Journal of Molecular Structure 966 (2010) 69–78

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Journal of Molecular Structure

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Photophysical investigations on supramolecular interaction of a C60 derivativewith free-base and metallo-phthalocyanines

Anamika Ray a, Shrabanti Bhattacharya Banerjee b, Subrata Chattopadhyay c, Sumanta Bhattacharya a,*

a Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, Indiab Department of Chemistry, Raja Rammohan Roy Mohavidyalaya, Radhanagore, Hooghly, West Bengal 712 406, Indiac Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 September 2009Received in revised form 21 November 2009Accepted 7 December 2009Available online 8 January 2010

Keywords:C60-derivativeH2- and Zn–PcCT absorption bandsQuenching of fluorescenceBinding constantsDFT calculations

0022-2860/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.molstruc.2009.12.010

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

The present paper reports the photophysical investigations on non-covalent interaction of a C60-deriva-tive, namely, tert-butyl-(1,2-methanofullerene)-61-carboxylate (1) with H2– (H2–Pc) and Zn–phthalocy-anine (Zn–Pc) in toluene medium. Well defined charge transfer (CT) absorption bands have been locatedin the ground state. Utilizing the CT absorption bands, various consequential physico-chemical parame-ters like oscillator strength, transition dipole moment, resonance energy, electronic coupling element andsolvent reorganization energy, have been estimated for the complexes of 1 with phthalocyanines. Theinfluence of 1 on the spectral characteristics of H2– and Zn–Pc are explained using a theoretical modelthat takes into account the interaction between electronic subsystems of 1 with the phthalocyanines(Pcs). Steady-state fluorescence experiment reveals large binding constants in the magnitude of 68,775and 31,750 dm3 mol�1, for the 1:1 complexes of 1 with H2- and Zn–Pc, respectively. Density functionaltheoretical calculations reveal significant redistribution of charges between 1 and Pcs.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

The weak molecular interactions, as found in natural donor–acceptor ensembles, bears a deep fascination as a superior meansto control the organization of photo- and redox-active componentsas well as their mutual electronic coupling. Stimulated by this vi-sion and considering the significant relevance to natural photosyn-thetic events, substantial efforts are devoted to the development ofnon-covalent linkages, associating the requisite donor and accep-tor [1,2]. In light of the above phenomena, self-assembly [3],molecular recognition [4] and inclusion phenomena [5] emergeas meaningful assets in devising supramolecular architectures.Since the key steps, either spontaneous or chemically induced,are often thermodynamically driven processes, the yields of self-assembled ensembles are likely to exceed those involving the for-mation of covalent bonds. Certainly, one of the major challengesthat still lie ahead of us is to regulate the weak forces, on a molec-ular basis, which will dictate two most important parameters,namely, size and shape in relation to function of the resulting com-posite. In the pursuit of artificial photosynthetic reaction centers,non-covalent supramolecular systems displaying photo-inducedenergy and electron transfer processes have attracted considerableattention [6,7]. Fullerenes make particularly suitable building

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ttacharya).

blocks for the construction of such multi-component systems be-cause of their three-dimensional structure, relatively low reduc-tion potentials, and strong electronic acceptor properties [8]. Onthe other hand, phthalocyanine (Pc) and its metallic complexesare remarkable both for their stability and potential use for obtain-ing new macrocyclic materials [9]. The variety of complexes of thePc or their derivatized form, make the compounds of interest fortheir applications in many fields as colorants, optical data storage,anti-cancer agents, solar energy conversion and catalysts, etc. [10].Thus, a fullerene-based/Pc aduct can be applied as a potentialmaterial for the construction of photovoltaic cells [11]. Recently,asymmetrical metallo phthalocyanines (Pcs) containing a singlefullerene (C60) substituent with rigid linkers at the periphery [12]and another asymmetrical C60/zinc phthalocyanine (ZnPc) hybridwith a flexible linker containing an azacrown [13] subunit havebeen synthesized to study the electron transfer and energy transferbetween Pc donor and C60 accepter. Very recently, Torres et al.have nicely demonstrated the self-organization phenomena ofC60/Pc dyads into liquid crystals by the use of blends in which amesogen induces mesomorphism into a non-mesogenic compound[14]. Kahnt et al. also show the photophysical properties of a newmulti component bis(C60)/bis(Pc) nanoconjugate [15]. Photo-phys-ical characterization of the conjugate has been carried out in differ-ent solvents, employing a bis(Pc) containing two ethynylphenylmoieties as reference compound [16]. However, according to ourbest of knowledge, there are no reports on both experimental

70 A. Ray et al. / Journal of Molecular Structure 966 (2010) 69–78

and theoretical approach for determination of electronic and struc-tural properties of the non-covalent fullerene/Pc systems based ona C60-derivative, namely, tert-butyl-(1,2-methanofullerene)-61-carboxylate (1) (Fig. 1). In this study, we have investigated theground and excited state properties of the supramolecular com-plexes of 1 (Fig. 1) with free-base (H2–) and Zn–Pc [17] by meansof UV–Vis absorption spectroscopic, steady-state fluorescenceand quantum chemical calculations in terms of density functionaltheory (DFT) methods.

