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Spectrochimica Acta Part A 90 (2012) 186– 192

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

j our na l ho me p age: www.elsev ier .com/ locate /saa

pectroscopic and theoretical insights on fullerene–octaethylporphyrin selfssembled non-covalent conjugates studied in solution

artha Mukherjee, Sumanta Bhattacharya ∗

epartment of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, India

r t i c l e i n f o

rticle history:eceived 8 September 2011eceived in revised form2 December 2011ccepted 16 January 2012

eywords:ullerenesctaethylporphyrin free baseV–Vis and fluorescence spectroscopic

nvestigations

a b s t r a c t

The present paper highlights the UV–Vis and fluorescence spectroscopic studies on spontaneousnon-covalent interaction between a designed monoporphyrin, namely, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin (1) and fullerenes (C60 and C70) in toluene medium. While UV–Vis studies revealconsiderable amount of fullerene/1 interaction in ground state, steady state fluorescence study estab-lishes remarkable quenching of fluorescence intensity of 1 in the presence of fullerenes C60 and C70.Stoichiometry of both the fullerene complexes of 1 is determined to be 1:1. The magnitude of averagebinding constant (K) values for C70/1 and C60/1 systems is estimated to be 8705 and 3440 dm3 mol−1,respectively, which also prove that moderate value of selectivity in binding between C70 and C60 com-plexes of 1 is resulted in our present work. The genetics of the photo-physical characteristics of fullerene/1complexes get tremendous support from lifetime experiment, which signifies the importance of static

inding constantsuantum mechanical calculations

quenching phenomenon for our presently investigated supramolecules. Life time experiment yields largermagnitude of charge separated rate constant for the fullerene/1 species in toluene. Theoretical calcula-tions at molecular mechanics level evoke the single projection geometric structures for the C60/1 andC70/1 systems in vacuo which also proves that interaction between C70 and 1 is governed by the disper-sive forces associated with �–� interaction rather than electrostatic mechanism associated with chargetransfer mechanism.

. Introduction

The pioneering work by Kroto et al. [1] and Krätschmer et al.2] regarding the initial discovery of fullerenes C60 and C70, andhe development of a large-scale method for their preparation,ave prompted a lively interest to employ these 3-dimensional car-on allotropes to undergo supramolecular interaction with variousost molecules like calix[n]arene [3,4], crown ethers [5], cyclodex-rin [6] and phthalocyanine [7–10]. Among various macrocycliceceptors, porphyrin molecule with highly delocalized �-electronystems, finds a unique place in forming host–guest or elec-ron donor–acceptor complexes with fullerenes. Porphyrins areery much suited in energy and/efficient electron transfer pro-ess(s) due to the minimal structural change of such entityuring the uptake or release of energy and/electrons, respec-ively. In order to understand the nature of interaction betweenullerene and porphyrin chromophores, the topology of the two

oieties in fullerene/porphyrin composite or dyad system haseen systematically varied and a wide range of covalently andon-covalently linked such systems have been reported [11–18].

∗ Corresponding author. Tel.: +91 332 6327254; fax: +91 342 2530452.E-mail address: sum 9974@rediffmail.com (S. Bhattacharya).

386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2012.01.031

© 2012 Elsevier B.V. All rights reserved.

�-Conjugated molecules like fullerenes and porphyrins with tun-able electronic properties may be employed as suitable buildingblocks for the construction of functional materials with excep-tional electrochemical and photophysical properties [19]. Oneof the most interesting aspects of fullerenes and porphyrins isthat they spontaneously attract to each other mainly throughorbital interactions, as determined through experiment in solution[20,21], in the solid state [22,23] and also confirmed by high-level theoretical calculations [24]. A variety of covalently linkedfullerene/porphyrin dyads of donor–acceptor type, have alreadybeen reported in this connection, in which the key feature isthat the porphyrin loses its characteristic luminescence propertyas a result of light induced energy and/or electron transfer tothe fullerenes [25]. The main objective of the present work isto study the extent of binding and selectivity of complexationof a newly designed monoporphyrin, namely, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin (1, Fig. 1) with fullerenes C60 andC70 in toluene. Spectroscopic investigations, viz., UV–Vis, steadystate and time resolved fluorescence measurements have beenemployed to determine the extent of binding and some other

physicochemical parameters related to fullerene/1 complexa-tion processes. The spectroscopic investigations enable us todetermine the selectivity of binding between C60/1 and C70/1 com-plexes. Molecular mechanics force field (MMMF) calculations well

P. Mukherjee, S. Bhattacharya / Spectrochimica Acta Part A 90 (2012) 186– 192 187

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Table 1Binding constants (K) and the selectivity in binding, i.e., KC70/1/KC60/1 , of thefullerene/1 complexes at 298 K.

