Spectroscopic and Theoretical Insights into the Origin of Fullerene

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Spectroscopic and Theoretical Insights into the Origin of Fullerene-Calix[4]pyrrole Interaction Debabrata Pal,† Dibakar Goswami,‡ Sandip K. Nayak,‡ Subrata Chattopadhyay,‡ and Sumanta Bhattacharya*,† Department of Chemistry, The UniVersity of Burdwan, Golapbag, Burdwan-713 104, India, and Bio-Organic DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India ReceiVed: NoVember 13, 2009; ReVised Manuscript ReceiVed: May 11, 2010

The present paper reports, for the first time, supramolecular interaction of meso-octamethyl calix[4]pyrrole (1) with fullerenes C60 and C70 in solutions having varying polarity (e.g., toluene, 1,2-dichlorobenzene and benzonitrile and chloroform). The interaction is facilitated through charge transfer (CT) transition as evidenced from well-defined CT absorption bands in the visible region of absorption spectroscopy. Utilizing the CT transition energy for the complexes of 1 with various electron acceptors, we have determined the ionization potential of 1. Estimation of degrees of CT, oscillator, and transition dipole strengths suggest that the complexes are almost of neutral character in ground state. Higher magnitude of electronic coupling element value for the C70-1 complex compared to C60-1 indicates strong binding between C70 and 1. Binding constants (K) of the fullerene-1 complexes have been determined from UV-vis investigations, which indicate high selectivity of 1 toward C70. Extraordinary large K value of the C70-1 complex in chloroform medium (K ∼ 1.43 × 106 dm3 · mol-1) establishes that a polar environment facilitates such interaction. Both proton NMR and liquid IR studies provide very good support in favor of strong binding between C70 and 1. 13C NMR study proves that C70 binds 1 with its equatorial belt, which substantiates the role of π-π interaction behind such strong interaction (i.e., high K value). Semiempirical theoretical calculations at the third parametric level (PM3) explore the stability difference between C60- and C70-1 complexes. PM3 calculations also reveal that approach of C70 toward 1 is directed in side-on manner rather than in a conventional end-on alignment. 1. Introduction Among representatives of a class of compounds that are the subject of supramolecular chemistry, calixpyrroles are gaining increasing interest as new macrocyclic receptor molecules.1 Pioneering work in this area by Sessler and co-workers has evidenced that calix[4]pyrroles can effectively be employed as suitable anion binding agents and may be used for the detection and sensing purposes in new anion separation technologies.2-8 Their appeal is furthered by the availability of various postmacrocyclization modifications.9,10 Anion binding properties of the calix[4]pyrroles depend on H bonding interactions between the pyrrole NH groups and the analyte anion. Also, the inherent flexibility of the porphyrinogen skeleton allows the calix[4]pyrroles to exist in conformations somewhat analogous with the calixarenes. Although a major shortcoming of using the calix[4]pyrrole is the difficulty involved in detecting the absorption spectra; the bands fall at energies that are too high to make them helpful in the binding process.11 Hence, various research groups have created functionalized calixpyrroles in which different varieties of electro-active species have been attached to the calix[4]pyrrole rim to utilize them for electrochemical anion recognition.12-17 Construction of supramolecular architectures involving electrondeficient fullerenes18,19 and various host compound such as calix[n]arene,20-23 crown ether,24,25 porphyrin,26-35 and phthalocyanine,36-40 is not only a topic of current interestsstudies along these lines have led to some significant practical applications.41-45 * To whom correspondence should be addressed. E-mail: sum_9974@ rediffmail.com. Fax: +91-342-2530452. † The University of Burdwan. ‡ Bhabha Atomic Research Centre.

They have been widely used in crystal engineering,41,42 in the synthesis of novel nanostructures,43,44 and in the development of host-guest chemistry for the purification of fullerenes.45 Photophysical and electrochemical studies have revealed that C60, C70, and their derivatives are excellent electron acceptors.46,47 Photoinduced electron transfer processes in fullerene-based diads, triads, and tetrads have been investigated by different research groups48-51 with the aim of designing light-harvesting systems. Through these studies, it has been established that large curved molecules such as fullerenes form strong noncovalent complexes with all of the above-mentioned hosts, whereby fullerene can reside endo- and/exo-relative to the cavity of the hosts, and this modulates the van der Waals connectivity and spatial interplay of the fullerenes. Thus size selectivity of the hosts plays a very significant role during host-guest complexation. In spite of several appealing properties of fullerenes and calix[4]pyrrole, chemists do not pay any great attention to the elucidation of their binding affinities toward each other. However, recently, Guldi and his collaborators have shown that treatment of a tetrathiafulvalene-functionalized calix[4]pyrrole 2 with chloride anions in CH2Cl2 produces bowl-like receptor 2•Cl-, which is able to encapsulate C60 in a 2:1 barrel-like manner.52 This group has also demonstrated the binding of the snake-like trinitrodicyanomethylenefluorene-C60 derivative (TNDCF-C60) to the dynamic receptor, tetrathiafulvalene calix[4]pyrrole (TTF-calix[4]pyrrole), which may be controlled via the use of a chloride anion as an external trigger.53 The purpose of the present investigations is to examine the nature of the complexation pattern and binding strength between mesooctamethyl calix[4]pyrrole (1, Figure 1) and fullerenes (C60 and

10.1021/jp910809s  2010 American Chemical Society Published on Web 06/04/2010

Origin of Fullerene-Calix[4]pyrrole Interaction

J. Phys. Chem. A, Vol. 114, No. 25, 2010 6777

Figure 1. Structure of 1.

