Photophysics and Structure of Inclusion Complex of 4,4

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Photophysics and Structure of Inclusion Complex of 4,4-Diaminodiphenyl Sulfone with Cyclodextrin Nanocavities Prosenjit Bhattacharya, Dibakar Sahoo, and Sankar Chakravorti* Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

bS Supporting Information ABSTRACT: The structure and dynamics of a charge transfer drug molecule 4,4-diaminodiphenyl sulfone (dapsone) inside the cyclodextrins (R-, β-, γ-CDs) in aqueous solution have been studied using steady state and time-resolved emission spectroscopies. The quantum yields were significantly larger in the presence of β- and γ-CDs than in water, wherein the β-CD confinement shows the largest effect. The results reveal that dapsone forms 1:1 complexes with both β-CD and γ-CD. At higher concentrations of β-CD a combination of 1:1 and 1:2 inclusion complexes could be observed. The average lifetime of the probe inside the CD cavity is larger than that observed in water due to hydrophobic and polarity effects of the nanocage. Anisotropy decay has been used to study the rotational dynamics of the molecule inside the cyclodextrin cavity. 1H NMR data also confirm shallow inclusion of dapsone in β-CD. PM3 semiempirical calculations indicate that for unimolar complex a partial (3.8 Å) encapsulation of the dapsone molecule in β-CD at an angle of 72
with the CD axis. The DFT calculations with solvent effect show that the formation of inclusion is spontaneous and enthalpy driven.

’ INTRODUCTION Cyclodextrins (CDs) are cyclic oligosaccharides that possess hydrophobic cavities which can encapsulate organic, organometallic, and inorganic molecules in aqueous solution.1,2 Neutral cyclodextrins are classified as R-, β-, and γ-cyclodextrins according to whether they have six, seven, or eight glucopyranose units.1 The internal cavity diameters accessible to the guest molecules are 4.5
5.3, 6.0
7.0, and 7.5
8.5 Å for R-CD, β-CD, and γ-CD, respectively, and for all CDs the depth (∼7.9 Å) is the same.3 They have a doughnut shape with a fairly hydrophobic interior, and the two rims formed by the primary and secondary hydroxyl groups are hydrophilic. The influence of the hydrophobic cavity and the limitation of the molecular mobility inside the CD cavity can alter the photochemistry and photophysics of an encapsulated guest molecule significantly. Over the past two decades increasing attention has been paid to supramolecular chemistry to understand host
guest interactions.4
6 It is generally accepted that CD nanocavities can constitute an efficient photoprotective agent for drug and medicine storage and delivery.7
9 The molecular assembly constrained microenvironment of CD provides a useful model to mimic the interactions of drugs with hydrophobic pockets of biological substrates. The nature of the molecular relaxation in the excited state and the deactivation process may be controlled by the nanocavity. Organic fluorophores that are sensitive to pH, polarity, and hydrogen bonding ability of the solvent medium, such as excited state charge transfer and proton transfer systems, have potential applications as probes for the study of microstructures of organized assemblies.10 4,4-Diaminodiphenyl sulfone (dapsone) is a wellknown antileprosy drug,11,12 and it is being used for HIV-positive patients.13 Recently the photophysics of dapsone in different solvents revealed that it is principally dominated by a charge transfer process.14,15 The entire outcomes from the photophysics r 2011 American Chemical Society

of dapsone have intrigued us to examine the nature of interactions with constrained media such as R-, β-, and γ-cyclodextrins. The aim of this paper is to investigate by electronic absorption, fluorescence, and time-resolved spectroscopies the interactions occurring between dapsone and R-, β-, and γ-CDs in aqueous media. The stoichiometric characteristics of the inclusion complex have been determined. The possible orientation of the probe inside the CD cavity from steady state and time-resolved emission spectroscopies has also been depicted. A 1H NMR study is contemplated to confirm inclusion. To understand the structure and thermodynamics of inclusion complexes, PM3 semiempirical and density functional theory (DFT) in the gas phase and in water are also envisaged to employ.

’ EXPERIMENTAL AND COMPUTATIONAL SECTION 4,4-Diaminodiphenyl sulfone (dapsone) was procured from Aldrich Chemical (USA) and was further purified by vacuum sublimation. Spectroscopic grade R-, β-, and γ-CDs (Fluka, USA) were used as received. For measurement of the absorption and emission spectra, deionized water (Millipore) was used. The concentration of dapsone used in all absorption and emission experiments was about 5 10
5 M, and the emission was corrected for all optical components. The absorption spectra (298 K) were recorded with an Agilent 8453 spectrophotometer, and the emission spectra were obtained with a Hitachi F-4500 fluorescence spectrophotometer. The steady state fluorescence anisotropy measurements were performed with the same steady state spectrophotometer fitted with a polarizer attachment. The steady state anisotropy was Received: March 10, 2011 Accepted: May 16, 2011 Revised: May 16, 2011 Published: May 16, 2011 7815

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calculated using the formula16 Æræ ¼

