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Spectrochimica Acta Part A 89 (2012) 284– 293

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

hysicochemical insights in supramolecular interaction of fullerenes C60 and C70

ith a monoporphyrin in presence of silver nanoparticles

atul Mitraa, Subrata Chattopadhyayb, Sumanta Bhattacharyaa,∗

Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, IndiaBio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

r t i c l e i n f o

rticle history:eceived 8 August 2011eceived in revised form7 November 2011ccepted 8 December 2011

eywords:ullerenesonoporphyrin (1)

ilver nanoparticlesinding constantsLSEMEMM3 calculations

a b s t r a c t

The present article reports for the first time on supramolecular interaction between fullerenes (C60 andC70) and a designed monoporphyrin in solution, e.g., 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine (1), in absence and presence of silver nanoparticles (AgNp) having varying diameter of rangebetween 3 and 7 nm. Ground state electronic interaction between fullerenes and 1 has been evidencedfrom the observation of decrease in the intensity of the Soret absorption band of 1 after complexationwith C60 and C70 in toluene. However, in presence of AgNp, extent of decrease in the intensity of Soretabsorption band of 1 has been reduced following its complexation with fullerenes. Steady state fluo-rescence measurements establish quenching of fluorescence of 1 by fullerenes and the most interestingaspect of the present work is that quenching efficiencies of C60 and C70 are found to be less in presence ofAgNp. Steady state fluorescence measurement reveals reduction in the binding constant (K) value for bothC60–1 (KC60–1 = 2355 dm3 mol−1) and C70–1 complex (KC70–1 = 11, 980 dm3 mol−1) in presence of AgNp(KC60–1 = 340 and KC70–1 = 7380 dm3 mol−1). The new physical insight of the present studies is that 1acts as excellent discriminator molecule for C70 in presence of AgNp as selectivity in binding is estimatedto be ∼21.7 in presence of AgNp compared to the situation when fullerene–1 mixture does not containany AgNp (i.e., selectivity in binding = ∼5.0) in solution. Time-resolved fluorescence studies establish therole of static quenching mechanism behind fluorescence decay of 1 by fullerenes in absence and pres-ence of AgNp. Magnitude of rate constant for charge separation and quantum yield of charge separationindicates that C70–1 complex exhibits highest value of such parameters in absence of AgNp compared tothe situation when AgNp particles are present in the composite mixture of C70 and 1. Dynamic light scat-tering (DLS) measurement reveals while particle size of AgNp is estimated to be ∼4.8–5.0 nm in presenceof 1, the size of the AgNp particles in 1 become larger in presence of C60 (∼13.0 nm) and C70 (∼37.0 nm)solution in toluene. Conductance measurement establishes that AgNp particles reduce the generation ofelectrical conductivity value for both C60–1 and C70–1 systems in toluene with respect to time; the rate ofdecrease of electrical conductivity become much slower in presence of C70–1 complex. Scanning electronmicroscopic experiment provides excellent support for DLS measurements regarding increase in the size

of the nanoparticles in presence of C60 and C70. Transmission electron microscope clearly demonstratesthat the electrostatic attraction between porphyrin-based supramolecules and silver nanoparticles is verymuch responsible behind the formation of larger aggregates. Semiempirical PM3 calculations in vacuoestablish the single projection structures for the fullerene–1 complexes and well interpret the stabilitydifference between C60- and C70-complexes of 1 in terms of heat of formation values of the respectivecomplexes.

. Introduction

Self-assembly of organic molecules on different substrates isne of the most studied approaches in order to obtain sur-ace supported supramolecular nanostructures with controlled

∗ Corresponding author. Tel.: +91 3356327254; fax: +91 3422530452.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.2011.12.013

© 2011 Elsevier B.V. All rights reserved.

dimensions and innovative properties. This strategy may be suit-ably utilized as a simple, versatile, and effective approach toobtain predetermined and defect-free nanostructured macroscopicmaterials. Recent developments in nanoscale carbon materials,such as fullerenes and carbon nanotubes, provide us with new

ways to design and construct energy conversion architectures.Porphyrin and fullerene have been found to be an ideal donor(D)–acceptor (A) couple in forming self-assembly phenomenon,because the combination of porphyrin and fullerene results in

R. Mitra et al. / Spectrochimica Act

N

NH N

HN

OCH3H3CO

OCH3

apclPutmi−amtutsnnsmctftot[tfFwtsCcItsrifnnn

OCH3

Fig. 1. Structure of 1.

