Covalently stabilized self-assembled chlorophyll nanorods by olefin metathesis

Article abstract of DOI:10.1039/c2cc32314a

A new chlorophyll derivative with peripheral olefinic chains has been synthesised and its self-assembly properties have been studied, revealing formation of well-defined nanorods. These nanorods were stabilized and rigidified by olefin metathesis reaction as confirmed by spectroscopic and microscopic methods. This journal is

Full text of DOI:10.1039/c2cc32314a

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Covalently stabilized self-assembled chlorophyll nanorods by
olefin metathesisw
Sanchita Sengupta and Frank Wurthner*
¨
Received 30th March 2012, Accepted 19th April 2012
DOI: 10.1039/c2cc32314a
A new chlorophyll derivative with peripheral olefinic chains has
been synthesised and its self-assembly properties have been studied,
revealing formation of well-defined nanorods. These nanorods were
stabilized and rigidified by olefin metathesis reaction as confirmed
by spectroscopic and microscopic methods.
One-dimensional self-assembled architectures such as nanorods
of functional molecules have attracted considerable interest as
1,2
materials for (opto)electronic devices. Since such devices have
to operate under changing and demanding environmental
conditions, it would be advantageous for durable application
to fix the morphology of electronically active self-assembled
2
,3
materials.
building blocks is often utilized for the enhancement of their
Post-polymerisation of self-assembled molecular
2–6
structural robustness. This has been achieved by a broad range
3–5
Scheme 1 Synthesis of zinc chlorin 1. DCC = dicyclohexylcarbodi-
imide, DMAP = 4-dimethylaminopyridine, DPTS = 4-(dimethyl-
amino)pyridinium p-toluenesulfonate.
of reaction strategies such as ring closing metathesis, acyclic
2
diene metathesis (ADMET), ring opening metathesis polymeriza-
2
tion (ROMP), [3+2] cycloaddition of azides and alkynes, and
2
6
radical or photopolymerisation.
The light harvesting (LH) systems of green bacteria, namely,
7
the chlorosomes, often provide the inspiration for the rational
For this purpose, we have synthesised a new zinc chlorin (ZnChl)
derivative 1 (Scheme 1) functionalized with peripheral olefinic
groups and first studied its self-assembly by spectroscopic and
microscopic methods to ensure formation of nanorod assemblies.
Subsequently, these assemblies were subjected to olefin metathesis
reaction to obtain stabilized nanorods of this chlorophyll derivative.
The target compound ZnChl 1 was synthesised from chlorin deri-
vative 2 in three steps according to the route depicted in Scheme 1.
The chlorin carboxylic acid compound 2 was prepared from natural
design of one-dimensional supramolecular architectures with
desirable optical and electronic properties to function as
artificial LH systems and (opto)electronic materials. Chloro-
somal LH systems are protein-free tubular ensembles of
bacteriochlorophyll (BChl) c, d, or e held together by non-
covalent interactions such as hydrogen bonding, metal–
7
oxygen coordination and p–p stacking. This self-assembly
mode of BChls has been mimicked by using semisynthetic zinc
8
chlorin (ZnChl) molecules that are pre-programmed for an
13
chlorophyll (Chl) a according to the literature procedure and was
esterified with 3,4,5-triundec-10-ene-oxybenzylalcohol in the
presence of coupling reagents dicyclohexylcarbodiimide (DCC),
4-dimethylaminopyridine (DMAP), 4-(dimethylamino)pyridinium
p-toluenesulfonate (DPTS) and N-ethyldiisopropylamine in dry
analogous tubular mode of self-assembly in nonpolar and
9
–12
aqueous media.
Although post polymerisation of nano-
2
–6
tubular assemblies based on p-conjugated systems such as
hexabenzocoronenes (HBCs) and triphenylenes is well-established,
to the best of our knowledge, covalently stabilized nanorods based
on chlorophyll derivatives, particularly zinc chlorins, have been to
date unprecedented.
1
dichloromethane as solvent in 56% yield. The 3 -aldehyde
functionality in ester 3 was subsequently reduced to the corres-
1
ponding 3 -alcohol with 10 equivalents of borane-tert-butylamine
complex (BH (t-Bu)NH
3
2
) in CH Cl
2
2
in 82% yield. The metalation
in
of the free base chlorin 4 with saturated solution of Zn(OAc)
2
methanol and in tetrahydrofuran (THF) as solvent afforded the
target chlorophyll derivative ZnChl 1 in 61% yield. The inter-