IVV
GIVH IVV þ 2GIVH

ð1Þ

where IVV and IVH are the intensities obtained with the excitation polarizer oriented vertically and the emission polarizer oriented in vertical and horizontal positions, respectively. The factor G was defined as G¼

IHV IHH

ð2Þ

where I terms refer to parameters similar to those mentioned above for the horizontal position of the excitation polarizer. All measurements other than the study of the temperature effect were done at room temperature (298 K). Quantum yields were determined using quinine sulfate in 0.1 N H2SO4 (jf = 0.54)17 as reference. Changes of the absorbance band upon addition of CDs have been incorporated in calculations of quantum yields. The fluorescence lifetime measurements and anisotropy decay were done by the method of time correlated single photon counting (TCSPC) in a HORIBA JOBIN YVON instrument. By use of a picosecond diode laser (IBN Nanoled-07), the system was excited at 295 nm. The data were collected using a DAQ card as a multichannel analyzer. The obtained spectra were analyzed with the software DAS6 at data station v2.3 through exponential fitting. The quality of the fit was determined in terms of a Durbin
Watson (DW) parameter, weighted residuals, and reduced χ2 values, and the acceptable values of χ2 and DW in our lifetime fitting18 were 0.8
1.2 and 1
2, respectively. 1H NMR spectra were recorded at room temperature on 500 MHz spectrometers (Bruker). All theoretical calculations were carried out by the Gaussian 03 software package. The initial geometry of dapsone was optimized by means of ab initio use of the B3LYP functional and 6-31þG basis set.19 The geometries of the β- and γ-CDs were fully optimized by PM3 without imposing any symmetrical restrictions based on the available crystallographic data determined by X-ray diffraction. The glycoside oxygen atoms of cyclodextrin were placed onto the XY plane, and their center was defined as the center of the Cartesian coordinate system. The primary hydroxyl groups are placed pointing toward the positive Z-axis. The inclusion complex was optimized by PM3. To get more accurate results, single point calculations at the level of DFT with the 6-31Gþ(d,p) basis set were also performed.

’ RESULTS AND DISCUSSION Steady State Absorption and Fluorescence Spectra. The UV
visible absorption spectra of dapsone in neat water and with gradual addition of β-CD are shown in Figure 1, and in the inset the absorption spectra in the presence of water, R-CD, β-CD, and γ-CD have been depicted. With the first addition of CDs in aqueous solution of dapsone the absorption bands showed an initial decrement along with a red shift, but with progressive increase of the CD concentrations an absorbance enhancement along with a red shift could be observed. The initial decrement in absorbance and the shifts are attributed to the formation of an inclusion/association complex of the probe with the CD cavity as CD itself has no absorption in the wavelength range of measurement (Figure 1). With progressive addition of CDs in dapsone solution, more and more formation of complex (dapsone:CD)

Figure 1. UV
visible absorption spectra of CDs in water: (i) only βCD (2 mM), (ii) only β-CD (8 mM), (iii) γ-CD (10 mM), and (iv) RCD (10 mM). Also the UV
visible absorption spectra of dapsone in water and in the presence of β-CD at different concentrations: (a) 0, (b) 0.4, (c) 0.8, (d) 1.2, (e) 1.6, and (f) 2.4 mM. Inset: UV
vis absorption spectra in water, 15 mM R-CD, 15 mM β-CD, and 15 mM γ-CD. The concentration of the dapsone molecule was 5 10
5 M.

results in an enhancement of the absorption band in a red-shifted position. Dapsone shows a room temperature unstructured emission spectrum as a charge transfer band around 559 nm in aqueous solution. Addition of R-CD changes the fluorescence intensity a little (∼2 times at 10 mM), whereas upon addition of β- and γ-CDs the emission intensity is largely enhanced [∼37 (at 15 mM) and 15 (at 15 mM) times, respectively] along with a blue shift. On photoexcitation at 290 nm Figure 2 shows the steady state emission spectra of dapsone in aqueous solution and in the presence of different β-CD and γ-CD concentrations. On the other hand, the big hypsochromic shift in β- and γ-CDs may suggest a total or partial encapsulation of the probe within the hydrophobic nanocavity of cyclodextrin. The hypsochromic shift indicates that dapsone experiences a less polar environment inside the β- and γ-CD cavities compared to the aqueous bulk solution. In water H-bonds and the twisting motion of the two aniline groups about the single bond may act as a nonradiative channel in the photophysics of the dapsone.20
22 The inclusion of dapsone inside both cavities reduces the abovementioned nonradiative channel and thereby increases the fluorescence quantum yield (Table 1). Because the cavity diameter of γ-CD is larger (∼9.5 Å) compared to β-CD (∼8 Å), one would normally expect better encapsulation of the probe in γ-CD than in β-CD. However, a surprisingly high quantum yield is observed in the β-CD environment. Ramadass et al.22 and Sahoo et al.20 recently showed that the hydrogen bond of the medium and the rotational movement of the probe diminish the fluorescence quantum yield and lifetime significantly. Therefore, in the present case greater rotational movement due to its large cavity diameter and hydrogen bonding between the probe and γ-CD may cause reduction of the fluorescence quantum yield.20,23 To authenticate the hydrogen bond effect, we add urea, which is known as a good hydrogen bond breaker, and a small increment in emission intensity was observed. This result indicates that the rotational movement of the probe inside the γ-CD cavity dominates the hydrogen bonding 7816

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Figure 3. Benesi
Hildebrand plot for 1:1 complexation of dapsone with β-CD in aqueous solution.