small reorganization energy, which allows one to acceleratehotoinduced electron and/energy transfer and to slow downharge recombination process, leading to the generation of a longived charge-separated state with a high quantum yield [1–14].orphyrins and fullerenes are also known to form supramolec-lar complexes, which contain close contacts between one ofhe electron-rich 6:6 bonds of the guest fullerene and the geo-

etric center of the host porphyrin. The fullerene–porphyrinnteraction energies are reported to be in the range of −16 to18 kcal mol−1 [15]. Such a strong interaction between porphyrinsnd fullerenes is likely to be a good driving force for the for-ation of supramolecular complexes [16,17]. Taking into account

he fact that porphyrin and fullerene tend to make a supramolec-lar complex in solutions as well as in the solid state [18–35],he fullerene–porphyrin self-assembly is expected to interact withilver nanoparticles (AgNp) which would yield D–A–AgNp typeanoclusters with an interpenetrating network. Thus, a combi-ation of fullerene–porphyrin–AgNp conjugate provides an idealystem for fulfilling an enhanced light harvesting efficiency of chro-ophores throughout the solar spectrum and a highly efficient

onversion of the harvested light into the high energy state ofhe charge separation by photoinduced energy and/electron trans-er. It is already explored by various researchers in recent past,hat, this type of model system is of great importance not only inpto-electronic technologies [36] but also in life science in rela-ion to respiration [37], photosynthesis [38] and photomedicine39]. Therefore, it would be a great idea to see whether any pho-ophysical changes take place in the composite mixture containingullerene and porphyrin when AgNp are added in such assembly.or the aforesaid reasons, we have chosen such a complex systemhich contains a monoporphyrin molecule, namely, 5,10,15,20-

etrakis(4-methoxyphenyl)-21H,23H-porphine (1) (Fig. 1) and toee the intermolecular interaction between fullerenes (C60 and70) and 1 in absence and presence of AgNp. Metal nanoparti-les have size dependent optical and electrical properties [40].n this respect, an important feature of metal nanoparticles ishe localized surface plasmon band resonance [41], which iseen as high extinction coefficients of metal nanoparticles. As aesult of this, smaller sizes of metal nanoparticles absorb lightntensively, whereas scattering of light becomes an important

actor for bigger nanoparticles. The surface plasmon band reso-ance causes enhancement of electromagnetic field near the metalanoparticles. Application utilizing the surface plasmon reso-ances of metal nanoparticles includes imaging, sensing, medicine,

a Part A 89 (2012) 284– 293 285

photonics and optics [42,43]. Although there are some reports oninteraction between fullerene and gold nanoparticles (AuNp) inpresence of porphyrin in recent past [44,45], up to date, thereis no such investigations on non-covalent interaction betweenfullerene and porphyrin in presence of silver nanoparticles (AgNp).In recent past, a novel two-step bottom-up approach to con-struct a 2-dimensional long-range ordered, covalently bondedfullerene–porphyrin binary nanostructure is presented in presenceof Ag(1 1 0) [46]. However, we anticipate that the combination ofphotoactive molecules, like fullerene and porphyrin, into formationof non-covalent assembly with AgNp may lead to some new physic-ochemical aspects. The motivation of the present work, therefore,deals with the non-covalent interaction between fullerenes andmonoporphyrin in presence of AgNp. Energy transfer is one of thegoal of our present studies other than to envisage the followingaspects, namely, perturbation of the Soret absorption band of 1in presence of fullerene + AgNp mixture, efficient quenching of thesinglet excited state of 1 in presence of fullerene + AgNp mixture,time-resolved decay kinetics of the excited singlet state of 1 in pres-ence of fullerene + AgNp mixture, binding constant of fullerene–1non-covalent complexes as well as selectivity in binding in pres-ence of AgNp, conductance measurement of fullerene + 1 + AgNpthree-component system, dynamic light scattering experimentof fullerene + 1 + AgNp three-component system in solution, andfinally, scanning electron microscope (SEM) along with trans-mission electron microscope (TEM) studies on fullerene–1–AgNpcomposite system. The complex three-component system, i.e.,fullerene + 1 + AgNp, is possibly advantageous on the basis of thefact that the selectivity in binding of C70–1 system over C60–1 sys-tem, i.e., KC70–1/KC60–1, increases dramatically in presence of AgNp.

2. Materials and methods

C60 and C70 are purchased from Sigma–Aldrich, USA and usedwithout further purification. 1 is procured from Tokyo ChemicalIndustry Co., Ltd., Japan and used without further purification.UV–vis spectroscopic grade toluene (Merck, Germany) is used assolvent to favor the intermolecular interaction between fullereneand 1, as well as to provide good solubility and photo-stability of thesamples. UV–vis spectral measurements have been performed ona Shimadzu UV-2450 model spectrophotometer using quartz cellwith 1 cm optical path length. Steady state emission spectra arerecorded with a Hitachi F-4500 model fluorescence spectropho-tometer. Fluorescence decay experiments have been measuredwith a HORIBA Jobin Yvon single-photon counting setup employingNanoled as excitation source. DLS measurements have been donewith Nano S Malvern instrument employing a 4 mW He–Ne laser(� = 413 nm) equipped with a thermostated sample chamber. Allthe scattered photons are collected at 173◦ scattering angle. SEMmeasurements are done in a S-530 model of Hitachi, Japan instru-ment having IB-2 ion coater with gold coating facility. Transmissionelectron micrographs (TEM) of the clusters have been recorded byapplying a drop of the sample onto a carbon-coated copper grid.Images are recorded using a JEOL JEM-200CX transmission electronmicroscope.

