Fast energy transfer within a self-assembled cyclic porphyrin tetramer

Article abstract of DOI:10.1039/b718628b

The structure of a cyclic self-assembled tetramer of an asymmetric meso-ethynylpyridyl-functionalized Zn(ii)-porphyrin was established by solution-phase X-ray scattering and diffraction; femtosecond transient absorption and anisotropy spectroscopies were used to (a) observe rapid energy transfer (3.8 ps^-1) between porphyrin subunits and (b) establish that the transfer occurs between adjacent units. The Royal Society of Chemistry.

Full text of DOI:10.1039/b718628b

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Fast energy transfer within a self-assembled cyclic porphyrin tetramerw
Rebecca A. Jensen,ab Richard F. Kelley,ab Suk Joong Lee,ab Michael R. Wasielewski,*ab
Joseph T. Hupp*ab and David M. Tiedebc
Received (in Berkeley, CA, USA) 3rd December 2007, Accepted 27th February 2008
First published as an Advance Article on the web 20th March 2008
DOI: 10.1039/b718628b
The structure of a cyclic self-assembled tetramer of an asym-
metric meso-ethynylpyridyl-functionalized Zn(II)–porphyrin was
established by solution-phase X-ray scattering and diffraction;
femtosecond transient absorption and anisotropy spectroscopies
were used to (a) observe rapid energy transfer (3.8 psÀ1) between
porphyrin subunits and (b) establish that the transfer occurs
between adjacent units.
and orientation between the donor and acceptor transition
dipole moments and the overlap of donor absorption and
acceptor emission spectra.8 Adjustments of these parameters
are achieved through modification of porphyrin unit and
assembly schemes for the optimization of energy capture and
transfer. Here, meso-ethynylpyridyl-functionalized Zn(II)–
porphyrin (EPZn), synthesized in 31% yield by Sonogashira
coupling of 4-ethynylpyridine and meso-bromo Zn(II)–
porphyrin (see ESIw), is used as an ensemble building block.
The resulting supramolecular complex has been characterized
Porphyrins have often been employed as light-harvest-
ers—both in bio-inspired model systems1 and in energy-con-
verting photoelectrochemical systems.2 Their appeal stems
from their large extinction coefficients and relative ease of
synthesis and modification, as well as their structural similarity
to naturally occurring pigments such as chlorophylls and
chlorins.3 Particularly interesting, for both fundamental and
applied reasons, are ensembles of porphyrins.4 From an ap-
plications perspective, the attachment of multi-porphyrin
structures to electrode surfaces, as opposed to single, non-
aggregated chromophores, may be useful for controlling dye
spacing and orientation and for modulating dye coverage and
dye-layer porosity.5 Additionally, if energy transfer (EnT)
between porphyrins is sufficiently rapid, ensembles can func-
tion as light-harvesting antennae in much the same fashion as
in natural photosynthetic systems.6 Here we report on EnT
within a photo-excited porphyrin ensemble comprising a self-
assembling cyclic tetramer. The solution-phase structure of the
tetramer is established via in situ X-ray scattering and diffrac-
tion measurements. We demonstrate that very small structural
changes, relative to previously investigated tetramers,7 lead to
substantially faster EnT. Additionally, we show that by com-
bining data from polarized transient absorption experiments
with data from singlet–singlet annihilation experiments, the
relative importance of EnT for adjacent versus parallel por-
phyrin subunits can be determined.
1
by solution-phase UV-Vis and fluorescence spectroscopy, H
NMR spectrometry, small-angle X-ray scattering (SAXS), and
wide-angle X-ray scattering (WAXS).
The tetrameric cyclic ensemble proposed for EPZn sponta-
neously assembles by coordination of substituent-ethynylpyr-
idyl nitrogens and porphyrin Zn(II) sites (Scheme 1). The
ground-state electronic absorption spectrum of EPZn in dry
toluene was observed to red-shift with increasing concentra-
tion up to 110 mM with Soret, Qx, and Qy bands at 447, 575,
and 629 nm, respectively (ESIw Fig. S2). The absorbance of
Zn–porphyrin is known to red-shift upon coordination of the
central porphyrin Zn(II) metal with electron-donating ligands,
such as pyridine.9 In dry toluene, the EPZn ethynylpyridyl
nitrogen is the only donor available for binding, and the
observed red-shift in absorbance is attributed to the formation
of a multi-porphyrin species. The Qx and Qy band positions at
110 mM are 15 and 25 nm to the red of those of a self-
assembled Zn–porphyrin cyclic tetramer featuring direct at-
tachment of pyridyl groups to meso porphyrin sites.7 In
addition, considerable intensification of the lower-energy Qy
band is seen relative to the Qx band. This is a consequence of
the greater degree of symmetry reduction introduced by the 4-
ethynylpyridyl substituent in EPZn which makes the normally
EnT within self-assembled porphyrin arrays generally oc-
curs via Forster resonance energy transfer (through-space
¨
transfer), the efficiency of which is determined by the distance
a Department of Chemistry, Northwestern University, 2145 Sheridan
Road, Evanston, IL 60208, USA. E-mail: j-hupp@northwestern.edu.
