Permittivity-Dependent Entropy Driven Complexation Ability of Cone

J. Phys. Chem. B 2008, 112, 11743–11749

11743

Permittivity-Dependent Entropy Driven Complexation Ability of Cone and Paco Tetranitro-calix[4]arene toward para-Substituted Phenols Sa´ndor Kunsa´gi-Ma´te´,*,† Zsolt Cso´k,‡ Angela Tuzi,§ and La´szlo´ Kolla´r| Department of General and Physical Chemistry, UniVersity of Pe´cs, Ifju´sa´g 6, H-7624 Pe´cs, Hungary, Departamento de Quı´mica da UniVersidade da Madeira, 9000-390 Funchal, Madeira, Portugal, Dipartimento di Chimica, UniVersita di Napoli ‘Federico II’, Via Cinthia, I-80126 Napoli, Italy, and Department of Inorganic Chemistry, UniVersity of Pe´cs, Ifju´sa´g 6, H-7624 Pe´cs, Hungary ReceiVed: April 22, 2008; ReVised Manuscript ReceiVed: July 2, 2008

Considering the importance of the polarizability of the rings of calixarenes in the entropy-driven interaction processes, we examined the effect of entropy compensation on the complex formation of cone and partial cone (paco) conformers of tetranitro-calix [4]arene, possessing O-ethyl substituents at the lower rim. Both calixarene conformers were fully characterized including X-ray crystallography. Various para-substituted phenols were used as guest molecules. Photoluminescence (PL) measurements and quantum-chemical (QC) investigations were used. A permittivity dependence of the molecular interactions was obtained in different alcohols as solvents. It was found that the cone conformer of the title calixarene derivative forms stable complexes with all phenols of the p-substituted series. The free enthalpy changes show very high complex stability of cone calixarene with p-nitro and p-chloro-substituted phenols. In the cases of parent phenol, p-cresol and p-tBu-phenol, the stability is significantly lower; however, it slightly increases with the increasing electron density on the aromatic ring of guest molecules. Similarly, the entropy changes are significantly different for these two separated groups: the entropy changes obtained in the former cases are nearly the same, while large differences in the formation entropy were obtained in the latter cases. Both the experimental and theoretical investigations revealed that no considerable interaction exists between phenols and the paco conformer of the title calixarene. It is probably due to the locking of the calixarene cavity by the bent O-ethyl chain. Introduction calixarenes1,2

The functionalized are a fascinating class of materials3,4 that are able to form complexes with both cationic and anionic forms of elements in usually regioselective complexation.5,6 Furthermore, the electron-rich cavity of calixarenes is known as a very good complexation unit toward small guest molecules.7 The complex formation of calixarenes with small molecules raises high interest since the steric hindering of the interacted species gives high selectivity character for complex formation. The nice fluorescent behavior of these compounds changes during complex formation making them highly sensitive sensor materials.8,9 In our previous work, the complexation behavior of calix[4]arene and 4-tert-butylcalix[6]arene (hosts) with pesticide related neutral π-electron deficient 1-trifluoromethyl-benzene derivatives (guests) in chloroform and dimethylformamide was reported.10,11 In addition to the cavity shape12 and solvent effect,13 the entropy was found to be the most important parameter of complex formation.14 This property is preferably based on the reordering of solvent molecules around the calixarene cavity under the effect of guestinduced polarization of calixarene aromatic rings.15,16 The ability of fine-tuning of the complex formation with entropy moderated effects results in the wide applicability of calixarenes in sensor chemistry and pharmaceutical and other material science applications. The above ongoing research on the systematic tuning * To whom correspondence should be addressed. † Department of General and Physical Chemistry, University of Pe ´ cs. ‡ Universidade da Madeira. § Universita di Napoli ‘Federico II’. | Department of Inorganic Chemistry, University of Pe ´ cs.

