ISSN : 0976-7630
EISSN : 0976-7649
RATUL MITRA1, SUMANTA BHATTACHARYA2*, BAURI A.K.3, MUKHERJEE D.C.4
1Department of Chemistry, The University of Burdwan, Golapbag, Burdwan- 713 104, India.
2Department of Chemistry, The University of Burdwan, Golapbag, Burdwan- 713 104, India.
3Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai- 400 085, India.
4Heritage Institute of Technology, Chowbaga Road, Anandapur, East Kolkata Township, Kolkata- 700 107, India.
* Corresponding Author : sum_9974@rediffmail.com
Received : 25-11-2011 Accepted : 09-03-2012 Published : 10-08-2012
Volume : 3 Issue : 1 Pages : 46 - 54
Nanotechnol Nanosci 3.1 (2012):46-54
Conflict of Interest : None declared
The present article reports the supramolecular interaction between fullerenes (C60 and C70) and a designed monoporphyrin, e.g., parafluorotetraphenylporphyrin (1), in presence of silver nanoparticles (AgNp) having diameter of ~3-7 nm in toluene. UV-Vis studies reveal an interesting new physicochemical aspect. It is observed that in presence of C60 and C70 solution (in toluene), reduction in the absorbance value of the Soret band of 1 takes place; however, increase in the absorbance value of 1 occurs when the same experiment is performed in presence of silver nanoparticles. The most remarkable feature of the UV-Vis studies is that in presence of AgNp, all the species, i.e., 1, 1 + C60 and 1 + C70 mixtures, undergo first order exponential decay and the rate constant values are estimated to be 0.068, 0.084 and 0.060 min-1, respectively. Steady state fluorescence measurement reveals that reduction in the binding constant (K) value takes place for both C60-1 (K = 1065 dm3×mol-1) and C70-1 systems (K = 12585 dm3×mol-1) in presence of AgNp (KC60-1 = 890 & KC60-1 = 11975 dm3×mol-1). Conductance measurements evoke that AgNp particle reduces the magnitude of conductance value for the fullerene-1 systems in presence of AgNp. Dynamic light scattering and scanning electron microscope measurements demonstrate that the electrostatic attraction between porphyrin-based supramolecules and AgNp is very much responsible behind the formation of nanorod in case of C60-1-AgNp composite.
C60 and C70, designed monoporphyrin (1), silver nanoparticles, UV-Vis and fluorescence investigations, conductance studies, DLS and SEM measurements.
Photoactive supramolecular systems, in which a donor and an acceptor moieties are non-covalently linked, are particularly appealing either as models for natural photosynthesis or for the conversion of light into electric current [1,2] . Toward constructing such systems, fullerenes [3] and porphyrins [4-6] have been utilized as important constituents owing to their rich redox [7] , optical [8] and photochemical [9] properties. After the first reports of the co-crystallization of C60 or C70 with the porphyrin unit, the search for highly efficient porphyrin receptors has become intensified [10,11] . A number of porphyrin receptors have been developed [12-18] and the resulting fullerene-porphyrin assemblies have brought forward many interesting photophysical properties [19,20] . In general, fullerenes are considered as excellent electron acceptors in forming electron donor-acceptor or charge transfer (CT) complexes for their ability to accept multiple electrons [21] . The primary component of the fullerene-porphyrin interaction is, therefore, driven by the dispersive forces associated with interaction, which is augmented by weak electrostatic or donor-acceptor stabilization. For efficient recognition of spherical guests like fullerenes, the recognition sites in the synthetic receptor are pre-organized for favorable interaction [22] . Taking into account the fact that fullerene and porphyrin undergo spontaneous interaction through self-assembly leading to make a supramolecular complex in solutions as well as in the solid state [23-26] , and as this type of model system is of great importance not only in opto-electronic technologies [27] but also in life science in relation to respiration [28] , photosynthesis [29] and photomedicine [30] , it would be a great idea to see whether any photophysical changes take place in the composite mixture containing fullerene and porphyrin when silver nanoparticles (AgNp) are added in such assembly. For the aforesaid reasons, we have selected a monoporphyrin molecule, namely, parafluorotetraphenylporphyrin (1) [Fig-1] to find out its intermolecular interaction with fullerenes C60 and C70 in absence and presence of AgNp. Metal nanoparticles have size dependent optical and electrical properties [31] . In this respect, an important feature of metal nanoparticles is the localized surface plasmon resonance [32] , which is seen as high extinction coefficients of metal nanoparticles. As a result of this, smaller sizes of metal nanoparticles absorb light intensively, whereas scattering of light becomes an important factor for bigger nanoparticles. The surface plasmon band resonance causes enhancement of electromagnetic field near the metal nanoparticles. Applications utilizing the surface plasmon resonances of metal nanoparticles include imaging, sensing, medicine, photonics and optics [33,34] . Although there are some reports on interaction between fullerene and gold nanoparticles (AuNp) in presence of porphyrin in recent past [35,36] , there is no such investigation on non-covalent interaction between fullerene and porphyrin in presence of silver nanoparticles (AgNp). In recent past, a novel two-step bottom-up approach to construct a 2-dimensional long-range ordered, covalently bonded fullerene-porphyrin binary nanostructure is presented in presence of Ag(110) [37] . However, we anticipate that the combination of photoactive molecules, like fullerene and porphyrin, into formation of non-covalent assembly in presence of AgNp may lead to some new physicochemical aspects. The motivation of the present work, therefore, deals with the non-covalent interaction between fullerenes and monoporphyrin in presence of AgNp. Binding and selectivity in binding between fullerenes and 1 is one of the goals of our present studies, other than to envisage various physicochemical aspects on host-guest chemistry of fullerene and porphyrin in presence of AgNp.
