Self-assembled zinc chlorin rod antennae powered by peripheral light-harvesting chromophores

Article abstract of DOI:10.1021/ja710253q

The multichromophoric dyads 1, 2 and triad 3 have been synthesized by coupling of the appropriately functionalized chlorin derivative with naphthalene diimide dyes through esterification, and subsequent metalation of the chlorin center with zinc acetate. The self-assembly properties of naphthalene diimide (NDI)-zinc chlorin (ZnChl) dyads 1, 2 and triad 3 have been studied in nonpolar, aprotic solvents by UV-vis, CD, and steady-state emission spectroscopy, revealing formation of rod-like structures by noncovalent interactions of zinc chlorin units, while the appended naphthalene diimide dyes do not aggregate at the periphery of the rod antennae. In all these systems, photoexcitation of the enveloping naphthalene diimides at 540 and 620 nm, respectively, leads to highly efficient energy-transfer processes (FRET; φ_ET ≥ 0.99) to the inner zinc chlorin backbone, as explored by time-resolved fluorescence spectroscopy on the picosecond time scale. The efficiencies of zinc chlorin rod aggregates for the harvesting of solar light are markedly increased from 26% for dyad 2 up to 63% for triad 3, compared to the LH capacity of the monochromophoric aggregates of model system ZnChl 6a. Thus, with the self-assembled zinc chlorin rod antenna based on triad 3, a highly efficient artificial LH system has been achieved.

Full text of DOI:10.1021/ja710253q

Published on Web 04/05/2008
Self-Assembled Zinc Chlorin Rod Antennae Powered by
Peripheral Light-Harvesting Chromophores
Cornelia Röger,† Yuliya Miloslavina,‡ Doris Brunner,† Alfred R. Holzwarth,*,‡ and
Frank Würthner*,†
Contribution from the UniVersität Würzburg, Institut für Organische Chemie and Röntgen
Research Center for Complex Material Systems, Am Hubland, D-97074 Würzburg, Germany,
and Max-Planck-Institut für Bioanorganische Chemie, Postfach 101365, D-45470 Mülheim an
der Ruhr, Germany
Received November 12, 2007; E-mail: wuerthner@chemie.uni-wuerzburg.de; holzwarth@mpi-muelheim.mpg.de
Abstract: The multichromophoric dyads 1, 2 and triad 3 have been synthesized by coupling of the
appropriately functionalized chlorin derivative with naphthalene diimide dyes through esterification, and
subsequent metalation of the chlorin center with zinc acetate. The self-assembly properties of naphthalene
diimide (NDI)-zinc chlorin (ZnChl) dyads 1, 2 and triad 3 have been studied in nonpolar, aprotic solvents
by UV–vis, CD, and steady-state emission spectroscopy, revealing formation of rod-like structures by
noncovalent interactions of zinc chlorin units, while the appended naphthalene diimide dyes do not aggregate
at the periphery of the rod antennae. In all these systems, photoexcitation of the enveloping naphthalene
diimides at 540 and 620 nm, respectively, leads to highly efficient energy-transfer processes (FRET; φET
g 0.99) to the inner zinc chlorin backbone, as explored by time-resolved fluorescence spectroscopy on the
picosecond time scale. The efficiencies of zinc chlorin rod aggregates for the harvesting of solar light are
markedly increased from 26% for dyad 2 up to 63% for triad 3, compared to the LH capacity of the
monochromophoric aggregates of model system ZnChl 6a. Thus, with the self-assembled zinc chlorin rod
antenna based on triad 3, a highly efficient artificial LH system has been achieved.
between the BChl molecules.2,3 However, chlorosomes take
advantage of their spartan constitution, since the tight packing
Introduction
In most of natural photosynthetic light-harvesting (LH)
complexes, sunlight is collected not only by chlorophyll (Chl)
or bacteriochlorophyll (BChl) chromophores, which are powerful
absorbers in the violet, blue, and red wavelength regions, but
also by accessory LH dyes such as phycobilines and carotenoids
to make use of the intensive green region of the solar spectrum.
In the majority of LH complexes, all the chromophores are
embedded in a rigid protein scaffold, providing a sophisticated
geometrical arrangement in order to guarantee efficient excita-
tion energy transport within the LH system.1 As an exception,
the LH complexes of green photosynthetic bacteria and of the
green filamentous bacterium Chloroflexus aurantiacus, the so-
called chlorosomes, do not contain such a variety of LH
chromophores since they lack a protein matrix to maintain a
diverse constitution.2 Chlorosomes show flattened sac-like
structures which mainly include, besides a few carotenoids,
cylindrical self-assemblies of BChl c, d, and e whose defined
arrangement is imparted merely by intermolecular forces
of BChls within the self-assembled rods provides a high
chromophore density and affords a large cross section for red/
NIR light absorption. Therefore, it is not surprising that the most
extremely low-sunlight-adapted phototrophic system known to
date is a population of green sulfur bacteria inhabiting the Black
Sea chemocline at depths of 100 m.4 Recently, another green
sulfur bacterial species was found to inhabit a deep-sea
hydrothermal vent at depths of even 2300 m, where the only
source of light is geothermal radiation.5
In recent years, it has been demonstrated that the self-
assembly process of BChl c, d, and e into rod-like structures
can be achieved, not only in ViVo but also in Vitro, by choosing
nonpolar, aprotic solvents.3a,6 Zinc chlorins (ZnChl), which are
readily available by stepwise derivatization of Chl a, serve as
appropriate model compounds for natural BChl by performing
(3) (a) Balaban, T. S.; Tamiaki, H.; Holzwarth, A. R. Top. Curr. Chem.
2005, 258, 1–38. (b) Holzwarth, A. R.; Schaffner, K. Photosynth. Res.
1994, 41, 225–233.
† Universität Würzburg.
(4) Manske, A. K.; Glaeser, J.; Kuypers, M. M. M.; Overmann, J. Appl.
EnViron. Microbiol. 2005, 71, 8049–8060.
‡ Max-Planck-Institut für Bioanorganische Chemie, Mülheim an der Ruhr.
(1) (a) Photosynthetic Light-HarVesting Systems: Organization and Func-
tion; Scheer, H.; Schneider, S., Eds.; de Gruyter: Berlin, 1988. (b)
Huber, R. Biosci. Rep. 1989, 9, 635–73. (c) Katz, J. J. Photosynth.
Res. 1990, 26, 143–160. (d) Cogdell, R. J.; Lindsay, J. G. New Phytol.
