Determination of binding strength for the supramolecular complexation of a designed bisporphyrin with C60, C70 and their derivatives employing absorption spectrophotometric, fluorescence and quantum chemical calculations

Article abstract of DOI:10.1016/j.saa.2011.05.099

The present paper reports the synthesis of a designed bisporphyrin (1), and its supramolecular complexes with C₆₀, C₇₀ and their derivatives, namely, tert-butyl-(1,2-methanofullerene)-61-carboxylate (2) and [6,6]-phenyl C₇₀

Full text of DOI:10.1016/j.saa.2011.05.099

Spectrochimica Acta Part A 79 (2011) 1952–1958
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Determination of binding strength for the supramolecular complexation of a
designed bisporphyrin with C60, C70 and their derivatives employing absorption
spectrophotometric, fluorescence and quantum chemical calculations
Sibayan Mukherjeea, Ajay K. Baurib, Sumanta Bhattacharyaa,∗
a Department of Chemistry, The University of Burdwan, Golapbag, Burdwan, West Bengal-713 104, India
b Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The present paper reports the synthesis of a designed bisporphyrin (1), and its supramolecular complexes
with C60, C70 and their derivatives, namely, tert-butyl-(1,2-methanofullerene)-61-carboxylate (2) and
[6,6]-phenyl C70 butyric acid methyl ester (3) in toluene medium. C60, C70 and their derivatives undergo
ground state non-covalent interaction with 1 is evidenced from absorption spectrophotometric study
in which it is observed that the intensity of the Soret absorption band of 1 decreases considerably in
presence of C60, C70 and their derivatives. Steady state fluorescence studies reveal efficient quenching of
fluorescence of 1 in presence of fullerenes. The binding constant (K) values of the fullerene/1 complexes
follows the trend: 2/1 < C60/1 < 3/1 < C70/1. However, selectivity in K values of the bisporphyrin complexes
is found to be higher for fullerene derivatives in comparison to C60 and C70. Time resolved emission
studies establish relatively long-lived charge separated state for the C70/1 complex. Molecular mechanics
calculations at force field model in vacuo evoke the single projection geometric structures of various
fullerene/1 complexes and interpret their stability differences in terms of heat of formation values.
© 2011 Elsevier B.V. All rights reserved.
Received 22 April 2011
Received in revised form 25 May 2011
Accepted 30 May 2011
Keywords:
C60
C70 and their derivatives
Designed bisporphyrin
UV–Vis
Steady state and time resolved fluorescence
investigations
Binding constant
MMMF calculations
1. Introduction
A small reorganization energy is one of the key factors in decreas-
ing energy waste. In general, two-dimensional electron acceptors
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]. In the artificial mod-
els, the donor–acceptor link plays an essential role and is a potent
means to restrain the electron donor–acceptor couples in their
positions [3–5]. Towards constructing such systems, fullerenes
[6] and porphyrins [7] have been utilized as important con-
stituents. Fullerenes have proved to be a rich platform on which
to conduct charge-separation (CS) studies [8–26]. These three-
dimensional building blocks are now readily available and exhibit
three characteristics of great relevance for energy transfer reac-
tions for the following reasons: (1) fullerenes are extremely good
electron acceptors [27,28]; (2) fullerenes undergo multiple deriva-
tization processes and can form a number of isomers in which two
hexagon–hexagon edges have reacted to provide the attachment
point for other molecules to be tethered [29–31]; and (3) primar-
ily because of its rigid structure, fullerenes possess a remarkably
low reorganization energy upon energy transfer reactions [32–34].
have much larger reorganization energies. The small reorganiza-
tion energy of C60 renders it an extremely useful building block
for exploring the modulation of rates and yields of charge sep-
aration (CS)/charge recombination (CR), for which conventional
electron acceptors, such as quinones, have proved unsatisfactory.
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 [35,36]. A number of porphyrin receptors
have been developed [37–43] and the resulting fullerene/porphyrin
assemblies have brought forward many interesting photophysical
properties [44,45]. 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
[46]. The primary component of the fullerene/porphyrin interac-
tion is, therefore, driven by the dispersive forces associated with
␲–␲ interaction, which is augmented by weak electrostatic or
donor–acceptor stabilization. For efficient recognition of spheri-
cal guests like fullerenes, the recognition sites in the synthetic
receptor are pre-organized for favorable interaction [47]. How-
ever, the synthesis of the above kind of receptors is usually of
low efficiency or time consuming [48]. Only, very recently, Liu
et al. has prepared a supramolecular architectures by fullerene-
bridged bis(permethyl-␤-cyclodextrin)s with porphyrins [49]. We
∗
Corresponding author. Tel.: +91 3422533(913-917)x424; fax: +91 342 2530452.
