A tightly coupled bis(zinc(ll) phthalocyanine)-perylenediimide ensemble to yield long-lived radical ion pair states

Article abstract of DOI:10.1021/ol0707968

A conjugated donor-acceptor array composed of two phthalocyanlnes connected to the bay region of a perylenedlimide is aseembled by using palladium chemistry. Excitation of the phthalocyanine produces a nanosecond lived charge-separated state.

Full text of DOI:10.1021/ol0707968

ORGANIC
LETTERS
2
007
Vol. 9, No. 13
481-2484
A Tightly Coupled Bis(zinc(II)
phthalocyanine) Perylenediimide
2
−
Ensemble To Yield Long-Lived Radical
†
Ion Pair States
¶
§
¶
‡
A
Ä
ngel J. Jim e´ nez, Fabian Spa
1
nig, M. Salom e´ Rodr ´ı guez-Morgade, Kei Ohkubo,
‡
,§
,¶
Shunichi Fukuzumi, Dirk M. Guldi,* and Tom a´ s Torres*
Departamento de Qu ´ı mica Org a´ nica, UniVersidad Aut o´ noma de Madrid,
Campus de Cantoblanco, 28049-Madrid, Spain, Institute for Physical Chemistry,
Friedrich-Alexander-UniVersit a¨ t Erlangen-N u¨ rnberg, Egerlandstrasse 3,
D-91058 Erlangen, Germany, and Department of Material and Life Science,
DiVision of AdVanced Science and Biotechnology, Graduate School of Engineering,
Osaka UniVersity, SORST (JST), Suita, Osaka 565-0871, Japan
tomas.torres@uam.es; dirk.guldi@chemie.uni-erlangen.de
Received April 3, 2007
ABSTRACT
A conjugated donor−acceptor array composed of two phthalocyanines connected to the bay region of a perylenediimide is assembled by
using palladium chemistry. Excitation of the phthalocyanine produces a nanosecond lived charge-separated state.
The quest for artificial photosynthetic reaction centers, as a
part of “the modular strategy”, constitutes an expanding
the selected building block functionalities, linkers, and
1
4
geometrical arrangements. The benefits of phthalocyanines
5
trend in the field of solar photochemistry and solar energy
(Pc’s) as components for such systems is beyond question.
2
conversion. In particular, there is considerable interest in
(3) (a) The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J.
the design of systems composed of electron donor and
acceptor entities displaying energy and/or electron-transfer
R., Eds.; Academic Press: New York, 1993. (b) Blankenship, R. E.
Molecular Mechanisms of Photosynthesis; Blackwell Science: Berlin,
Germany, 2002. (c) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, Germany, 2001.
3
events. The efficiency of those processes is governed by
(
4) Hasobe, T.; Fukuzumi, S.; Hattori, S.; Kamat, P. V. Chem. Asian J.
†
Dedicated to Professor Miguel Yus on the occasion of his 60th birthday.
Universidad Aut o´ noma de Madrid.
Friedrich-Alexander-Universit a¨ t.
Osaka University.
2007, 2, 265-272.
¶
(5) (a) de la Torre, G.; Claessens, C. G.; Torres, T. Chem. Commun.
2007, 2000-2015. (b) Phthalocyanines: Properties and Applications;
Leznoff, C. C., Lever, A. B. P., Eds.; VCH: Weinheim, Germany, 1989,
1993, 1996; Vols. 1-4. (c) The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2003; Vols.
15-20. (d) de la Torre, G.; V a´ zquez, P.; Agull o´ -L o´ pez, F.; Torres, T. Chem.
ReV. 2004, 104, 3723-3750.
§
‡
(
1) Eisenberg, R.; Nocera, D. G. Inorg. Chem. 2005, 44, 6799-6801
and references cited therein.
2) (a) Kay, E. R.; Leigh, D. A. Nature 2006, 440, 286-287. (b)
Matyushov, D. V. J. Phys. Chem. B 2006, 110, 10095-10104.
