Synthesis, electrochemical and complexation studies of Zn(II) aryloxyporphyrins with fullerene C60

Article abstract of DOI:10.1142/S1088424616500656

A new series of aryloxyporphyrins bearing benzyl and naphthyl substituents viz., ZnTBPP (1), ZnTNPP (2) and ZnONPP (3) has been synthesized and characterized by UV-vis, fluorescence and ¹H NMR spectroscopic techniques and mass spectrometry. 1-3

Full text of DOI:10.1142/S1088424616500656

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2016; 20: 744–751
DOI: 10.1142/S1088424616500656
Published at http://www.worldscinet.com/jpp/
Synthesis, electrochemical and complexation studies
of Zn(II) aryloxyporphyrins with fullerene C60
#
#
Tawseef Ahmad Dar , Mandeep K. Chahal , R. Ashwin Kumar
and Muniappan Sankar*◊
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India
Received 14 February 2016
Accepted 17 March 2016
ABSTRACT: A new series of aryloxyporphyrins bearing benzyl and naphthyl substituents viz., ZnTBPP
(
1), ZnTNPP (2) and ZnONPP (3) has been synthesized and characterized by UV-vis, fluorescence and
1
H NMR spectroscopic techniques and mass spectrometry. 1–3 have been utilized as donors to interact
with fullerene (C ) acceptor to form 1:1 complex in toluene at 298 K. The subsequent investigation
6
0
into quenching measurements with concomitant increase in fullerene concentration revealed effective
3
-1
quenching constants. The calculated association constants were in the order of 10 M . However,
ZnTNPP (2) exhibited higher binding constant as compared to other analogs due to effective p–p
interactions. ZnONPP (3) exhibited 3.4 times lower association constant than ZnTNPP (2) due to steric
hindrance offered by meso-(3,5-dinapthyloxyphenyl) groups. The geometric and electronic structure
of Zn(II) porphyrin-fullerene dyad was probed by DFT calculations which suggested the possibility of
charge transfer from meso-aryloxyporphyrin core to fullerene C .
6
0
KEYWORDS: meso-aryloxyporphyrins, supramolecular dyad, porphyrin-fullerene host–guest
complexes.
high electronic excitation energies generally exceeding
INTRODUCTION
2
.0 eV which favor energy releasing electron transfer
Porphyrins and their metal derivatives are of
considerable importance owing to their applications in
various fields such as self-assembly, sensors, non-linear
optics, light emitting diodes and dye sensitized solar cells
to name a few. Non-covalent interactions like hydrogen
bonding, labile metal–ligand bonds, ion–ion, dipole–
dipole, ion–dipole and p–p interactions play crucial role
in developing various inter-molecular building blocks
which can be utilized for the construction of advanced
functional materials [1, 2]. One aim of developing such
assembled materials is to have artificial systems which
can trap and process solar energy convincingly [3]. Both
porphyrins and fullerenes are electrochemically active,
purple chromophores [4]. Porphyrins have typically
and thereby pave a way for conversion of light energy
into chemical/electrical energy [5]. Good and efficient
electron acceptors are hugely desired and in this regard
fullerenes are assuring species. The birth of fullerene
in the early twentieth century brought to limelight a
versatile acceptor [6]. Curved p-surface on fullerene
makes it a naturally tailored moiety to be exploited for
architecturing supramolecular assemblies. Fullerene
has three degenerate LUMOs which are low on energy
scale making it a strong electron acceptor in general
and for solar stimulated processes in particular [6a, 6b].
Among the earliest known conjugates of fullerene
were its cocrystallites with ferrocene, S and SbPh [7].
Porphyrins are easily oxidizable species and give
intense spectra in the visible region [8]. Also porphyrins
are tetragonal and fullerenes are spherical which makes
their spontaneous attraction all the more feasible [8a].