Figure 3. Gaussian analysis plot of the C60-1 system recorded in chloroform.

Figure 2. CT absorption spectra of (a) C60 (8.30 × 10-6 mol · dm-3) + 1 (5.20 × 10-3 mol · dm-3), (b) DDQ (2.70 × 10-4 mol · dm-3) + 1 (1.25 × 10-3 mol · dm-3), (c) TCNE (3.90 × 10-4 mol · dm-3) + 1 (1.35 × 10-3 mol · dm-3), (d) C70 (1.43 × 10-5 mol · dm-3) + 1 (6.42 × 10-3 mol · dm-3), (e) uncomplexed 1, (f) uncomplexed C60 (8.30 × 10-6 mol · dm-3), and (g) uncomplexed C70 (1.43 × 10-5 mol · dm-3) recorded in chloroform medium.

C70) in solvents having varying polarity, viz., toluene, 1,2dichlorobenzene, benzonitrile and chloroform, employing various spectroscopic tools. Semiempirical calculations at the third parametric level (PM3) have explored the orientation of the host-guest complex as well as the stability difference between the C60 and C70 complexes of 1. 2. Materials and Methods Both C60 and C70 were purchased from Aldrich, USA. 1 was collected from Aldrich, USA. p-Chloranil was obtained from Fluka and purified by sublimation just before use. 2,3-Dichloro5,6-dicyano-p-benzoquinone (DDQ) was collected from Sigma and used without further purification. Tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) were purchased from Aldrich, USA. UV-vis spectroscopic grade toluene (Merck, Germany) and chloroform (Spectrochem, India) were used as solvent to favor noncovalent interaction between fullerene and 1. Toluene-d8 was collected from Aldrich, USA. UV-vis spectral measurements were performed on a Shimadzu UV-1601 model spectrophotometer fitted with a TB-85 Peltier controlled thermobath using a quartz cell with a 1 cm optical path length. Proton NMR spectra were recorded with a Bruker 200 MHz NMR spectrometer. IR spectra were recorded with a JASCO FT/IR-5300 model spectrometer. Theoretical calculations were done with a Pentium IV computer using SPARTAN’06 V1.1.0 Windows version software. 3. Results and Discussion 3.1. Observation of Charge Transfer (CT) Absorption Bands. Figure 2 shows the electronic absorption spectra of CHCl3 solution of 1 with C60, DDQ, TCNE, and C70. Spectra of the above solutions have been recorded against the pristine

acceptor solution as reference to find out the existence of CT bands in the visible region. Since more concentrated solution of 1 compared to acceptor is described to detect the CT absorption bands, the absorption spectra are measured in the concentration range of 1.2 × 10-3 and 2.7 × 10-4 to 8.3 × 10-6 mol · dm-3 for 1 and acceptors, respectively. In Figure 2a-d, the CT peaks are identified to be the peaks in the region of 350-525 nm because they normally have the longest wavelength among the peaks, different from those obtained from the spectra of the components. Similar spectral features are obtained with mixtures of 1 with all of the electron acceptors in toluene medium (Figure 1S, Supporting Information). However, we have not observed any CT peak for C60-1 and C70-1 systems recorded in other solvents, such as toluene, 1,2dichlorobenzene, and benzonitrile. This is because, even though fullerenes and 1 undergo CT interaction with great ease, one may still argue that the way they approach each other is dictated by packing consideration rather than by the special affinity we have highlighted above. The choice of a polar solvent undoubtedly promotes the aggregation of the hydrophobic fullerene and 1 entities, but this may be viewed as an artificial enhancement of their affinity. The demonstration of fullerene-1 binding in chloroform is therefore considered to be a particularly stringent test of the spontaneous attraction. The CT absorption spectra are analyzed by fitting to the Gaussian function y ) y0 + [A/(w(π/2)]exp[-2(x - xc)2/w2], where x and y denote wavenumber and molar extinction coefficient, respectively. One typical Gaussian analysis plot is shown in Figure 3. The wavelengths at these new absorption maxima (hνmax ) xc) and the corresponding transition energies (hν) are summarized in Tables 1 and 2. The Gaussian analysis fitting is done in accordance with the method developed by Gould et al.54 One important point to mention here is that Gaussian analysis of a curve generally gives a decent result near the maxima of the curve spread over a very small region. For this reason, although the errors in the center of the CT spectra for the complexes of 1 with various electron acceptors are very small, there are appreciable errors in the y0 value. 3.2. Determination of Vertical Ionization Potential (IDv) of 1. For complexes with neutral ground state, a CT band corresponds to a transfer of an electron from a donor (D) to an acceptor (A) with the absorption of a quantum. The relationship between the energy (hνCT) of the lowest energy intermolecular

