Journal of Molecular Structure 889 (2008) 352–360

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Journal of Molecular Structure

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UV–vis spectrophotometric and theoretical investigations on charge transfercomplexes of a designed mesotetraphenylporphyrin with C60 and C70

Partha Mukherjee a, Sandip K. Nayak b, Shrabanti Banerjee (Bhattacharya) c, Subrata Chattopadhyay b,Sumanta Bhattacharya a,*

a Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, Indiab Bio-organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, Indiac Raja Rammohun Roy Mahavidyalaya, Radhanagore, Nangulpara, Hooghly, West Bengal, India

a r t i c l e i n f o

Article history:Received 18 December 2007Received in revised form 4 February 2008Accepted 16 February 2008Available online 29 February 2008

Keywords:MesotetraphenylporphyrinFullereneUV–vis investigationsCharge transfer absorption bandBinding constantsAb initio theoretical investigations

0022-2860/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.molstruc.2008.02.026

* Corresponding author. Fax: +91 342 2530452.E-mail address: sum_9974@rediffmail.com (S. Bha

a b s t r a c t

The present investigations report the UV–vis absorption studies on electron donor–acceptor (EDA) com-plexes of 5,10,15,20-tetrakis(octadecyloxyphenyl)-21H,23H-porphyrin (TP) with C60, C70 and some otherelectron acceptors in chloroform medium. Charge transfer (CT) absorption bands of the above mentionedcomplexes have been located in the visible region and vertical ionization potential of TP has been deter-mined from the trend in CT transition energy. CT is very weak in these complexes as revealed from verylow value of oscillator and transition dipole strengths. All the complexes are found to stable with 1:1 stoi-chiometry. Binding constants (K) for the fullerene complexes of TP are determined at four different tem-peratures from which enthalpies and entropies of binding of the processes have been estimated. The highK value for the C70/TP complex as well as C70/C60 selectivity ratio at room temperature supports theunderstanding of EDA complex binding between fullerenes and porphyrin. Ab initio theoretical investiga-tions provide a rational support in favor of strong complexation between C70 and TP.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

After generating an avalanche of research activity [1–4], earlyinvestigations on two novel third generation carbon allotropes,viz., C60 and C70 [5,6], evoked various aspects in fullerene chemis-try like synthesis, redox chemistry, understanding of the new typeof bonding presented by their curved surfaces [7] followed by theintroduction of suitable function group in their surfaces [8–10].However, the supramolecular chemistry of fullerenes is gainingcurrent impetus day by day [11–19].

On the other hand, porphyrins are among the most frequentlyemployed building blocks as electron donors and sensitizers inartificial photosynthetic models [20,21]. It has been envisaged thatcombination of porphyrin with fullerenes can lead to the develop-ment of advance materials which has potential application indeveloping light-harvesting devices [22,23], molecular magnets[24,25], medicine [26] and porous metal-organic framework [27].In this connection, many fullerene/porphyrin supramolecular sys-tems have been proposed to achieve high-performance photovol-taic devices [28–30]. Recently Imahori et al. has reportedfullerene/porphyrin linked system as artificial photosynthetic

ll rights reserved.

ttacharya).

mimics [31]. The first observation of a fullerene/porphyrin closeapproach was reported by Sun et al. [32] in octa-ethyl-porphyrin/and tetra-phenyl-porphyrin/fullerene complexes. In these com-plexes, the traditional paradigm of supramolecular chemistry isnot followed, i.e., there is no necessity to match a concave hostwith a convex guest. It was proposed that the fullerene/porphyrinp–p interaction involved some degree of electrostatic attraction orcharge transfer (CT) [33]. In particular, the electron rich doublebond at the 6:6 ring juncture of C60 and C70 was reported to be at-tracted to the protic center of the free-base porphyrin which iscounter to the prevailing notion of fullerenes acting as electronacceptors. Guldi et al. have nicely demonstrated the feature of elec-tron donor–acceptor concept in their designed fullerene/ruthe-nium ensemble [34]. Òapinski et al. also carried out a detailedsolid state UV–vis electronic and vibration (IR) spectra of the fuller-ene/zinc porphyrin dyad [35]. But no CT absorption bands havebeen found in their particular fullerene/porphyrin system. In orderto address the CT phenomena in fullerene/porphyrin complex, wehave carried out a detailed UV–vis absorption spectrophotometricmeasurements of the complexes of 5,10,15,20-tetrakis(octadecyl-oxyphenyl)-21H,23H-porphyrin (TP, Fig. 1) with C60 and C70 inchloroform medium. The spectroscopic investigations enable usfor to determine the extent of binding as well as complexationenthalpy.