2000, 145, 167–196. (e) Holzwarth, A. R. In Series on PhotoconVer-
sion of Solar Energy; Archer, M. D., Barber, J., Eds.; Imperial College
Press: London, U.K., 2002; pp 43–115. (f) Scholes, G. D.; Fleming,
G. R. AdV. Chem. Phys. 2006, 132, 57–129.
(5) Beatty, J. T.; Overmann, J.; Lince, M. T.; Manske, A. K.; Lang, A. S.;
Blankenship, R. E.; Van Dover, C. L.; Martinson, T. A.; Plumley,
F. G. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9306–9310.
(6) (a) Bystrova, M. I.; Mal’gosheva, I. N.; Krasnovsky, A. A. Mol. Biol.
(Moscow) 1979, 13, 440–451. (b) Staehelin, L. A.; Golecki, J. R.;
Drews, G. Biochim. Biophys. Acta 1980, 589, 30–45. (c) Smith, K. M.;
Kehres, L. A. J. Am. Chem. Soc. 1983, 105, 1387–1389. (d) Worcester,
D. L.; Michalski, T. J.; Katz, J. J. Proc. Natl. Acad. Sci. U.S.A. 1986,
83, 3791–3795.
(2) Olson, J. M. Photochem. Photobiol. 1998, 67, 61–75.
9
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A R T I C L E S
Röger et al.
donating character of the core substituents.12 We have now
extended our novel concept to the newly developed triad ZnChl-
NDINN-NDINO 3 and synthesized the new dyad 1 (Chart 1) and
thoroughly investigated the self-assembly behavior of these
multichromophoric conjugates as well as energy-transfer pro-
cesses in their monomers and aggregates.
Results and Discussion
Synthesis of Dyad ZnChl-NDINO 1 and Triad ZnChl-NDINN
-
NDINO 3. The dyad ZnChl-NDINO 1 was synthesized according
to the route outlined in Scheme 1. Chlorin propionic acid 10,
which is a key compound for the synthesis of ZnChl-NDI
conjugates, was obtained by stepwise derivatization of natural
Chlorophyll (Chl) a,7c,13 the latter of which was extracted from
the cyano bacterium Spirulina platensis.13a The precursor of
all 2,6-core-disubstituted NDI dyes used in this work, namely
N,N′-bis-(2′,6′-diisopropylphenyl)-2,6-dichloronaphthalene-1,4,5,8-
tetracarboxylic acid diimide, was prepared by stepwise trans-
formations of pyrene as reported previously.12c Nucleophilic
substitution of one chlorine atom of the 2,6-dichloro-core-
substituted NDI with 6-hydroxyhexylamine according to litera-
ture provided the starting material NDI 7.11
The hydoxy group of NDI 7 was protected by reaction with
tert-butyldiphenylsilyl chloride to afford NDI 8 quantitatively
(Scheme 1, step i). In the next step, the remaining chlorine atom
was replaced by nucleophilic substitution with 1,6-hexanediol
in the presence of potassium carbonate, followed by esterifi-
cation of the newly introduced hydroxy group with 3,5-
bis(dodecyloxy)benzoic acid (step ii). Cleavage of the silyl
protecting group with TBAF gave the pink NDINO 4 (step iii;
for structure, see Chart 1). The esterification of chlorin propionic
acid 10 with the hydroxyl-functionalized NDINO 4 by using
DCC, DMAP, and DPTS in the presence of N-ethyldiisopro-
pylamine gave the precursor 11 (step iv). Selective reduction
of the aldehyde group of compound 11 was achieved with
borane-tert-butylamine complex, and the central zinc ion in
chlorin was introduced by metalation in the presence of saturated
methanolic zinc acetate solution to afford the hitherto unknown
dyad ZnChl-NDINO 1. Dyad 2 was prepared by a similar route
as reported before.11
Figure 1. Concept for the improvement of natural BChl c LH antennae
by means of additional peripheral red and blue chromophores that harvest
the green and orange fraction of solar light. FRET denotes “Förster
Resonance Energy Transfer”.
an analogous self-assembly process into rod-like structures.3a,7
BChl and ZnChl self-assemblies have drawn considerable
attention as they show very high exciton mobilities,8 which make
these aggregates potentially interesting for applications in
supramolecular photonic and electronic devices. A decisive
advancement toward the preparation of well-defined nanorods
for the development of materials based on artificial ZnChl LH
systems was achieved by functionalization of ZnChl monomers
with solubilizing alkyl chains, leading to well-soluble self-
assembled ZnChl nanorods.9
However, for an application of such supramolecular systems
in light harvesting and photovoltaics, not only well-defined
nanostructures but also optimal utilization of the terrestrial solar
spectrum is required.10 Since ZnChl aggregates are not well-
suited for absorption of the dominant green and orange regions
of solar irradiance due to the above-mentioned lack of appropri-
ate absorption bands, we intended to design multichromophoric
LH systems consisting of the cylindrical ZnChl antenna and
additional light-absorbing LH chromophores in the periphery
of the nanorod (Figure 1).
In a recent communication, we have demonstrated on the basis
of naphthalene diimide-zinc chlorin dyad 2 (Chart 1) that the
concept depicted in Figure 1 is indeed viable.11 We have chosen
2,6-core-disubstituted naphthalene diimides (NDIs) as additional
LH chromophores, because the absorption maxima of these
strong and photostable fluorophores can be easily tuned within
the whole range of the “green gap” by variation of the electron-
The synthetic route to triad 3 is outlined in Scheme 2 that
starts from NDIs 12 and 16; the latter dyes were synthesized
according to literature.12a The amino functionality of NDI 12
was protected by a tert-butylcarbonyl (BOC) group, followed
by substitution of the chlorine atom with 1,6-hexanediol,
esterification with 3,5-bis(dodecyloxy)benzoic acid, and cleav-
age of the BOC group to yield the pink colored NDI building
block 15 (steps i-iii). The blue-colored NDI building block 17
was synthesized from NDI 16 by nucleophilic substitution with
6-hydroxyhexylamine (step iv). Coupling of amine 15 with
carboxylic acid 17 in the presence of HATU gave NDINN-NDINO
18 (step v) which was subsequently esterified with chlorin
propionic acid 10 to obtain compound 19 (step vi). Reduction
of the aldehyde group in the latter compound and subsequent
metalation of the chlorin center with zinc acetate (step vii) finally
afforded the desired triad ZnChl-NDINN-NDINO 3.
(7) (a) Tamiaki, H.; Holzwarth, A. R.; Schaffner, K. J. Photochem.
