17848
J. Phys. Chem. B 2005, 109, 17848-17854
Effect of Nanocavity Confinement on the Relaxation of Anesthetic Analogues: Relevance to Encapsulated Drug Photochemistry Laura Tormo, Juan Angel Organero, and Abderrazzak Douhal* Departamento de Quı´mica Fı´sica, Seccio´ n de Quı´micas, Facultad de Ciencias del Medio Ambiente, UniVersidad de Castilla-La Mancha, AVda. Carlos III, S.N., 45071, Toledo, Spain ReceiVed: March 10, 2005; In Final Form: July 14, 2005
We report on UV-vis absorption and picosecond emission studies of methyl 2-amino-4,5-dimethoxy benzoate in neutral water and complexed to R-, β-, and γ-cyclodextrin (CD). Upon encapsulation, the emission intensity and the fluorescence lifetime increase, indicating a hydrophobic effect of the nanocages on the photophysical behavior of the guest. β-CD confinement shows the largest effect. The time-dependent frequency shift of the emission (∼720 cm-1) in β-CD nanocavity is larger than the one observed in water (∼490 cm-1) due to the hydrophobic and polarity effect of the nanocage and reflects a strengthening of the intramolecular H-bond of the encapsulated dye upon electronic excitation. Anisotropy measurements indicate a free motion of the guest into the nanocavity. The observed results are relevant to the hydrophobic as well as hydrophilic interactions which govern photochemistry and photophysics of caged drugs, organic, and biological systems.
1. Introduction Cyclodextrin (CD) nanocavities have been and are being used in a wide field of science and technology for inclusion complexes of organic and inorganic molecules.1-5 For example, for drug and medicine storage and delivery, it is generally accepted that CDs can constitute an efficient photoprotective agent regarding drug/medicine photoinduced damages on biological targets.1-3,6-10 A number of factors influence drugCD complexation. Among them, “the goodness of fit” between host and guest and the hydrophobic effects are probably the most important. The weak binding forces responsible for association to the constrained microenvironment of CD provide a useful model to mimic the interactions of drugs with hydrophobic pockets of biological substrates. The nature of the relaxation of the excited guest molecules, the efficiency of its deactivation pathways, the fate of the reaction intermediates, and the opening of new photoreactive channels are some of the parameters that may be controlled by the nanocavities. Recently, we have used CDs to study proton-transfer reactions and twisting motion in confined systems and have shown the role of twisting on the photodynamics of these systems in solution and in chemical and biological nanocages.5,11-16 However, not much effort has been directed toward understanding the fast and ultrafast dynamics of drugs in these nanocavities. Here we report on the fast relaxation of a caged molecule, methyl 2-amino-4,5-dimethoxy benzoate (ADMB, Chart 1), an anesthetic analogous to procaine and tetracaine in order to shine some light on the role played by the R-, β-, and γ-cyclodextrin (R-, β-, and γ-CD) confinement effect on the photorelaxation of caged drugs. The results show a large increase of emission quantum yield and lifetime upon encapsulation by β-CD. Within this cavity, the guest moves almost freely. These results are explained in terms of the hydrophobic effect of the host on the * To whom correspondence should be addressed. Fax: + 34-925-268840. E-mail:
[email protected].
CHART 1. Schematic Representation of the Conformers I and II of ADMB in Water and of Their 1:1 Inclusion Complexes with β-CD in Water. For Clarity of the Structures, the Intramolecular H-bonds between the Amino and the Ester (ether or carbonyl atom) Groups Are Not Drawn.