2. Experimental

1 has been purchased from Sigma–Aldrich, USA. Both H2– andZn–Pc are obtained from Sigma–Aldrich, USA. The solvent tolueneis of UV spectroscopic grade and has been collected from Merck,Germany. All UV–Vis spectral measurements are recorded on aUV 2450 PC model Shimadzu spectrophotometer fitted with a Pel-tier controlled thermo bath. Steady-state fluorescence measure-ments have been performed in a F-4500 Hitachispectrofluorimeter. DFT calculations are done using SPARTAN ’06Windows version software.

3. Results and discussion

3.1. Observation of CT bands

The UV–Vis absorption spectrum of 1 in toluene shows verysimilar sort of spectral characteristics of parent C60 (Fig. 1S(a)).On the contrary, both H2– and Zn–Pc exhibit two different set ofabsorption bands, namely, Q and Soret (Fig. 1S(b) and (c)). TheseQ and Soret features correspond to the S1 S0 and S2 S0 transi-tions, respectively [18]. Location of Q and Soret-absorption bandsfor H2– and Zn–Pc in toluene medium are listed in Table 1. Groundstate electronic interaction between 1 and Pcs has been first evi-denced from UV–Vis experiment. Addition of 1 (in toluene) to a tol-uene solution of both H2– (Fig. 2S) and Zn–Pc (Fig. 2) producesappearance of additional absorption peak in the visible region. Thisis a clear sign of a strong electronic interaction between the twoparent molecules in the ground state. Figs. 2S & 2 show the elec-tronic absorption spectra of the mixtures containing 1 + H2–Pcand 1 + Zn–Pc, respectively, in toluene medium along with thespectrum of 1. The newly obtained peaks can be assigned as chargetransfer (CT) absorption peak. To obtain the CT peaks, spectra of

Fig. 1. Structure of 1.

Table 1Q and Soret-absorption bands of H2– and Zn–Pc recorded in toluene medium. Temp.298 K.

Phthalocyanine Q absorption band (nm) Soret absorption band (nm)

H2–Pc 655, 693 342Zn–Pc 606, 642, 671 336

above solutions (in toluene medium) are recorded against the pris-tine donor (Pc) solution as reference to cancel out the donor’sabsorbance. The solvent toluene does not absorb in the visible re-gion. Formidable support in favor of CT peaks come from the factthat the absorbance value at CT peak maxima, viz., kCT, increasesgradually with the increasing concentration of 1 (see Figs. 2S &2). The CT peak can be utilized further for the determination of var-ious physico-chemical parameters. The CT absorption spectra havebeen analyzed by fitting to the Gaussian function y = y0 + [A/(wp

(p/2)] exp[�2(x � xc)2/w2] where x and y denote wavelengthand absorbance, respectively. A is defined as a constant; w and xc

can be recognized as width of the curve and center of the CT max-ima, respectively. One typical Gaussian analysis plot of complex of1 with Zn–Pc is shown in Fig. 3. The wavelengths at these newabsorption maxima (kmax = xc) and the corresponding CT transitionenergies (hmCT) are summarized in Table 2.

3.2. Determination of oscillator (f’s) and transition dipole strengths(lEN’s)

From the CT absorption spectra, we can estimate the value ofoscillator strength. The oscillator strength, f, is calculated usingthe formula [19]

f ¼ 4:32� 10�9Z

eCT dm ð1Þ

where eCT andReCT dm are the molar extinction coefficient of the CT

complex obtained at CT maxima and the area under the curve of theextinction coefficient of the absorption band in the above equationversus the frequency, respectively. The range of integration remainsbetween zero and infinity. To a first approximation

f ¼ 4:32� 10�9emaxDm1=2 ð2Þ

where emax is the maximum molar extinction coefficient of the CTband and Dm1/2 is the half-width, i.e., the width of the band at halfthe maximum molar extinction. The observed oscillator strengths ofthe CT bands are summarized in Table 2. Table 2 reveals that thecomplex of Zn–Pc with 1 exhibits higher value of oscillator strengthcorresponding to free-base Pc. It is worth mentioning that we needa proper calculation of oscillator strengths for the 1/Pc CT com-plexes. This is because oscillator strength is very much sensitiveto the molecular configuration and the electron charge distributionin the CT complex. In case of the presently investigated 1/Pc com-plexes, we cannot use simple model assuming a charge localizedat a certain cite of fullerene sphere; this is because the fullerenehave p-bonds, which are directed radially, with a node on themolecular cage.

The extinction coefficient is related to the transition dipole by[19]

lEN ¼ 0:0952½emaxDm1=2=Dm�1=2 ð3Þ

where at emax and lEN is defined as �eRwex(ri)wg ds. Values of lEN

for the complexes of 1 with H2– and Zn–Pc are furnished in Table 2.

3.3. Determination of resonance energy (RN)

Resonance energy of the non-covalent complexes of 1 with Pcshave been determined by using the formula derived by Briegleband Czekalla [20]:

emax ¼ 7:7� 104=½ðhmCT=jRNjÞ � 3:5� ð3Þ

where hmCT and RN are the CT transition energy and the resonanceenergy of the complex in the ground state, respectively, which obvi-ously is a contributing factor to the binding constant (K) of the

Fig. 2. UV–Vis absorption spectra of uncomplexed 1 (a) and 1 + Zn–Pc mixtures recorded in toluene medium against the pristine donor solution as in reference; theconcentration of Zn–Pc is kept fix at a concentration of 5.77 � 10�6 mol dm�3 and 1 varies in the following concentrations range: (b) 4.0 � 10�6 mol dm�3, (c) 5.35 � 10�6 moldm�3, (d) 6.70 � 10�6 mol dm�3, (e) 8.0 � 10�6 mol dm�3, (f) 9.40 � 10�6 mol dm�3, (g) 10.7 � 10�6 mol dm�3, (h) 12.0 � 10�6 mol dm�3, (i) 13.3 � 10�6 mol dm�3 and (j)16.7 � 10�6 mol dm�3.