N

HN

NH

NO(CH2)17CH3CH3(CH2)17O

O(CH2)17CH3

O(CH2)17CH3

Fig. 1. Structure of TP.

P. Mukherjee et al. / Journal of Molecular Structure 889 (2008) 352–360 353

2. Materials and methods

C60 and C70 were purchased from Aldrich. TP was preparedaccording to the reported procedure. O- and p-chloranil were ob-tained from Sigma and Fluka, respectively. These two compoundswere purified by sublimation just before use. 2,3-Dichloro-5,6-dicyano-p-benzoquinone (DDQ) and vitamin K were collected fromSigma and used without further purification. 2,3-Dichloro-1,4-naphthoquinone and fluorescein isothiocyanate were obtainedfrom Aldrich and Koch Light Laboratories Ltd., respectively. Thesolvent, chloroform, was of UV spectroscopic grade. All UV–visspectral measurements were performed on a UV 1601 PC modelspectrophotometer fitted with a Peltier controlled thermal bath.All computational calculations were carried out using SPARTAN’06Windows version software.

3. Results and discussion

3.1. Observation of CT bands

Fig. 2 shows the electronic absorption spectra of CHCl3 solutionof TP with o-chloranil, p-chloranil, DDQ, C60 and C70. Spectra of theabove solutions were recorded against the pristine acceptor solu-tion as reference to find out the existence of CT bands in the visibleregion. Since more concentrated solution of porphyrin compared toacceptor is described to detect CT absorption bands, the absorptionspectra were measured in the concentration range of 10�4 and10�6 mol dm�3 for TP and acceptors, respectively. In Fig. 1a–e,the CT peaks are identified to be the peaks in the region of 690–700 nm, because they normally have the longest wavelengthamong the peaks different from those obtained from the spectra

Fig. 2. Electronic absorption spectra of mixtures of TP and acceptors: (a) C70 + TP;(b) fluorescein isothiocyanate + TP; (c) p-chloranil + TP; (d) o-chloranil + TP; and (e)DDQ + TP recorded against the respective pristine acceptor solutions as reference.

of the components. Similar spectral features were obtained withmixtures of porphyrin with 2,3-dichloro-1,4-naphthoquinone andfluorescein isothiocyanate. The CT absorption spectra were ana-lyzed by fitting to the gaussian function y = y0 + [A/(w

p(p/2 ex-

p[�2(x–xc)2/w2] where x and y denote wavenumber and molarextinction coefficient, respectively. One such plot is shown inFig. 3. The results of the Gaussian analysis for all the TP/acceptorsystems under study are shown in Table 1. The wavelengths atthese new absorption maxima (kmax = xc) and the correspondingtransition energies (hm) are summarized in Table 2. The gaussiananalysis fitting is done in accordance with the method developedby I.R. Gould et al. [36]. One important point to mention here isthat Gaussian analysis of a curve generally gives a decent resultnear the maxima of the curve spread over a very small region.For this reason, although the errors in the center of the CT spectrafor the complexes of TP with various electron acceptors are verysmall, there are appreciable errors in the y0 value. Even though ful-lerenes and TP undergo CT interaction with great ease, one couldstill argue that the way they approach to each other is dictatedby packing consideration than by the special affinity we high-lighted above. The choice of a polar solvent undoubtedly promotesthe aggregation of the hydrophobic fullerene and porphyrin enti-ties but this could be viewed as an artificial enhancement of theiraffinity. The demonstration of fullerene/TP binding in chloroform istherefore considered to be a particularly stringent test of the spon-taneous attraction. All the spectra were recorded against the pris-tine fullerene solution as reference. Absorption maxima at thecenter of the curves are calculated by Gaussian analysis. Resultsof Gaussian analysis for the above systems are shown in Table 1.

