Photophysical insights on effect of gold nanoparticles over fullerene–porphyrin interaction in solution

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 61–69

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Photophysical insights on effect of gold nanoparticles over fullerene–porphyrin interaction in solution Ratul Mitra a, Ajoy K. Bauri b, Shrabanti Banerjee c,⇑, Sumanta Bhattacharya a,⇑ a

Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, India Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India c Department of Chemistry, Raja Rammohun Roy Mahavidyalaya, Radhanagore, Hooghly 712 406, India b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Fullerene–porphyrin (1)

a r t i c l e

i n f o

Article history: Received 22 October 2013 Received in revised form 6 February 2014 Accepted 8 March 2014 Available online 20 March 2014 Keywords: Fullerenes C60 and C70 Monoporphyrin Gold nanoparticle Binding constant DLS SEM

1000

(a)

(b)

Counts (log)

complexation is examined in presence of AuNp in toluene.  AuNp causes effective reduction in the value of binding constant for C70–1 system.  Fluorescence studies elicit quenching of 1 in presence of fullerene-AuNp mixture.  DLS study reveals increase in the particle size of C70–1–AuNp nanocomposite.  Energy transfer phenomenon takes place.

100

(c) (d)

(e)

10 (f)

1 0

10

20

40

a b s t r a c t The present article reports the role of gold nanoparticles, i.e., AuNp (having diameter 2–4 nm), in noncovalent interaction between fullerenes (C60 and C70) and a monoporphyrin (1) in toluene. Both UV–vis and fluorescence measurements reveal considerable reduction in the average value of binding constant (Kav) for the C70–1 system (KC70–1(av) = 19,300 dm3 mol1) in presence of AuNp, i.e., KC70–1–AuNp(av) = 13,515 dm3 mol1 although no such phenomenon is observed in case of C60–1 system, viz., KC60–1(av) = 1445 dm3 mol1 and KC60–1–AuNp(av) = 1210 dm3 mol1. DLS study reveals sizeable amount of increase in the particle size of C70–1–AuNp nanocomposite, i.e., 105 nm, compared to C60–1–AgNp system, e.g., 5.5 nm which gives very good support in favor of decrease in the value of Kav for the former system. SEM study reveals that nanoparticles are dispersed in larger extent in case of C70–1–AuNp system. Time-resolved fluorescence study envisages that deactivation of the excited singlet state of 1 by C70 takes place at a faster rate in comparison to C60 in presence of gold nanoparticles. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Gold (Au) nanoparticles (AuNp) play important roles in different branches of science, such as in nanoelectronics, nonlinear optics, biological labeling and oxidation catalysis [1–5]. The ⇑ Corresponding authors. Fax: +91 3422530452. E-mail addresses: [email protected] (S. Banerjee), rediffmail.com (S. Bhattacharya). http://dx.doi.org/10.1016/j.saa.2014.03.014 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

30

Time, ns

sum_9974@

application of nanoparticles in nanoelectronic devices, exploiting the organoelectronic p-orbital interactions, which are generally used in electron-conductive polymers and organic transistors etc., are quite important in light of the reduction of the tunneling resistance of the surrounding ligands. In this sense, porphyrin is one of the most important p-conjugated compound and strong coupling between porphyrin and AuNp is observed due to orbital overlap in which porphyrin is attached with multiple linkers parallel to the surface [6]. Fullerenes also show high affinity for

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AuNp and mixing of fullerene with TOABr-protected AuNp produces larger aggregates, where the individual particles are linked together by fullerenes [7]. Fullerenes modified with a thiol linker and attached to AuNp as mixed layers with dodecanethiols show energy transfer from photoexcited fullerene to the particle [8]. As a result of this, photovoltaic cells are developed using composite nanoclusters of porphyrins and fullerenes in presence of AuNp [9]. When a nanosize metal particle is activated by light, the collective oscillation of conduction electrons occurs on the particle surface, which is called localized surface plasmon resonance (SPR) [10,11]. It exhibits unique phenomena such as intense absorption at a wavelength resonant with the electronic transition of molecules in the ultraviolet–visible (UV–vis) region. While the analogies between nano particulate building blocks at the nanoscale and the atomic building blocks at the molecular-scale appear appealing [12,13], it must be remembered that nanoparticles – unlike atoms, are never mono dispersed, and no two particles are ever identical. This inherent polydispersity proves self-assembly, and affects the overall characteristics deriving from the sizedependent properties of individual NPs (e.g., SPR) [14,15] or magnetic susceptibility [16]. In spite of several appealing properties, researchers did not pay their attention in exploring the photophysical insights during selfassembly phenomenon between fullerene and porphyrin in presence of AuNp in past few decades. Only very recently, our research group have explored that the binding between fullerene and a monoporphyrin, namely, 1 (Scheme 1), is reduced considerably in presence of silver nanoparticle (AgNp) in solution [17]. In continuation of our earlier work [17], we have presented the photophysical