Figure 4. Benesi
Hildebrand plot for 1:1 complexation of dapsone with γ-CD in aqueous solution. Figure 2. Emission spectra of dapsone in water and in the presence of (a) β-CD and (b) γ-CD at different concentrations: (a) 0.0, (b) 0.2, (c) 0.5, (d) 0.8, (e) 1.2, (f) 2.0, (g) 4.0, (h) 6.0, (i) 8.0, (j) 10.0, and (k) 15.0 mM.

Table 1. Spectral Characteristics and Quantum Yields of Dapsone in Water and in the Presence of β- and γ-CDs at 298 K λmax (nm) solvent

concn (mM)

water

absorption

emission

quantum yield (j) 0.009

291

459

R-CD

15

291

444

0.022

β-CD

10

293

437

0.264

γ-CD

15

294

439

0.118

effect. However, in the case of β-CD the hydrogen bonding between the adjacent
OH groups makes a complete secondary belt, forming a more rigid structure than γ-CD.23 Because the cavity diameter is less, the rotational relaxation is hindered in β-CD, and as a result the fluorescence quantum yield is greater than that of γ-CD. To understand the nature of complexation, the stoichiometry of the probe
CD complexes along with the equilibrium constant double-reciprocal Benesi
Hildebrand plot as described by the following equation for 1:1 complexation24,25

(eq 3) has been used. 1 1 1 ¼ þ I
I0 Im
I0 K½CDðIm
I0 Þ

ð3Þ

Here, I0 and Im are the fluorescence intensities at zero and maximum concentrations of CD, I denotes the fluorescence intensities at different concentrations of CD, [CD] is the total CD concentration, and K is the binding constant. The Benesi
Hildebrand plots of 1/(I
I0) vs 1/[ R-CD] and vs [R-CD]2 do not yield straight lines (Supporting Information, Figures S2 and S3), so some sort of association rather than a 1:1 or 1:2 inclusion complex may be inferred. The analysis of fluorescence data indicates that the dapsone:β-CD stoichiometry is different at lower concentration than at higher β-CD concentrations. The plot of 1/(I
I0) vs 1/[CD] at lower concentration (inset of Figure 3) confirms that a 1:1 complex is formed at lower concentration range (up to concentration e 2.4 mM), while at higher β-CD concentration range (Figure 3) the nonlinearity indicates that along with 1:1 a different type of complex is also present. However, with γ-CD the Benesi
Hildebrand plot (Figure 4) shows linear variation, justifying the validity of eq 3 and hence establishing the formation of a 1:1 complex between the probe and the γ-CD. The binding constant comes out to be 295  103 L M
1 for γ-CD and 10  103 L M
1 for β-CD (e2.4 mM) (Table 2). The high binding constant for γ-CD indicates a better inclusion complex 7817

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Table 2. Binding Constant (K), Stern
Volmer Constant (KSV), and Steady State Anisotropy (r) of Dapsone in β- and γ-CDsa binding constant concn (mM)

K  103 (L M
1)

β-CD

2

10

0.9997

β-CD

10







γ-CD

10

295

environment

a

correln coeff

0.9996

stand dev

KSV (mol L
1)

r

0.0064

9.89

0.05

6.59

0.09

20.25

0.06


0.0004

The excitation wavelength was 290 nm. Cu2þ ion has been used for the quenching study.

Scheme 1

Figure 5. Stern
Volmer fluorescence quenching of dapsone by Cu2þ ions in aqueous β-CD (2 and 10 mM) and γ-CD (10 mM) solutions.

than that for β-CD at low concentration, but in high β-CD concentration as both types of complexes are present, it is difficult to predict a deeper inclusion comparable to γ-CD on the basis of a huge increase in intensity. Study of Fluorescence Quenching. To determine the accessibility of CD-bound dapsone to an external quencher, copper ion induced fluorescence quenching has been exploited. The lower micropolarity in the hydrophobic core of the CD cavity restricts the availability of the ionic quencher in the abovementioned region. Therefore, it is expected that the quencher is available in the aqueous solution as well as in the CD
water rim. The Stern
Volmer plots (Figure 5) for the quenching of the probe by Cu2þ ion in the presence of CDs have been done to assess the accessibility of the encapsulated fluorophore toward the quencher which is in the order γ-CD > β-CD (1:1) > higher concentration of β-CD. With β-CD the quenching of the fluorophore is less than that in γ-CD, which indicates a partial encapsulation of the probe molecule. However, at high concentration of β-CD the smaller Stern
Volmer constant (Table 2) suggests greater encapsulation of the probe by one or two β-CD molecules. To get an insight about the orientation of the probe inside the CDs, we compared the spectrum of protonated species15 of dapsone (with mild acid (HCl)) in water with those in the presence of β-CD and γ-CD, respectively. In all cases a gradual decrement of charge transfer emission with lowering pH could be observed. Probably the Hþ ion of acid binds to the lone pair of one or both nitrogen atoms of the amino group (
NH2) of the probe, and after photoexcitation charge migration from nitrogen to the sulfur atom was hindered and as a result fluorescence intensity decreased. The quenching rate in aqueous solution is higher (KSV = 22.15 mol L
1) than that in the restricted