3.2. Determination of vertical ionization potential (IBv) of TP

For complexes with neutral ground state, a CT band correspondsto a transfer of an electron from a donor (B) to an acceptor (A) withthe absorption of a quantum. The relationship between the energy(hmCT) of the lowest energy intermolecular CT band and the verticalionization potential (IB

v) of the donor for a series of complexeswith a common donor species has been source of much discussion.According to Mulliken’s theory [37] CT transition energies in thesecomplexes are related to vertical ionization potential of the donorby the relation

hmCT ¼ IBv � C1 þ ½C2=ðIB

v � C1Þ� ð1ÞHere C1 ¼ EA

v þ G0 þ G1 ð2Þ

where EAv is the vertical electron affinity of the acceptor, G0 is the

sum of the several energy terms (like dipole–dipole, van der Waals

13600 13800 14000 14200 14400 14600 14800 15000

500

1000

1500

2000

2500

ε, d

m3 m

ol-1

cm

-1

Wavenumber, cm-1

Fig. 3. Gaussian analysis of the CT absorption band for fluorescein isothiocyanate/TP system.

Table 1Gaussian curve analysis for the CT spectra of TP with different electron acceptors including C60 and C70

System 10�5 Area of the curve (A) Width of the curve (w) Centre of the curve (xc), cm�1 y0

C60/TP 36.4 ± 0.5 779 ± 7.4 14444 ± 3.5 5607 ± 129C70/TP 1075 ± 29 885 ± 19 14474 ± 7.0 1303 ± 981o-Chloranil/TP 2.8 ± 0.02 833 ± 5.0 14453 ± 2.0 12 ± 0.7p-Chloranil/TP 1.5 ± 0.06 866 ± 21 14478 ± 4.0 17 ± 3.0DDQ/TP 21 ± 0.3 826 ± 9 14314 ± 6.0 55 ± 5.52,3-Dichloro-1,4-naphthoquinone/TP 10.0 ± 0.8 883 ± 41 14482 ± 8. 60 ± 36Fluorescein isothiocyanate/TP 31 ± 6.2 900 ± 83 14464 ± 7 �423 ± 320

Table 2CT absorption maxima and transition energies of the CT complexes of TP with various electron acceptors; experimentally determined ionization potential of the donor (IB

v);resonance energy (RN), oscillator strength (f) and transition dipole strengths (lEN) of the complexes of TP with different electron acceptors

System kCT, nm hmCT, eV IBv, eV jRNj f lEN (Debye)

C60/TP 690.3 1.797 6.520 0.340 0.072 14.0C70/TP 691.0 1.795 0.390 0.130 17.5o-Chloranil/TP 692.0 1.792 0.0036 0.0005 1.134p-Chloranil/TP 690.7 1.7956 0.0035 0.0003 0.83DDQ/TP 698.6 1.775 0.044 0.0037 3.12,3-Dichloro-1,4-naphthoquinone/TP 690.5 1.796 0.021 0.0018 2.1Fluorescein isothiocyanate/TP 691.4 1.794 0.050 0.0046 3.3

45.64 45.68 45.72 45.76 45.80

13.625

13.630

13.635

13.640

13.645

13.650

13.6552C

1 + h

ν CT, e

V

C1(C1 + hνCT), (eV)2

Fig. 4. Plot of 2C1 + hmCT vs. C1(C1 + hmCT) for the CT complexes of TP.

