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Reversible Self-Organization of Semisynthetic Zinc Chlorins into Well-Defined Rod Antennae.

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Reversible Self-Organization of Semisynthetic
Zinc Chlorins into Well-Defined Rod Antennae**
Valerie Huber, Martin Katterle, Marina Lysetska, and
Frank Wrthner*
Dedicated to Professor Franz Effenberger
on the occasion of his 75th birthday
The light-harvesting rod-shaped antennae in the chlorosomes
of green phototrophic bacteria (e.g. Chloroflexus aurantiacus)
are one of the most fascinating examples of self-organized
functional assemblies. In contrast to other plant or bacterial
light-harvesting systems, chlorosome antennae solely consist
[*] V. Huber, Dr. M. Katterle, Dr. M. Lysetska, Prof. Dr. F. Wrthner
Institut fr Organische Chemie
Universitt Wrzburg
Am Hubland, 97074 Wrzburg (Germany)
Fax: (+ 49) 931-888-4756
[**] This work was supported by the Fonds der Chemischen Industrie.
Angew. Chem. Int. Ed. 2005, 44, 3147 –3151
DOI: 10.1002/anie.200462762
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
of bacteriochlorophylls (BChl) and do not require any protein
to control their self-organization. Holzwarth, Schaffner,
Tamiaki, and co-workers have investigated these natural
systems and developed a simple model compound 1 for the
main constituents, namely BChl c,
of these light-harvesting antennae.[1]
Both BChl c and the model
compound 1 self-organize in nonpolar solvents, such as n-hexane,
into extended dye aggregates
which exhibit the highest exciton
diffusion length of all dye assemblies reported to date.[2] This property is of fundamental importance
for the high efficiency of the lightharvesting antennae in chlorosomes of green bacteria and
instructive for the design of organic solar cells.
The micelle-type aggregates of BChl c and its model
compound 1 are prone to further macroscopic aggregation in
solution leading to precipitation of less-defined agglomerates.[3] Thus, it is difficult to handle these compounds and to
deduce their structure–function relationships. Herein we
introduce compounds developed on the basis of 1 which
self-organize into well-defined rod aggregates of excellent
solubility. The formation of these aggregates is completely
reversible and can be triggered simply by variation of solvent
polarity. Hence, for the first time an easy to use biomimetic
antennae model for the more complex natural systems is
made available.
Scheme 1 illustrates our strategy for the design of highly
soluble artificial rod-shaped antennae which do not agglomerate. In natural chlorosome systems, each chlorin unit
Scheme 1. Left: Model of a chlorosome: Lipid monolayer (black), rod
aggregates of BChl c containing one nonpolar side chain per chlorin
unit (red and blue). Right: Isolated rod aggregate in solution containing two nonpolar side chains per chlorin unit (compare 6 b).
contains only one nonpolar chain (e.g. farnesyl).[1b] The rod
aggregates formed by self-organization of these chlorins are
surrounded by neighboring rod aggregates as well as by a lipid
monolayer membrane covering the chlorosome. In both cases
the neighboring alkyl chains interpenetrate and thereby
accomplish a “fusion” which holds the aggregate together.[4]
Consequently, isolated BChl c dissolved in water or organic
solvents tend to agglomerate. In contrast, increasing the
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
number of alkyl chains in 173 position of the chlorin unit
should lead to highly soluble rod aggregates in the form of
cylindrical micelles with a significantly reduced propensity for
agglomeration. For the evaluation of this concept, the zinc
chlorins 6 a–c containing one (6 a), two (6 b), or three (6 c)
alkyl chains were prepared. These new biomimetic compounds may be helpful for the structural elucidation of the
natural antennae aggregates, and also provide spectroscopically uniform functional structures.
The zinc chlorins were synthesized by derivatization of
Chl a which was extracted from the algae Spirulina maxima.[5]
Scheme 2 shows the synthesis of the desired zinc chlorins 6 a–c
starting with the pheophorbide 2, which was obtained from
Chl a.[6]
Scheme 2. a) DCC, DMAP, DPTS, N-ethyldiisopropylamine, CH2Cl2, RT,
3 h, 40–61 %; b) OsO4, NaIO4, AcOH, THF/H2O, RT, 5 h, 80–98 %;
c) BH3(tBu)NH2, THF, RT, 4 h; Zn(OAc)2, MeOH, RT, 3 h, 60–85 %.
DCC = dicyclohexylcarbodiimide, DMAP = 4-dimethylaminopyridine,
DPTS = 4-dimethylaminopyridinium-p-toluenesulfonate.