3. Results and discussion

3.1. UV–vis investigations

The ground state absorption spectrum of 1

(5.130 × 10−6 mol dm−3, Fig. 2(a)) in toluene recorded againstthe solvent as reference displays one broad Soret absorption band(�max = 422 nm) corresponding to the transition to the secondexcited singlet state S2. As for the Q absorption bands, 1 shows

286 R. Mitra et al. / Spectrochimica Acta Part A 89 (2012) 284– 293

F p in toa rptiont ene.

t5ptam

F(irs

ig. 2. UV–vis absorption spectrum of (a) 1 (5.130 × 10−6 mol dm−3), (b) 0.02 ml AgNgainst the solvent containing the AgNp as reference; for clarity, inset shows the absohe 0.02 ml AgNp in toluene as reference along with spectrum of pure AgNp in tolu

wo major absorption bands 518 at 554 nm and two minor bands95 and 650 nm (Fig. 2(a)). Q absorption bands in metallopor-

hyrin correspond to the vibronic sequence of the transition tohe lowest excited singlet state S1. Fig. 2(b) shows the electronicbsorption spectrum of 0.02 ml AgNp solution in 5 ml tolueneeasured against toluene. The distinctive colour of AgNp is due

ig. 3. UV–vis absorption spectrum of uncomplexed 1 (5.130 × 10−6 mol dm−3) in to5.130 × 10−6 mol dm−3) system in toluene recorded against the pristine acceptor solutn toluene recorded against the pristine acceptor solution as in reference, mixture of

ecorded against the mixture of pristine acceptor solution and AgNp in toluene as in refeystem + AgNp in toluene recorded against the mixture of pristine acceptor solution and A

luene and (c) 1 (5.130 × 10−6 mol dm−3) + 0.02 ml AgNp mixture in toluene recorded spectrum of 1 (5.130 × 10−6 mol dm−3) + AgNp mixture in toluene recorded against

to a phenomenon known as plasmon absorbance. Incident lightcreates oscillations in conduction electrons on the surface of the

nanoparticles and electromagnetic radiation is absorbed. Thespectrum of the clear yellow colloidal silver shows one broadabsorption band near the region of 452 nm due to its surfaceplasmon band resonance character [47]. When 0.02 ml solution of

luene recorded against the solvent as reference, C60 (2.035 × 10−5 mol dm−3)–1ion as in reference, C70 (2.0 × 10−5 mol dm−3)–1 (5.130 × 10−6 mol dm−3) system

C60 (2.035 × 10−5 mol dm−3)–1 (5.130 × 10−6 mol dm−3) system + AgNp in toluenerence and finally, mixture of C70 (2.0 × 10−5 mol dm−3)–1 (5.130 × 10−6 mol dm−3)gNp in toluene as in reference.

ca Act

AtasaooftdfIa(1paoclbtasbtCioiaibofTpmas

3

asudaposptndtdtfmflt(t

R. Mitra et al. / Spectrochimi

gNp is added to the solution of 1 (5.130 × 10−6 mol dm−3) andhe electronic absorption spectrum of the mixture is recordedgainst the same concentration of AgNp in toluene, the inten-ity of the Soret absorption band is found to decrease frombsorbance value of 1.3775–0.9870 (Fig. 2(c)). Notable featuref the UV–vis investigations is that a new absorption peak isbserved at 458.0 nm, which is explained on the basis of theact that AgNp particles impart some electrostatic character onhe surface of 1 which results new electronic transition. Evi-ence in favor of ground state electronic interaction betweenullerenes and 1 first comes from the UV–vis titration experiment.t is observed that addition of a C60 (2.035 × 10−5 mol dm−3)nd/C70 solution (2.0 × 10−5 mol dm−3) to a toluene solution of 15.130 × 10−6 mol dm−3, Fig. 3) decreases the absorbance value of

at its Soret absorption maximum (Fig. 3) recorded against theristine acceptor solution as reference. However, no additionalbsorption peaks are observed in the visible region. The formerbservation extends a good support in favor of the non-covalentomplexation between fullerenes and 1 in ground state. Theatter observation indicates that the interaction is not controlledy charge transfer (CT) transition. Another important feature ofhe UV–vis investigations is the larger extent of decrease in thebsorbance value of 1 in presence of C70 in comparison to C60. Thispectroscopic observation suggests greater amount of interactionetween C70 and 1. However, measurements of UV–vis spectrum ofhe C60 (2.035 × 10−5 mol dm−3) + 1 (5.130 × 10−6 mol dm−3) and70 (2.0 × 10−5 mol dm−3) + 1 (5.130 × 10−6 mol dm−3) mixtures