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TABLE 1: Various Physicochemical Parameters of the Complexes of 1 with Different Electron Acceptors Including C60 and C70 in Toluene at 298 K, CT Absorption Maxima (λCT), CT Transition Energy (hνCT), Oscillator Strength (f), Degrees of Charge Transfer (r), Transition Dipole Strength (µEN), Resonance Energy (RN), and Vertical Ionization Potential (IDv) of 1 system C60-1 C70-1 TCNE-1 TCNQ-1 p-chloranil-1 DDQ-1

λCT, nm

hνCT, eV

R × 103

f × 104

µEN, Debye

RN

IDv of 1, eV 9.15

528.0 515.5 303.5 448.0

2.35 2.40 4.10 2.77

2.52 1.12 4.43 5.0

0.29 0.11 30.1 3.4

0.728 0.295 1.179 0.146

6.7 × 10-4 1.0 × 10-3 1.8 × 10-4 1.9 × 10-4

TABLE 2: Various Physicochemical Parameters of the Complexes of 1 with Different Electron Acceptors Including C60 and C70 in Chloroform at 298 K, CT Absorption Maxima (λCT), CT Transition Energy (hνCT), Oscillator Strength (f), Degrees of Charge Transfer (r), Transition Dipole Strength (µEN) and Resonance Energy (RN) system

λCT, nm

hνCT, eV

R × 103

f × 104

µEN, Debye

RN

IDv of 1, eV

C60-1 C70-1 TCNE-1 TCNQ-1 p-chloranil-1 DDQ-1

447.5 452.3 461.0 471.0 453.0 483.5

2.77 2.75 2.70 2.63 2.75 2.55

6.65 6.30 4.0 5.45 3.2 3.25

7.65 14.95 2.95 6.0 2.4 4.6

0.348 0.317 0.391 0.544 0.237 0.521

1.1 × 10-3 2.6 × 10-3 1.8 × 10-3 4.8 × 10-4 8.2 × 10-4 9.5 × 10-4

7.84

CT band and the vertical ionization potential (IDv) of the donor for a series of complexes with a common donor species has been the source of much discussion. According to Mulliken’s theory,55 CT transition energies in these complexes are related to vertical ionization potential of the donor by the relation

hνCT ) (IDv - C1) + {C2 /(IDv - C1)}

(1)

C1 ) EAv + G0 + G1

(2)

where EAv is the vertical electron affinity of the acceptor, G0 is the sum of the several energy terms (such as dipole-dipole, van der Waals interaction, etc.) in the “no-bond” state, and G1 is the sum of number of energy terms in the “dative” state. In most cases, G0 is small and can be neglected, while G1 is largely the electrostatic energy of attraction between D(1-δ)+ and A(1-δ)-. The term C2 in eq 1 is related to the resonance energy of interaction between the no-bond and dative forms in the ground and excited states, and for a given donor species, it may be supposed to be constant.55 A rearrangement of eq 1 yields

2C1 + hνCT ) {C1(C1 + hνCT)/IDv} + {(C2 /IDv) + IDv} (3) The vertical electron affinities of C60, C70, p-chloranil, DDQ, TCNE, and TCNQ are collected from the literature.56-60 Neglecting G0 and taking the typical donor-acceptor distance in π-type EDA complexes to be 3.5 Å, we have estimated the major part of G1 to be e2/4πε0r ) 4.13 eV. Using these values, C1 is obtained from eq 2 for each of the acceptors. A plot of 2C1 + hνCT versus C1(C1 + hνCT) for a given donor and various acceptors yields a slope of 1/IDv, from which the value of IBv is determined for 1. The following linear regression equation had been obtained with the present data:

2C1 + hνCT ) (0.1092 ( 0.0050)C1(C1 + hνCT) + (9.2857 ( 0.3340) in toluene (4)

2C1 + hνCT ) (0.1275 ( 0.0027)C1(C1 + hνCT) + (8.0210 ( 0.1715) in chloroform (5) 3.3. Degrees of CT (r) for the Fullerene Complexes of 1. In the Mulliken two-state model,55 the ground state (ψg) and excited state (ψex) wave functions of the CT complexes are described by a linear combination of dative ψ(D0, A0) and ionic ψ(D+, A-) states

ψg ) {√(1 - R)}ψ(D0, A0) + (√R)ψ(D+, A-)

(6)

ψex ) {√(1 - R)}ψ(D+, A-) - (√R)ψ(D0, A0)

(7)

where R is the degree of charge transfer. The function ψ(D+, A-) differs from ψ(D0, A0) by the promotion of an electron from the donor to the acceptor; R is given by55

R ) (C2 /2)/[(IDv - EAv + C1)2 + (C2 /2)]

(8)

The values of R (calculated by using eq 8 and given in Tables 1 and 2) indicates that an appreciable amount of ground state CT took place between C70 and 1 in chloroform. The dependence of R on electron affinity of the acceptors is shown in Figure 4. An excellent parabolic relationship has been obtained in the present case.61 3.4. Determination of Oscillator (f) and Transition Dipole Strengths (µEN). From the CT absorption spectra, we can enumerate the magnitude of the oscillator strength. The oscillator strength f was estimated using the formula

f ) 4.32 × 10-9

∫ εCTdν

(9)

where the term ∫εCTdν is the area under the curve of the extinction coefficient of the absorption band in question versus frequency. To a first approximation

f ) 4.32 × 10-9εmax∆ν1/2

(10)