The carboxylic acid in 173 position of 132-demethoxycarbonylmethylpheophorbide a (2) was esterified with the alcohols 3 a–c by activation with DCC, DMAP, DPTS, and Nethyldiisopropylamine in dichloromethane. Oxidative cleavage of the 31 vinyl group of 4 a–c by osmium tetroxide and
sodium periodate afforded the respective formyl derivates
5 a–c. The formyl group in 5 a–c was selectively reduced with
borane tert-butylamine to the alcohol, and the desired zinc
Angew. Chem. Int. Ed. 2005, 44, 3147 –3151
chlorins 6 a–c were obtained by subsequent addition of a
methanolic solution of zinc acetate.[7, 8]
Characteristic for the dye aggregates in chlorosomes of
Chloroflexus aurantiacus and for the in vitro aggregates of
BChl c is a bathochromic shift of the Qy band with respect to
the corresponding absorption in the monomer.[1b] This red
shift can be ascribed to the interaction between metallochlorins which is comparable to that in Scheibe aggregates (Jaggregates).[9] Three structural characteristics of metallochlorins are important for the formation of tubular Jaggregates:[3] A central metal ion for the coordination with
the oxygen atom of the 31 hydroxy group, an extended
p system for effective p–p interactions, and hydrogen-bond
donor (31 OH) and hydrogen-bond acceptor (131 keto)
groups. The first two interactions mentioned above lead to a
slipped type of p–p stacking as in the case of J-aggregates (see
Scheme 3), whereas hydrogen bonding induces the tubular
Scheme 3. Interactions between zinc chlorins 6 a–c. Zn O coordination bonds that direct J-type stacks are marked in gray. Hydrogen
bonds that connect these stacks to give tubes are shown by arrows.
structure in which all the alkyl chains are oriented towards the
outside (inverse cylinder micelle).[10] Like BChl c, the zinc
chlorins 6 a–c also have these structural characteristics and,
thus, their Qy-band should also be bathochromically shifted
because of J-aggregate formation. Indeed, for the zinc
chlorins 6 a–c the characteristic red shift of the Qy-band
from 648 nm to 742 nm was observed as the UV/Vis spectra
show (Figure 1). Also characteristic is the very small Stokes
Figure 1. UV/Vis spectra of 6 b. Monomer dissolved in THF,
c = 2.3 10 5 mol L 1 (a); aggregate dissolved in hexane with < 1 %
THF, c = 4.6 10 5 mol L 1 (c). Fluorescence spectrum
(lex = 690 nm) of 6 b dissolved in hexane with < 1 % THF (g).
Angew. Chem. Int. Ed. 2005, 44, 3147 –3151
shift (< 1 nm) which indicates resonance fluorescence behavior of J-aggregates of the zinc chlorins 6 a–c (see fluorescence
spectrum in Figure 1).
In contrast to model compound 1, both zinc chlorins 6 b
and c with two and three alkyl chains, respectively, form in
nonpolar solvents, soluble aggregates that are stable for a
prolonged time. As expected from our concept, at least two
alkyl chains are necessary to enable good solubility, since zinc
chlorin 6 a as well as model compound 1 agglomerate within
hours to days and finally precipitate (Figure 2).
Figure 2. Color and solubility of zinc chlorin 6 b in THF (left) and in
heptane with 1 % THF after 8 days (right) compared to that of model
compound 1 (middle, also in heptane with 1 % THF after 8 days).
To gain more insight into the aggregation process, zinc
chlorin 6 b was dissolved in di-n-butyl ether and heptane
(20:80) and temperature-dependent (15–95 8C) UV/Vis spectroscopic measurements were performed. Preliminary experiments showed that a steady state is achieved after about 1 h at
lower temperatures; therefore, the samples were equilibrated
for 1.5 h prior to each measurement. The variable temperature UV/Vis absorption studies have revealed that with
increasing temperature, the Qy-band of aggregates at 742 nm
decreases and accordingly the monomer band of chlorins at
648 nm increases (Figure 3). On subsequent cooling of the
sample, the Qy-band of the aggregate is completely recovered,
demonstrating the reversibility of aggregate formation. The
fact that no precipitation was observed on warming or on
cooling indicates that the aggregate structures of zinc chlorins
are thermodynamically stable and, thus, they differ from
numerous other self-organized dye aggregates.[11] As can been
seen in the inset of Figure 3, the absorption versus temperature (A vs. T) plot provides sigmoidal curves with a melting
temperature of about 60 8C for the aggregates of zinc chlorin
6 b. The dissociation of the aggregates can also be induced by
addition of small amounts of protic solvents, such as
methanol, which weaken hydrogen bonding as well as
coordinative interactions (Scheme 3).
A distinct feature for natural BChl c rod aggregates is,
besides bathochromic shift of the long-wavelength UV/Vis
absorption bands, an induced circular dichroism (CD) effect.
This effect occurs on chiral excitonic coupling of transition
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The data presented reveal that the aggregates of zinc
chlorins 6 b and c have spectral properties comparable to
those of the natural light-harvesting systems in chlorosomes.