n presence of AgNp recorded against the same concentrationf fullerene in presence of AgNp, make new physical insight. Its observed that in presence of AgNp, the intensity of the Soretbsorption peak of the C60 + 1 and C70 + 1 mixtures increase (Fig. 3),n comparison to the situation when complexation takes placeetween 1 and fullerenes in absence of AgNp. From the abovebservations we may infer that binding phenomenon betweenullerenes and 1 takes place at less extent in presence of AgNp.his new photophysical feature of the fullerene–1 mixtures inresence of AgNp prompt us to measure the quantitative esti-ation of the binding phenomenon between fullerenes and 1 in

bsence and presence of AgNp employing steady state fluorescencepectroscopic tool.

.2. Steady state fluorescence investigations

The photoinduced behavior of the complexes of the C60–1nd C70–1 complexes has been investigated by steady-state emis-ion measurements. It is observed that the fluorescence of 1pon excitation at Soret absorption band diminishes graduallyuring titration with C60 and C70 solutions in toluene (Fig. 4(a)nd (b), respectively). This indicates that there is a relaxationathway from the excited singlet state of the porphyrin to thatf the fullerene in toluene. It is already reported that chargeeparation can also occur from the excited singlet state of the por-hyrin to the C60 in toluene medium [48]. Competing betweenhe energy and electron transfer processes is a universal phe-omenon in donor molecule–fullerene complexes [48,49], solventependent photophysical behavior is a typical phenomenon ofhe most fullerene–porphyrin non-covalent system studied toate [50]. Photophysical studies already prove that in conforma-ionally flexible fullerene–porphyrin dyad, �-stacking interactionsacilitate the through space interactions between these two chro-

ophores which is demonstrated by quenching of 1porphyrin*

uorescence and formation of fullerene excited states (by energyransfer) or generation of fullerene−porphyrin+ ion-pair statesby electron transfer) [51]. However, in non-polar solvent, energyransfer generally dominates (over the electron transfer process)

a Part A 89 (2012) 284– 293 287

the photo physical behavior in deactivating the photo excitedchromophore 1porphyrin* of fullerene–porphyrin non-covalentsystem. Similar sort of rationale is already proved by Yin et al.for their particular cis-2,5-dipyridylpyrrolidino[3,4:1,2]C60-zinctetraphenylporphyrin supramolecule [52]. In the present investi-gations, therefore, the quenching phenomenon may be ascribed tophotoinduced energy transfer from porphyrins to fullerenes. As weuse the Soret absorption band as our source of excitation wave-length in fluorescence experiment, the 2nd excited singlet state of1 is deactivated by singlet–singlet energy transfer to the fullerene.The spectral changes finally reach a plateau by the complexationwith C60 and C70 (Fig. 1S(a) and (b), respectively). Although 1exhibits fluorescence quenching upon the addition of fullerenes,the quenching efficiency of C70 is found to be higher than that ofC60. As ground state complex formation between 1 and fullerenesis evidenced from the steady state fluorescence studies, let us con-sider the formation of a non-fluorescent 1:1 complex according tothe following equilibrium:

1 + fullerene � fullerene–1 (1)

The fluorescence intensity of the solution decreases upon addi-tion of fullerenes C60 and C70. Using the relation of binding constant(K) we obtain,

K = [fullerene–1][1][fullerene]

(2)

Using the mass conservation law, we may write

[1]0 = [1] + [fullerene–1] (3)

where [1]0, [1] and [fullerene–1] are the initial concentrations of 1,1 in presence of fullerene, and fullerene–1 complex, respectively.Eq. (3) can be rearranged as

[1]0

[1]= 1 + [fullerene–1]

[1](4)

Using the value of K in place of [fullerene–1]/[1] from Eq. (2), wecan write Eq. (4) as follows

[1]0

[1]= 1 + K[fullerene] (5)

Considering the fluorescence intensities are proportional to theconcentrations, Eq. (5) is expressed as

F0

F= 1 + K[fullerene] (6)

where F0 is the fluorescence intensity of 1 in the absence of fullereneand F is the fluorescence intensity of 1 in the presence of quencher(i.e., fullerene). In our present investigations, steady-state fluores-cence quenching studies afforded excellent linear plot for boththe C60–1 and C70–1 systems as demonstrated in Fig. 5, which isexplained by fluorescence of 1 is being quenched only by staticmechanism, as opposed to diffusional quenching process. Eq. (6),therefore, represents a Stern-Volmer (SV) type plot for investi-gated supramolecules. K values of the C60–1 and C70–1 systemsare given in Table 1. K values of various fullerene–1 systems aredetermined employing Eq. (6); K values of the C60–1 and C70–1systems are estimated to be 2355 and 11,980 dm3 mol−1, respec-tively. It is interesting to note that the increase in magnitude of K ledto increase of the fluorescence quenching efficiency. Although thedetails are not clear at this point, the well-defined structure of 1 hasafforded tight fixing of C60 should give rise to correct host-guest ori-entation. The steady state fluorescence emission measurements of