(
1
0.1021/ol0707968 CCC: $37.00
© 2007 American Chemical Society
Published on Web 06/02/2007

Thus, their extended π-conjugated structure provides them
with strong absorption in the visible and redox features that
make them specially suitable for photoinduced charge
Scheme 1. Synthesis of ZnPc-PDI-ZnPc 5
6
separation. In the past few years we have directed our
attention to construct Pc-based multinuclear systems with
additional π-extended conjugation, as well as multifunctional
donor-acceptor hybrids in which the complementary elec-
troactive constituents (i.e., C60) are connected through a
variety of junctions.7 Remarkably, ZnPc-C60 systems tend
to yield shorter radical ion pair state lifetimes, when
compared with analogous ZnP-C60 systems.9
,8
Recently, perylenediimides (PDI’s) have attracted our
1
0
attention as the complementary oxidizing moieties, and
hence, we have reported a supramolecular RuPc-PDI-RuPc
that forms upon photoexcitation a long-lived radical ion pair
1
1
state. Herein, we wish to describe a novel, covalently
linked, conjugated array 5 (Scheme 1) composed of two
Zn(II)-phthalocyanines attached through ethynyl functions
to the 1,7-positions of a perylenediimide moiety.
Previous reports on the Pc-PDI motif are very scarce, and
1
2
include organization of the two dyes in thin films, or
covalent hybrids of the two chromophores connected through
the PDI imido positions.13 In the triad 5 the two phthalo-
cyanines are bound to the PDI-bay region so that the system
was expected to be electronically coupled,14 with the PDI
component profoundly influenced by the presence of the two
phthalocyanines.15
The ZnPc-PDI-ZnPc triad 5 can be assembled by Sono-
gashira coupling of suitable Pc and PDI derivatives. In a
first approach (Scheme 1, pathway A) the intermediate 2
(6) Reddy, P. Y.; Giribabu, L.; Lyness, C.; Snaith, H. J.; Vijaykumar,
C.; Chandrasekharam, M.; Lakshmikantam, M.; Yum, J.-H.; Kalyana-
sundaram, K.; Gr a¨ tzel, M.; Nazeeruddin, M. K. Angew. Chem., Int. Ed.
2
007, 46, 373-376.
(
7) (a) Garc ´ı a-Frutos, E. M.; Fern a´ ndez-L a´ zaro, F.; Maya, E. M.;
V a´ zquez, P.; Torres, T. J. Org. Chem. 2000, 65, 6841-6846. (b) Mart ´ı nez-
D ´ı az, M. V.; Fender, N. S.; Rodr ´ı guez-Morgade, M. S.; G o´ mez-L o´ pez, M.;
Diederich, F.; Echegoyen, L.; Stoddart, J. F.; Torres, T. J. Mater. Chem.
was attained by bromination of perylene bisanhydride,
1
6
2
002, 12, 2095-2099. (c) de la Escosura, A.; Mart ´ı nez-D ´ı az, M. V.;
followed by imidation in propionic acid. Under these
conditions, the 1,7-dibrominated compound 2 was obtained
as the major product, even though it was accompanied by
the corresponding 1,6-regioisomer and 1,6,7-tribrominated
Thordarson, P.; Rowan, A. E.; Nolte, R. J. M.; Torres, T. J. Am. Chem.
Soc. 2003, 125, 12300-12308.
(8) (a) Guldi, D. M.; Gouloumis, A.; V a´ zquez, P.; Torres, T.; Georgakilas,
V.; Prato, M. J. Am. Chem. Soc. 2005, 127, 5811-5813. (b) Gouloumis,
A.; Gonz a´ lez-Rodr ´ı guez, D.; V a´ zquez, P.; Torres, T.; Liu, S.; Echegoyen,
L.; Ramey, J.; Hug, G. L.; Guldi, D. M. J. Am. Chem. Soc. 2006, 128,
17
product, arising from the bromination step. We could easily
1
2674-12684.
remove the minor tribrominated component (4% of the
(
9) Guldi, D. M. Phys. Chem. Chem. Phys. 2007, 9, 1400-1420.
10) (a) W u¨ rthner, F. Chem. Commun. 2004, 1564-1579. (b) Prodi, A.;
17
mixture) by standard column chromatography. However,
(
1
7
the 1,7-derivative 2 (76% of the mixture) could only be
separated from its 1,6-regioisomer by repetitive recrystalli-
zation from a 3:1 mixture of methanol and dichloromethane.
Once isolated, the two subunits 1 and 2 were assembled in
Chiorboli, C.; Scandola, F.; Lengo, E.; Alessio, E.; Dobrawa, R.; W u¨ rthner,
F. J. Am. Chem. Soc. 2005, 127, 1454-1562. (c) Elemans, J. A. A. W.;
van Hameren, R.; Nolte, R. J. M.; Rowan, A. E. AdV. Mater. 2006, 18,
1
251-1266.