E-mail: m-wasielewski@northwestern.edu; Fax: +1-847-467-1425;
Tel: +1-847-491-3504
b Argonne-Northwestern Solar Energy Research (ANSER) Center,
Northwestern University, 2145 Sheridan Road, Evanston, IL 60208,
USA
c Chemistry Division, Argonne National Laboratory, Argonne, IL
60439, USA
w Electronic supplementary information (ESI) available: Details of
synthesis, characterization, and additional transient absorption and
anisotropy data. See DOI: 10.1039/b718628b
Scheme 1 Assembly of EPZn cyclic tetramer in a non-coordinating
solvent. Solubilizing groups of the space-filling tetramer model (right)
have been omitted for clarity.
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forbidden Q bands, especially the Qy band, more allowed.10
The red shift and increase in oscillator strength of the lowest
energy transition enhance the overlap of the absorption and
emission spectra (Fig. 1).
Additional evidence for self-assembly is provided by com-
1
parisons of EPZn H NMR spectra in non-coordinating and
coordinating solvents (ESIw Fig. S3). In toluene-d8, coordina-
tion between the ethynylpyridyl nitrogen and Zn(II) places the
a and b pyridyl-protons within the ring current of the co-
ordinated porphyrin, resulting in shielded proton resonances
(5.42 and 2.69 ppm).11 Upon addition of 10 molar equivalents
of pyridine-d5, the structure disassembles, and the pyridyl-
proton resonances shift downfield (9.83 and 7.36 ppm).
Definitive evidence for the proposed tetrameric ensemble
was obtained from solution-phase SAXS measurements car-
ried out over a q range of 0.01 to 0.43 AÀ1. In the low q region
(q o 0.2 AÀ1), the scattering intensity follows the Guinier
relation that is parameterized in terms of the forward scatter-
ing amplitude, I(0), and the radius of gyration, Rg:12
Fig. 2 Experiment scattering intensity, I(q), versus scattering vector,
q, data for EPZn in dry toluene. Inset: Scattering intensity versus q2.
Guinier fit to the data is also shown.
2
tained using the X-ray scattering fitting program GNOM,14
matched with peaks at 13.6 and 19.4 A that correspond to the
distance between adjacent and diagonal porphryin units, respec-
tively (Fig. 3). These values are B6% larger than the Zn–Zn
distances of the tetramer model (12.9 and 18.2 A) due to electron
density contributions from the porphyrin rings and substituents.
The sinusoidal behavior observed in the experimental PDF plot
around 23 A is due to truncation artifacts of the Fourier trans-
form. Model Zn–Zn distances are displayed in the PDF derived
from only the Zn-metal atoms in the model (ESIw Fig. S5).
Transient absorption data were collected for EPZn in dry
toluene and in pyridine following excitation with 632 nm,
120 fs laser pulses. The decay kinetics of 1*EPZn in dry
toluene were biexponential, with t = 3.2 Æ 0.2 ps and t =
2.5 ns (Fig. 4). In pyridine, the decay kinetics of 1*EPZn were
monoexponential with t = 2.5 ns (ESIw Fig. S6). The 2.5 ns
decay component in both dry toluene and pyridine matches
the measured fluorescent lifetime in pyridine (ESIw Fig. S8).
I(q) = I(0)exp(Àq2Rg /3)
(1)
Experimental Rg values were determined from least squares
fits to the Guinier scatter plots. In dry toluene, the Rg value for
EPZn is 10.5 Æ 0.2 A (Fig. 2). This value decreased with the
addition of 2 molar equivalents of pyridine to 4.6 Æ 0.1 A
(ESIw Fig. S4). These results matched Rg values calculated
from the simulated scattering of geometry-optimized models13
for the tetramer (10.3 A) and pyridine-ligated monomer (4.6
A). Strain minimization clearly should favor a tetramer over
other cyclic structures. Nevertheless, scattering was also mod-
eled for hypothetical trimer and pentamer assemblies; these
gave Rg values of 8.3 A and 11.6 A, respectively.