E-mail address: sum 9974@rediffmail.com (S. Bhattacharya).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.05.099

S. Mukherjee et al. / Spectrochimica Acta Part A 79 (2011) 1952–1958
1953
anticipate that the introduction of suitable spacer (or group) to
rationally designed folding scaffolds would lead to gable type por-
phyrin receptors with selective recognition ability. As the design
of supramolecular ensembles with high binding constants (K)
always remains an important challenge in supramolecular chem-
istry, therefore, tailoring of porphyrins with judiciously chosen
functionalities plays an important role in controlling or modify-
ing the electronic structure. It should be mentioned at this point
that very recently, we have done the supramolecular interaction
between fullerenes and Zn2-diporphyrin having phenylene spacer
unit [50]. In the present work, we are very much interested to see
the effect of the absence of Zn metal in the bisporphyrin moiety
during complexation with C60 and C70. The present investiga-
tions also highlight, for the first time, supramolecular interaction
of a free base bisporphyrin molecule with derivatized fullerenes,
namely, tert-butyl-(1,2-methanofullerene)-61-carboxylate (2) and
[6,6]-phenyl C70 butyric acid methyl ester (3). We report the syn-
thesis and photo-processes of a designed bisporphyrin, 1 (see
Scheme 5), with fullerene C60, C70 and their derivatives in terms of
optical spectroscopy, steady state and time resolved fluorescence
along with the computational chemistry work done by molecular
mechanics calculations.
zylic), 1.39 (s, 18H tert-butyl), 0.85 (s, 2H, >NH), −2.86 (s, 2H, pyrrol
H).
3.1.2. Synthesis of 5-bromo [10,20-di
(1ꢀ-tert-butoxycarbonylamino-2ꢀ-phenylethyl)]-porphyrin
N-bromosuccinamide (26.7, 0.15 mM) is added to a 100 mL
CHCl3 solution containing pyridine (1.0 mL) and 5,15-di (1ꢀ-
tert-butoxycarbonylamino-2ꢀ-phenylethyl) porphyrin (112 mg,
0.15 mM) at 0 ◦C. The mixture is stirred over a period of 45 min
and quenched the reaction by the addition of 10 mL acetone.
After stirring for 5 min, the mixture was diluted with water and
extracted with CHCl3. The combined extract is washed with water
three times and dried over anhydrous Na2SO4. The crude product
is passed through short silica column. The reaction is depicted
(Scheme 2S).
3.1.3. 5-Bromo-[10,20-di
(1ꢀ-tert-butoxycarbonylamino-2ꢀ-phenylethyl])-porphyrinato
zinc (II)
A mixture of Zn (OAc)2 in methanol and solution of bromopor-
phyrin in CHCl3 is refluxed for 3 h. The reaction is allowed to cool at
room temperature, thoroughly washed with water and dried over
anhydrous Na2SO4. This extract is then filtered through silica a bed
to afford crude product. This reaction was depicted in Scheme 3S.
2. Experimental
C60, C70, 2 and 3 have been collected from Aldrich, USA and their
purity levels are of ∼99.8%. Toluene (spectroscopic grade, Merck)
is used as solvent, because it is sufficiently apolar to favor non-
covalent interaction between fullerene and porphyrin and, at the
same time, ensures good solubility and photo-stability of the sam-
ples. UV–Vis spectral measurements are performed on a Shimadzu
UV-2450 spectrophotometer using quartz cell with 1 cm optical
path length. Steady state fluorescence spectral measurements have
been recorded in a Hitachi F-4500 fluorescence spectrophotome-
ter. Fluorescence decay curves are measured with a HORIBA Jobin
Yvon Single Photon Counting set up employing Nanoled as exci-
tation source. Theoretical calculations are done using SPARTAN’06
Windows Version software.