of the electron density of calixarene aromatic moieties encouraged us to investigate the host properties of the electron deficient nitro-substituted calixarenes. There are different methods for the selective introduction of nitro groups at the upper rim of calixarenes. The most widely used approach is the direct replacement of the tert-butyl groups (ipso-nitration17) in readily available p-tert-butyl-calixarene derivatives. The outcome of ipso-nitration, i.e., the number of the substituted ring(s), depends largely on both the employed reaction conditions and the presence of electron-donating groups (OH and/or OR) at the lower rim.18,19 The application of harsh conditions (fuming nitric acid) is generally necessary to achieve the nitration of all aromatic rings.20-22 Recently, Kumar et al. studied the effect of different nitrating agents on yield to establish the best reaction conditions for the ipso-nitration of 4-tert-butyl-calix[n]arene (n ) 4, 6, 8) derivatives.23 Because of the importance of the polarizability of the rings of calixarenes in the entropy-driven processes, we examined the effect of entropy compensation on complex formation of cone (1a) and paco (1b) conformers of tetranitro-calix[4]arene with different para-substituted derivatives of phenols (2a-e) in different alcoholic solutions. In this work, the thermodynamic parameters of complex formation was determined by temperature-dependent PL and solvent-relaxation studies. The relevant conformations of the formed complexes were examined at molecular level by DFT/6-31++G calculations, where the solvent effect is considered by the PCM (polarizable continuum method).

10.1021/jp803498v CCC: $40.75  2008 American Chemical Society Published on Web 08/20/2008

11744 J. Phys. Chem. B, Vol. 112, No. 37, 2008

Figure 1. Charge density map of phenol derivatives 2a-e. Calculations were performed with the ab initio DFT/6-31G** method. From left to right: p-nitrophenol (2a), p-chlorophenol (2b), phenol (2c), p-cresol (2d), and p-tBu-phenol (2e).

Figure 2. Host calixarene compounds. Cone (1a) and paco (1b) conformers of tetranitro-calix[4]arene.

Experimental Methods and Calculation Details Materials and Methods. 4-tert-Butyl-calix[4]arene (99%) was purchased from Acros Organics; solvents and reagents were used as received. 5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetraethoxy-calix[4]arene was synthesized according to Gutsche.24 In a subsequent paper, he described the synthesis of 1a as well.25 The guest compounds, p-nitrophenol 2a, p-chlorophenol 2b, phenol 2c, p-methylphenol 2d, and p-tert-butyl-phenol 2e (p.a. grade) were purchased from Merck (Germany) and used without further purification (Figure 1). Solvents (methanol, ethanol, n-propanol, and n-butanol; spectroscopic grade, Panreac, Spain) were used without further purification. The 1H and 13C spectra were recorded at 293 K in CDCl3 on a Bruker Avance 400 spectrometer. The 1H chemical shifts (δ), reported in parts per million (ppm) downfield, are referenced to residual CDCl3 (7.27 ppm). The 13C{1H} chemical shifts were reported in ppm relative to the carbon resonance of CDCl3 (77.00 ppm). Mass spectra were obtained on a Shimadzu Axima CFRplus MALDI-TOF spectrometer, using 2,5-dihydroxybenzoic acid as the matrix. Elemental analyses were performed on a Carlo Erba EA 1110 CHN instrument. The variable temperature 1H NMR spectra were recorded in DMSO-d6 on a Varian Inova 400 spectrometer at 400.13 MHz. Chemical shifts δ are reported in ppm relative to the residual peak of DMSO-d6 (2.54 ppm). All data collections were performed in flowing N2 at 173 K on a Bruker-Nonius KappaCCD diffractometer (Mo KR radiation, λ ) 0.71073 Å, CCD rotation images, thick slices, φ scans and ω scans to fill the asymmetric unit). Crystal data for 1a (Figure 2): C36H36N4O12 · CHCl3, M ) 836.06, monoclinic, C2/ c, Z ) 8, a ) 31.596(9) Å, b ) 13.172(5) Å, c ) 18.798(8) Å, β ) 95.59(6)°, V ) 7786(5) Å3, Dx ) 1.426 g/cm3, µ ) 0.303 mm-1, 6820 unique reflections, 1805 observed [I > 3σ(I)], 509 parameters, R ) 0.0774. Crystal data for 1b: C36H36N4O12 · 2CHCl3, M ) 955.42, monoclinic, P21/c, Z ) 4, a ) 19.675(5) Å, b ) 12.383(4) Å, c ) 20.808(8) Å, β ) 121.66(2)°, V ) 4315(2) Å3, Dx ) 1.471 g/cm3, µ ) 0.463 mm-1, 7577 unique reflections, 3400 observed [I > 2σ(I)], 556 parameters, R ) 0.0536. Semiempirical absorption corrections (multiscan SADABS) were applied. The structures were solved by direct methods (SIR 97 package26) and refined by the full matrix least-