insights on non-covalent interaction between 1 and fullerenes (C60 and C70) in presence of AuNp (having diameter in the range of 2–4 nm) in toluene. We anticipate that there is a strong possibility that fullerenes may undergo interaction with AuNp, but the nature of the interaction seems to be dependent on the nature of solvent matrix, structure of the fullerene-complexes of porphyrin and concentration of all the interacting species like fullerene, porphyrin and AuNp. Other than binding studies employing absorption spectrophotometric and steady state fluorescence methods, we have employed time-resolved fluorescence, dynamic light scattering (DLS) and scanning electron microscope (SEM) probes to study the physicochemical insights behind fullerene–porphyrin interaction in presence of AuNp.

Materials and methods C60 and C70 are purchased from Sigma–Aldrich, USA and used without further purification. 1 is synthesized according to the method reported in literature [17]. AuNp (2–4 nm) is purchased from Sigma–Aldrich, USA (Catalogue No. 660426) and used without further purifications. UV–vis spectroscopic grade toluene (Merck, Germany) is used as solvent to favour the intermolecular interaction between fullerene and 1, as well as to provide good solubility and photo-stability of the samples. UV–vis spectral measurements have been performed on a Shimadzu UV-2450 model spectrophotometer using quartz cell with 1 cm optical path length. Fluorescence decay curves have been obtained with a HORIBA Jobin Yvon Single Photon Counting Setup employing Nanoled as excitation source. DLS

Scheme 1. Synthesis of Zn-5, 15-di (para- methoxy phenyl)-porphyrin.

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measurements have been done with Malvern Zeta Seizer instrument of Model No. NANOZS90. All the scattered photons are collected at 90° scattering angle. SEM measurements are done in a S-530 model of Hitachi, Japan instrument having IB-2 ion coater with gold coating facility. Ab initio calculations in vacuo by Hartree–Fock method using Slater type of orbitals (STO) in 3-21G basis set are performed with the help of SPARTAN’06 Windows version software. Results and discussion UV–vis investigations The ground state absorption spectrum of 1 (1.0  105 mol dm3, Fig. 1S(i) in toluene recorded against the solvent as reference displays one broad Soret absorption band (kmax = 413 nm) corresponding to the transition to the second excited singlet state S2. As for the Q absorption band, 1 shows two absorption bands at 503 and 540 nm (Fig. 1S(i)). Q absorption bands in metalloporphyrin correspond to the vibronic sequence of the transition to the lowest excited singlet state S1. Fig. 1S(ii) shows the electronic absorption spectrum of 0.005 ml AuNp solution in 4.0 ml toluene measured against the solvent as reference. More recent results have shown that the color of AuNp is due to the collective oscillations of the electrons in the conduction band, known as surface plasmon oscillation. The oscillation frequency is usually in the visible region for gold giving rise to strong surface plasmon resonance absorption [18]. The spectrum of the deep red colloidal gold shows one broad absorption band near the region of 508 nm (Fig. 1S(ii)). When 0.005 ml solution of AuNp is added to the solution of 1 (1.0  105 mol dm3) and the electronic absorption spectrum of the mixture is recorded against the same concentration of AuNp in toluene, the intensity of the Soret absorption band is found to decrease from the absorbance value of 3.270–2.916 (Fig. 1S(iii)). Evidence in favour of ground state electronic interaction between fullerenes and 1 first comes from the UV–vis titration experiment. It is observed that addition of a C60 (0–9.5  105 mol dm3, Fig. 2S) and C70 solution (0–3  105 mol dm3, Fig. 3S) to a toluene solution of 1 (5.7  106 mol dm3) decreases the net absorbance value of 1, i.e., Dabs, at its Soret absorption maximum recorded against the solvent as reference. The net decrease in the absorbance value of 1 may be calculated with the help of following equation:

respectively; Dabs is the corrected absorbance of the fullerene–1 mixture recorded at the wavelength of measurement against the solvent as reference. The quantity De (=ecomplexeFullerenee1) means the corrected molar absorptivity of the complex, e1 and eFullerene being those of the 1 and fullerenes, respectively. K is the binding constant of the fullerene–1 complexes. Eq. (2) is valid under 1:1 approximation for 1 and fullerene systems. It should be mentioned at this point that the corrected molar extinction coefficient, De, is not quite that of the complex. The BH method [19] is an approximation that we have used many times and it gives decent answers. But the extinction coefficient is really a different one between the complex and free species that absorbs at the same wavelength. Experimental data for spectrophotometric determination of K for the supramolecular complexes of 1 with C60 and C70 are provided in Tables 1S and 2S, respectively. In all the cases very good linear plots are obtained for C60–1 and C70–1 systems, as shown in Figs. 4S and 5S, respectively. The value of K for C60–1 and C70–1 systems are listed in Table 1. When the UV–vis experiment is performed in presence of AuNp, new interesting finding is observed. It is observed that in presence of AuNp, there is an enhancement in the absorbance value of 1, i.e., D/abs for the C60 + 1 and C70 + 1 mixtures (Fig. 1) in comparison to the situation when complexation takes place between only 1 and fullerenes. The inset of Fig. 1 is clearly demonstrating the variation in the absorbance value of 1 in absence and presence of fullerene and fullerene + AuNp mixture. This observation triggers us to perform a detailed UV–vis titration experiment on C60–1 and C70–1 systems in presence of AuNp. The mathematical expression of D/abs may be formulated as follows: Table 1 Binding constants (K) for the non-covalent complexes of 1 with C60 and C70 in absence and presence of AuNp recorded in toluene. Temp. 298 K. System

UV–vis C60–1 C60–1–AuNP C70–1 C70–1–AuNP C60–ZnTPP C70–ZnTPP a b

Dabs ¼ Abs1  AbsC60ðand=C70Þ  AbsC60ðand=C70Þ1

ð1Þ

The terms Abs1, AbsC60(and/C70) and AbsC60(and/C70)–1 may be ascribed as absorbance value of uncomplexed 1 with the same concentration as in the mixture containing fullerene and 1, absorbance value of uncomplexed C60 (and C70) with the same concentration as in the mixture containing fullerene and 1 and finally, absorbance value of fullerene–1 mixture recorded in toluene, respectively. In the UV–vis experiment, no additional absorption peaks are observed in the visible region. The former observation extends a good support in favor of the non-covalent complexation between fullerenes and 1 in the ground state. The latter observation indicates that the interaction is not dominated by charge transfer (CT) transition. Another important feature of the UV–vis investigations is the larger extent of decrease in the absorbance value of 1 in presence of C70 than that of C60. This spectroscopic observation clearly suggests that greater amount of interaction between C70 and 1 takes place in solution. The binding constant (K) of the fullerene–1 systems are evaluated with the help of modified Benesi–Hildebrand (BH) type equation as described in Eq. (2) [19].

ð½1=Dabs Þ ¼ ð1=DeÞ þ ð1=K De½FullereneÞ

K, dm3 mol1 Fluorescence

a

1560b 1090a 14,790b 12,875a 1870 9670

1330 1330a 23,810a 14,155a – –

In present work. Reported in literature [17].

1 1 + C60 + AuNp 1 + C60

1 + C70 + AuNp 1 + C60

ð2Þ

Here [Fullerene] and [1] are the initial concentrations of the acceptor (i.e., C60 and C70) and donor (1) solutions in toluene,

Fig. 1. UV–vis spectral variation of 1 in presence of C60, C70 and composite mixture of C60 + 1 + AuNp and C70 + 1 + AuNp.

