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

Article abstract of DOI:10.1016/j.saa.2014.03.014

The present article reports the role of gold nanoparticles, i.e., AuNp (having diameter ～2-4 nm), in non-covalent interaction between fullerenes (C₆₀ and C₇₀) and a monoporphyrin (1) in toluene. Both UV-vis and fluorescence measureme

Full text of DOI:10.1016/j.saa.2014.03.014

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 61–69
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Photophysical insights on effect of gold nanoparticles over
fullerene–porphyrin interaction in solution
a,
Ratul Mitra a, Ajoy K. Bauri b, Shrabanti Banerjee c, , Sumanta Bhattacharya
⇑
⇑
a Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, India
b 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
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)
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.
1000
(a)
(b)
100
(c)
(d)
(e)
10
(f)
1
0
10
20
Time, ns
30
40
a r t i c l e i n f o
a b s t r a c t
Article history:
The present article reports the role of gold nanoparticles, i.e., AuNp (having diameter ꢁ2–4 nm), in non-
covalent 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
Received 22 October 2013
Received in revised form 6 February 2014
Accepted 8 March 2014
(Kav) for the C70–1 system (KC70–1(av) = 19,300 dm3 molꢂ1) in presence of AuNp, i.e., KC70–1–AuNp(av)
13,515 dm3 molꢂ1 although no such phenomenon is observed in case of C60–1 system, viz., KC60–1(av)
=
=
Available online 20 March 2014
1445 dm3 molꢂ1 and KC60–1–AuNp(av) = 1210 dm3 molꢂ1. 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.
Keywords:
Fullerenes C60 and C70
Monoporphyrin
Gold nanoparticle
Binding constant
DLS
SEM
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
application of nanoparticles in nanoelectronic devices, exploiting
the organoelectronic
p-orbital interactions, which are generally
Gold (Au) nanoparticles (AuNp) play important roles in differ-
ent branches of science, such as in nanoelectronics, nonlinear
optics, biological labeling and oxidation catalysis [1–5]. The
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
⇑
Corresponding authors. Fax: +91 3422530452.
E-mail addresses:
shrabanti_05@rediffmail.com (S. Banerjee), sum_9974@
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.

62
R. Mitra et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 61–69
AuNp and mixing of fullerene with TOABr-protected AuNp pro-
duces 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
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 spec-
trophotometric 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 interac-
tion in presence of AuNp.
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 nano-
scale 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 size-
dependent 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 self-
assembly phenomenon between fullerene and porphyrin in pres-
ence 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 continu-
ation of our earlier work [17], we have presented the photophysical
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.

R. Mitra et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 61–69
63
respectively; Dabs is the corrected absorbance of the fullerene–1
mixture recorded at the wavelength of measurement against the
measurements have been done with Malvern Zeta Seizer instru-
ment 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 Har-
tree–Fock method using Slater type of orbitals (STO) in 3-21G basis
set are performed with the help of SPARTAN’06 Windows version
software.
solvent as reference. The quantity
D
e
(=ecomplex
ꢂ
eFullerene e1) means
ꢂ
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 approxima-
tion 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
Results and discussion
UV–vis investigations
The ground state absorption spectrum of
1
(1.0 ꢃ 10ꢂ5
mol dmꢂ3, Fig. 1S(i) in toluene recorded against the solvent as ref-
erence 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 metalloporphy-
rin correspond to the vibronic sequence of the transition to the low-
est 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 ꢃ 10ꢂ5 mol dmꢂ3) 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.
C
70–1 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
K, dm3 molꢂ1
UV–vis
Fluorescence
C
60–1
1330a
1330a
23,810a
14,155a
–
1560b
1090a
It is observed that addition of a C60 (0–9.5 ꢃ 10ꢂ5 mol dmꢂ3
,
C60–1–AuNP
C70–1
C70–1–AuNP
C60–ZnTPP
Fig. 2S) and C70 solution (0–3 ꢃ 10ꢂ5 mol dmꢂ3, Fig. 3S) to a toluene
solution of 1 (5.7 ꢃ 10ꢂ6 mol dmꢂ3) 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:
14,790b
12,875a
1870
C70–ZnTPP
–
9670
a
In present work.
Reported in literature [17].
b
Dabs ¼ Abs1 ꢂ AbsC60ðand=C70Þ ꢂ AbsC60ðand=C70Þꢂ1
ð1Þ
The terms Abs1, AbsC60(and/C70) and AbsC60(and/C70)–1 may be
ascribed as absorbance value of uncomplexed 1 with the same con-
centration 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 indi-
cates that the interaction is not dominated by charge transfer
(CT) transition. Another important feature of the UV–vis investiga-
tions 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].
1
1 + C60 + AuNp
1 + C60
1 + C70 + AuNp
1 + C60
ð½1ꢄ=DabsÞ ¼ ð1=
D
eÞ þ ð1=K
D
e½FullereneꢄÞ
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.