3.1.4. Synthesis of 5 [4ꢀ,4ꢀ,5ꢀ,5ꢀ-tetramethyl-[1ꢀ,3ꢀ,2ꢀ]
dioxaborolan-2ꢀ-yl-10,20-di (1ꢀ-tert-butoxy carbonyl
amino-2ꢀ-phenylethyl)-porphyrinato zinc (II)]
In a 50 mL three neck round bottom flask is charged with
crude bromide (112 mg), 4,4ꢀ,5,5ꢀ-octamethyl-[2,2ꢀ] bi [1,3,2] diox-
aborolane (76.2 mg, 0.30 mM), trans-dichloro-bis (triphenylphos-
phine) palladium (II) (3.0 mg, 0.0043 mM), KOtBu (56.0 mg, 50 mM)
and 30 mL of 1,2-dichloroethane under argon gas. The mixture
is stirred at 85 ◦C for 3 h under argon atmosphere. The reaction
is quenched by the addition of 10 mL of saturated aqueous KCl
solution washed with water and dried over MgSO4. The solvent
is removed under reduced pressure to afford a crude product.
This crude product is subjected to purify by column chromatog-
raphy over silica eluting with dichloromethane to give porphyryl
boronate (yield 50%). ꢀmax (CHCl3) 418, 550, 550 and 583 nm; 1H
NMR (CD4OD, 200 MHz) 10.15 (s, meso-H). 9.85–9.92 (4H, ␤-H),
9.80 (2H, ␤-H), 9.41 (2H, ␤-H), 8.17 (2H methane), 7.62 (2H phenyl),
7.18 (4H phenyl), 6.96 (4H, phenyl), 4.31 (4H, benzylic), 1.89 (s, 12H,
methyl), 1.30 (s, 18H, tert-butyl), 0.85 (s, 2H, NH). This reaction is
depicted in Scheme 4S.
3. Results and discussions
3.1.1. Synthesis of (R) 5,15-di
(1ꢀ-tert-butoxycarbonylamino-2ꢀ-phenylethyl) porphyrin
This reaction is carried out to a solution of dipyrromethane
(292 mg, 2.0 mM and (R)) phenylalaninal (500 mg, 2.0 mM) in dry
dichloromethane (300 mL) in presence of catalytic amount of triflu-
oroacetic acid (0.1 mL) in argon atmosphere at room temperature
under dark with continuous magnetic stirring for 48 h. The reac-
tion product is treated with DDQ (740 mg, 3.3 mM) and stirred
for additional 3 h. The reaction is quenched by the addition of a
base triethyl amine. The reaction product is poured into water
and extracted with dichloromethane followed by usual worked
up. The reaction is depicted (Scheme 1S). The product is extracted
with dichloromethane to afford crude product, which is contam-
inated with large quantity of polymeric mugs. It is subjected
to purify by column chromatography over silica and basic alu-
mina with increase in the polarity of eluting solvent system. The
desired product is characterized by means UV–Vis, 1H NMR and
MALDI-TOF-MS, HR ESI-MS and assigned as (R) 5,15-di (1ꢀ-tert-
butoxycarbonylamino-2ꢀ-phenylethyl) porphyrin (yield ∼ 10%).
MALDI-TOF mass (m/z) 746.6 (found), 748.4 (calcd). ꢀmax
(CHCl3) 406, 504, 537, 576 and 630 nm; 1H NMR (CDCl3, 200 MHz)
10.20 (s, 2H, meso-H), 9.57 (4H, ␤-H), 9.38 (4H, ␤-H), 7.37 (2H,
phenyl), 6.80–7.00 (m, 8H, phenyl), 6.25 (2H, methane), 4.33 (ben-
3.1.5. Synthesis of 1,3-bis[10ꢀ, 20ꢀ-di(1ꢀꢀ-tert-
butoxycarbonylamino-2ꢀꢀ-phenylethyl)-porphyrinato zinc
(II)-5ꢀ-yl]- benzene by Suzuki coupling reaction
5-[4ꢀ,4ꢀ,5ꢀ,5ꢀ-Tetramethyl-[1ꢀ,3ꢀ,2ꢀ] dioxaborolan-2ꢀ-yl-10,20-di
(1ꢀ-tert-butoxy carbonyl amino-2ꢀ-phenylethyl)-porphyrinato zinc
(II)] (2.0 equiv.), Cs2CO3 (1.2 equiv.), Pd(PPh3)4 (II) (10% mM) are
dissolved in dry DMF (5.0 mL) and dry toluene (5.0 mL). After deoxy-
genating, the mixture is heated to 80 ◦C with stirring for 3 h under
argon atmosphere. After completion of the reaction, the reaction
product is quenched by the addition of water. This crude product is
purified over silica by column chromatographic technique eluting
with dichloromethane. The desired product is characterized by con-
ventional methods (yield ∼ 50%). The reaction scheme is depicted
(Scheme 5S).