Photobiol. B 1992, 15, 355–360. (b) Cheng, P.; Lidell, P. A.; Ma,
S. X. C.; Blankenship, R. E. Photochem. Photobiol. 1993, 58, 290–
295. (c) Tamiaki, H.; Amakawa, M.; Shimono, Y.; Tanikaga, R.;
Holzwarth, A. R.; Schaffner, K. Photochem. Photobiol. 1996, 63, 92–
99. (d) Hildebrandt, P.; Tamiaki, H.; Holzwarth, A. R.; Schaffner, K.
J. Phys. Chem. 1994, 98, 2191–2197.
(8) (a) Prokhorenko, V. I.; Steensgard, D. B.; Holzwarth, A. R. Biophys.
J. 2000, 79, 2105–2120. (b) Prokhorenko, V. I.; Holzwarth, A. R.;
Müller, M. G.; Schaffner, K.; Miyatake, T.; Tamiaki, H. J. Phys. Chem.
B 2002, 106, 5761–5768. (c) Psencik, J.; Ma, Y.-Z.; Arellano, J. B.;
Hála, J.; Gillbro, T. Biophys. J. 2003, 84, 1161–1179.
(9) Huber, V.; Katterle, M.; Lysetska, M.; Würthner, F. Angew. Chem.,
Int. Ed. 2005, 44, 3147–3151.
(10) (a) For reviews on supramolecular LH systems, see: Balzani, V.;
Ceroni, P.; Maestri, M.; Vicinelli, V. Curr. Opin. Chem. Biol. 2003,
7, 657–665. (b) Fréchet, J. M. J. J. Polym. Sci., Polym. Chem. Ed.
2003, 41, 3713–3725. (c) Würthner, F. Chem. Commun. 2004, 1564–
1579. (d) Hoeben, F. J. M.; Jonkheim, P.; Meijer, E. W.; Schenning,
A. P. H. J. Chem. ReV. 2005, 105, 1491–1546. (e) Wasielewski, M. R.
J. Org. Chem. 2006, 71, 5051–5066. (f) For recent articles on
multichromophoric supramolecular nanorods, see: Yamamoto, Y.;
Fukushima, T.; Suna, Y.; Ishii, N.; Saeki, A.; Seki, S.; Tagawa, S.;
Taniguchi, M.; Kawai, T.; Aida, T. Science 2006, 314, 1761–1764.
(g) Miller, R. A.; Presley, A. D.; Francis, M. B. J. Am. Chem. Soc.
2007, 129, 3104–3109.
(12) (a) Würthner, F.; Ahmed, S.; Thalacker, C.; Debaerdemaeker, T.
Chem.sEur. J. 2002, 8, 4742–4750. (b) Thalacker, C.; Miura, A.; De
Feyter, S.; De Schryver, F. C.; Würthner, F. Org. Biomol. Chem. 2005,
3, 414–422. (c) Thalacker, C.; Röger, C.; Würthner, F. J. Org. Chem.
2006, 71, 8098–8105.
(13) (a) Smith, K. M.; Goff, D. A.; Simpson, D. J. J. Am. Chem. Soc. 1985,
107, 4946–4954. (b) Kosaka, N.; Tamiaki, H. Eur. J. Org. Chem. 2004,
11, 2325–2330.
(11) Röger, C.; Müller, M. G.; Lysetska, M.; Miloslavina, Y.; Holzwarth,
A. R.; Würthner, F. J. Am. Chem. Soc. 2006, 128, 6542–6543.
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Zinc Chlorin Rod Antennae Powered by Chromophores
A R T I C L E S
Chart 1. Chemical Structures of ZnChl-NDINO 1, ZnChl-NDINN 2, ZnChl-NDINN-NDINO 3, and Reference Chromophores 4–6a
a NN and NO in compound codes donate “nitrogen, nitrogen” and “nitrogen, oxygen” substituents, respectively, at the 2,6-positions of naphthalene
diimide (NDI) unites.
The new dyad 1 and triad 3 are properly characterized by 1H
NMR and UV–vis spectroscopy and high resolution mass
spectrometry. The detailed synthetic procedures and character-
ization data, also for the key unknown intermediates, are given
in the Supporting Information.
These stacks are held together by hydrogen bonding between
the 31 hydoxy group and the 131 keto functionality, resulting
in rod-like supramolecular structures.6d,7d,8a,9 In Figure 2, such
a typical rod aggregate absorption spectrum is depicted for
ZnChl 6a (green line). In contrast, the absorption and emission
spectra of the additional antenna chromophores used in this
work, namely NDINO 4 (pink line) and NDINN 5 (blue line), in
cyclohexane/tetrachloromethane (1%) exhibit the characteristic
shapes and maxima of 2,6-core-substituted NDI monomers,
respectively.12 This observation can be explained in terms of
the bulky 2,6-diisopropylphenyl imide substituents that prevent
NDIs 4 and 5 from intermolecular π-π interactions.
To explore the self-assembly behavior of multichromophoric
dyads 1 and 2, UV–vis spectra were recorded in the polar solvent
tetrahydrofuran (THF) and in the nonpolar solvent mixture
cyclohexane/tetrachloromethane (1%). Figure 3 shows the
UV–vis absorption and normalized fluorescence spectra of dyads
1 and 2. If THF was used as a solvent, UV–vis spectra of both
dyads 1 and 2 revealed the characteristic Qy-band of ZnChl
monomers7,9 (λmax ) 647 and 646 nm, respectively), since in
this solvent metal ion coordination and hydrogen bonding
Steady-State Spectral Properties of Dyads 1 and 2 Monomers
and their Aggregates. One of the most characteristic features
of BChl c, d, e and related ZnChl dyes is that they form
J-aggregates with a pronounced bathochromic shift of the S0-S1
transition band, namely the Qy-band, of about 80–90 nm
compared to the respective monomer spectra.7,9,13 The formation
of such aggregates was observed in water, or in nonpolar aprotic
solvents such as n-hexane or cyclohexane already at a low
concentration (∼10-6 M) owing to the interplay of three
different types of noncovalent interactions between the molec-
ular building blocks.7,9 The combination of π-π stacking of
the extended aromatic core and coordination of the 31 hydroxy
group to the central metal ion of a neighboring molecule leads
to the formation of one-dimensional chromophore stacks.14
(14) Huber, V.; Lysetska, M.; Würthner, F. Small 2007, 3, 1007–1014.