guest, where ultrafast relaxation due to H-bonding interactions with water molecules is prevented for the deeply encapsulated dye. 2. Experimental Section ADMB was synthesized by a simple esterification procedure of its acid derivative (Sigma-Aldrich). The purity of the sample was checked by 1H NMR, IR, elemental analysis, and thin-layer chromatography. In addition to that, emission data in noninteracting solvents, like methyl cyclohexane, show only one structure at both ground and electronically first excited states
10.1021/jp0512457 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/31/2005
Nanocavity Confinement Effect on Anesthetic Analogues
J. Phys. Chem. B, Vol. 109, No. 38, 2005 17849
and were used as a proof of its purity. CDs (Acros) were used as received. Steady-state absorption and emission spectra were recorded on Varian (Cary E1) and Perkin-Elmer LS 50B spectrophotometers, respectively. Emission lifetimes were measured by using a time-correlated single-photon-counting picosecond spectrophotometer (FluoTime 200, PicoQuant). The sample was excited by a 40 ps pulsed (20 MHz) laser centered at 371 or at 393 nm (PicoQuant), and the emission signal was collected at the magic angle. The pulse width measured by the apparatus was typically 65 ps. The emission decay data were convoluted with the cross-correlation signal and fitted to a multiexponential function using the Fluofit package (Picoquant). The time-dependent anisotropy was constructed using the expression r(t) ) (I| - G⊥)/(I| + 2G⊥), where G is the ratio between the fluorescence intensity at parallel (I|) and perpendicular (I⊥) polarizations of the emission with respect to the excitation beam. The value of G was measured at a gating time of 2-3 ns, at which the fluorescence is almost completely depolarized (tail matching technique). The quality of the fits was characterized in terms of residual distribution and reduced χ2 value. The magic-angle time-resolved emission spectra were recorded at different wavelengths of observation and were constructed using the Fluofit package. Details on the apparatus and procedure of data analysis were described.15 All the measurements were done at an ADMB concentration of 5 × 10-5 M and at 293 ( 1 K. 3. Results and Discussion 3.1. Steady-State Observation and Nature of the Complex. Previously,17 we have shortly reported on picosecond emission dynamics of ADMB in solution and shown that in organic solvents, the NsH‚‚‚OdC intramolecular H-bond in the ground state (maximum of the first UV-vis transition between 335 and 345 nm) becomes stronger upon electronic excitation leading to an emission band whose peak positions (398-440 nm) depend on the polarity and H-bonding ability of the solvent. Figure 1A shows UV-vis absorption spectra of 5 × 10-5 M ADMB in neutral water and upon addition of β-CD. Addition of CD induces negligible changes; any change in the absorption spectrum suggests no interaction of CD with the dye or no change in the molar absorption coefficient of the interacting species. In both media, the absorption bands at 260 nm (S0 f S2) and at 332 nm (S0 f S1) are due to (π,π*) transitions. Figure 1B shows the emission spectra of the same solutions upon excitation at 340 nm and reflects the interaction between ADMB and β-CD. In the absence of CD, the spectrum contains a single emission band centered at 440 nm, and the quantum yield is 5 × 10-3. The Stokes shift (energy difference between the maxima of absorption and emission intensity bands) is large, about 7400 cm-1, and is explained in terms of an intramolecular charge-transfer (ICT) reaction from the amino group to the phenyl part.17 In fact, solvent polarity effect on the Stokes shift suggests an increase of the dye dipole moment upon electronic excitation.17 For anthranilic acid (AA) in the gas phase, a molecule having similar groups involved in the intramolecular H-bond of ADMB, theoretical calculations predict at the S1 state a shortening of the IHB between the N-H and CO groups. In addition to that, electronic and IR spectroscopy of AA in a supersonic jet, together with DFT calculations, show a strengthening of the IHB without the occurrence of proton or hydrogen atom transfer and in-plane bending motions of the amino and carboxylic groups which change their relative positions to that in the ground state.18 Therefore, for ADMB in water, the observed emission band does not result from a tautomer formed
Figure 1. UV-vis absorption (A) and emission (B) spectra of ADMB in the presence of 0-15 mM β-CD and upon excitation at 340 nm. (C) Excitation spectra observed at 440 nm of ADMB in water (solid line) and in the presence of 15 mM β-CD (dashed line).
17850 J. Phys. Chem. B, Vol. 109, No. 38, 2005
Tormo et al.