C60- and C70 are purchased from Sigma-Aldrich, USA and used without further purification. 1 is synthesized in our laboratory according to the method reported in literature [38] . UV-Vis spectroscopic grade toluene (Merck, Germany) is used as solvent to favor the intermolecular interaction between fullerene and 1, as well as to provide good solubility and photostability 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. Steady state emission spectra are recorded with a Hitachi F-4500 model fluorescence spectrophotometer. DLS measurements have been done with Nano S Malvern instrument employing a 4 mW He-Ne laser (λ = 413 nm) equipped with a thermostated sample chamber. All the scattered photons are collected at 173° scattering angle. SEM measurements are done in a S-530 model of Hitachi, Japan instrument having IB-2 ion coater with gold coating facility.
A. UV-Vis investigations
The ground state absorption spectrum of 1 (4.50 ´ 10-6 mol×dm-3, [Fig-2a] ) in toluene recorded against the solvent as reference displays one broad Soret absorption band (lmax = 417 nm) corresponding to the transition to the second excited singlet state S2. As for the Q absorption bands, 1 shows two major absorption bands at 510 and 588 nm along with two minor absorption bands at 543 and 655 nm [Fig-2a] . Q absorption bands in metalloporphyrin correspond to the vibronic sequence of the transition to the lowest excited singlet state S1. [Fig-2] shows the electronic absorption spectrum of 0.02 ml AgNp solution in 4 ml toluene measured against toluene. The distinctive colour of AgNp is due to a phenomenon known as plasmon absorbance. Incident light creates oscillations in conduction electrons on the surface of the nanoparticles and electromagnetic radiation is absorbed. The spectrum of the clear yellow colloidal silver shows one broad absorption band near the region of 451 nm due to its surface plasmon band resonance character [39] When 0.02 ml solution of AgNp is added to the solution of 1 (4.50 ´ 10-6 mol×dm-3) and the electronic absorption spectrum of the mixture is recorded against the same concentration of AgNp in toluene, the intensity of the Soret absorption band is found to decrease from absorbance value of 1.395 to 1.190 [Fig-2c] . 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 (2.10 ´ 10-5 mol×dm-3) and/ C70 solution (2.10 ´ 10-5 mol×dm-3) to a toluene solution of 1 (4.50 ´ 10-6 mol×dm-3, [Fig-3] decreases the absorbance value of 1 at its Soret absorption maximum [Fig-3] recorded against the pristine acceptor solution as reference; the extent of decrease is considerably larger in magnitude compared to AgNp in toluene. However, 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 ground state. The latter observation indicates that the interaction is not controlled 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 in comparison to C60. This spectroscopic observation suggests greater amount of interaction between C70 and 1. However, measurements of UV-Vis spectrum of the C60 (2.10 ´ 10-5 mol×dm-3) + 1 (4.50 ´ 10-6 mol×dm-3) and C70 (2.10 ´ 10-5 mol×dm-3) + 1 (4.50 ´ 10-6 mol×dm-3) mixtures in presence of AgNp recorded against the same concentration of fullerene in presence of AgNp, make new physical insight. It is observed that in presence of AgNp, the intensity of the Soret absorption peak of the C60 + 1 and C70 + 1 mixtures increase [Fig-3] , in comparison to the situation when complexation takes place between 1 and fullerenes in absence of AgNp. From the above observations we may infer that binding phenomenon between C60 and 1 takes place at less extent in presence of AgNp. Similar observation is observed in case of C70-1 complexation process. This new photophysical feature of the fulllerene-1 mixtures in presence of AgNp prompt us to measure the quantitative estimation of the binding phenomenon between fullerenes and 1 in absence and presence of AgNp employing steady state fluorescence spectroscopic tool.
