Singlet and triplet energy transfer rate acceleration by additions of clusters in supramolecular pigment-organometallic cluster assemblies

Article abstract of DOI:10.1039/c1cc11174d

Both S₁ and T₁ energy transfer rates (porphyrin → cluster) increase from mono- to di- to tetracarboxylate[tetraphenyl- (zinc)porphyrin] adducts with [Pd₃(dppm)₃(CO)] ²⁺ clusters.

Full text of DOI:10.1039/c1cc11174d

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COMMUNICATION
Singlet and triplet energy transfer rate acceleration by additions of
clusters in supramolecular pigment-organometallic cluster assemblieswz
Bin Du,ab Christine Sternb and Pierre D. Harvey*ab
Received 28th February 2011, Accepted 23rd March 2011
DOI: 10.1039/c1cc11174d
Both S1 and T1 energy transfer rates (porphyrin - cluster)
increase from mono- to di- to tetracarboxylate[tetraphenyl-
(zinc)porphyrin] adducts with [Pd3(dppm)3(CO)]2+ clusters.
In past decades, tremendous efforts have been devoted to mimic
natural photo-induced electron or energy transfer processes1,2 and
the development of charge-separating devices.3–5 However, a large
Fig. 1 Left: space filling models showing two possible cavity sizes
number of these models including porphyrin-based complex
architectures, were based on covalent linkages.1 Owing to the
virtues of non-covalent systems, such as versatility, flexibility in
design, and ease of synthesis, increasing attentions have recently
been paid to the design and analysis of non-covalent systems,
formed by hydrogen bonds,6 salt bridges,7 metal–ligand bonds8 to
assemble donors and acceptors for energy transfers or charge
separations. The assembling of carboxylate-porphyrins, which can
bind cations, have widely been used as a convenient mean of
forming noncovalent systems, including expanded porphyrins.9
The use of the unsaturated metal clusters M3(dppm)3(CO)2+
(M32+; M = Pd, Pt; dppm = Ph2PCH2PPh2)10 has not attracted
attention in the context of energy transfer until recently.11 These
dicationic clusters, which also bind carboxylates via ionic inter-
actions (as evidenced by the PdꢀꢀꢀO separations measured by
X-ray diffraction techniques for Pd3(dppm)3(CO)(O2CCF3)+ that
are too long to be coordination bonds but range in the typical
window for ionic interactions),10 are attractive because of a
‘‘picket fences’’ like cavity built with dppm-phenyl groups (Fig. 1).
Nature uses the antenna effect where many energy donors, like
chlorophylls, capture light and transport this excitation energy
towards the special pair placed inside the reaction centre protein
prior to the primary photo-induced electron transfer.1 In the
depending on the dppm-conformations. Right: drawing of the
Pd3(dppm)3(CO)2+ cluster.
J integral which is the surface between the donor fluorescence
intensity and the absorptivity of the acceptor.12
The experimental verification of the relationship between kET
and the donor–acceptor separation for zinc-porphyrin as the
donor has been confirmed by us,13 but to the best of our
knowledge not for this integral also called J. If the number of
donors increases, as Nature strategically does, then J increases,
but this should also be true for the acceptor. One easy way to
increase the number of acceptors is to employ supramolecular
assemblies organized via electrostatic interactions placing the
donor and acceptors in a predictable manner. At the same time,
one ensures that the energy transfer event takes place via a
through space process (not through bond), as in Nature.
We now wish to report the design of three supramolecular
donor-(acceptor)n systems (n = 1, 2, 4), MCO2ZnP-(Pd32+),
DCO2ZnP-(Pd32+)2 and TCO2ZnP-(Pd32+)4 where the tetra-
phenyl-(zinc)porphyrin and cluster act as S1 and T1 energy
donor and acceptor, respectively (Scheme 1). kET increases by
2+
about an order of magnitude going from MCO2ZnP-(Pd3
2+
)
to DCO2ZnP-(Pd32+)2, and from DCO2ZnP-(Pd3
)
to
2
TCO2ZnP-(Pd32+)4. See ESI for experimental details.