Figure 2. Variation of the relative emission intensity at 440 nm of ADMB with the initial β-CD concentration. I0 and I are the emission intensities without and with β-CD, respectively. The solid curve is the result of the fit using eq 1 which assumes a 1:1 stoichiometry association complex between M and β-CD. The deduced equilibrium constant is 12 ( 3 M-1 at 295 K.
by an intramolecular proton-transfer reaction at S1. Recently, time-dependent DFT calculations on AA at S1 suggest a shortening of the IHB (by 0.322 Å) but the absence of proton motion in the gas phase.19 Results of theoretical calculations (B3LYP-631+G*) on H-bonded complexes of ADMB with water at S0 suggest several types of complexes where the specific interaction through the amino group is the most stabilizing one and in which this group acts as an acceptor and a water molecule as a donor.17 In this complex, charge redistribution in the phenyl part has occurred. The N-C2 bond distance between the nitrogen atom and the phenyl part increases by 0.077 Å. In addition to that, the value of the dipole moment changes from 1.21 to 2.95 D upon water complexation. Addition of β-CD induces a clear increase of the emission intensity without any change in the spectral position. Figure 2 shows the variation of the relative emission intensity at 440 nm with the initial concentration of β-CD. To get information on the stoichiometry of the formed complexes between the dye and CD, we analyzed the observed change using models assuming 1:1 and 1:2 stoichiometries (dye/CD). A BenesiHildebrand plot (not shown) gives a straight line suggesting the formation of a 1:1 inclusion complex between ADMB and β-CD.20 From these data it is not possible to distinguish between structures involving I and II in the complexes (Chart 1) as both are of 1:1 stoichiometry and emit at the same spectral region. The fluorescence lifetime will shine some light. The experimental data were fitted using eq 1 assuming a 1:1 stoichiometry21
I/I0 ) (1 + KG(φ/φ0) [CD])/(1 + K[CD])
(1)
where I and I0 are the emission intensities with and without CD, respectively. G is the ratio of the molar absorption coefficients (complex/free) of the dye upon interaction with CD, and φ0 and φ are the emission quantum yields of the free and complexed dye, respectively. [CD] was replaced by [CD]0 in
eq 1 because the values of the used concentrations of β-CD (2-15 mM) are larger than that of ADMB (5 × 10-5 M). Furthermore, as the absorption spectra suggest no change in the molar absorption coefficient of the dye upon complexation with β-CD, we fixed G ) 1 in the fit. Using the mean value of the emission lifetime (2.9 ns, vide infra) of the complexes and a radiative lifetime of 10 ns (comparable to that in dimethyl sulfoxide, for which the quantum yield is 0.98), we obtained the emission quantum yield of the complex, φ ) 0.29. Fixing this value in the fit together with the experimental value of φ0 ) 5 × 10-3, the best fit to eq 1 gives K ) 12 ( 3 M-1 and R2 ) 0.994. When G and φ were free in the fit, we obtained K ) 30 M-1, φ ) 0.20, G ) 0.68, and R2 ) 0.999. Because the values of φ and G of this fit are very different from the expected ones using the absorption and emission lifetime experiments, we suggest using the value of K deduced from the first fitting procedure, K ) 12 M-1, in which the fixed values should be very close to the real ones. Therefore, from the deduced inclusion equilibrium constant (12 M-1) at 293 K, at 15 mM β-CD, and 5 × 10-5 M ADMB, about 15% of the dye is complexed and the estimated fluorescence quantum yield of the complexes, φ ) 0.33. This value, close to the calculated one (0.29), is larger than that measured in pure water (5 × 10-3). This indicates that some of the radiationless