Kunsa´gi-Ma´te´ et al. squares method (SHELXL program of SHELX97 package27) on F2 against all independent measured reflections using anisotropic thermal parameters for all non-hydrogen atoms. All H atoms were placed in calculated positions with Ueq equal to those of the carrier atom and refined by the riding method. Crystals of 1a were of poor quality and were weakly diffracting. The Fourier maps gave evidence of statistical disorder for the Cl atoms of the chloroform solvent molecule, and it was not possible to solve it. In 1b, two statistical positions for one methyl group were found and refined to 0.8 and 0.2 occupancy factor; some restraints were also applied. All crystallographic data have been deposited at the Cambridge Crystallographic Data Centre (CCDC). Deposition numbers are CCDC 684966 for 1a and CCDC 684967 for 1b. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (internat.) +44-1223/336-033; E-mail: [email protected]]. Synthesis of 5,11,17,23-Tetranitro-25,26,27,28-tetraethoxycalix[4]arene. Fuming nitric acid (4.5 mL) was added dropwise under vigorous stirring at 0 °C to the dichloromethane (30 mL)/ glacial acetic acid (15 mL) solution of the 5,11,17,23-tetra-tertbutyl-25,26,27,28-tetraethoxy-calix[4]arene (980 mg; 1.29 mmol). The reaction mixture was stirred at room temperature until its deep violet color changed to yellow (approximately 3 h). At this point, it was quenched with water (100 mL), the organic phase was separated, and the water layer was extracted with dichloromethane (2 × 25 mL). The combined organic layers were washed with brine (2 × 25 mL), water (2 × 25 mL), and dried over Na2SO4. After evaporation of the volatiles, the residue was triturated with isopropanol to yield a white-off precipitate (750 mg; 81%). The 1H NMR analyses showed an approximately 1:1 ratio of the cone and the partial cone conformational isomers of the product. The two conformers were isolated by column chromatography (silica gel; particle size, 60, 200 µm; eluent, dichloromethane) in analytically pure forms. The samples were dried in Vacuo (∼0.01 mbar) for several hours. Single crystals suitable for X-ray analyses were obtained from a chloroform/methanol chamber. Cone conformer (1a): 1H NMR (CDCl3): 1.48 (t, J ) 7.0 Hz, 12H, ArOCH2CH3); 3.41 (d, J ) 14.0 Hz; 4H, ArCHaHbAr); 4.12 (q, J ) 7.0 Hz, 8H, ArOCH2CH3); 4.53 (d, J ) 14.0 Hz; 4H, ArCHaHbAr); 7.61 (s, 8H, Ar); 13C NMR (CDCl3): 15.53 (ArOCH2CH3); 31.18 (ArCH2Ar); 71.44 (ArOCH2CH3); aromatic carbons: 123.98; 135.64; 143.02; 161.28; MALDI-TOF: m/z ) 717.08 [M + H]+; 739.43 [M + Na]+; 755.42 [M + K]+; Elem. Anal., calculated for C36H36N4O12: C, 60.33; H, 5.06; N, 7.82; found C, 60.33; H, 4.89; N, 7.59. Partial-cone conformer (1b): 1H NMR (CDCl3): 0.88 (t, J ) 7.0 Hz, 3H, ArOCH2CH3); 1.57 (t, J ) 7.0 Hz, 6H, ArOCH2CH3); 1.67 (t, J ) 7.0 Hz, 3H, ArOCH2CH3); 3.31 (d, J ) 13.8 Hz, 2H, ArCHaHbAr); 3.52 (q, J ) 7.0 Hz, 2H, ArOCH2CH3); 3.80-4.12 (complex multiplets, 12H, ArCH2Ar overlapping with ArOCH2CH3); 7.21 (d, J ) 2.6 Hz, 2H, Ar); 7.90 (d, J ) 2.8 Hz, 2H, Ar); 8.17 (s, 2H, Ar); 8.23 (s, 2H, Ar); 13C NMR (CDCl3): 14.40 (ArOCH2CH3); 15.75 (ArOCH2CH3); 15.79 (ArOCH2CH3); 30.51 (ArCH2Ar); 35.84 (ArCH2Ar); 69.15 (ArOCH2CH3); 70.43 (ArOCH2CH3); 70.95 (ArOCH2CH3); aromatic carbons: 124.03; 125.03; 125.41; 126.56; 132.27; 134.07; 134.23; 136.95; 142.45; 142.91; 143.37; 160.32; 161.93; 162.20; MALDI-TOF: m/z ) 739.54 [M + Na]+; 755.52 [M + K]+; Elem. Anal., calculated for C36H36N4O12: C, 60.33; H, 5.06; N, 7.82; found C, 60.32; H, 4.77; N, 7.67.