Figure 4. Variation of R vs EAv of various electron acceptors recorded in chloroform medium.

where εmax is the maximum extinction coefficient of the band and ∆ν1/2 is the half-width, that is, the width of the band at half the maximum extinction. The observed oscillator strengths of the CT bands are summarized in Tables 1 and 2. It is worth mentioning that we need a proper calculation of oscillator strengths of fullerene-1 CT complexes. This is because oscillator strength is very much sensitive to the molecular configuration and the electron charge distribution in the CT complex. In the C60-1 and C70-1 complexes, we could not use a simple model assuming a charge localized at a certain site of the fullerene sphere because π-bonds of the fullerenes are directed radially with a node on the molecular cage. Values of f demonstrate that the complex containing C70 shows a slightly larger degree of CT and greater interaction energies than that of C60. The extinction coefficient is related to the transition dipole by

µEN ) 0.0952[εmax∆ν1/2 /∆ν]

(11)

where ∆ν ) ν at εmax and µEN is defined as -e∫Ψex∑iriΨgdτ. The transition dipole strengths (µEN) for the complexes of 1 with various electron acceptors are given in Tables 1 and 2. It has been observed that the value of µEN for the C70-1 complex is somewhat higher compared to that of C60-1 complex. This trend in µEN is in conformity with the fact that C70 has a higher electron affinity value than C60,62 but it should be mentioned at this point that the difference in electron susceptibilities of C60 and C70 are very small. The observed trend in µEN values of the fullerene-1 complexes coincide fairly well with the binding constant (K) values of the respective complexes, which will be discussed in section 3.6. 3.5. Determination of Resonance Energy (RN). Briegleb and Czekalla63 theoretically derives the relation

εmax ) 7.7 × 104[hνCT /|RN | - 3.5]

(12)

where εmax is the molar extinction coefficient of the complex at the maximum of the CT absorption, hνCT is the frequency of the CT peak, and RN is the resonance energy of the complex in the ground state. RN values for the complexes of 1 with various electron acceptors are summarized in Tables 1 and 2.

Figure 5. UV-vis titration curve of (i) C60-1 and (ii) C70-1 systems recorded in chloroform medium: (i) curve a, uncomplexed 1 (3.92 × 10-5 mol · dm-3); curve b, uncomplexed C60 (1.36 × 10-6 mol · dm-3), and curves c-j, 5.984 × 10-7, 1.197 × 10-6, 2.992 × 10-6, 3.59 × 10-6, 5.386 × 10-6, 5.984 × 10-6, 6.582 × 10-6, and 7.779 × 10-6 mol · dm-3; (ii) curve a, uncomplexed 1 (3.80 × 10-5 mol · dm-3); curve b, uncomplexed C70 (1.10 × 10-6 mol · dm-3), and curves c-i, 4.40 × 10-7, 8.80 × 10-7, 1.32 × 10-6, 1.75 × 10-6, 2.20 × 10-6, 2.65 × 10-6, and 3.080 × 10-6 mol · dm-3.

3.6. Spectrophotometric Study of Formation Equilibria of the Complexes of 1 with C60 and C70. For the determination of the binding constant (K) of fullerene-1 complexes, we have employed UV-vis spectroscopic measurements. It is observed that the intensity in the visible portion of the absorption band of the C60 + 1 and C70 + 1 mixtures, measured against the fullerene (i.e., C60 and C70) solution of same concentration as reference, increases systematically with the gradual addition of 1. Two typical UV-vis titration plots for the C60-1 and C70-1 systems in chloroform medium are shown in Figure 5. Thus, it is definitely established in this work that the systematic increase in intensity of the broad 400-700 nm absorption band (resulting from a forbidden singlet-singlet transition in C60 and C7064,65) is due to molecular complex formation between fullerenes and 1. Job’s method of continuous variation method establishes 1:1 molecular complexes between fullerenes and 1. One typical Job’s plot of continuation variation for the C70-1 system in toluene is demonstrated in Figure 6. K values of the C60-1 and C70-1 complexes are determined in toluene, 1,2-dichlorobenzene and benzonitrile chloroform using the Benesi-Hildebrand (BH)66 equation. It should be mentioned at this point that the binding constant values calculated using the BH model are only an approximation. Sibley et al.67 have also estimated the values of K for the complexes of C60 with aniline and substituted anilines using the BH equation. In all of the cases studied, very good linear plots are obtained. One typical BH plot of the C60-1 system in chloroform is shown in Figure 7. BH plots of various