64
R. Mitra et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 61–69
D=abs ¼ Abs1 ꢂ AbsC60ꢂAuNpðor C70ꢂAuNpÞ ꢂ AbsC60ðandC70Þꢂ1ꢂAuNp
ð3Þ
may be attributed due to the presence of intermolecular interaction
between the two graphitic 6:6 planes of the C70 molecule with the
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 ful-
lerenes 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.
flat
tive interaction between C70 and the monoporphyrin is driven by
the presence of dispersive forces associated with interactions.
p-plane of the monoporphyrin units in 1. Primarily, the attrac-
p–p
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 regard-
ing 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 (
mol. However, the end-on pattern of C70 molecule produces
D
H0f ) value of ꢂ3.630 kcal/
D
Hf0
value of ꢂ3.210 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 pres-
ence of 6:5 ring-juncture bond [20,21]; In present case, the side-
on 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. There-
fore, the equatorial face of C70 is centered over the electropositive
part of the porphyrin plane, which may be viewed as an enhance-
ment in van der Waals interaction due to availability of greater sur-
face area and volume favouring strong p–p interactions. It should
be noted at this point that C60–1 system exhibits lower value of
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
D
H0f , viz., ꢂ2.313 kcal/mol compared to C70–1 system. Single pro-
jection 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.

R. Mitra et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 61–69
65
1 þ Fullerene ꢀ Fullerene ꢂ 1
ð4Þ
Steady state fluorescence investigations
The fluorescence intensity of the solution decreases upon addi-
tion of fullerenes C60 and C70. Using the relation of binding con-
stant (K) we obtain,
The photoinduced behavior of the complexes of the C60–1 and
70–1 complexes in presence of AuNp has been investigated by
C
steady-state emission measurements. It is observed that the fluo-
rescence 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 porphy-
rin 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 fuller-
ene–porphyrin dyads studied to date [23]. Photo physical studies
already prove that in conformationally flexible fullerene–porphyrin
K ¼ ½Fullerene ꢂ 1ꢄ=½1ꢄ½Fullereneꢄ
ð5Þ
And the mass conservation law (where [1]0 is the total concentra-
tion of 1)
½1ꢄ0 ¼ ½1ꢄ þ ½Fullerene ꢂ 1ꢄ
ð6Þ
We obtain the fraction of the uncomplexed fluorophore (herein 1):
ð½1ꢄ=½1ꢄ0Þ ¼ f1=ð1 þ K½Fullereneꢄg
ð7Þ
dyad,
p-stacking interactions facilitate the through space interac-
Considering that the fluorescence intensities are proportional to
the concentrations (as the solution is dilute), this relationship can
be rewritten as
tions between these two chromophores which is demonstrated by
quenching of 1porphyrinꢅ fluorescence and formation of fullerene
excited states (by energy transfer) or generation of fullereneꢂpor-
phyrin+ 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 deactivat-
ing the photo excited chromophore 1porphyrinꢅ of fullerene–por-
phyrin dyad. Similar sort of rationale is already proved by Yin
ðF0=FÞ ¼ 1 þ K½Fullereneꢄ
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 fluo-
rescence 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 demon-
strated in inset of Fig. 3(a). K value of the C60–1–AuNp system is tab-
ulated in Table 1 and corroborates fairly well with that reported
from UV–vis investigations. However, in case of C70–1–AuNp sys-
tem, 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., 1ꢅ is not sup-
posed 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 supramole-
cules. We anticipate that in presence of C70, the size of the nanopar-
ticle becomes much higher such that diffusion controlled
mechanism plays key role behind the photo-excited decay of the
et al. for their particular cis-2,5-dipyridylpyrrolidino[3,4:1,2] C60
-
zinctetraphenylporphyrin supramolecule [25]. In the present inves-
tigation, 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 wave-
length 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
C
60. As ground state complex formation between 1 and fullerenes
is evidenced from the steady state fluorescence studies, let us con-
sider the formation of a non-fluorescent 1:1 complex according to
the equilibrium:
(a)
800
600
400
200
0
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
18
(a)
700
600
500
400
300
200
100
0
15
12
9
1.0x10-5 2.0x10-5 3.0x10-5 4.0x10-5 5.0x10-5
0.0
5.0x103
1.0x104
1.5x104
2.0x104
2.5x104
-3
[C60], mol.dm
1/[C60] dm3.mol-1
520
570
620
670
720
770
820
870
520
620
720
820
Emission Wavelength, nm.
Emission Wavelengthg, nm.
6
5
4
3
2
1
(b)
700
600
500
400
300
200
100
0
3.45x104
3.30x104
3.15x104
3.00x104
2.85x104
2.70x104
2.55x104
(b)
750
550
350
150
-50
0
2.0x104 4.0x104 6.0x104 8.0x104 1.0x105
1/[C70] dm3.mol-1
0.0
2.50x10-5
5.00x10-5
[C70], mol.dm
7.50x10-5
-3
1.00x10-4
520
570
620
670
720
770
820
525
625
725
825
Emission Wavelength, nm.
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.
Fig. 4. Steady state fluorescence titration experiment of (a) C60–ZnTPP and (b)
70–ZnTPP systems recorded in toluene; inset of this figure depicts BH plot for the
evaluation of K for C60–ZnTPP and C70–ZnTPP systems.
C