1.40x104 1.44x104 1.47x104 1.50x104 1.54x104 1.58x104

0.00

2.50x104

5.00x104

7.50x104

1.00x105

1.25x105

1.50x105

ε , d

m3 .m

ol-1.c

m-1

ν, cm-1

Fig. 3. Gaussian analysis plot for 1/Zn–Pc system.

Table 2Various physico-chemical parameters of the complexes of 1 with H2– and Zn–Pc intoluene medium at 298 K; CT transition energy (hmCT), Oscillator strength (f),transition dipole strength (lEN), resonance energy (RN), electronic coupling element(V) and solvent reorganization energy (RS).

System kmax (nm) hmCT (eV) f lEN (Debye) RN (eV) V (cm�1) RS (eV)

1/H2–Pc 671 1.85 0.0024 2.34 0.02 420 �0.5051/ZnPc 671 1.85 0.0317 9.76 0.26 1555 �0.530

A. Ray et al. / Journal of Molecular Structure 966 (2010) 69–78 71

complex (a ground-state property). The values of RN for the com-plexes under study are given in Table 2.

3.4. Determination of electronic coupling element (V) andreorganization energy (R) for the complexes of 1 with H2– and Zn–Pc

The electronic coupling element (V) is related to the extent ofoverlap between the appropriate donor or host and acceptor or

guest orbitals, and scales the dependency of free energy of electrontransfer (i.e., driving force) on such rate constant. The electronicinteraction between 1 and Pcs in 1/Pcs systems, mix electroniccharacter and induces electron transfer (ET). It also creates an elec-tronic basis for inducing dipole-allowed optical ET with the magni-tude of the perturbation dictating the intensities of the intervalence transfer bands. This assumption implies that the value ofelectronic coupling element between 1 and Pc is small or moderateassuming an ET process in terms of Marcus formalism [21] In thepresent investigation, V has been estimated from the absorptionmaxima of 1/Pc complexes by applying the model proposed byVerhoeven et al. [22] Values of V for the complexes 1 with H2–and Zn–Pc are listed in Table 2. Estimated electronic coupling ele-ments obtained for the above systems are very much comparableto the other donor–acceptor and host–guest systems found in theliterature [23]. V values determined in the present investigationare significantly larger than typical value for the solvent-separatedradical–ion pair (12 cm�1) [23], indicating that 1 and Pcs are inclose contact to form contact radical–ion pair. The higher V valuein case of Zn–Pc complex arises due to the through space energytransfer pathway which originates from the Zn–Pc moiety. Thereare some recent observations of large solvent dependence of en-ergy transfer rate constants and electronic coupling elements inelectron donor–acceptor systems where the donor and acceptorare in close proximity. Zimmt et al. have also reported the largesolvent dependent electronic coupling matrix element for theirC-clamp-shaped molecule [24]. They propose that solvent medi-ated super exchange coupling phenomenon accounts for higher Vvalues. Therefore, the effect of solvent over V is likely to be impor-tant in the present systems. This can be better understood by theestimation of solvent reorganization energies (Rs) for the 1/Pcscomplexes. In general, the total reorganization energy, R, is asum of the two terms, i.e., inner-sphere reorganization energy (sol-vent-independent) R0, and outer-sphere reorganization energy(solvent-dependent) Rs. In case of fullerene, contributions fromR0, which is related to the differences in nuclear configurations be-tween an initial and a final state of the fullerene, are very small, i.e.,�4.3 � 10�5 eV. This observation implies that the structure of 1 inthe ground and excited states is very much similar, which relates

0.0 5.0x10-6 1.0x10-5 1.5x10-5 2.0x10-5 2.5x10-5 3.0x10-5

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Δ Abs

.

[1], mol.dm-3

Fig. 4. Variation of absorbance vs. concentration of 1 for 1/Zn–Pc system.

72 A. Ray et al. / Journal of Molecular Structure 966 (2010) 69–78

to the rigidity of these spherical carbon structures. As far as Rs con-tribution is concerned, this is also believed to be small, as well.Thus the symmetrical shape and large size of the fullerene frame-work requires little energy for the adjustment of an excited or re-duced state to the new solvent environment. In the presentinvestigation, Rs of 1/H2– and 1/Zn–Pc complexes have been esti-mated applying the dielectric continuum model developed by Hau-ke et al. [25]. Values of Rs for the above supramolcular systems aregiven in Table 2. It is to be mentioned that solvent reorganizationenergies obtained in the present investigation do not corroboratewell with that observed for quinone/porphyrin system [24]. Thediscrepancy in the value of Rs for quinone/porphyrin and fuller-ene/Pc systems may be due to the subtle structural change in thehost–guest complexes which exert a large influence upon thephoto-induced electron and/ energy transfer process.

