. T.; Stoddart, J. F. Acc. Chem. Res. 1997, 30, 393-401. (6) Sauvage, J.-P. Acc. Chem. Res. 1998, 31, 611-619. (7) Seidel, S. R.; Stang, P. J. Acc. Chem. Res. 2002, 35, 972-983. (8) Holliday, B. J.; Mirkin, C. A. Angew. Chem. 2001, 40, 2022-2043.

chemistry, such as molecular recognition, pre-organization, cooperativity, multivalency, reversibility, and self-assembly.1,11-24 The first step after the synthesis of a new system is to prove its structure, where X-ray crystallography, mass spectrometry, and NMR are the methods of choice of the synthetic chemist. Further characterization is obtained from the acquisition of thermodynamic information, such as equilibrium constants and stoichiometries for the molecular units incorporated into a discreet system. The choice of technique, such as potentiometry, NMR, and absorption or fluorescence spectroscopy, is dictated by the magnitude of the equilibrium constants and observables that differentiate between free and complexed building blocks. Thermodynamic characterization is frequently followed by investigation of specific supramolecular function. Since its beginning over four decades ago, the field of supramolecular chemistry has been driven by structural modification. Systematic changes in structure have led to a wealth of information, which is applied in the design of more complex (9) Murakami, Y. Supramolecular Reactivity and Transport: Bioorganic Systems. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Lehn, J. M., Eds.; Pergamon: New York, 1996; Vol. 4. (10) Reinhoudt, D. N. Supramolecular Technology. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Lehn, J. M., Eds.; Pergamon: New York, 1996; Vol. 10. (11) Vo¨gtle, F. Molecular Recognition: Receptors for Molecular Guests. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Lehn, J. M., Eds.; Pergamon: New York, 1996; Vol. 2. (12) Sauvage, J.-P.; Hosseini, M. W. Templating, Self-Assembly, and SelfOrganization. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Lehn, J. M., Eds.; Pergamon: New York, 1996; Vol. 9. (13) De Fetyer, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139-150. (14) Badjic, J. D.; Nelson, A.; Cantrill, S. J.; Turnbull, W. B.; Stoddart, J. F. Acc. Chem. Res. 2005, 38, 723-732. (15) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. Angew. Chem. 2001, 40, 2382-2426. (16) Lavigne, J. J.; Anslyn, E. V. Angew. Chem. 2001, 40, 3118-3130. (17) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.; Stoddart, J. F. Angew. Chem. 2002, 41, 898-952. (18) Ercolani, G. J. Phys. Chem. B 1998, 102, 5699-5703. (19) Ercolani, G. J. Am Chem. Soc. 2003, 125, 16097-16103. (20) Ercolani, G.; Mandolini, L.; Mencarelli, P.; Roelens, S. J. Am. Chem. Soc. 1993, 115, 3901-3908. (21) Mulder, A.; Huskens, J.; Reinhoudt, D. N. Org. Biomol. Chem. 2004, 2, 3409-3424. (22) Linton, B.; Hamilton, A. D. Chem. ReV. 1997, 97, 1669-1680. (23) Rebek, J., Jr. Acc. Chem. Res. 1999, 32, 278-286. (24) Rebek, J., Jr. Angew. Chem. 2005, 44, 2068-2078.

10.1021/la061227w CCC: $33.50 © 2006 American Chemical Society Published on Web 09/12/2006

Supramolecular Dynamics Studied Using Photophysics

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small guest molecules. The focus is on information from dynamic studies that has consequences on function that cannot be uncovered from thermodynamic investigations, and a comprehensive review is not provided.

Theoretical Background

Figure 1. Schematic representation of the relationship between structure, thermodynamics and dynamics in supramolecular systems.

systems. In contrast, the understanding of dynamics is less developed. Structure, thermodynamics, and dynamics are interrelated (Figure 1). Knowledge of structure provides a snapshot of the system where, in general, the most abundant species is depicted. Thermodynamics provides data on the system at equilibrium, and it provides information after all action is over. Frequently, the equilibrium state is related to the state for which structural information is obtained. For this reason, one can infer structural aspects from thermodynamic information and viceversa. Dynamics provide information on how the system evolves over time. Kinetic information cannot be determined from structural or thermodynamic studies; that is, the magnitude of an equilibrium constant does not include any information on how fast a complex is formed or dissociates. In contrast, the knowledge on rate constants can be used to derive thermodynamic parameters. Therefore, the relationship between thermodynamics and structure with dynamics is unidirectional. Dynamic studies are necessary to provide the “movie” in addition to the “snapshots” taken from structural and thermodynamic measurements. It is important to discuss why the understanding of supramolecular dynamics is less developed than the ability to design new supramolecular structures, in view of the essential role that dynamics and reversibility play in supramolecular chemistry. Characterization of the dynamics of a system relies on the measurement of kinetics in real time, since studies of relative rates are limited by the assumption of similar mechanisms for different systems. The field of supramolecular chemistry started with the investigation of small host-guest systems, and in this context it is useful to discuss the relationship between length scales and time scales for events involving two or more components. Any bimolecular event in solution is limited by diffusion, dictating the fastest time scale for which an event can occur. The dimension of 1 nm is comparable to the height of host systems having a cavity, such as cyclodextrins and calixarenes. The time for a small molecule, such as glucose in water at room temperature, to diffuse over 1 nm is 0.003 µs (3 ns). Larger supramolecular systems can have dimensions of hundreds of nanometers. Even for such length scales, the diffusion times are shorter than milliseconds (e.g., it takes 800 µs (0.8 ms) for glucose to diffuse over a distance of 500 nm). This simple analysis shows that the dynamics of the host-guest systems that started the field of supramolecular chemistry was in a time domain not accessible to common kinetic techniques, such as NMR and stopped-flow. Therefore, the lack of kinetic information on supramolecular systems is a consequence of the lack of appropriate techniques and methodology. The objective of this feature article is to highlight the type of information that can be obtained from dynamic studies. A theoretical background is followed by examples using cyclodextrins (CDs) and bile salt aggregates as host systems that bind