B. Kinetic Studies
Absorption spectrophotometric investigations evoke a new physical insight on interaction between monoporphyrin, i.e., 1 with AgNp in absence and presence of fullerenes C60 and C70. It is observed that when AgNp solution in toluene is added to the solution of 1 in same solvent, peak intensity of 1 at its electronic transition, i.e., Soret and Q absorption maximum, found to decrease with increasing time [Fig-1S] . For example, the absorbance value at 417 nm (Soret peak) suffers considerable extent of decrease from 2.1720 (0 minute) to 0.5638 in 130 minutes. Plot of absorbance value against time fits excellently well according to the following equation:
Absorbance = (0.55343 ± 0.00785) + (1.7027 ± 0.02752) exp{-(Time)/ (14.5815 ± 0.4530)} (1)
From [Equ-1] , the rate constant (k) for above process is estimated to be 0.068 min-1. Plot of [Equ-1] for 1-AgNp system is demonstrated in [Fig-2S] . In case of C60-1 and C70-1 composite mixtures, when AgNp solution is added similar sort of spectral observations are noticed as depicted in [Fig-3S(a)] & [Fig-4a] , respectively. Variation of absorbance value with time is found to obey the following equations:
C60-1-AgNp system:
Absorbance = (0.92616 ± 0.00733) + (1.1773 ± 0.02958) exp{-(Time)/(11.8737 ± 0.5634)} (2)
C70-1-AgNp system:
Absorbance = (0.59060 ± 0.00742) + (1.4314 ± 0.02112) exp{-(Time)/(16.5900 ± 0.5900)} (3)
k for C60-1-AgNp and C70-1-AgNp systems are estimated to be 0.084 and 0.060 min-1, respectively; the exponential plots are demonstrated in [Fig-3S(b)] & [Fig-4b] , respectively.
C. Steady State Fluorescence Investigations
It is observed that the fluorescence of 1 upon excitation at Soret absorption band diminishes gradually during titration with C60 and C70 solutions in toluene [Fig-5a] & [Fig-5b] , respectively). This indicates that there is a relaxation pathway from the excited singlet state of the porphyrin to that of the fullerene in toluene. It is already reported that charge separation can also occur from the excited singlet state of the porphyrin to the C60 in toluene medium [40] Competing between the energy and electron transfer processes is a universal phenomenon in donor molecule-fullerene complexes [41] solvent dependent photo physical behavior is a typical phenomenon of the most fullerene-porphyrin dyads studied to date [42] 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 fullerene-porphyrin+ ion-pair states (by electron transfer) [43] 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 [3,4:1,2] tetraphenylporphyrin supramolecule [44] 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:
1 + Fullerene Fullerene-1 (4)
The fluorescence intensity of the solution decreases upon addition of fullerenes C60 and C70. Using the relation of binding constant (K) we obtain,
K = (Fullerene-1) / (1)(Fullerene) (5)
Using the mass conservation law, we may write
(1)0 = (1) + (Fullerene-1) (6)
where (1)0, (1) and (Fullerene-1) are the initial concentrations of 1, 1 in presence of fullerene, and Fullerene-1 complex, respectively. [Equ-6] can be rearranged as
(1)0/(1) = 1 + (Fullerene-1)/(1) (7)
Using the value of K in place of (Fullerene-1)/(1) from [Equ-5] , we can write the [Equ-7] as follows
(1)0/(1) = 1 + K (Fullerene (8)
Considering the fluorescence intensities are proportional to the concentrations, [Equ-8] is expressed as
F0/F = 1 + K (Fullerene) (9)
where, F0 is the fluorescence intensity of 1 in the absence of fullerene and F is the fluorescence intensity of 1 in the presence of quencher (i.e., fullerene). In our present investigations, steady-state fluorescence quenching studies afforded excellent linear plot for both the C60-1 and C70-1 systems as demonstrated in [Fig-6a] & [Fig-6b] , respectively, which is explained by fluorescence of 1 is being quenched only by static mechanism, as opposed to diffusional quenching process. [Equ-9] , therefore, represents a stern-volmer (SV) type plot for investigated supramolecules. K values of the C60-1 and C70-1 systems are given in Table 1. The binding constant is determined by the [Equ-9] for the C60-1 and C70-1 systems are estimated to be 1065 and 12585 dm3mol-1, respectively. It is interesting to note that the increase in magnitude of binding constant led to increase of the fluorescence quenching efficiency. The steady state fluorescence emission measurements of C60-1 and C70-1 systems in presence of AgNp, however, evoKe new physicochemical insight regarding perturbation in binding between fullerenes and 1 in solution.