Figure 6. Steady state fluorescence anisotropy variation as a function of β-CD concentration. Inset: fluorescence anisotropy variation with γ-CD concentration.

environment. In the high β-CD concentration region the quenching rate is lower than 1:1 complex formation of the probe with β- and γ-CDs, as we observed in copper ion quenching. The above results indicate that one part of the donor side (N-atom) of the guest molecule forming 1:1 complex resides outside and gets protonated in mild acid.15,26 However, in higher β-CD concentration region possibly a combination of 1:1 and the next possible 1:2 inclusion complex results in greater encapsulation (both N-atoms of the donor) of the probe molecule, resulting in less quenching of the fluorescence. The possible orientation of dapsone may be conceived as in Scheme 1. Steady State Anisotropy. The rotational diffusion of entrapped or encapsulated compounds can be well accounted for by the steady state fluorescence anisotropy measurements.27 The fluorescence anisotropy should increase if the rigidity of the surrounding environment of the fluorophore increases. Figure 6 represents the fluorescence anisotropy (r) variation of dapsone in β- and γ-CD environments at different concentrations. A small change in fluorescence anisotropy values in the presence of 7818

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Table 3. Values of the Emission Lifetimes (τf) and Normalized Pre-exponential Factors (ai) from the Multiexponential Fit to the Fluorescence Decay of Dapsone (5  10
5 M) in Water, and in the Presence of Different Concentrations of β- and γ-CDsa concn

τ1

solvent (mM) (ns)

τ3 a2

(ns)

a3

Æτæ

DW

χ2

0.11 21.29 0.90 76.78
0.72 60.36 0.95 39.64






0.71 1.48 1.02 0.81 1.51 0.98

2.0

0.72 57.93 0.98 42.07





0.83 1.75 1.00

6.0

0.74 27.98 0.97 54.15 1.18 17.87 0.94 1.54 0.99

10.0

0.75 21.26 1.00 58.41 1.21 20.33 0.99 1.72 1.08

γ-CD

γ-CD could be observed, and in β-CD the variation of anisotropy indicates two different regions of differing motional restriction at lower and higher concentrations, respectively. The fluorescence anisotropy values of dapsone in the presence of β- and γ-CDs are presented in Table 2. The observed results indicate that the probe molecule experiences a little restriction in β-CD at lower concentration (e2.4 mM) and in γ-CD, which indicates that some portion of the fluorophore remains exposed to the bulk aqueous solution. On the other hand, at higher concentration (g2.4 mM) of β-CD the probe experiences greater restriction and fluorescence anisotropy increases due to a different type of inclusion complex along with 1:1 probe
β-CD complex. Time-Resolved Emission and Anisotropy Decay. Since it is well-known that the motional restriction of the fluorophore is directly reflected from the fluorescence decay, time-resolved fluorescence experiments were performed for a better view of the changes in the photophysics of the probe molecule in different concentrations of the CD environment. The lifetimes of the probe were measured at different concentrations of β- and γ-CDs. The excitation and monitoring wavelengths were 295 and 450 nm. About 5000 counts were collected for all the CD environments. Figure 7 demonstrates the fluorescence decay profile of dapsone inside β- and γ-CDs. Table 3 depicts the fluorescence decay data at different CD concentrations. The dual fluorescence of dapsone in different solvents is ascribed to the emission from the locally excited (LE) state and from the charge transfer (CT) state.14 The component of the lifetime which changes much with the environment is assigned to originate from the charge transfer state, and the other is attributed to the locally excited state.16,22 Therefore, the two components in neat water are due to decay from the locally excited state and from the charge transfer state. However, in the presence of CDs we found that the fast component of the lifetime in CDs may be due to free probe molecule in the form of the average decay time of locally excited and charge transfer states in aqueous solvent. The other slower component may be due to the decay from the probe
CD complex. The decay pattern is complicated in the β-CD environment. In different ranges of β-CD concentration the probe shows different types of probe
β-CD complex. In the case of 1:1 partial inclusion complex, the rotational freedom of one aniline group is hindered and a slightly greater lifetime due to encapsulated

(ns)

0.5

water β-CD

Figure 7. Time-resolved decay profiles of the probe molecule in water and aqueous solutions of β- and γ-CDs at room temperature monitoring at their corresponding steady state emission wavelengths. IRF = instrument response function.

τ2 a1

0.5

0.68 44.48 0.95 55.52





0.83 1.58 1.06

2.0

0.67 34.07 0.96 65.93





0.86 1.62 0.99

6.0

0.70 31.26 0.96 68.74





0.88 1.64 1.09

15.0

0.72 30.63 0.97 69.37





0.89 1.78 1.07

a

The excitation and observation wavelengths were 295 and 440
459 nm, respectively.