System K (dm3 mol−1) KC70 /KC60

Fig. 1. Structure of 1.

nterpret the stability difference between complexes C60/1 and70/1.

. Experimental

C60, C70 and 1 have been purchased from Aldrich, USA. Toluenespectroscopic grade, Merck) is used as solvent, because it is suffi-iently apolar to favor non-covalent interaction between fullerenend porphyrin and, at the same time, ensures good solubilitynd photo-stability of the samples. UV–Vis spectral measurementsre performed on a Shimadzu UV-2450 spectrophotometer usinguartz cell with 1 cm optical path length. Steady state fluorescencepectral measurements are recorded in a Hitachi F-4500 fluores-ence spectrophotometer. Fluorescence decay curves are measuredith a HORIBA Jobin Yvon Single Photon Counting set up employ-

ng nanoled as excitation source. Theoretical calculations are donesing SPARTAN’06 software.

. Results and discussions

.1. UV–Vis absorption studies

The ground state electronic absorption spectrum of 1 (Fig. 2(a))n toluene shows one intense absorption band, i.e., B or Soret band,t 401 nm corresponding to the transition to the second excited sin-let state (S0← S2) and four absorption bands of lower intensity at00 nm, 530 nm, 560 nm and 623 nm corresponding to the vibronicequence of transition to the lowest excited singlet state (S0← S1).ccording to the Gouterman’s four-orbital model, the intensity of

he Q band is directly related to the energy separation between theearly degenerate (a1u,eg) and (a2u,eg) one-electron excited config-ration [26]. Evidence in favor of ground state electronic interactionetween fullerenes and 1 come from UV–Vis titration experiment.

t is observed that the gradual addition of a C60 solution to a tolueneolution of 1 decreases the absorbance of the Soret band and it suf-ers little amount of red-shift, i.e., from 401 to 402 nm (Fig. 2(b)).owever, no additional absorption peaks are observed in the visi-le region. The former observation extends a good support in favorf the complexation between C60 and 1 in ground state. The latterbservation imparts that the interaction is not primarily controlledy charge transfer (CT) type transition in ground state. The stoi-

hiometry of the C60/1 complex is found to be 1:1 as confirmed byobs plot of continuous variation experiment (Fig. 3(a)). Similar sortf absorption spectral features are observed in case of C70/1 systemFig. 1S), though the Soret absorption band has been red-shifted by

C60/1 5600 (UV–Vis) 1280 (fluorescence) 1.9(UV–Vis)

5.2(fluorescence)C70/1 10,710 (UV–Vis) 6680 (fluorescence)

2 nm. Jobs plot of continuous variation indicates the 1:1 stoichiom-etry for C70/1 complex (Fig. 2S). All of the above phenomena provethat 1 undergoes substantial amount of interaction with both C60and C70 solution. The gradual decrease in the absorbance value atthe Soret band of 1 has been utilized to determine the binding con-stant (K) of C60/1 and C70/1 systems using Benesi–Hildebrand (BH)equation [27] of following type:

[A]0[D]0

d′= [D]0

ε′+ 1

Kε′(1)

Here [A]0 and [D]0 are the initial concentrations of the acceptor (i.e.,C60 and C70) and donor or host (1) solutions (in toluene), respec-tively; d′ is the corrected absorbance of the fullerene/1 mixtureat the wavelength of measurement (i.e., 401 nm, Soret absorptionband of 1) recorded against the same concentration of 1 as refer-ence. The quantity ε′ means the corrected molar absorptivity ofthe complex. K is the binding constant of the fullerene/1 complex.Eq. (1) is valid under 1:1 approximation for fullerene/1 systems. Itshould be mentioned at this point that the corrected molar extinc-tion coefficient, ε′, is not quite that of the complex. The BH method[27] is an approximation that we have used many times, and itgives decent answers. But the extinction coefficient is really a dif-ferent one between the complex and free species that absorbs atthe same wavelength. Data for spectrophotometric determinationof stoichiometry of the C60/1 and C70/1 systems recorded in toluenemedium are provided in Tables 1S and 2S, respectively. The mostformidable support in favor of evaluation of K by BH method comesfrom the work of Haino et al. on fullerene encapsulation by varioustypes of calix[5]arene derivatives [28] and also from CT complex-ation of C60 with some amines reported by Mizyed et al. [29]. Inall the cases studied, very good linear plots have been obtainedaccording to the present data. Typical BH plots for C60/1 and C70/1systems in toluene medium are shown in Figs. 3(b) and 3S, respec-tively. Values of K determined from BH method for the fullerene/1complexes are listed in Table 1. The larger extent of decrease inthe absorbance value of the Soret absorption band for C70/1 systemexemplifies the strong interaction between C70 and 1 compared toC60 and 1. The most important observation in the present investi-gation is that the Soret absorption bands get affected more than theQ-absorption bands, which is already observed by Guldi et al. [30]for their particular fullerene/porphyrin systems.