C60–1 and C70–1 systems in presence of AgNp, however, evoke newphysicochemical insight regarding magnitude of binding betweenfullerenes and 1 in solution. It is observed that the K value of C60–1and C70–1 systems in presence of AgNp are estimated to be 340 and

288 R. Mitra et al. / Spectrochimica Acta Part A 89 (2012) 284– 293

0

500

1000

1500

2000

2500

3000

800750700650600Emission wavelength,nm

Fluo

resc

ence

Inte

nsity

,a.u

.

0

500

1000

1500

2000

2500

3000

3500

770720670620Emission Wavelength, nm

Fluo

resc

ence

inte

nsity

,a.u

.0

500

1000

1500

2000

2500

3000

770720670620Emission Wavelength,n.m.

Fluo

resc

ence

Inte

nsity

,a.u

.

0

400

800

1200

1600

770720670620

Emission Wavelength,n.m.Fl

uore

scen

ce In

tens

ity,a

.u.

(a) (b)

(d)(c)

Fig. 4. Steady state fluorescence quenching experiment of 1 (5.15 × 10−6 mol dm−3) by (a) C60 (0–8.0 × 10−5 mol dm−3) and (b) C70 (0–9.40 × 10−5 mol dm−3) in absenceof AgNp recorded in toluene; and steady state fluorescence quenching experiment of 1 (5.15 × 10−6 mol dm−3) by (c) C60 (0–9.20 × 10−5 mol dm−3) and (d) C70

(

7ptpuiga

3

fsfiuctmcCt

TBC

0–7.80 × 10−5 mol dm−3) in presence of AgNp recorded in toluene.

380 dm3 mol−1, respectively, which are found to be much lower inresence of AgNp (see Table 1). This phenomenon clearly suggestshat AgNp particles partially cover the surface area of the mono-orphyrin which renders the fullerenes C60 and C70 molecules tondergo interaction with 1. The steady state fluorescence quench-

ng spectral variation of 1 by C60 and C70 in presence of AgNp isiven as Fig. 4(c) and (d), respectively; corresponding SV type plotsre provided in Fig. 3S(a) and (b), respectively.

.3. Time resolved fluorescence investigations

The time-resolved fluorescence spectral features of theullerene–1 systems in absence and presence of AgNp track theteady state measurements. The fluorescence decay-time pro-le of the investigated complexes of 1 (monitored at 422 nmsing 462 nm LASER diode) shows an enhanced decay rate asompared to uncomplexed 1 (Fig. 6). In toluene, all of the inves-igated species, viz., 1, C60–1 (Fig. 6(a)) and C70–1 (Fig. 6(b)) reveal

ono-exponential decay. Substantial quenching of the fluores-ence lifetime is observed for the investigated supramolecules; the70–1 system clearly shows a higher efficiency of quenching thanhat of C60–1 complex. Similar sort of mono-exponential feature

able 1inding constants (K), life time (�) values, rate constants of charge separation (kCS) and qu60 and C70 in absence and presence of AgNp recorded in toluene (temperature 298 K).

System In absence of AgNp

K, dm3 mol−1 �, ns 106 kCSa, sec−1 ˚CS

b

1 – 7.490 – –

C60–1 2,355 ± 120 7.210 5.200 0.0375C70–1 11,980 ± 600 6.540 19.410 0.127

a kCS = (1/�)complex–(1/�)ref.b ˚CS = [(1/�)complex–(1/�)ref]/(1/�)complex.

is observed for the same systems, i.e., C60–1 and C70–1, in pres-ence of AgNp (Fig. 6(a) and (b)). While the insufficient polarity oftoluene prevents an appreciable stabilization of the radical pair,we can assume that the quenching is due to energy transfer fromthe excited singlet state of porphyrin to fullerene. In our presentinvestigations, we have determined the rates of charge separation(ksinglet

CS ) and quantum yields (�singletCS ) in an usual manner employed

for the intermolecular energy/electron-transfer process and thedata are provided in Table 1. An examination of Table 1 revealsthe following: (i) the experimentally determined values of ksinglet