processes acting in pure water are not present or less efficient in the encapsulated dye. Compared with the results observed in alcohols (φ ) 0.1 in methanol and 0.6 in 1-decanol) and in non-hydroxylic apolar and polar solvents (φ ) 0.6-0.9),17 we anticipate that the main nonradiative channel of excited ADMB involves two processes: (1) intermolecular H-bonding interactions with the solvent when this one acts as a H-bond donating partner and (2) twisting motion of the amino and ester groups. For methanol, we obtained fluorescence lifetimes of 0.9 ns (70%) and 2.2 ns (30%), and for 1-decanol, a 20 times more viscous solvent, we obtained 1.8 ns (10%) and 6.3 ns (90%). In water, H-bonds with the amino group will affect the photophysics of ADMB. Hydrophobicity and confinement of the β-CD interior reduces the efficiency of these channels. For the confined geometries, we suggest that in some structures, the amino and ester groups are still exposed to water found at the gate of CD (Chart 1). In fact, the excitation spectrum gated at 410-480 nm (Figure 1C), is similar in shape to that found in water and only shows a small, blue shift of 4 nm upon encapsulation. 3.2. Picosecond Time-Resolved Emission Decays and Theoretical Calculations. To get information on the time scale of the emission lifetime of the free and complexed dye, we recorded the emission decay at different wavelengths of observation. Figure 3 shows the emission decay in water and in the presence of different β-CD concentrations upon excitation at 371 nm and observation at 450 nm. The inset shows the decay at a larger window of observation in the presence of 15 mM β-CD. Table 1 gives the obtained data of the fits. To begin with water solution, the decay could be fitted using a biexponential function giving time constants of 47 ( 10 ps (93%) and 110 ( 15 ps (7%). The contribution (preexponential factors) of these components does not show a significant change with the emission wavelength (Table 1). Comparable values were obtained under 393 nm excitation wavelength. The first picosecond time is assigned to the conformer II type while the second one is assigned to the I type (Chart 1). This is based on the following discussion. B3LYP (6-31+G**) theoretical calculations suggest two structures of ADMB in which the intramolecular H-bonds are
Nanocavity Confinement Effect on Anesthetic Analogues
J. Phys. Chem. B, Vol. 109, No. 38, 2005 17851 TABLE 2: Values of the Emission Lifetimes and Normalized (to 1) Pre-exponential Factors from the Multiexponential Fit of the Fluorescence Decays of 5 × 10-5 MADMB at Different Wavelengths (as indicated) and in the Absence and Presence of 15 mM β-CD λem/ [β-CD]/ nm mM τ1/ps 410 450 500
Figure 3. Emission decays (magic angle) of ADMB in neutral water (b) and in the presence of β-CD upon ps excitation (the IRF signal, ∼65 ps, is indicated) at 371 nm and observation at 450 nm. The inset shows the used CD concentrations and a larger window of observation for the β-CD 15 mM solution. The continuous curves are the fit of the experimental data. The data of the multiexponential fit are shown in Tables 1 and 2. The bottom shows the distribution of the residuals for water and 15 mM CD solutions.
TABLE 1: Values of the Emission Lifetimes and Normalized (to 1) Pre-exponential Factors from the Multiexponential Fit of 450 nm Emission of ADMB 5 × 10-5 M in Absence and Presence of Different β-CD Concentrationsa [β-CD]/mM τ1/ps 0 2 6 10 15
47 49 45 43 40
a1
τ2/ps
a2
τ3/ns
a3
τ4/ps
a4
χ2
0.93 0.83 0.80 0.72 0.67
110 110* 110* 110* 110*
0.07 0.09 0.15 0.20 0.22
0.9 0.8 0.9 0.8
0.01 0.01 0.02 0.03
4.0 3.7 3.8 3.7
0.01 0.04 0.06 0.08
1.05 1.02 1.01 1.03 1.10
a The asterisk at 110 ps time indicates a fixed value in the fit (see text).