3.2. Fluorescence investigations

3.2.1. Steady stateThe photo-induced behavior of the fullerene complexes of 1 has

been investigated by steady-state emission measurements, whichalso accounts the binding of 1 with fullerenes. The large molarextinction coefficient value of 1 with respect to the fullerenes inthe UV–Vis spectral region allows us to preferentially excite 1,keeping the porphyrin concentration at much lower level com-pared to fullerenes. In toluene solution, upon excitation at 401 nm,1 exhibits two emission bands at 623 and 690 nm corresponding to(0, 0) and (0, 1) transitions, respectively. According to Gouterman

[26], the (0, 0) band results from a transition between the groundstate and the lowest exited state, and the (0, 1) band is related to thetransition between the lowest exited state and the vibronic stateinvolving the most active vibrations. The fluorescence spectral

188 P. Mukherjee, S. Bhattacharya / Spectrochimica Acta Part A 90 (2012) 186– 192

Fig. 2. (a) UV–Vis absorption spectra of 1 (5.0 × 10−6 mol dm−3) recorded against the solvent as reference; (b) UV–Vis absorption spectra of uncomplexed1 60 mii 60 ((ii)

cFaptotobpdmscat

(5.0 × 10−6 mol dm−3) recorded against the solvent as reference along with 1 + Cndicates UV–Vis absorption spectra of 1 (5.0 × 10−6 mol dm−3) in the presence of C

hanges of 1 upon addition of C70 and C60 solutions are shown inigs. 4(a) and 4S, respectively. As the concentrations of fullerenesre increased, the emission intensity of 1 is reduced. The titration iserformed at a constant concentration of 1. This finding indicateshat there is some relaxation pathway from the excited singlet statef the porphyrin to the fullerene in toluene. It is already reportedhat charge separation can also occur from the excited singlet statef the porphyrin to the C60 in toluene medium [31]. Competingetween the energy and electron transfer processes is a universalhenomenon in fullerene/donor molecule complexes [32], solventependent photo physical behavior is a typical phenomenon of theost fullerene/porphyrin dyads studied to date [33]. Photophysical

tudies as well as theoretical calculations already prove that, inonformationally flexible fullerene/porphyrin systems, �-stackingbility facilitates the through-space interactions between thesewo chromophores. This type of interaction is clearly demonstrated

xtures recorded against the pristine C60 solution as reference. Here, set (ii)–(viii) 1.0 × 10−5 to (viii) 1.0 × 10−4 mol dm−3).

by quenching of 1porphyrin* fluorescence through formation offullerene-excited states (energy transfer) and/or generation offullerene−•/porphyrin+• ion-pair states (electron transfer) [34].In a non-polar medium like toluene, generally, energy transferdominates over the electron transfer in deactivating the photo-excited chromophore (here 1porphyrin*). Similar sort of rationaleis already provided by Yin et al. for their particular cis-2′,5′-dipyridinylpyrrolidino[3′,4′:1,2]C60/zinc(II)tetraphenylporphyrinsupramolecule [35]. However, very recently it is reported that, forC60/zincporphyrin (ZnP) dyad system, a new band correspondingto C60 fluorescence develops at 710 nm in addition to ZnP emis-sion at 600 and 650 nm [36]. This phenomenon indicates that

rapid intramolecular transduction of singlet excitation energytakes place from the excited ZnP to fullerene. In our presentinvestigations, however, the fullerene fluorescence at 710 nmis not observed, although the fullerene/1 complexes exhibit the

P. Mukherjee, S. Bhattacharya / Spectrochimica Acta Part A 90 (2012) 186– 192 189

Fs

csfifiiaiCe[

I1iccrlCTt1d

Table 2Fluorescence lifetimes (�f), charge separation rate constant (ksinglet

CS ) and charge sep-

aration quantum yield (˚singletCS ) for the designed monoporphyrin 1 in the absence

and in the presence of C60 and C70. Temp. 298 K.

