Ac

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b

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ARRA

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rsca“stahhlcpmbecllh

1d

Spectrochimica Acta Part A 78 (2011) 1364–1375

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journa l homepage: www.e lsev ier .com/ locate /saa

bsorption spectrophotometric, fluorescence, transient absorption and quantumhemical investigations on fullerene/phthalocyanine supramolecular complexes

namika Raya, Kotni Santhoshb, Sumanta Bhattacharyaa,∗

Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, West Bengal, IndiaSchool of Chemistry, University of Hyderabad, Hyderabad, AP 500 046, India

r t i c l e i n f o

rticle history:eceived 18 November 2010eceived in revised form 1 January 2011ccepted 14 January 2011

a b s t r a c t

The present paper reports the photophysical investigations on supramolecular interaction of a phthalo-cyanine derivative, namely, 2,9,16,23-tetra-tert-butyl-29H,31H-Pc (1) with C60 and C70 in toluene. Thebinding constants of the C60 and C70 complexes of 1 are estimated to be 27,360 and 25,205 dm3, respec-tively. Transient absorption measurements in the visible region establishes that energy transfer fromTC∗ (and TC∗ ) to 1 occurs predominantly in toluene which is subsequently confirmed by the consecu-

eywords:ullereneshthalocyanineV–visteady state and time resolved fluorescenceeasurements

ransient absorption study

60 70tive appearance of the triplet states of 1. Quantum chemical calculations at DFT level of theory explorethe geometry and electronic structure of the supramolecules and testify the significant redistribution ofcharge between fullerenes and 1.

© 2011 Elsevier B.V. All rights reserved.

FT calculations

. Introduction

After the initial discovery in 1984 [1], the fortuitous contempo-ary growth of two apparently independent research lines, namelyynthetic fullerene chemistry and supramolecular fullerene photo-hemistry, has been reciprocally beneficial and contributed to boostctivity in both fields. However, the formation of multi-componentsupermolecules” acting as artificial photosynthetic centers repre-ents one of the most active research areas in fullerene science forhe last fifteen years [2]. While this area of research is broad, theddition of novel building blocks such as phthalocyanine (Pc) [3]as led to numerous breakthroughs. Phthalocyanines (Pcs) withighly delocalized �-electron systems are the best known ana-

ogue of azaporphyrin. They are very versatile and stable aromatichromophores with unusual physicochemical properties that canlay a crucial role in different fields of materials science. Therefore,uch attention has been devoted to the study of Pcs as organic

uilding blocks applicable as active components in gas sensors,lectrochemical devices, field effect transistors and photovoltaic

ells [4,5]. The first report on fullerene/Pc arrays dates back toate 1995, when these two redox-active moieties are covalentlyinked following different synthetic strategies to form fullerene/Pcybrids [6]. Fullerenes have been linked to Pcs as a consequence

∗ Corresponding author. Tel.: +91 342 2533917x424; fax: +91 342 2530452.E-mail address: sum 9974@rediffmail.com (S. Bhattacharya).

386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2011.01.011

of their ability to delocalize charges over an extended spherical�-surface and for their low reorganization energy [7]. As a conse-quence of their outstanding light-harvesting properties combinedwith their redox features, Pcs are most commonly used as thedonor units in such photoactive molecules. As a result of this, inrecent past, some covalently and non-covalently linked arrays com-prising of fullerenes and Pcs have been prepared [8–10] some ofwhich exhibited photoinduced electron transfer [11]. An interest-ing aspect of the chemistry of fullerenes and Pcs is that they arespontaneously attracted to each other, as a result of ground statecomplexation [9,10]. Our rationale, which is based on molecularinteraction of fullerenes with a designed Pc, namely 1 (Fig. 1),addresses to this issue. Various spectroscopic tools like UV–vis,steady-state and time-resolved fluorescence measurements givecredence to the relation between molecular complex formationand stability. We feel that the above studies will certainly pro-vide direct information for such type of interactions at groundstate. Apart from that, as electron donor–acceptor phenomenonis attended with the changes in electron density in donor or hostas well as in the acceptor or guest, it is anticipated that perturba-tion of electronic environments in the equilibrium configurationof Pc due to molecular interactions with C60 and C70 will influ-

ence the original absorption as well as fluorescence emission bandof the designed Pc in solution. The purpose of the present paperis to investigate some important physicochemical properties aswell as the extent of non-covalent interaction between fullerenes(C60 and C70) and 1 by means of various spectroscopic measure-

A. Ray et al. / Spectrochimica Acta P

mtao

2

p(iesmtssaie7rtaVw

3

3

gavidte[iPaPnoabI

Fig. 1. Structure of 1.

ents, like UV–vis, steady state and time resolved fluorescence,ransient absorption studies and quantum chemical calculationst ab initio and density functional theory (DFT) levels of the-ry.