3.5. Theoretical model in favor of electric dipole–dipole interactionbetween 1 and Pcs

Consider the interaction between 1 and Zn–Pc. The interactionbetween the dipole–dipole transitions for the above mentionedmolecules can be represented in the form

H ¼XNmax

i¼1

d1r1x di

Zn—PcrZn—Pcx;i

� �ð1� 3 cos2 hiÞ=e1r3

i ð4Þ

where d1 and di[Zn–Pc] are dipole moments of the corresponding

transitions in 1 and the ith number of Zn–Pc molecule, respectively;rx and rx,i

Zn–Pc are the corresponding Pauly matrices; ri is the dis-tance between 1 and Zn–Pc molecules; and e1 in Eq. (4) is thehigh-frequency dielectric constant. The reconstruction of the result-ing spectrum, taking into account Eq. (4), is determined by mixingof the states of the derivatized fullerene molecule and the surround-ing Zn–Pc molecule.

Ei� ¼ ðE1 þ EZn—PcÞ=2� ½ðE1 � EZn—PcÞ=2�2 þ Vij2

n o1=2ð5Þ

For one 1/Zn–Pc pair, Eq. (4) gives Eq. (5), where E1 and EZn–Pc

are the energies of dipole transitions of 1 and Zn–Pc, respectively,and Vi (= [d1di

Zn–Pc – (1 � 3 cos2hi)]e1�1ri�3) is the matrix element

of the state mixing. The final expression has the form

e� ¼ eð0Þ� � ðjVijN1=2Þ=2 ð6Þ

where V is the amplitude of the non-diagonal flip-flop dipole–dipolematrix element for 1 and of the dipole transitions in neighboringZn–Pc molecule and N is the number of neighboring Zn–Pc mole-cule. Such a dependence of the absorption band edge is valid onlyunder the condition N < Nthr, where Nthr is the maximum numberof 1 molecule that can take part in the dipole–dipole flip–flop inter-action with Zn–Pc. A further increase in the concentration of 1 doesnot increase the number of these molecules in the nearest environ-ment of Zn–Pc. Fig. 4 and Fig. 3S show that the dependences are sat-urated when the concentration of Zn–Pc and H2–Pc exceed2.0 � 10�5 and 9.0 � 10�6 mol.dm�3, in case of 1/Zn–Pc and 1/H2–Pc complexes, respectively, in agreement with the theory. Thismechanism, thus, allows us to explain the formation of the CT com-plexes in the systems under study.

3.6. Fluorescence investigation

The photo-induced behavior of the complexes of 1 with H2– andZn–Pc has been investigated by steady-state emission measure-ments. It is observed that the fluorescence of Pcs (both H2– andZn–Pc) upon excitation at Soret-absorption band diminishes grad-ually during titration with 1 in toluene medium. This indicates thatthere is a relaxation pathway from the excited singlet state of the

Pcs to that of 1 in the said solvent. It is already reported that chargeseparation can also occur from the excited singlet state of the Pc tothe fullerene in fullerene/Pc hybrid system [26]. Photo-physicalstudies already prove that in case of conformationally flexibledyads comprising fullerenes and macrocyclic receptor moleculeslike porphyrin, where p-stacking interactions facilitated thethrough-space interactions between these two chromophores,which is demonstrated by quenching of 1*porphyrin fluorescenceand formation of fullerene-excited states (by energy transfer) orgeneration of fullerene�/porphyrin+ ion-pair states (by electrontransfer) [27]. However, in non-polar solvent like toluene, energytransfer generally dominates (over the electron transfer process)the photo physical behavior in deactivating the photo excited chro-mophore 1*Pc. Similar sort of rationale has already been providedby Yin et al. for their particular cis-2

0,50-dipyridinylpyrrolidi-

no[30,40:1,2]C60/zinc(II)tetraphenylporphyrin supramolecule [28].

In the present investigations, therefore, the quenching phenome-non can be ascribed to photo-induced energy transfer from Pc tofullerene entity in the supramolecular 1/H2–Pc and 1/Zn–Pc sys-tems. In case of 1/Zn–Pc system, it is observed that fluorescenceintensity at 680 (strong) and 745 nm (weak) nm upon excitationat 336 nm has been quenched by the addition of 1. The titrationexperiment is carried out maintaining a constant concentrationof H2– and Zn–Pc. This phenomenon certainly ascribes due to thecomplexation between 1 and Zn–Pc. Variation of the fluorescencespectra of Zn–Pc by the addition of 1 is shown in Fig. 5(a). Similarsort of luminescence feature is observed for 1/H2–Pc system(Fig. 4S(a)). The spectral changes finally reach a plateau, indicatingthat the fluorescence quenching is induced by the complexation(inset of Fig. 5(a) and Fig. 4S).