1 �f (ns) 109 ksingletCS (s−1)a ˚singlet

CSb

C60/1 C70/1 C60/1 C70/1 C60/1 C70/1

0.131 0.1158 0.1031 1.001 2.065 0.1159 0.2129

resorcarene (K = 5010 dm mol ) [37] and terphenyl porphyrin

ig. 3. (a) Jobs plot of continuous variation for C60/1 system and (b) BH plot of C60/1ystem estimated in toluene at 298 K.

haracteristic emission of 1 at 572 and 625 nm. This findinguggests that singlet energy transduction to give 1fullerene*/1ollowed by generation of 3fullerene*/1 (by intersystem crossing)s not operative in our present investigations. The spectral changesnally reach a plateau, indicating that the fluorescence quenching

s induced by the complexation with C70 and C60 (inset of Figs. 4(a)nd 4S(b), respectively). Although 1 exhibits fluorescence quench-ng upon the addition of fullerenes, the quenching efficiency of70 is much higher than that of C60. The binding constants arevaluated according to a modified Benesi–Hildebrand equation27] (see Eq. (1)):

F0

F0 − F= 1

A+

{(1

KA

) (1

[fullerene]

)}(2)

n the above equation, F0 and F are the fluorescence intensity of without and with the fullerenes, respectively, and [fullerene]

ndicates the molar concentration of fullerene; A is a constant asso-iated with the difference in the emission quantum yield of theomplexed and uncomplexed porphyrin. By plotting relative fluo-escence intensity, viz., F0/(F0− F) versus 1/([fullerene]), excellentinear plots have been obtained for both the complexes C70/1 and60/1 (Figs. 4(b) and 5S, respectively). The values of K are given inable 1. Trend in the K values for the fullerene/1 complexes suggests

hat tight fixation of C70 takes place in the well defined structure of

giving rise to correct host–guest orientation. The order of K valuesetermined by fluorescence method corroborates fairly well with

a ksingletCS = (1/�f)sample − (1/�f)ref.

b ˚singletCS = [(1/�f)sample − (1/�f)ref]/(1/�f)sample.

those obtained from the UV–Vis absorption studies (see Section 3.1and Table 1).

3.2.2. Time resolved fluorescence studiesIn our present investigations, we have performed time-

resolved fluorescence measurements for all the fullerene/1complexes in toluene medium. The time-resolved fluorescenceof 1 (5.6 × 10−6 mol dm−3), reveals a single exponential decaywith a life-time of 1.30 × 10−10 s (Fig. 5). Addition of C60(5.4 × 10−5 mol dm−3) and C70 (5.4 × 10−5 mol dm−3) solution tothe toluene solution of 1 (5.6 × 10−6 mol dm−3), accelerates thedecay process and the decay curve may satisfactorily be fitted toa bi-exponential decay (Fig. 5). The measured life time (�) value of1 in the presence of C60 and C70 is found to be 1.158 × 10−10 s and1.030 × 10−10 s, respectively. The rate of charge separation (kCS)calculated from the fast decaying component is estimated to be1.0 × 109 s−1 and 2.1 × 109 s−1 for C60/1 and C70/1 systems, respec-tively, and the charge separation quantum yield (˚CS) is calculatedto be in the range of 0.1160–0.2130 indicating efficient charge sep-arations for the presently investigated supramolecule. Values of kCSand ˚CS obtained from the time-resolved fluorescence experimentsare listed in Table 2. Table 2 indicates that ˚CS(C70/1) is estimated tobe higher than that of ˚CS(C60/1). This feature supports the fact thatthere is a mark reduction in the fluorescence intensity of the 1 in thepresence of C70 than that of C60 as we observe in steady-state fluo-rescence experiments. The time resolved fluorescence experimentsestablish that at least in toluene, where there is a weaker over-lap between the porphyrin fluorescence and fullerene absorption,singlet–singlet energy transfer dominates.

We have determined the rates of charge separation of theexcited singlet (kCS) and quantum yields of charge separation ofthe excited singlet (˚CS) in an usual manner employed for theintermolecular process taking place between fullerenes and 1, andthe corresponding data are provided in Table 2. An examination ofTable 2 reveals the following: (i) the estimated values of kCS and˚CS are found to be higher for C70/1 system as compared to theC60/1 system. This observation is consistent with the close prox-imity of the octaethylporphyrin molecule 1 and fullerene entityin the C70/1 system as obtained from the magnitude of K value ofsuch system; and (ii) in agreement with the steady state emissionresults, time-resolved fluorescence experiment suggests that withthe increasing value of electron affinity of the acceptor (here C70),kCS and ˚CS increase.