In addition to scattering, by extending the X-ray studies to
wide-angle (large q: 0.11 to 2.37 AÀ1), WAXS diffraction data
were obtained. There data are most conveniently interpreted via
atomic pair distribution (PDF) analyses, in either real or recipro-
cal space. PDF plots are often dominated by heavy atoms (in this
case, zinc atoms). Experimental and tetramer-model PDFs, ob-
Fig. 1 Ground-state electronic absorption spectrum of EPZn in
pyridine (blue) with peak positions: 444 nm (Soret), 573 nm (Qx),
625 nm (Qy), and excited-state electronic emission spectrum of EPZn
in pyridine (red) with 553 nm excitation and total intensity normalized
to unity, with peak positions: 634 nm (Qy) and 688 nm.
Fig. 3 Experiment (black) and tetramer model (red) pair-distribution
plots, calculated from corresponding X-ray scattering intensity versus
q (AÀ1) plots. Inset: EPZn tetramer model (solubilizing groups
omitted) with adjacent and diagonal Zn–Zn distances indicated.
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porphyrin units by B2.6 A, but also enhances the strength of
the transition dipole moment along the direction of the
ethynylpyridyl substituent. The porphyrin symmetry is re-
duced which enhances the overlap of absorption and emission
spectra. A similar EnT enhancement was observed within a
cyclic chlorophyll tetramer,17 where the rate of energy transfer
(1.2 Æ 0.1 ps) was observed to be B20 times faster than that of
the meso-pyridyl Zn(II)–porphyrin tetramer. This difference
was attributed to the larger transition dipole moment found in
chlorophyll as compared to porphyrin. Further increasing the
conjugation within the porphryin tetramer units and/or vary-
ing the transition dipole moment orientations within the
assembly may result in even faster EnT rates. We are currently
designing new porphyrin systems to investigate this possibility.
We thank the Office of Science, US Department of Energy
for support of our work (Grants DE-FG02-87ER13808 and
DE-FG02-99ER14999 to Northwestern and contract W-31-
109-ENG-38 to the Argonne National Laboratory). We are
especially appreciative of the expert help of the Sector 12 staff
at the Advanced Photon Source.
Fig. 4 Transient absorption spectra of EPZn in dry toluene following
excitation with 632 nm, 120 fs laser pulses. Inset: power dependent
transient absorption kinetics of 1*EPZn monitored at 466 nm using
1.00 (black), 0.66 (red), and 0.33 (green) mJ per 632 nm excitation pulse.
The amplitude of the 3.2 ps decay component in dry toluene
was found to be laser power dependent, indicative of single-
t–singlet annihilation (inset Fig. 4). The following formula
relates the annihilation lifetime (ta) to the EnT lifetime (th) for
cyclic arrays made up of N chromophores:15
Notes and references
1 (a) H. Imahori, Y. Sekiguchi, Y. Kashiwagi, T. Sato, Y. Araki, O. Ito,
H. Yamada and S. Fukuzumi, Chem.–Eur. J., 2004, 10, 3184; (b) A.
Harriman, Angew. Chem., Int. Ed., 2004, 43, 4985; (c) M. J. Ahrens, R.
F. Kelley, Z. E. X. Dance and M. R. Wasielewski, Phys. Chem. Chem.
Phys., 2007, 9, 1469–1478; (d) R. F. Kelley, W.-S. Shin, B. Rybtchinski
and M. R. Wasielewski, J. Am. Chem. Soc., 2007, 129, 3173–3181.
2 (a) Y. Tachibana, S. A. Haque, I. P. Mercer, J. R. Durrant and D.
R. Klug, J. Phys. Chem. B, 2000, 104, 1198–1205; (b) F. Odobel, E.
Blart, M. Lagree, M. Villieras, H. Roujtita, N. E. Murr, S.
Caramori and C. A. Bignozzi, J. Mater. Chem., 2003, 13,
502–510; (c) W. M. Campbell, A. K. Burrell, D. L Officer and K.
W. Jolley, Coord. Chem. Rev., 2004, 248, 1363–1379.
3 L. R. Milgrom, The Colours of Life, Oxford University Press,
Oxford, UK, 1997.
4 K. Ogawa and Y. Kobuke, J. Photochem. Photobiol. C: Photo-
chem. Rev., 2006, 7, 1–16.