Table 4. Values of Rotational Time (τrot) and Normalized Pre-exponential Factor (a) of Fluorescence Anisotropy Decay Fitting of Dapsone in Water, β-CD, and γ-CDa τrot (ns)

a

χ2

water

0.89

100

1.11

β-CD (2 mM) β-CD (10 mM)

1.16 1.38

100 100

0.92 0.85

γ-CD (10 mM)

0.97

100

0.96

solvent

a

The excitation wavelength was 290 nm and monitored at the corresponding steady state emission wavelength maxima.

molecule results but in the high concentration region both aniline group rotations may hindered; as a result, a larger decay component arises (∼20%) along with the 1:1 (∼60%) inclusion complex and (∼20%) free probe molecule decay component (Table 3). The larger decay component may be due to 1:2 inclusion complex, which possibly explains the nonlinearity in the Benesi
Hildebrand plot (vide supra). In the case of dapsone: γ-CD inclusion complex one aniline group always remains outside. The rotational freedom is less hindered than that in β-CD environment and the molecule shows less decay time. A small increment of the decay time of the inclusion complex compared to the free probe molecule was observed. This observation indicates that the molecule is not fully encapsulated in the CD cavity and a major part of the probe is exposed to water. However, a significant change in the average lifetime explains the large enhancement in steady state spectra as the latter is a kind of time-averaged data of emission from all the components. To get further information on confined structure and rotational times (τrot) of the cyclodextrin inclusion complex, timeresolved emission anisotropy (r(t)) measurements (Figure 8) were performed by exciting the probe at 295 nm. In pure water as well as in different CD environments the decay time fitted to a single exponential function. In neat water the rotational decay time is about 890 ps (Table 4). The long relaxation time (∼1000 ps) in the presence of CD environment suggests that the molecules are caged in the cavity. In the case of higher concentration, the 7819

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Figure 8. Anisotropy (r(t)) decay of dapsone in aqueous solution of 10 mM β-CD upon excitation at 295 nm and monitored at 540 nm.

rotational relaxation time value is greater than that of the 1:1 inclusion complex in β-CD as well as that in γ-CD. Considering the host
guest complex as a prolate ellipsoid and nonhydrate rotor, the rotational relaxation can be described by the hydrodynamic model.28,29 The rotational relaxation time in the Debye
Stokes
Einstein (DSE)30 model is expressed as τrot ¼

4πηfcRH 3kB T

ð4Þ

In eq 4, RH is the hydrodynamic radius of the rotor, η is the viscosity of the solvent, kB is the Boltzmann constant, T is the absolute temperature, and c is a constant depending on the solute
solvent friction, the relative size of the solute compared to the solvent molecules, and the boundary conditions (stick and slip). The factor f accounts for the shape of the solute.31 For a nonspherical solute molecule, the value of c is 0 < c e 1 and f > 1 (f = 1 for a sphere). Under limiting conditions the value of f was determined from the molecular dimensions using the following expression:30 f ¼

2 1
β4  qffiffiffiffiffiffiffiffiffiffiffiffiffi  3 2β2
β4 1 pffiffiffiffiffiffiffiffiffiffiffiffi2ffi ln ð1 þ 1
β2 Þ
β2 β 1
β

ð5Þ

where β = b/a; a and b are the semimajor and semiminor axes, respectively. The hydrodynamic radius RH of the inclusion complex is estimated from its optimized geometry to be 9.12 Å; the value of f for β-CD complex is computed as 1.07. As the volume of the inclusion complex (dapsone:β-CD) is much larger than that of a water molecule, we may take the value of c as 1 and η(298 K) ∼ 0.0011 Pa 3 s.32 The theoretically computed value of τrot = 0.91 ns for 1:1 complexation with β-CD is very close to the experimental value of 0.81 ns. NMR Spectroscopy. To get proof for an inclusion complex of dapsone inside β-CD, the most effective tool is proton nuclear magnetic resonance (1H NMR) spectroscopy.33
35 The selective line broadening or chemical shift of 1H NMR of guest and host protons confirms the association or inclusion of the guest inside the host.34,35 It is reported that the 1H NMR spectrum of β-CD in D2O consists of six types of protons.36 Out of six protons H-3 and H-5 protons are well inside the β-CD cavity. If