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Pal et al. between these species, which we do not observe in toluene. Other than this, this unexpectedly close fullerene-1 contact can be ascribed to be due to an electrostatic attraction of the electrondeficient 6:5 ring-juncture bond of the fullerene to the N-H centre of the calix[4]pyrrole. This highlights the importance of an electrostatic component augmenting the π-π interaction. Thus, a polar medium, such as chloroform, facilitates the CT interaction between fullerenes (C60 and C70) and calix[4]pyrrole. Another interesting feature of the present investigation is that 1 binds C70 in preference of C60. The average selectivity of C70 over C60 is found to be ∼5.0 (Table 3). Highest value of selectivity in the binding constant of C70 over C60 (i.e., KC70/ KC60) takes place in 1,2-dichlorobenzene and is least observed in benzonitrile. Thus, we can find a straightforward relationship between selectivity and solubility in our present investigations as fullerenes are highly soluble in 1,2-dichlorobenzene and least in benzonitrile.68,69 Higher magnitude K values for the C70-1 complex compared to the C60-1 complex in chloroform can also be explained in terms of oscillator strength for such complexes. For the C70-1 system, strong oscillator strengths have been reported in toluene (fC70-1 ) 1.50 × 10-3) and in chloroform (fC70-1 ) 7.65 × 10-4). These values may be considered to calibrate the current systems, as far as the donor (1)-acceptor (fullerene) interaction is considered. We have provided the corrected value of molar extinction coefficient (ε′) for all of the fullerene-1 complexes in Table 4. It is observed that, in all of the solvents studied, the supramolecular complex of C70 with 1 exhibits higher value of ε′ compared to the C60-1 system. Greater magnitude of ε′ in chloroform medium validates the higher value of K as obtained in our present investigations. The present selectivity is found to be comparable to calixarene bisporphyrin (∼4.3)70 and even larger than those of azacalix[m]arene[n]pyridine (∼1.9)71 and cyclotriveratrylenophane (∼2.5)72 hosts. However, the present selectivity is lower than those observed for that of bridged calix[5]arene (∼10.2),73 carbon nanoring (∼16),74 cyclic dimers of Zn-porphyrins (∼25.5),75 and H2-diporphyrin (∼32).76 From the above observations, it can be said that such a simple molecule as 1 is found to exhibit relatively high selectivity towards C70. Practical applications of such high selectivity are observed in the preferential precipitation of C70 over C60 with a host molecule such as p-halohomooxacalix[3]arenes.77 The larger value of K for the C70 complex of 1 compared to C60 is commonly observed for macrocyclic receptors such as calixarene78 and porphyrin.79,80 Another important point is that in the covalently linked systems, in which a flexible linker allows configurational freedom, this is not observed in the current works in terms of donor-acceptor approach. This clearly indicates that rigidity of the calix[4]pyrrole unit in 1 is very much responsible for producing high values of K with the fullerenes in both toluene and chloroform. In prior work of Guldi and collabo-

Figure 6. Job plot of continuous variation method for C70-1 system done in toluene medium.

Figure 7. BH plot of C60-1 system estimated in chloroform medium.

fullerene-1 systems in 1,2-dichlorobenzene and benzonitrile are shown as Figures 2S-5S (Supporting Information). Binding constants for the complexes of 1 with C60 and C70 (determined from the BH plots) in four different solvents are summarized in Table 3. Table 3 shows that values of K are exceptionally high, considering the type of interactions. The conjugate effect of the π-electrons of the calix[4]pyrrole rim would probably increase the electron density of the endo-surface of 1 and, therefore, enhance the binding affinity toward the curved π-surfaces of fullerenes. It is also observed that the K values of fullerene-1 complexes in chloroform medium are found to exhibit much higher value than that observed in toluene. The additional intermolecular interaction between fullerenes and 1 in chloroform may be viewed as an enhanced CT interaction

TABLE 3: Binding Constants (K) of the C60- and C70-1 Complexes Measured in Various Solvents at 298 K along with the Selectivity in K of C70 over C60 (KC70/KC60) and Heat of Formation (∆Hf0) Values of the Same Complexes Obtained by PM3 and Ab Initio Calculations Done In Vacuo K, dm3 · mol-1 system

toluene

C60-1

43700

303725

84980

153400

C70-1

247350

1430335

586650

479150

KC70/KC60

5.65

chloroform

4.70

1,2-dichlorobenzene

6.90

benzonitrile

3.10

∆Hf0, kJ · mol-1 -5.77 (N side) 8.47 (methyl side) -4.55 (N side, end-on) -5.82 (N side, side-on) 9.03 (methyl side, end-on) 8.55 (methyl side, side-on)



TABLE 4: Corrected Molar Extinction Coefficient (ε′) of the Fullerene Complexes of 1 in Various Solvents at 298 K ε′, dm3 · mol-1 · cm-1 solvent