Permittivity-Dependent Entropy Driven Complexation

Figure 3. ORTEP view of 1a. Ellipsoids are drawn at 30% probability level. H atoms are omitted for clarity.

Photoluminescence Measurements and Calculations. A highly sensitive Fluorolog τ3 spectrofluorometric system (JobinYvon/SPEX) was used to investigate the photoluminescence (PL) spectra of the different solutions. For data collection, a photon counting method with 0.2 s integration time was used. Excitation and emission bandwidths were set to 1 nm. A 1 mm layer thickness of the fluorescent probes with front face detection was used to eliminate the inner filter effect. For the phase fluorometric measurements, the modulation frequency was varied within the 50-250 MHz range, and the calixarene emission was applied to data evaluation using the DataMax 2.20 software. The probe was calixarene, and aqueous solution of Glycogen was used as the Rayleigh-scattering reference. In order to investigate the interaction of 1a and 1b hosts with 2a-e guests, 10-4 M stock solutions were prepared in alcoholic (methanol, ethanol, n-propanol, and n-butanol) solvents. According to Job’s method, samples were prepared as mixture of the appropriate stock solutions of 1 and 2 with 1:9,...9:1 molar ratios. The PL spectra were recorded in 13 different temperatures within the range from 283 to 307K with 2K steps. The data evaluation method was described earlier.10-16 The equilibrium conformations of calixarene derivatives and their complexes with phenols were studied with B3LYP/DFT/ 6-31++G calculations. The Berny geometry optimization method was used for the investigation of the conformers. The solvents were considered by the PCM (polarizable continuum method). All calculations were performed by the Gaussian 03 program package.28 Results Crystal Structure of 1a and 1b. The molecular structures of 1a and 1b are shown in Figures 3 and 4, respectively. Both in 1a and 1b, solvent molecules of chloroform are present and located at the upper rim of the calixarene skeleton. The bond lengths and angles are in the normal range in both structures,29 and the overall geometry of the calixarene skeleton is in good agreement with similar compounds.15,30-32 The sequence of torsion angles around Ar-CH2-Ar bonds at C7, C14, C21, and C28 are in accordance with pinched cone conformation for 1a (-122.3(9), 60.4(9), -63.4(9), 125.1(9), -120.6(8), 62.2(9), -63.3(9), and 128.2(9)°) and partial cone conformation for 1b (-118.7(4), 70.7(5), -68.4(5), 119.8(4), -68.4(5), -63.5(5), 59.5(5), and 67.4(5)°). 1a possesses a local C2 symmetry; two opposite nitrophenyl groups face each other, their mean planes bent inward (the distance between the ring centroids is 4.68(1) Å, N1 · · · · N3 ) 3.67(1) Å). In contrast to 1a, the two facing nitrophenyl groups in 1b are almost parallel,

J. Phys. Chem. B, Vol. 112, No. 37, 2008 11745

Figure 4. ORTEP view of 1b. Ellipsoids are drawn at 30% probability level. H atoms are omitted for clarity.