The presence of porphyrin moiety in various biological
processes such as photosynthesis led researchers to
8
3
◊
SPP full member in good standing
*
Correspondence to: Muniappan Sankar, email: sankafcy@iitr.
ac.in, tel: +91 1332-284753, fax: +91 1332-273560
#
The authors contributed equally
Copyright © 2016 World Scientific Publishing Company

SYNTHESIS, ELECTROCHEMICAL AND COMPLEXATION STUDIES OF ZN(II) ARYLOXYPORPHYRINS WITH FULLERENE C60
745
1
exploit the redox properties of these macrocycles and
their interaction with light [9]. Intricately covalently
conceived porphyrin-fullerene dyads with auxiliary light
harvesting electron donor/withdrawing substituents
both chiral and achiral, linear or bulky, acyclic or ring
shaped have variable light harvesting capabilities and
possess the ability to mimic photosynthetic systems
H NMR spectroscopic techniques, mass spectrometry
and cyclic voltammetric studies.
The synthetic methods for the preparation of 1–3
are described in the supporting information. The NMR
spectra and MALDI spectra of synthesised porphyrins
are given in Figs S1–S11 in the Supporting information.
[
10]. A significantly promising candidate in this field is a
Crystal structure of ZnTBPP (1)
porphyrin-fullerene dyad in which a fullerene moiety is
adjusted as a guest into the porphyrin cavity because of
its exceptional photo-acceptor properties. The insertion
of fullerene into the porphyrin core can be utilized in the
design of new encouraging photo-active systems [11].
The capability of the fullerene molecule to complex with
bothfreebaseaswellasmetalloporphyrinsonlyenhances
its candidature as an able solar energy harvesting device
X-ray quality single crystals of ZnTBPP (1) were
obtained by direct solvent diffusion of hexane into the
saturated toluene of solution 1 containing a few drops
of ethyl acetate. The crystallographic data of ethyl
acetate coordinated ZnTBPP (1-EtOAc) is listed in
Table S1 in the Supporting information. The Zn(II) ion
is deviated 0.312 Å from porphyrin mean plane due to
the coordination of keto group of ethyl acetate to the
Zn(II) center. ORTEP views (top and side) of 1-EtOAc
are shown in Fig. 1. 1-EtOAc has shown almost planar
confirmation (also observed in DFT calculations) as
evidenced from the displacement of 24 core atoms
from the mean plane, D24 = 0.088 Å and displacement
[
12, 13]. Upon irradiation, the formation of radical
•
+
•-
ion pairs (i.e. porphyrin -fullerene ) in high yields
exhibit longer lifetime [14]. The ability of fullerenes
to demonstrate small reorganization energy following
an electron transfer reaction can be further explored in
this regard [15, 16]. The close association of fullerene
and porphyrin was first recognized in the solid-state
assembly of a covalent fullerene-porphyrin conjugate
of b-carbon from mean plane, DC = 0.162 Å. The four
b
meso-phenyl rings are oriented perpendicularly to the
porphyrin plane and three phenyl rings of benzyl groups
are oriented perpendicularly to porphyrin plane while
the fourth one lies within the porphyrin macrocyclic
plane. The benzyloxy side arms stay apart from the inner
porphyrin core.
[
17]. In naturally occuring porphyrin-fullerene co-
crystallites, the contact distance has been found to be
pretty small (3.0–3.5 Å) [17a]. Meso, meso-linked
porphyrin arrays nurture a strong potential for increasing
quantum yield as well as efficiency because their length
can very easily be extended by oligomerization of
porphyrin monomers [18, 19]. Electron transfer between
Absorption spectral studies
an octaethylporphyrin and a fullerene (C60 or C ) was
The ground state absorption spectra of ZnTBPP (1),
ZnTNPP (2) and ZnONPP (3) display (a) a very intense
Soret band (lmax = 425 nm) corresponding to the transition
70
successfully reported by Ito et al. [20]. Similarly C70
acts as an electron acceptor in photoinduced processes
and also has the tendency to act as mediator for
electron transport by diffusing across semipermeable
membranes [21]. Gross et al. reported a non-covalent
to the second excited state S , and (b) single Q-band
2
of lower intensity at 552, 552 and 550 nm respectively
corresponding to the vibronic sequence of the transitions
supramolecular assembly between
porphyrin conjugate and a methanofullerene derivative
22]. D’Souza et al. recently reported a triphenylamine-
a
crown-ether
to the lowest excited singlet state S (Table 1). Figure S12
in Supporting information represents the UV-vis spectral
1
[
profiles of H TBPP and ZnTBPP in toluene at 298 K. The
2
phthalocyanine-Zn porphyrin conjugate with fairly high
binding constants [23a]. As a continuing effort to build
novel supramolecular donor–acceptor systems [23b], in
the present study, we have developed donor–acceptor
dyads using aryloxyporphyrin Zn(II) complexes (1–3)
with fullerene C60 and studied their spectroscopic as
well as electrochemical redox properties.