1954
S. Mukherjee et al. / Spectrochimica Acta Part A 79 (2011) 1952–1958
Fig. 2. Fluorescence quenching of
1
(1.32 × 10−6 mol dm−3
)
)
in presence of:
and (b) (from
(a) C70 (from 1.32 × 10−5 mol dm−3 to 4.63 × 10−5 mol dm−3
3
Fig. 1. (a) UV–Vis absorption spectrum of only
1
(1.32 × 10−6 mol dm−3
)
1
1.02 × 10−5 mol dm−3 to 4.75 × 10−5 mol dm−3) recorded in toluene medium at
along with
1
(1.32 × 10−6 mol dm−3) + C60 (6.22 × 10−5 mol dm−3
)
and
(1.32 × 10−6 mol dm−3) + C70
(4.63 × 10−5 mol dm−3
)
recorded
in
toluene
298 K.
against the solvent as reference; and (b) UV–Vis absorption spectrum
of only
1
(1.32 × 10−6 mol dm−3
)
along with
1
(1.32 × 10−6 mol dm−3) + 2
(6.29 × 10−5 mol dm−3
)
and
1
(1.32 × 10−6 mol dm−3) + 3 (4.75 × 10−5 mol dm−3
)
recorded in toluene against the solvent as reference.
of ground state electronic interaction between fullerenes and 1
first comes from the absorption spectrophotometric experiment.
It is observed that gradual addition of a C60 and/C70 solutions to a
toluene solution of 1 decreases the absorbance of the Soret band
(Fig. 1(a)). However, no additional absorption peaks are observed
in the visible region. The former observation extends a good sup-
port 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 CT type transition. Similar sort of
absorption spectral characteristics is observed with the 2/1 and 3/1
mixtures in toluene medium (Fig. 1(b)). The interesting observation
of the UV–Vis experiment is the larger extent of decrease in the
absorbance value of 1 in presence of C70 and 3 takes place in com-
parison to C60 and 2, respectively. However, it should be mentioned
at this point that larger spectroscopic change do not always means
stronger value of binding constant (K) for the various fullerene/1
complexes.
3.1.6. Synthesis of the free base analogue of 1,3-bis[10ꢀ,20ꢀ-di(1ꢀꢀ-
tert-butoxycarbonylamino-2ꢀꢀ-phenylethyl)-porphyrinato zinc
(II)-5ꢀ-yl]-benzene (1)
The solution of the above tweezers in CHCl3 (purple) along with
20% aqueous HCl is taken into round bottom flask and it is stirred
using magnetic stirrer for 45 min (Scheme 6S). The colour of the
resultant solution appears green. The extract is washed with water
to remove the residual portion of the acid and it looks as purple
colour. It is washed with brine and dried over Na2SO4. The solvent
is removed under reduced pressure to afford a purple residue. It
passes through a glass column over neutral alumina eluting with
CHCl3 to obtain the free base tweezers. The product is confirmed
by 1H NMR spectra. 1H NMR (CDCl3, 200 MHz): 10.10 (s, 2H, meso-
H), 9.71(broad m, 4H, ␤-H), 9.52 (broad m, 4H, ␤-H), 9.39 (4H, ␤-
H), 9.30 (4H, ␤-H), 9.10 (dd,1H, Ar–H), 8.58 (dd,2H, Ar–H), 8.11
(brs,1H, Ar–H), 7.04 (8H, phenyl-H), 6.90 (8H, m, Phenyl-H), 6.20 (m,
4H, CH–CH2Ph), 4.35(m, 8H, CH–CH2Ph), 1.32 (s, 36H, tert-butyl),
0.85(4H, NH), −2.84 (s, 4H, NH of pyrrole ring).