. Materials and methods

Both C60 and C70 are purchased from Aldrich, USA. Pc, 1, isurchased from Aldrich, USA. UV–vis spectroscopic grade tolueneMerck, Germany) has been used as solvent to favor non-covalentnteraction between fullerene and Pc and, at the same time, tonsure good solubility and photo-stability of the samples. UV–vispectral measurements are performed on a Shimadzu UV-2450odel spectrophotometer fitted with TB-85 Peltier controlled

hermo-bath using quartz cell with 1 cm optical path length. Emis-ion spectra have been recorded with a Hitachi F-4500 modelpectrofluorimeter. Fluorescence decay curves are measured with

HORIBA Jobin Yvon single photon counting set up employ-ng nanoled as excitation source. C60 and C70 are selectivelyxcited by 532 nm light from a Nd:YAG laser (6 ns fwhm) withmJ power. For the transient absorption spectra in the visible

egion, a photomultiplier tube has been used as a detector forhe continuous Xe-monitor light (150 W). Theoretical calculationsre performed with a Pentium IV computer using SPARTAN’061.1.0 Windows version, Gaussian 03W and Gauss view 04W soft-ares.

. Results and discussions

.1. UV–vis absorption studies

The extensively conjugated aromatic chromophoric system of Pcenerates intense bands in its absorption spectrum. The strongernd the most well-resolved absorption band of 1 detected in theisible region (ranging from 550 to 690 nm (Fig. 1S)). Generally,n case of Pc, the four fused benzene rings break the accidentalegeneracy of the top-filled molecular orbitals as well. Because ofhe influence of the configuration interactions, only the 1a1u → 1e∗

glectronic transition is responsible for the generation of the Q-band12]. It is already observed that peripheral substituents play anmportant role in the tuning of the absorption bands of free-basec. The Q-band can be shifted when the same kind of substituentsre introduced at the same position of each benzene ring in ac macrocycle. The effect of introducing substituents of different

ature on each benzene ring has been examined, recently [13]. Thebserved Q-bands of unsubstituted free-base Pc macrocycle at 656nd 694 nm, recorded in toluene, are shifted by 6 nm when tert-utyl groups are introduced in the peripheral positions of Pc, i.e., 1.n our present investigations, the first evidence in favor of ground

art A 78 (2011) 1364–1375 1365

state complexation phenomena between fullerenes and 1 comesfrom UV–vis absorption spectrophotometric titration measure-ments. Addition of varying concentrations of C60 and C70 solutions(in toluene medium) to 1 (fixed concentration) produce remark-able change in the absorbance value of the fullerene solutions. It isobserved that the absorbance value of the mixture of fullerene/1solution increases systematically following the increasing amountof addition of fullerene (Fig. 2(a) and Fig. 2S for C60/1 and C70/1 sys-tems, respectively). Thus, it is definitely established in this workthat the systematic increase in intensity of the broad 400–700 nmabsorption band of fullerene solution (resulting from a forbiddensinglet–singlet transition in C60- and C70) [14] is due to molecularcomplex formation between fullerenes and 1. The solvent toluenedoes not absorb in the visible region. In our present investigations,we have done Jobs continuous variation experiment to determinethe stoichiometry of fullerene/1 complex. It is observed that bothC60 and C70 form 1:1 complexes with 1. One typical Jobs analy-sis plot for C60/1 system is shown in Fig. 2(b). The notable featureof such experiment is that we have observed very good isobesticpoint (at 375 nm and 570 nm) in the corresponding UV–vis spec-tra of the mixture of fullerenes and 1 (Fig. 3S). K values of thefullerene/1 systems have been estimated in accordance with theBenesi–Hildebrand (BH) equation [15] for cells with 1 cm opticalpath length. Excellent linear BH plots are obtained for both thesystems studied in present investigations. Typical BH plots for theC60/1 and C70/1 systems are shown in Fig. 4S. Values of K are listedin Table 1.

3.2. Theoretical model in favor of electric dipole–dipoleinteraction between the fullerene molecules and 1

Consider the interaction of C60 and 1. The interaction betweenthe dipole-dipole transitions of C60 and 1 can be represented in theform:

H =Nmax∑i=1

(dC60 �C60x di

1�1x,i)

(1 − 3 cos2 �i)

∈ ∞r3i

(1)

where dC60 and di1 are dipole moments of the corresponding transi-

tions in C60 and the ith 1 molecule, (x and �1x,i

are the correspondingPauly matrices, ri is the distance between the C60 and 1 molecules,and (∞ in Eq. (1) is the high-frequency dielectric constant. Thereconstruction of the resulting spectrum, taking into account Eq.(1), is determined by mixing of the states of the C60 molecule andthe surrounding 1 molecule.

Ei± = EC60 + E1

2±

{[EC60 − E1

2

]2

+ |Vi|2}1/2

(2)

For one C60/1 pair, Eq. (1) gives Eq. (2), where EC60 and E1 arethe energies of dipole transitions of C60 and 1, respectively, andVi =

[dC60 di

1 − (1 − 3 cos2 �i)]

ε−1∞ r−3i

is the matrix element of thestate mixing. The final expression has the form:

∈ − = ∈ (0)− − |Vi|N1/2

2(3)

where V is the amplitude of the non-diagonal flip-flopdipole–dipole matrix element for C60 and of the dipole tran-sitions in neighboring 1 molecules, and N is the number ofneighboring 1 molecules. Such a dependence of the absorptionband edge is valid only under the condition N < Nthr, where Nthr

is the maximum number of 1 molecules that can take part in thedipole–dipole flip-flop interaction with C60. A further increasein the concentration of 1 does not increase the number of thesemolecules in the nearest environment of C60. Fig. 5S shows thatthe dependence is saturated when the concentration of C70 and

1366 A. Ray et al. / Spectrochimica Acta Part A 78 (2011) 1364–1375

0

0.04

0.08

0.12

0.16

0.2

0.24

750700650600550500450400350

Wavelength, nm

Abs.