3.7. Determination of K

(a) Fluorescence investigationsAs ground state complex formation between 1 and Pc is evi-

denced from observation of CT bands (as discussed in Section (3.1)),let us consider the formation of a non-fluorescent 1:1 complexaccording to the equilibrium:

1þ Pc�1=Pc ð7Þ

The fluorescence intensity of the solution decreases upon additionof 1. Using the relation of binding constant (K) we obtain

K ¼ ½1=Pc�=½1�½Pc� ð8Þ

Imposing the mass conservation law, we can write

0

2000

4000

6000

8000

10000

640 660 680 700 720 740 760 780 800

Emission wavelength (nm)

Flu

ores

cenc

e in

tens

ity

0.0 4.0x10-6 8.0x10-6 1.2x10-5 1.6x10-50.0

0.4

0.8

1.2

1.6

2.0

2.4

F 0/F

[1], mol.dm-3

[1]

0.0 4.0x10-6 8.0x10-6 1.2x10-5 1.6x10-5 2.0x10-52

4

6

8

10

12

14

16

Rel

ativ

e fl

uore

scen

ce in

tens

ity

[1], mol.dm-3

(a)

(b)

Fig. 5. (a) Fluorescence spectral variation of Zn–Pc (4.750 � 10�6 mol dm�3) in presence of 1; the concentration of 1 increases from top to bottom according to the arrowmark as indicated in the figure and the inset of Fig. 5(a) also shows the plot of relative fluorescence intensity vs. concentration of 1 for 1/Zn–Pc system and (b) SV plot for 1/H2–Pc system in toluene medium.

A. Ray et al. / Journal of Molecular Structure 966 (2010) 69–78 73

½Pc�0 ¼ ½Pc� þ ½1=Pc� ð9Þ

where [Pc]0, [Pc] and [1/Pc] are the initial concentrations of Pc, Pc inpresence of fullerene, and 1/Pc complex, respectively. Eq. (9) can berearranged as

½Pc�0=½Pc� ¼ 1þ ½1=Pc�=½Pc� ð10Þ

Using the value of K in place of [1/Pc]/[Pc] from Eq. (8), we can writeEq. (10) as follows

½Pc�0=½Pc� ¼ 1þ K½1� ð11Þ

Considering the fluorescence intensities are proportional to the con-centrations, Eq. (11) is expressed as

F0=F ¼ 1þ K½1� ð12Þ

where, F0 is the fluorescence intensity of Pc in absence of 1 and F isthe fluorescence intensity of Pc in presence of quencher (i.e., 1). Inour present investigations, steady-state fluorescence quenchingstudies afforded excellent linear plots (Fig. 5(b) and Fig. 4S(b)),which is explained by fluorescence of both H2– and Zn–Pc are beingquenched only by static mechanism, as opposed to diffusionalquenching process. All the data of steady-state fluorescence investi-

gations are given in Table 3. K values of the 1/H2–Pc and 1/Zn–Pcsystems are given in Table 4.

(b) UV–Vis investigationsK values of the 1/H2–Pc and 1/Zn–Pc complexes are also deter-

mined in the present investigations from the UV–Vis titrationexperiment. It is observed that the value of absorbance of the com-plexes of 1 with both H2– and Zn–Pc, at kCT, increases systemati-cally with the increasing concentration of 1 (Figs. 2 & 2S). Kvalues for the presently investigated supramolecules have beenestimated in accordance with the Benesi–Hildebrand (BH) Eq.[29]. Excellent linear BH plots are obtained with the present data.One typical BH plot of 1/Zn–Pc system is shown in Fig. 6. Values ofK are listed in Table 4.

3.8. Binding constant (K)

Table 4 shows that 1 undergoes considerable amount of com-plexation with both H2– and Zn–Pc as K1/H2–Pc(av) and K1/Zn–Pc(av)

are estimated to be 68,775 and 31,750 dm3 mol�1, respectively.However, very recently, we have reported the K values of the com-plexes of C60 with H2– and Zn–Pc as 47,440 and 23,500 dm3 mol�1,

Table 3Variation of fluorescence intensities of the complexes of 1 with H2– and Zn–Pc alongwith the concentration of 1 in toluene recorded at 298 K.

System Concentration of 1(mol dm�3)

Concentration of Pc(mol dm�3)

Fluorescenceintensity

1/H2–Pc 0.0 6.206 � 10�6 44752.057 � 10�6 41134.114 � 10�6 38756.170 � 10�6 34088.230 � 10�6 3233

10.284 � 10�6 291612.340 � 10�6 272714.400 � 10�6 269416.455 � 10�6 2434

1/Zn–Pc 0.0 4.760 � 10�6 93891.955 � 10�6 88013.915 � 10�6 83955.870 � 10�6 79087.830 � 10�6 75439.786 � 10�6 7123

11.743 � 10�6 680413.700 � 10�6 651115.660 � 10�6 609817.615 � 10�6 578119.570 � 10�6 5379

0.0 4.0x10-6 8.0x10-6 1.2x10-5 1.6x10-5 2.0x10-5 2.4x10-5

0.00

3.50x10-11

7.00x10-11

1.05x10-10

1.40x10-10

1.75x10-10

2.10x10-10

2.45x10-10

[1][

ZnP

c]/Δ

Abs

., (m

ol.d

m-3)2

[1], mol.dm-3

Fig. 6. BH plot of for 1/Zn–Pc system recorded in toluene medium.