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Scheme 1. Synthesis of ZnChl-NDINO 1a
a Reagents and conditions: (i) tert-Butyldiphenylsilyl chloride, imidazol, DMAP, CH2Cl2, rt, 30 min, >99% of 8; (ii) 1,6-hexanediol, K2CO3, 100 °C,
12 h; 3,5-bis(dodecyloxy)benzoic acid, DCC, DMAP, DPTS, CH2Cl2, rt, 3 h, 68% of 9; (iii) tetra-n-butylammonium fluoride (TBAF), THF, rt, 4 h, 64% of
4; (iv) DCC, DMAP, DPTS, N-ethyldiisopropylamine, CH2Cl2, Ar, rt, 24 h, 39% of 11; (v) BH3(t-BuNH2), CH2Cl2, 0 °C, 3 h; Zn(OAc)2 × 2H2O, MeOH,
THF, rt, 6 h, 50% of ZnChl-NDI 1. DCC ) dicyclohexylcarbodiimide, DMAP ) 4-(dimethylamino)pyridine, DPTS ) 4-(dimethylamino)pyridinium
4-toluenesulfonate.
between ZnChl units are prevented due to the ligation of THF
as a fifth ligand at the zinc ion and its hydrogen-bond acceptor
ability. The absorption maxima of the respective NDI units are
located in the “green gap” of the ZnChl absorption spectrum
(λmax ) 550 nm for 1 and 607 nm for 2, respectively). Both of
the NDI absorption bands match the UV–vis spectra of the
individual NDI chromophores 4 (λmax ) 547 nm, THF) and 5
(λmax ) 614 nm, THF), respectively, indicating that there are
no ground-state interactions between NDI and monomeric ZnChl
units.
The fluorescence spectra of 1 and 2 monomers do not exhibit
any NDI emission bands, although NDI reference components
4 and 5 are highly fluorescent (φFl ) 0.58 and 0.39, respectively,
in THF). Likewise, the fluorescence quantum yields of dyad 1
and 2 monomers (φFl ) 0.12 and 0.11, respectively), which were
determined by selective excitation of the respective NDI S0-S1
absorption band, are in the same range as the quantum yield of
ZnChl 6a monomers (φFl ) 0.19, in THF), suggesting almost
complete quenching of NDI emission by the appended ZnChl
chromophore.
In the nonpolar aprotic solvent system cyclohexane/tetra-
chloromethane (1%), good solubility for dyads 1 and 2 is given,
despite the fact that the fifth coordination site of the zinc ion
and the hydroxyl functionality are not well solvated. As a
consequence, self-assembly into J-aggregates takes place for
these dyads in this solvent system as revealed by a bathochromic
shift of the ZnChl Qy-band to 729 and 731 nm, respectively
(see Figure 3). It is noticed that the absorption spectra of NDI
units in 1 and 2 aggregates (λmax ) 545 and 611 nm,
respectively) are only slightly bathochromically shifted com-
pared to those of the isolated NDI chromophores 4 and 5 (λmax
) 540 and 604 nm, respectively, Figure 2). This observation
suggests that in aggregates of dyads 1 and 2 no excitonic
coupling between NDI units occurs.
Further evidence for the J-type excitonic coupling of ZnChl
units of 1 and 2 in cyclohexane/tetrachloromethane (1%) is
provided by steady-state fluorescence spectra (Figure 3). Ag-
gregated species of 1 and 2 show a very small Stokes shift of
the ZnChl Qy-band (∆λ ) 8 and 6 nm, respectively), which is
characteristic for BChl and ZnChl J-aggregates, signifying
thermal equilibration of the Qy excitonic states.2 Moreover, for
aggregates of 1 and 2, complete quenching of the fluorescence
of the respective NDI units is observed, and the quantum yields
of the emission from the ZnChl subunits have decreased to φFl
e 0.001. These results imply that not only the emission of the
NDI units is quenched but also the aggregated ZnChl species
is poorly fluorescent. Such low fluorescence quantum yields are
characteristic for ZnChl and BChl c, d, e aggregates, since
efficient quenching processes originate from the presence of
small amounts of oxidized chlorin,8b which cannot be excluded
for the present system. However, it had been demonstrated for
both chlorosomes and artificial chlorin aggregates that their LH
efficiencies are not adversely affected by these quenching
processes if effective energy acceptors are present.8b,15 For
instance, in chlorosomes the excitation energy of the rod
(15) (a) Brune, D. C.; King, G. H.; Infosino, A.; Steiner, T.; Thewalt,
M. L. W.; Blankenship, R. E. Biochemistry 1987, 26, 8652–8658. (b)
Holzwarth, A. R.; Griebenow, K.; Schaffner, K. J. Photochem.
Photobiol. A: Chem. 1992, 65, 61–71. (c) Savikhin, S.; Zhu, Y.;
Blankenship, R. E.; Struve, W. S. J. Phys. Chem. 1996, 100, 3320–
3322. (d) Miyatake, T.; Tamiaki, H.; Holzwarth, A. R.; Schaffner, K.
HelV. Chim. Acta 1999, 82, 797–810.
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Scheme 2. Synthesis of ZnChl-NDINN-NDINO 3a
a Reagents and conditions: (i) BOC2O, CH2Cl2, rt, 30 min, 87% of 13; (ii) 1,6-hexanediol, K2CO3, 80 °C, 6 h; 3,5-bis(dodecyloxy)benzoic acid, DCC,
DMAP, DPTS, CH2Cl2, rt, 3 h, 55% of 14; (iii) CF3COOH, CH2Cl2, rt, 30 min, 97% of 15; (iv) 6-aminohexanol, 80 °C, 2 h, 91% of 17; (v) HATU,
N-ethyldiisopropylamine, CH2Cl2, rt, 5 min, 62% of 18; (vi) chlorin propionic acid 10, DCC, DMAP, DPTS, N-ethyldiisopropylamine, CH2Cl2, Ar, 35 °C,
24 h, 26% of 19; (vii) BH3(t-BuNH2), CH2Cl2, 0 °C, 3 h; Zn(OAc)2 × 2H2O, MeOH, THF, rt, 16 h, 66% of ZnChl-NDINN-NDINO 3. HATU ) O-(7-
azabenzotriazole-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate.