1.00.80.60.40.20.0

0.00

0.02

0.04

0.06

0.08

0.10

Δ A

bso

rban

ce

Mole fraction of C60

0

0.04

0.08

0.12

0.16

0.2

430410390

Wavelength, nm

Abs.

i

ii

iii vii

a

b

F (i) una ge ofa bs con

Cca(

TBt

ig. 2. UV–vis titration curve of (a) C60/1 system recorded in toluene medium. In (a)nd (iii)–(vii) mixture of C60 and 1 in which the C60 concentration varies in the ranbsorption band of C60 in the presence of 1 is shown in inset of (a) and (b) shows Jo

60 exceeds 5.0 × 10−5 mol dm−3 concentrations, in case of C60/1

omplex in agreement with the theory. This mechanism, thus,llows us to explain the formation of the electron donor–acceptorEDA) type study.

able 1inding constants (K) of fullerene/Pc complexes along with our systems reported inhe present investigations in toluene.

System K, dm3 mol−1 Selectivity KC60 /KC70

UV–vis Fluorescence UV–vis Fluorescence

C60/1 30,300 ± 0.9600 24,380 ± 1120 1.17 1.0C70/1 25,930 ± 17.87 24,800 ± 1200 – –ZnPc-L1

a – 14,000 – –ZnPc-L1-ZnPca – 19,000 – –ZnPc-L2

a – 4800 – –C60Im-ZnNcb 62,000 – – –K4[ZnTCPc]2

− – – –[pyC60NH3

+]c 2500 ± 500 – – –

a Collected from literature [26].b Collected from literature [27].c Collected from literature [28].

complexed 1 (1.10 × 10−6 mol dm−3); (ii) uncomplexed C60 (4.70 × 10−6 mol dm−3)1.88 × 10−5 to 5.15 × 10−5 mol dm−3; variation in the absorbance value at 407 nmtinuous variation plot for C60/1 system in toluene.

3.3. Steady state and time resolved fluorescence studies

To study the photo-induced behavior of C60/1 and C70/1supramolecular complexes and the recognition motif of C60 and C70towards 1, steady-state emission measurements are carried out intoluene. From above discussions, we can say that direct �-stackingeffect plays a very minor role as the fullerene core may not get inclose contact with the Pc. This can be demonstrated in terms of theelectrostatic interactions prevailing between positively charged 1ı+

and the negatively charged fullereneı− components with a trendtowards formation of well-directed and oriented assembly of 1:1supramolecular fullerene/1 complexes. The simple mixing of theindividual components, i.e., fullerene and 1, leads to a novel super-structure, for which we can expect that the highly fluorescentstate of the singlet excited 1* is quenched by an inter-complexenergy and/electron transfer to fullerene forming fullereneı−. Ithas been reported earlier that charge separation can occur from

the excited singlet state of the Pc to fullerene in fullerene/Pc hybridsystem [16]. Photo-physical studies prove that in case of confor-mationally flexible dyads comprising fullerenes and macrocyclicreceptor molecules, like porphyrin (Por), �-stacking interactionsare facilitated due to through-space interactions between these

A. Ray et al. / Spectrochimica Acta Part A 78 (2011) 1364–1375 1367

0

1000

2000

3000

4000

5000

6000

7000

8000

850830810790770750730710690670650

Emission wavelength (nm)

Flu

ore

scen

ce i

nte

nsi

ty

25201510

Em

. In

ten

sit

y

Time (ns)

[C60] + 1

[1]

i

ii

iii

0.0 4.0x10-5 8.0x10-5 1.2x10-4 1.6x10-4

1.0

1.5

2.0

2.5

Rel

ativ

e fl

uore

scen

ce i

nte

nsi

ty

[C60

], mol.dm-3

a

b

Fig. 3. Fluorescence spectral variation of 1 (1.050 × 10−5 mol dm−3) in the presence of (a) C60 in toluene medium; the concentrations of C60 from top to bottom in the arrowmark (indicated in the figure) are as follows: 0, 2.204 × 10−5, 4.410 × 10−5, 6.615 × 10−5, 8.820 × 10−5, 11.0 × 10−5, 13.23 × 10−5, 15.43 × 10−5 and 17.65 × 10−5 mol dm−3;p ium i( nd (iib For int

t1

e(si7(tttbmtmtcpmrdrfmcotes

lot of relative fluorescence intensity vs. [C60] for C60/1 system in toluene med1.0 × 10−5 mol dm−3), (ii) C60 (75.00 × 10−5 mol dm−3) + 1 (1.0 × 10−5 mol dm−3) alue and pink color, respectively. The instrument response function is also shown. (he web version of the article.)