(
11) Rodr ´ı guez-Morgade, M. S.; Torres, T.; Atienza Castellanos, C.;
Guldi, D. M. J. Am. Chem. Soc. 2006, 128, 15145-15154.
12) (a) W o¨ hrle, D.; Kreienhoop, L.; Schnurpfeil, G.; Elbe, J.; Tennigkeit,
1
6% yield by using palladium chemistry to afford 5.
(
B.; Hiller, S.; Schlettwein, D. J. Mater. Chem. 1995, 5, 1819-1829. (b)
Abe, T.; Nagai, K.; Kabutomori, S.; Kaneko, M.; Tajiri, A.; Norimatsu, T.
Angew. Chem., Int. Ed. 2006, 45, 2778-2781.
Alternatively, the array 5 can be prepared by the palladium
cross-coupling reaction of an iodo-substituted phthalocyanine
3
and a PDI derivative 4 endowed with two ethynyl rests
(
13) (a) Signerski, R.; Jarosz, G.; Godlewski, J. Synth. Met. 1998, 135-
1
37. (b) Liu, S.-G.; Liu, Y.-Q.; Xu, Y.; Jiang, X.-Z.; Zhu, D.-B. Tetrahedron
(Scheme 1, pathway B). The strategy relies on the premise
that functionalization of PDI’s at the bay region with bulky
Lett. 1998, 39, 4271-4274. (c) Fukuzumi, S.; Ohkubo, K.; Ortiz, J.;
Guti e´ rrez, A. M.; Fern a´ ndez-L a´ zaro, F.; Sastre-Santos, A. Chem. Commun.
2
005, 3814-3816. (d) Li, X.; Sinks, L. E.; Rybtchinski, B.; Wasielewski,
M. R. J. Am. Chem. Soc. 2004, 126, 10810-10811.
14) The presence of nodes at the imido nitrogens in the HOMO and
(16) (a) B o¨ hm, A.; Arms, H.; Henning, G.; Blaschka, P. (BASF AG)
German Patent DE 19547209 A1, 1997; Chem. Abstr. 1997, 127, 96569g.
(b) Chen, S.; Liu, Y.; Qiu, W.; Sun, X.; Ma, Y.; Zhu, D. Chem. Mater.
2005, 17, 2208-2215.
(17) W u¨ rthner, F.; Stepanenko, V.; Chen, Z.; Saha-M o¨ ller, C. R.; Kocher,
N.; Stalke, D. J. Org. Chem. 2004, 69, 7933-7939.
(
LUMO of the PDI molecule prevents electronic coupling between subunits,
when such subunits are attached through the PDI imido positions.
(15) Shibano, Y.; Umeyama, T.; Matano, Y.; Tkachenko, N. V.;
Lemmetyinen, H.; Imahori, H. Org. Lett. 2006, 8, 4425-4428.
2482
Org. Lett., Vol. 9, No. 13, 2007

groups prevents aggregation and makes separation by chro-
matography easier. The alkyne linkers were introduced via
reaction of triphenylsilylacetylene and 2, as a 1,6- and 1,7-
regioisomeric mixture. Here, the choice of the large tri-
phenylsilane as the protecting group for the alkyne function
is critical, since this allowed the straightforward separation
of the 1,7- from the 1,6-PDI regioisomer by column
chromatography. Subsequent one-pot deprotection and So-
nogashira coupling with phthalocyanine 3, following a
protocol reported by Grieco and co-workers,18 afforded the
hybrid 5 in quite good yield (33%), taking into account that
this yield is referred to four chemical reactions, namely two
alkyne deprotections and two Sonogashira couplings.
Kasha. The phthalocyanine B-band has two perpendicular
components of B and B . In 1, they are degenerate, but in
ZnPc-PDI-ZnPc (5) they couple differently with the adjacent
perylene, so that only the B transitions couple and, in turn,
shift to the blue. The other dipole interactions (i.e., B ) should
x
y
x
y
be zero leaving the corresponding transitions unchanged.
Importantly, the large splitting of the B-bands (0.36 eV)
is mainly due to exciton-coupling between neighboring ZnPc
and PDI moieties. Much weaker splitting (0.1 eV), as well
as smaller red shifts, indicating a weak coupling of transition
dipoles, is seen for the Q-bands. Moreover, the PDI transi-
tions, which occur in the reference at 464, 495, and 529 nm,
are red shifted to 540 and 570 nm. Part of these changes
arise from the substitution by conjugated ethynyl groups at
the PDI bay (see the Supporting Information). In addition,
the presence of two bulky phthalocyanines in 5 must
necessarily have a strong influence on the planarity of the
red chromophore. Still, all this evidence speaks for strong
electronic communication between the two redox-active
components. It is likely that a partial redistribution of charge
densitystransfer from ZnPc (i.e., electron donor) to PDI (i.e.,
acceptor)sis active.