the probe molecule enters well inside, then the H-3 and H-5 protons would be affected more; otherwise deep inclusion may be ruled out. Figure 9a shows the 1H NMR spectrum of β-CD in D2O, which consists of H-1 doublet at δ 5.014, H-3 triplet at δ 3.907, H-5 and H-6 at δ 3.821 and at δ 3.807, two doublets of H-2 at δ 3.603 and δ 3.583, and H-4 triplet centered at δ 3.527. All these observed results are very close to the previously reported results ((0.04 ppm).33,34,36 With a ddition of dapsone to β-CD solution the 1H NMR (Figure 9b) shows upfield shifts of the H-1, H-3, and H-4 protons by 0.071, 0.093, and 0.03 ppm, respectively. The two doublets of the H-2 proton of pure β-CD drastically become a single doublet with addition of dapsone centered at δ 3.636 and δ 3.616. Also, the H-5 and H-6 protons, which were slightly resolved in pure β-CD become unresolved and centered at δ 3.745, having an upfield shift of about 0.1 ppm with addition of dapsone. It is clear that the H-2 proton, which belongs to the periphery,37 is most affected in terms of downfield shift. The peak area and the nature of coupling clearly indicate that the probe molecule does not enter well inside the cyclodextrin cavity, but rather stays a small distance inside the cavity. Theoretical Modeling. To get information about the geometry and mode of inclusion of dapsone in a CD cavity, semiempirical calculations in the gas phase were performed. For the inclusion of the guest molecule inside the cavity there are two possibilities: one is the donor group (aniline) inside the cavity and other is the sulfone group inside the cavity. The possibility of inclusion of either of the aniline groups is the same due to geometric symmetry. Interestingly, the stable structure is formed only when the one of the two aniline groups is inside the cavity, but the other conformation is not stable. The PM3 calculated results reveal that the energy of the complex found from the optimized geometry is lower (
36.25 kJ mol
1) than the energy sum of isolated host (β-CD) and guest (dapsone). The dipole moment of dapsone:β-CD was only 4.37 D, compared to that of free dapsone molecule (∼8.43 D), which possibly indicates that the guest molecule experiences less polarity inside the β-CD cavity.38 Furthermore, the optimized geometry of inclusion complex shows that a length 6.8 Å (out of 10.6 Å) of the probe is exposed to the water and it enters the hydrophobic cavity at an angle of 72
with the cyclodextrin axis (Figure 10). The optimized structure shows that the angle between two aniline groups changes from 90
(free dapsone) to 125
(dapsone:CD complex) due to encapsulation inside the β-CD cavity. Also, the dihedral angle of the encapsulated aniline group changes from 105
(free dapsone) to 127
(complex form). These geometric changes cause the decrease in dipole moment of the 1:1 dapsone
β-CD complex. All these data support our proposed model of an inclusion complex of dapsone molecule inside the cyclodextrin cavity on the basis of experimental results. Also, the geometry of 1:1 complex indicates that 1:2 guest:host complex is possible at higher concentrations of CD as there exists a sufficiently large distance between two cyclodextrins (∼6 Å) to avoid steric effect. Just as it was already indicated in the experimental discussion (vide supra) that the presence of a mixture of 1:1 and 1:2 complexes at higher concentration may be possible, the same possibility is observed from the theoretically optimized geometry. In addition, the results from the single point B3LYP/6-31Gþ(d,p) calculations further confirm that the complex of 1:1 dapsone
β-CD is much more stable. The DFT calculated results reveal that the energy of the complex is lower (
53.5 kJ mol
1) than the energy sum of isolated host and guest. 7820

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Figure 9. 1H NMR spectra in D2O: (a) β-CD only and (b) β-CD containing dapsone.

Figure 10. Optimized geometry of 1:1 inclusion complex inside β-CD in the gas phase.

The thermodynamics of the binding process and the statistical thermodynamic calculations were carried out in a vacuum

by PM3, and the results are listed in Table 5. The positive Gibbs free energy (40.26 kJ mol
1) change of the complex in a vacuum implies that at room temperature the reaction is not spontaneous. We also calculated the thermodynamic parameters at the DFT level using the B3LYP 6-31Gþ(d,p) basis set in a vacuum. Tomasi’s polarizable continuum model (PCM)), as implemented in Gaussian 03, is also used to incorporate the solvent effect in energy calculations.39 In a vacuum we found the changes in enthalpy, Gibbs free energy, and entropy to be
72.3 kJ mol
1, 21.2 kJ mol
1, and
0.31 kJ mol
1 K
1, which were similar to the results we obtained at the PM3 level. However, in water using the PCM model, the changes in enthalpy, Gibbs free energy, and entropy are
69.6 kJ mol
1,
14.9 kJ mol
1, and
0.18 kJ mol
1 K
1 (Table 5), which imply that in water the complex formation is spontaneous and enthalpy driven. Therefore, the obtained thermodynamic parameters also suggest that the acting force was mainly the hydrophobic association force.40 In experiments simply with addition of CD to aqueous solution of dapsone, we observe a complex formation in the form of spectral change 7821

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Table 5. Energies upon Inclusion Complexation of β-CD with Dapsone in the Gas Phase Using Semiempirical (PM3) and DFT (B3LYP/6-31Gþ(d, p)) Methodsa species

Eb (kJ mol
1)

Hf (kJ mol
1)

Gf (kJ mol
1)

ΔEb (kJ mol
1)

ΔH
c (kJ mol
1)

ΔG
c (kJ mol
1)

ΔS
d (kJ mol
1 K
1)

