A 'two-point' bound zinc porphyrin-zinc phthalocyanine-fullerene supramolecular triad for sequential energy and electron transfer

Article abstract of DOI:10.1039/c3cc43510e

A novel supramolecular triad composed of a zinc porphyrin-zinc phthalocyanine dyad and fullerenes has been assembled using a 'two-point' axial binding approach, and occurrence of efficient photoinduced energy transfer followed by electron transfer is demonstrated. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc43510e

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A ‘two-point’ bound zinc porphyrin–zinc
phthalocyanine–fullerene supramolecular triad for
sequential energy and electron transfer†‡
Chandra B. KC,a Kei Ohkubo,b Paul A. Karr,c Shunichi Fukuzumi*bd and
Francis D’Souza*a
Cite this: Chem. Commun., 2013,
49, 7614
Received 10th May 2013,
Accepted 1st July 2013
DOI: 10.1039/c3cc43510e
www.rsc.org/chemcomm
A novel supramolecular triad composed of a zinc porphyrin–zinc
phthalocyanine dyad and fullerenes has been assembled using a
‘two-point’ axial binding approach, and occurrence of efficient photo-
induced energy transfer followed by electron transfer is demonstrated.
In natural photosynthesis, photon energy is efficiently funnelled
with the help of antenna pigments to the reaction centre to generate
charge separated states as a result of the occurrence of sequential
energy and electron transfer events.1 Inspired by this, there have
been a number of artificial photosynthetic models capable of
mimicking these events with an ultimate aim of converting light
energy into solar fuels, as well as to build optoelectronic devices.2–4
Self-assembled assemblies are more appealing compared to
covalently linked ones due to their close resemblance to natural
Fig. 1 Structure of the zinc porphyrin–zinc phthalocyanine–fullerene triad held
by a ‘two-point’ axial coordination binding approach.
systems. Additionally, although synthetically challenging, by intro- transfer7 and binding to an electron acceptor fullerene function-
ducing multiple modes of binding between the entities, the desired alized to possess two pyridine entities (C60Py2) via ‘two-point’
distance and orientation between the donor–acceptor entities could binding as shown in Fig. 1. Occurrence of sequential light induced
be achieved.2–4 In this regard, a ‘two-point’ axial coordination energy and electron transfer events as probed spectroscopic and
approach was introduced by us to link a covalently linked porphyrin transient spectral studies is presented.
dimer to fulleropyrrolidine via metal–ligand axial coordination,5
The synthesis of a ZnP–ZnPc dyad was carried out according to
and photoinduced electron-transfer processes in relevant systems the procedure given in Scheme 17b (see experimental details in
were demonstrated.6 In the present study, we have extended this ESI‡). Here, a meta-carboxy functionalized ZnP was used to react
approach by employing a zinc porphyrin–zinc phthalocyanine with hydroxy functionalized ZnPc. The meta derivative provided the
(ZnP–ZnPc) dyad capable of undergoing singlet–singlet energy necessary molecular orientation of the dimer to accommodate
C60Py2 via the two-point binding (vide infra).
Fig. 2a shows the absorption spectrum of the dyad along with
the spectrum of a 1 : 1 mixture of ZnP and ZnPc in o-dichloro-
a Department of Chemistry, University of North Texas, 1155 Union Circle, #305070,
Denton, TX 76203-5017, USA. E-mail: Francis.dsouza@unt.edu
b Graduate School of Engineering, Osaka University, ALCA, Japan Science and
Technology Agency (JST), Suita, Osaka 565-0871, Japan.
E-mail: Fukuzumi@chem.eng.osaka-u.ac.jp
benzene (DCB). A small red shift (1–2 nm) of the ZnP Soret and
broadening of ZnPc bands were observed suggesting occurrence of
some intramolecular interactions between the two entities of the
dyad. Fig. 2b shows the fluorescence spectrum of the dyad along
with that of the 1 : 1 mixture at the excitation at 425 nm corre-
sponding to ZnP. The ZnP emissions at 600 and 650 nm were
c Department of Physical Sciences and Mathematics, Wayne State College,
111 Main Street, Wayne, Nebraska 68787, USA
d Department of Bioinspired Science, Ewha Womans University, Seoul, 120-750,
Korea
† In loving memory of Prof. Erach Talaty, a beloved teacher and dedicated quenched over 80% of their values observed for the 1 : 1 mixture
researcher.