Soret and Q-band of these aryloxyporphyrins exhibited
marginal bathochromic shift as compared to meso-
tetraphenylporphyrin (MTPP, M = 2H and Zn(II)).
Fluorescence spectral studies
The steady state emission measurements have been
investigated in accordance with the photo-induced
behavior of C with porphyrins. It was observed that
6
0
RESULTS AND DISCUSSION
the fluorescence intensity of synthesized porphyrins
upon excitation at Q-band gradually diminished during
titration with concomitant increase of C concentration
in toluene at 298 K as shown in Figs 2 and S13–S14
(Supporting information). This indicated the possibility
of a relaxation pathway from the excited singlet state
of the porphyrin to that of fullerene in toluene. It has
also been proposed that charge separation might also
6
0
Synthesis and characterization
Meso-aryloxy porphyrins and their zinc(II) complexes
(1–3) were synthesized according to the modified
procedure as shown in Scheme 1 [24]. All the synthesized
porphyrins were characterized by UV-vis, fluorescence,
Copyright © 2016 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2016; 20: 745–751

746
T. A. DAR ET AL.
OH
OR
1
. 18-C-6, K2CO3,
R-Br, dry DMF
NH
N
N
N
N
N
N
Zn
HO
OH
RO
OR
2
. Zn(OAc)2 2H2O
CH3Cl/CH3OH
HN
OH
OR
R =
(1),
(2)
HO
RO
OR
OH
HO
HO
OH
OH
RO
OR
1
. 18-C-6, K2CO3,
NH
N
N
N
N
N
R-Br, dry DMF
Zn
2
. Zn(OAc)2 2H2O
CH3Cl/CH3OH
HN
N
OR
RO
HO
OH
RO
OR
R =
(3)
Scheme 1. Synthesis of meso-aryloxyporphyrins and their Zn(II) complexes (1–3)
Fig. 1. ORTEP top (a) and side (b) views of ethyl acetate coordinated ZnTBPP (1-EtOAc). In side view, meso-phenyl ring and ethyl
acetate (except coordinated oxygen) are not shown for clarity
Copyright © 2016 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2016; 20: 746–751

SYNTHESIS, ELECTROCHEMICAL AND COMPLEXATION STUDIES OF ZN(II) ARYLOXYPORPHYRINS WITH FULLERENE C60
747
a
Table 1. Optical absorption and emission spectral data of synthesized porphyrins in CH Cl at 298 K
2
2
Compound
B-band, nm
Q-band(s), nm
Emission, nm
H TBPP
421 (4.54)
425 (4.42)
421 (4.33)
425 (3.72)
422 (4.34)
425 (4.40)
517 (3.01), 554 (3.11), 594 (3.76), 652 (2.12)
552 (3.04)
663, 725 (sh)
609, 654
2
ZnTBPP (1)
H TNPP
518 (3.26), 555 (3.13), 595 (3.55), 653 (2.31)
552 (2.39)
663, 725 (sh)
609, 654
2
ZnTNPP (2)
H ONPP
516 (3.22), 552 (3.34), 592 (3.66), 647 (2.40)
550 (3.09)
657, 720
2
ZnONPP (3)
606, 651
a
-1
-1
l/nm. The values in parenthesis refer to loge (e = M .cm ) values.