Steady-state fluorescence helped to shed light onto the
excited state interactions. Taking the photo- and redox-
activity of both ZnPc and PDI units into account, their high
fluorescence quantum yields of 0.3 and 0.75 are particularly
helpful. In this context, we compared PDI with ZnPc-PDI-
ZnPc (5) and ZnPc with 5 by exciting them in the 450-525
and 600-675 nm ranges, respectively. In both cases,
although seeing a similar ZnPc and PDI fluorescence pattern,
strong fluorescence quenching is noted: For the PDI unit a
quenching of 200 evolves around the emission in the visible
Figure 1. UV/vis spectra of phthalocyanine 1 (ZnPc, blue), N,N′-
di-(2-ethylhexyl)perylene-3,4,9,10-tetracarboxylic acid bisimide
(PDI, green), and ZnPc-PDI-ZnPc (5) (black) in chloroform. The
red spectrum shows 5 in a THF solution containing 1% pyridine.
(
i.e., shifted from 540 nm at 2.3 eV to 590 nm at 2.1 eV),
while for the ZnPc component, emission in the near-infrared
i.e., shifted from 690 nm at 1.79 eV to 700 nm at 1.77 eV),
Figure 1 shows the electronic spectra of the hybrid 5
together with those corresponding to its Pc and PDI frag-
ments. Compound 5 strongly aggregates in solution of
noncoordinating solvents such as chloroform, as is clearly
evidenced by its broad, abnormally low intense Q-band. The
use of coordinating solvents such as THF or pyridine
produces stable coordinative complexes with the phthalo-
cyanine metal ion, thus reducing aggregation in diluted
solutions (c < 10 M). Under these conditions the
broadening still lingers, although to a much lesser extent.
This speaks for a significant redistribution of charge density
in the ground state, that is, from the electron donating ZnPc
to the electron accepting PDI. Moreover, the characteristic
(
the quenching is 250-fold.
Prior to conducting photolytic tests with ZnPc, PDI, and
ZnPc-PDI-ZnPc (5), the possible redox products, namely,
one-electron oxidized ZnPc radical cation and one-electron
reduced PDI radical anion, were generated pulse radiolyti-
cally. The corresponding spectra are gathered in Figure S-20
(Supporting Information). For PDI, we see in the visible a
-
5
19
set of minima at 460, 490 and 530 nm, followed in the near-
infrared by a broad maximum at 625 nm, as features of the
one-electron reduced radical anion. For ZnPc, maxima at 530
and 840 nm are well-known attributes of the one-electron
oxidized radical cation. We also tested the PDI triplet excited
state features via triplet-triplet energy transfer from the
radiolytically generated anthracene triplet excited state.
Q- and B-absorption bands, corresponding to the S
states, split in ZnPc-PDI-ZnPc (5) with 330/351 nm and 678/
10 nm components respectively in THF/pyridine. Compare
1 2
and S
7
Next, the excited state properties of ZnPc shall be
discussed, since they emerge as important reference points
for the interpretation of the features expected in the triad 5.
The differential spectrum, recorded immediately after the
laser pulse for the ZnPc reference, is characterized by
bleaching of the Q-band absorption at 686 nm and broad
absorption between 400 and 600 nm (Figure S-21, Supporting
Information). These spectral attributes are indicative of the
ZnPc singlet excited state and are formed with a rate constant
those to maxima at 361 and 686 nm for the phthalocyanine
fragment 1.
The spectral changes are rationalized by referring to the
simple point-dipole exciton coupling theory developed by
(
18) Mio, M. J.; Kopel, L. C.; Braun, J. B.; Gadzikwa, T. L.; Hull, K.
L.; Brisbois, R. G.; Markworth, C. J.; Grieco, P. A. Org. Lett. 2002, 4,
199-3202.
19) However, the H NMR spectrum (see the Supporting Information)
3
1
(
shows that 5 is still aggregated at higher concentrations (3 mM).
Org. Lett., Vol. 9, No. 13, 2007
2483

1
2
-1
of >1 × 10 s . The singlet excited state features decay
840 and 1000 nm, which correspond to the ZnPc radical
cation. In other words, we have accomplished a full spectral
8
-1
slowly (i.e., 3.0 ns; 3.3 × 10 s ) to the energetically lower-
lying triplet excited state, predominantly via intersystem
crossing (ZnPc triplet quantum yield, ΦTriplet ) 0.7).