It is observed that the K value of C60-1 and C70-1 systems in presence of AgNp are estimated to be 890 and 11975 dm3×mol-1, respectively, which are found to be lower in absence of AgNp [Table-1] . The steady state fluorescence quenching spectral variation of 1 by C60 and C70 in presence of AgNp are given as [Fig-5c] & [Fig-5d] , respectively; corresponding SV type plots are provided in [Fig-6c] & [Fig-6d] , respectively.
D. Dynamic Light Scattering Experiment
Dynamic light scattering (DLS) is the most versatile and useful set of techniques for measuring in situ on the sizes, size distributions, and (in some cases) the shapes of nanoparticles in liquids [45-48] In our present investigations, DLS measurements clearly demonstrate that in presence of C60 and C70, the size of the nanoparticles do not change much. [Fig-7a] [Fig-7c] show the variation of scattering intensity vs. size of the nanoparticles for various systems. It shows that size of the nanoparticles remains within the limit of ~4.8 nm [Fig-7a] when fullerenes are not present in the mixture of 1 + AgNp. In presence of C60 [Fig-7b] and C70 [Fig-7c] , the particle size of AgNp becomes ~4.35 nm. The above findings provide strong support in favor of the evaluation of K value of fullerene-1 systems in absence of AgNp, i.e., slight reduction in the K value of fullerene-1 systems takes place in presence of AgNp. It also envisages that AgNp partially covers the surface area of porphyrin, which renders the fullerene molecule to undergo interaction with the whole surface area of 1 leading to the reduction in the binding constant value for fullerene-1 systems.
E. Conductance Measurements
Conductance measurements of the 1 in presence of only AgNp, only C60, only C70, C60 + AgNp and C70 + AgNp mixtures reveal very important new physicochemical aspect. It is observed that in presence of AgNp, the conductance value of 1 in toluene gets very little change with increasing time. After ~2 hours of measurement, the conductance value of 1 + AgNp mixture in toluene is determined to be 1.6 mS compared to the initial value of 0.2 mS in absence of AgNp [Table-1S] ; from this observation, it may be inferred that AgNp molecule generates some degree of photocurrent in 1 + AgNp mixture due to electrostatic deposition of silver nanoparticles over the surface of 1. The most promising aspect of the present investigations is that in presence of C60 and C70, the composite solution containing C60 + 1 + AgNp and C70 + 1 + AgNp mixtures do not produce any great degree of photocurrent. For example, after ~2 hours of measurement, the conductance value of C60 + 1 + AgNp mixture in toluene [Table-2S] is determined to be 0.4 mS compared to the initial value of 0.2 mS. This is very much comparable with the C60 + 1 system in absence of AgNp [Table-3S] ; the conductance value gets little shifted from 0.1 mS to 0.3 mS. In case of C70, however, slight increase in the value of conductance is observed both in absence [Table-4S] and in presence of AgNp [Table-5S] . This is quite consistent with the fact that C70 forms much stronger complex with 1 either AgNp is absent or present in the solution.
F. SEM Measurements
SEM measurements of 1 in presence of AgNp reveals uneven sized particles [Fig-8a] . The SEM image of (C60 + 1 + AgNp) composite mixture [Fig-8b] obtained from drop-casted films of the clusters {(1):(C60)=1:1} on a stab made of copper reveals the formation of rod-like clusters with a well-controlled size (4-5 mm in the long axis). In sharp contrast, the SEM image of the clusters of (C70 + 1 + AgNp) composite mixture [Fig-8c] shows ill-defined sizes and shapes. In particular, the SEM image of clusters consisting of (C60 + 1 + AgNp) reveals both rods of different sizes and small random-shaped structures. In addition, variation of the (1):(C60) ratio is found to affect the cluster size and shape [Fig-8b] . 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. These results clearly demonstrate that the shape and the size of the surface holes on the 1-C(60 or 70)-AgNp structure play an important role in controlling the formation of molecular clusters with either C60 or C70 molecules.