3.3. Fluorescence investigations
Steady state: The photo induced behavior of the complexes
of C60, C70 and their derivatives, namely, 2 and 3, with the
bisporphyrin, i.e., 1, has been investigated by steady-state fluores-
cence measurements. It is observed that the fluorescence intensity
of 1 at 633 and 700 nm upon excitation at Soret-absorption band
(407 nm) diminishes gradually following the addition of increas-
ing concentration of C60 (Fig. 1S) and C70 (Fig. 2(a)) solution 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. Although 1 exhibits fluorescence quenching upon the
addition of fullerenes, the quenching efficiency is found to be
3.2. UV–Vis investigations
The ground state absorption spectrum of 1 (Fig. 1) displays one
broad Soret absorption band (ꢀmax = 407 nm) corresponding to the
transition to the second excited singlet state S2. As for the Q absorp-
tion bands, 1 shows four minor bands at 505 nm, 536 nm, 577 nm
and 631 nm (Fig. 1) corresponding to the vibronic sequence of the
transition to the lowest excited singlet state S1. Evidence in favor

S. Mukherjee et al. / Spectrochimica Acta Part A 79 (2011) 1952–1958
1955
much higher in case of C70 than that of C60. Similar sort of
a
7.5
6.0
4.5
3.0
1.5
0.0
quenching phenomenon is observed for 1 in presence of both
2/1 and 3/1 systems (Fig. 2S and Fig. 2(b), respectively). It is
already reported that charge separation can also occur from the
excited singlet state of the porphyrin to the C60 in toluene medium
[51]. Competing between the energy and electron transfer pro-
cesses is a universal phenomenon in donor molecule–fullerene
complexes [52]. Solvent dependent photo physical behavior is a
typical phenomenon of the most fullerene/porphyrin dyads stud-
ied to date [53]. Photo physical studies already prove that in
conformationally flexible fullerene/porphyrin dyads, ␲-stacking
interactions facilitate the through space interactions between
these two chromophores which is demonstrated by quenching
of 1*porphyrin fluorescence and formation of fullerene-excited
states (by energy transfer) or generation of porphyrin+/fullerene−
ion-pair states (by electron transfer) [34]. Generally, in non-polar
solvent, energy transfer dominates (over the electron transfer
process) the photo physical behavior in deactivating the photo
excited chromophore 1*porphyrin of fullerene/porphyrin dyad.
Similar sort of rationale has already provided by Yin et al.
2.0x104
4.0x104
6.0x104
8.0x104
0.0
1/[C70], dm3.mol-1
10
8
b
for their particular cis-20,50-dipyridinylpyrrolidino[30,40:1,2]C60
-
zinctetraphenylporphyrin supramolecule [54]. In the present
investigation, however, we have not observed any peak of
fullerenes and their derivatives near 710 nm regions which is
indicative of the triplet state generation of such species. In this
circumstance, the quenching phenomenon could not be ascribed
to photo-induced energy transfer from porphyrins to fullerenes in
the supramolecular complexes of C60, C70 and their derivatives with
1. In our present investigations, binding constant (K) values are
evaluated according to a modified BH equation (see Eq. (1)) [55]:
6
4
ꢀꢁ ꢂꢁ
ꢂꢃ
F0
1
A
1
KA
1
2
=
+
(1)
F0 − F
fullerene
In Eq. (1), F0 and F are the fluorescence intensities of 1 in absence
and presence of fullerenes, respectively; [fullerene] indicates the
molar concentration of fullerene; A is a constant associated with
the difference in the emission quantum yield of the complexed and
uncomplexed porphyrin. By plotting F0/(F0 − F) (i.e., relative fluo-
rescence intensity) against 1/[fullerene], K values are obtained for
various fullerene/1 complexes (see Table 1). Two typical BH fluo-
rescence plots of C70/1 and 3/1 systems are shown in Fig. 3(a) and
(b), respectively. BH fluorescence plot of C60/1 and 2/1 systems are
provided as Figs. 3S and 4S, respectively. The following correlations
are obtained with the present data:
0
4.0x104
8.0x104
1.2x105
0.0
1/[3], mol-1.dm3
Fig. 3. BH fluorescence plots for: (a) C70/1 and (b) 3/1 systems recorded in toluene
medium.