The kinetics of supramolecular systems can be measured in real time when the system is removed from equilibrium by a perturbation that is faster than relaxation back to a new equilibrium. The fastest convenient way of mixing solutions with components at different concentrations is ca. 1 ms using stopped-flow experiments. The generation of a new chemical or changes of external conditions, such as temperature or pressure, can be employed to study fast kinetic events. Ultrasonic relaxation is a technique where periodic oscillations of temperature and pressure are used to recover kinetic information, and this technique has been employed to study the complexation kinetics with CDs.25-29 Recently, fluorescence correlation spectroscopy, which is a technique based on concentration fluctuations around an equilibrium state, has also been used to investigate the CD binding dynamics.30 However, the most widely employed methods involve the use of lasers to create a perturbation of an equilibrium. Lasers are useful because they can deliver photons at specific energies in short periods of time. For example, laser temperature jump experiments (LTJ)31-34 make it possible to heat a solution in times as short as picoseconds. Alternatively, excitation of the guest or host creates a new chemical (i.e. excited states or reactive intermediates) that can interact differently with the supramolecular system when compared to the interactions of its ground state.35 The kinetics for the reactive intermediate are measured using laser flash photolysis (LFP) experiments. The relationship between the lifetime of the excited state and the host-guest dynamics is essential for the determination of the association and dissociation rate constants between the guest and the host. In the discussion below, it is assumed that the excited state of the guest is formed, but the theoretical framework is equally valid when the host is excited. The excited-state guest will not move between the supramolecular system and the homogeneous phase when its lifetime is much shorter than its binding dynamics. In this case, the excited-state guests in the homogeneous phase and bound to the host are isolated and no information on the association and dissociation processes can be obtained. This situation is ideal to obtain thermodynamic parameters, such as “equilibrium constants”, and this condition is met for most singlet excited states that fluoresce. Triplet excited states and reactive intermediates, such as radical and radical (25) Fukahori, T.; Kondo, M.; Nishikawa, S. J. Phys. Chem. B 2006, 110, 4487-4491. (26) Jobe, D. J.; Verrall, R. E.; Junquera, E.; Aicart, E. J. Phys. Chem. 1994, 98, 10814-10818. (27) Nishikawa, S.; Yamaguchi, K.; Fukahori, T. J. Phys. Chem. A 2003, 107, 6415-6418. (28) Nishikawa, S.; Yamaguchi, S. Bull. Chem. Soc. Jpn. 1996, 69, 24652468. (29) Rohrbach, R. P.; Rodriguez, L. J.; Eyring, E. M.; Wojcik, J. F. J. Phys. Chem. 1977, 81, 944-948. (30) Al-Soufi, W.; Reija, B.; Novo, M.; Felekyan, S.; Ku¨hnemuth, R.; Seidel, C. A. M. J. Am Chem. Soc. 2005, 127, 8775-8784. (31) Ballew, R. M.; Sabelko, J.; Reiner, C.; Gruebele, M. ReV. Sci. Instrum. 1996, 67, 3694-3699. (32) Callender, R.; Dyer, R. B. Curr. Opin. Struct. Biol. 2002, 12, 628-633. (33) Holzwarth, J. F.; Schmidt, A.; Wolff, H.; Volk, R. J. Phys. Chem. 1977, 81, 2300-2301. (34) Turner, D. H.; Flynn, G. H.; Sutin, N.; Beitz, J. V. J. Am Chem. Soc. 1972, 94, 1554-1559. (35) Kleinman, M. H.; Bohne, C. Use of Photophysical Probes to Study Dynamic Processes in Supramolecular Structures. In Molecular and Supramolecular Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker Inc.: New York, 1997; Vol. 1, pp 391-466.

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decay of the excited guest is much slower than the binding dynamics ((k0 + kH0 ) , (k/- + k/+[H]) the exponential factor for the fast component can be approximated to35,38

kobs ) k/- + k/+[H]

Figure 2. Kinetic scheme for the interaction of a guest (G, circle) in the ground state (open circle) and the excited state (blue circle) with a host (H, square). The guest binding dynamics for the excited state is defined by the association (k/+) and dissociation (k/-) rate constants (red). The competitive excited-state decay pathways achieved by quenching in the aqueous phase (kq) and within the supramolecular system (kHq ) are shown in green.

anions, can have lifetimes of several hundreds of microseconds and therefore are the species of choice to study the dynamics of host-guest binding. The mechanism shown in Figure 2 corresponds to a system containing two compartments that are linked by their binding dynamics. Excitation and deactivation processes as well as the association/dissociation processes in the excited state have to be considered. In addition, quenching is useful to broaden the scope of the kinetic studies by increasing the structural variability of the guests that can be employed (see below). Concentrations of excited states are in general much lower than the concentrations of host ([H]) or quencher ([Q]) and bimolecular reactions involving the host or quenchers can be assumed to fulfill pseudofirst-order conditions. The excited guests in each compartment, i.e., water or inside the host, decay with an intrinsic rate constant (k0, kH0 ) and the two lifetimes can be shortened with the addition of quenchers (kq, kHq ). The general solution for this mechanistic scheme leads to a decay of the excited state that corresponds to the sum of two exponentials, where the two exponential factors of the decay include the rate constants for the binding dynamics (k/+[H], k/-) and the intrinsic lifetimes for the free (k0 + kq[Q]) and bound excited states (kH0 + kHq [Q]). The solution for the time constants for this kinetic scheme for photophysical experiments was derived many years ago for excimer emission.36 When the exponential factors of the decay are measured at different host concentrations a linear relationship exists between the sum and the product of the exponential factors with the host concentration.37 For specific experimental conditions the kinetics can be simplified by applying useful approximations, which are described below. The kinetics of long-lived reactive intermediates is measured in LFP experiments where the change in the absorbance of the species of interest is detected. The relaxation kinetics after the formation of the excited state can be followed directly when the molar extinction coefficients of the excited guest in water and bound to the host are different, and when the equilibrium constant for the excited state is significantly different from the equilibrium constant in the ground state. Depending on the relationship between the molar extinction coefficients, the kinetics corresponds to the decay of two exponentials or to an exponential growth followed by an exponential decay. The exponential factors follow the general solution mentioned above. However, when the intrinsic (36) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970. (37) Cheung, S. T.; Ware, W. R. J. Phys. Chem. 1983, 87, 466-473.

(1)

This approximation should be used with caution, and as a guideline, it is valid when the difference between the two exponential factors measured from the kinetics are different by at least 2 orders of magnitude.35 An additional test for the validity of the approximation is to compare the values for the exponential factor of the fast decay with the value for the sum of the two exponential factors and check how different these two parameters are. If the value for the sum of the two exponential factors is within the error for kobs, eq 1 can be employed.38 Unfortunately, only a few excited states have very different spectroscopic signatures when bound to a host system from those in the bulk solvent. A more general methodology is the use of a quencher to differentially modulate the lifetimes of the guest in water and when bound to the host. This quenching methodology was first developed for the binding of arenes with micelles, where it was assumed that no quenching occurred for the excited state in the micelle (kHq ) 0).39 This method was generalized for the binding of guests with cyclodextrins using a quencher that mainly resided in the aqueous phase and had a diminished quenching efficiency inside the host (kq > kHq ).40 When the concentration of the excited state in the aqueous phase is much smaller than the concentration in the host the decay follows a monoexponential function and steady-state can be assumed to derive the equation for the observed first-order rate constant (eq 2)

kobs ) k0H + k/- + kHq [Q] -

k/-k/+[H] k0 + k/+[H] + kq[Q]

(2)

The parameters k0 and kq are determined independently from quenching studies for the excited guest in water, and kH0 is the lifetime of the excited guest in the presence of host before the addition of any quencher. These three parameters are fixed when the experimental data are fit to eq 2, and the recovered rate constants from the fit are those for the association and dissociation processes and for the quenching of the excited guest when bound to the host. Conceptually, the guest binding to the host sequesters the excited state guest into an environment in which the quenching efficiency is decreased when compared to quenching in water. Therefore, the lifetime for the excited guest in water can be decreased more readily than the lifetime for the guest in the host. As a consequence, the quenching plot in the presence of a host, as defined by eq 2, deviates from the linear behavior observed in water (Figure 3). The curvature of the quenching plot is influenced by the magnitude of all rate constants and the most important requirement is that the quenching rate constant in water be much higher than the rate constant for quenching inside the host system. The curvature can also be influenced by changing the host concentration.35 At high quencher concentrations, the lifetime of the excited-state guest in water is so short that this excited guest cannot reassociate with the host, and the ratelimiting step becomes exit of the guest from the host or quenching (38) Liao, Y.; Frank, J.; Holzwarth, J. F.; Bohne, C. J. Chem. Soc., Chem. Commun. 1995, 2435-2436. (39) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 279-291. (40) Turro, N. J.; Okubo, T.; Chung, C.-J. J. Am. Chem. Soc. 1982, 104, 1789-1794.