354 P. Mukherjee et al. / Journal of Molecular Structure 889 (2008) 352–360

interaction, etc.) in the ‘no-bond’ state and G1 is the sum of numberof energy terms in the ‘dative’ state. In most cases G0 is small andcan be neglected while G1 is largely the electrostatic energy ofattraction between B(1�d)+ and A(1�d)�. The term C2 in Eq. (1) is re-lated to the resonance energy of interaction between the ‘no-bond’and ‘dative’ forms in the ground and excited states and for a givendonor species it may be supposed to be constant [37]. A rearrange-ment of Eq. (1) yields

2C1 þ hmCT ¼ fC1ðC1 þ hmCTÞ=IBvg þ fðC2=IB

vÞ þ IBvg ð3Þ

The vertical electron affinities of C60, C70, o-chloranil, p-chloranil,DDQ, 2,3-dichloro-1,4-naphthoquinone and fluorescein isothiocya-nate were collected from the literatures [38–42]. Neglecting G0

and taking the typical B–A distance in p-type EDA complexes tobe 3.5 Å, the major part of G1 is estimated to be e2/4pe0r = 4.13 eV.Using these values C1 is obtained from Eq. (2) for each of the accep-tors. A plot of 2C1 + hmCT versus C1(C1 + hmCT) for a given donor andvarious acceptors yields a slope of 1=IB

v from which the value ofIB

v has been obtained for the TP. The following linear regressionequation has been obtained with the present data:

2C1 þ hmCT ¼ ð0:1534� 6:52� 10�5Þ½C1ðC1 þ hmCTÞ� þ ð6:63

� 0:003Þ; correlation coefficient

¼ 0:99: ð4Þ

Result is given in Table 2. The above plot is demonstrated in Fig. 4.

3.3. Determination of oscillator (f) and transition dipole strengths(lEN)

From the CT absorption spectra, we can enumerate the magni-tude of the oscillator strength. The oscillator strength f is estimatedusing the formula

f ¼ 4:32� 10�9Z

eCTdm ð5Þ

whereR

eCTdm is the area under the curve of the extinction coeffi-cient of the absorption band in question vs. frequency.

To a first approximation f ¼ 4:32� 10�9emaxDm1=2 ð6Þ

where emax is the maximum extinction coefficient of the band andDm1/2 is the half – width, i.e., the width of the band at half the max-

imum extinction. The observed oscillator strengths of the CT bandsare summarized in Table 2. It is worth mentioning that we need aproper calculation of oscillator strengths of fullerene/TP CT com-plexes. This is because oscillator strength is very sensitive to themolecular configuration and the electron charge distribution in CTcomplex. In the C60/ and C70/TP complexes, we can not use simplemodel assuming a charge localized at a certain cite of fullerenesphere; because p-bonds of the fullerenes are directed radially witha node on the molecular cage. Values of f also show that the com-plex containing C70 show a slightly larger degree of CT and greaterinteraction energies than that of C70.

The extinction coefficient is related to the transition dipole by

lEN ¼ 0:0952½emaxDm1=2=Dm�1=2 ð7Þ

where Dm � m at emax and lEN is defined as �eR

wexP

iriwgds. Thetransition dipole strengths (lEN) for the complexes of TP with vari-ous acceptors are given in Table 2. It has been observed that the lEN

for the C70/TP complex is somewhat higher than that of the corre-sponding C60/TP complex. This trend in lEN is in conformity withthe fact that C70 has a higher electron affinity value than that ofC60 [43]. But it should be mentioned at this point that the differencein electron susceptibility of C60 and C70 are very small.

0.0 1.0x10-5 2.0x10-5 3.0x10-5 4.0x10-5 5.0x10-5

0.0

2.0x10-10

4.0x10-10

6.0x10-10

8.0x10-10

1.0x10-9

1.2x10-9

1.4x10-9

[C70

] 0[1]

0/d ,

(mol

.dm

-3)2

[1]0 , mol.dm-3

Fig. 5. Benesi–Hildebrand plot for the complex of C70 with TP at 303 K.