of amino‚‚‚carbonyl (conformer I) and amino-ester (conformer II) types.23 In the gas phase, conformer I is more stable than II by 2.68 kcal/mol, and their dipole moments are 1.17 and 3.51 D, respectively. In water, the dye may coexist under two conformers having intermolecular H-bonds with the surrounding water molecules. Using the isodensity surface-polarized continuum model IPCM24 for water (to define the cavity water, we used an electronic density of 0.0005 au and a water dielectric constant of 78), the energy gap between I and II is reduced to 1.84 kcal/mol. The values of the dipole moments are 1.53 and 4.89 D, respectively. As both conformers will form intermolecular H-bonds with the solvent molecules, and taking into
0 15 0 15 0 15
46 41 47 40 50 46
a1
τ2/ps
a2
0.93 0.67 0.93 0.67 0.95 0.71
101 100* 110 110* 135 130*
0.07 0.23 0.07 0.22 0.05 0.16
τ3/ns 0.6 0.8 1.1
a3
τ4/ps
a4
χ2
1.05 0.04 3.4 0.03 0.98 1.05 0.03 3.7 0.08 1.10 1.17 0.03 3.9 0.10 0.97
account the small energy gap and the largest dipole moment of II, we suggest that in water solution, II will be more stabilized than I. Previously, we have shown that intermolecular H-bonds of this kind of molecules make the conformer II type more stable than the conformer I type by about 7.2 kcal/mol.17 Because the intramolecular H-bond in II (d(amino‚‚‚carbonyl) ) 1.933 Å) is weaker than in I (d(amino‚‚‚carbonyl) ) 1.972 Å), nonradiative rates due to a twisting motion (as the viscosity effect shows) are expected to be faster in II than in I. Thus, we suggest that the (shortest) 47 ps component is due to II, while the (longest) 115 ps one is due to I. On the basis of this assignment, the largest contribution to the emission decay is due to the conformer II type. In the presence of CD, the decays become longer and the fitting procedure took into consideration the presence of free dye molecules in water (Figure 3). At the used concentrations of β-CD (2-15 mM) and of ADMB (5 × 10-5 M), the value of K (12 M-1) suggests that a small (2-15%) fraction of the dye is complexed to β-CD. Thus, when fitting the decays we have taken into account the components due to noncomplexed molecules. For example, by gating the emission of the 15 mM β-CD solution at 450 nm a fourexponential fit gives 40 ps (67%), 110 ps (22%), 0.8 ns (3%), and 3.7 ns (8%). Tables 1 and 2 contain the data for observed wavelengths and concentration dependence, respectively. The first picosecond components are then assigned to the free dye (conformers II and I, respectively) in water, while the 0.8 and 3.7 ns contributions are due to the confined complexes. Note that the value of the preexponential factors of the longest components (11% in total) is close to the calculated one from the steady-state emission spectrum and equilibrium constant (15%). We observed a similar trend for the other three used in the β-CD concentrations (Table 2). The values of the preexponential factors of the complexes emission components increase with CD concentration. At the same time that of the 40-50 ps component decreases while that of 110 ps one relatively increases. This last behavior might be the result of complexing conformer II rather than conformer I, and therefore the preexponential factor for the component of I becomes larger than that of II upon addition of CD. Another possibility is the photoformation of conformer I upon decomplexation of the inclusion complex at the excited state. However (vide infra), if such a dissociation occurs, the emission decays should also show a rising component which we did not observe in the experiment. Dissociation rate constants of CD-inclusion complexes of aromatic molecules at S0 or at T1 states ranged from 10-2 to 106 s-1.25-28 This suggests that with the emission lifetime (ns regime) of the complexes, no decomplexation occurs. From the point of view of theory, molecular dynamics calculations on complexes of phenol and benzoic derivatives with R-CD suggest a possible moving in and out of the guest in a 10-100 ps time scale.29 However, we notice also that for smaller guests (aliphatic alcohols and leucine for example) of β-CD, ground-state