wo chromophores. This has been demonstrated by quenching of*Por fluorescence and formation of fullerene-excited states (bynergy transfer) or generation of fullerene−/Por+ ion-pair statesby electron transfer) [17]. In our present investigations, the steadytate experiments reveal that the fluorescence of 1 is character-zed by maxima at 711 nm followed by two shoulder peaks at40 and 785 nm, upon excitation at the Soret band maximumsee Fig. 3(a) and Fig. 6S(a)). Evidence in favor of the energy-ransfer deactivation obtains from the titration experiment of aoluene solution of 1 with variable C60 and C70 concentration inhe range of 2.20 × 10−5 mol dm−3 to 35.40 × 10−5 mol dm−3. It haseen observed that upon excitation at 345 nm, i.e., Soret bandaxima of 1, a C60 or C70 concentration dependent decrease in

he intensity of the fluorescence maxima of 1 is seen in tolueneedium. The relative fluorescence intensity of 1 vs. the concen-

ration of C60 is shown in the inset of Fig. 3(a). At high fullereneoncentration, a plateau feature is observed, at which the com-lexation of 1 is assumed to be complete (Fig. 3(a)). It should beentioned at this point that a purely diffusion-driven process is

uled out, on the basis of the applied fullerene concentration. Theecrease of fluorescence intensity of 1 and the shift of the 1 fluo-escence, suggest a static quenching event inside the well-definedullerene/1 supramolecular complexes. On the basis of the afore-

entioned results, we reach the conclusions that in the fullerene/1

omplexes, the fluorescence state of 1 is quenched by the additionf electron-accepting C60 and C70. K values are evaluated accordingo a modified BH equation [15]. Steady state fluorescence titrationxperiment of C70/1 system along with BH fluorescence plot of theame system and C60/1 systems is shown in Fig. 6S.

s shown in inset of (a), and (b) shows fluorescence decay profiles of only (i) 1i) C70 (21.43 × 10−5 mol dm−3) + 1 (1.0 × 10−5 mol dm−3) in toluene shown in red,terpretation of the references to color in this figure legend, the reader is referred to

To validate the presence of static quenching mechanism in ourpresent investigations, we have performed detailed pico-secondtime-resolved fluorescence measurements for fullerene/1 com-plexes in toluene medium. The titration experiment has beencarried out at a fixed concentration of 1 and variable concentra-tion of C60 and C70. The time resolved fluorescence of 1 reveals asingle exponential decay with a lifetime of 7.259 ns (Fig. 3(b(i))).It is observed that upon the gradual addition of fullerenes C60and C70, there is practically no change in the lifetime comparedto the uncomplexed 1, and mono exponential decay is followed(Fig. 3(b(ii)) and (b(iii)), respectively). Since the lifetime is unaf-fected by the presence of the quencher in cases of pure staticquenching event, a plot of �0/� vs. [Q] should give a straight lineof parallel to x-axis, i.e., [Q]. Here, �0 = lifetime of 1 in the absenceof quencher; � = lifetime of 1 in the presence of quencher, viz.,fullerene C60 or C70; [Q] = quencher concentration. In our presentcase, excellent linear plots having very small slope values have beenobtained for C60/1 and C70/1 systems. Variation of �0/� vs. concen-tration of the quencher for the fullerene/1 systems, are shown inFig. 7S. Lifetime data of C60/1 and C70/1 systems are tabulated inTable 1S.

3.4. Solvent reorganization energy (RS) for the fullerene/1complexes

The effect of solvent over electronic coupling phenomenonbetween fullerenes and 1 can be better understood by the esti-mation of solvent reorganization energy (RS) for the fullerene/1complexes. The total reorganization energy, in general, is a sum of

1368 A. Ray et al. / Spectrochimica Acta Part A 78 (2011) 1364–1375

Table 2Solvent reorganization energies (RS) free energies of charge separation (�GCS) andcharge recombination (�GCR) values for C60/1 and C70/1 systems in toluene.

ti(icesCc

R

wosteshpfotet

opt[a

�

hpretE

�

ep

TEi

System RS, eV �GCS, eV �GCR, eV

C60/1 −0.455 −2.165 −0.360C70/1 −0.405 −2.120 −0.440

he two terms, i.e., inner-sphere reorganization energy (solvent-ndependent) R0 [18], and outer-sphere reorganization energysolvent-dependent) RS. In case of fullerene, as far as RS contributions concerned, this is believed to be small as well. The symmetri-al shape and large size of the fullerene framework requires littlenergy for the adjustment of an excited or reduced state to the newolvent environment. In the present investigations, RS of C60/1 and70/1 complexes have been estimated by applying the dielectricontinuum model developed by Hauke et al. (see Eq. (4)) [19]:

S =(

e2

4�ε0

)[{(1

2R1

)+

(1

2Rfullerene

)−

(1

RD–A

)}(1εS

)

−{(

12Rfullerene

)+

(1

2R1

)}(1εR

)](4)

ith the following parameters: radius of donor, R1 = 6.88 A; radiusf acceptor (Rfullerene): RC60 = 4.2 Å , RC70 = 4.4 Å ; donor–acceptoreparation (RD–A): RC60/1 = 4.52 Å , RC70/1 = 4.79 Å ; solvent dielec-ric constant, εS (εtoluene = 2.39); solvent dielectric constant forlectrochemical measurements, εR = 9.93. Values of RS for the aboveupramolecular systems are given in Table 2. It is to be mentionedere, that the solvent reorganization energies obtained in theresent investigation do not corroborate well with that observedor quinone/porphyrin system [20]. The discrepancy in the valuef RS for quinone/porphyrin and fullerene/Pc systems may be dueo the subtle structural change in the host–guest complex whichxert a large influence upon the photo-induced electron and/energyransfer process.