74 A. Ray et al. / Journal of Molecular Structure 966 (2010) 69–78

respectively [17]. Very large value of K for the non-covalentlylinked C60/ZnPc dyad system, in which two photoactive units arebrought together by a phenylenevinylene spacer, is already beenreported by Torres et al. (K = 2.9 � 105 dm3 mol�1) [30]. Apart fromthat, Martinez-Diaz et al. have estimated high K value(K = 1.53 � 104 dm3 mol�1) for their particular system comprisingC60 pseudorotaxane and Pc moieties [31]. Thus, selectivity in K va-lue between H2– and Zn–Pc is totally reversed using derivatizedfullerene in our present investigations (see Table 4). It is clear thata number of small effects conspire to affect the magnitude of K indifferent ways that do not always lend themselves to easy decon-volution. The report that the order of K changes slightly betweenH2– and Zn–Pc for the same host shows that subtle difference insolvation energies is an important factor in this matter [32]. An-other interesting feature of the present investigation is that 1 can-not serve as a good discriminator between free-base and Zn–Pc, asselectivity in K values of 1/H2–Pc complex over 1/Zn–Pc complex isestimated to be 2.15. The binding of 1 to both H2– and Zn–Pcreceptors is often comparable strength to Pd bisporphyrin cleft(K = 37,000 dm3 mol�1) [33], stronger than a designed porphyrinhosts like Zn(por)[RuPor]4 box (K = 9650 dm3 mol�1) [34] and fora number of other first row transition metalloporphyrin host likeJAWS porphyrin (2), i.e., Zn-2 (1950 dm3 mol�1), Mn-2(2760 dm3 mol�1), Fe-2 (490 dm3 mol�1), Co-2 (2980 dm3 mol�1),Cu-2 (4860 dm3 mol�1) and Pd-2 (815 dm3 mol�1) [35]. For thecomplexation with 1, both H2– and Zn–Pc are shown to exhibit29 and 12 times larger K than H2– and Zn–thiacalixarene bispor-phyrin, respectively [36]. The K value of 1/Zn–Pc system corrobo-rates fairly well with the C60 � Im ? ZnNc conjugate(K = 6.2 � 104 dm3 mol�1) developed by Souza and Ito et al. [37].From above discussions, it can be concluded that the simple free-base and Zn–Pc are much more competent in forming productivesupramolecular complexes with derivatized fullerene. We propose

Table 4Binding constants (K, dm3 mol�1) estimated by UV–Vis and steady-state fluorescence invesenergies of charge recombination (DGCR, eV) for the complexes of 1 with H2– and Zn–Pc.

System K (dm3 mol�1) Kav (dm3 mol�1)

UV–Vis Fluorescence

1/H2–Pc 85,930 51,620 68,7751/Zn–Pc 26,150 37,350 31,750

that, like fullerene/porphyrin interaction [38], the higher electrondensity in the ‘double’ bond at the 6:6 ring juncture of 1 is at-tracted to the protic center of the free-base Pc or the electroposi-tive center of the metallo Pc. It is assumed that at a global level,fullerene is expected to behave as an electron acceptor in formingCT complex, but at the local level, a specific fullerene bond may do-nate electron density to the positive center of the free-base ormetallophthalocyanine.

3.9. Theoretical calculations

Accurate quantum chemical calculations for the elucidation ofelectronic structures as well as association energies of the 1/Pcensembles is a tedious job due to the presence of large numberof basis functions resulting from the large size of the system whichlimits the use of methods. We will describe here an approach thatappears to be capable of precisely mimicking such results with afar-less expensive treatment of electron correlation.

In our present investigations, the existence of the electrostaticinteractions between the Pc macrocycle and fullerene moiety havebeen evidenced by the results obtained on frontier molecular orbi-tals, viz., highest occupied molecular orbital (HOMO) and lowestunoccupied molecular orbital (LUMO) using density functional the-ory (DFT) calculations. The frontier orbitals, namely, HOMO andLUMO, have more importance because they can help to explainthe reactivity and physicochemical properties of the molecule.The HOMO–LUMO energy separation has been used as an indexof kinetic stability for fullerenes, as for other type of molecules.DFT calculations reveals that, in the ground state, the majority ofthe HOMO is located on Pcs (both H2– and Zn–Pc) (Fig. 7 andFig. 5S, respectively), while the LUMO is positioned in fullerene en-tity of the complex (Fig. 7 and Fig. 5S, respectively). The absence ofHOMO on fullerene and LUMO on the Pc macrocycles suggest weakCT interactions between 1 and Pc, which is consistent with the factthat CT bands of low intensity are observed for the presently inves-

tigations, heat of formation (DH0f , kJ nmol�1) values, static energies (DGS, eV) and free

KH2—Pc=KZn—Pc DGCR (eV) DH0f (kJ�mol�1) DGS (eV)

2.15 �0.673 �0.730 1.340�0.450 5.490 1.375

Fig. 7. HOMOs and LUMOs of 1/H2–Pc complex at different electronic states; e.g., (a) HOMO, (b) HOMO – 1, (c) HOMO – 2, (d) HOMO – 3, (e) LUMO, (f) LUMO + 1, (g)LUMO + 2 and (h) LUMO + 3 done by DFT/B3LYP/6-31G* calculations.