17852 J. Phys. Chem. B, Vol. 109, No. 38, 2005 dissociation rate constants of 108 s-1 were experimentally observed.30,31 Now, let us discuss the values of the lifetime in nanosecond components (0.6-1.1 and 3.4-3.9 ns) assigned to the complexes emission decay. First of all, the emission lifetimes of the complexes are longer than those of the free dye in agreement with the effect of CD confinement on the emission intensity (Figure 1). Lifetime values of 6-9 ns were observed in non-hydroxylic media.17 In addition to that, while the contribution of the 0.6-1.1 ns component almost does not change with the emission wavelength, that of the 3.4-3.9 ns one increases at longer wavelengths of observation and with β-CD concentration as said above (Table 1). The observation of two lifetimes for the complexes is explained in terms of different heterogeneous environments at the gate of CD leading to different interactions of water molecules with the amino and ester groups of the guest as said above. The longest time (3.7 ( 0.5 ns) is comparable to those observed (6-9 ns) in non-hydroxylic solvents, and therefore the corresponding fluorophore should be more embedded into β-CD, rather than the one having 0.8 ( 0.3 ns. This latter is more exposed to water interactions or may have a conformation like that of II/β-CD in which the twisting of the ester group is more favorable than in I/β-CD (Chart 1). The longest lifetimes are 1.5 ns (7%) and 7 ns (15%) using heptakis-(2,6-di-O-methyl)-β-CD for which the gates are more hydrophobic than those of β-CD. Within the time resolution of the used apparatus (65 ps, for instrumental response function, and 10 ps, a limiting value for a decay after deconvolution procedure), we did not observe any rising component, as said above. As discussed above, we suggest that the complexes at the excited state do not undergo any dynamics by moving the guest out from the CD cavity. This conclusion is also supported by the contribution (Table 1) of the complexes emission in the decay upon changing CD concentration, which is comparable to the fraction of the complex calculated from the steady-state emission changes (2, 6, 12, and 15% for [β-CD] ) 2, 6, 10, and 15 mM, respectively). 3.3. Picosecond Time-Resolved Emission Spectra of the Complex. Figure 4 shows picosecond time-resolved emission spectra (TRES) of ADMB in water in the presence of 15 mM β-CD and upon excitation at 371 nm. The bottom of the figure is a projection of the emission intensity change with time and wavelength of observation, while the inset shows the spectral position changes for a few gating times. When time proceeds, the maximum shifts to longer wavelengths by almost 720 cm-1 when the gating time is 7 ns. For a water solution (not shown), the complete spectral shift (in 300 ps) is smaller (about 485 cm-1). From the spectral relaxation difference, it is clear that the complexes emit at the red side of the spectrum. For AA, the line (of the IR spectrum in the gas phase) corresponding to the amino-carbonyl IHB shifts from 3514 cm-1 in S0 to 3019 cm-1 in S1.18 Excited AA in the gas phase shows a shortening of the IHB. We expect a similar process in ADMB in the hydrophobic interior of CD leading to a molecular relaxation (frequency shift in the emission spectrum) at the excited state. From the TRES, ADMB in β-CD shows a shift of 720 cm-1, which is larger than that in water (485 cm-1). Thus, the shift indicates a confinement hydrophobic effect of CD on the molecular relaxation of the guest. Because of the heterogeneity of the system in the presence of CD, it is not easy to extract the solvation times using the solvation function inside the cavity. It is worth noting that ultrafast water solvation occurs in less than 1 ps,32,33 a very short time not possible to measure by the present apparatus. Femtosecond experiments (in progress) will elucidate this point. Solvation times in the sub-
Tormo et al.
Figure 4. Time-resolved emission spectra (TRES) of ADMB in water solution containing 15 mM β-CD. The excitation wavelength was 371 nm. The inset shows some TRES at different times of observation and the confined structure of the complex.