mediates 3 and 4 and the final product ZnChl 1 were purified
by silica gel column chromatography, followed by HPLC and
Universita¨t Wu¨rzburg, Institut fu¨r Organische Chemie and
Ro¨ntgen Research Center for Complex Material Systems,
Am Hubland, 97074 Wu¨rzburg, Germany.
E-mail: wuerthner@chemie.uni-wuerzburg.de
w Electronic supplementary information (ESI) available: Synthetic
details, spectroscopic and microscopic characterisation. See DOI:
1
characterized by H NMR spectroscopy and high-resolution mass
10.1039/c2cc32314a
spectrometry (HRMS) (see ESIw for details).
5
730 Chem. Commun., 2012, 48, 5730–5732
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0
À1
0
We have first studied the self-assembly properties of this new
zinc chlorin 1 by UV/Vis and CD (circular dichroism) spectro-
scopy. Molecularly dissolved ZnChl 1 in polar solvent such as THF
the DH value of À54.4 kJ mol
and the DS value of
are obtained from the linear relationship.
À1
À1
À57.8 J mol
K
0
À1
Gibbs free energy, DG of À37.7 kJ mol , was obtained at
0
0
exhibits characteristic Soret and Q absorption bands at 428 nm
y
288 K using the DH and DS values. Comparison of the
enthalpy and entropy contributions to the Gibbs free energy
changes for aggregation leads to the conclusion that the self-
assembly process of ZnChl 1 is enthalpically driven, whereas it
is entropically disfavored. Previously, dimerization constants
and 647 nm, respectively (see ESIw). For aggregation studies,
monomer stock solution of 1 was prepared in THF and sub-
sequently added to nonpolar solvents like n-hexane. The UV/Vis
spectrum of a solution of 1 in n-hexane/THF (100 : 1, v/v) indeed
revealed the formation of aggregates as typical strongly red-shifted
4
À1
4
À1
for Chl a (1.0 Â 10 M ), Chl b (0.8 Â 10 M ), and BChl
4
À1
(
up to 100 nm) absorption maxima, compared to that of mono-
a (2.2 Â 10
M
) were derived from absorption spectra
14
16
mers, which was observed due to J-type excitonic coupling
between the S –S transition dipole moments of the chlorin Q
y
according to a monomer–dimer model. However, binding
constants for extended aggregates of chlorophylls have not
been assessed so far and thermodynamic data for self-assembly
of zinc chlorins have not been reported before.
0
1
10
bands (see ESIw). For further characterization of the self-
assembly of zinc chlorin 1, the aggregation process was investi-
gated by temperature-dependent UV/Vis spectroscopy in the range
between 15 and 95 1C in a solution of di- n-butyl ether (20%) and
n-heptane (80%). With increasing temperature, the aggregate band
at 741 nm decreases and the monomer band at 647 nm increases
Angle-dependent dynamic light scattering (DLS) studies of 1
revealed a broad intensity distribution, which is indicative of
anisotropic one-dimensional extended aggregates in solution (for
details see ESIw). Atomic force microscopy (AFM) confirmed
formation of nanorod aggregates with heights of B6 nm
(vide infra), in very good accordance with those of previously
y
(Fig. 1a). Upon cooling, the Q band was completely recovered,
which is indicative of the reversibility of the aggregation process.
The CD spectra of ZnChl 1 aggregates show a strong bisignate
1
10
studied 3 -hydroxy zinc chlorin nanorod assemblies.
signal in the region of the Q
y
band. With increasing temperature,
Once the self-assembly of ZnChl 1 into nanorod structures was
confirmed, olefin metathesis reaction was carried out in situ
the intensity of the CD signals decreases which suggests dissocia-
tion of aggregates, and the observed exciton couplet reversibly
arises and disappears upon temperature change, validating again
the reversibility of the self-assembly process (see ESIw).
Moreover, temperature-dependent UV/Vis studies suggest
an isodesmic self-assembly pathway of 1 (Fig. 1b) according to
1
the model published recently (for details and the equations
5
using Grubbs catalysts. To a highly diluted aggregate solution
À6
of ZnChl 1 (1.1 Â 10 M) in n-hexane/THF (100 : 1, v/v),
a catalytic amount of Grubbs catalyst benzylidenebis(tricyclohexyl-
phosphine)dichlororuthenium complex (B1 mol%) was added at
room temperature. The reaction mixture was stirred in the dark for
3–4 hours and subsequently quenched with few drops of ethyl vinyl
5
4
applied for Fig. 1b–d see ESIw). The number-averaged degree
ether. The reaction mixture was then filtered to remove the residual
of polymerisation, DP
