Efficient energy transfer from peripheral chromophores to the self-assembled zinc chlorin rod antenna: A bioinspired light-harvesting system to bridge the "green gap"

Article abstract of DOI:10.1021/ja0584469

An artificial light-harvesting rod aggregate based on zinc chlorin and covalently linked naphthalene bisimide chromophore has been realized by self-assembly. Efficient energy transfer (φ_ET ≥ 0.99) takes place upon excitation at 620 nm from peri

Full text of DOI:10.1021/ja0584469

Published on Web 05/03/2006
Efficient Energy Transfer from Peripheral Chromophores to the
Self-Assembled Zinc Chlorin Rod Antenna: A Bioinspired Light-Harvesting
System to Bridge the “Green Gap”
Cornelia Ro¨ger,§ Marc G. Mu¨ller,‡ Marina Lysetska,§ Yulia Miloslavina,‡ Alfred R. Holzwarth,*,‡ and
Frank Wu¨rthner*,§
Institut fu¨r Organische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany, and
Max-Planck-Institut fu¨r Bioanorganische Chemie, Postfach 101365, D-45413 Mu¨lheim an der Ruhr, Germany
Received December 13, 2005; E-mail: wuerthner@chemie.uni-wuerzburg.de; holzwarth@mpi-muelheim.mpg.de
In the course of evolution, nature has developed highly complex
light-harvesting (LH) systems.1 In particular, the protein-free
bacteriochlorophyll c (BChl c), d, and e aggregates in the chlo-
rosomes of green phototrophic bacteria exhibit the highest chro-
mophore density of all LH systems2 and show the most extended
exciton diffusion length known for dye aggregates.2c,3 Zinc chlorins
(ZnChl), which can be prepared by derivatization of Chl a, serve
as appropriate model compounds for BChl c since they self-
assemble into rod-shaped structures in the same way as their natural
analogues.4
Assemblies of BChl c and ZnChl are very efficient in harvesting
blue and red light; however, they cannot utilize the significant green
region of the solar spectrum. It would be a major advancement
toward efficient utilization of solar energy if such assemblies,
especially those of zinc chlorins, could be connected functionally
with auxiliary LH chromophores to bridge the “green gap”. Here
we report on such a biomimetic LH system based on the ZnChl-
NBI dyad 1 that self-assembles into rod antennae, in which efficient
energy transfer occurs from peripheral blue naphthalene bisimide
(NBI) dyes to the core ZnChl rod. The synthesis and characteriza-
tion of dyad 1 and isolated NBI 3 (Chart 1) is described in detail
in the Supporting Information. The reference chromophore ZnChl
Figure 1. (A) UV-vis (solid lines) and normalized fluorescence (dashed
2 was prepared according to literature.4c
lines, λex ) 620 nm) spectra of 1. Monomers in THF (black); aggregates in
cyclohexane/tetrachloromethane (1%) (red) at room temperature. (B) UV-
Chart 1. Chemical Structures of ZnChl-NBI 1 and Isolated
Chromophores 2 and 3
vis (blue solid line) and normalized fluorescence spectrum (blue dashed
line, λex ) 575 nm) of NBI 3 in cyclohexane/tetrachloromethane (1%),
and UV-vis spectrum of ZnChl 2 (green) in cyclohexane/tetrachloromethane
(1%)/THF (0.1%).
of 1 (Figure 1A) revealed the characteristic bathochromic shift4b,c
of the zinc chlorin QY-band from 646 nm in THF (monomer of 1)
to 731 nm in cyclohexane/tetrachloromethane (1%), indicating
J-type excitonic coupling of ZnChl units in the aggregates of 1.
For comparison, the UV-vis spectrum of the aggregate of the
isolated chromophore ZnChl 2 is given in Figure 1B. Notably, the
absorption maximum of the NBI unit in the 1 aggregate (λmax
)
611 nm) is slightly changed compared to that of the isolated NBI
chromophore 3 (λmax ) 604 nm, Figure 1B), suggesting insignificant
interaction between the NBI units at the periphery of ZnChl rods.
This interpretation is supported by the fact that an exciton couplet
was observed in the circular dichroism (CD) spectrum for the
ZnChl-related optical transition at 731 nm of the self-assembly
(Figure S1 in Supporting Information) due to the induced CD
effect,4c but no CD signal is present for the NBI absorption band
at 611 nm. Furthermore, the fluorescence spectrum of the aggregated
species (Figure 1A) revealed a small Stokes shift of only 6 nm,
which is characteristic for chlorin J-aggregates.4
The self-assembly of BChl c and its ZnChl model system into
tubular dye aggregates with J-type electronic coupling is facilitated
by the interplay of three noncovalent intermolecular interactions:
hydrogen bonding, π-π stacking, and metal ion coordination.5
Since the present ZnChl-NBI dyad possesses the required structural
features for such interactions, it should also form self-assembled
aggregates in nonpolar aprotic solvents. Indeed, UV-vis spectra
Not only the spectroscopic properties (vide supra) but also the
structural features of ZnChl-NBI 1 self-assemblies are quite similar
§ Universita¨t Wu¨rzburg.
‡ Max-Planck-Institut fu¨r Bioanorganische Chemie.