C60-1

C70-1

toluene chloroform 1,2-dichlorobenzene benzonitrile

405 935 175 30

1070 1450 840 200

rators,52,53 it is found that the cone conformation of calix[4]pyrrole binds C60 appreciably. Therefore, a good competition between species such as TCNQ would be expected in native, 1,3-alternate form, which is the target of the present investigations. For this reason, we have performed the UV-vis titration experiment on a TCNQ-1 complexation process in toluene medium (Figure 6S, Supporting Information). A K value of only 3690 dm3 · mol-1 is obtained for the 1:1 complex of TCNQ with 1 having an excellent correlation coefficient of 0.99 (Figure 7S). The estimated binding constant value for the TCNQ-1 system is much lower in comparison to those reported for fullerene-1 systems in the same solvent and also in chloroform. From this observation, we may infer that quite possibly chloroform, usually contaminated with a bit of hydrochloric acid, provides a source of chloride; this changes the conformation of calix[4]pyrrole to “cone” from its native “1,3-alternate” position, resulting in a high value of K. It is well-known that the stability of the supramolecular complexes depends upon attractive interactions between host and guest and on solvation of the binding partners.81 In the present investigations, since the host and guest are separately solvated in solution, the desolvation of the host and guest is requisite in the association process. However, the process of desolvation is energetically an uphill task, and hence, an association takes place only when the energy gain between the host and guest interaction exceeds this unfavorable energy. From the trends in the K values of the fullerene-1 complexes, we may infer that the extent of solvation and desolvation of the binding partners plays an important role in forming weak or strong supramolecular complexes. Haino et al.82 show that the stability of the complex increases as the solubility of fullerene in a solvent decreases because less energy is required for the desolvation of fullerene, which must necessarily precede before its complexation with a host molecule. This would be one of the reasons for the higher K values of the C60 and C70 complexes in chloroform because the solubility of the fullerenes is higher in toluene than that in chloroform.83 3.7. Computational Studies. In order to gain insight into the preferred molecular geometry and to measure the heat of formation (∆Hf0), a detailed conformational analysis of the individual components as well as the fullerene-1 adducts has been performed in vacuo. The geometric parameters of the complexes are obtained after complete energy minimization. Figure 8 shows the space-filling models of the C60 and C70 complexes of 1, optimized by semiempirical PM3 calculations. The distances between the -NH atoms of 1 and the nearest carbon atom of the C60 and C70 spheroids are computed to be 3.153 and 3.106 Å, respectively. These data validate the presence of CT as well as van der Waals interaction between the acceptor (i.e., fullerene) and donor (i.e., 1). The most important feature of all of these structures is their unexpectedly close fullerene-1 contact, which is manifested in the C-H · · · π interaction between the fullerene carbon atom and the H atom of the methyl groups of 1. As shown in Figure 8b, C70 is positioned over the calix[4]pyrrole rim in end-on

Figure 8. Space-filling models of (a) C60-1, (b) C70-1 (in end-on orientation of C70), and (c) C70-1 (in side-on orientation of C70) systems, done in vacuo by semiempirical PM3 calculations.

orientation, which allows the electron-deficient pole region of C70 to come close to the H atom of the (CH3)2C unit of 1. The above distances are found to be observed in the range between 2.548 and 2.558 Å. In the case of C60-1 complex, however, the estimated C-H distance is found to be 2.562 Å only (Figure 8a). The larger distance in the latter case is due to weaker interaction between 1 and C60, which is also reflected in the trend of the K values as reported in Table 3. These findings indicate that C70-1 complex is stabilized more compared to C60-1 complex in terms of C-H · · · π interactions. The side-on approach of C70 towards 1 can also be established from the fact that the 6:5 ring juncture of C70, that is, the electron-deficient region, is positioned very close (2.529 Å) to

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TABLE 5: Electronic Coupling Element (V) and Solvent Reorganization Energies (RS) of the Fullerene-1 Systems in Various Solvents Having Varying Polarity at 298 K V, cm-1 system

toluene, 1,2-dichlorobenzene, benzonitrile

C60-1 C70-1

RS, eV CHCl3

toluene

CHCl3

1,2-dichlorobenzene

benzonitrile

506.5 715.5

-1.120 -1.180

0.755 0.780

-1.038 -1.092

-0.458 -0.456

the H atom of the -(CH3)2C- group of 1. The above structural analysis justifies that the 1 and C70 π-electrons, in the presently investigated C70-1 supramolecule, would strongly interact owing to their close proximity. The side-on orientation of C70 toward 1 also gets tremendous support from the heat of formation (∆Hf0) values of C70-1 complexes (Table 5). It is observed that, while ∆Hf0 for C70-1 complex exhibits -5.82 kJ · mol-1 of energy value at its side-on orientation, ∆Hf0 value of the same complex is estimated to be -4.55 kJ · mol-1 when C70 is oriented in a end-on manner. Thus, C70-1 complex gains 1.27 kJ · mol-1 of additional energy when the fullerene guest aligned in an end-on manner with the -NH plane of 1. Table 5 demonstrates that the trend in ∆Hf0 value for the fullerene-1 complexes, viz., ∆Hf0 (C70-1 < C60-1), corroborates well with the trend in K values of the respective complexes in all the solvents studied. Evidence in favor of sideon orientation of C70 toward the plane of the calix[4]pyrrole gets strong experimental support from 13C NMR experiment of the C70-1 system recorded in toluene-d8 medium (Figure 9). It is already reported that chemical shifts of free C70 appear at 150.07, 147.52, 146.82, 144.77, and 130.28 ppm, due to the existence of five different type of carbon atoms, namely, polar, R, β, γ, and equatorial, respectively.84-86 In our present investigations, it has been observed that, although the peak belonging to the pole region has only 4.414 ppm perturbation in terms of difference in chemical shift value, the peak at the equatorial region suffers a considerable amount

Figure 9.