When turning to the assembly 5, photoexcitation at 387
nm, seeing the ZnPc singlet excited state features right after
the laser pulse confirms the nearly exclusive excitation of
the ZnPc moietyssee Figure 2.
•+
characterization of the radical ion pair state, namely, ZnPc -
•
-
PDI -ZnPc (1.2 eV), as it has evolved from a thermody-
namically allowed (0.52 eV) electron transfer. A deceleration
of the charge separation is seen when looking at toluene/
pyridine, due to a lower driving force.
By following the temporal changes of the aforementioned
radical ion pair featuressthroughout the visible and near-
9
-1
infrared regionssa lifetime of 224 ps (4.4 × 10 s ) was
derived. In toluene/pyridine, the lifetime increases to 517
9
-1
ps (1.9 × 10 s ). Some time absorption profiles are
illustrated in Figure 2. With the conclusion of this charge
recombination process the triplet excited state spectrum of
ZnPc evolves in, however, minute yields (i.e., 7% and 2%
in toluene/pyridine and THF/pyridine, respectivelysrelative
to ZnPc). This decay pathway helps to explain the unusually
long lifetime of the radical ion pair states, when compared
to analogous, but electronically decoupled ZnP-PDI-ZnP
1
0
-1
conjugate (i.e., 77 ps; 1.3 × 10 s ). A markedly higher
lying radical ion pair state (1.61 eV) affords, however, the
20
triplet excited states to be formed quantitatively. It should
be noted that photoexcitation of a different ZnPc-PDI
conjugate (i.e., linked at the imido positions) affords only
1
3c
triplet excited states.
Figure 2. Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of ZnPc-
PDI-ZnPc (5) in nitrogen saturated THF/pyridine with several time
delays (i.e., 0 ps, black; 1 ps, red; 15 ps, green; 3000 ps, blue) at
room temperature. Inset: Time-absorption profiles of the spectra
shown above at 620 nm, monitoring the charge separation and
charge recombination in toluene/pyridine (red) and THF/pyridine
In summary, we have reported the synthesis and physico-
chemical characterization of a ZnPc-PDI-ZnPc ensemble,
which yields long-lived radical ion pair states. Key to this
success is a charge recombination that leads mainly to the
recovery of the singlet ground state. We are currently
investigating related multinuclear Pc-PDI-based systems in
(black).
11
which both covalent and supramolecular motifs are in-
volved.
We rule out an excitation of PDI due to the low extinctions
Acknowledgment. Financial support by MEC (CTQ2005-
08933/BQU), C. de Madrid (S-0505/PPQ/000225), EU
(MRTN-CT-2006-035533, Solar-N-type and COST Action
D35), SFB 583, DFG (GU 517/4-1), FCI and the Office of
Basic Energy Sciences of the U.S. Department of Energy is
gratefully acknowledged. M.S.R.-M. thanks the Spanish
MEC for a R & C contract.
in this range of wavelength (i.e., less than 5%). In line with
this assumption, the differential spectra lack any measurable
PDI singlet excited state contributions. The ZnPc singlet
1
1
excited state features decay rapidly (i.e., 7.2 ps; 1.4 × 10
-
1
s ) to yield a transient product that looks distinctly different
from any possible excited states (i.e., ZnPc triplet, PDI
singlet, or PDI triplet). First, let us look at the visible range,
where minima at 540 and 565 nm and maxima at 600 and
Supporting Information Available: Synthetic procedures
for the preparation of unreported compounds, as well as
NMR, IR, UV/vis and MS, radiolytic transient absorption,
femtosecond transient absorption, and fluorescence spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
7
60 nm are discernible. Notably, the minima mirror image
the ground state absorption maxima of PDI and resemble
equally that part of the radical anion spectrum. The maxima,
on the other hand, are only the blue and red flanks of what
would be the 625 nm maximum of the radical anion band.
However, the strong bleaching of the ZnPc radical cation
affects the net changes in this part of the differential
absorption spectrum. In the NIR part we note maxima at
OL0707968
(20) Kelley, R. F.; Shin, W. S.; Rybtchinski, B.; Sinks, L. E.; Wasielew-
ski, M. R. J. Am. Chem. Soc. 2007, 129, 3173-3181.
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Org. Lett., Vol. 9, No. 13, 2007