It is customary to estimate the driving forces for the free energiesf charge-separation (�GCS) and charge-recombination (�GCR)rocess for the fullerene/1 supramolecular complexes. �GCR forhe fullerene/1 complexes are calculated using the Weller equation21]. In this equation, the static energy (�GS) has been calculatedccording to the following equation:

GS = e2

4�ε0εRRfullerene/1(5)

ere the terms e, ε0 and εR refer to elementary charge, vacuumermittivity and static dielectric constant of the solvent used forate measurements, respectively. Based on �GCR and excited statenergy (E0,0) values of the C60 and C70, the free-energy changes ofhe charge separation process (�GCS) have been calculated usingq. (6) and are listed in Table 2.

GCS = −(−�GCR) + E0,0 (6)

Table 2 reveals that the charge-separation process of 1 via thexcited singlet state of C60 (1C∗

60) is sufficiently exothermic com-ared to C70 by 0.045 eV. The faster charge recombination in case

able 3nthalpies of formation (�Hf

◦) values for the complexes of 1 with C60 and C70

n vacuo obtained by explicit theoretical calculations at different levels.

System �Hf◦ , kcal mol−1

ab initio DFT/B3LYP/STO3G

C60/1 −0.270 −0.784C70/1 (side-on) −1.130 −5.181C70/1 (end-on) −1.157 −6.045

Fig. 4. Stereoscopic structures of (a) C70/1 (end-on orientation of C70), (b) C70/1(side-on orientation of C70) and (c) C60/1 systems done by DFT/B3LYP/6-31G* calcu-lations.

of C70/1 complex can result from one important factor: solventreorganization energy. This affects the activation energy of elec-

tron transfer. Table 2 indicates that RS(C60/1) < RS(C70/1). However,it should be mentioned at this point that although �GCR > RS forC60/1 complex, the reverse trend is observed for C70/1 complex(Table 2). From this we can infer that the activation energy shouldbe larger and the charge recombination is slower for C60/1 com-

A. Ray et al. / Spectrochimica Acta Part A 78 (2011) 1364–1375 1369

F OMOD

pTut

3

(aefhmlis[

ig. 5. HOMOs and LUMOs of C60/1 system in various electronic states, e.g., (a) HFT/B3LYP/6-31G* calculations.

lex compared to C70/1 on the basis of the reorganization energy.he chemical reason for this can be explained on the basis that thenpaired electron in the C60 anion radical is more localized thanhat in the C70 anion radical.

.5. Binding constants and computational calculations

Binding constants for the complexes of 1 with C60 (KC60 ) and C70KC70 ) are summarized in Table 1. It is observed that 1 undergoesppreciable amount of complexation with both C60 and C70. How-ver, very low selectivity in K value, viz., KC70 /KC60 , suggests that 1ails to discriminate C60 from C70. However, it is interesting to noteere that the reported K value of C60/1 complex is shown to exhibit

uch higher K value compared to other host molecules like metal-

odiporphyrins, JAWS porphyrin and porphyrin tetramer reportedn literature [22–24]. The remarkable decrease in K value of C70/1ystem in comparison to C70/Por system (reported in literature)25] as well as very low KC70 /KC60 selectivity ratio in our present

, (b) HOMO − 1, (c) LUMO, (d) LUMO + 1, (e) LUMO + 3 and (f) LUMO + 4 done by

investigations (Table 1) can be ascribed by the end-on approach ofC70 towards 1.

We have made a good comparison between the binding con-stants determined in present investigations and those obtainedby other research workers [26–28]. These are summarized inTable 1. It is observed that the K values estimated by Guldi et al.[26] for their designed fullerene/Pc, namely, ZnPc-L1, ZnPc-L1-ZnPc and ZnPc-L2, exhibit lower value in comparison to C60/1and C70/1 ensembles. Similar sort of phenomenon is observed forK4[ZnTCPc]2-pyC60NH3

+ system reported by D’Souza et al. [27]However, C60Im-ZnNc dyad system exhibits much greater valueof K compared to fullerene/1 systems [28]. It should be men-tioned at this point that very recently, we have estimated K values

for some fullerene/Pc complexes using zinc-2,3,9,10,16,17,23,24-octakis-(octyloxy)-29H,31H-phthalocyanine (2) [10]. The averageKC70 /KC60 ratio for fullerene/2 complex is estimated to be 2.1 intoluene [10]. From the above observations, it is very much clearthat presence of octyloxy group in 2 make an octopus like embrace

1370 A. Ray et al. / Spectrochimica Acta Part A 78 (2011) 1364–1375

F ous ela

waa

egpcaboiteasg

ig. 6. HOMOs and LUMOs of C70/1 system (in the end-on orientation of C70) in varind (f) LUMO + 4 done by DFT/B3LYP/6-31G* calculations.

hich is very much favorable for fullerene encapsulation. Thebove molecular pattern is absent in 1 and for this reason it fails toct as a good discriminator between C60 and C70.