and the distance between N atoms is longer (N1 · · · · N3 ) 4.98 Å, the angle between the ring mean planes is 6.52(6)°, and the distance between the ring centroids is 5.16(5) Å). The plane of the alternate ring is almost perpendicular to the plane defined by the C atoms of the CH2 units (the angle between the mean planes is 87.22(8)°). Temperature-Dependent Measurements on the O-Tetraethyl-tetranitro-calix[4]arene (Figure 5). The variable temperature 1H NMR measurements of 1a have shown an extremely high conformational stability of the cone conformer in the temperature range of 30-90 °C. The pair of doublets with a geminal coupling constant of 2J ) 14.0 Hz, assigned to the methylene groups, undoubtedly reflect the cone conformer. The spectra obtained at 30, 60, and 90 °C in DMSO-d6 are almost identical (Figure 5). That is, neither the chemical shifts of the signals nor the line shapes have substantially been changed. Unexpectedly, the chemical shift difference between the characteristic pair of doublets assigned to the benzylic methylene groups has been slightly increased upon increasing temperature. At 30 °C, the equatorial and axial methylene protons are at 4.38 ppm and at 3.67 ppm, respectively (∆δ ) 0.71 ppm). The same protons appeared at 4.44 ppm and 3.66 ppm at 90 °C (∆δ ) 0.78 ppm). Consequently, the system is fixed in the cone conformer and is far from coalescence even at the highest temperature investigated. Host-guest Chemistry of 1a and 1b Investigated by Photoluminescence in Solution. Comparing the PL spectra of stock solutions, no significant differences between the shape of PL spectra of 1a and 1b are observed. As expected, while a slight decrease of the PL spectra at higher temperatures was found, the PL intensity decreased significantly in the presence of guest molecules. Figure 6 shows the typical optical changes observed on the PL spectra of calixarene 1a or 1b under the presence of the parent phenol 2c in the solutions. Considering our earlier results,10,12,16 it is expected that the spectral changes reflect complex formation between the calixarene and phenol molecules. Evaluating the changes observed in the intensity, the thermodynamic parameters were calculated using a combination of the Job’s method and van’t Hoff theory.10-16 Job’s plot of the change observed in PL intensity validates complex formation of 1a calixarene with 2a-2e phenols by 1:1 stoichiometry. Furthermore, as can be seen in Figure 6, no considerable changes were observed in the spectra of 1b calixarene in the presence of 2c phenol. This observation was found to be the same when the effect of other different phenol molecules in the PL spectra of 1b calixarene is studied. Here, we concluded

11746 J. Phys. Chem. B, Vol. 112, No. 37, 2008

Figure 5. (bottom).

1

Kunsa´gi-Ma´te´ et al.

H NMR spectra of O-tetraethyl-tetranitro-calix[4]arene (cone conformer, 1a) at 90 °C (top), at 60 °C (middle), and at 30 °C

Figure 6. PL spectra of calixarene hosts 1a and 1b in the absence and in the presence of 2c phenol.

that very probably the 1b host cannot form stable complexes with 2 phenols. This statement is supported later by quantum chemical investigations. Table 1 summarizes the thermodynamic parameters observed during the interaction of 1a calixarene with 2a-e phenols in different alcoholic solvents, while Figures 7 and 8 show the main tendencies of the change of the thermodynamic parameters as a function of Hammett parameters and solvent permittivity, respectively. Figure 7 shows the free enthalpy, enthalpy, and entropy change, observed during the formation of calixarene-phenol host-guest complexes, as a function of Hammett parameter of the p-substituents of phenols. Figure 2 shows accordingly the effect of the substituents on the electron density of the guest’s aromatic ring. A systematic change of the electron-withdrawing character to electron-releasing character of the p-substituents, results in an increase in the electron density of aromatic ring, and the total charge of the ring changes from slightly positive to negative. However, the free enthalpy changes show significantly more stable 1a-2a and 1a-2b complexes compared to the stability of 1a-2c, 1a-2d, and 1a-2e complexes. Similarly, the entropy changes are significantly different for these two separated groups: the entropy change associated with the

formation of 1a-2a and 1a-2b complexes are nearly the same, while large differences in the formation entropy were obtained in the other three cases. Comparing the formation thermodynamics of 1a-2c, 1a-2d, and 1a-2e complexes, the result shows the opposite tendency of change of enthalpy compared to the change of free enthalpy. It could only be explained by the entropy term. Namely, the entropy term overcompensates the decreased enthalpy changes in those cases when the aromatic rings of the guest have higher electron density. This result fits well with our earlier observation when the calixarene phenol interactions were studied in solvents having high15 or low16 permittivities. The thermodynamic parameters related to the complex formation of 1a with 2a-e were slightly changed with the permittivity of the solvents, while, considering the Hammett parameters of the guest’s substituents, the thermodynamic parameters show the same trend in all solvents (Figure 8). These results show that the decrease of the permittivity of the solvents resulted in higher stability of the complexes formed. By the application of all solvents, significant difference in the formation free enthalpies of 1a-2a and 1a-2b complexes was obtained compared to the formation enthalpies of 1a-2c, 1a-2d, and 1a-2e complexes. It can also be seen in Figure 7 that the free enthalpy changes of the latter group of complexes show higher solvent dependence. To discuss this observation, we assumed a similar situation here as was observed earlier in water15 and carbontetrachloride.16 In both cases, it was found that the decreased entropy change observed during complex formation of calixarenes with phenols can be associated with less reordering of solvent molecules around the calixarene cavity. The reordering of solvent molecules is induced by the guests when they enter into the cavity and induce slight polarization of calixarene’s rings. However, the building blocks of 1a calixarene are electron-deficient aromatics, which have very low polarizability. Therefore, the 2a and 2b phenols, having electron-deficient rings as well, cannot considerably vary the polarizability of the calixarene skeleton. In contrast, when the guest molecules have higher electron densities on their aromatic rings (2c, 2d, and 2e phenols), the polarizability of