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D=abs ¼ Abs1  AbsC60AuNpðor C70AuNpÞ  AbsC60ðandC70Þ1AuNp

ð3Þ

The trend in the decrease of absorbance value for the fullerene– 1–AuNp nanocomposite is much more pronounced compared to fullerene–1 mixture. Thus, we may infer that binding between fullerenes and 1 is inhibited in presence of AuNp. The gradual decrease in the absorbance of the Soret band of 1 has been utilized to determine the value of binding constant (K) of the fullerene–1 complexes in presence of AuNp employing modified BH equation [19] as stated in Eq. (2). Excellent linear correlations are obtained with the present data and they are demonstrated in Figs. 6S & 7S. Experimental data for spectrophotometric determination of K for the supramolecular complexes of 1 with C60 and C70 in presence of AuNp are provided in Tables 3S and 4S, respectively. Table 1 reports the value of K for C60–1 and C70–1 systems in absence and presence of AuNp determined by UV–vis method. Table 1 envisages although there is practically no reduction in the K value of C60–1 system in presence of AuNp, marked reduction in the value of K for C70–1 system take place when AuNp is added in such mixture. Very good selectivity in binding between C70–1–AuNp and C60–1–AuNp composites (10.6) give clear indication in favour of employing AuNp as a photosensitizer to study C70–porphyrin complexation process in solution. Large value of K for the C70–1 system, both in absence and in presence of AuNp, provides additional stabilization to that system and such a stabilization of the C70–1 supramolecular complex

may be attributed due to the presence of intermolecular interaction between the two graphitic 6:6 planes of the C70 molecule with the flat p-plane of the monoporphyrin units in 1. Primarily, the attractive interaction between C70 and the monoporphyrin is driven by the presence of dispersive forces associated with p–p interactions. The most concrete evidence of the above statement is illustrated by the side-on rather than end-on binding of C70 with the plane of 1. Ab initio calculations in vacuo well reproduce the above feature regarding orientation of bound guest (here C60 and C70) with the plane of 1. Thus, in the case of C70–1 system, the side-on interaction of C70 with 1 generates heat of formation (DH0f ) value of 3.630 kcal/ mol. However, the end-on pattern of C70 molecule produces DH0f value of 3.210 kcal/mol. It is already well established that the side-on or equatorial portion of C70 contains 6:6 ring-juncture bond, while polar or end-on part of C70 is dominated by the presence of 6:5 ring-juncture bond [20,21]; In present case, the sideon part of C70 lies closest to the porphyrin plane as the 6:6 ‘‘double’’ bonds of C70 are more electron-rich than 6:5 ‘‘single’’ bonds. Therefore, the equatorial face of C70 is centered over the electropositive part of the porphyrin plane, which may be viewed as an enhancement in van der Waals interaction due to availability of greater surface area and volume favouring strong p–p interactions. It should be noted at this point that C60–1 system exhibits lower value of DH0f , viz., 2.313 kcal/mol compared to C70–1 system. Single projection geometric structures for all the fullerene–1 complexes at their different orientations are visualized in Fig. 2.

Fig. 2. Ab initio optimized single projection structures for (a) C70–1 (side-on orientation of C70), (b) C70–1 (end-on orientation of C70) and (c) C60–1 systems done in vacuo.

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1 þ Fullerene Fullerene  1

Steady state fluorescence investigations The photoinduced behavior of the complexes of the C60–1 and C70–1 complexes in presence of AuNp has been investigated by steady-state emission measurements. It is observed that the fluorescence of 1 upon excitation at Soret absorption band diminishes gradually during titration with C60 (Fig. 3(a)) and C70 (Fig. 3(b)) solutions in presence of AuNp in toluene. This indicates that there is a relaxation pathway from the excited singlet state of the porphyrin to that of the fullerene in toluene. Competing between the energy and electron transfer processes is a universal phenomenon in donor molecule-fullerene complexes [22], solvent dependent photo physical behavior is a typical phenomenon of the most fullerene–porphyrin dyads studied to date [23]. Photo physical studies already prove that in conformationally flexible fullerene–porphyrin dyad, p-stacking interactions facilitate the through space interactions between these two chromophores which is demonstrated by quenching of 1porphyrin fluorescence and formation of fullerene excited states (by energy transfer) or generation of fullereneporphyrin+ ion-pair states (by electron transfer) [24]. However, in non-polar solvent, energy transfer generally dominates (over the electron transfer process) the photo physical behavior in deactivating the photo excited chromophore 1porphyrin of fullerene–porphyrin dyad. Similar sort of rationale is already proved by Yin et al. for their particular cis-2,5-dipyridylpyrrolidino[3,4:1,2] C60zinctetraphenylporphyrin supramolecule [25]. In the present investigation, therefore, the quenching phenomenon can be ascribed to photoinduced energy transfer from porphyrins to fullerenes. As we use the Soret absorption band as our source of excitation wavelength in fluorescence experiment, the 2nd excited singlet state of 1 is deactivated by singlet–singlet energy transfer to the fullerene. Although 1 exhibits fluorescence quenching upon the addition of fullerenes, the quenching efficiency of C70 is higher than that of C60. As ground state complex formation between 1 and fullerenes is evidenced from the steady state fluorescence studies, let us consider the formation of a non-fluorescent 1:1 complex according to the equilibrium:

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

K ¼ ½Fullerene  1=½1½Fullerene

And the mass conservation law (where [1]0 is the total concentration of 1)

½10 ¼ ½1 þ ½Fullerene  1

ð6Þ

We obtain the fraction of the uncomplexed fluorophore (herein 1):

ð½1=½10 Þ ¼ f1=ð1 þ K½Fullereneg

ð7Þ

Considering that the fluorescence intensities are proportional to the concentrations (as the solution is dilute), this relationship can be rewritten as

ðF 0 =FÞ ¼ 1 þ K½Fullerene

ð8Þ

In Eq. (8), F0 and F are the fluorescence intensities of 1 in absence and presence of fullerenes, respectively; [fullerene] indicates the molar concentration of fullerene. By plotting F0/F (i.e., relative fluorescence intensity) against [fullerene], K value is obtained. In our present investigations, steady-state fluorescence quenching studies afforded excellent linear plot for the C60–1–AuNp system as demonstrated in inset of Fig. 3(a). K value of the C60–1–AuNp system is tabulated in Table 1 and corroborates fairly well with that reported from UV–vis investigations. However, in case of C70–1–AuNp system, plot of relative fluorescence intensity versus concentration of quencher deviates significantly from the linear nature of the plot. Therefore, deactivation of the excited singlet of 1, i.e., 1 is not supposed to be primarily controlled by static quenching mechanism in presence of C70–AuNp system. Rather, both static and dynamic quenching mechanism is operative for the investigated supramolecules. We anticipate that in presence of C70, the size of the nanoparticle becomes much higher such that diffusion controlled mechanism plays key role behind the photo-excited decay of the

200

(a)

1.0x10-5 2.0x10-5 3.0x10-5 4.0x10-5 5.0x10-5 -3

[C60], mol.dm

520

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700 600 500 400 300 200 100 0

15 F0/(F0-F)

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2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0

Fluorescence Intensity, a.u

F0/F

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3.15x104 3.00x104 4

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[C70], mol.dm-3

625

725

1.5x10 4 1/[C 60] dm 3.mol -1

720

2.0x10 4

2.5x10 4

820

825

Emission Wavelength, nm. Fig. 3. Steady state fluorescence titration experiment of (a) C60–1 and (b) C70–1 systems recorded in toluene in presence of AuNp; inset of Fig. 3 depicts plot for the evaluation of K for C60–1 and C70–1 systems in presence of AuNp.

6

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5 4 F0/(F0-F)

3.30x10

2.55x104

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Emission Wavelengthg, nm.

3.45x104

600

{(Fo/F)-1}/[C70], dm3.mol-1

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700

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(b)

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1/[C 70] dm 3.mol -1

520

570

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Emission Wavelength, nm. Fig. 4. Steady state fluorescence titration experiment of (a) C60–ZnTPP and (b) C70–ZnTPP systems recorded in toluene; inset of this figure depicts BH plot for the evaluation of K for C60–ZnTPP and C70–ZnTPP systems.

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Scattering Intensity, a.u.