Langmuir, Vol. 22, No. 22, 2006 9103 Chart 1

Figure 3. Schematic representation for the dependence of the observed first-order decay rate constant for an excited state in water (black, straight line) and for an excited-state bound to a host (blue) when the quenching in water is more efficient than the quenching of the excited state in the host. The dashed straight line represents the limiting case at high quencher concentration (see text).

of the guest in the host. Mathematically, for this scenario, the last term in eq 2 is negligible and the dependence of kobs with the quencher concentration is linear (dashed line in Figure 3), where the slope corresponds to kHq and the intercept of the Y-axis corresponds to the sum of kH0 and k/-. When the intrinsic lifetime of the excited guest is longer than the residence time in the host (kH0 , k/-), the intercept of the dashed line with the Y axis is a good estimate for the value of the dissociation rate constant. The curvature of the quenching plot in the presence of host contains information on both the association and the dissociation processes. The larger the difference between the straight line observed in water and the curved quenching plot the slower the binding dynamics. Finally, the value of kHq can provide information on the access of the quencher to the binding site inside the host. When quenching in water is diffusion controlled, the intrinsic rate constant for quenching is high. If one assumes that this intrinsic rate constant does not decrease by many orders of magnitude when quenching occurs inside the host, the value of kHq corresponds to the association rate constant of the quencher to the binding site in the host.

Binding Dynamics of Guests to Cyclodextrins Cyclodextrin (CD) is a cyclic host where guests can be included within its internal cavity. CDs have been employed to study fundamental aspects of host-guest chemistry, were used as enzyme mimetic systems or as active components for selfassembled systems, and have technological applications in the food industry to sequester undesirable flavors and in the drug industry as potential drug delivery systems.41,42 The thermodynamics of guest complexation has been extensively studied,43,44 and in general terms, steric constraints and hydrophobicity dictate the binding efficiency of guests to CDs. The three common CDs, i.e., R-, β-, and γ-CD, have increasingly wider cavities where the smallest internal diameter varies between 5 and 8 Å and the internal volume varies between 170 and 430 Å3. Complex formation can be driven by favorable enthalpy changes or favorable entropy changes and an enthalpy-entropy compensation effect has been observed.44 (41) Szejtli, J. Chem. ReV. 1998, 98, 1743-1753. (42) Szejtli, J.; Osa, T., Cyclodextrins. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies, J. E., MacNicol, D. D., Vo¨gtle, F., Lehn, J.-M., Eds.; Elsevier Science Ltd.: New York, 1996; Vol. 3. (43) Connors, K. A. Chem. ReV. 1997, 97, 1325-1357. (44) Rekharsky, M. V.; Inoue, Y. Chem. ReV. 1998, 98, 1875-1917.

Comparison of Ground State and Excited-State Guest Binding. When applying the quenching methodology described above, it is inherently easier to obtain values with higher precision for the dissociation rate constant than for the association rate constant. It is tempting, and sometimes necessary, to calculate the values for the association rate constants from the measured values of the dissociation rate constants and equilibrium constants (K ) k+/k-).39,45 Unfortunately, the equilibrium constants measured are those for the ground state (KGS eq ), whereas the rate constants (k/+,k/-) are for the triplet excited states of the guests. It is important to establish if such a calculation is warranted considering that excited states are different chemicals when compared to their ground states. Xanthone (Chart 1) was employed as a guest to compare the behavior for the binding dynamics of its ground and triplet excited states with CDs. Xanthone is suitable for such studies because its triplet-triplet absorption spectrum depends on the solvent polarity making it possible to follow directly the relocation of the triplet state between different environments.46-48 In addition, triplet xanthone has a higher dipole moment48,49 and basicity50 than its ground state. These properties are expected to alter the binding interactions of the excited state with CDs when compared to the behavior of the ground state. The relocation of triplet xanthone from the cavity of CDs was established by the observation of a shift to shorter wavelengths of the triplettriplet absorption spectra as the delay after the excitation laser pulse increased.49 Measurements of the relocation kinetics when the concentration of CD was varied led to the determination of the values for k/+ and k/- where the data were analyzed using eq 1 (Table 1).38,51,52 The association rate constants are slightly lower than the rate constant for a diffusion controlled process in water (6.5 × 109 M-1 s-1),53 which is expected since inclusion of the guest into the CD cavity will depend on the directionality of the approach of the guest toward the CD, i.e., collisions with the side of the CD are unproductive with respect to complex formation. The residence time of triplet xanthone within β-CD, which corresponds to the inverse of k/-, is fairly short, ca. 120 ns. The equilibrium constant for the triplet state is significantly lower than measured for the ground state showing that in the case of xanthone any calculation of k/+ from the measured k/- and KGS eq values would have led to an error of at least an order of magnitude in the determination of the association rate constant.51,52 The dynamics of ground-state xanthone binding to CDs was (45) Hashimoto, S.; Thomas, J. K. J. Am. Chem. Soc. 1985, 107, 4655-4662. (46) Abuin, E. B.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 6274-6283. (47) Evans, C. H.; Prud′homme, N.; King, M.; Scaiano, J. C. J. Photochem. Photobiol. A: Chem. 1999, 121, 105-110. (48) Scaiano, J. C. J. Am. Chem. Soc. 1980, 102, 7747-7753. (49) Barra, M.; Bohne, C.; Scaiano, J. C. J. Am. Chem. Soc. 1990, 112, 80758079. (50) Ireland, J. F.; Wyatt, P. A. H. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1053-1058. (51) Liao, Y.; Frank, J.; Holzwarth, J. F.; Bohne, C. J. Chem. Soc., Chem. Commun. 1995, 199-200. (52) Okano, L. T.; Barros, T. C.; Chou, D. T. H.; Bennet, A. J.; Bohne, C. J. Phys. Chem. 2001, 105 B, 2122-2128. (53) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd ed.; Marcel Dekker: New York, 1993.

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T Table 1. Equilibrium Constants for the Ground (KGS eq ) and Triplet Excited States (Keq) of Xanthone and 1-NpOH as Guests of β-CD / / and Association (k+, k+) and Dissociation (k-, k-) Rate Constants Determined for the Binding Dynamics of the Ground and Triplet Excited States

guest

method

-1 KGS eq /M

xanthone

direct direct quenching (Cu2+)

1100 ( 200a

1-NpOH quenching (Mn2+)

500 ( 80e

KTeq/M-1 70 ( 10c 90 ( 10d 1000 ( 600f

k/+ /108 M-1 s-1 6 ( 1c 11 ( 1d 5 ( 2f

k+/108 M-1 s-1 4 ( 1b

k/-/106 s-1 8.1 ( 0.1c 12 ( 1d

k-/106 s-1 0.3 ( 0.1b

0.5 ( 0.2f

a Fluorescence binding isotherms from ref 51. b LTJ from ref 51. c LFP from ref 52. d LFP from ref 55. e Fluorescence binding isotherms from ref 56. f LFP from ref 56.