For 3/1 system:
F0
= (0.413 0.08) + (6.80 × 10−5 1.61 × 10−6
)
F0 − F
ꢁ
ꢂ
For C60/1 system:
1
×
;
R = 0.997
(5)
fullerene
F0
= (2.499 0.65) + (4.47 × 10−4 2.30 × 10−5
)
)
)
F0 − F
It is interesting to note that the increase in magnitude of binding
constant led to increase of the fluorescence quenching efficiency.
Although the details are not clear at this point, the well-defined
structure of 1 affording tight fixation of C70 in comparison to
C60 and other derivatized form of fullerenes, which ultimately
resulted correct host–guest orientation. This phenomenon is cer-
tainly indicative of ground state complexation between 1 and
fullerenes, which also gets substantial evidence from UV–Vis stud-
ies.
ꢁ
ꢂ
1
×
;
R = 0.993
(2)
(3)
(4)
fullerene
For C70/1 system:
F0
= (0.756 0.13) + (7.27 × 10−5 3.11 × 10−6
F0 − F
ꢁ
ꢂ
1
×
;
R = 0.992
fullerene
3.4. Time resolved fluorescence studies
For 2/1 system:
The time-resolved fluorescence spectral features of the
fullerene/1 track the steady state measurements. The fluorescence
decay-time profiles of the investigated complexes of 1 (monitored
at 407 nm using 462 nm LASER diode) show an enhanced decay rate
as compared to uncomplexed 1. In toluene, all of the investigated
species, viz., 1, C60/1, C70/1, 2/1 and 3/1 complexes, reveal mono-
F0
= (0.824 1.53) + (5.00 × 10−4 3.69 × 10−5
F0 − F
ꢁ
ꢂ
1
×
;
R = 0.989
fullerene

1956
S. Mukherjee et al. / Spectrochimica Acta Part A 79 (2011) 1952–1958
Table 1
Binding constants (K) and the selectivity ratio of various fullerene/1 systems along with fluorescence lifetimes (ꢁf), charge separation rate constant (ksinglet), charge separation
CS
quantum yield (ꢂCsiSnglet) and heat of formation (ꢃH0) values for the designed bisporphyrin 1 in absence and presence of C60 and C70. Temp. 298 K.
f
singlet
CS
108 kCsiSnglet (s−1
)
ꢂ
ꢄHf0 (kJ mol−1
)
a
b
System
K (dm3 mol−1
)
KC70/KC60
ꢁf (ns)
1
–
5595
–
0.2944
0.2898
–
0.54
–
–
C60/1
0.0155
−200.8787
1.86
3.68
C70/1
2/1
10,400
1650
0.2841
0.2916
1.23
0.32
0.0350
0.0093
−475.3059c
−181.5672
3/1
6080
0.2880
0.74
0.0213
−185.9011d
singlet
a
k
= (1/ꢁf)sample − (1/ꢁf)ref
.
CS
singlet
CS
b
c
ꢂ
= [(1/ꢁf)sample − (1/ꢁf)ref](1/ꢁf)sample
.
ꢄHf0 (kJ mol−1) of C70/1 system in end-on direction of C70 = −453.9161.
ꢄHf0 (kJ mol−1) of 3/1 system in end-on direction of 3 = −168.5475.
d
exponential decay. Decay profile of 1 in presence of fullerenes C60
and C70 as well as 2 and 3 are shown in Fig. 4(a) and (b), respec-
tively. However, we do not observe substantial quenching of the
fluorescence lifetime of 1 in presence of fullerenes. For example,
the life time of uncomplexed 1, viz., 0.2944 ns, becomes 0.2841 ns
in presence of C70. This indicates that static quenching mecha-
nism is operative for the investigated supramolecules. To validate
the static quenching phenomenon, we have performed explicit
time-resolved titration experiment on fullerene/1 complexation
process in toluene. Typical plots of relative variation in life time
of 1 in uncomplexed and complexed form against the concentra-
tion of fullerenes are shown in Fig. 5S. Very small slope value of the
straight line, as depicted in Fig. 5S, clearly points out the rationale
behind static quenching mechanism in our present investigations.
The interesting observation of the present investigations is that the
life time of the uncomplexed 1 gets perturbed following the addi-
tion of fullerenes in accordance with the K value of the respective
fullerene/1 complexes.
