Light-harvesting and energy-transfer system based on self-assembling perylene diimide-appended hexaazatriphenylene

Article abstract of DOI:10.1021/ol050919t

(Chemical Equation Presented) Six perylene diimide-appended hexaazatriphenylene, HAT-PDI₆, was self-assembled in both solution and film state to form a highly stabilized dimer aggregate, in which an efficient energy transfer occurs from the hexaazatriphenylene core to the perylene-diimide moiety.

Full text of DOI:10.1021/ol050919t

ORGANIC
LETTERS
2005
Vol. 7, No. 15
3175-3178
Light-Harvesting and Energy-Transfer
System Based on Self-Assembling
Perylene Diimide-Appended
Hexaazatriphenylene
Tsutomu Ishi-i,*,† Koh-ichi Murakami,‡ Yusuke Imai,§ and Shuntaro Mataka†
Institute for Materials Chemistry and Engineering (IMCE) and Interdisciplinary
Graduate School of Engineering Sciences, Kyushu UniVersity, 6-1 Kasuga-koh-en,
Kasuga 816-8580, Japan, and Inorganic-Organic Hybrid Materials Group, National
Institute of AdVanced Industrial Science and Technology Kyushu, 807-1 Shuku-machi,
Tosu 841-0052, Japan
ishi-i@cm.kyushu-u.ac.jp
Received April 25, 2005
ABSTRACT
Six perylene diimide-appended hexaazatriphenylene, HAT-PDI6, was self-assembled in both solution and film state to form a highly stabilized
dimer aggregate, in which an efficient energy transfer occurs from the hexaazatriphenylene core to the perylene-diimide moiety.
Energy transfer in a donor-acceptor system is an important
process in a natural photo synthetic system,1 as well as an
artificial photovoltaic.2 In the natural system, antenna
chlorophyll moieties are arranged in a highly ordered self-
assembling manner to achieve light harvesting and energy
transfer with extremely high efficiency.3 Recently, artificial
energy-transfer systems were created by using long-range
ordering in well-defined supramolecular aggregates com-
posed of π-conjugated components.4,5 In the self-assembled
systems, the acceptor molecules, which possess structural
similarity with the donor molecules, such as perylene
diimides with different substituents4 and oligo(p-phenyl-
enevinylene)s with different repeating units,5 are efficiently
incorporated in the well-defined columnar-type aggregates
composed of donor molecules without destabilization of self-
assembled ordering. However, quantitative analysis of the
energy transfer in the systems was difficult due to significant
overlap between donor and acceptor emissions.
† IMCE, Kyushu University.
‡ Interdisciplinary Graduate School of Engineering Sciences, Kyushu
University.
Recently, we reported that fluorescent hexaazatriph-
enylenes (HATs) can be self-assembled in both solution and
bulk state to form one-dimensional columnar-type ag-
§ National Institute of Advanced Industrial Science and Technology.
(1) Deisenhofer, J.; Michel, H. Angew. Chem., Int. Ed. Engl. 1989, 28,
829-847.
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40-48.
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614-621. (b) McDermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaite-
Lawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995,
374, 517-521.
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43, 1229-1233.
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A. P. H. J.; Silva, C.; Meskers, S. C. J.; Beljonne, D.; Phillips, R. T.; Friend,
R. H.; Meijer, E W. Angew. Chem., Int. Ed. 2004, 43, 1976-1979.
10.1021/ol050919t CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/28/2005