decreases with increasing temperature (Fig. 1c).
N
, determined by applying eqn S2w,
catalyst and the filtrate was subjected to UV/Vis, CD, Fourier
transform infrared (FTIR) spectroscopy and AFM measurements
without further purification. Since dissociation of ZnChl aggregates
The average binding constant K at 288 K at two different
wavelengths of the aggregate bands (741 and 448 nm) was deter-
10
takes place at increased temperature, and in the presence of polar
solvents such as THF or methanol, temperature- and solvent-
dependent UV/Vis studies were performed on the aggregate
solution of 1 after metathesis reaction. Addition of THF to
aggregate solution of ZnChl 1 in n-hexane/THF (100 : 1 vol%)
6
À1
mined to be 4.5 Â 10 M (see ESIw). By plotting the logarithm of
K versus the reciprocal temperature (van’t Hoff plot, Fig. 1d),
before cross-linking leads to a gradual decrease in the Q aggregate
y
band at 741 nm and a concomitant increase in the monomer band
at 647 nm with increasing amount of THF. Above 10% THF a
complete disruption of the nanorod aggregate structures was
observed (Fig. 2a) owing to the strong coordination of THF to
the Zn(II) metal ion. In contrast, the cross-linked nanorods of 1
after metathesis exhibit strong resistance towards aggregate
disrupting solvents like THF as no significant spectral changes
were observed upon addition of up to 10% of THF (Fig. 2b),
thus providing evidence for the presence of covalently stabilized
nanorods. Moreover, upon increasing the temperature up to
5
0 1C, virtually no changes in UV/Vis and CD spectra of cross-
linked aggregates were observed, revealing their thermal stability
see ESIw). Cross-linking of the peripheral olefinic groups of rod
(
aggregates of 1 could also be evidenced by FTIR spectroscopy as
a strong decrease in the intensity of the CQC vibrational band at
1646 cm was observed (see ESIw).
AFM measurements were performed on highly oriented
Fig. 1 (a) Temperature-dependent UV/Vis spectra of ZnChl 1 in
À6
À1
n-heptane/di-n-butyl ether (80 : 20; c = 3.1 Â 10 M). (b) Degree
of aggregation, a, calculated from the UV/Vis absorption at 448 nm vs.
temperature (T) fitted to isodesmic model (black line). (c) Number-
pyrolytic graphite (HOPG) surfaces with aggregate solution
À6
averaged degree of polymerisation, DP
N
, as a function of temperature.
(c = 1.1 Â 10 M) before and after metathesis reaction.
(d) van’t Hoff plot (ln K vs. 1/T) for ZnChl 1.
As shown in Fig. 3a and b, well-defined, flexible nanorods of
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5730–5732 5731

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We thank the Volkswagen Foundation for the financial
support of this work and Dr S. Uemura and Dr V. Stepanenko
for AFM measurements.
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À6
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1
¨
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¨
(b) corresponding phase image. Inset in (a) shows a cross section
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À6
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(
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(d) corresponding phase image.
2
1
with large contour lengths were observed for aggregates
¨
before metathesis reaction, while the aggregates appear much
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images indicate intra-aggregate cross-linking since isolated
nanorods were observed and no bundles or network structures
were visible. These observations also suggest that the nanorods
are cross-linked and stabilized by the olefin metathesis reaction.
In conclusion, a novel chlorophyll derivative ZnChl 1
functionalized with peripheral olefinic groups was synthesised,
which self-assembles into well-defined nanorods. For the first
time, binding constants and thermodynamic data for the self-
assembly of a ZnChl building block have been determined.
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¨
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7
18
exciton transport and promising photovoltaic properties
of zinc chlorin aggregates, the present covalently stabilized
one-dimensional nanorods appear to be highly promising for
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2
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732 Chem. Commun., 2012, 48, 5730–5732
This journal is c The Royal Society of Chemistry 2012