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10.1021/ja0584469 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
nents are observed with the lifetimes τ ) 88 and 18 ps and a minor
one of 710 ps. These components reflect the decay of the ZnChl
excited states in the tubular self-assemblies of 1 that are generated
by energy transfer from excited NBI dyes. Multiexponential
fluorescence decay with similar lifetimes, attributed to emission
from several exciton states with high oscillator strength, has been
reported earlier for ZnChl aggregates without appended dyes in
L-R-lecithin vesicles.3 In previous work3, short lifetime (<100 ps)
components were assigned to quenching processes caused by small
amounts of oxidized chlorin. Such quenching processes cannot be
excluded for the present system. We note that such lifetimes are
characteristic for intact chlorosomes. It has been shown both for
chlorosomes as well as artificial chlorin aggregates that these
quenching processes do not adversely affect their LH efficiencies
if effective acceptors are present.3
Figure 2. (A) Tapping mode AFM image of a ZnChl-NBI 1 sample, which
was prepared by spin-coating from a solution of 1 (c ∼ 1 × 10-5 M) in
cyclohexane/THF (1%) onto highly ordered pyrolytic graphite (HOPG) and
measured under ambient conditions. (B) Profile of the red line in (A); the
vertical distance between the red triangles provides the height of the rod
aggregate.
It is a remarkable feature of the bichromophoric system 1 that
self-assembly leads to a well-defined rod aggregate composed of
strongly coupled ZnChl chromophores, while the second NBI dye
at the periphery of the rod does not form aggregates. Thus, NBI
provides additional LH functionality to the ZnChl antenna and
thereby improves its efficiency. From the ratio of the respective
cross sections of solar light absorption by aggregates of ZnChl-
NBI 1 and ZnChl 2, an increase of 26% of the total LH efficiency
of ZnChl-NBI aggregates lent by appended NBI dyes is calcu-
lated.9 The present antenna system is promising for utilization of a
wider part of the solar spectrum in potential LH devices, in which
the subsequent processes should be much faster than the observed
decay times of the aggregate excitation. Obviously, dyad 1 differs
from other known bichromophoric systems which form either
amorphous materials,10 charge transfer complexes,11 or coaggregated
stacks.12
Figure 3. Fluorescence decay-associated spectra of aggregates of 1 (λex
) 620 nm; c ∼ 0.76 × 10-5 M) in cyclohexane/tetrachloromethane (1%).
Acknowledgment. C.R. is grateful to the Degussa Stiftung for
a Ph.D. scholarship. We thank the Fonds der Chemischen Industrie
for financial support.
to those of the natural BChl c and its ZnChl model systems.4,6 Thus,
rod aggregates are observed for self-assembled dyad 1 by atomic
force microscopy (AFM), as shown in Figure 2A. Direct measure-
ments of the contour lengths of 362 rods revealed a mean value of
93 ( 75 nm. The longest observed aggregates show contour lengths
in the range of 300-380 nm, and the longer rods (>100 nm) are
slightly curved. The persistence length of curved rods has been
estimated to be 200 ( 30 nm, indicating a pronounced stiffness of
these assemblies. All rods possess a height of 7.3 ( 0.2 nm (Figure
2B), which is slightly higher than that observed previously for
ZnChl rods without additional NBI dyes (5.8 ( 0.4 nm).4c
Having established the structural features of ZnChl-NBI self-
assemblies to be a ZnChl rod bearing NBI dyes at the periphery,
their suitability as artificial LH antennae was addressed. To get a
detailed insight into the excited state properties of 1 aggregates,
time-resolved fluorescence studies were carried out using the single
photon timing technique. Selective excitation of the NBI moiety
with 620 nm picosecond laser pulses resulted in three positive
amplitude decay-associated (DAS) components in the wavelength
region of the NBI emission (λ e 670 nm) (Figure 3). The dominant
component shows a lifetime of 5 ps, which reflects energy transfer
from NBI to the ZnChl aggregate, as evidenced by its large negative
amplitude in the ZnChl emission band. Further, two small amplitude
components with τF ) 11 ns and τ ) 2.6 ns occur in the NBI
emission region. The former can be attributed to a small amount
of unquenched NBI (τ ∼ 10.5 ns), while the latter is apparently
due to small amounts of residual 1 monomer present in equilibrium
with the aggregate.7 The 5 ps component clearly describes an energy
transfer process with a quantum efficiency φΕΤ ) kΕΤ/(kΕΤ + kF)
g 0.99 (if the small amount of unquenched NBI is ignored).8 In
the ZnChl emission region, two large positive amplitude compo-
Supporting Information Available: Synthetic procedures, char-
acterization, spectroscopic data of 1 and 3, and complete ref 12. This
material is available free of charge via Internet at http://pubs.acs.org.
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