with the appearance of strong ZnPc emission at 700 nm indicating
‡ Electronic supplementary information (ESI) available: Synthetic details and the
occurrence of intramolecular singlet excitation transfer in the
mass spectrum of the ZnP–ZnPc dyad, experimental procedures and instrumen-
dyad.7,8 The excitation spectrum recorded for the dyad by holding
tation, the excitation spectrum of the dyad, the femtosecond transient spectrum
the emission wavelength at 695 nm and scanning the excitation
wavelength (Fig. S1 in ESI‡) revealed bands of both ZnPc and ZnP.
of C60Py2 in DCB, and the nanosecond transient spectrum of the ZnP–ZnPc:Py2C60
triad in DCB. See DOI: 10.1039/c3cc43510e
c
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Scheme 1 Synthetic procedure adopted for the ZnP–ZnPc dyad (DMAE
=
=
N,N-dimethylaminoethanol, DCC
=
N,N0-dicyclohexyl-carbodiimide, DMAP
Fig. 3 (a) Absorption spectral changes observed for C60Py2 (3.7 mM, 2 mL each
addition) binding to ZnP–ZnPc (3.9 mM) in DCB, (b) Benesi–Hildebrand plot
to calculate the binding constant, (c) mole-ratio plot to establish the molec-
ular stoichiometry, (d) fluorescence changes observed for binding of C60Py2 to
ZnP–ZnPc in DCB; lex = 412 nm corresponding to one of the isosbestic points.
4-(dimethylamino)pyridine).
studies were performed. Fig. 4a and b show B3LYP/6-31G* opti-
mized geometries10 of the ZnP–ZnPc dyad and C60Py2. The two
macrocycles of ZnP–ZnPc were tilted due to the meta-substitution of
ZnP (dihedral angle = 961). At least two geometries for the triad were
possible owing to an extra connecting methylene group for one of
the Py entities of C60Py2 as shown in Fig. 4c and d. The structure
shown in Fig. 4c is more stable by ca. 2.3 kJ molꢁ1 compared to the
one shown in Fig. 4d. The centre-to-centre distance between Zn and
C60 for the triad having C60-pyridine with connecting –CH2– was
about 0.5 Å more than the one without a –CH2– fragment, both with
a distance of 11.5 ꢂ 0.5 Å.
Fig. 2 (a) Absorption (3.9 mM each) and (b) emission (3.9 mM each) spectrum of
(i) ZnP–ZnPc dyad and (ii) 1 : 1 mixture of ZnP and ZnPc in DCB. lex = 425 nm.
However, the ZnP bands were missing in the spectrum recorded for
the 1 : 1 mixture providing evidence for the occurrence of excitation
transfer in the dyad.
Further free-energy calculations for charge-separation (DGCS)
were performed according to the Weller approach.11 These values,
The supramolecular triad was formed by complexing C60Py2 to
the ZnP–ZnPc dyad. Fig. 3a shows spectral changes observed during
titration of C60Py2 to a solution of the ZnP–ZnPc dyad in DCB. A ZnP
Soret band at 425 nm revealed characteristic features of axial
coordination with diminished intensity accompanied by a red shift.6
Similarly, the ZnPc band at 690 nm revealed diminished intensity in
addition to three isosbestic points. The binding constant calculated
from the spectral data using the Benesi–Hildebrand method9 was
found to be 1.7 ꢀ 105 Mꢁ1 (Fig. 3b) which was over an order of
magnitude higher than that reported for ‘single-point’ metal–ligand
coordination.3b Further, a plot of the mole ratio revealed a break at
0.5 (Fig. 3c) confirming a 1 : 1 stoichiometry of the supramolecular
triad involving binding of both metal centers to C60Py2.