TABLE 3: Values of the Emission Lifetimes and Normalized (to 1) Pre-exponential Factors from the Multiexponential Fit of the Fluorescence Decays of 5 × 10-5 M ADMB at 450 nm in the Absence and Presence of 15 mM CDs CD
τ1/ps
a1
τ2/ps
a2
τ3/ps
a3
τ4/ps
a4
χ2
0 R-CD β-CD γ-CD
47 47 40 49
0.93 0.75 0.67 0.71
110 110 110* 110*
0.07 0.18 0.22 0.17
0.4 0.8 0.4
0.02 0.03 0.09
1.8 3.7 1.5
0.04 0.08 0.03
1.05 0.98 1.10 1.02
nanosecond and nanosecond regimes were reported using restricting media (CD, micelles, Sol-gel, and proteins).4,5,32-36 For example, 4-aminophthalimide in a β-CD water solution, solvation dynamics of N,N,dimethyl formamide (DMF) with the cavity is found to occur in 400 ps (25%) and 8 ns (75%).34 These times are largely longer than the solvation time (about 1 ps) in bulk solvent. The bimodal solvation dynamics due to free and bound water molecules in CD complexes and organized media has been discussed in several reviews.4,5,32 The slow component is controlled by the rotational motion of water molecules found at the gates of CD. Confined water molecules in such microenvironments exhibit a larger degree of spatial and orientational order than in the bulk phase. Therefore, the freezing of translational motions by restriction of rotational processes of water at/or inside the CD gates makes its solvation dynamics slower. 3.4. Cavity Size Effect on the Complex Photorelaxation. We have also studied the cavity size effect using R- and γ-CD (having larger diameters of 5.7 and 9.5 Å, respectively). Figure 5 shows the emission spectra and decays at 450 nm of ADMB in water and in the presence of 15 mM of each CD. Table 3 gives the data from the multiexponential fits. For R- and γ-CD cavities, the effect on the emission intensity is weak and the lifetimes of the complexes are shorter (0.4 and 1.7 ns) than those
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J. Phys. Chem. B, Vol. 109, No. 38, 2005 17853
Figure 6. Anisotropy (r(t)) decay of ADMB in water (•••) and in the presence of 15 mM β-CD (ooo) upon excitation at 371 nm and observation at 460 nm. The solid curves are the result of the fit: 97 ps in water, 53 ps (40%), and 510 ps (60%) in the presence of 15 mM β-CD.{/PICK;crtI;84480n;;block;;;;yes;4;;4224n;;;;}
Figure 5. (A) Emission spectra of ADMB in neutral water and in the presence of 15 mM R-, β-, and γ-CD, upon excitation at 340 nm. (B) Magic-angle emission decays of the same solution of (A) gated at 450 nm and upon excitation at 371 nm. The data of the multiexponential fits are shown in Table 3.
of the β-CD complexes. One possible explanation is that the confinement effect is weaker because of the small cavity size for R-CD and the too large cavity size for γ-CD. Within the former, the guest is not deeply embedded into the cage, and therefore the amino and ester groups should be more exposed to water molecules; while for the latter because of a larger available space, a twisting motion may easily happen leading to faster decays of the complexes. 3.5. Confinement Effect on the Anisotropy Decay. To get information on the rotational times (φ) of the complex, we performed time-resolved anisotropy (r(t)) measurements at different wavelengths of observation and excitation at 371 and 393 nm. Figure 6 shows typical r(t) decays in water and in the presence of 15 mM β-CD upon excitation at 371 nm and gating at 460 nm. For the dye in pure water, the decay fits to a singleexponential function giving a rotational time of φ ) 97 ( 15 ps (82 ps when exciting at 393 nm). Rotational times of 50-130 ps have been found for comparable (in size) aromatic molecules.5,15,16,37 Modeling ADMB as a prolate ellipsoid and