The main findings of our attempt to develop a new and improved methodology of characterization of fullerene-porphyrin non-covalent assembly in presence of silver nanoparticles are surmised as follows:
• The absorbance value of the Soret absorption peak gets reduced when fullerene-1 non-covalent complexation process is monitored in presence of AgNp in solution.
• It is observed that the quenching efficiencies of both C60 and C70 have been reduced in presence of AgNp in toluene.
• Estimation of binding constants applying steady state fluorescence spectroscopic techniques reveal that 1 could not serve as an efficient complexing agent towards C60 and C70 Âin presence of AgNp; however, 1 serves as a better discriminator molecule towards C70 in presence of AgNp.
• Conductance measurements evoke that AgNp particle reduces the magnitude of conductance value for the fullerene-1 systems in presence of AgNp.
• DLS studies establish the fact that particle size of AgNp becomes shorter when complexation takes place between fullerenes and 1, and it is the primary reason behind getting lesser magnitude of K value for fullerene-1 systems.
• SEM measurements, for the first time, establish the formation of C60-porphyrin-AgNp nanorod and prove that the shape and the size of the surface holes on the fullerene-porphyrin-AgNp structure play an important role in controlling the formation of molecular clusters with fullerenes.
• The results emanating from present investigations would be of potential interest in studying the interaction between fullerenes and diporphyrin in presence of gold and silver nanoparticles in near future.
RM thanks Department of Atomic Energy, Board of Research in Nuclear Science (BRNS), Bhabha Atomic Research Centre, Mumbai, India for providing Junior Research Fellowship to him. Financial assistance through the project of Ref. No. 2010/20/37P/2/BRNS is also gratefully acknowledged.
UV-Vis absorption spectral variation in the kinetics experiment of 1 in presence of AgNp, plot of absorbance vs. time for 1-AgNp kinetics experiment, UV-Vis absorption spectral variation in the kinetics experiment of 1 in presence of C60 + AgNp and plot of absorbance vs. time for C60-1-AgNp system, conductance value of 1 + AgNp mixture in toluene, conductance value of C60 + 1 + AgNp mixture in toluene, conductance value of C60 + 1 system in absence of AgNp, conductance value of C70 + 1 + AgNp mixture in toluene and conductance value of C70 + 1 system in absence of AgNp are given as [Fig-1S] - [Fig-3S] and [Tables-1S] - [Tables-5S] , respectively. [Fig-1S] - [Fig-3S] and [Tables-1S] - [Tables-5S] are provided as supporting information. Supporting information associated with this paper are available free of charge via the internet.
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 | Fig. 1 |
 | Fig. 2 |
 | Fig. 3 |
 | Fig. 4a |
 | Fig. 4b |
 | Fig. 5a |
 | Fig. 5b |
 | Fig. 5c |
 | Fig. 5d |
 | Fig. 6a |
 | Fig. 6b |
 | Fig. 6c |
 | Fig. 6d |
 | Fig. 7a |
 | Fig. 7b |
 | Fig. 7c |
 | Fig. 8a |
 | Fig. 8b |
 | Fig. 8c |
 | Fig. 1S- UV-Vis absorption spectral variation of the 1 (4.75 ´ 10-6 mol×dm-3)-AgNp system recorded in toluene with time; the inset of Fig. 1S demonstrates the clear view of such spectral variation at two different wavelengths, namely, 417 nm and 453 nm. |
 | Fig. 2S- Plot of the variation of the absorbance vs. time for 1-AgNp system recorded in toluene. |
.jpg) | Fig. 3S(a)- demonstrates the clear view of such spectral variation at two different wavelengths, namely, 417 nm and 453 nm. |
.jpg) | Fig. 3S(b)- Plot of the variation of the absorbance vs. time for C60-1-AgNp system. |
 | Table 1- Binding constants (K) for the non-covalent complexes of 1 with C60 and C70 in absence and presence of AgNp recorded in toluene Temp. 298K. |
 | Table 1S- Conductance value of 1 + AgNp mixture in toluene. Temp. 298K. |
 | Table 2S- Conductance value of C60 + 1 + AgNp mixture in toluene. Temp. 298K. |
 | Table 3S- Conductance value of C60 + 1 mixture in toluene. Temp. 298K. |
 | Table 4S- Conductance value of C70 + 1 mixture in toluene. Temp. 298K. |
 | Table 5S- Conductance value of C70 + 1 + AgNp mixture in toluene. Temp. 298K. |