antennae is transferred efficiently to BChl a protein complexes
in the so-called baseplate,15a–c while for artificial ZnChl LH
systems coaggregated BChl a functionalities work as efficient
energy acceptors.15d
To further elaborate the aggregation process of ZnChl-NDI
dyads 1 and 2, temperature-dependent UV–vis spectroscopic
measurements were carried out in cyclohexane/tetrachlo-
romethane (1%). Figure 4A shows the temperature-dependent
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Examination of an aggregate solution of ZnChl-NDINO 1 in
cyclohexane/tetrachloromethane (1%) by CD spectroscopy
reveals such a Cotton effect for the chlorin Qy absorption band
as well (Figure 4B). The displayed effect consists of an exciton
couplet, whose shape resembles the signals observed previously
for aggregates of ZnChls 6.9 By contrast, no Cotton effect of
the peripheral NDINO units could be detected; thus, these
chromophores do not interact with each other in the self-
assembled dyad 1 which corroborates the results of UV–vis
spectroscopy. After increasing the temperature of the aggregate
solution of 1 to 65 °C and an equilibration time of 1 h, the
intensity of the induced CD effect is diminished, indicating
dissolution of the aggregated species into monomers. On
stepwise decrease of the temperature to 25 °C in intervals of
10 °C and equilibration times of 1 h, the amplitude of the exciton
couplet rises again. This increase can be taken as further
evidence for the reversibility of the aggregation process of dyad
1 in cyclohexane/tetrachloromethane (1%). Temperature-de-
pendent CD spectroscopy of dyad 2 aggregates revealed similar
aggregation behavior as observed for dyad 1(see Figure S1 in
the Supporting Information).
Figure 2. UV–vis (solid lines) and normalized fluorescence spectra (dotted
lines) of ZnChl 6a rod aggregate in cyclohexane/tetrachloromethane (1%)/
THF (0.1%) (green lines, λex ) 700 nm), NDINO 4 in cyclohexane/
tetrachloromethane (1%) (pink lines, λex ) 510 nm), and NDINN 5 in
cyclohexane/tetrachloromethane (1%) (blue lines, λex ) 575 nm).
Steady-State Spectral Properties and Aggregation Process
of Triad 3. The absorption spectrum of triad 3 in THF exhibits
the absorption maxima of the two different NDI components at
552 and 610 nm, respectively, along with the typical ZnChl
monomer Qy-band at 647 nm (Figure 5A). In cyclohexane/
tetrachloromethane (1%) the characteristic bathochromic shift
of the Qy-band to 731 nm signifies J-type excitonic coupling of
the ZnChl units in a similar manner as observed for dyads 1
and 2 (λmax ) 729 and 731 nm, respectively). Importantly, the
“green gap” of ZnChl absorption is now effectively covered by
the absorption bands of the NDINO and NDINN units, whose
absorption maxima (λmax ) 551 and 619 nm, respectively) are
only slightly shifted compared to those of the NDINN-NDINO
component 18 (λmax ) 543 and 612 nm), indicating that there
are no significant interactions between peripheral NDI chro-
mophores within the self-assembly of triad 3. This result is
further supported by the CD spectrum of 3 aggregates (Figure
5B), which shows the characteristic bisignate Cotton effect
resembling the exciton couplet of 1 and 2 aggregates (Figure 4B
and Figure S1), but lacks any signal originating from the NDI
units.
Stationary fluorescence spectra of 3 monomers that have been
measured at the selective excitation of either the NDINN or
NDINO unit only reveal an emission band originating from the
Qy-band of ZnChl (λmax ) 648 and 704 nm), while the emission
of both NDI chromophores is completely quenched. The
fluorescence quantum yield of 3 monomers (φFl ) 0.11) is
independent of the excitation wavelength and resembles the
values obtained for 1, 2, and 6a monomers.
Aggregates of 3 show a small Stokes shift (∆λ ) 4 nm) and
are only slightly fluorescent, since the emission of both NDI
units is quenched again by the ZnChl aggregate. Moreover, the
emission spectrum of dyad 18 that was recorded at a selective
excitation of the pink NDINO unit (λex ) 520 nm) resembles
exclusively the emission spectrum of NDINN, indicating efficient
energy transfer from NDINO to the blue NDINN building block.
On the basis of the results obtained from steady-state studies
on ZnChl-NDI conjugates 1-3, one can reasonably draw the
Figure 3. (A) UV–vis (solid lines) and normalized fluorescence (dashed
lines, λex ) 510 nm) spectra of 1. Monomers in THF (black); aggregates in
cyclohexane/tetrachloromethane (1%) (red) at room temperature. (B) UV–vis
(solid lines) and normalized fluorescence (dashed lines, λex ) 530 nm)
spectra of 2. Monomers in THF (black); aggregates in cyclohexane/
tetrachloromethane (1%) (red) at room temperature.
absorption spectra of a solution of dyad 1 which was first heated
to 65 °C. After a 1 h equilibration time, the UV–vis spectrum
revealed not only a strongly decreased aggregate absorption band
at 729 nm but also an absorption maximum at 646 nm, indicating
a risen amount of monomers of 1 in the equilibrium at this
temperature. On subsequent cooling of the sample from 65 to
25 °C in 10 °C intervals and an equilibration time of again 1 h,
the Qy-band of aggregates regained its initial intensity, while
the monomer band disappears completely, confirming the
reversibility of aggregate formation. As shown in the inset of
Figure 4A, the absorption versus temperature plot provides a
sigmoidal curve with a melting temperature of about 64 °C for
the aggregates in the solvent system applied. Thus, the tem-
perature dependence of monomer-aggregate equilibria of dyad
1 properly resembles the characteristics of dyad 2.11
The self-assembly process of ZnChl dyes such as ZnChls 6
into cylindrical structures goes along with an induced circular
dichroism (CD) effect due to excitonic coupling, as the transition
dipole moments of chiral ZnChls are arranged helically.16
(16) (a) Berova, N.; Nakanishi, K. In Circular Dichroism: Principles and
Applications; Berova, N., Nakanishi, K., Woody, R., Eds.; VCH: New
York, 2000; pp 337–368. (b) Prokhorenko, V. I.; Steensgaard, D. B.;
Holzwarth, A. R. Biophys. J. 2003, 85, 3173–3186.
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5934 J. AM. CHEM. SOC. VOL. 130, NO. 18, 2008

Zinc Chlorin Rod Antennae Powered by Chromophores
A R T I C L E S
Figure 4. Temperature-dependent (A) UV–vis (c ) 2.5 × 10-6 M) and (B) CD (c ) 3.0 × 10-6 M) spectra of ZnChl-NDINO 1 in cyclohexane/
tetrachloromethane (1%). The initial temperature of 65 °C was successively decreased to 25 °C in 10 °C steps, and at each temperature the solution was
equilibrated for 1 h; the arrows indicate the changes upon decreasing temperature. Inset in (A): Increase of the Qy-band of aggregates at 729 nm (red) and
the decrease of the monomer band at 646 nm (black) upon decreasing temperature. Inset in (B): Magnification of the CD signals.