Table 4Binding constants (K/mol�1 � dm3) and thermodynamic parameters (DH0

f =kJ mol�1

and DS0f =kJ�1 mol�1) for the complexes of C60 and C70 with TP at four different

temperatures

Fullerene Temp./K

10�4 K/mol�1 dm3

DH0f =kJ mol�1 DS0

f =kJ�1 mol�1 DG0f =kJ mol�1

C60 298 9.72 ± 0.9 �6.6 ± 0.75 73.3 ± 2.5 �29.0 ± 1.5303 9.50 ± 0.84308 8.93 ± 0.76313 8.60 ± 0.45

C70 298 22.85 ± 1.1 �59.4 ± 7.5 �97.2 ± 24.7 �30.4 ± 0.14303 12.50 ± 0.15308 7.48 ± 0.11313 7.03 ± 0.25

P. Mukherjee et al. / Journal of Molecular Structure 889 (2008) 352–360 355

3.4. Determination of resonance energy (RN)

Briegleb and Czekalla [44] theoretically derived the relation

emax ¼ 7:7� 104=½hmCT=jRNj � 3:5� ð8Þ

where emax is the molar extinction coefficient of the complex at themaximum of the CT absorption, mCT is the frequency of the CT peakand RN is the resonance energy of the complex in the ground state,which, obviously is a contributing factor to the stability constant ofthe complex (a ground state property).

3.5. Spectrophotometric study of binding equilibria of the complexesof TP With C60 and C70

The binding constants of the fullerene complexes of TP weredetermined at four different temperatures using the Benesi–Hilde-brand (BH) [45] equation in the

form ½fullerene�0½TP�0=d ¼ ½TP�0=eþ 1=Ke ð9Þ

Here [fullerene]0 and [TP]0 are the initial concentrations of theacceptor and donor, respectively, d is the absorbance of the do-nor–acceptor mixture at kCT against the pristine acceptor solutionas reference, The extinction coefficient, e, is not quite that of thecomplex. BH method is an approximation that we have used manytimes and it gives decent answers. But the extinction coefficient isreally a different one between the complex and the free species thatabsorbs at the same wavelength. K is the binding constant of thecomplex. Eq. (9) is valid [45] under the condition [TP]0� [fuller-ene]0 for 1:1 fullerene/TP complexes. The intensity in the visibleportion of the absorption band, measured against the acceptor solu-tion of concentration [fullerene]0 as reference, increases systemati-cally with gradual addition of TP which shows complex binding.One typical absorption data for C70/TP system are given in Table3. In all the cases very good linear plots according to Eq. (9) are ob-tained, one typical case being shown in Fig. 5. The correlation coef-ficients of all such plots were above 0.90. Binding constantsdetermined from the BH plots at four different temperatures aresummarized in Table 4. Sibley et al. [46] have also estimated the

Table 3Data for spectrophotometric determination of stoichiometry and molar absorptivities(e) of the C70/TP complex at four different temperatures

Temp., K 105 Donorconcentration,mol dm�3

106 [A]0,mol dm�3

Absorbanceat kCT

10�4 e,dm3 mol�1 cm�1

298 1.374 3.401 0.1665 8.3 ± 0.72.060 0.19173.440 0.17244.120 0.22644.810 0.1963

303 0.687 0.09701.374 0.15522.060 0.21043.440 0.19424.120 0.20794.810 0.2078

308 0.687 0.10051.374 0.14012.060 0.21593.440 0.19874.810 0.2213

313 0.687 0.10391.374 0.12212.060 0.21363.440 0.19694.810 0.2323

binding constants for the complexes of C60 with aniline and thederivatives using BH equation. The most interesting feature of thepresent investigation is that TP binds C70 in preference to C60

[47]. Thus, the receptor TP with K of 2.285 � 105 dm3 mol�1 and9.72 � 104 dm3 mol�1 for C60 and C70, respectively, gives the C70/C60 selectivity of about 2.3. The trend in the binding constantsshows that the order of binding constants changes slightly betweenC60 and C70 [48]. The preference in the fullerene complexation alsoindicates that the subtle difference in solvation energy is also animportant factor affecting affinities. Previous study on fullerene/porphyrin system suggests that the attraction of fullerene to por-phyrin is largely van der Waals in nature [49]. During complexationbetween C70 and TP, the orientation of C70 molecule towards theporphyrin is ‘‘side-on” rather than ‘‘end-on”, confirming structuraldeductions based on theoretical calculations [50]. The van derWaals attraction is considered to be maximized in the side-onstructure of the complex since the flatter equatorial regions of C60

have more contact with the porphyrin than the more highly curvedpolar region.