nonhydrated rotor and using the hydrodynamics theory,38,39 one gets orientation relaxation times of 45 and 25 ps under stickand slip-boundary condition limits, respectively. The experimental value is close to the expected one under stick conditions suggesting an attachment of the solvation shell (water molecules) to the dye. This agrees with the existence of strong H-bonding interactions between ADMB and water molecules affecting its relaxation to the ground state. For ADMB/β-CD solution excited at 371 nm and observed at 460 nm, the fit of r(t) decay gives two times: φ1 ) 53 ( 20 ps (40%) and φ2 ) 510 ( 50 ps (60%). The values of the times do not change (within the errors of experiment and fitting) when excited at 393 nm and observed at 480 nm, while the contribution of the φ1 component is 56% and that of the φ2 one is 44%. On the basis of these data, we attribute φ1 to the rotational relaxation time of the caged dye. This value is not different from 55 ps (single decay) observed in tetrahydrofurane. The caged drug analogous in the nanocavity is almost free to rotate within the cavity, indicating a weak docking of the guest. However, because 85% of the dye is not complexed, this component should contain a contribution from the 97 ps rotational time in pure water. The second relaxation time, φ2, is an overall rotational time of the confined complex. In fact, by modeling β-CD as a prolate ellipsoid, the stickcondition limit gives 520 ps as a rotational time, comparable to the observed value (510 ps), while the slip condition gives 340 ps. For γ-CD solution, we obtained rotational times of 56 ps (49%) and 870 ps (51%). While the shorter time is similar to that found in β-CD and in THF suggesting an almost free rotation of the caged dye, that of a 0.9 ns time is assigned to a global motion of the nanostructure. As expected, this time is longer for a larger volume of CD. Finally, the initial value of r(t) for the complexes is ∼0.30, not very different from the ideal one (0.4), indicating that the emission transition moment of caged ADMB has rotated by about 23° to that of its absorption, while for the water solution this angle is slightly larger (28°), and it may reflect the fast and ultrafast solvation dynamics acting in water.
17854 J. Phys. Chem. B, Vol. 109, No. 38, 2005 4. Conclusion The present work shows a clear effect of CD confinement on the photophysical behavior of a drug analogue. Compared to water, into the β-CD nanocage the emission quantum yield increases by a factor of 60 while the lifetime changes from the picosecond regime to the nanosecond one. Therefore, encapsulating a drug by CD makes its excited-state lifetime longer. The multiexponential behavior of the emission decays is explained in terms of heterogeneity of the complexes having a different conformation of the guest and different H-bonding interactions with water molecules. The time-resolved emission spectra of CD solutions show a larger spectral relaxation (Stokes shift) within the nanocavity, suggesting a shortening of the IHB bond of the guest at the S1 state. The anisotropy experiment suggests that the trapped guest can still move within the cage. We believe that the results reported here are relevant to those found in photochemistry of encapsulated drugs and give some new insights for a better understanding of the photochemical events of medicines used in phototherapy. Acknowledgment. This work was supported by the MCYT (Spain) and the JCCM through projects MAT-2002-001829 and PAI-02-004. We wish to thank M. Moreno (UAB, Spain) for his help in the theoretical calculations. References and Notes (1) Szejtli, J. L., Ed. In Cyclodextrin Technology; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1988. (2) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 803. (3) (a) Special issue of Chem. ReV. 1998, 98, 1743. (b) Special issue of J. Photochem. Photobiol., A 2005, 173 (3). (4) Nandi, N.; Bhattacharyya, K.; Bagchi, B. Chem. ReV. 2000, 100, 2013. (5) Douhal, A. Chem. ReV. 2004, 104, 1955. (6) Partyka, M.; Au, B. H.; Evans, C. H. J. Photochem. Photobiol., A 2001, 140, 67. (7) Duchen, D., Ed. In Cyclodextrin and Their Industrial Uses; Editions de Sante: Paris, 1987; p 448. Formming, K. H.; Szejtli, J. Cyclodextrins in Pharmacy; Kluwer Academic Publishers: Dordrechet, The Netherlands, 1994; p 224. Special issue of Chem. ReV. 1998, 98, 5. (8) (a) Bortolus, P.; Monti, S. AdV. Photochem. 1996, 21, 1. (b) Monti, S.; Sortino, S. Chem. Soc. ReV. 2002, 31(5), 287. (c) Li, N.; Duan, J.; Chen, H.; Chen, G. Talanta 2003, 59, 493. (9) Trzos, W.; Reed, J. K. FEBS Lett. 1981, 127, 196.
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