Table 1. Fluorescence Lifetimes and Corresponding Amplitudes at
Indicated Wavelengths for Monomers of Dyads 1 and 2 in
2-Methyltetrahydrofuran
ZnChl-NDINO 1 (λex ) 540 nm)
ZnChl-NDINN 2 (λex ) 620 nm)
amplitudesa
amplitudesa
τ
@ 575 nm
@ 650 nm
τ
@ 625 nm
@ 655 nm
13 ns
0.30
0.15
0.35
1.4
0.14
4.3
0.58
12 ns
0.14 ns
75 ps
7 psb
0.10
0.22
0.27
2.2
0.22
3.7
0.40
0.85 ns
160 ps
25 psb
-2.0
-4.1
a Negative amplitudes indicate rise components. b Predominant energy
transfer component.
Table 2. Fluorescence Lifetimes for Reference Chromophores
NDINO 4, NDINN 5, and ZnChl 6b Monomers and Aggregates
compd
λex (nm)
τ1
τ2
τ3
NDINO 4a
540
620
640
720
12 ns
10 ns
4 ns
-
-
-
-
-
-
NDINN 5a
ZnChl 6b mon.b
ZnChl 6b agg.c
13 ps
45 ps
131 ps
a Cyclohexane/tetrachloromethane (1%). b 2-Methyltetrahydrofuran/
methanol (1%). c n-Hexane/2-methyltetrahydrofuran (1%).
Figure 5. (A) UV–vis spectra (solid lines) and normalized fluorescence
spectra (dotted lines) of triad 3 monomers in THF (black, λex ) 510 nm)
and of triad 3 aggregates (red, λex ) 540 nm) and NDINN-NDINO 18 (blue,
λex ) 520 nm) in cyclohexane/tetrachloromethane (1%). (B) CD spectrum
of triad 3 aggregates in cyclohexane/tetrachloromethane (1%) (red).
The results for dyads 1 and 2 are summarized in Table 1
(see Table 2 for lifetimes of the pure reference compounds),
and the corresponding figures for the time-resolved data are
given in the Supporting Information (Figure S4). The data show
that ultrafast energy transfer with predominant amplitudes for
lifetimes of 25 ps (for 1) and 7 ps (for 2) takes place in these
monomeric units. Very small amplitudes of longer lifetimes (ca.
100 ps) reflect less efficiently transferring conformations.
For the examination of excited-state properties of triad 3
monomers, at first, predominantly the NDINO (and slightly the
NDINN) units were excited with 540 nm ps laser pulses (Table 3
and Figure S5 in the Supporting Information). Again a highly
efficient energy transfer takes place with a lifetime of τ ) 22 ps in
the emission region of NDINO, while the contribution of the
amplitude component of unquenched NDINO (τ ) 8.0 ns) is
negligibly small. Evidence for a direct energy-transfer process,
instead of a sequential process via the middle NDINN chromophore,
is obtained from the time-resolved fluorescence measurements of
the NDINN-NDINO reference compound 18 in 2-methyltetrahydro-
furan. Excitation of 18 at 540 nm provided a 29 ps lifetime
conclusion that the ZnChl aggregates formed in cyclohexane/
tetrachloromethane (1%) from ZnChl-NDI dyads 1, 2 and from
triad 3 are of a similar type, although the bulkier NDINN-NDINO
functionalities at the periphery of 3 aggregates appear to be more
impedimental toward rod formation than the NDI units at the
circumference of 1 and 2 aggregates.
Time-Resolved Fluorescence Measurements. The elevation
of the LH capacity of 1–3 aggregates by peripheral NDI
chromophores depends on the efficiency of energy transfer from
these NDI units to the ZnChl rod antenna. Stationary fluores-
cence spectroscopy suggests that complete energy-transfer
quenching of the NDI units takes place in both the monomers
and the aggregates (Vide supra). To gain more insight into the
excited-state properties of the monomeric compounds 1–3, time-
resolved fluorescence studies were carried out using 2-meth-
yltetrahydrofuran as a solvent by the single photon timing
technique for various excitation wavelengths and the results were
analyzed by global analysis.17
(17) O’Connor, D. V.; Phillips, D. Time-correlated single photon counting;
Academic Press: London, 1988.
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A R T I C L E S
Röger et al.
Table 3. Fluorescence Lifetimes and Corresponding Amplitudes at
Indicated Wavelengths for Monomers of Triad 3 in
2-Methyltetrahydrofuran
Table 4. Fluorescence Lifetimes and Corresponding Amplitudes at
Indicated Wavelengths for Aggregates of ZnChl-NDI 1 and 211 in
Cyclohexane/Tetrachloromethane (1%)a
triad 3 (λex ) 540 nm)
triad 3 (λex ) 621 nm)
ZnChl-NDINO 1 (λex ) 540 nm)
ZnChl-NDINN 2 (λex ) 620 nm)
amplitudes
amplitudes
amplitudes
amplitudes
τ
@ 575 nm
@ 650 nm
τ
@ 629 nm
@ 655 nm
τ
@ 560 nm
@ 745 nm
τ
@ 625 nm
@ 740 nm
8.0 ns
2.0 ns
150 ps
22 psa
0
0.46
7.7
0.58
7.8 ns
2.0 ns
200 ps
8 psa
0.06
0.26
0.25
3.2
0.09
3.4
0.69
10 ns
87 ps
18 ps
6 ps
0.15
0.03
-0.86
11
0.01
13
108
12 ns
88 ps
18 ps
5 ps
0.09
0
0
0
2.9
11
-15
0.01
0.09
1.5
-4.6
-3.9
-99
0.69
a Dominant energy transfer component.
a Note that the nearly negligible amplitudes of the longer lifetimes
are not shown in Figure 6 for the reason of better clarity.
of ZnChl rod aggregates, since for aggregates of ZnChl reference
compound 6b (see Table 2) and for natural BChl aggregates
multiexponential decay characteristics have also been observed.8
Furthermore, it had been reported earlier for ZnChl self-
assemblies without appended dyes in L-R-Lecithin vesicles that
the shortest lifetime component of the multiexponential decay
is due to energy-transfer processes between different excitonic
levels of the aggregate characterized by different radiative
lifetimes.8b The other short lifetime components (<100 ps) are
provoked by small amounts of oxidized chlorin that act as
energy traps in model systems as well as in intact chlorosomes.8b
These quenching processes are responsible for the generally low
fluorescence quantum yields of ZnChl aggregates (Vide supra).