studied using LTJ experiments (Table 1).51 The association rate constants for the ground and excited states were the same, and the lower binding efficiency for the triplet state is solely due to an increase in k- (ca. 27 times). Studies with different guests showed that the association process is not very sensitive to changes in the guest’s structure, suggesting that the rate limiting step for complex formation is related to processes mainly involving the CD, which, for example, could be the desolvation step of the CD cavity. The large increase for the dissociation rate constant of triplet xanthone from CDs has to be related to changes in properties of the guest, and both the higher dipole moment and basicity for triplet xanthone will enhance the driving force for its exit from the CD. Dynamic studies in D2O54 provide some support to the assignment that the rate-limiting step for the association rate constant of xanthone with β-CD is related to the desolvation of the CD cavity, because in D2O the k/+ values decreased to (0.88 ( 0.06) × 108 M-1 s-1 whereas the k/- value was unchanged ((7.6 ( 0.1) × 106 s-1). This example shows that the equilibrium constants for the ground and excited state guests can be very different and this difference can only be uncovered through kinetic measurements. In addition, knowledge of the dynamics uncovers mechanistic details, such as the importance of the desolvation of the CD cavity for the association process, which are not easily accessible from thermodynamic studies. Xanthone is the only guest for which relocation of the excited state has been directly measured to date. Structure-dynamics relationship studies, where the structure of the guest is modified, rely on the use of the quenching methodology. For this reason, xanthone was the ideal guest to compare the direct and quenching methodologies. The triplet state of xanthone is efficiently quenched by Cu2+ cations ((8.6 ( 0.5) × 107 M-1 s-1)49 and the quenching plot in the presence of β-CD is curved.55 However, the curved quenching plot does not reach the linear region predicted at high quencher concentrations because the hostguest binding dynamics is fast and the time-resolution of the LFP system precludes the addition of higher quencher concentrations. The k/+ and k/- values determined from the quenching methodology are both higher than determined in the direct measurements (Table 1). The equilibrium constants calculated from the rate constants for both methodologies were the same, suggesting that the increase for the rate constants determined using the quenching methodology were comparable. The reasons for the overestimation of the rate constants are unknown; however, comparison of the binding dynamics for different guests is possible when the same or structurally similar ions are used as quenchers. The large differences for the equilibrium constants in the ground and excited states of xanthone are not a general phenomenon for other guests. Naphthalenes also have π,π* triplet excited states, as is the case for xanthone, and for this reason, the excited state (54) Barra, M. Supramol. Chem. 1997, 8, 263-266. (55) Christoff, M.; Okano, L. T.; Bohne, C. J. Photochem. Photobiol. A: Chem 2000, 134, 169-176.

Figure 4. Calculated structures for the complexes of xanthone (A) and (R)-1-NpOH (B, one of the two possible incorporation modes) with β-CD showing the binding of both guests to the rim of the CD. Figure adapted from refs 57 and 58.

of naphthalenes have larger dipole moments than their ground states. The comparison of the binding dynamics of xanthone with that for 1-naphthyl-1-ethanol (1-NpOH, Chart 1) is illustrative because both guests are bound to the rim of β-CD without being deeply included in the cavity (Figure 4).57,58 The value of KGeq is smaller for 1-NpOH compared to xanthone, probably because the fit of 1-NpOH with β-CD is less complementary than for xanthone. The values for the association rate constant for the triplet states of both guests are the same; a result in line with the assignment that the association process is largely independent of the structure of the guest. However, the k/- value for 1-NpOH is much smaller than for xanthone, showing that the higher dipole moment for the excited state of these guests is not sufficient to explain the fast exit for xanthone. In addition, the value for KTeq for 1-NpOH did not decrease when (56) Barros, T. C.; Stefaniak, K.; Holzwarth, J. F.; Bohne, C. J. Phys. Chem. A 1998, 102, 5639-5651. (57) Murphy, R. S.; Barros, T. C.; Barnes, J.; Mayer, B.; Marconi, G.; Bohne, C. J. Phys. Chem. A 1999, 103, 137-146. (58) Murphy, R. S.; Barros, T. C.; Mayer, B.; Marconi, G.; Bohne, C. Langmuir 2000, 16, 8780-8788.

Supramolecular Dynamics Studied Using Photophysics Scheme 1

Chart 2

compared to the equilibrium constant for its ground state. Similar equilibrium constants were also observed for the ground and triplet excited states of naphthalene derivatives substituted with positively charged alkyl chains of different length.40 The faster exit for triplet xanthone is probably due to its higher basicity in the triplet excited state. This conclusion is similar to studies on the binding of guests to micelles where the importance of the guest’s basicity has been highlighted.59 Equilibria and Kinetics. Once an equilibrium is achieved, the rates for formation and dissociation of a complex are the same. The equilibrium constant is defined by a ratio of concentrations or a ratio of rate constants as shown in Scheme 1 for the sequential formation of host-guest complexes with 1:1 and 2:2 host:guest stoichiometries. The equilibrium constant is a thermodynamic parameter, which does not provide any information on how fast a complex is formed or dissociates. Even small changes in the structures of the guest or the host can have a large effect on the association and dissociation rate constants without significantly affecting the equilibrium constants. Increases or decreases in equilibrium constants can be a consequence of changes to either the association or dissociation rate constants and in the absence of kinetic studies one cannot know which rate constant is affected. The importance of kinetic studies to understand the complexation of guests with CD was realized early on, and temperature jump experiments were performed to correlate the kinetics of complex formation with the equilibrium constants for a series of azo dyes (Chart 2) with R-CD.60 Similar equilibrium constants were observed for the dyes that formed host-guest complexes with 1:1 stoichiometries, where the inclusion of the phenyl moiety into the R-CD cavity was proposed. Despite the similar (59) Quina, F. H.; Alonso, E. O.; Farah, J. P. S. J. Phys. Chem. 1995, 99, 11708-11714.

Langmuir, Vol. 22, No. 22, 2006 9105 Table 2. Equilibrium Constants for the Ground State (KG eq) of Azo Dyes with r-CD and the Association (k+) and Dissociation (k-) Rate Constants Determined from Temperature Jump Experiments60

guest

KGeq/ M-1

k+/ 107 M-1 s-1

k-/ 104 s-1

KGeq/ M-1 from kinetics

Azo dye R ) H Azo dye R ) CH3 Azo dye R ) CH2CH3

270 417 455

1.3 0.012 0.0006

5.5 0.035 0.0019

236 343 316

equilibrium constants the association and dissociation rate constants varied by 7 orders of magnitude. A small subset of the data from this study is shown in Table 2 to underline how subtle changes in structure have significant consequences on the guest binding dynamics. The equilibrium constants were obtained either from binding isotherms determined in UV-vis absorption studies or were calculated from the ratio of the association and dissociation rate constants determined using eq 1, where the association and dissociation rate constants correspond to the dynamics in the ground-state (k+, k-) (Table 2). The calculated values are in reasonable agreement with the values determined from binding isotherms with the same trend of an increase in the equilibrium constants being observed for the methyl and ethyl substituted azo dyes. The addition of one methyl group to the azo dye decreases significantly the binding dynamics of the host-guest complex. The effect is similar for k+ (decrease of ca. 100) and k- (decrease of ca. 150) so that the equilibrium constant is affected to a much smaller extent (increase of 1.5). Increasing the size of the alkyl group to an ethyl group further decreases the dynamics (ca. 20). The decrease of the association rate constant with alkyl substitution could be related to additional distortions of the CD cavity necessary to accommodate the larger guest, whereas the decrease for the dissociation rate constants could be related to the increased hydrophobicity of the guest. Therefore, formation of the complex is more difficult for the larger guests, but once the penalty for its formation is paid, the dissociation is slower because the larger guests are also more hydrophobic. The complexation dynamics cannot be slowed indefinitely by increasing the guest size because at one point the guest is too large to fit into the CD cavity. For example, the azo dye with methyl substituents at the 3- and 5-positions of the phenol does not form a complex with R-CD. Finally, this early study showed that to investigate the dynamics of guests that readily fit into CD cavities techniques for kinetic measurements in the nanosecond to tenth of a microsecond time domain are required because the dissociation rate constants are high. Addition of cosolvents, such as alcohols, or surfactants, influences the equilibrium constants of guests with cyclodextrin, and increases or decreases of equilibrium constants were observed depending on the guest/cosolvent pair investigated.45,61-69 Ternary complexes, containing xanthone, β-CD, and alcohols, such as (60) Cramer, F.; Saenger, W.; Spatz, H. C. J. Am. Chem. Soc. 1967, 89, 1420. (61) Hamai, S. J. Phys. Chem. 1990, 94, 2595-2600. (62) Hamai, S.; Ikeda, T.; Nakamura, A.; Ikeda, H.; Ueno, A.; Toda, F. J. Am. Chem. Soc. 1992, 114, 6012-6016. (63) Kano, K.; Takenoshita, I.; Ogawa, T. Chem. Lett. 1982, 321-324. (64) Matsui, Y.; Mochida, K. Bull. Chem. Soc. Jpn. 1979, 52, 2808-2814. (65) Munõz de la Penã, A.; Ndou, T. T.; Zung, J. B.; Greene, K. L.; Live, D. H.; Warner, I. M. J. Am. Chem. Soc. 1991, 113, 1572-1577. (66) Nelson, G.; Patonay, G.; Warner, I. M. Anal. Chem. 1988, 60, 274-279. (67) Nelson, G.; Warner, I. M. J. Phys. Chem. 1990, 94, 576-581. (68) Ponce, A.; Wong, P. A.; Way, J. J.; Nocera, D. G. J. Phys. Chem. 1993, 97, 11137-11142. (69) Schuette, J. M.; Ndou, T. T.; Munõz de la Penã, A.; Mukundan, S., Jr.; Warner, I. M. J. Am. Chem. Soc. 1993, 115, 292-298.