PM3 in a Vacuum dapsone β-CD β-CD:dapsone

32.73

642.12

501.29


6 024.89
6 028.41


2 715.13
2 149.36


3 235.78
2 694.23


36.25


76.35

40.26


0.39


72.3

21.2


0.31


69.6


14.9


0.18

B3LYP/6-31G (d,p) in a Vacuum dapsone β-CD β-CD:dapsone

5 312 219.61

6 194 264.75

3 625 695.26


11 162 624.94


9 050 141.80


10 432 957.44


5 850 458.83


2 855 949.35


6 807 240.98

5 312 198.53

6 193 759.46

3 195 475.32


11 162 617.39


9 049 895.52


9 648 785.13


5 850 476.26


2 856 205.66


6 453 324.71


53.5

B3LYP/6-31G (d,p) in Water dapsone β-CD β-CD:dapsone


57.4

a

Tomasi’s polarizable continuum model (PCM)) is also used to incorporate the solvent effect in energy calculations. Room temperature T = 300 K. b E is the HF energy; ΔE is the stabilization energy upon complexation. ΔE = Ecomplex
Ehost
Eguest. c ΔA
= Af,complex

Af,host

Af,guest
; A = H, G. d ΔS
= (ΔH

ΔG
)/T.

(steady state and time resolved), which is thermodynamically favored due to hydrophobicity as the driving force.

’ CONCLUSION The photophysical behavior of dapsone is modified significantly upon encapsulation of the probe inside CD cavities. The emission quantum yield increases and the average lifetime of the drug molecule inside the cyclodextrin cavity becomes longer. The 1H NMR experiment indicates the inclusion of dapsone just inside β-CD cavity, and that is also reflected in the theoretical calculations. The results obtained from the semiempirical PM3 calculation in the gas phase suggest that the complex process is favored and enthalpy driven in nature. DFT calculations of dapsone in water show that the Gibbs free energy is negative; i.e., the inclusion process is spontaneous in nature, which is also reflected in the experimental results. The time-resolved anisotropy experiments gave more insight into the nature and structure of the complex. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1 showing emission spectra of dapsone in water and in the presence of R-CD at different concentrations; Figures S2 and S3 showing Benesi
Hildebrand plots for 1:1 and 1:2 complexation of dapsone with R-CD in aqueous solution. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors express thanks to Prof. Narayan Pradhan and Prof. P. K. Mukherjee for their kind help, and the authors also thank Mr. Subrata Das, Department of Spectroscopy, IACS, for taking and analyzing picosecond time-resolved data.

’ REFERENCES (1) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; SpringerVerlag: New York, 1978. (2) Saenger, W. Cyclodextrin Inclusion Compounds in Research and Industry. Angew. Chem., Int. Ed. Engl. 1980, 19, 344–362. (3) Li, S.; Purdy, W. C. Cyclodextrins and their applications in analytical chemistry. Chem. Rev. 1992, 92, 1457–1470. (4) Schneider, H. J.; D€urr, H. Frontiers in SupramolecularChemistry and Photochemistry; VCH: Weinheim, Germany, 1992. (5) Balzani, V.; DeCola, L. Supramolecular Chemistry; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992. (6) Komiyama, M.; Shigekawa, H. Comprehensive Supramolecular Chemistry; Pergamon: New York, 1996; (7) Wenz, G. Cyclodextrins as Building Blocks for Supramolecular Structures and Functional Units. Angew. Chem., Int. Ed. Engl. 1994, 33, 803–822. (8) Duchen, D. Cyclodextrin and Their Industrial Uses; Editions de Sante: Paris, 1987. (9) Formming, K. H.; Szejtli, J. Cyclodextrins in Pharmacy; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994. (10) Kalyanasundaram, K. Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH: New York, 1991. (11) Venkatesan, K. Clinical Pharmacokinetic Considerations in the Treatment of Patients with Leprosy. Clin. Pharmacokinet. 1989, 16, 365–386. (12) McGeer, P. L.; Schulzer, M.; McGeer, E. G. Arthritis and antiinflammatory agents as possible protective factors for Alzheimer’s disease: A review of 17 epidemiologic studies. Neurology 1996, 47, 425–432. (13) Antinori, A.; Murary, R.; Ammassari, A.; Luca, A. D. Aerosolized pentamidine, cotrimoxazole, and dapsone-pyrimethamine for primary prophylaxis of Pneumocystis carinii pneumonia and toxopalasmic encephalitis. AIDS 1995, 9, 1343–1350. (14) Rettig, W.; Chandross, E. A. Dual fluorescence of 4,40 -dimethylamino and 4,40 -diaminophenyl sulfone. Consequences of d-orbital participation in the intramolecular charge separation process. J. Am. Chem. Soc. 1985, 107, 5617–5624. (15) Enoch, I. V. M. V.; Swaminathan, M. Unusual twisted intramolecular charge transfer processes of 4,40 -diamino-diphenylsulfone in β-cyclodextrin: a study by electronic spectra. J. Chem. Res. 2006, No. Aug, 523–526. (16) Lakowicz, J. R. Principle of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006; Chapters 1
3. 7822