CS

and �singletCS are found to be higher for C70–1 complex as compared

to the C60–1 complex both in absence and presence of AgNp. Thisobservation is consistent with the close proximity of the mono-porphyrin molecule 1 and fullerene entity in the C70–1 complexas obtained from the magnitude of K value of such complex; (ii)in agreement with the steady state emission results, time-resolvedfluorescence experiment suggests that with the increasing valueof electron affinity of the acceptor (here C70) [53,54], ksinglet

CS and

�singletCS increase; and, (iii) magnitude of �singlet

CS for the C60–1 andC70–1 complexes are found to be small in presence of AgNp whichhints strongly that feasibility of energy transfer between porphyrinand fullerene decreases due to the adsorption of such particles at

antum yields for charge separation (˚CS) for the non-covalent complexes of 1 with

In presence of AgNp

K, dm3 mol−1 �, ns 106 kCSa, sec−1 ˚CS

b

– 7.490 – 340 ± 17 7.280 3.920 0.0285

7380 ± 370 6.780 13.995 0.0950

R. Mitra et al. / Spectrochimica Acta Part A 89 (2012) 284– 293 289

0.0 2.0x10-5 4.0x 10-5 6.0x10-5 8.0x1 0-50.0

0.4

0.8

1.2

1.6

2.0F 0/F

[C60], mol.dm-3

(a)

(b)

0.0 2.0x1 0-5 4.0x1 0-5 6.0x10-5 8. 0x10-5 1. 0x10-40.0

0.4

0.8

1.2

1.6

2.0

2.4

F 0/F

[C70], mol.dm-3

FA

ttfltomi

3

stlcave∼taipa

Fig. 6. Time resolved fluorescence decay plots of (a) (i) uncomplexed 1(5.105 × 10−6 mol dm−3) in toluene, (ii) in presence of C60 (6.25 × 10−5 mol dm−3)in toluene and (iii) C60 (6.25 × 10−5 mol dm−3) + AgNp mixture in tolueneand; and (b) (i) uncomplexed 1 (5.105 × 10−6 mol dm−3) in toluene, (ii) 1

3.6. SEM measurements

ig. 5. SV type fluorescence plots of (a) C60–1 and (b) C70–1 systems in absence ofgNp measured in toluene.

he surface of 1. It should be noted at this point that actually inoluene, where there is a weaker overlap between the porphyrinuorescence and the fullerene absorption, singlet–singlet energyransfer dominates over electron transfer phenomenon. The abovebservations give very good support in favor of static quenchingechanism for the decay of 1 in presence of only fullerene and also

n presence of fullerene + AgNp mixture.

.4. Dynamic light scattering experiment

Dynamic light scattering (DLS) is the most versatile and usefulet of techniques for measuring in situ on the sizes, size dis-ributions, and (in some cases) the shapes of nanoparticles iniquids [55–58]. In our present investigations, DLS measurementslearly demonstrate that presence of AgNp inhibit the complex-tion between fullerenes and 1 in toluene. Fig. 7(a) shows theariation of scattering intensity vs. size of the AgNp solution in pres-nce of 1. It shows that size of AgNp remains within the range of5–7 nm. However, in presence of C60 (Fig. 7(b)) and C70 (Fig. 7(c)),

he particle size of AgNp becomes ∼13.0 nm and ∼37.0 nm. Thebove scenario provides strong evidence in favor of the fact that

n presence of fullerenes, AgNp partially covers the surface area oforphyrin, which renders the fullerene molecule to undergo inter-ction with the whole surface area of 1. This is the actual reason

(5.105 × 10−6 mol dm−3) + C70 (6.490 × 10−5 mol dm−3) mixture in absence of AgNpand (iii) 1 (5.105 × 10−6 mol dm−3) + C70 (6.490 × 10−5 mol dm−3) + AgNp mixturerecorded in toluene medium.

behind the fact that the magnitude of K of the C60–1 and C70–1systems decreases in presence of AgNp in toluene.

3.5. Conductance measurements

Conductance measurements of the 1 in presence of only AgNp,only C60, only C70, C60 + AgNp and C70 + AgNp mixtures reveal newphysicochemical aspect. It is observed that in presence of AgNp,the conductance value of 1 in toluene gets very little change withincreasing time. After 3 h of measurement, the conductance valueof the mixture of 1 and AgNp in toluene is determined to be 0.6 �Scompared to the initial value of 0.2 �S in absence of AgNp (seeTable 1S). This observation is quite consistent with the fact thatAgNp fails to generate any photocurrent during its interaction withporphyrin. On the other hand, the conductance measurement ofC60 + 1 (Table 2S) and C70 + 1 (Table 3S) mixtures in absence ofAgNp, reveal considerable increase in the value of such parame-ter with increase in time. The conductance value of C60–1 systemis estimated to be 0.6 �S after 85 min of measurement. In presenceof AgNp, the conductance value of the same system is reduced to0.5 �S after same time interval (see Table 4S). This phenomenongives clear evidence in favor of the inhibition of complexationbetween 1 and C60 in presence of AgNp. Similar sort of experimen-tal observation is noticed in case of C70–1 system in absence andpresence of AgNp (see Table 5S).