9106 Langmuir, Vol. 22, No. 22, 2006

Bohne Chart 3

Figure 5. (A) Effect of alcohol addition (2-butanol ) 2-BuOH, cyclopentanol ) c-PeOH, 1-pentanol ) 1-PeOH) on the equilibrium constants for the ternary complexes between xanthone, β-CD, and alcohol and on the apparent equilibrium constant for xanthone with γ-CD. (B) Effect of alcohol addition on the dissociation rate constant of the triplet excited state of xanthone from β-CD and γ-CD. The inset shows a cartoon where the green areas indicate the preferential solvation at the entrances of the CD cavity. Data from ref 70.

2-butanol, tert-butyl alcohol, pentanol, cyclopentanol, and cyclohexanol, are formed with equilibrium constants that are smaller (13-68 times) than observed for the xanthone/β-CD complex (see Figure 5 for selected alcohols). In contrast, in the case of the xanthone/γ-CD/alcohol ternary complexes, the binding efficiencies increased by factors of 1.5-2.0 when compared to the binary system.70 An additional slow decay was observed in the kinetics for triplet xanthone in the presence of γ-CD and alcohols, when compared to the kinetics in the absence of alcohols (k/- ) (7.3 ( 0.5) × 106 s-1), showing that the exit of the guest was slowed by the presence of the cosolvent in the host-guest complex. The decrease of k/- ranged from a factor of 11-25, and there was no clear correlation between the structure of the alcohols and the decrease in the dissociation rate constant. In the presence of β-CD, the dissociation rate constant also decreased despite the decrease in the equilibrium constants for the ternary complexes (Figure 5). These results indicate that irrespective to the opposite effect of alcohols on the thermodynamics for complex formation of xanthone/alcohols with β- and γ-CD, the binding dynamics of the guest was slowed. The combination of slower dynamics and insensitivity of the kinetics to the structure of the alcohol led to the proposal that alcohols, in addition to filling the empty spaces inside the CD cavity, also provide a shielding of the cavity by preferential solvation of the CD rim.70 This example (70) Liao, Y.; Bohne, C. J. Phys. Chem. 1996, 100, 734-743.

shows that even trends in the changes of equilibrium constants, decreases vs increases in the KGeq values, cannot be related to trends in the binding dynamics. The information that alcohols lead to slower binding dynamics of guests is, for example, useful when developing chromatographic separation methods using CD columns where the addition of alcohols could improve the separation efficiency of analytes. The addition of cosolvent can also lead to changes of the guest’s association rate constants. Addition of alkyl sulfates to the naphthalene/β-CD complex (Chart 2) led to a decrease in the association rate constant of the guest by a factor of 27 without altering the value for the dissociation rate constants. In contrast, in the case of pyrene bound to β-CD, the dissociation rate constant was decreased.45 The different effect of surfactant addition on the guest’s binding dynamics was explained by the fact that naphthalene is deeply included in the CD cavity and the presence of the surfactant hinders its entrance, whereas pyrene is bound closer to the rim and the surfactant bound to β-CD creates a more hydrophobic binding environment for the guest. In this respect, the pyrene case is akin to the xanthone binding discussed above. The singlet excited state of DBO (Chart 2) has a very long lifetime and fluorescence can be used to measure its binding dynamics with CDs.71 The special feature of this molecule is that the singlet excited state of DBO is deactivated by an “aborted” hydrogen transfer and its lifetime in water is longer than when bound to CDs. Once incorporated into the CD, the DBO singlet excited state is too short-lived for exit from the host to occur. Therefore, the quenching rate constant for DBO in water by CDs is a direct measure of the association rate constants of DBO with CDs. The k/+ values were 1.9 × 108, 4.0 × 108, and 0.8 × 108 M-1 s-1 for R-, β-, and γ-CD, respectively. These values parallel the KGeq values (50 ( 10 M-1 for R-CD, 1100 ( 300 M-1 for β-CD, and 6 ( 3 M-1 for γ-CD), where a higher association rate constant was observed for the more efficiently formed complex. However, the changes for the k/+ values with the size of the CD cavity were smaller than the differences in the equilibrium constant, indicating that a larger selectivity exists for the DBO exit rate constants (k/-,β < k/-,R < k/-,γ). Structure and Binding Dynamics. Structure-dynamics relationship studies provide mechanistic information that is relevant for the rational optimization of the function of supramolecular systems. A systematic approach in which the structures of the relevant components are changed is necessary to uncover the relevant parameters that influence the dynamics of a supramolecular system. The size of the guest is an important parameter for CD complexation. In general terms, if the guest is too small for the CD cavity the equilibrium constant is low, whereas for large guests, binding occurs to the rim of the CD and in some cases involves the binding of more than one CD molecule. Guests that fill the cavity have the largest equilibrium constants. Flavone, xanthone, and 2-NpOH (Charts 1 and 3) have similar ground-state equilibrium constants with β-CD (Tables 1 and 3), (71) Nau, W. M.; Zhang, X. J. Am. Chem. Soc. 1999, 121, 8022-8032.


Langmuir, Vol. 22, No. 22, 2006 9107

Table 3. Equilibrium Constants for the Ground (KG eq) and Triplet State (KTeq) of Flavone and 2-NpOH with β-CD and Association (k/+) and Dissociation (k/-) Rate Constants for the Triplet States Determined by the Quenching Methodolology55,56 guest

KGeq/ M-1

k/+ / 108 M-1 s-1

k/-/ 106 s-1

KTeq/ M-1

flavone 2-NpOH

1090 ( 80 1800 ( 200

24 ( 12 2.9 ( 1.6

4.4 ( 1.8 0.18 ( 0.07

550 ( 350 1600 ( 1100

a Triplet flavone was quenched by Cu2+ and triplet 2-NpOH was quenched by Mn2+.