occur from the excited state of the porphyrin to C60 in
toluene medium [25]. The mutual competition between
energy and electron transfer processes is a universal
phenomenon in donor molecule-fullerene acceptor dyads
[26]. In conformationally flexible porphyrin-fullerene
dyads, the p-stacking interactions are assisted through
space interactions between the two chromophores which
has been postulated and proved via quenching studies
on porphyrin fluorescence and formation of fullerene
excited states (by energy transfer) as established
through photo-physical experiments [27]. The photo-
physical studies revealed that the energy transfer process
generally dominates over the electron transfer process
in non-polar solvents and is ascribed to deactivation
of the photo-excited porphyrin-fullerene dyad. As an
inference to the ongoing experiments, the quenching
phenomenon can be attributed to photo-induced energy
transfer from porphyrin moiety to the fullerene acceptor
in the supramolecular assemblies of 1–3 with C . The
6
0
association constant K for 1–3 with C was determined
6
0
by the use of Benesi–Hildebrand equation (Table 2). A
straight line was obtained by plotting F /(F –F) vs. 1/
0
0
[
C ], where F and F are the fluorescence intensity at 602
60 0
nm of ZnTNPP (2) without and with C , respectively,
6
0
indicating the 1:1 stoichiometry of the complex of 2 and
C (Fig. 2b). The association constant K was calculated
6
0
3
from ratio for the intercept vs. slope to be 4.99 × 10
-
1
M . This value was broadly in agreement with that
was determined by Stern–Volmer method, i.e. F /F = 1
0
+
K [fullerene], where F is the fluorescence intensity
0
of the porphyrins in the absence of fullerene, F is the
fluorescence intensity of the porphyrins (viz., 1, 2 and 3)
in the presence of a quencher (i.e. fullerene). The order of
association constants was found to be 2 > 1 > 3. 2 showed
higher association constant as compared to other analogs
due to effective p–p interactions whereas 3 exhibited 3.4
times lower association constant than that of 2 due to
steric hindrance offered by naphthyloxy moieties.
Benesi–Hildebrand plots were constructed and the
3
binding constants were calculated to be 1.85 × 10 ,
3
3
-1
4
.99 × 10 and 1.47 × 10 M for 1, 2 and 3, respectively.
Fig. 2. (a) Fluorescence spectral changes of porphyrin 2 while
Byvaryingthenumberandpositionofaryloxygroupslead
to varying contact areas with fullerenes, thereby altering
the magnitude of van der Walls interaction components
increasing the concentration of fullerene (C ) in toluene at
60
2
98 K; (b) Bensei-Hildebrand plot of 2:C host–guest complex;
60
(c) Stern-Volmer plot of 2:C60 host–guest complex
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J. Porphyrins Phthalocyanines 2016; 20: 747–751

748
T. A. DAR ET AL.
Table 2. Association constants of porphyrin-C60
supramolecular dyads in toluene at 298 K
the benzyl protons showed marginal shift on addition of
C . Figure S15 in the Supporting information represents
6
0
1
the comparative H NMR spectra of 2 and its C adduct,
-1
60
Porphyrin
Association constant (K), M
respectively. These results show that upfield shift of
3
ZnTBPP (1)
ZnTNPP (2)
ZnONPP (3)
1.85 × 10
b-protons is due to interaction of porphyrins with C .
6
0
3
Only marginal changes were observed for aromatic
naphthyl protons, an evidence that fullerene interacts
mainly with the porphyrin 24 core-atom macrocycle
effectively.
4.99 × 10
1.47 × 10
3
causing a shift in binding energy and consequently the
binding constant. Quantum yields were also calculated
for all the system in the presence and absence of fullerene
Electrochemistry
To investigate the impetus of the meso-aryloxyphenyl
substituents on the p-electronic properties of the
porphyrin ring system, the electrochemical redox
behaviors of these porphyrins were analyzed by cyclic
voltammetry (Fig. 4). As shown in Table 4, the first
oxidation potential is 80–90 mV anodically shifted in
case of ZnTBPP (1) and ZnTNPP (2) as compared to
ZnTPP. The reduction potential is not much influenced
by the meso-substitution. The obtained redox potentials
of 1–3 and C60 entities revealed small changes upon
supramolecular dyad formation suggesting the existence
of weak charge-transfer (CT) interactions [28].