Quantum chemical calculations may provide some light for thelucidations of electronic structures as well as association ener-ies of the C70/1 ensembles for different orientations of C70. In theresent investigations, we have performed explicit theoretical cal-ulations at DFT levels of theory using Slater type of orbitals (STO)t 3G basis set other than ab initio calculations using STO 3-21Gasis set for fullerene/1 complexes. We have employed Slater typerbitals for our calculations, which are capable of precisely predict-ng the optimized geometric structures with a far-less expensive

reatment of electron correlation. In our present investigations,nthalpy of formation (�Hf

◦) values for the fullerene/1 complexesre estimated from the difference between the total energy of theupramolecular complex and the sum of the individual host anduest entities separated from the optimized structure (single-point

ectronic states, e.g., (a) HOMO, (b) HOMO − 1, (c) LUMO, (d) LUMO + 1, (e) LUMO + 3

calculation). The estimated �Hf◦ values of the fullerene/1 com-

plexes at the DFT level of theory are provided in Table 3 other thanHF/3-21G calculations. Among all the calculations, DFT methodsexhibit most reliable data and prove that C70 is oriented in end-on binding motif with the flat pyrrolic plane of 1 compared to itsside-on geometrical pattern. This is because C70/1 complex enjoys0.864 kcal mol−1 of energy of stabilization when it approaches 1in end-on manner rather than in side-on orientation estimatedfrom DFT calculations (Fig. 4(a) and (b), respectively). Table 3 alsoreveals that the binding between C70 and 1 is favored over C60and 1 (Fig. 4(c)). This can be viewed in terms of subtle differencebetween the electron affinity values of these two fullerenes. As

C70 is known to have good electron accepting ability comparedto C60 [14], extent of electrostatic interactions is higher in case ofC70/1 complex than that of C60/1 complex. It is already proved thatpossibility of electrostatic interactions in fullerene/Pc systems aremuch more higher compared to dispersive forces associated with

A. Ray et al. / Spectrochimica Acta Part A 78 (2011) 1364–1375 1371

F ous ela

�ohbatftvbtvmHzfopmzqo

ig. 7. HOMOs and LUMOs of C70/1 system (in the side-on orientation of C70) in varind (f) LUMO + 4 done by DFT/B3LYP/6-31G* calculations.

–� interactions [9,10]. As electrostatic interactions are primarilyriginated from the electron distribution in donor–acceptor typeost–guest system, it may be anticipated that C70/1 complex woulde stabilized more in comparison to C60/1 system. The negativessociation energies determined in both ab initio and DFT calcula-ions suggest stable supramolecular complex formation betweenullerene and 1. Typical stereoscopic structures of fullerene/1 sys-ems done by ab initio calculations are shown in Fig. 8S. Values ofarious bond moments for the complexes of 1 with C60 and C70 haveeen calculated by DFT calculations. Typical bond moment data forhe fullerene/1 complexes estimated by DFT calculations are pro-ided in Tables 2S–4S. It is known that charged species have a dipoleoment and even higher (quadrupole, octapole, etc.) moments.owever, according to quantum mechanics, only the highest non-ero ‘pole’ is independent of the chosen origin. This means thator ions, only the monopole (overall charge) is independent of therigin. For most neutral molecules or complexes, the dipole is inde-

endent of the origin. For neutral complex that have a zero dipoleoment by symmetry, the quadrupole moment is the highest, non-

ero ‘pole’. There are probably molecules out there that have a zerouadrupole moment, making the octapole moment independentf origin. In the present case, the various bond moments data are

ectronic states, e.g., (a) HOMO, (b) HOMO − 1, (c) LUMO, (d) LUMO + 1, (e) LUMO + 3

shown in Tables 2S–4S which also give good view of separation anddistribution of charges within the molecule.

Electrostatic interactions originating from the electron densityat surface of the fullerenes and 1 of the fullerene/1 supramoleculesare supposed to play a vital role in the interaction betweenfullerenes and 1. Molecular electrostatic potential (MEP) maps havebeen generated for C60, C70, 1, C60/1 and C70/1 systems to visualizethe electrostatic interactions (Fig. 9S). The MEP for 1 shows neg-ative electrostatic potential (shown in red) on the Pc ring (mostlylocated on the nitrogen atoms). The MEPs for the fullerenes areblue–green indicating positive electrostatic potential; blue–greencolor of fullerenes corresponds to the center regions of the five-and six-membered rings. However, along the 6:6 bonds, regions ofnegative potentials (shown in red) can be observed. Interestingly, inthe supramolecular complexes, C60/1 and C70/1, the original blue-green color of the separated C60 and C70 are changed to green,and deep red color of Pc changed to reddish-yellow, indicating

possibility of electron transfer between these two chromophoresupon photo-excitation. It should be mentioned at this point, thatelectrostatic interaction is one of the important components (andthe most important one), which can contribute to the stabiliza-tion of the complexes through the van der Waals interaction since

1372 A. Ray et al. / Spectrochimica Acta Part A 78 (2011) 1364–1375

400 500 600 700 8000.00

0.01

0.02

0.03

0.04

Δ A

bs

1.0 μs

5.0 μs

10.0 μs

0 10 20 30 40

0.00

0.01

0.02

0.03

0.04

0.05

Δ A

bs

Decay at 750 nm

400 500 600 700 800

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

Δ A

bs

1 μs

12.5 μs

0 10 20 30 40

0.00

0.01

0.02

0.03

Decay at 750 nm

Δ A

bs

Decay at 500 nm

a b

dc

Fig. 8. (a) Transient absorption spectra obtained by 532 nm laser photolysis of C60 (1.388 × 10−4 mol dm−3) in the presence of 1 (2.0 × 10−6 mol dm−3) in toluene in timei nt abs −4 −3