singlet state, the Forster theory states that the rate for energy
¨
2+
The host–guest interactions between carboxylates and Pd3
transfer, kET, is a function of the radiative rate constant of the
donor, the relative orientation factor of the transition moments
of the donor vs. acceptor, the donor–acceptor separation, and the
was previously established from X-ray crystallography.10a More-
over, the 1 : 1 binding constant, K11, between MCO2ZnP and
Pd32+ (MCO2ZnP + Pd32+ 2 MCO2ZnP-(Pd32+)) was also
a
´
Departement de Chimie, Universite de Sherbrooke,
2550 Boulevard de l’Universite, Sherbrooke, Quebec,
´
´
Canada J1K 2R1. E-mail: Pierre.Harvey@USherbrooke.ca;
Fax: 001-819-821-8017; Tel: 001-819-821-7092
b Institut de Chimie Mole´culaire de l’Universite´ de Bourgogne
´
(ICMUB,UMR 5260), Universite de Bourgogne, Dijon, France
w This article is part of a ChemComm ‘Supramolecular Chemistry’
web-based themed issue marking the International Year of Chemistry
2011.
z Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c1cc11174d
Scheme 1
c
6072 Chem. Commun., 2011, 47, 6072–6074
This journal is The Royal Society of Chemistry 2011

Fig. 2 Space filling model for TCO2ZnP-(Pd32+)4.
measured using UV-vis spectroscopic methods and these
interactions are found to be strong and reversible (K11 B 2 ꢁ
104
M
ꢂ1).11 The binding constants, K1n, for DCO2ZnP-
2+
2+
2+
(Pd3
)
and TCO2ZnP-(Pd3
2+
) (i.e. DCO2ZnP+2 Pd3
4
2+
2
2 DCO2ZnP-(Pd3
)
and TCO2ZnP + 4 Pd3
2
2
TCO2ZnP-(Pd32+)4) were measured and are also in the same
range (K1n B 2 ꢁ 104 Mꢂ1; see ESIz for spectral changes, Scott
and Scatchard graphs)14 indicating that K1n for all clusters and
sites is about the same. This means that the binding of the first
cluster does not induce steric hindrance to the binding of the
2+
Fig. 3 Top: UV-vis spectra for the addition of Pd3 (as PF6 salt)
into a TCO2ZnP 0.25 ꢁ 10ꢂ5 M methanol solution. Curves D–Z are
ꢂ
obtained with successive additions of 0.1 ml of 7 ꢁ 10ꢂ5 M Pd3
2+
next ones. Careful attempts to detect other isosbestic points in
2+
solution. Bottom: spectral shift of the Pd32+ band as a function of the
Pd32+/TCO2ZnP ratio.
the spectra upon the change in DCO2ZnP/Pd3
and
2+
TCO2ZnP/Pd3 ratio failed, clearly indicating that the binding
sites are relatively electronically and sterically independent.
Indeed, modeling was used in the absence of X-ray structure.
Using X-ray data for Oꢀ ꢀ ꢀPd distances for related adducts,10a
a model for TCO2ZnP-(Pd32+)4 was built showing absence of
2+
2+
steric Pd3 ꢀ ꢀ ꢀPd3 interactions (Fig. 2).
MALDI-TOF and ESI-TOF also failed to confirm the mass of
2+
these assemblies. Instead, the Pd3
UV-vis spectra were
monitored when it is added to a DCO2ZnP and a TCO2ZnP
2+
2+
solution. The lmax for the free Pd3 and RCO2-Pd3 adducts
are B487 and B467 nm, respectively. As the cluster is added,
adducts are formed and the resulting band is located at B467 nm.
When the sites are saturated, the excess addition of Pd32+ to the
solution adds a new feature at B487 nm resulting in a gradual
Fig. 4 Comparison of the UV-vis absorption spectrum of Pd32+ and
fluorescence spectra of DCO2ZnP and TCO2ZnP in MeOH at 298 K.
shift of the spectral envelope in the 465–490 nm range. The graph
2+
of lmax vs. the number of Pd3
equivalents for TCO2ZnP-
(Pd32+)4 (Fig. 3) gives a flat line for the number of equivalents
lower than 4, and increases after this saturation point (see ESIz for
DCO2ZnP-(Pd32+)2). At large excesses, lmax is 467 nm as
expected. When the number of equivalents is 8 (meaning that
2+
the relative amount of free and associated Pd3 is nearly the
same), lmax is precisely placed halfway between 467 and 487 nm.
2+
The Pd3
UV-vis band spreads to 650 nm, whereas the
Fig. 5 Comparison of the fluorescence spectra of TCO2ZnP and
TCO2ZnP-(Pd32+)4 in a 2MeTHF/MeOH mixture (20 : 1) at the same
concentration at 298 K.
0–0 peak of DCO2ZnP (see ESIz) and TCO2ZnP are seen at
B595 nm placing the cluster and the porphyrin unit as the
energy donor and acceptor, respectively. The Pd3 absorption
2+
and DCO2ZnP and TCO2ZnP fluorescences exhibit an overlap
suitable for singlet energy transfer (Fig. 4).