Addition of C60Py2 to the solution of the ZnP–ZnPc dyad
quenched ZnPc emission almost quantitatively while such changes
for the weak ZnP emission were minimal (Fig. 3d). To further
understand this, additional computational and electrochemical
Fig. 4 B3LYP/6-31G* optimized structures of the ZnP–ZnPc dyad (a), C60Py2 (b)
and the two forms of self-assembled triads (c) and (d).
c
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relatively long-lived charge-separated states. The lifetime of the
charge-separated state was determined from the decay curve of
ꢁ
ꢃ
absorbance at 1000 nm due to the C60 moiety to be 1.7 ns
(Fig. 6c). Additional nanosecond transient studies performed
revealed the absence of transient features of the charge separated
species indicating occurrence of charge recombination well before
the 10 ns detection limit of the instrument (Fig. S4 in ESI‡).
In summary, using a ‘two-point’ axial binding approach, we have
assembled a novel triad comprised of a ZnP–ZnPc dyad and C60 to
probe sequential photoinduced energy and electron transfer events.
The two-point binding approach resulted in a triad of defined shape
and relatively high stability. Free-energy calculation suggested that
Fig. 5 (a) Femtosecond transient spectra of the ZnP–ZnPc dyad at the indicated
time intervals in DCB. The kinetic profiles of the 500 and 900 nm bands of 1ZnP*
are shown in b and c, respectively.
1
electron transfer from ZnPc* is energetically more favorable than
1
that from the ZnP* in the triad although the acceptor (C60) was
disposed at an almost equal distance. Consequently, efficient energy
determined from redox potential (Fig. S2 in ESI‡ for voltammograms)
and singlet state energy of each zinc tetrapyrrole (2.06 eV for ZnP*
transfer from 1ZnP* to ZnPc in the triad followed by formation of a
1
+
ZnP–ZnPcꢃ –C60 ꢁ radical ion-pair was witnessed in the triad. These
and 1.83 eV for 1ZnPc*), were found to be ꢁ0.72 eV for 1ZnPc* and
ꢁ0.58 eV from 1ZnP* originated electron transfer in the triad indicat-
ing dominance of ZnPc over ZnP in the electron transfer process.
Femtosecond transient absorption spectral studies were per-
formed to probe mechanistic and kinetic details of energy and
electron transfer events. The acceptor, C60Py2, upon excitation by
the 393 nm laser light revealed formation of 1C60Py2* at 900 nm that
ꢃ
findings prove to be significant for future design of tetrapyrrole
donor–nanocarbon acceptor hybrids for wide-band capture and
efficient light energy harvesting.
FD acknowledges support from the NSF (Grant No. 1110942).
SF and K.O. acknowledge support from Grants-in-Aid (No. 20108010
to S.F. and 23750014 to K.O.) from MEXT, Japan, and KOSEF/MEST
through the WCU project (R31-2008-000-10010-0), Korea. The com-
putational work was completed utilizing the Holland Computing
Center of the University of Nebraska.
3
decayed at a rate of 2.3 ꢀ 109 sꢁ1 to populate C60Py2* at 700 nm
(Fig. S3 in ESI‡).12 The ZnP–ZnPc dyad upon excitation by the 393 nm
laser revealed initial population of 1ZnP* with characteristic transient
peaks at 500 and 950 nm with a rate constant >5 ꢀ 1012
s
ꢁ1; these
1
Notes and references
peaks decayed to populate ZnPc* at 830 nm (Fig. 5) within 1.5 ps.
The rate of singlet energy transfer was close to that reported recently
for the ZnP–ZnPc dyad with a para linkage7b revealing very small
influence from the mode of porphyrin substitution (meta versus para).
The supramolecular triad formed upon binding of C60Py2
revealed features confirming the occurrence of photoinduced elec-
tron transfer. As shown in Fig. 6, excitation of the triad at 393 nm
had features of singlet–singlet energy transfer from 1ZnP* to ZnPc at
a rate not much different from that of the ZnP–ZnPc dyad. However,
the newly formed 1ZnPc* underwent photoinduced electron transfer
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ꢁ
ꢃ
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1
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1
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Fig. 6 (a) Femtosecond transient absorption spectra of the ZnP–ZnPc:Py2C60
triad at the indicated time intervals in DCB. The kinetic profiles of absorbance at
(b) 900 nm and (c) 1000 nm are at the right hand side panel.
c
7616 Chem. Commun., 2013, 49, 7614--7616
This journal is The Royal Society of Chemistry 2013