A. Ray et al. / Journal of Molecular Structure 966 (2010) 69–78 75

tigated supramolecules in the UV–Vis experiments as mentionedabove. It can be seen that LUMO energy (ELUMO) levels of 1/Pc com-plexes compare well with the fullerene guest, while the HOMO en-ergy (EHOMO) levels are similar to those of the uncomplexed Pcreceptor. For example, in the case of 1/Zn–Pc complex, ELUMO iscomputed to be 3.0435 eV, which is comparable to the ELUMO of1, i.e., 3.1031 eV. Also, the EHOMO of the same complex was esti-mated to be 4.8674 eV, which corroborates excellently with thatof Zn–Pc, viz., 4.8911 eV. EHOMO and ELUMO of all the 1/Pcs systems

along with uncomplexed 1, H2– and Zn–Pc are given in Table 5. Themost fascinating feature of the present investigations is that theHOMO–LUMO gap (HLG) for both the 1/H2–Pc and 1/Zn–Pc sys-tems corroborate fairly well with the CT absorption maxima thatwe obtain in UV–Vis experiment. For example, HLG of 1/Zn–Pc sys-tem is estimated to 1.8239 eV done by DFT/B3LYP/6–31G* calcula-tions which is very much close to the experimentally obtain hmCT

value, viz., 1.85 eV. Slight difference between the HLG and hmCT val-ues for the 1/H2–Pc and (�0.17 eV) and 1/Zn–Pc systems

Table 5Comparison of five highest occupied and five lowest unoccupied molecular orbitallevels of H2–Pc, Zn–Pc, 1, 1/H2–Pc and 1/Zn–Pc systems done by DFT/B3LYP/6-31G*calculations.

State Energy (eV)

H2–Pc Zn–Pc 1 1/H2–Pc 1/Zn–Pc

HOMO – 4 �6.9607 �6.7709 �6.2022 �5.8659 �5.9030HOMO – 3 �6.8169 �6.7599 �5.9624 �5.8214 �5.8568HOMO – 2 �6.5574 �6.6296 �5.9167 �5.8103 �5.8481HOMO – 1 �6.4533 �6.5204 �5.9075 �5.6590 �5.6972HOMO �5.0150 �4.8911 �5.7563 �5.0200 �4.8674LUMO �2.8227 �2.7862 �3.1031 �3.0066 �3.0435LUMO + 1 �2.8054 �2.7597 �3.0205 �2.9233 �2.9605LUMO + 2 �1.2287 �1.1024 �2.7937 �2.8261 �2.7702LUMO + 3 �1.0611 �0.9372 �1.8783 �2.8078 �2.7381LUMO + 4 �0.6718 �0.7429 �1.8305 �2.6986 �2.7333

76 A. Ray et al. / Journal of Molecular Structure 966 (2010) 69–78

(�0.03 eV), are resulted from the fact that the theoretical calcula-tions are done in vacuo. As a result of this, it fails to correlate ex-actly with the experimentally obtain CT transition energy. Lackof solvation energy calculation in the former case may be ac-counted for this slight difference. In these particular systems,therefore, the electrostatic interaction primarily determined theabsorption geometry of Pcs with respect to 1. It is also observed

Fig. 8. Space filling midels of the complexes of 1 with (a) H2–Pc and (b) Zn-Pc.

that, in the charged state, all the decisive molecular orbitals ofthe complex are leaning on one of the components, i.e., either onthe Pc or on the fullerene subunit.

In the present investigations, heat of formation (DH0f ) values for

the H2– and Zn–Pc complexes of 1, also extend a good support ininterpreting the stability differences between corresponding H2–and Zn–Pc complexes (Table 4). The geometric parameters of thecomplexes have been obtained after complete energy minimiza-tion at the B3LYP/6-31G* level of theory (DFT calculations). Typicalspace filling models of the supramolecular complexes of 1 withH2– and Zn–Pc are shown in Fig. 8. For the energy classificationof the optimized structures, we have employed the following(standard) DH0

f definitions. DH0f have been estimated within the

supramolecular approach: DH0f ¼ Ecomplex � ½E1 þ EPc�, where Ecomplex,

E1 and EPc, are the energy of 1/Pc complex, uncomplexed 1 andfree Pc, respectively, in order to determine the total strength ofthe various interaction patterns between the derivatized fullerenesand the receptor, e.g., H2- and/ Zn-Pc. The value of DH0

f becomesnegative as the K values increases in case of 1/H2-Pc complex. Thepositive and negative sign of DH0

f indicates that the complexationprocesses of 1 with Zn- and H2-Pc are exo- and endothermic in

Fig. 9. MESP plots for the complexes of 1 with (a) H2– and (b) Zn–Pc along withuncomplexed 1 in (c) front and (d) back view, done by DFT/B3LYP/6-31G*calculations.

A. Ray et al. / Journal of Molecular Structure 966 (2010) 69–78 77

nature, respectively. Table 4 also suggests that the complexation of1 with H2-Pc is much more enthalpy favored than that of Zn-Pc. Pre-sumably, attractive p....p interaction is responsible for greater sta-bility in case of the 1/H2-Pc complex. The higher DH0

f valueobserved for the 1/H2–Pc complex can also be rationalized bydesolvation of the fullerene 1 occurring to a lesser degree withthe Zn–Pc molecule as compared with the H2–Pc.

3.10. Calculation of free energy of charge recombination (�DGCR)

The driving forces for charge recombination (�DGCR) was calcu-lated according to Eq. (13)

�DGCR ¼ Eox � Ered � DGS ð13Þ

where Eox is the first oxidation potential of the Pcs, Ered is the firstreduction potential of the fullerene 1 and DGS refers to the static en-ergy, calculated by using the ‘Dielectric continuum model’ accord-ing to Eq. (14) [39].