DPS spacer (diphenylthiophene, Cmeso–Cmeso distance = 6.33 A)
with phenyl-etio-porphyrin (P(2H); Scheme 2).15 Indeed, these
show no changes between P(2H) and DPS(4H) and between
DPS(4H) and DPS(Pd,2H) indicating that both ic and isc are
not affected by these structure modification at this distance.
Evidence for fluorescence quenching (and phosphorescence
at 77 K; ESIz) of the central zinc-porphyrin is observed when
2+
ꢂ
Pd3 clusters bind the ꢂCO2 sites (Fig. 5; see the ESIz for
the other examples). Quenching by intersystem crossing, isc,
and internal conversion, ic, are most unlikely. A convincing
argument comes from the comparison of the photophysical
data of the free base in cofacial bis-etio-porphyrins using the
2+
For comparison purposes, the Cmeso–Pd3 distance is 9.0 A
in the TCO2ZnP-(Pd32+)4 model (Fig. 2).
This quenching is therefore due to singlet energy transfer
2+
from the porphyrin to Pd3 and kET (Table 1) is determined
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6072–6074 6073

o
Table 2 tT, tT and triplet kET(T1) data at 298 K
tTo/ms
tT/ms
kET/sꢂ1
2.1 ꢁ 104
1.1 ꢁ 105
5.6 ꢁ 107
2+
MCO2ZnP
DCO2ZnP
TCO2ZnP
125
603
617
MCO2ZnP-(Pd3
2+
DCO2ZnP-(Pd32+
)
35
8.8
0.072
)
)
2
TCO2ZnP-(Pd3
4
2+
magnitude per Pd3 complexation. Since the Forster theory
¨
Scheme 2
Table 1 tF, tF and singlet kET(S1) data at 298 and 77 K
does not apply to triplet energy transfer (but the Dexter
double electron exchange theory does),1,13 the J integral alone
cannot explain the very large acceleration for kET(S1).
While the exciton energy migration across energy donors is
well known,1 this same exciton behaviour for acceptors is not.
Is there an exciton effect on kET? This needs to be explored in
detail in order to explain this current impressive acceleration
in kET with the number of acceptors.
o
tF/ns
kET/nsꢂ1
298 K
77 K
298 K
77 K
MCO2ZnP
1.78 ꢃ 0.01
1.74 ꢃ 0.01
1.83 ꢃ 0.05
0.78 ꢃ 0.05
1.65 ꢃ 0.02
0.17 ꢃ 0.02
1.59 ꢃ 0.01
1.10 ꢃ 0.25
2.85 ꢃ 0.06
0.29 ꢃ 0.05
2.19 ꢃ 0.01
o0.1
—
0.016
—
0.73
—
—
0.28
—
3.1
—
410
2+
MCO2ZnP-(Pd3
DCO2ZnP
DCO2ZnP-(Pd3
TCO2ZnP
TCO2ZnP-(Pd3
)
2+
)
PDH thanks the Natural Sciences and Engineering
Research Council of Canada for funding and the Agence
National de la Recherche for the grant of a Research Chair
of Excellence.
2
2+
)
5.3
4
according to kET = (1/te) ꢂ (1/teo) where te and teo are the
emission lifetimes of the donor in the presence and absence of
energy acceptor, respectively.1 First, kET(S1) increases at 77 K
contrasting with previous data on cofacial bis-etio-porphyrins
similar to that shown in Scheme 2.13 Here, the donor–acceptor
systems are help by electrostatic interactions whereas the cofacial
bis-etio-porphyrins are more rigidly held by covalent bonds with
Notes and references
1 (a) P. D. Harvey, C. Stern and R. Guilard, in Handbook of
Porphyrin Science With Applications to Chemistry, Physics,
Materials Science, Engineering, Biology and Medicine, ed.
K. M. Kadish, K. M. Smith and R. Guilard, World Scientific
Publishing, Singapore, 2011, pp. 1–180; (b) P. D. Harvey, in
The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, San Diego, 2003, vol. 18, ch, 113.
2 E. Baranoff, J.-P. Collin, L. Flamigni and J.-P. Sauvage, Chem.
Soc. Rev., 2004, 33, 147.
3 (a) J. S. Connolly and J. R. Bolton, in Photoinduced Electron
Transfer, ed. M. A. Fox and M. Chanon, Elsevier, Amsterdam,
The Netherlands, 1988, part D, pp. 303–393; (b) S. Fukuzumi and
D. M. Guldi, in Electron Transfer in Chemistry, ed. V. Balzani,
Wiley-VCH, Weinheim, Germany, 2001, vol. 2, pp. 270-337.