3.6. Enthalpies (DHf0) and entropies of binding (DHf

0) of thecomplexes of TP with C60 and C70

Evaluation of K for the fullerene/TP complexes at four differenttemperatures allows the determination of enthalpies (DHf

0) andentropies of binding (DHf

0) by a van’t Hoff plot of ln K vs. 1/T(Eq. 10). As measured, these terms will represent the net changein enthalpy and entropy for the solvated species.

lnK ¼ �DHf0=RTþ DSf

0=R ð10Þ

The following linear regression equations have been obtained withthe present data:

Fig. 7. Stereoscopic structure of (a) C60/and (b) C70/TP complexes optimized byHF/3-21G calculation.

356 P. Mukherjee et al. / Journal of Molecular Structure 889 (2008) 352–360

C60/TP complex:

ln K ¼ ð794� 92Þ=T þ ð8:8� 0:3Þ; correlation coefficient ¼ 0:99

ð11aÞ

C70/TP complex:

ln K¼ð7148�908Þ=T�ð11:7�2:97Þ;correlation coefficient¼0:99

ð12bÞ

The positive slope in each case indicates that the complexation pro-cess is exothermic and thus enthalpy favored. The H0

f and DSf0 val-

ues of the complexes are listed in Table 4. One typical plot of ln Kagainst 1/T is shown in Fig. 6. The negative enthalpy changes result-ing from the fact that binding of the CT complex predominate overthe other opposing factors (i.e., dissociation of CT complex). Thisphenomenon has been observed in the present cases. The large en-thalpy for the C70 is consistent with the larger value for K in the C70/TP system. DHf

0 value is more negative in case of C70/TP complexindicating that above complexation process is enthalpy favored.The trend in magnitude of DS0

f i.e., DSf0ðC60=TPÞ > DSf

0ðC70=TPÞ can be

rationalized as follows; randomness of C60 and TP molecules areso high that these donor and acceptor species are failed to approachquite close to each other. For this reason, effective overlap betweentheir molecular orbital could not take place. In contrast, muchstronger association is predicted computationally for C70/TP com-plex in which the aromatic moiety of TP molecule containing Natom appear to interact with the graphitic equatorial belt of C70

[50]. For this reason, DSfo value is very low for C70/TP complex.

Moreover, greater extent of negative free energy of binding valuein case of C70/TP complex also suggests that C70 binds itself morestrongly with TP compared to C60.

3.7. Theoretical calculations on fullerene/TP molecular complexes

It was already indicated that the porphyrin and fullerene p-elec-trons would strongly interact in their close proximity [51]. How-ever, the relative stability of the C60 and C70 complexes is notapparent. Thus, it is very much essential to study the combiningstability and electronic structures for the corresponding com-plexes. The extent of relative stability for both the fullerenes com-plexes have been measured in terms of enthalpies of binding(DHf

o) value. For ease of computational work, we did not considerthe contribution of n-alkane chain in the TP structure. It is verymuch interesting to find that the C70/TP complex is found to bemore stable than the C60/TP complex by 2.87 kJ/mol at the HF/3-21G optimization (Fig. 7). However, the 3-21G basis set is oftenmisleading to more or less higher energy conformers due to the

0.00320 0.00325 0.00330 0.00335

11.32

11.36

11.40

11.44

11.48

11.52

11.56

11.60

lnK

1/T , K-1

Fig. 6. Plot for determination of enthalpy of binding of the C60/TP complex.