Time-resolved fluorescence spectroscopic measurements on
aggregates of ZnChl-NDINN 2 were performed before,11 and
for comparison, the results are listed in Table 4. As for dyad 1,
nearly complete quenching of the excited NDINN chromophores
(λex ) 620 nm) has been observed for 2 and the dominant
amplitude component (τ ) 5 ps) in the short wavelength region
turns strongly negative in the emission region of ZnChl
aggregates (λ > 700 nm), indicating fast energy transfer. The
lifetimes of the multiexponential ZnChl aggregate decay (18
and 88 ps) match the decay characteristics of dyad 1 aggregates.
Prior to investigating the photophysical processes in triad 3
aggregates, we have examined a solution of the NDINN-NDINO
reference compound 18 in cyclohexane/tetrachloromethane
(1%). Clear evidence for FRET from the excited NDINO
chromophore to the NDINN unit in this solvent system has
already been obtained from the steady-state emission spectrum
of 18, as only an emission band of NDINN has been detected
(Figure 5A). To gain insight into the kinetics of this process,
NDINN-NDINO 18 was irradiated with 540 nm laser pulses and
the fluorescence decay was recorded in the range 575–680 nm
(decay-associated spectra are shown in Figure S3). Global
analysis revealed a 15 ps lifetime component, which turns from
positive amplitudes in the wavelength region of the NDINO
emission region (λ < 615 nm) to negative amplitudes in the
NDINN emission region. This lifetime component is attributed
to the energy transfer from NDINO to the blue NDINN dye.
The excited-state properties of triad 3 aggregates in cyclo-
hexane/tetrachloromethane (1%) have been first investigated by
exciting mainly the NDINO (and slightly NDINN) units at 540
nm. The resulting amplitude decay-associated (DAS) results are
depicted in Figure 7A, and the amplitude components and their
lifetimes are collected in Table 5. The spectrum of the
component with lifetime τ ) 6 ps in the short wavelength range
(λ e 700 nm) is characteristic for an efficient energy-transfer
process from NDINO to the ZnChl antenna. A time constant of
6 ps for this process is considerably smaller than that observed
for the energy transfer from NDINO to NDINN in reference
Figure 6. Fluorescence decay-associated spectra of aggregates of ZnChl-
NDINO 1 (λex ) 540 nm; c ≈ 7.5 × 10-6 M) in cyclohexane/tetrachlo-
romethane (1%).
component, which turns from positive amplitude values in the
wavelength range of the NDINO emission (λ < 620 nm) to negative
amplitudes in the NDINN emission region (λ > 620 nm), indicating
intramolecular energy transfer from NDINO to NDINN (for decay-
associated spectra of 18, see the Supporting Information, Figure
S2). Thus, the time constant of FRET between NDINO and NDINN
in 18 (τ ) 29 ps) exceeds the time constant of the energy transfer
observed for triad 3 (τ ) 22 ps). Accordingly, in triad 3 the faster
energy-transfer process is the direct transfer from NDINO to the
ZnChl dye.
In a further experiment 621 nm excitation was used to excite
directly the NDINN chromophore (Figure S5 in Supporting
Information and Table 3). The main negative amplitude
component (τ ) 8 ps) indicates that highly efficient energy
transfer from the NDINN units to the ZnChl chromophores takes
place.
Time-Resolved Fluorescence of Aggregate Solutions of 1–
3. To investigate the energy-transfer processes in self-assembled
multichromophoric compounds 1–3, time-resolved fluorescence
measurements were conducted in cyclohexane/tetrachloromethane
(1%) solutions. The NDINO unit of dyad 1 aggregates was
excited with 540 nm laser pulses, which resulted in one
dominant positive amplitude component with the lifetime τ )
6 ps in the wavelength region of the NDINO emission (λ < 640
nm) (Figure 6 and Table 4). The 6 ps amplitude component is
negative in the emission region of ZnChl aggregates (λ > 700
nm), demonstrating that an ultrafast energy transfer from the
NDINO chromophores to the self-assembled ZnChl aggregate
takes place. Two positive amplitude components with lifetimes
τ ) 87 and 18 ps match the emission spectrum of the ZnChl
6b aggregates (Table 2), reflecting the decay of those excited
states that are generated by energy transfer from excited NDINO
dyes. Multiexponential fluorescence decay is a typical feature
9
5936 J. AM. CHEM. SOC. VOL. 130, NO. 18, 2008

Zinc Chlorin Rod Antennae Powered by Chromophores
A R T I C L E S
Figure 7. Fluorescence decay-associated spectra of aggregates of triad 3 in cyclohexane/tetrachloromethane (1%). (A) λex ) 540 nm; c ≈ 6.9 × 10-6 M;
(B) λex ) 621 nm; c ≈ 7.9 × 10-6 M.
Table 5. Fluorescence Lifetimes and Corresponding Amplitudes at
Indicated Wavelengths for Aggregates of Triad 3 in Cyclohexane/
NDINN, respectively, overlaps with the rather narrow emission band
Tetrachloromethane (1%)a
wavelength, since the broad S0-S1 emission band of NDINO and
of the ZnChl monomer. Therefore, all the recorded excitation
triad 3 (λex ) 540 nm)
amplitudes
triad 3 (λex ) 621 nm)
amplitudes
spectra of 1–3 monomers did not exactly match with the respective
absorption spectrum but showed a too small contribution of the
ZnChl band. Moreover, the fluorescence intensities of 1-3
aggregates are too low to obtain excitation spectra of these species
that would be suitable for a quantitative analysis. Hence, we decided
to calculate energy-transfer efficiencies from time-resolved data
by using the following relationship:
τ
@ 565 nm @ 640 nm @ 740 nm
τ
@ 640 nm @ 740 nm
10 ns
76 ps
14 ps
6 ps
0.02
0.02
-0.39
3.8
0.69
-0.10
1.3
0.12
1.8
47
-43
10 ns
110 ps
12 ps
4 ps
0.14
0.03
0
0
0.06
3.4
8.4
0.49
-4.5
a Note that the nearly negligible amplitudes of the longer lifetimes
are not shown in Figure 7 for the reason of better clarity.
kET
1
τ
φET
)
; k )
(1)
kET + kD
compound 18 (τ ) 15 ps). Thus, upon excitation of NDINO in
triad 3 the excitation energy is directly transmitted, at least in
part, to the ZnChl antenna rather than being transferred along
a cascade via the NDINN unit. The decay of the excited ZnChl
aggregate, which is generated by this energy transfer, shows
again the characteristic multiexponential behavior with a major
14 ps and a minor 76 ps amplitude component.