i d) dec

bsmPwf

tsLwaHLfLcisftiio[oC−it−tm

ntervals of 1.0, 5.0 and 10.0 �s; (b) time profile plot observed at 750 nm; (c) transien the presence of 1 (2.0 × 10−6 mol dm−3) in toluene observed at 1.0 and 12.5 �s; (

oth components, viz., fullerenes and 1, are not charged (the verymall differences in electronic density distribution over the neutralolecule cannot provide strong electrostatic interaction between

c and fullerenes). This interaction is important for the complexesith charged components or it is important in the excited states of

ullerene/Pc assemblies.One fascinating observation takes place when we visualize

he electron distribution in various orbitals at different electronictates, like, HOMO, HOMO − 1, LUMO, LUMO + 1, LUMO + 3 andUMO + 4 for the complexes of C60 (Fig. 5) and C70 (Figs. 6 and 7)ith 1 as evidenced from DFT calculations. It is observed that for

ll the studied complexes except C70/1 at its end-on orientation,OMO and HOMO − 1 are centered on the 1 while LUMO, LUMO + 1,UMO + 3 and LUMO + 4 are positioned almost exclusively on theullerene, as illustrated in Figs. 6 and 7. Neither HOMO nor theUMO is observed to be in the tert-butyl unit of 1. However, inase of C70/1 complex (end-on orientation of C70, Fig. 6), HOMO − 1s positioned exclusively in the C70 unit. This phenomenon clearlyuggests that binding between C70 and Pc takes place in a moreacile manner when C70 is aligned in end-on binding pattern withhe place of 1. This phenomenon validates our findings regard-ng trend of �GCS(C60/1) and �GCS(C70/1) as already mentionedn Section 3.4. It could be seen that LUMO energy (ELUMO) levelsf fullerene/1 complexes compare well with the fullerene guest29], while the HOMO energy (EHOMO) levels are similar to thosef the uncomplexed 1 receptor (Table 5S). For example, in case of70/1 complex at end-on orientation of C70, ELUMO is computed to be0.037 eV, which is comparable to the ELUMO of uncomplexed C70,

.e., −0.040 eV. Also, the EHOMO of the same complex is estimatedo be −0.123 eV, which corroborates excellently with that of 1, viz.,0.123 eV, obtained by DFT calculations. EHOMO and ELUMO of all

he fullerene/1 complexes along with 1 are given in Table 5S (esti-ated from DFT calculations). It should be mentioned at this point

orption spectra obtained by 532 nm laser photolysis of C60 (1.388 × 10 mol dm )ay time profile plot at 500 and 750 nm.

that, electrostatic interaction is only one of the important com-ponents (and not the most important one), which can contributeto the stabilization of molecular van der Waals complex between1 and fullerenes since both components are not charged (verysmall differences in electronic density distribution over the neutralmolecule cannot provide strong electrostatic interaction betweenfullerene and Pc). This interaction is important for the complexeswith charged components [30]. Together with electrostatic inter-action other types of fullerene/Pc interaction like polarizationinteraction, �–� interaction, d–� interaction, etc. between neutralmolecules also play vital role in stabilizing the complex.

3.6. Transient absorption study

The steady state UV–vis spectra of C60 and 1 are shown inFig. 10S. Similarly, the steady state UV visible spectra of C70 and1 are demonstrated in Fig. 11S. Both Figs. 10S and 11S reveal that 1does not have any appreciable absorption intensity at 532 nm. Forthis reason, we have predominantly excited C60 and C70 moleculeby 532 nm laser light in our present investigations. It is already evi-denced from the spectrum of the mixture of C60 with 1 in Fig. 10Sthat appreciable CT type interaction does not exist in the groundstate under the concentration range of (0.5–1.38) × 10−4 (M), wherethe laser experiments are performed. Transient absorption spectraobtained by 532 nm laser light exposure on C60 in the presence of1 in toluene are shown in Fig. 8(a). The absorption band appearsat 750 nm is attributed to the formation of TC∗

60 [31–33]. However,we fail to detect any absorption bands due to the formation of 1+

and C60−. From the above observations, we may infer that photo

induced energy transfer phenomenon via TC∗60 from 1 is confirmed

in our present investigations. Fig. 8(b) indicates the time profiledecay plot at 750 nm. However, from Fig. 8(c) it is clear that thenegative absorbance at shorter wavelength region (in comparison

A. Ray et al. / Spectrochimica Acta Part A 78 (2011) 1364–1375 1373

F and (bs

tiotifi1ttdlo(tftstt(CdoAott(�tnpo(l

which the decay time is estimated to be 20 �s. Fig. 10(a) and (b)shows the C70 sensitized triplet–triplet absorption spectra of 1 intoluene; among these, Fig. 10(a) shows the “rise” in the character-

ig. 9. Triplet–triplet absorption spectra of 1 in toluene sensitized by C60 in (a) risepectra of 1 in toluene.