Figure 6. Quenching plot with Cu2+ of the triplet excited states of xanthone (blue) and flavone (red) in water (circles) and in the presence of 10 mM β-CD (squares). The quenching plots in water are shown only for low quencher concentrations. Data from ref 55.

but the binding dynamics are very different as can be readily seen for the differences between the quenching plots for xanthone and flavone in water and in the presence of β-CD (Figure 6).51,55,56 In addition to the different dynamics, the decrease of KTeq when compared to the ground-state value was much more pronounced for xanthone than for flavone. Both these guests have higher dipole moments and basicity in their excited states55 when compared to their ground states and, therefore, the difference in the binding dynamics has to reflect differences in the structure of the complex. In the case of flavone, the pedant phenyl ring could be included into the CD cavity and could provide an anchor, which makes exit of the guest from the host more difficult.55 The decrease in the k/- value for flavone when compared to xanthone is then responsible for the smaller decrease in the equilibrium constant for the excited state when compared to the ground state. 2-NpOH is located inside the β-CD for the complex with 1:1 stoichiometry (Figure 7),58 and a much slower dynamics is observed than in the case of the ketones (see structure for the complex with xanthone in Figure 4), showing that the structure of the guest and its location inside the host are important factors in determining the binding dynamics. Modifications to the CD structures affect the binding dynamics of guests.52 The binding dynamics of xanthone was compared for modified β-CDs, where 2/3 of the hydroxyl groups were randomly methylated (CDMet) or one secondary hydroxyl group was substituted with a 2-ethanoic acid moiety (CDAcid) or was substituted with the ethyl ester of the 2-ethanoic acid moiety (CDEster). Methylation of the CD hydroxyl groups led to an increase in the equilibrium constant of xanthone by a factor of 2.8 and an increase of k/- for triplet xanthone. As a consequence, a larger difference for the values of KGeq and KTeq was observed than for the binding of xanthone to β-CD. The shorter residence time of triplet xanthone in CDMet was interpreted to be due to the larger difference in hydrophobicity between the rim of the

Figure 7. Calculated structures for the complexes between (R)2-NpOH and β-CD, showing the possible incorporation into the CD cavity of the ethanol (A) and naphthyl (B) moieties of (R)-2-NpOH. Figure adapted from ref 58.

cavity and water in the case of CDMet. A larger effect on the binding dynamics was observed for CDEster than for CDAcid. The value for k/+ increased and the value for k/- decreased for the binding of triplet xanthone with CDEster when compared to binding to the unsubstituted CD, indicating that the addition of a pendant arm that cannot fold backward into the CD cavity increases the probability of complex formation and provides an extended cavity for the guest. The less prominent effect observed for CDAcid showed that addition of a charge offsets some of the gains from adding a pendant arm. Higher Order CD Complexes. Complexes that include more than one CD molecule are called higher order complexes. 2-NpOH forms 1:1 and 2:2 complexes with β-CD. The 2:2 complex (Figure 8) is characterized by the excimer emission of the two 2-NpOH molecules in the complex, where the excited state of one guest interacts with the ground state of the second guest.56 Broadening of the 1H NMR peaks corresponding to the aromatic hydrogens of 2-NpOH in the presence of β-CD were observed, which are absent for guests, such as 1-NpOH, that do not form 2:2 complexes. A long lived triplet state for 2-NpOH was observed in the presence of β-CD. This triplet state was quenched 70 times less efficiently by nitrite anions than triplet 2-NpOH in the 1:1 complex and 700 times less efficiently than triplet 2-NpOH in water. This efficient protection is in line with the formation of a 2:2 complex. Analysis of the triplet quenching plot and the broadening of the NMR spectra led to an estimate of k/- for the 2:2 complex between 2 × 102 and 2.5 × 103 s-1. This estimate shows that dissociation from the 2:2 complex is much slower than guest dissociation from the 1:1 complex. Unfortunately, the laser flash photolysis system was not suitable to measure precise lifetimes in the millisecond time domain. A more detailed analysis of the dynamics for a higher order complex was feasible for pyrene complexation to γ-CD, because

9108 Langmuir, Vol. 22, No. 22, 2006

Figure 8. Calculated structure (one of three possible structures58) for the 2:2 complex between (R)-2-NpOH and β-CD, which leads to the excimer emission of 2-NpOH. Figure adapted from ref 58.

this dynamics could be studied using stopped-flow experiments.72 Excimer like emission is observed for the pyrene in the 2:2 complex.72,73 Our original interpretation of the binding isotherms measured by steady-state fluorescence was that 1:1, 1:2 and 2:2 (pyrene:γ-CD) complexes were formed,72 which agreed with a previous assignment.73 The formation of the 1:2 complex has been recently revised (see below). The dynamics of the 2:2 complex was shown to be unaffected by this revision. When pyrene and γ-CD are mixed in the stopped-flow experiment, the formation of the 1:1 complex occurs during the mixing time (ca. 1 ms), and the kinetics observed is that for the 2:2 complex formation from the association of two 1:1 complexes (see Scheme 1 above). The increase in excimer emission is concomitant with a decrease in the monomer emission, showing that changes in the pyrene monomeric species and formation of the 2:2 complex are coupled. The value for the association rate constant (k + 22) was determined to be (6 ( 1) × 107 M-1 s-1 and the value for k-22 was 73 ( 5 s-1. The ratio of rate constants led to an equilibrium constant of (0.9 ( 0.2) × 106 M-1, which compares well with the value recovered from binding isotherms (1.3 ( 0.5) × 106 M-1. The remarkable feature of these results is the extensive decrease (103-105 times) of the dissociation rate constant for the 2:2 complex when compared to the 1:1 complex, whereas the association rate constant is only decreased by a factor of 10. Therefore, a small change in complexity from 2 components to 4 components changed the time scale of the dynamics by 3 orders of magnitude. This result shows that even for simple supramolecular systems, such as cyclodextrin complexes, the dynamics can span several orders of magnitude. Temperature studies were performed to establish if the slower dynamics for the 2:2 pyrene:γ-CD complex was of an enthalpic or entropic nature. During these studies, we uncovered that the pyrene:γ-CD complexes present in solution, using the standard preparation methodology of stirring the solutions at room temperature for 12 h, were not at thermodynamic equilibrium.74 This conclusion was reached when the pyrene/CD solutions were annealed at 80 °C for 20 min (Figure 9). Increasing the temperature of pyrene/γ-CD aqueous solutions led to a decrease in the excimer emission and an increase in the monomer fluorescence intensity. (72) Dyck, A. S. M.; Kisiel, U.; Bohne, C. J. Phys. Chem. B 2003, 107, 1165211659. (73) Hamai, S. J. Phys. Chem. 1989, 93, 6527-6529. (74) Wright, P. J.; Bohne, C. Can. J. Chem. 2005, 83, 1440-1447.

Bohne

Figure 9. Pyrene fluorescence spectra in the presence of 20 mM γ-CD at 7 °C (black and red, a and d) and 47 °C (blue and green, b and c) before (black and blue, a and b) the temperature was raised to 60 °C. The sample was annealed at 80 °C for 20 min and then cooled (green and red, c and d). The inset shows the change in the I/III ratio for the heating and cooling cycle. The solid lines in the inset were included to guide the eye and do not have a physical meaning. Data from ref 74.