Permittivity-Dependent Entropy Driven Complexation

J. Phys. Chem. B, Vol. 112, No. 37, 2008 11747

TABLE 1: Thermodynamic Parameters Obtained during Interactions of 1a Calixarene with p-Substituted Phenols 2a-e p-substituent of the guest phenol

enthalpy kJ/mol

entropy J/Kmol

free enthalpy kJ/mol

log K (298.16 K)

methanol

NO2ClHCH3tBu-

-52.1 (2) -48.1 (2) -35.6 (3) -34.2 (3) -33.6 (3)

-36.3 (2) -35.8 (2) -77.2 (2) -66.3 (2) -62.5 (3)

-41.3 (3) -37.4 (3) -12.6 (3) -14.4 (4) -15.0 (4)

7.24 6.55 2.21 2.52 2.63

ethanol

NO2ClHCH3tBu-

-52.0 (2) -48.1 (2) -37.2 (3) -35.8 (3) -35.1 (3)

-36.1 (2) -35.6 (2) -74.3 (2) -64.2 (3) -59.3 (3)

-41.2 (3) -37.5 (3) -15.0 (3) -16.7 (3) -17.4 (3)

7.22 6.57 2.63 2.93 3.05

n-propanol

NO2ClHCH3tBu-

-52.4 (2) -48.3 (2) -37.9 (3) -36.4 (3) -35.4 (3)

-35.4 (2) -35.2 (2) -71.6 (2) -61.1 (2) -56.8 (3)

-41.8 (3) -37.8 (3) -16.6 (3) -18.2 (3) -18.5 (3)

7.32 6.62 2.91 3.19 3.24

n-butanol

NO2ClHCH3tBu-

-52.6 (2) -48.4 (2) -38.7 (3) -37.2 (3) -35.9 (3)

-35.2 (2) -34.9 (2) -68.2 (2) -58.4 (2) -53.2 (3)

-42.1 (3) -38.0 (3) -18.4 (3) -19.8 (3) -20.0 (3)

7.38 6.66 3.22 3.47 3.50

solvent

the calixarene skeleton can be increased due to the charge transfer interactions between the rings of the calixarene and the guest molecule. Furthermore, the electrostatic (high-permittivity solvents) or dispersive (low permittivity solvents) forces make an order

on the solvent molecules closest to the solute. Since the electrostatic forces are stronger than the dispersive ones, weaker ordering can be observed in solvents having lower permittivity. In a parallel effect, the shift of the emission spectra is also weaker due to the smaller change induced in the electric field when the permittivity of the solvent is lower. In addition, the weaker forces result in slower motion of solvent molecules. This property can be observed in the solvent relaxation times measured by the optical shift of the emission spectra of the solute in the time domain.16 The solvent relaxation time of the 1a calixarene is found as 234 ps. This value does not show considerable change when 1a-2a or 1a-2b complexes are formed. It is probably due to the very low polarizability of the calixarene building blocks as described above. In contrast, the solvent relaxation times observed for the other complexes vary from 327 ps (1a-2c complexes in methanol) to 466 ps (1a-2e complexes in n-butanol). Figure 9 shows the correlation between the solvent relaxation times and the entropy changes of complex forma-

Figure 7. Change of the thermodynamic parameters during the complex formation of 1a with 2a-e obtained in methanol.

Figure 8. Free enthalpy change during the complex formation of 1a with 2a-e obtained in different alcoholic solvents.

Figure 9. Correlation of the entropy change (open symbols, right scale) with the solvent relaxation times (solid symbols, left scale), associated with the complex formation of 1a with 2c-e. Data are plotted against the permittivity of the solvents. Precisions of entropies are given in Table 1, while the accuracy of the solvent relaxation times is within (15 ps.