inherent accuracy in calculating weak interactions such as p–pand C–H� � �p interactions [52]. Thus we have added additionalpolarization function, like, 6-31G* at the HF level. The HF/6-31G*

single-point energy calculations have been performed at the HF/3-21G optimized geometries and the C70/TP complex is still foundto be more stable than the C60/TP complex by 2.92 kJ/mol (Fig. 7).Considering that we did not carry out the full optimization at the6-31G* level of theory due to computational limit, the relativeenergies of the C60/ and C70/TP complexes are expected to be quitesimilar. The negative association energies suggest stable supramo-lecular complex, and the trend in the magnitude of these valuestrack the K values in Table 4. In our present investigations, theoret-ical calculations reveal that the five-membered ring in the C60 moi-ety is closely and facially oriented towards the porphyrin five-membered p-ring at a distance of 4.529–4.756 Å for the C60/TPcomplex at HF/6-31G* level of theory (Fig. 8). These distances,however, are somewhat greater than the typical p–p stacking dis-tances in a benzene dimer ranging from 3.4 to 4.0 Å by the alreadyreported theoretical results [51]. In contrary, the distance betweenthe hydrogen atom of pyrrole nitrogen and five-membered ring ofC60/TP complex lie in the range of 4.174–4.490 Å. It is very muchinteresting to observe the plane of the TP consisting all the pyrrolegroups is almost covered by graphitic plane of C70 whereas C60

remain closely far away form the porphyrin p-ring (Fig. 8). This

Fig. 8. HF/6-31G* structure of the (a) C60/and (b) C70/TP complexes optimized atHF/3-21G geometry.

ig. 9. Molecular electrostatic potential maps of (a) TP, (b) C60/TP and (c) C70/TPystems.

P. Mukherjee et al. / Journal of Molecular Structure 889 (2008) 352–360 357

has been reflected in the close positioning of plane of the porphyrinmolecule to the C70; the distances lie in the interval of 4.453–4.481 Å. Orientation of C70 towards TP is aligned equatoriallywhere the p-electron density is much greater than the pole areadue to greater number of 6:6 bonds. This has been reflected inthe larger binding constant value between C70 and TP comparedto C60/TP complex. The equatorial face of C70 is centered over theelectro positive center of the porphyrin plane which can be viewedas enhancement in van der Waals interaction due to availability ofgreater surface area which will favor strong-interaction.

C–H� � �p interactions are clearly shown between p-ring in fuller-enes and the phenyl group substituted at the pyrrole moiety ofporphyrin (TP). However, the present results exhibit somewhathigher value compared to previously reported C–H� � �p distancesof 2.35–2.70 Å depending on the complex orientation and alsothe way of calculation levels in benzene–water complex showingtypical C–H� � �p interactions [53].

Electrostatic interactions originating from the highest occupiedmolecular orbitals (HOMOs) and lowest unoccupied molecularorbitals (LUMOs) of the fullerene/TP supramolecules are consid-ered to play a vital role in the interaction between fullerenes andTP. It was already envisaged that electrostatic assemblies of fuller-ene/porphyrin hybrids may lead to generation of long-lived

charged separated species [54]. In this connection, molecular elec-trostatic potential (MEP) maps were generated TP, C60/ and C70/TPsystems to visualize the electrostatic interactions (Fig. 9). The MEPfor TP showed negative electrostatic potential (shown in red) onthe porphyrin ring (mostly located on the nitrogen atoms). TheMEPs for the fullerenes are blue–green indicating positive electro-static potential; blue–green color of fullerenes corresponds to thecenter regions of the five- and six-membered rings [55]. However,along the 6:6 bonds, regions of negative potentials (shown in red)can be observed [55]. Interestingly, in the supramolecular com-plexes, C60/ and C70/TP, the original blue–green color of the sepa-rated C60 and C70 are changed to green, and deep red color ofporphyrin changed to reddish-yellow, indicating possibility of

Fs

Fig. 10. HOMOs and LUMOs of C60/TP complex at various electronic states: (a) HOMO, (b) HOMO � 1, (c) LUMO, (d) LUMO + 1, (e) LUMO + 2, (f) LUMO + 3 and (g) LUMO + 5.