If the system was at thermodynamic equilibrium, cooling of the solution should recover the same spectra. However, the spectra are different. After the annealing process at high temperatures, the amount of excimer decreased significantly and a higher intensity was observed for the monomer emission, whereas at low temperatures, the excimer intensity did not change but the ratio of emission bands for the monomer emission was different. The sensitivity of the pyrene fluorescence to the solvent polarity is measured as the ratio of the I (373 nm) and III (383 nm) emission bands.75,76 The I/III ratio is high in water (1.75) and low in cyclohexane (0.58). During the annealing process, the I/III ratio was irreversibly increased showing that pyrene on average was located in a more hydrophilic environment after the heating cycle. The pyrene monomer emission is a composite of the emission for pyrene in water and for monomers in the CD complexes. The annealing effect and increase in the I/III ratio was attributed to the break up of γ-CD aggregates which included pyrene in very hydrophobic environments and were previously assigned to the formation of a 1:2 complex. Preliminary experiments for the 2:2 complex formation showed that the kinetics previously described is not significantly altered because of the presence of CD aggregates. However, the slow kinetics observed over tenths of seconds, which was different for the excimer and monomer emissions,72 is likely due to the presence of aggregates. This diversion into the annealing experiments showed that seemingly very well characterized systems, such as cyclodextrin complexes for which preparation techniques have converged to common methods, can uncover unusual phenomena. The formation of aggregates in solution had been previously reported in the literature,77-81 but its consequences on the characterization of CD complexes has yet to be fully explored. In the case of the pyrene/γ-CD, annealing the cyclodextrin samples (75) Dong, D. C.; Winnik, M. A. Photochem. Photobiol. 1982, 35, 17-21. (76) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 20392044. (77) Bonini, M.; S., R.; Karlsson, G.; Almgren, M.; Lo Nostro, P.; Baglioni, P. Langmuir 2006, 22, 1478-1484. (78) Coleman, A. W.; Nicolis, I.; Keller, N.; Dalbiez, J. P. J. Incl. Phenom. Mol. Recog. Chem. 1992, 13, 139-143. (79) Gaitano, G. G.; Brown, W.; Tardajos, G. J. Phys. Chem. B 1997, 101, 710-719. (80) Gonza´lez-Gaitano, G.; Rodrı´guez, P.; Isasi, J. R.; Fuentes, M.; Tardajos, G.; Sańchez, M. J. Incl. Phenom. Macroc. Chem. 2002, 44, 101-105. (81) Miyajima, K.; Sawada, M.; Nakagani, M. Bull. Chem. Soc. Jpn. 1983, 56, 3556-3560.


Langmuir, Vol. 22, No. 22, 2006 9109

Figure 11. Quenching plot with nitrite anion of the triplet excited states of 1-EtNp (red) and 1-NpOH (blue) in water (solid circles) and in the presence of 10 mM (open circles) and 40 mM (solid squares) of sodium cholate. The quenching plots in water and for 1-NpOH in the presence of 10 mM cholate are shown only for low quencher concentrations. Data from ref 91. Chart 4

Figure 10. Cartoon representation of the aggregate formed from sodium cholate monomers, where the blue areas correspond to the hydrophobic convex face of the monomer, the white faces correspond to the hydrophilic concave faces and the red circles represent the carboxylate moieties. The number of monomers in the primary aggregates is not known and the aggregate continuously increases in size as the cholate concentration is raised.

even in the absence of guest leads to the break up of the aggregates and simplifies the complexation dynamics (unpublished results).

Binding Dynamics of Guests to Bile Salt Aggregates Bile salts, such as sodium cholate, are amphiphilic molecules that aggregate in solution. The size of the aggregate continuously increases when the monomer concentration is raised. The molecular structure of bile salt aggregates is not known in detail. We adopt the primary-secondary aggregation model,82-87 where at low bile salt concentrations a small number of monomers forms primary aggregates with a hydrophobic interior and, as the concentration of bile salt monomers increases, the primary aggregates cluster to form larger structures called secondary aggregates (Figure 10). Bile salts were initially described as micelles because they efficiently solubilize hydrophobic molecules. Some specific interactions of the guest with the aggregate were inferred84,88 because the primary aggregates are formed from a much smaller number of monomers (e10)85 than conventional micelles, such (82) Hinze, W. L.; Hu, W.; Quina, F. H.; Mohammadzai, I. U., Bile Acid/Salt Surfactant Systems: General Properties and Survey of Analytical Applications. In Organized Assemblies in Chemical Analysis; Hinze, W. L., Ed.; JAI Press Inc.: Stamford, 2000; Vol. 2: Bile Acid/Salt Surfactant Systems, pp 1-70. (83) Kawamura, H.; Murata, Y.; Yamaguchi, T.; Igimi, H.; Tanaka, M.; Sugihara, G.; Kratohvil, J. P. J. Phys. Chem. 1989, 93, 3321-3326. (84) Li, G.; McGown, L. B. J. Phys. Chem. 1993, 97, 6745-6752. (85) O’Connor, C. J.; Wallace, R. G. AdV. Colloid Interface Sci. 1985, 22, 1-111. (86) Small, D. M. The Physical Chemistry of Cholanic Acids. In The Bile Salts; Nair, P. P., Kritchevsky, D., Eds.; Plenum Press: New York, 1971; Vol. 1, pp 249-256. (87) Small, D. M.; Penkett, S. A.; Chapman, D. Biochim. Biophys. Acta 1969, 176, 178-189. (88) Li, G.; McGown, L. B. J. Phys. Chem. 1994, 98, 13711-13719.

as sodium dodecyl sulfate (N ) 62).89 Our studies on the binding dynamics of guests with bile salt aggregates unequivocally showed that these aggregates have two binding sites with different properties90,91 and should be viewed as a more complex structure than conventional micelles. Triplet naphthalene was shown to be bound to a site which is very well protected from nitrite ion quenching, whereas triplet xanthone was located in a less protected site and its dissociation from the aggregate was faster than observed for naphthalene.90 The assignment made was that naphthalene was bound to the primary site, whereas xanthone was bound to the secondary site. However, based on the differences in the complexation dynamics of these two guests with CDs, it was necessary to compare guests with similar chromophores to ensure that two binding sites were present in bile salt aggregates. The binding dynamics of 1-EtNp and 1-NpOH (Charts 1 and 4) with cholate aggregates are strikingly different (Figure 11).91 The dynamics for 1-EtNp is much slower than the dynamics observed for 1-NpOH. A curved quenching plot is observed for 1-EtNp, but no significant decrease in the quenching efficiency is seen for 1-NpOH, at low cholate concentrations (10 mM), when only primary aggregates are present in solution. In the presence of secondary aggregates (40 mM), the curved quenching plot for 1-NpOH is much closer to the quenching plot for this guest in water than observed for 1-EtNp, indicating that the dynamics for 1-NpOH is faster when compared to the dynamics for 1-EtNp. Analysis of the quenching plots using eq 2 showed that the dissociation rate constant of 1-EtNp from cholate aggregates is more than 1 order of magnitude smaller than for 1-NpOH (Table 4). In addition, 1-EtNp in the presence of cholate aggregates is much better protected from quenchers in the aqueous phase than (89) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: Orlando, 1987; p 388. (90) Ju, C.; Bohne, C. J. Phys. Chem. 1996, 100, 3847-3854. (91) Rinco, O.; Nolet, M.-C.; Ovans, R.; Bohne, C. Photochem. Photobiol. Sci. 2003, 2, 1140-1151.

9110 Langmuir, Vol. 22, No. 22, 2006

Bohne

Table 4. Dissociation (k/-) Rate Constants for the Triplet States of Several Guests from Cholate Aggregates and Quenching Rate Constants by Nitrite Anions for the Guests in the Cholate 90,91,93 Aggregates (kH q ) and in Water (kq) guest 1-EtNp 1-NpOH naphthalene biphenyl