66
R. Mitra et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 61–69
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:
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
fðF0=F ꢂ 1Þ=½Fullereneꢄg ¼ ðKSV þ KÞ þ ðKSVKÞ½Fullereneꢄ
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.
C70–1 complex is found to be 1.52 times higher than that of
C70–ZnTPP complex in toluene.
DLS experiment
To study and compare the intermolecular interaction of the ful-
lerene complexes of 1 with some model compound like zinc tetra-
phenylporphyrin (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-
DLS is the most versatile and useful set of technique for measur-
ing in situ on the sizes, size distributions, and (in some cases) the
shapes of nanoparticles in liquids [26,27]. In our present investiga-
tions, 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
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
C
60 (5.0 ꢃ 10ꢂ5 mol dmꢂ3) and C70 (5.0 ꢃ 10ꢂ5 mol dmꢂ3), the par-
ticle size of AuNp becomes ꢁ5.5 nm and ꢁ105.0 nm, respectively;
the concentration of 1 is kept fixed at 5.0 ꢃ 10ꢂ5 mol dmꢂ3. It may
be anticipated that AuNp covers the surface area of monoporphyrin
1 which reduces the possibility of fullerenes to come close to inter-
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 complex-
ation 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 sur-
face 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.
act with the flat
p-region of 1. Thus, we may anticipate lesser
extent of interaction between fullerenes and 1 in presence
p–p
of AuNp. This is the actual reason behind the fact that the magni-
tude 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.
SEM measurements
Time-resolved fluorescence study
SEM measurements of 1 (3.8 ꢃ 10ꢂ6 mol dmꢂ3) 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 ꢃ 10ꢂ5
mol dmꢂ3) + 1 (3.8 ꢃ 10ꢂ6 mol dmꢂ3) + 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 fuller-
ene–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 (s
singlet) of 1 is estimated to be 2.388 ns. In presence of

68
R. Mitra et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 61–69
porphyrin in two different cases, viz., in absence and presence of
1000
100
10
AuNp. The key findings are summarized below:
(a)
(1) The average value of K of the C70–1 system suffers consider-
able 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 pho-
tosensitizer 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.
(b)
(c)
(d)
(e)
(f)
(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-aggrega-
tion takes place.
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.
(4) Theoretical calculations well reproduce the single projection
geometric structures of the fullerene–1 complexes and
interpret the stability difference between C60 and C70 com-
plexes of 1 in vacuo.
(5) Time-resolved fluorescence study evokes that energy trans-
fer is the only path for the deactivation of the excited singlet
state of 1 in presence of fullerenes in toluene.
Table 2
Fluorescence lifetime of the excited singlet state (
absence and presence of AuNp, C60
mixtures. Temp. 298 K.
s
singlet) of 1 in
, C70 and fullerene–AuNp
System
s
singlet, ns
(6) Finally, the results emanating from present investigations
contemplate that AuNp acts as an effective quencher mole-
cule in modulating the selectivity of binding ratio between
1
2.388
2.355
2.195
2.217
2.149
2.189
1–AuNp
C
C
C
C
60–1
60–1–AuNp
70–1
C60–1 and C70–1 complexes in solution.
70–1–AuNp
Acknowledgements
AuNp, the s
singlet 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 system-
atic decrease is observed according to following trend:
S. Banerjee acknowledges UGC, New Delhi for providing finan-
cial assistance to her through Major Research Project of Ref. No.
F. No. 41-307/2012 (SR). Financial assistance through the project
of Ref. No. 2010/20/37P/2/BRNS is also gratefully acknowledged.
We also wish to record our gratitude to Prof. S.C. Bhattacharya,
Jadavpur University for his valuable guidance in DLS measure-
ments and Dr. Srikanta Chakraborty, U.S.I.C., Burdwan University
for his helpful co-operations during SEM measurements.
ssingletðC60 ꢂ 1 ꢂ AuNpÞ > ssingletðC60 ꢂ 1Þ > ssingletðC70 ꢂ 1 ꢂ AuNpÞ
>
ssingletðC70 ꢂ 1Þ:
Fluorescence decay profiles of 1, 1–AuNp, C60–1–AuNp, C60–1,
70–1–AuNp and C70–1 systems are shown in Fig. 7(a)–(f), respec-
C
tively. 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., 1ꢅ is found to be more faster in case of C70 when AuNp are
present in such systems while 1ꢅ decays at a slower rate when
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.03.014.
C
60 is added in presence of AuNp. The above experimental findings
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