We have determined the rates of charge separation of the
excited singlet (kCsiSnglet) and quantum yields of charge separation
of the excited singlet (ꢂCsiSnglet) in an usual manner employed for
the intermolecular process taking place between fullerenes and 1,
and the corresponding data are provided in Table 1. An examina-
tion of Table 1 reveals the following: (i) the estimated values of
singlet
CS
singlet
are found to be higher for C70/1 system as com-
CS
k
and ꢂ
pared to the C60/1 system. This observation is consistent with the
close proximity of the bisporphyrin molecule 1 and fullerene entity
in the C70/1 system as obtained from the magnitude of K value of
such system; and (ii) in agreement with the steady state emission
results, time-resolved fluorescence experiment suggests that with
the increasing value of electron affinity of the acceptor (here C70),
singlet
CS
singlet
increase.
CS
k
and ꢂ
3.5. Binding constants
Binding constants (K) of various fullerene/1 complexes are
summarized in Table 1. It is observed that 1 undergoes appre-
ciable amount of complexation with both C60, C70 and their
derivatives. The average value of binding constant of C60/1 com-
plex (K = 5.595 × 103 dm3 mol−1) is found to be large compared
to those of the previously reported thiacalixarene bisporphyrin
(K = 2340 dm3 mol−1) [56], C60 complexes of zinc(II) resorcinarene
(K = 5010 dm3 mol−1) [57] and comparable with terphenyl por-
phyrin tetramer (K = 5800 dm3 mol−1) [58]. The binding of C60
Fig. 4. Time resolved fluorescence spectrum of: (a) 1 (1.32 × 10−6 mol dm−3) in
absence and presence of C60 (6.22 × 10−5 mol dm−3) and C70 (4.63 × 10−5 mol dm−3
)
in toluene medium; here, green line represents fit to the decay curve; black
line represents prompt; red colour represents uncomplexed 1; pink line rep-
resents C60/1 system and wine red colour represents C70/1 system; and (b) 1
(1.32 × 10−6 mol dm−3) in absence and presence of 2 (6.29 × 10−5 mol dm−3) and
3 (4.75 × 10−5 mol dm−3); here, green line represents fit to the decay curve; black
line represents prompt; blue line represents uncomplexed 1; purple line represents
2/1 system; wine red colour line represents 3/1 system. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version
of this article.)
with
1
is also found to exhibit much larger value to that
[59] and,
reported for JAWS porphyrin (K = 1950 dm3 mol−1
)
somewhat surprisingly, stronger than a number of other first
row transition metalloporphyrins (K = 815 dm3 mol−1) [59]. How-
ever, both C60 and C70 exhibit much lower value of
K for

S. Mukherjee et al. / Spectrochimica Acta Part A 79 (2011) 1952–1958
1957
their complexes with
1
compared to cyclic dimers of Zn-
this reason, effective overlap between their molecular orbital may
not take place. In contrast, much stronger association is predicted
computationally for C70/1 complex in which the aromatic moiety
of 1 molecule containing four N atoms appear to interact with the
graphitic equatorial belt of C70. During complexation with 1, the
orientation of C70 molecule towards the porphyrin is proved to be
side-on rather than end-on, confirming structural deductions based
on MMMF calculations (see Fig. 6S). The van der Waals attraction
is considered to be maximized in the side-on structure of the com-
plex since the flatter equatorial regions of C70 have more contact
with the porphyrin than the highly curved polar region. This can
also be viewed in terms of higher number of 6:6 bonds present in
the equatorial region of C70 compared to C60. In addition to this
dispersion force, the attraction must also be subject to the subtle
interplay of small effect from differences in solvation, and electro-
static interactions.