11748 J. Phys. Chem. B, Vol. 112, No. 37, 2008 tion. The close correlation supports our earlier idea15,16 stating that the unexpected entropy change observed during the complexation of calixarenes with phenols is originated from the reorientation of the solvent molecules during complex formation. DFT Calculations on the Host-Guest Complexes of 1a and 1b. To clarify the relevant conformations of the formed complexes, DFT/6-31++G calculations were performed where the solvent effect is considered by the PCM (polarizable continuum method). These calculations revealed that the complex formation of 1a with 2a and 2b is preferably based on Coulomb interactions between the electron-rich nitro-groups and the electron-deficient aromatic rings of calixarenes and those of the 2a and 2b guests. Accordingly, the most stable conformations of such complexes are when the nitro or chloro substituents of the p-substituted phenols enters deeply into the calixarene cavity. However, in the cases of 2c-e guests, the phenolic OH group enters first into the cavity; therefore, the interaction between the calixarene and the guest is preferably based on the π-π interaction between the aromatic rings of the host and guest. However, this π-π interaction is moderated by the Coulomb interaction of the electron-rich nitro group of the host and the slightly electron-deficient H, Me, and tBu groups of 2c-e guests. Because of the known limitation of the PCM method regarding the protic solvents, the calculations were repeated using the TIP3P method for explicit consideration of the solvent molecules. The Hamiltonian was calculated with the AM1 (Austin model) method. The results of these calculations show conformations similar to those derived by applying the PCM. Furthermore, this calculation validates the desolvation of the nitro and chloro substituents of the phenol guest molecules, supporting the low entropy changes associated with the complex formation of 1a calixarene with the 2a and 2b guest molecules. Both the experimental and theoretical investigations revealed that no considerable interaction exists between phenols and 1b. It is probably due to the locking of the calixarene cavity by the bent O-ethyl chain. Furthermore, on the basis of these results, any further interaction of phenols with 1a can be excluded. Conclusions The formation of cone and partial cone (paco) conformers of tetranitro-calix[4]arene derivative with different parasubstituted phenols were investigated by photoluminescence (PL) measurements and quantum-chemical (QC) investigations in different alcoholic solutions. The results show that cone calixarene forms stable inclusion complexes with all phenol molecules from the p-substituted series, while no considerable outer complexation of phenols with calixarene was observed. Both the experimental and theoretical investigations revealed that no considerable interaction exists between phenols and 1b, a property probably due to the locking of the calixarene cavity by the bent O-ethyl chain. The free enthalpy changes show very high complex stability of cone calixarene with 2a and 2b phenols. In the cases of 2c, 2d, and 2e guest compounds, the stability is significantly lower; however, it slightly increases with increasing electron density on the aromatic ring. The thermodynamic parameters are slightly changed with the permittivity of the solvents, while the trends of the thermodynamic parameters, observed as a function of the Hammett parameters of the substituents, remained substantially unchanged in different solvents. The good correlation obtained between the entropy changes and the solvent relaxation times supports the fact that