13

C NMR spectrum of C70 in the presence of 1.

of chemical shift value (i.e., 7.533 ppm). This phenomenon clearly demonstrates that the equatorial region of C70 approaches closely toward the plane of the 1, resulting in a side-on orientation of such molecule. In the case of C60-1 complex, the lone peak of C60 suffers 4.010 ppm upfield shift (i.e., 138.670 ppm) (Figure 10), which is considerably lower than that observed in the case of C70-1 complex. Thus, the approaching surface area of both C60 and C70 toward the plane of 1 does not remain the same, which is reflected in binding constant values of such complexes. Both theoretical and experimental observations envisage that such a binding motif of C70 toward 1 takes place to maximize the π-π interactions. This feature also suggests that, like C70-porphyrin interaction,87,88 the presence of dispersive forces associated with π-π interactions is playing the key role in our present investigations. One important point to mention here is that the approach of fullerene guests toward calix[4]pyrrole host takes place from the side containing the -NH group of 1 (see Table 5). The present observation substantiates our rationale in favor of C-H · · · π interaction between the fullerene carbon atom and the H atom of the methyl groups of 1. 3.8. Determination of Electronic Coupling Element (V) and Reorganization Energy (R) for Fullerene/1 Complexes. The electronic coupling element (V) is related to the extent of overlap between the appropriate donor or host and acceptor or guest orbitals and scales the dependency of free energy of electron transfer (i.e., driving force) on such a rate constant.


Figure 10.

13


C NMR spectrum of C60 in the presence of 1.

The electronic interaction between 1 and fullerenes in fullerene-1 systems used mix electronic character and induces electron transfer (ET). It also created an opportunity for the estimation of electronic basis inducing dipole-allowed optical ET with the magnitude of the perturbation dictating the intensities of the intervalence transfer bands. This assumption implies that V between fullerenes and 1 are small or moderate assuming an ET process in terms of Marcus formalism.89 In the present investigations, values of V have been estimated from the absorption maxima of fullerene-1 complexes applying the model provided by Verhoeven et al.90 Values of V for C60-1 and C70-1 complexes are listed in Table 5. Estimated electronic coupling elements obtained for the above systems are very much comparable to the other donor-acceptor and host-guest systems found in literature.91 V values determined in the present investigation are significantly larger than typical values for the solvent-separated radical ion pair (∼12 cm-1),91 indicating that fullerenes and 1 remain in close contact to form contact radical ion pair. The higher V values in the case of the C70 complex arise due to the through-space energy transfer pathway which originates from the mixing pathway induced by a very low lying antibonding orbital in the C70 moiety. There are some recent observations of large solvent dependence of energy transfer rate constants and electronic coupling elements in electron donoracceptor systems, where the donor and acceptor are in close proximity. Zimmt et al. have reported the large solventdependent electronic coupling matrix element for their C-clampshaped molecule.92 They propose that the solvent-mediated superexchange coupling phenomenon is the main contributing

factor behind such higher V values. Therefore, the effect of solvent over V is likely to play an important role in the present systems. For this reason, solvent reorganization energies (Rs) have been calculated for the fullerene complexes of 1. The total reorganization energy, R, is a sum of the two terms (i.e., innersphere reorganization energy (solvent-independent) R0 and outersphere reorganization energy (solvent-dependent) Rs. In the case of fullerene, contributions from R0, which is related to the differences in nuclear configurations between an initial and a final state, are very small, ∼4.3 × 10-5 eV.93 This observation implies that the structures of C60 and C70 in the ground and excited states are very similar, which related to the rigidity of these spherical carbon structures. As far as Rs contribution is concerned, this is also believed to be small, as well. Thus, the symmetrical shape and large size of the fullerene framework requires little energy for the adjustment of an excited or reduced state to the new solvent environment. In the present investigations, Rs of C60-1 and C70-1 systems are estimated applying the dielectric continuum model developed by Hauke et al.94 Values of Rs for the C60- and C70-1 systems are given in Table 5. It should be mentioned at this point that solvent reorganization energies obtained in the present investigations do not coincide well with those observed for the porphyrin-quinone system found in the literature.94 The discrepancy in the value of Rs for porphyrin-quinine and fullerene-1 systems might be due to the subtle structural change in the host-guest complex, which exerted a large influence upon the photoinduced electron and energy transfer process.

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Figure 12. 1H NMR spectrum of uncomplexed 1 (3.50 × 10-2 mol · dm-3) recorded in toluene-d8 medium.

Figure 11. IR spectra of (a) uncomplexed 1 (3.5 × 10-5 mol · dm-3), (b) C60 (5.0 × 10-5 mol · dm-3) + 1 (3.5 × 10-5 mol · dm-3), and (c) C70 (5.0 × 10-5 mol · dm-3) + 1 (3.5 × 10-5 mol · dm-3) systems recorded in toluene medium.