gregates.6 This finding led us to design and prepare a new
type of energy transfer system based on the incorporation
of HAT possessing an acceptor moiety into HAT-based
columnar-type aggregate. In this strategy, we have selected
perylene diimide as the acceptor moiety7 because of its strong
accepting nature,8 suitable overlap of its absorption spectrum
with the fluorescence spectrum of the HAT chromophore
used as donor moiety,9 and good separation between the
fluorescence spectra of the HAT and PDI chromophores.9
In this Letter, we report the preparation of a six perylene
diimide-appended HAT derivative (HAT-PDI6) and its self-
assembling nature in both solution and film state. Finally,
the energy transfer in the HAT-based aggregate system
incorporating HAT-PDI6 is discussed.
groups 3 was obtained by deprotection of the corresponding
N-Boc derivative 2, which was derived from HAT having
six bromophenyl groups 16 by coupling reaction with N-Boc-
4-aminophenylboronic acid in the presence of a palladium-
(0) catalyst. Finally, HAT-PDI6 was obtained by condensa-
tion reaction of 3 with six molecules of 4 in quinoline in the
presence of zinc acetate.12 Identification of HAT-PDI6 was
performed by means of spectroscopic methods and MALDI-
TOF mass spectrometry. HAT-PDI6 can dissolve in various
solvents from polar to nonpolar ones such as toluene,
dichloromethane, chloroform, 1,1,2,2-tetrachloroethane, and
THF. A stable film of HAT-PDI6 was prepared by conven-
tional spin-coating technique.
A direct evidence for the aggregation of HAT-PDI6 is
provided from the MALDI-TOF mass spectrometry13,14 and
gel permeation chromatography (GPC).11,15 In the MALDI-
TOF mass spectrum, a multiple of the parent ion of HAT-
PDI6 (m/z 5929) is seen at m/z 11,858, which is assigned to
a dimer aggregate of HAT-PDI6 molecules (S-Figure 2 in
Supporting Information). The molecular weight of 10,100
for the aggregate in THF solution, which was estimated from
GPC analysis using polystyrene references, is very similar
to that (11,858) of the dimer species of HAT-PDI6 (S-Figure
3 in Supporting Information).
Six perylene diimide-appended hexaazatriphenylene HAT-
PDI6 was prepared according to Scheme 1. Perylene monoim-
Scheme 1. Preparation of HAT-PDI6
The UV-vis spectrum of HAT-PDI6 in toluene shows two
bands at 511 and 544 nm arising from PDI chromophores
and a band around 390 nm from the HAT chromophore
(Figure 1). The former two bands are assigned to the
Figure 1. UV-vis spectra of HAT-PDI6 at 20 °C in toluene [1.0
mM (solid line) and 0.001 mM (dotted line)] and at the spin-coating
film (dashed line, a.u. unit).
transition from the ground state to the ν ) 0 level of the
lowest excited state (0-0 vibronic transition) and to the ν )
1 level (0-1 vibronic transition), respectively.13,17,18 The
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in three steps via bromination,10 aryloxylation,11 and imida-
tion.11 In the final imidation step, the desired 1,7-diaryloxy
isomer 4 was isolated from the reaction mixture including
other isomers.10b HAT with six amino-containing biphenyl
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Org. Lett., Vol. 7, No. 15, 2005