porphyrins (K = 1.1 × 105 dm3 mol−1) [47] and porphyrin hexamer
(K = 1.4 × 108 dm3 mol−1) [60]. However, the most interesting fea-
ture of the present investigation is that 1 binds C70 in preference
to C60 [37] and the average KC /KC selectively is estimated to
70
60
be ∼1.85 in toluene. Although, the average value of present selec-
tivity is found to be comparable with azacalix[m]arene[n]pyridine
(∼1.9), [61], it exhibits lower selectivity than that of cyclotrivera-
trylenophane (∼2.5) [62], calixarene bisporphyrin (∼4.3) [56] and
bridged calix[5]arene (∼10.2) [63]. The present selectivity is found
to be much lower than those of observed for cyclic bisporphyrins
(∼25–32) [47,64]. Practical applications on such high selectivity to
C70 is also observed in the preferential precipitation of C70 over
C60 with host molecule like p-halohomooxacalix[3]arene [65]. The
larger values of the K for C70 compared to C60 with porphyrins are
commonly observed in the work of Tashiro and Aida [66] or Boyd
and Reed [12]. One noticeable factor in our studies is the moderate
value of selectivity in binding constant between 3/1 and 2/1 sys-
tems. This is mainly because of the restricted motion of both C60 and
C70 moieties in their derivatized form due to the presence of bulky
substitutions in the periphery of such molecules which results
better docking of the moelcules. The preference in the fullerene
C70 complexation also indicates that subtle difference in solvation
energies between C60 and C70 is the key factor behind the variation
of affinities. It is well known that the stability of supramolecular
complexes depends upon attractive interactions between host and
guest, and on the solvation of the binding partners [67]. Since the
host and guest are separately solvated in solution, the desolvation
is energetically an uphill task and, hence, an association takes place
only when the energy gain between the host and guest interaction
exceeds this unfavorable energy. From the trends in the K values
of fullerene/1 complexes, we may infer that the extent of solvation
and desolvation of the binding partners might play an important
role in forming weak or strong supramolecular complexes. Haino
et al. [68] show that the stability of the complex is increased as
the solubility of fullerene in solvent decreases, since less energy
is required for the desolvation of fullerene which must necessar-
ily precedes its complexation with a host molecule. This would be
one of the reasons for the higher K values of the C70 complexes in
toluene, since the solubility of C60 is higher in toluene (4.1 mg/mL)
than that of C70 (2.0 mg/mL) [69].
4. Conclusions
From the foregoing discussions we can surmise the results of
the following investigations as follows:
(1) The present investigations report ground state non-covalent
interactions of a newly designed bisporphyrin, namely 1, with
fullerenes C60, C70 and derivatized fullerenes, viz., 2 and 3, in
toluene medium.
(2) Estimation of binding constants applying steady state fluo-
rescence spectroscopic techniques reveal that 1 may not be
effectively and selectively employed as molecular tweezerss for
both C70 and 3.
(3) Time resolved fluorescence experiment reveals fluorescence
decay of 1 in presence of all the form of fullerenes take place
through static quenching mechanism.
(4) Quantum chemical calculations at MMMF level of theory sug-
gest that C70 is encapsulated in side-on orientation within the
cavity of 1.
(5) The results emanating from present investigations would cer-
tainly enhance the probabilities to apply 1 for encapsulation of
nanotubes having small diameter in near future.
Acknowledgements
3.6. Computational studies
S.M. thanks UGC, New Delhi for providing financial assistance to
him through the project of Ref. No. F. No. 34-336/2008 (SR). Finan-
cial assistance from the same organization through the DSA project
in Chemistry, Burdwan University, is also gratefully acknowledged.
The authors thank the Editor of this journal and the learned review-
ers for making valuable comments.
To visualize the geometry and electronic structure of the
individual components as well as the fullerene/1 adducts, compu-
tational studies are performed at the molecular mechanics level
using the force field model (MMMF), in vacuo. For this reason,
the fullerene/1 systems are optimized on the Born–Oppenheimer
potential energy surface, and a global minimum for each of the
fullerene complexes are recognized (see Fig. 6S). In all of the struc-
tures reported here, two porphyrin units of 1 are arranged in such a
manner so that they can form cavity for the approaching fullerene
surface, facilitating a close face-to-face fullerene/porphyrin con-
tact. Heat of formation values (ꢄHf0) for the supramolecular
complexes of 1 with C60, C70, 2 and 3 are measured in vacuo. All
the systems are shown to exhibit negative ꢄHf0 value. The nega-
tive heat of formation values are resulted from the formation of the
complexes predominate over the other opposing factors (i.e., dis-
sociation of complexes). Theoretically estimated ꢄHf0 values (see
Table 1) also support the high K value observed in case of C70/1
complex. ꢄHf0 value is more negative in case of C70/1 complex
indicating that above complexation process is enthalpy favored.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.05.099.
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