3.3. Binding constants

Binding constants (K) obtained from UV–Vis absorption andfluorescence studies of the fullerenes/1 complexes are summa-rized in Table 1. It is observed that the K values for C60/1complex is comparable to those of the C60 complexes of zinc(II)

3 −1

tetramer (K = 5800 dm3 mol−1) [38]. However cyclic dimer ofZn–porphyrin (K = 110,000 dm3 mol−1) [39] and porphyrin hex-amer (K = 1.4 × 108 dm3 mol−1) [40] shows much higher binding

190 P. Mukherjee, S. Bhattacharya / Spectrochimica Acta Part A 90 (2012) 186– 192

F 0−5 mr scenc

aif[wtdaclhpi

ig. 4. (a) Fluorescence quenching of 1 (5.6 ×10−6 mol dm−3) by C70 (from 3.2 × 1elative fluorescence intensity versus [C70] for C70/1 complex at 298 K; (b) BH fluore

ffinity towards C60. The most important finding of the presentnvestigations is that extent of binding of 1 towards C60 and C70 isound to be small than the corresponding Zn(II)octaethylporphyrin41] molecule which is analogous to our earlier investigationith free base and metalated bisarylmonoporphyrin [42] and

etrahexylporphyrin–fullerene [43] experiments. We assume that airect interaction between the Zn metal of the porphyrin moleculend the surface of the fullerene may led to enhancement in bindingonstant. It is well known that the stability of the supramolecu-

ar complexes depends upon attractive interactions between theost and guest and on salvation of the binding partners [44]. In theresent investigations, since the host and guest separately solvated

n solution, the desolvation of the host and guest is requisite in the

ol dm−3 to 1.76 × 10−4 mol dm−3) in toluene medium; inset of (a) depicts plot ofe plot of C70/1 system in toluene at 298 K.

association process. However the process of desolvation is an ener-getically uphill task and hence an association takes place only whenthe energy gain between the host and guest interaction exceeds thisunfavorable energy. From the trends in K values of the fullerenes/1complex, we may infer that the extent of solvation and desolva-tion of the binding partner might play an important role in formingweak or strong supramolecular complexes. Haino et al. [45] showedthat the stability of complex increases as the solubility fullerene insolvents decreases, since less energy is required for the desolvation

of fullerene which must necessarily precede its complexation witha host molecule. This would be one of reasons for higher K valuesof C70/1 complex in toluene medium, since the solubility of C60 ishigher in toluene (4.1 mg L−1) than that of C70 (2.0 mg L−1) [46].

P. Mukherjee, S. Bhattacharya / Spectrochim

Fig. 5. Time resolved fluorescence decay profile of 1 (5.6 × 10−6 mol dm−3)in the absence and in the presence of C60 (5.4 × 10−5 mol dm−3) and C70

(5.4 × 10−5 mol dm−3). In this figure blue, magenta and wine red color indicate decaypit

3

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THeM

rofile of uncomplexed porphyrin (1), C60/1 and C70/1 systems, respectively. (Fornterpretation of the references to color in this figure legend, the reader is referredo the web version of the article.)

.4. Computational studies

Table 1 reports the K values of various fullerene/1 complexes.t is observed that for all the complexes studied, C70 exhibitsarger value of K in comparison to C60. The most important find-ng of the present investigations is the selectivity ratio in termsf K value between C70 and C60, i.e., KC70 /KC60 . KC70 /KC60 is esti-ated to be in our present investigations. The moderate value of

verage selectivity in binding, i.e., ∼3.5, suggests that 1 may dis-riminate C60 from C70 in solution. Such a stabilization of the C70/1upramolecular complexes may be attributed due to the presencef intermolecular interaction between the graphitic 6:6 plane of70 and the monoporphyrin. Primarily, the attractive interactionsetween C70 and the monoporphyrin are driven by the presence ofispersive forces associated with �–� interactions. The most con-rete evidence of the above statement is illustrated by the side-onather than end-on binding of C70 with the plane of 1. Molecu-ar mechanics force field (MMMF) calculations well reproduce thebove feature regarding orientation of bound guest (here C60 and70) with the plane of 1. Thus, in the case of C70/1 complex, theide-on interaction of C70 with 1 generates enthalpies of forma-ion (�H0