DGS ¼ e2=ð4pe0esRCT—CTÞ ð14Þ

The symbols e0 and es represent vacuum permittivity anddielectric constant of the solvent, respectively. RCT indicates dis-tance of separation between 1 and Pc macrocycles, viz., H2– andZn–Pc, during CT interaction. The difference in Eox � Ered corre-sponds to the HOMO–LUMO energy gap calculated by DFT method.The calculated DGCR values, corresponding to the energy of theradical ion pairs, are summarized in Table 4. Table 4 nicely demon-strates that 1/Zn–Pc complex is stabilized more in charge-sepa-rated state compared to 1/H2–Pc complex as DGCR(1/Zn–Pc)< DGCR(1/H2–Pc). Thus, greater extent degree of charge recombina-tion in case of 1/H2–Pc complex justifies the formation of produc-tive non-covalent complex of 1 with H2–Pc compared to 1 with Zn–Pc. Since the reorganization energy for fullerene is reported to beas small, we can surmise that the charge separation process fromthe singlet excited H2– or Zn–Pc would take place be in the Marcus‘inverted region’. Molecular electrostatic potential (MESP) map cal-culation by DFT method demonstrates that the regions of high neg-ative electrostatic potential of the Pcs, viz., H2– and Zn–Pc, (thefour N atoms) are facing the regions of strong positive electrostaticpotential of the fullerene, i.e., 1, (the centers of hexagons and pen-tagons) and vice versa (6:6 bond is high negative electrostatic po-tential and Zn is high positive electrostatic potential) (Fig. 9). Thesame reasoning can be done in terms of high charge density andlow regions. Therefore, in these particular systems, the electro-static interaction eventually determined the absorption geometryof the Pcs with respect to functionalized fullerene 1 (Fig. 9(c) and(d)).

4. Conclusions

Summarizing the results of the present investigations, we reachthe following conclusions:

(1) Both H2– and Zn–Pc undergo effective ground state com-plexation with a novel fullerene derivative, namely, tert-butyl-(1,2-methanofullerene)-61-carboxylate, i.e., 1, in tolu-ene medium.

(2) Well defined CT absorption bands of the complexes of 1 withboth H2– and Zn–Pc enable us to calculate various importantphysico-chemical parameters. It is observed that the inten-sity of the CT peak maxima increases systematically withthe increasing concentration of the acceptor. This phenome-non has been utilized nicely to ellucidate the values of K forthe complexes of H2– and Zn–Pc with 1 in our presentinvestigations.

(3) Steady-state emission studies reveal energy transfer fromthe excited fluorophore (H2– and/ Zn–Pc) to the fullereneentity for all the studied supramolecular complexes.

(4) Quenching studies enables us to estimate the values of K forthe complexes of 1 with H2– and Zn–Pc operated by staticcontrol mechanism. Inspection of K values revealed thatboth free-base and metallo-phthalocyanines do not exhibitany sort of selectivity towards 1 in toluene medium.

(5) The free-energy calculations on charge recombination evokethat electron transfer from the excited Pcs to the 1 in the 1/Pcs complexes is an unlikely process.

(6) The geometry and electronic structures of the supramolecularcomplexes are visualized by DFT/B3LYP/6-31G* calculations.Theoretical investigations testify a significant redistributionof charge between the HOMOs and LUMOs of 1 upon complex-ation with Pcs. Presence of weak CT absorption bands pointout that the electronic interaction between 1 and Pcs aremainly governed by electrostatic mechanism.

(7) Large negative DH0f value for the 1/H2–Pc complex indicates

that other than the electrostatic mechanism, weak disper-sive forces associated with p–p interactions also play animportant role in forming stable ground state non-covalentcomplex.

(8) Finally, we can say that the results emanating from the pres-ent investigations will open up new possibilities for modu-lating the photo-physical characteristics of functionalizedfullerene/phthalocyanines host–guest systems.

Acknowledgements

A.R. acknowledges The University of Burdwan for providing ajunior research fellowship to her through state funded project ofthe Government of West Bengal, India. This work is financially sup-ported by the Department of Science and Technology, New Delhi,India through Fast Track scheme of Sanction No. SR/FTP/CS-22/2007. The authors of this manuscript wish to express their sinceregratitude to the Editor and the learned reviewers for making valu-able comments.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.molstruc.2009.12.010. Elec-tronic spectra of 1, H2– and Zn–Pc in toluene; UV–Vis titrationcurve of the 1/H2–Pc mixture in toluene; sigmoidial curve of1/H2–Pc system; fluorescence spectral variation of H2–Pc(6.210 � 10�6 mol dm�3) in presence of 1 and plot of relative fluo-rescence intensity vs. concentration of 1 for 1/H2–Pc system; SVplot of 1/H2–Pc system; and HOMOs and LUMOs of 1/Zn–Pc com-plex at different electronic states done by DFT/B3LYP/6-31G* cal-culations are demonstrated as Fig. 1S–5S, respectively. Fig. 1S–5Sare provided as supporting information. Supporting informationis available free of charge via the Internet in the online version ofJournal of Molecular Structure.

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