(Table 3). All the system showed decrement in quantum
yields upon addition of C . Experimental studies
6
0
regarding quantum yields further proved a testament
to the acquired binding constants calculated before.
From the data given below, we can summarize that the
highest quantum yield observed is in accordance with the
theoretical data supporting the charge transfer between
porphyrins and C systems.
6
0
1
H NMR studies
The association of aryloxyporphyrins (1–3) with
1
C60 was monitored by H NMR titrations in benzene-d
6
Computational studies
at 298 K. Addition of 1.1 equivalent of C60 into the
solution of Zn(II) porphyrin results in shifting of only
b and meso-phenyl protons as shown in Fig. 3 whereas
DFT calculations on the basis of B3LYP functional
and 3-21G setup was used to analyze geometry and
Table 3. Quantum yield data of 1–3 in presence and absence of fullerene C60 in toluene at 298 K
Porphyrin
Quantum yield (without C )
Quantum yield (with 40 equiv. C60)
Percentage decrement
60
ZnTPP
0.11
0.14
0.21
0.10
0.06
0.08
0.11
0.06
45
46
48
41
1
2
3
a
a
a
1
Fig. 3. H NMR spectra of ZnTBPP (1) in presence and absence of C in benzene-d at 298 K
6
0
6
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SYNTHESIS, ELECTROCHEMICAL AND COMPLEXATION STUDIES OF ZN(II) ARYLOXYPORPHYRINS WITH FULLERENE C60
749
Fig. 4. Cyclic voltammetric studies of 1–3 in presence and absence of C60 in PhCN at 298 K
a
Table 4. Electrochemical redox data of 1–3 in presence and absence of C in benzonitrile containing
6
0
0
.1 M TBAPF as supporting electrolyte at 298 K
6
Porphyrin
Oxidation, V
Reduction, V
I+/II+
0/I+
0/-
I-/II-
0/-
I-/II-
P
P
P
P
C60
C60
ZnTPP
1.140
1.092
1.092
1.086
1.097
1.136
1.120
0.835
0.753
0.753
0.747
0.747
0.840
0.822
-1.360
-1.345
-1.345
-1.121
-1.357
-1.105
-1.334
-1.765
-1.706
-1.736
-1.564
ZnTBPP (1)
ZnTBPP:C60
ZnTNPP (2)
ZnTNPP:C60
ZnONPP (3)
ZnONPP:C60
a
-0.422
-0.405
-0.376
-0.847
-0.841
-0.810
-1.511
vs. Ag/AgCl reference electrode, Pt working and Pt wire auxiliary electrodes were used. Aliquots
of 0.01 M solution of C (excess with respect to porphyrins) in PhCN were added.
60
electronic structure of the porphyrin-fullerene assemblies.
The advantages of 3-21G setup to predict the structure of
donor–acceptor porphyrin-fullerene dyads have already
been reported by D’Souza et al. [29]. Porphyrin donor
and fullerene acceptor were initially optimised and
allowed to interact and used for calculations. Figure S16
shows that all the porphyrins have planar structure with
aryloxy side arms staying away from 24-inner core atoms.
Figure 4 represents optimised version of porphyrin-
fullerene dyad. Geometric parameters were established
after complete energy optimization. The optimized
structure for all the three porphyrin systems with C60
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T. A. DAR ET AL.
Fig. 5. (a) B3LYP/3-21G optimized structure of ZnTBPP (1):C supramolecular dyad (b) HOMO and (c) LUMO orbitals
6
0
revealedthatthearyloxysidearmsstayapartfromtheinner
porphyrin core and C60 interacts with central porphyrin
core only. The B3LYP/3-21G revealed the presence of
HOMO on porphyrin moiety (Fig. 5a) and LUMO on
fullerene (Fig. 5b). The absence of LUMO on porphyrin
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