SEM measurements of 1 in presence of AgNp reveals unevensized particles (Fig. 8(a)). The SEM image of C70 + 1 + AgNp

290 R. Mitra et al. / Spectrochimica Acta Part A 89 (2012) 284– 293

10090807060504030201000.0

0.5

1.0

1.5

2.0

Scat

terin

g In

tens

ity,a

.u.

Size, nm

(a)

(b) (c)

10090807060504030201000

1

2

3

4

5

Scat

terin

g In

tens

ity, a

.u.

Size, nm1009080706050403020100

0

10

20

30

40

50

60

Scat

terin

g In

tens

ity,a

.u.

Size, nm

F for

( −6 mo

cte(tsrsrtsCtaw

3

bmtCtc

ig. 7. Plot of scattering intensity vs. size of the silver nanoparticle7.125 × 10−5 mol dm−3) + AgNp and (c) 1 (5.105 × 10−6 mol dm−3) + C70 (7.050 × 10

omposite mixture (Fig. 8(b)) obtained from drop-casted films ofhe clusters ([1]:[C70] = 1:1) on a stab made of copper reveals thexclusive formation of rod-like clusters with a well-controlled size35–40 nm in the long axis). In sharp contrast, the SEM image ofhe clusters of (C60 + 1 + AgNp) (Fig. 8(c)) shows ill-defined sizes andhapes. In particular, the SEM image of clusters of (C60) reveals bothods of different sizes (5–10 nm in the long axis) and small random-haped structures (1–5 nm). In addition, variation of the [1]:[C60]atio is found to affect the cluster size and shape (Fig. 8(c)). Thisrend is consistent with the size of the large and bucket-shapedurface holes, which are expected to incorporate up to as many as60 molecules. These results clearly demonstrate that the shape andhe size of the surface holes on the 1–C(60/70)–AgNp structure playn important role in controlling the formation of molecular clustersith either C60 or C70 molecules.

.7. TEM measurements

The electrostatic attraction and coordination interactionetween porphyrin-based supramolecules and silver nanoparticlesay cause the formation of larger aggregates. One typical TEM pho-

ograph of AgNp in the situation of before and after coated with70–1 supramolecular system is shown in Fig. 9(a) and (b), respec-ively; Fig. 9 nicely demonstrates that presence of AgNp in C70–1omposite mixture creates formation of larger aggregates.

(a) 1 (5.105 × 10−6 mol dm−3) + AgNp, (b) 1 (5.105 × 10−6 mol dm−3) + C60

l dm−3) + AgNp systems recorded in toluene.

3.8. Theoretical calculations

Table 1 reports the K values of various fullerene–1 complexes.The notable feature of the binding studies is that presence of AgNpin the fullerene–1 composite mixture causes dramatic increasein the selectivity of binding of C70 over C60, i.e., KC70 /KC60 . Thus,KC70 /KC60 is estimated to be ∼21.7 in presence of AgNp, while thesame parameter exhibits value of only ∼5.0 when it is measuredin absence of AgNp. It is observed that for all the complexes stud-ied, C70 exhibits larger value of K in comparison to C60. Very largevalue of average selectivity in binding in absence of AgNp suggeststhat 1 may discriminate C60 and C70 from their mixture in solu-tion. Large value of K for the C70–1 system, both in absence andin presence of AgNp, provides additional stabilization to that sys-tem and such a stabilization of the C70–1 supramolecular complexmay be attributed due to the presence of intermolecular interactionbetween the graphitic 6:6 plane of C70 and the monoporphyrin.Primarily, the attractive interaction between C70 and the mono-porphyrin is driven by the presence of dispersive forces associatedwith �–� interactions. The most concrete evidence of the abovestatement is illustrated by the side-on rather than end-on bind-ing of C70 with the plane of 1. Semiempirical calculations at third

parametric level (PM3) in vacuo well reproduce the above featureregarding orientation of bound guest (here C60 and C70) with theplane of 1. Thus, in the case of C70–1 complex, the side-on interac-tion of C70 with 1 generates enthalpies of formation (�Hf

0) value

R. Mitra et al. / Spectrochimica Acta Part A 89 (2012) 284– 293 291

Fig. 8. SEM photographs of (a) 1 (5.105 × 10−6 mol dm−3) + AgNp, (b) C70 (7.050 × 10−5 mol dm−3) + 1 (5.105 × 10−6 mol dm−3) + AgNp and (c) C60 (7.125 × 10−5 mol dm−3) + 1(5.105 × 10−6 mol dm−3) + AgNp systems recorded in toluene.

Fig. 9. TEM photographs of AgNp (a) before and (b) after complexation with C70 (7.050 × 10−5 mol dm−3) recorded in toluene.