12

9

6

3

0 0

2

4

6

8

10 12 14 16 18 20

Size, nm

(a)

(b)

(c) Fig. 5. Particle size analyzing experiment of (a) 1–AuNp, (b) C601–AuNp and (c) C70–1–AuNp composites.

excited singlet state of 1 apart from static process. The value of K for the C70–1–AuNp system is determined according to the equation as described below:

fðF 0 =F  1Þ=½Fullereneg ¼ ðK SV þ KÞ þ ðK SV KÞ½Fullerene

ð9Þ

A very good linear plot is obtained which is shown in the inset of Fig. 3(b). It is interesting to note that the increase in magnitude of K led to increase of the fluorescence quenching efficiency. Although the details are not clear at this point, the well-defined structure of 1 has afforded tight fixing of C70 which should give rise to correct host–guest orientation. To study and compare the intermolecular interaction of the fullerene complexes of 1 with some model compound like zinc tetraphenylporphyrin (ZnTPP), we have measured the value of K for the complexes of zinc tetrephenylporphyrin (ZnTPP) with C60 and C70 in toluene. Steady state fluorescence titration method evokes although C60 and ZnTPP does not undergo very strong binding, sig-

nificant interaction persists between C70 and ZnTPP in toluene. Fig. 4(a and b) demonstrates the quenching behavior of ZnTPP in presence of C60 and C70. Excellent linear correlations are obtained in the BH plot [19] from which K value of fullerene–ZnTPP systems are evaluated (inset of Fig. 4). Values of K for the fullerene–ZnTPP systems are listed in Table 1. Table 1 envisages while ZnTPP forms strong complex with C60 compared to 1, the magnitude of K for the C70–1 complex is found to be 1.52 times higher than that of C70–ZnTPP complex in toluene. DLS experiment DLS is the most versatile and useful set of technique for measuring in situ on the sizes, size distributions, and (in some cases) the shapes of nanoparticles in liquids [26,27]. In our present investigations, DLS measurements clearly demonstrate that in presence of AuNp, the extent of complexation between fullerenes and 1 is

R. Mitra et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 61–69

67

(a)

(b)

(c)

Fig. 6. SEM images of (a) 1 + AuNp, (b) C60 + 1 + AuNp and (c) C70 + 1 + AuNp mixtures.

inhibited in toluene. DLS picture of 1–AuNp mixture shows scattering intensity of 9 having particle size of 3 nm (Fig. 5(a)). Fig. 5(b and c) clearly demonstrate that in presence of C60 (5.0  105 mol dm3) and C70 (5.0  105 mol dm3), the particle size of AuNp becomes 5.5 nm and 105.0 nm, respectively; the concentration of 1 is kept fixed at 5.0  105 mol dm3. It may be anticipated that AuNp covers the surface area of monoporphyrin 1 which reduces the possibility of fullerenes to come close to interact with the flat p-region of 1. Thus, we may anticipate lesser extent of p–p interaction between fullerenes and 1 in presence of AuNp. This is the actual reason behind the fact that the magnitude of K of the C60–1 and C70–1 systems decreases in presence of AuNp in toluene. As the particle size of the nanoparticles become much larger in presence of C70 compared to C60, i.e., by 100 nm, larger extent of decrease in the K value of C70–1–AuNp system is observed in present work.

nanoparticles having ill-defined size and shapes; actually, small random-shaped structures having diameter in the range of 4– 6 nm are formed. In addition, variation of the [1]:[C60] ratio is found to affect the cluster size and shape (Fig. 6(b)). In case of C70–1–AuNp system, however, formation of beautiful crystal shaped nano-aggregate is noticed (Fig. 6(c))). The formation of nano-aggregate provides very good support in favour of the increase in the particle size of the nanoparticle in case of complexation between C70 and 1 as evidenced from the DLS measurements (see Section DLS experiment). The size of the nanoparticles ranges in the region of 100–110 nm. The above interesting photophysical finding clearly demonstrates that the shape and the size of the surface holes on the 1–C70–AuNp nano-structure play an important role in controlling the formation of molecular cluster and affect the binding process between C70 and 1. This trend is consistent with the size of the large and bucket-shaped surface holes, which are expected to incorporate up to as many as C60 molecules.