358 P. Mukherjee et al. / Journal of Molecular Structure 889 (2008) 352–360

electron transfer or charge transfer between these two chromoph-ores upon photoexcitation. In all of the studied complexes, HOMOand HOMO � 1 were centered on the TP while LUMO, LUMO + 1and LUMO + 2 were centered almost exclusively on the fullerene,as illustrated in Figs. 10 and 11. The above results suggest thatthe charge-separated state in photoinduced electron-transfer reac-tions of the supramolecular complex is TP+/fullerene�. EHOMO and

ELUMO of all the fullerene/TP complexes along with TP at variouselectronic states are summarized in Table 5. Some pictures ofHOMOs and LUMOs of the C70/TP complex are demonstrated inFig. 10. Figs. 10 and 11 also reveal that HOMO � 1, HOMO,LUMO + 3 and LUMO + 4 of C60/ and C70/TP complexes are of por-phyrin character and correspond to the four frontier orbitalsaccording to the Gouterman four-orbital model for porphyrin [56].

Fig. 11. HOMOs and LUMOs of C70/TP complex at various electronic states: (a) HOMO, (b) HOMO � 1, (c) LUMO, (d) LUMO + 1, (e) LUMO + 2, (f) LUMO + 3 and (g) LUMO + 5.

P. Mukherjee et al. / Journal of Molecular Structure 889 (2008) 352–360 359

4. Conclusions

The present study gives a clear evidence of attractive fullerene/porphyrin interaction in ground states. CT bands of porphyrin com-plexes with different electron acceptors have been located and thevertical ionization potential of the porphyrin has been estimatedusing the full form of Mulliken’s equation. Very low values of oscil-lator strength for the fullerene complexes of TP and some other

electron acceptors indicate that the CT complexes studied herehave almost neutral character in their ground states. Very high val-ues of binding constants for the fullerene/TP complexes indicatethe surprising affinity in the interaction of a curved molecular sur-face to a planar surface, which differs from traditional p–p interac-tions between concave and convex species. Theoreticalcalculations envisage close approach of C70 to the plane of the por-phyrin compared to C60 which was reflected in the higher binding

Table 5Comparison of the five highest occupied and five lowest unoccupied molecularorbitals of C60/and C70/TP complexes done by HF/3-21G and HF/6-31G* calculations

States Energy (eV)

HF (3-21G) HF (6-31G*)

C60/TP C70/TP TP C60/TP C70/TP TP

HOMO � 4 �8.2420 �7.8898 �8.5789 �7.6107 �7.2788 �8.3721HOMO � 3 �8.2324 �7.8840 �8.5185 �7.6021 �7.2694 �8.3594HOMO � 2 �8.2301 �7.8131 �8.4847 �7.5989 �7.2371 �8.3257HOMO � 1 �6.7440 �6.7678 �6.7147 �6.7740 �6.7979 �6.7449HOMO �6.5585 �6.5732 �6.5253 �6.2934 �6.3079 �6.2594LUMO �0.7475 �0.8636 0.6803 �0.2741 �0.3423 0.8696LUMO + 1 �0.7378 �0.8622 0.8370 �0.2665 �0.3412 0.9337LUMO + 2 �0.7321 �0.6827 3.2062 �0.2600 �0.0989 3.3426LUMO + 3 0.6509 �0.0749 3.7322 0.8402 0.3347 3.7141LUMO + 4 0.8078 0.5568 3.7458 0.9048 0.8191 3.7642

360 P. Mukherjee et al. / Journal of Molecular Structure 889 (2008) 352–360

constant value of C70/TP complex as well as large selectivity of C70

over C60 at room temperature. We anticipate that the supramolec-ular complex like fullerene/TP may have potential applications inthe field of molecular magnets and conductors if we can properlytune the photophysical properties of such complex.

Acknowledgements

One of the authors, S. Bhattacharya, acknowledges BurdwanUniversity for providing basic research infrastructure facility tohim. The authors also acknowledge the Editor of Journal of Molec-ular Structure and the learned reviewer for their valuable com-ments in improving the structural aspect of the paper.

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