spectral pattern with intense PDI 0-1 vibronic transition and
a shoulderlike 0-0 one is very similar to those of perylene
diimide based aggregates with H-type stacking mode, which
is rationalized by the molecular exciton model.13,14,17,18
Characterization in the absorption band of the HAT chro-
mophore is difficult because of overlap with the weak band
of PDI chromophores. The UV-vis spectra in toluene and
THF changed slightly depending on concentration (0.001-
1.0 mM) and temperature (20-75 °C), indicating high
stability of the dimer aggregate in the solution (Figure 1 and
S-Figure 4 in Supporting Information). The spectrum of the
spin-coating film is very similar to those of the solution
(Figure 1). The results show that in the film state the HAT-
PDI6 molecules are self-assembled with similar H-aggrega-
tion mode created in the solution state.
range of 0.001-1.0 mM concentration, which is assigned
to emission from the dimer aggregate (Figure 2a).17,19 The
Concentration- and temperature-dependent UV-vis spec-
tral change was observable in 1,1,2,2-tetrachloroethane. The
absorbance of the PDI 0-0 vibronic transition at 553 nm
increased with lowering concentration and increasing tem-
perature, whereas the PDI 0-1 vibronic transition at 514 nm
weakened (S-Figure 5 in Supporting Information). The
phenomena are attributed to the dynamic exchange between
the monomer and dimer species.
1
In the H NMR spectra of HAT-PDI6, a line-broadening
effect arising from the π-stacked aggregation was observed
in chloroform-d1 and 1,1,2,2-tetrachloroethane-d2. The chemi-
cal shifts and the shape of the broad resonances are
independent of concentration in the range of 0.01-1.0 mM,
indicating the dominance of a single aggregate species (S-
Figure 6 in Supporting Information).15,16 At dilute concentra-
tion and high temperature, a set of sharp resonances is newly
generated in addition to the broad ones in 1,1,2,2-tetrachlo-
roethane-d2 (S-Figures 6 and 7 in Supporting Information).
The results indicate that the dynamic exchange between the
monomer and dimer species is slow in the NMR time scale.
The aggregation of HAT-PDI6 is reflected in fluorescence
spectra. In toluene solution, a broad emission around 660
nm arising from the PDI chromophore is seen in the whole
Figure 2. Fluorescence spectra of HAT-PDI6 at 20 °C (a) in toluene
[0.1 mM (solid line), 0.001 mM (dotted line)], and at the spin-
coating film (dashed line), and (b) in 1,1,2,2-tetrachloroethane [0.1
mM (solid line), 0.001 mM (dotted line)]. Excited at 394 nm in
toluene, at 403 nm in 1,1,2,2-tetrachloroethane, and at 400 nm in
the film.
dimer emission is supported by time-resolved fluorescence
spectra, from which a long-lived species with a lifetime of
18.5 ns was detected in toluene at 0.001 mM (S-Figure 10
in Supporting Information). The value is as large as those
of perylene diimide based H-aggregates reported so far.19
Similarly, the THF solution also provides the dimer emission
(S-Figure 8 in Supporting Information). In contrast, in the
dilute 1,1,2,2-tetrachloroethane solution (0.001 mM), mono-
mer emission is observable around 590-630 nm in addition
to the weak dimer emission around 690 nm as a shoulder
(Figure 2b). With increasing concentration (0.1 mM), the
intensity of the monomer emission decreased whereas the
dimer emission was enhanced, indicating again the dynamic
exchange between the monomer and dimer species. The
broad emission around 650 nm found in the film state
indicates once again that in the film state the HAT-PDI6
molecule is self-assembled with H-aggregation mode (Figure
2a).
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Interestingly, red PDI emission is provided upon direct
excitation of the PDI chromophore as well as upon indirect
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Org. Lett., Vol. 7, No. 15, 2005
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excitation of the HAT chromophore (S-Figure 9 in Support-
ing Information). Upon excitation of the HAT chromophore,
no emission from the HAT core was seen in either the
monomer or dimer species, indicating the energy transfer
from the HAT core to the PDI moiety. The energy transfer
was supported also by a good agreement between UV-vis
and excitation spectra (S-Figures 11 and 12 in Supporting
Information). These results provide the valuable information
that an efficient light-harvesting and energy-transfer system
can be created if HAT-PDI6 is incorporated in HAT-based
columnar-type aggregate.
As a preliminary test, a film of HAT derivative with six
octyloxy-containing biphenyl groups (HAT-OC8 in Scheme
1)20 doped with 5 mol % HAT-PDI6 was prepared by spin
coating from cyclohexane solution, and the energy transfer
from the HAT-OC8-based medium to the HAT-PDI6 incor-
porated in the medium was examined. The spin-coating
samples provide a clear overlap of the fluorescence spectrum
of HAT-OC8 and the absorption spectrum of HAT-PDI6 (S-
Figure 13 in Supporting Information). In the presence of
HAT-PDI6, the emission from HAT-OC8 around 510 nm
was quenched significantly even upon excitation of the HAT
chromophore (Figure 3). In addition, the PDI emission
around 650 nm upon indirect excitation of the HAT chro-
mophore is enhanced by a factor of 4 times compared to
that observed upon direct excitation of the PDI chromophore,
indicating the light-harvesting effect of the HAT-OC8-based
medium (Figure 3).21 The energy-transfer phenomenon was
easily visualized by change in the emission color from green
HAT emission to red PDI emission (Figure 3).
Figure 3. Fluorescent images under UV light (365 nm) and
fluorescence spectra excited at the HAT chromophore (395 nm) of
HAT-OC8/HAT-PDI6 mixtures with 100:0 (solid line), 100:5
(dotted line), and 0:5 (dashed line) molar ratio in the spin-coating
film at 20 °C. The fluorescence spectrum of the 100:5 mixture
excited at the PDI chromophore (at 517 nm) is indicated by the
wide dotted line.
that the present system will be developed to provide self-
assembled light harvesting and energy transfer systems.
In conclusion, we have shown that six perylene diimide-
appended hexaazatriphenylene can be self-assembled to form
a stable dimer aggregate in both solution and film state. In
the aggregate structure, an efficient energy transfer from the
HAT core to the peripheral PDI moiety proceeds. We believe
Acknowledgment. We thank Professor Dr. Shigeori
Takenaka and Dr. Keiichi Otsuka (Kyushu University) for
the measurement of MALDI-TOF-MS, and Professor Dr.
Hideo Nagashima and Dr. Kishiro Yasuhara (Kyushu
University) for the GPC analysis. This work was partially
supported by a Supporting Young Researchers with Fixed-
term Appointments, Special Coordination Funds for Promot-
ing Science and Technology.
(20) Both in film state and in nonpolar solution such as cyclohexane,
HAT-OC8 forms stable columnar-type aggregate. In contrast, HAT-OC8
dissolved molecularly in polar solvents such as dichloromethane and
chloroform. The details for preparation and aggregation property of HAT-
OC8 will be published elsewhere, and the selected spectral data are shown
in Figures 15 and 16 in Supporting Information.
(21) When the spin-coating film was prepared from polar dichlo-
romethane and chloroform solutions including molecularly dissolved HAT-
OC8, the energy-transfer efficiency was reduced significantly, suggesting
an importance of the HAT-OC8-based columnar-type aggregate as a good
light-harvesting antenna as well as an energy-transfer reagent.
Supporting Information Available: Experimental pro-
1
cedure, GPC trace, and MALDI-TOF-MS, H NMR, UV-
vis, and fluorescence spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL050919T
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