3.9. Solution IR and Proton NMR Studies. The ground state interaction between fullerenes and 1 receives tremendous experimental support from solution state IR measurements. It is observed that the -NH peak of 1 appeard at 3086.38 cm-1 (Figure 11a) in the absence of C60 and C70. However, it is shifted to 3028.51 cm-1 (Figure 11b) and 2924.35 cm-1 (Figure 11c) in the presence of C60 and C70, respectively. Thus, a considerable amount of IR shift in the case of the C70-1 complex, viz., 162.03 cm-1 compared to C60-1 complex (i.e., 57.87 cm-1), supports the higher value of K in the case of the former system. We have also carried out proton NMR investigations to substantiate the larger extent of binding between C70 and 1. The methyl proton of calix[4]pyrrole molecule in toluene-d8 generally appears as doublets at 1.461 and 1.589 ppm (Figure 12). It is observed that, although we get only 0.003 ppm downfield shift (i.e., 0.9 Hz) in the case of C60-1 complex (Figure 13), C70-1 complex exhibits 0.008 ppm (2.4 Hz, Figure 14) downfield shift. Larger amount of downfield shift in case of C70-1 complex in comparison to C60-1 complex substantiates our findings as reported in Table 3. However, it should be mentioned at this point that, because C60 and C70 display very similar features in terms of redox potential, excited state energies, etc., it is not obvious that there should be any great difference in binding constant between these two species.

Figure 13. 1H NMR spectrum of 1 (3.50 × 10-2 mol · dm-3) in the presence of C60 (3.50 × 10-3 mol · dm-3) recorded in toluene-d8 medium.

Figure 14. 1H NMR spectrum of 1 (3.50 × 10-2 mol · dm-3) in the presence of C70 (3.50 × 10-3 mol · dm-3) recorded in toluene-d8 medium.

However, the most important and distinct difference between C60 and C70 is their structure. C60 is spherically symmetrical, whereas C70 is egg-shaped. For this reason, although electron density around the surfaces of C60 is equivalent at all places, there is a marked difference in such parameter when one moves from the side-on position of C70 (i.e., electron-rich) to the endon corner (i.e., electron-deficient part). Apart from electrostatic interaction, as fullerene-calix[4]pyrrole interaction is also dictated by preorganization of the host toward the guest molecule

Origin of Fullerene-Calix[4]pyrrole Interaction during a host-guest complexation process, C60 and C70 behave differently in forming supramolecular complexes with this particular macrocyclic receptor. Because C70 is oriented in a side-on manner toward 1, more π-π interaction is facilitated in the C70-1 complexation process in comparison to C60-1 binding. This is reflected in their corresponding binding constant values. Conclusions Various important findings as discussed in sections 3.1 to 3.9 can be summarized as follows: (1) meso-octamethyl calix[4]pyrrole (1) is shown to behave as an efficient macrocyclic receptor in forming productive ground state noncovalent complexes with fullerenes C60 and C70 in solution. (2) Formation of CT complexes between 1 and fullerenes is facilitated in solvent having more dielectric constant. (3) Vertical ionization potential of 1 was determined in solution utilizing the CT bands. (4) Order of the various important physicochemical parameters, such as degrees of charge transfer, oscillator strength, transition dipole moment, and resonance energy of interaction, suggest the neutral character of the complexes in the ground state. (5) Trend in the electronic coupling elements for the fullerene-1 complexes coincide fairly well with the trend in the binding constant (K) value of the same systems. Sign of the solvent reorganization energy of the fullerene-1 systems indicate the role of exothermic electron transfer phenomenon in our present investigations. (6) Very large value of K for the noncovalent complex of 1 with C70 compared to C60 strongly suggest the role of size-selective interaction for presently investigated supramolecular complexes. We anticipate that CT interaction plays a very important role for obtaining a high K value of the C70-1 system in chloroform medium. Selectivity in the K value (viz., KC70-1/KC60-1) establishes that functionalized 1 could effectively be employed as a selective host molecule for encapsulation of C70 in solution. Acknowledgment. D.P. thanks The University of Burdwan, Burdwan, India, for providing the basic research facility to him. Financial assistance provided by the Department of Scientific Assistant (DSA) in Chemistry from UGC, New Delhi, is also gratefully acknowledged. The authors wish to record their sincere gratitude to the Editor and the learned reviewers for making valuable comments. Supporting Information Available: CT absorption spectra for the noncovalent complexes of 1 with electron acceptors such as TCNE, TCNQ, p-chloranil, and DDQ, BH plots of fullerene-1 systems in 1,2-dichlorobenzene and benzonitrile, UV-vis titration curve of TCNQ-1 system in toluene, and BH plot of TCNQ-1 system in the same solvent are provided as Figure 1S-7S, respectively. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gale, P. A.; Anzenbacher, P., Jr.; Sessler, J. L. Coord. Chem. ReV. 2001, 222, 57. (2) Gale, P. A.; Sessler, J. L.; Kra’l, V.; Lynch, V. J. Am. Chem. Soc. 1996, 118, 5140. (3) Allen, W. E.; Gale, P. A.; Brown, C. T.; Lynch, V. M.; Sessler, J. L. J. Am. Chem. Soc. 1996, 118, 12471. (4) Gale, P. A.; Sessler, J. L.; Allen, W. E.; Tvermoes, N. A.; Lynch, V. Chem. Commun. 1997, 665. (5) Custelcean, R.; Delmau, L. H.; Moyer, B. A.; Sessler, J. L.; Cho, W.-S.; Gross, D.; Bates, G. W.; Brooks, S. J.; Light, M. E.; Gale, P. A. Angew. Chem., Int. Ed. 2005, 44, 2537. (6) Gale, P. A.; Sessler, J. L.; Kr’al, V. Chem. Commun. 1998, 1.

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Spectroscopic and Theoretical Insights into the Origin of Fullerene

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