SEM measurements Time-resolved fluorescence study SEM measurements of 1 (3.8  106 mol dm3) in presence of AuNp reveals the formation of scattered particles directed in the long axis (Fig. 6(a)). The SEM image of (C60 (4.0  105 mol dm3) + 1 (3.8  106 mol dm3) + AuNp) composite mixture (Fig. 6(b)) obtained from drop-casted films of the clusters ([1]:[C60] = 1:1) on a stab made of copper reveals formation of

The time resolved fluorescence spectral features of the fullerene–1 complexes in absence and presence of AuNp track the steady state measurements. The fluorescence decay-time profile of 1 reveals mono-exponential decay (Fig. 7(a)); life time of the excited singlet state (ssinglet ) of 1 is estimated to be 2.388 ns. In presence of

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porphyrin in two different cases, viz., in absence and presence of AuNp. The key findings are summarized below:

1000 (a)

Counts (log)

(b)

100

(c)

(d) (e)

10

(f)

1 0

10

20

30

40

Time, ns Fig. 7. Time correlated single photon counting decay of 1 in (a) absence and presence of (b) AuNp, (c) C60–AuNp (d) C60, (e) C70–1–AuNp and (f) C70 in toluene.

Table 2 Fluorescence lifetime of the excited singlet state (ssinglet) of 1 in absence and presence of AuNp, C60, C70 and fullerene–AuNp mixtures. Temp. 298 K. System

ssinglet, ns

1 1–AuNp C60–1 C60–1–AuNp C70–1 C70–1–AuNp

2.388 2.355 2.195 2.217 2.149 2.189

AuNp, the ssinglet of 1 is suffered very little reduction in its value, i.e., 2.355 ns. This phenomenon proves that AuNp acts as a quencher. In presence of C60, C70, C60–AuNp and C70–AuNp mixtures, a systematic decrease is observed according to following trend:

ssinglet ðC60  1  AuNpÞ > ssinglet ðC60  1Þ > ssinglet ðC70  1  AuNpÞ > ssinglet ðC70  1Þ: Fluorescence decay profiles of 1, 1–AuNp, C60–1–AuNp, C60–1, C70–1–AuNp and C70–1 systems are shown in Fig. 7(a)–(f), respectively. Table 2 lists the value of ssinglet of 1 in absence and presence of various fullerenes and fullerene–AuNp mixtures. Table 2 also indicates that rate of deactivation of the excited singlet state of 1, i.e., 1 is found to be more faster in case of C70 when AuNp are present in such systems while 1 decays at a slower rate when C60 is added in presence of AuNp. The above experimental findings prevail while the insufficient polarity of toluene prevents an appreciable stabilization of the radical pair, we may assume that the quenching is due to energy transfer from the singlet excited state of 1 to fullerene. Thus, it may be envisaged that in toluene, where there is a weaker overlap between the porphyrin fluorescence and the fullerene absorption, singlet–singlet energy transfer dominates over electron transfer phenomenon. It should be mentioned at this point that El-Khouly et al. have nicely demonstrated various physicochemical aspects related to charge separation in the molecular and supramolecular fullerenes and porphyrins [28,29].

Conclusions The goal of our present studies is to characterize the photophysical insights behind fullerene–porphyrin interaction in presence of AuNp and to compare the extent of binding between fullerene and

(1) The average value of K of the C70–1 system suffers considerable reduction in magnitude in presence of AuNp although such phenomenon could not be observed in case of C60–1 system under similar condition. This interesting observation gives strong propensity to employ AuNp as an effective photosensitizer towards C70 during host–guest interaction with porphyrin. (2) DLS study establishes that particle size of AuNp becomes much larger when complexation takes place between C70 and 1, and it is the primary reason behind getting lesser magnitude of K value for such system. (3) The shape and the size of the surface holes on the fullerene– 1–AuNp nano-structure play an important role in controlling the formation of molecular clusters with fullerenes. It is observed that in case of C70–1–AuNp system, nano-aggregation takes place. (4) Theoretical calculations well reproduce the single projection geometric structures of the fullerene–1 complexes and interpret the stability difference between C60 and C70 complexes of 1 in vacuo. (5) Time-resolved fluorescence study evokes that energy transfer is the only path for the deactivation of the excited singlet state of 1 in presence of fullerenes in toluene. (6) Finally, the results emanating from present investigations contemplate that AuNp acts as an effective quencher molecule in modulating the selectivity of binding ratio between C60–1 and C70–1 complexes in solution.

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