Kunsa´gi-Ma´te´ et al. the entropy change is probably due to the reorientation of solvent molecules during complex formation. Acknowledgment. S.K.-M. wishes to thank the Hungarian Academy of Sciences for a Ja´nos Bolyai Fellowship. Z.C. thanks the Fundacao para a Ciencia e a Tecnologia (FCT) for a postdoctoral grant (SFRH/BPD/20663/2004). The financial support of the European Union-Hungarian National Development Program (Grant GVOP-3.2.1-2004-04-0200/3.0) is highly appreciated. Thanks are also due to the Centro Interdipartimentale di Metodologie Chimico Fisiche (CIMCF) of University of Naples “Federico II” for the use of the X-ray equipment. References and Notes (1) Gutsche, C. D. Calixarenes ReVisited: Monographs in Supramolecular Chemistry; Stoddart, J. F., Ed.; Royal Society of Chemistry: London, 1998. (2) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713–745. (3) Kuwabara, T.; Nakajima, H.; Nanasawa, M.; Ueno, A. Anal. Chem. 1999, 71, 2844–2849. (4) Beer, P. D.; Gale, P. A.; Chen, G. Z. Coord. Chem. ReV. 1999, 186, 3–18. (5) Mohammed-Ziegler, I. Spectrochim. Acta A 2003, 59, 19–27. (6) Calixarenes 2001; Asfari, Z.; Bo¨hmer, V.; Harrowfield, J.; Vicens. J., Eds., Kluwer Academic: Dordrecht, The Netherlands, 2001. (7) Mohammed-Ziegler, I.; Billes, F. J. Incl. Phenom. Macrocycl. Chem. 2007, 58, 19–42. (8) Kim, J. S.; Quang, D. T. Chem. ReV. 2007, 107, 3780–3799. (9) Buie, N. M.; Talanov, V. S.; Butcher, R. J.; Talanova, G. G. Inorg. Chem. 2008, 47, 3549–3558. (10) Kunsa´gi-Ma´te´, S.; Nagy, G.; Kolla´r, L. Anal. Chim. Acta 2001, 428, 301–307. (11) Kunsa´gi-Ma´te´, S.; Nagy, G.; Kolla´r, L. Sens. Actuators, B 2001, 76, 545–550. (12) Kunsa´gi-Ma´te´, S.; Bitter, I.; Gru¨n, A.; Nagy, G.; Kolla´r, L. Anal. Chim. Acta 2001, 443, 227–234. (13) Kunsa´gi-Ma´te´, S.; Bitter, I.; Gru¨n, A.; Nagy, G.; Kolla´r, L. Anal. Chim. Acta 2002, 461, 273–279. (14) Kunsa´gi-Ma´te´, S.; Szabo´, K.; Lemli, B.; Bitter, I.; Nagy, G.; Kolla´r, L. J. Phys. Chem. A 2005, 109, 5237–5242. (15) Kunsa´gi-Ma´te´, S.; Szabo´, K.; Lemli, B.; Bitter, I.; Nagy, G.; Kolla´r, L. J. Phys. Chem. B 2004, 108, 15519–15522. (16) Kunsa´gi-Ma´te´, S.; Szabo´, K.; Desbat, B.; Brunneel, J. L.; Bitter, I.; Kolla´r, L. J. Phys. Chem. B 2007, 111, 7218–7223. (17) Moodie, R. B.; Schofield, K. Acc. Chem. Res. 1976, 9, 287–292. (18) Verboom, W.; Durie, A.; Egberink, R.; J, M.; Asfari, Z.; Reinhoudt, D. N. J. Org. Chem. 1992, 57, 1313–1316. (19) Mogck, O.; Böhmer, V.; Ferguson, G.; Vogt, W. J. Chem. Soc., Perkin Trans. 1 1996, 1711–1715. (20) Kumar, S.; Chawla, H. M.; Varadarajan, R. Synth. Commun. 2001, 31, 775–779. (21) Kenis, P. J. A.; Noordman, O. F. J.; Houbrechts, S.; van Hummel, G. J.; Harkema, S.; van Veggel, F. C. J. M.; Clays, K.; Engbersen, J. F. J.; Persoons, A.; van Hulst, N. F.; Reinhoudt, D. J. Am. Chem. Soc. 1998, 120, 7875–7883. (22) Jakobi, R. A.; Böhmer, V.; Gruttner, C.; Kraft, D.; Vogt, W. New J. Chem. 1996, 20, 493–501. (23) Kumar, S.; Varadarajan, R.; Chawla, H. M.; Hundal, G.; Hundal, M. S. Tetrahedron 2004, 60, 1001–1005. (24) Gutsche, C. D.; Dhawan, B.; Levine, J. A.; Hyun No, K.; Bauer, L. J. Tetrahedron 1983, 39, 409–426. (25) Sharma, S. K.; Gutsche, C. D. J. Org. Chem. 1996, 61, 2564– 2568. (26) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119. (27) Sheldrick, G. M. SHELX-97; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,

Permittivity-Dependent Entropy Driven Complexation S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford CT, 2004. (29) Allen, F. H. Acta Crystallogr. 2002, 58B, 380–388.

J. Phys. Chem. B, Vol. 112, No. 37, 2008 11749 (30) Klimentova, J.; Vojtisek, P.; Sklenarova, M. J. Mol. Struct. 2007, 871, 33–41. (31) Vysotsky, M. O.; Böhmer, V.; Bru Roig, M.; Bolte, M. Acta Crystallogr. 2005, E61, o3526–o3528. (32) Lu, G.; Song, W.; Wan, X. J. Chem. Cryst. 2000, 30, 185– 188.

JP803498V