dx.doi.org/10.1021/ie2004797 |Ind. Eng. Chem. Res. 2011, 50, 7815–7823

Industrial & Engineering Chemistry Research

ARTICLE

(17) Demas, J. N.; Crosby, G. A. Measurement of photoluminescence quantum yields. Review. J. Phys. Chem. 1971, 75, 991–1024. (18) Lakowicz, J. R. Topics in Fluorescence Spectroscopy; Kluwer Academic Publishers: New York, 2002; Vol. 1, Chapter 1. (19) Yang, C.; Lee, W.; Parr, R. G. Development of the Colle-Salvetti correlation energy formula into a functional of the electron density. Phys. Rev. B 1988, 37, 785–789. (20) Sahoo, D.; Chakravorti, S. Orientational dynamics of a charge transfer complex in cyclodextrin cavity as receptor. Phys. Chem. Chem. Phys. 2008, 10, 5890–5897. (21) Panja, S.; Chakravorti, S. Photophysics of 4-(N,N-dimethylamino) cinnamaldehyde/R-cyclodextrin inclusion complex. Spectrochim. Acta, Part A 2002, 58, 113–122. (22) Ramadass, R.; Hahn, J. B. Photophysical Properties of DASPMI as Revealed by Spectrally Resolved Fluorescence Decays. J. Phys. Chem. B 2007, 111, 7681–7690. (23) Szejtli, J. Introduction and general overview of cyclodextrin chemistry. Chem. Rev. 1998, 98, 1743–1754. (24) Li, G.; McGown, L. B. Molecular Nanotube Aggregates of β- and γCyclodextrins Linked by Diphenylhexatrienes. Science 1994, 264, 249–251. (25) Connors, K. A. The Measurement of Molecular Complex Stability; Wiley: New York, 1987. (26) Panja, S.; Bangal, P. R.; Chakravorti, S. Modulation of photophysics due to orientational selectivity of 4-N,N-dimethylamino cinnamaldehyde β-cyclodextrin inclusion complex in different solvents. Chem. Phys. Lett. 2000, 329, 377–385. (27) Small, E. W.; Isenberg, I. Hydrodynamic properties of a rigid molecule: Rotational and linear diffusion and fluorescence anisotropy. Biopolymers 1977, 16, 1907–1928. (28) Hu, C.; Zwanzig, R. Rotational friction coefficients for spheroids with the slipping boundary condition. J. Chem. Phys. 1974, 60, 4354–4357. (29) Baskin, J. S.; Zewail, A. H. Molecular Structure and Orientation: Concepts from Femtosecond Dynamics. J. Phys. Chem. A 2001, 105, 3680–3692. (30) Fleming, G. R. Chemical Applications of Ultrafast Spectroscopy; Oxford: New York, 1986. (31) Dutt, G. B. Molecular Rotation as a Tool for Exploring Specific Solute
Solvent Interactions. ChemPhysChem 2005, 6, 413–418. (32) Sen, P.; Roy, D.; Kumar, S.; Sahu, M. K.; Ghosh, S.; Bhattacharyya, K. Fluorescence Anisotropy Decay and Solvation Dynamics in a Nanocavity: Coumarin 153 in Methyl β-Cyclodextrins. J. Phys. Chem. A 2005, 109, 9716–9722. (33) Abdel Shafi, A. A. Effect of β-cyclodextrin on the excited state proton transfer in 1-naphthol-2-sulfonate. Spectrochim. Acta, Part A 2001, 57, 1819–1828. (34) Schneider, H. J.; Hacket, F.; Rudiger, V. NMR Studies of Cyclodextrins and Cyclodextrin Complexes. Chem. Rev. 1998, 98, 1755–1786 and references therein. (35) Turro, N. J.; Okubo, T.; Chung, C. J. Analysis of static and dynamic host-guest associations of detergents with cyclodextrins via photoluminescence methods. J. Am. Chem. Soc. 1982, 104, 1789–1794. (36) Abdel Shafi, A. A. Spectroscopic studies on the inclusion complex of 2-naphthol-6-sulfonate with β-cyclodextrin. Spectrochim. Acta, Part A 2007, 66, 732–738. (37) Rekharsky, M. V.; Goldberg, R. N.; Schwarz, F. P.; Tewari, Y. B.; Ross, P. D.; Yamashoji, Y.; Inoue, Y. Thermodynamic and Nuclear Magnetic Resonance Study of the Interactions of R- and β-Cyclodextrin with Model Substances: Phenethylamine, Ephedrines, and Related Substances. J. Am. Chem. Soc. 1995, 117, 8830–8840. (38) Yang, E. C.; Zhao, X. J.; Hua, F.; Hao, J. K. Semi-empirical PM3 study upon the complexation of β-cyclodextrin with 4,40 -benzidine and o-tolidine. THEOCHEM 2004, 712, 75. (39) Frisch, M. J.; et al. Gaussian 03, revision D.01; Gaussian. Inc.: Wallingford, CT, USA, 2004. (40) Ross, P. D.; Subramanian, S. Thermodynamics of protein association reactions: forces contributing to stability. Biochemistry 1981, 20, 3096–3102. 7823

dx.doi.org/10.1021/ie2004797 |Ind. Eng. Chem. Res. 2011, 50, 7815–7823