[cholate]/ mM

k/-/ 106 s-1

kHq / 10 M-1 s-1

kq/ 107 M-1 s-1

10 40 20-40 20-40 20-40

0.20 ( 0.04 0.14 ( 0.03 8(1 1.0 ( 0.4 1.5 ( 0.4

3.7 ( 0.5 2.4 ( 0.1 ca. 10 2.4 ( 0.4 6.9 ( 0.6

270 ( 20

7

340 ( 20 380 ( 20 550 ( 40

1-NpOH. The quenching rate constants for the triplet states of all guests in water are close to the diffusion controlled limit (6.5 × 109 M-1 s-1)53 indicating that the intrinsic rate constant for quenching after the encounter complex is formed is high. The quenching of the triplet states by nitrite anions occurs through energy transfer,92 and the polarity of the environment will not significantly affect the energy transfer rate constant. Therefore, the decrease observed for the quenching efficiency of the triplet guests in the presence of bile salt aggregates is due to the restricted access of the quencher (nitrite anion) to the guest in the aggregate, and this rate constant is a measure of the association rate constant of nitrite with the aggregate. In the presence of primary aggregates, only 1-EtNp binds to the cholate aggregate, whereas 1-NpOH is located in the aqueous phase. In the presence of secondary aggregates, 1-EtNp continues to be located in the primary aggregate because relocation of 1-EtNp to secondary sites would lead to an increase in kHq to a value similar to that observed for 1-NpOH. The small decrease observed for the k/- and kHq values for 1-EtNp in the presence of 40 mM cholate can be explained by the clustering of primary aggregates in the secondary structure where the exit of 1-EtNp from a primary aggregate located in the center of a secondary aggregate will have a higher probability of encountering another primary aggregate than a 1-EtNp released into the aqueous phase. The higher dissociation rate constant for 1-NpOH and higher triplet quenching rate constant showed that the secondary site is more accessible to ionic species and the guest binding dynamics to this site are faster. Therefore, the addition of a hydrophilic moiety to the guest, in this case a hydroxyl group, precludes the incorporation of the guest into the primary aggregate, suggesting that the guest has to be hydrophobic for binding to the primary site. Biphenyl is a hydrophobic molecule, which in the ground state is nonplanar but achieves planarity in the singlet and triplet excited states.94-96 Quenching experiments for the singlet and triplet excited states of biphenyl in the presence of cholate aggregates established that this guest is bound to the primary site.93 The value for kHq was higher for biphenyl when compared to the quenching rate constant for naphthalene and 1-EtNp, which are also bound to the primary site. The same trend was observed for fluorescence quenching experiments. The value of kHq is a measure of the association rate constant of the quencher to the aggregate containing the excited-state guest. The kHq values are ca. 1 order of magnitude smaller than the association rate constants of the guests,90,91,93 which is due to the electrostatic repulsion of the negative quencher anion and the negatively charged cholate aggregates. If the structure of the binding site does not change (92) Treinin, A.; Hayon, E. J. Am. Chem. Soc. 1976, 98, 3884-3891. (93) Waissbluth L. O.; Morales C. M.; Bohne, C. Photochem. Photobiol. 2006, accepted for publication (http://phot.allenpress.com/photonline/?request)get-tocaop, March 23, 2006). (94) Berlman, I. B. J. Chem. Phys. 1970, 52, 5616-5621. (95) Lim, E. C.; Li, Y. H. J. Chem. Phys. 1970, 52, 6416-6422. (96) Mispelter, J. Chem. Phys. Lett. 1971, 10, 539-542.

for the incorporation of different guests, one would expect the same kHq values for all guests. The larger value observed for biphenyl indicates that the primary site in the cholate aggregates has a different structure from that for the primary site containing naphthalene and 1-EtNp. This result suggests that the primary aggregate around biphenyl is more flexible facilitating the access of the nitrite anions, or the aggregate containing biphenyl is larger leading to a smaller negative charge density and therefore to less repulsion of the negative nitrite anion by the aggregate. In contrast to the trend observed for the quenching rate constant in the presence of cholate aggregates, the dissociation rate constant for triplet biphenyl is similar to the rate constant measured for naphthalene, both of which are 5-10 times larger than measured for 1-EtNp. This result shows that the effect of changes to the structure of the guest is more prominent for the dissociation process of the guest than for the accessibility of the ionic quencher. The different trends observed for kHq and k/- indicate that the mechanism for nitrite anion entry into the cholate aggregate is different from the mechanism for exit of guests. The latter could be either due to the guest’s exit from an intact aggregate or due to the break up of the aggregate. The latter possibility has to be considered because primary aggregates are formed from a small number of monomers, and the dissociation of one or two monomers could expose the binding site to the aqueous phase and make release of the guest possible. A distinction between these two mechanisms is not possible at this point.

Conclusions The understanding of the dynamics of supramolecular systems lags behind the knowledge on how to synthesize complex and elegant structures. This state of affairs is a reflection of the time scale for which dynamics occur. Understanding and rationally manipulating the dynamics of supramolecular systems will be necessary to fulfill the promise of achieving chemistry not accessible to molecular structures. This feature article provided examples in the case of cyclodextrin host-guest complexes, which showed that the knowledge of an equilibrium constant, i.e., the thermodynamics of a system, does not provide any information on the dynamics for complexation. For very similar equilibrium constants, the rate constants for guest complexation can vary over a factor of a million. Even trends in changes for equilibrium constants are not useful to predict the dynamics. Opposite effects were observed for the changes in the hostguest equilibrium constants after alcohols were added to β- and γ-CD complexes, but in both cases, the residence time of the guest in the CD cavity was increased in the presence of cosolvent. The second feature from the CD studies that points to the importance of dynamic studies is the fact that a modest change in the complexity of the system can move the dynamics to very different time scales. While the residence time for guests that fit within the cavity of CDs and form 1:1 complexes is of hundreds of nanoseconds to tenths of microseconds, the dynamics of a guest in a 2:2 complex occurs in the millisecond time domain. This fairly simple example shows that in order to characterize systems with the complexity currently being synthesized it will be necessary to develop methodology that covers the whole time domain. Unfortunately, there are currently only few techniques that span more than 3 or 4 orders of magnitude. The simultaneous presence of two binding sites with different properties in bile salt aggregates makes them more complex structures than host-guest systems and conventional micelles. The dual sites were not noticed in the many studies centered on the thermodynamics of guest complexation and dynamic studies were essential to uncover this feature. The presence of a fairly


hydrophobic site that selectively binds guests and a hydrophilic site inside a water soluble aggregate can be, in principle, employed as a microreactor with dual binding chambers. Finally, changes in the guest structure showed that the primary aggregate is adaptable to the structure of a hydrophobic guest and therefore this host system is more flexible in its formation of complexes than macrocycles, such as cyclodextrins and calixarenes. The challenges for the future in supramolecular dynamics lie in developing methodology to study binding of guests to multiple binding sites with different properties, such as those in DNA and proteins and to expand these types of studies from systems in solution to systems on surfaces. The latter is critical because many of the devices in nanostructures will be built on surfaces. In addition, methodologies will have to be developed for studies on time scales that vary by factors of 109 to 1012 s, where ideally the same methodology covers the widest time range possible. It will also be important to develop methodologies that do not employ triplet excited states because, in some cases, these excited states can only be rendered nonreactive with the host by decreasing their triplet energy to such an extent that the quenching (97) Pace, T. C. S.; Monahan, S. L.; MacRae, A. I.; Kaila, M.; Bohne, C. Photochem. Photobiol. 2006, 82, 78-87.

Langmuir, Vol. 22, No. 22, 2006 9111

methodology cannot be applied. This situation was recently described for the binding dynamics of xanthone derivatives with DNA.97 It is evident from the simple systems described in this article that the acquisition of dynamic information requires the development of appropriate methodology followed by a systematic study in which the structure of components are changed. While the synthetic community is able to provide us with colored and 3-D pictures of elegant structures, dynamic studies can now provide black and white movies of relatively low resolution, but the level of sophistication will catch up soon. Acknowledgment. I would like to thank my co-workers at the University of Victoria and my collaborators at other institutions for their contributions in the development of my research program in supramolecular dynamics. Their enthusiasm and hard work make this research possible. I would like to thank the Natural Sciences and Engineering Research Council of Canada for the financial support in the form of discovery (operating) and equipment grants. The donors of the Petroleum Research Fund, administered by the American Chemical Society, are thanked for the partial funding of the research on bile salt aggregates. LA061227W

Supramolecular Dynamics Studied Using Photophysics - Langmuir

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