In a further experiment, 621 nm laser pulses were chosen to
excite the NDINN unit (Figure 7B and Table 5). The large positive
amplitude of a τ ) 4 ps component in the NDINN emission region
(wavelengths λ e 670 nm) turns negative in the ZnChl aggregate
emission (λ g 710 nm) region and can, therefore, be assigned to
the fast energy-transfer process from NDINN chromophores to the
ZnChl rod antenna. The multiexponential decay of ZnChl ag-
gregates is dominated by a 12 ps lifetime component and a few
additional longer wavelength components with much smaller
amplitudes.
The decay pathways that followed after excitation of the
respective NDI units in 1-3 aggregates are illustrated in energy-
level diagrams shown in Figure 8. The energy-level diagram and
photophysical processes of triad 3 aggregates nearly resemble the
sum of the diagrams of dyads 1 and 2. The most remarkable feature
in triad 3 is the ultrafast energy transfer, directly from the outer
red NDI unit to the central ZnChl rod aggregate.
Light-Harvesting Efficiencies. The singlet–singlet excitation
energy-transfer efficiencies from NDI to ZnChl chromophores in
1–3 monomers and ZnChl rod antenna aggregates can in principle
be estimated either from steady-state or from time-resolved
fluorescence data. To calculate energy-transfer efficiencies from
steady-state spectroscopic measurements, it is necessary to record
excitation spectra at an acceptor emission wavelength which is
separated from the donor emission region. However, for monomers
of 1-3, it was not possible to find an appropriate detection
where φET is the energy-transfer efficiency, kET is the energy-
transfer rate, and kD is the donor fluorescence decay rate in the
absence of the acceptor. Values for kET and kD can be easily
calculated from the respective lifetimes that have been obtained
from spectroscopic measurements.
The resulting values are summarized in Table 6. The energy-
transfer rates for 1-3 monomers are calculated to be in the
range (4.0–4.5) × 1010 s-1 if NDINO acts as an energy donor
and (1.3–1.4) × 1011 s-1 with NDINN as the donor unit. Energy-
transfer rates from peripheral NDI chromophores to the ZnChl
rod aggregate are even larger showing values of 1.7 × 1011
s-1 at excitation of the NDINO chromophore and (2.0–2.5) ×
1011 s-1 when NDINN units are excited. Energy-transfer
processes from NDINN donors are slightly faster than those from
NDINO units for both 1-3 monomers and aggregates, which
can be related to the larger overlap integral of the NDINN
emission spectrum with the ZnChl monomer or aggregate
absorption spectrum compared to the respective overlap area
observed for ZnChl-NDINO. Due to the high energy-transfer
rates, we obtained energy-transfer efficiencies of φET g 99%
for monomers as well as for aggregates of 1–3.
As we have shown that all excited NDI units transfer their energy
directly and completely to the ZnChl rod, the increase of the
sunlight harvesting capacity of the multichromophoric LH systems
1-3compared to the monochromophoric aggregates of reference
system ZnChl 6a can be calculated from the ratio of the respective
cross sections of terrestrial solar light absorption. For aggregates
of dyad ZnChl-NDINN 2, an increase of 26% of the total LH
efficiency lent by appended NDI dyes was calculated.11,18 For the
new dyad ZnChl-NDINO 1, an improvement to 36% is reached,
(18) The cross sections can be calculated by integration of the products of
the respective absorption values and the terrestrial solar irradiance (for
details, see the Supporting Information).
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A R T I C L E S
Röger et al.
Figure 8. Energy-level diagram and photophysical behavior of aggregates of (A) ZnChl-NDINO 1, (B) ZnChl-NDINN 2, and (C) ZnChl-NDINN-NDINO 3 in
cyclohexane/tetrachloromethane (1%).
Table 6. Energy-Transfer Rates and Efficiencies for 1-3 Monomers in 2-Methyltetrahydrofuran and 1–3 Rod Aggregates in Cyclohexane/
Tetrachloromethane (1%) Derived from Time-Resolved Emission Data
monomers
aggregates
kD
kD
compd
NDINO (s-1
)
NDINN (s-1
)
kET (s-1
)
φET (%)
NDINO (s-1
)
NDINN (s-1
)
kET (s-1
)
φET (%)
1
2
3
3
7.7 × 107
1.3 × 108
4.0 × 1010
1.4 × 1011
4.5 × 1010
1.3 × 1011
g99
g99
g99
g99
1.0 × 108
1.0 × 108
1.7 × 1011
2.0 × 1011
1.7 × 1011
2.5 × 1011
g99
g99
g99
g99
8.3 × 107
1.3 × 108
9.1 × 107
1.0 × 108
and the best result is achieved for triad 3 revealing an increase of
copy revealed that in nonpolar aprotic solvents such as cyclohexane/
tetrachloromethane (1%) self-assembled ZnChl J-aggregates are
formed by intermolecular interactions of ZnChl units of ZnChl-
NDI conjugates 1–3, whereas the appended NDI chromophores
remain not aggregated at the periphery.
63% of the total LH efficiency.
Conclusions
We have demonstrated that it is possible to equip the biomimetic
ZnChl rod antennae with additional peripheral LH chromophores,
without the assistance of a structure-stabilizing protein matrix. This
goal has been achieved by self-assembly of multichromophoric
arrays based on covalently linked ZnChl-NDI dyads 1, 2 and, more
pleasingly, with the triad ZnChl-NDINN-NDINO 3 containing
additional chromophores to cover the green and orange fractions
of solar light. UV–vis, CD, and stationary fluorescence spectros-
Time-resolved fluorescence experiments revealed that upon
selective excitation of peripheral blue NDINN or pink NDINO
units their excitation energy is conveyed quantitatively and
directly to the inner green ZnChl rod antenna by highly efficient
energy-transfer processes. Thus, the “green gap” can be ef-
ficiently covered by the attached NDI antenna chromophores,
providing black dye aggregates whose sunlight harvesting
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5938 J. AM. CHEM. SOC. VOL. 130, NO. 18, 2008

Zinc Chlorin Rod Antennae Powered by Chromophores
A R T I C L E S
efficiency is increased by up to 63% compared to the parent
ZnChl or natural BChl c rod aggregates.
Supporting Information Available: Experimental information.
This material is available free of charge via Internet at http://
pubs.acs.org.
Acknowledgment. C.R. is grateful to the Degussa Stiftung
for a PhD scholarship. We thank the Fonds der Chemischen
Industrie for financial support.
JA710253Q
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J. AM. CHEM. SOC. VOL. 130, NO. 18, 2008 5939