5 (a) A. B. F. Martinson, A. M. Massari, S. J. Lee, R. W. Gurney, K.
E. Splan, J. T. Hupp and S. T. Nguyen, J. Electrochem. Soc., 2006,
153(3), A527–A532; (b) A. M. Massari, R. W. Gurney, C. P.
Schwartz, S. T. Nguyen and J. T. Hupp, Langmuir, 2004, 20,
4422–4429; (c) K. E. Splan, A. M. Massari and J. T. Hupp, J. Phys.
Chem. B, 2004, 108, 4111–4115; (d) J. Libera, R. Gurney, C.
Schwartz, H. Jin, T.-L. Lee, S. T. Nguyen, J. T. Hupp and M.
Bedzyk, J. Phys. Chem. B, 2005, 109, 1441–1450.
ta = th/(8 sin2(p/2N))
(2)
Using this model, N = 4, and the measured ta, an EnT lifetime
of 3.8 Æ 0.2 ps was determined.
In the limit where the transition dipole moment becomes
completely polarized along the ethynylpyridyl axis, the moments
of adjacent porphyrins would be essentially orthogonal, with the
dynamics of EnT greatly decreased. Under those conditions, EnT
conceivably could be dominated instead by dipolar coupling
between parallel porphyrins, i.e. porphyins comprising opposite
sides of a tetrameric assembly. Disfavoring the coupling, however,
would be the comparatively large separation distance between the
candidate energy donor and acceptor. To differentiate between
the two possible pathways of EnT, transient absorption aniso-
tropy measurements were conducted. The depolarized nature of
the experiment allows only EnT between the perpendicularly
oriented adjacent EPZn tetramer units to be observed. The
transient absorption depolarization lifetime (tdepolarization) is re-
lated to the EnT lifetime by the following equation:16
6 G. McDermott, S. M. Prince, A. A. Freer, A. M.
Hawthornthwaite-Lawless, M. Z. Papiz, R. J. Cogdell and N. W.
Isaacs, Nature, 1995, 374, 517.
7 (a) M. M. Yatskou, R. B. M. Koehorst, H. Donker and T. J.
Schaafsma, J. Phys. Chem. A, 2001, 105, 11425–11431; (b) M. M.
Yatskou, R. B. M. Koehorst, A. van Hoek, H. Donker, T. J.
Schaafsma, B. Gobets, I. van Stokkum and R. van Grondelle, J.
Phys. Chem. A, 2001, 105, 11432–11440.
th = 4(1 À cos2 a)tdepolarization
(3)
where a is the angle between the donor and acceptor species.
The value of tdepolarization obtained for EPZn in dry toluene
was 1.1 Æ 0.1 ps, revealing an EnT lifetime of 4.4 Æ 0.4 ps
(ESIw Fig. S7). This matches (within experimental error) the
EnT lifetime result obtained from the annihilation experiment,
demonstrating that EnT occurs chiefly between adjacent units
of the EPZn cyclic tetramer.
8 J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer,
Dordrecht, 1999.
9 (a) P. N. Taylor and H. L. Anderson, J. Am. Chem. Soc., 1999, 121,
11538–11545; (b) H. L. Anderson, Inorg. Chem., 1994, 33, 972–81.
10 M. Gouterman, J. Phys. Chem., 1959, 30, 1139–1161.
11 G. S. Wilson and H. L. Anderson, Chem. Commun., 1999, 1539–1540.
12 A. Guinier and G. Fournet, Small-Angle Scattering of X-rays,
Wiley, New York, 1955.
13 Hyperchem, version 5.02, Hypercube Inc., Gainesville, FL, 1997.
14 D. I. Svergun, J. Appl. Crystallogr., 1992, 25, 495–503.
15 G. Trinkunas, J. Lumin., 2003, 102–103, 532–535.
16 S. E. Bradforth, R. Jimenez, F. van Mourik, R. van Grondelle and
G. R. Fleming, J. Phys. Chem., 1995, 99, 16179–16191.
17 R. F. Kelley, R. H. Goldsmith and M. R. Wasielewski, J. Am.
Chem. Soc., 2007, 129, 6384–6385.
The rate of EnT within the EPZn cyclic self-assembled
tetramer is B8 times faster than that reported for a closely
analogous Zn(II)–porphyrin tetramer with direct meso-pyridyl
functionalization.7 Introduction of an ethynyl group between
the porphyin and pyridyl rings increases the distance between
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This journal is The Royal Society of Chemistry 2008
1888 | Chem. Commun., 2008, 1886–1888