292 R. Mitra et al. / Spectrochimica Act

Table 2Heat of formation values (�Hf

0) of the fullerene–1 complexes done by PM3 methodin vacuo.

System �Hf0, kJ mol−1

C60–1 −1.35C70–1 (side-on orientation of C70) −1.95C70–1 (end-on orientation of C70) −1.52

FCi

omaCptcwaTtvSp

4

mcUmsfi

ig. 10. Single projection structures of (a) C60–1, (b) C70–1 (in side-on orientation of70) and (c) C70–1 (in end-on orientation of C70) systems done by PM3 calculations

n vacuo.

f −1.95 kJ mol−1 (Table 2). However, the end-on pattern of C70olecule produces �Hf

0 value of −1.52 kJ mol−1 (Table 2). It islready well established that the 6:6 ring–juncture bond of the70, rather than 6:5 ring–juncture bond, lies closest to the por-hyrin plane as the 6:6 “double” bonds of C70 are more electron-richhan 6:5 “single” bonds [19,26]. Thus, the equatorial face of C70 isentered over the electropositive center of the porphyrin plane,hich can be viewed as an enhancement in van der Waals inter-

ction due to availability of greater surface area and volume (seeable 2) favoring strong �−� interactions. It should be noted athis point that C60–1 complex exhibits much lower value of �Hf

0,iz., −1.35 kJ mol−1 compared to C70–1 complex as listed in Table 2.ingle projection geometric structures for all the fullerene–1 com-lexes at their different orientations are visualized in Fig. 10.

. Conclusions

The main focus of our attempt to develop a new and improvedethodology for the characterization of fullerene–porphyrin non-

ovalent assembly in presence of silver nanoparticles supported byV–vis, steady state fluorescence, time resolved fluorescence, DLSeasurements, conductance experiments along with SEM and TEM

tudies are discussed elaborately in present investigations. The keyndings are summarized below:

(1) Here, we report the solution phase ground state non-covalent interactions between a monoporphyrin, namely 1,and fullerenes, viz., C60 and C70, in absence and presence of

AgNp having diameter of ∼3–7 nm.

(2) The two new photophysical insights are the increase in theabsorbance value of the Soret absorption band of 1 after com-plexation with C60 and C70 in presence of AgNp relative to the

a Part A 89 (2012) 284– 293

situation when complexation between fullerenes and 1 takesplace in absence of AgNp, and secondly, quenching efficienciesof both C60 and C70 have been reduced in presence of AgNp intoluene; these two observations are certainly indicative of thefact that AgNp renders the complexation between fullerenesand 1.

(3) Estimation of binding constants applying steady state fluores-cence spectroscopic techniques reveal that 1 could not serve asan efficient complexing agent towards C60 and C70 in presenceof AgNp; however, 1 serves as a better discriminator moleculetowards C70 in presence of AgNp as selectivity in binding isestimated to be ∼21.7 in presence of AgNp compared to the sit-uation when fullerene–1 mixture does not contain any AgNp(i.e., the selectivity in binding ∼5.0).

(4) Time resolved fluorescence experiment reveals fluorescencedecay of 1 by both C60 and C70 in the absence and presence ofAgNp, and the mechanism of fluorescence decay is governedby static quenching process.

(5) DLS studies establish that particle size of AgNp becomes largerwhen complexation takes place between fullerenes and 1, andit is the primary reason behind getting lesser magnitude of Kvalue for fullerene–1 systems.

(6) Conductance measurements evoke new physicochemicalaspect on fullerene–porphyrin complexation process in pres-ence of AgNp; it envisages that AgNp particle reduces themagnitude of conductance value for the fullerene–1 systemsin presence of AgNp.

(7) The shape and the size of the surface holes on theporphyrin–fullerene–AgNp structure play an important rolein controlling the formation of molecular clusters withfullerenes.

(8) TEM measurements clearly explain the role of the electrostaticattraction and coordination interaction between porphyrin-based supramolecules and silver nanoparticles.

(9) Theoretical calculations at PM3 level provide very good sup-port in favor of strong binding between C70 and 1.

(10) The results emanating from present investigations wouldbe of potential interest in studying the interaction betweenfullerenes and diporphyrin in presence of silver nanoparticlesin near future.

Acknowledgements

RM thanks Department of Atomic Energy, Board of Research inNuclear Science (BRNS), Bhabha Atomic Research Center, Mumbai,India for providing Junior Research Fellowship to him. Financialassistance through the project of Ref. No. 2010/20/37P/2/BRNS isalso gratefully acknowledged. We also wish to acknowledge Dr. A.K. Ghosh, Department of Chemistry, The University of Burdwan forhis helpful co-operations in performing the conductance measure-ments.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.saa.2011.12.013.

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