o 750 nm) may be due to the depletion of 1, since the decay of TC∗60

s accelerated on addition of 1. Since the decay of TC∗60 is accelerated

n addition of 1, this is clearly indicative of the fact that reac-ion other than electron transfer is taking place for our presentlynvestigated C60/1 complex. Fig. 8(d) shows the decay time pro-le plot of the C60 sensitized triplet–triplet absorption spectra ofin toluene. The decay time at 500 nm and 750 nm is estimated

o be (80.0 ± 2.0) �s and (4.4 ± 0.1) �s, respectively, while the riseime at 500 nm is determined to be (5.4 ± 0.1) �s. From Fig. 8(d), theaughter–mother relationship in energy transfer is clearly estab-

ished. Fig. 9(a) and (b) shows the triplet–triplet absorption spectraf 1 in toluene sensitized by C60 in two different modes, i.e., riseFig. 9(a)) and decay in transient spectra (Fig. 9(b)). The absorp-ion band appears at 505 nm is ascribed to be of T1* [34,35]. Theast rise in the absorption intensity at 505 nm is attributed to behe absorption band of TC∗

60 having considerable absorption inten-ity in this region. Contribution from the direct excitation of 1 tohe formation of T1* is small; the initial absorbance of 0.01 due tohe formation of T1* is observed at the concentration of 2 × 10−5

M) in the absence of C60 after 1 �s. The decay time profile plot of60 sensitized triplet–triplet absorption spectra of 1 in toluene isemonstrated in Fig. 9(c). Considering all the above findings, thebserved reactions may be illustrated as Scheme 1 (see Appendix). Thus, in toluene, energy transfer from TC∗

60 to 1 takes place with-ut electron transfer. It should be mentioned at this point that theriplet–triplet absorption spectrum of 1 obtained on direct excita-ion in toluene after 1 �s does not provide any new physical insightFig. 12S(a)). The decay time profile plot, i.e., �Abs. vs. time (ins), is found to be also blurred (Fig. 12S(b)) which also indicates

hat C60 plays key role behind getting facile energy transfer phe-

omenon. In case of C70/1 complexation process, similar sort ofhenomenon is observed. However, transient absorption spectrumf C70 in toluene is found to be very weak in the visible regionFig. 13S). Decay profile of C70 is observed at two different wave-engths, i.e., 410 and 600 nm (Fig. 14S(a) and (b), respectively) from

) decay mode; (c) decay time profile plot of C60 sensitized triplet–triplet absorption

Fig. 10. C70 sensitized triplet–triplet absorption spectra of 1 in toluene in (a) risingand (b) decay mode.

1 Acta

iFoa5ptaCeB

4

(

(

(

A

vptaiottc

A

C

w

[

[

[

[

[

[[

[

374 A. Ray et al. / Spectrochimica

stic absorption spectrum of 1 in the time range of 1–12.5 �s whileig. 10(b) demonstrates nicely the characteristic decay spectrumf the same species in the time interval of 12.5–150 �s. From thebove spectra, the rise time and decay time are calculated to be�s and 70 �s, respectively. Fig. 15S shows the plot of decay timerofile, i.e., �Abs. vs. time (in �s) at 490 nm. Similar to C60 sensi-ized triplet–triplet absorption spectra of 1, negative absorbancet the shorter wavelength than 750 nm is observed (Fig. 16S).onsidering all the above mentioned photophysical changes, thenergy transfer diagram may be constructed as shown in Appendix.

. Conclusions

From above discussions, the following conclusions are reached:

(a) Both C60 and C70 form ground state molecular complexes withthe Pc derivative, 1, in non-polar solvent.

b) The influences of 1 on the absorption spectra of C60 and C70 areexplained using a theoretical model.

(c) Efficient quenching of fluorescence intensity of 1 in the pres-ence of fullerenes takes place in our present investigations.

d) Magnitude of binding constant for fullerene/1 complexes asestimated from UV–vis and steady state fluorescence studiessuggest that 1 may not be selectively employed as moleculartweezers for either C60 or C70.

(e) Lifetime measurements of 1 in the absence and presence offullerenes establish the presence of static quenching mecha-nism in our present investigations.

(f) DFT calculations well reproduce the geometry and binding pat-tern of C70 towards 1 in forming C70/1 supramolecular complex.

(g) Photoinduced energy transfer via TC∗60 and TC∗

70 from 1 is con-firmed by observing the transient absorption spectra in toluenemedium.

h) Finally, we can infer that the foregoing spectroscopic andtheoretical studies on fullerene/Pc model systems may beof immense interest for interpreting various photo-physicaland physicochemical parameters of fullerene/Pc hybridsystems.

cknowledgments

A.R. thanks The University of Burdwan, Burdwan, India, for pro-iding a junior research fellowship to her. Financial assistancerovided by the Department of Science & Technology, New Delhi,hrough the FAST TRACK Project of Ref. No. SR/FTP/CS-22/2007 islso gratefully acknowledged. We also wish to record our grat-tude to Prof. Anunay Samanta, School of Chemistry, Universityf Hyderabad, Hyderabad, India for his helpful co-operations inhis work. We also take this opportunity to express our thanks tohe honorable editor and learned reviewers for making valuableomments.

ppendix A.

See Scheme 1.

60SC60

* TC60

*;

TC60

* + 1 C60 +

T1

*

here ISC: inter system crossing; Ent: energy transfer.

hν

532

ISC Ent

Scheme 1.

[[

[[

[

[

[

[

Part A 78 (2011) 1364–1375

Appendix B.

C70 + 1 in toluene

hν

C70 + T1*

TC70

*+ 1

SC70

*+ 1

hν (532 nm)

ISC

T1

Appendix C. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.saa.2011.01.011.

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