Encouraged by these results, atomic force microscopy (AFM)
studies were carried out to elucidate the structural properties
of 6 b and c. Figure 5 a shows an AFM image of a sample,
Figure 3. Temperature-dependent UV/Vis spectra of 6 b in di-n-butyl
ether/heptane (20:80; c = 3 10 6 mol L 1). Initial temperature 15 8C,
increased successively by 10 8C in 1.5 h intervals up to 95 8C; arrows
indicate changes upon increasing temperature. Inset: the decrease of
the Qy-band of aggregates at 742 nm and the increase of the monomer
band at 648 nm upon increasing temperature.
dipole moments and consists of two bands with opposite signs
(exciton couplet).[12] Surprisingly, different CD spectra of
isolated chlorosomes and in vitro generated aggregates of
BChl c have been reported.[2, 13] Griebenow et al. have categorized the observed CD spectra into three types (type I: the
sign of the CD curve changes from positive at shorter
wavelengths to negative at longer wavelengths [ + / ],
type II: opposite behavior to type I [ / + ], and the mixedtype: [ / + / ]).[14] In a theoretical approach, Holzwarth and
co-workers proposed that the different types of CD spectra
result from a size effect.[15] According to this approach, at an
aggregate length of more than 30 molecules of BChl c, the
type II converts into the mixed-type. On the other hand,
previously mentioned uncontrolled agglomeration of in vitro
aggregates of BChl c and 1 could lead to artifacts in CD
spectra. Therefore, it is interesting to note that zinc chlorin 6 b
exhibits an exciton couplet, which is very similar to that of the
model compound 1. This exciton couplet reversibly arises and
disappears on temperature change (see Figure 4) and does not
undergo any time-dependent changes at a particular temperature.
Figure 4. Temperature-dependent (15–95 8C) CD spectra of 6 b
(c = 1 10 5 mol L 1) in di-n-butyl ether/heptane (20:80). The temperature was increased in 10 8C intervals after 1.5 h equilibration time;
arrows indicate changes upon increasing temperature.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 5. a) AFM image in tapping mode of 6 b. The sample was prepared by spin-coating of a solution of 6 b (c = 4.6 10 5 mol L 1) in
hexane/THF (100:1) onto HOPG and measured in air. b) Profile of the
red line in (a), details are given in the text.
which was prepared by spin-coating of a solution of 6 b in
hexane/THF onto HOPG (highly ordered pyrolytic graphite).
As can be seen in this image, zinc chlorin 6 b self-organizes
into rigid rods. The length of the rods is 300 97 nm and the
height is 5.8 0.4 nm (vertical distance between the black and
green triangle, Figure 5 b). It is remarkable that all the
observed rod-like antennae are isolated. This structural
behavior corroborates our concept illustrated in Scheme 1.
The measured height of about 6 nm of these biomimetic zinc
chlorin aggregates is in agreement with electron microscopy
data of the BChl c rod aggregates in chlorosomes (Chloroflexus aurantiacus)[16] and also comply with the tubular model
postulated by Holzwarth and Schaffner.[10] Spectroscopic data
and theoretical calculations[10, 15] already suggested the presence of rod-shaped structures, but now, by AFM measurements, microscopic evidence is obtained for the first time for
the supramolecular organization of this class of chromophores.
Besides rod-shaped structures, smaller (3.1 0.6 nm in
height, vertical distance between the black and blue triangle,
Angew. Chem. Int. Ed. 2005, 44, 3147 –3151
Figure 5 b) and bigger (5.6 0.7 nm in height, vertical distance between the black and orange triangle) globular objects
are visible in the AFM image shown in Figure 5. Future
studies should clarify, whether these globular micelle structures are caused by preparation process of the sample or if
they already co-exist with the rod aggregate in solution.
Furthermore, a molecular monolayer of 0.5 0.1 nm thickness (vertical distance between the black and red triangle,
Figure 5 b) is visible on HOPG surface, which may indicate
the self-organization of zinc chlorin 6 b monomers induced by
the graphite surface.
Preliminary experiments indicate that our strategy demonstrated in Scheme 2 can also be applied to prepare rod
aggregates of zinc chlorins in water, since the Qy-band of zinc
chlorin 7 (in water/THF 100:1) is also red-shifted from 648 nm
to 744 nm. Further investigations with this compound are in
In summary, with the newly developed compounds 6 b and
6 c, zinc chlorins are made available whose self-organized rodshaped antennae exhibit excellent solubility in nonpolar
solvents. This favorable property enabled spectroscopic
investigations that confirm the reversible formation of these
biological important dye assemblies. For the first time AFM
enabled isolated rod aggregates to be observed whose
diameter is in good agreement with electron microscopic
data of BChl c aggregates in chlorosomes.
Received: November 30, 2004
Published online: April 21, 2005
Keywords: aggregation · chlorins · dyes/pigments ·
scanning probe microscopy · self-organization
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