4 L. Sanchez, N. Martin and D. M. Guldi, Angew. Chem., Int. Ed.,
2005, 44, 5374.
5 F. D’Souza, S. Gadde, Amy L. Schumacher, Melvin E. Zandler,
Atula S. D. Sandanayaka, Y. Araki and O. Ito, J. Phys. Chem. C,
2007, 111, 11123.
6 (a) P. J. F. de Rege, S. A. Williams and M. J. Therien, Science, 1995,
269, 1409; (b) J. L. Sessler, B. Wang, S. L. Springs and C. T. Brown, in
Comprehensive Supramolecular Chemistry, ed. Y. Murakami,
Pergamon Press, Oxford, 1996, vol. 4, pp. 311–336;
2+
the spacer. Upon cooling, the RCO2ꢂꢀꢀꢀPd3 interactions get
stronger and the assemblies are in a more rigid environment.
Second, kET(S1) increases going from MCO2ZnP-(Pd32+) to
DCO2ZnP-(Pd32+)2 to TCO2ZnP-(Pd32+)4. This trend appears
to be due to the overlap integral J. As the number of acceptors
increases, J is bound to increase. However, one expects a B2- and
B4-fold increase in kET(S1), respectively, but these are much
larger. In an attempt to shine some light onto this question,
the triplet state behaviour was studied as well. Because the
donor porphyrin is not phosphorescent at 298 K, transient
absorption spectroscopy was used to extract kET(T1) (Fig. 6 for
DCO2ZnP-(Pd32+)2 for example and ESIz).
2+
The transient absorption spectra for DCO2ZnP-(Pd3
)
2
and DCO2ZnP are identical meaning that the transient
(c) J. M. Hodgkiss, N. H. Damrauer, S. Presse, J. Rosenthal and
´
Daniel G. Nocera, J. Phys. Chem. B, 2006, 110, 18853.
T1–Tn signal belongs to the central unit. The T1 lifetimes
obtained from the transient decays indicate a large decrease
2+
7 (a) J. L. Sessler, E. Karnas, S. K. Kim, Z. P. Ou, M. Zhang, Karl
M. Kadish, K. Ohkubo and S. Fukuzumi, J. Am. Chem. Soc.,
2008, 130, 15256; (b) B. Grimm, E. Karnas, M. Brettreich,
K. Ohta, A. Hirsch, Dirk M. Guldi, T. Torres and J. L. Sessler,
J. Phys. Chem. B, 2010, 114, 14134.
8 (a) A. Prodi, Cornelis J. Kleverlaan, M. Teresa Indelli and
Franco Scandola, Inorg. Chem., 2001, 40, 3498; (b) P. J. Stang,
J. Fan and B. Olenyuk, Chem. Commun., 1997, 1453.
upon complexation with Pd3
o
(Table 2). The kET(T1)’s
(kET = (1/te) ꢂ (1/te )) show a large increase going from
2+
MCO2ZnP-(Pd32+) to DCO2ZnP-(Pd3
)
to TCO2ZnP-
2
(Pd32+)4. Again, kET increases by approximately an order of
9 V. Kral, Stacy L. Springs and J. L. Sessler, J. Am. Chem. Soc.,
´
1995, 117, 8881.
10 (a) R. Provencher, Khin T. Aye, M. Drouin, J. Gagnon,
N. Boudreault and P. D. Harvey, Inorg. Chem., 1994, 33,
3689; (b) P. D. Harvey and R. Provencher, Inorg. Chem., 1993, 32, 61.
11 S. M. Aly, C. Ayed, C. Stern, R. Guilard, A. S. Abd-El-Aziz and
P. D. Harvey, Inorg. Chem., 2008, 47, 21.
12 T. Forster, Ann. Phys., 1948, 2, 55.
¨
13 S. Faure, C. Stern, R. Guilard and P. D. Harvey, J. Am. Chem.
Soc., 2004, 126, 1253.
14 K. A. Connors, Binding Constants: The Measurements of
Molecular Complex Stability, J. Wiley & Sons, New York, 1987.
15 S. Faure, C. Stern, E. Espinosa, R. Guilard and P. D. Harvey,
Chem.–Eur. J., 2005, 11, 3469.
Fig. 6 Transient spectrum of DCO2ZnP-(Pd32+)2 in methanol at 298 K
exc = 355 nm. The pump and probe pulse delay times are in the inset.
l
c
6074 Chem. Commun., 2011, 47, 6072–6074
This journal is The Royal Society of Chemistry 2011