f ) value of −403.6 kJ mol−1 (Table 3). However, the end-onattern of C70 molecule becomes side-on towards the plane of 1uring geometry optimization of the C70/1 complex in MMMF cal-ulations. It is already well established that the 6:6 ring-junctureond of the C70, rather than 6:5 ring-juncture bond, lies closest

o the porphyrin plane as the 6:6 “double” bonds of C70 are morelectron-rich than 6:5 “single” bonds [12,47]. Thus, the equatorialace of C70 is centered over the electropositive center of the por-hyrin plane, which can be viewed as an enhancement in van der

able 3eat of formation value (�H0

f), surface area (A), volume (V), solvent reorganization

nergies (Rs) and dipole moment (�) of the fullerene/1 complexes at 298 K done byMMF method.

System (�H0f)

(kJ mol−1)A (A2) V (A3) Rs (eV) � (D)

C60/1 −83.5 1059.95 1214.90 −0.259 2.26C70/1 −403.6 1121.12 1310.95 −0.7588 1.90

ica Acta Part A 90 (2012) 186– 192 191

Waals interaction due to availability of greater surface area andvolume (see Table 3) favoring strong �–� interactions. It shouldbe noted at this point that C60/1 complex exhibits much lowervalue of �H0

f , viz., −83.5 kJ mol−1 compared to C70/1 complex aslisted in Table 3. Single projection geometric structures for all thefullerene/1 complexes are visualized in Fig. 6S.

3.5. Solvent reorganization energies (Rs)

The effect of solvent over interaction between fullerenes and1 can be better understood by the estimation of solvent reor-ganization energy (Rs) for the fullerene/1 complexes. The totalreorganization energy, in general, is a sum of the two terms,i.e., inner-sphere reorganization energy (solvent-independent) R0,and outer-sphere reorganization energy (solvent-dependent) Rs

[11,48]. In case of fullerene, contribution from R0, which is relatedto the differences in nuclear configurations between an initial andfinal state of the fullerene, is very small, i.e., ∼4.3 × 10−5 eV. Thisfinding implies that the structures of both C60 and C70 are very muchsimilar in the ground and excited states, which relates to the rigid-ity of these spherical carbon structures. As far as Rs contribution isconcerned, this is believed to be small as well [49]. The symmet-rical shape and large size of the fullerene framework require littleenergy for the adjustment of an excited or reduced state to thenew solvent environment. In the present investigations, Rs of C60/1and C70/1 complexes has been estimated by applying the dielectriccontinuum model [50] developed by Hauke et al. (see Eq. (3)).

Rs =(

e2

4�ε0

)[{1

2R1+ 1

2Rfullerene− 1

RD/A

}1εs

−{

12Rfullerene

+ 12R1

}1εR

](3)

With the following parameters: radius of donor, R1 = 4.233 A; radiusof acceptor (Rfullerene): RC60 = 4.2 A, RC70 = 4.4 A; donor–acceptorseparation (RD–A): RC60–1 = 4.490 A, RC70–1 = 3.312 A; solventdielectric constant, εs (εtoluene = 2.39); solvent dielectric constantfor electrochemical measurements, εR = 9.93. Values of Rs for theabove supramolecular systems are given in Table 3. Table 2 clearlysuggests that C70/1 system is stabilized convincingly in comparisonto C60/1 system.

4. Conclusions

From the foregoing discussions, we may conclude that thedesigned monoporphyrin, namely 1, forms effective ground statenon-covalent complexes with both C60 and C70 in toluene medium.Steady state quenching experiment reveals that static quenchingis the key factor behind the decrease in fluorescence intensity ofuncomplexed 1 in the presence of fullerenes. Magnitude of averageselectivity in binding i.e., KC70/1/KC60/1 (∼3.5) suggests that 1 maybe employed as a selective receptor molecule for C70. MMMF cal-culations well interpret the favorable binding between C70 and 1 incomparison to C60 and 1 in vacuo. Solvent reorganization energiesclearly demonstrate that C70/1 system is stabilized convincingly incomparison to C60/1 system.

Acknowledgments

PM thanks The University of Burdwan for providing basicresearch facility to him. SB wishes to record sincere gratitude toUGC, New Delhi and DST, New Delhi for providing financial assis-tance to him.

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92 P. Mukherjee, S. Bhattacharya / Spec

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.saa.2012.01.031.

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