Self-assembled fluorescent hexaazatriphenylenes that act as a light-harvesting antenna

Article abstract of DOI:10.1021/jo060768n

In this paper we report the self-assembling nature of fluorescent hexaazatriphenylenes (HATs) 6a-d with six alkyl/alkoxy-chain-containing biphenyl groups and their application to light-harvesting antennae. In a nonpolar solvent and the film state, the HAT

Full text of DOI:10.1021/jo060768n

Self-Assembled Fluorescent Hexaazatriphenylenes That Act as a
Light-Harvesting Antenna
Tsutomu Ishi-i,*,†,‡ Koh-ichi Murakami,§ Yusuke Imai,| and Shuntaro Mataka*,†
Institute for Materials Chemistry and Engineering (IMCE), Kyushu UniVersity,
6-1 Kasuga-koh-en, Kasuga 816-8580, Japan, Department of Biochemistry and Applied Chemistry,
Kurume National College of Technology, 1-1-1 Komorino, Kurume 830-8555, Japan, Interdisciplinary
Graduate School of Engineering Sciences, Kyushu UniVersity, 6-1 Kasuga-koh-en,
Kasuga 816-8580, Japan, and On-site Sensing and Diagnosis Research Laboratory, National Institute of
AdVanced Industrial Science and Technology (AIST), 807-1 Shuku-machi, Tosu, Saga 841-0052, Japan
ishi-i@kurume-nct.ac.jp
ReceiVed April 12, 2006
In this paper we report the self-assembling nature of fluorescent hexaazatriphenylenes (HATs) 6a-d
with six alkyl/alkoxy-chain-containing biphenyl groups and their application to light-harvesting antennae.
In a nonpolar solvent and the film state, the HAT derivatives form one-dimensional aggregates with an
1
H-type parallel stacking mode, which were analyzed by H NMR, UV-vis, and steady-state and time-
resolved fluorescence spectroscopy. When HAT derivative 7 with six perylenediimide moieties is
incorporated into the one-dimensional aggregates, an efficient energy transfer takes place from the self-
assembled HAT moiety as a light-harvesting antenna to the perylenediimide moiety as an energy acceptor.
Further, when HAT derivative 8 with six triphenylamino moieties is newly added to the light-harvesting
system, an intermolecular electron transfer occurs subsequently between the electron-accepting perylene-
diimide molecule and the electron-donating triphenylamino molecule.
Introduction
efficiency.3 The noncovalent synthesis used in the natural system
has been widely developed as a novel strategy to construct
artificial antenna moieties such as self-assembled multiporphyrin
systems.4-7 Recently, long-range ordering in well-defined
Energy and electron transfer in a donor-acceptor system has
been of much interest in recent years because it is an important
process in a natural photosynthetic system1 as well as artificial
photovoltaic.2 In the natural system, antenna chlorophyll
moieties are self-assembled in a highly ordered manner to
achieve light harvesting and energy transfer with extremely high
(3) (a) Ku¨hlbrandt, W.; Wang, D. N.; Fujiyoshi, Y. Nature 1994, 367,
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. (c) Karrasch, S.; Bullough, P. A.; Ghosh, R. EMBO J. 1995,
14, 631-638. (d) Koepke, J.; Hu, X.; Muenke, C.; Schulten, K.; Michel,
H. Structure 1996, 4, 581-597. (e) McLuskey, K.; Prince, S. M.; Cogdell,
R. J.; Isaacs, N. W. Biochemistry 2001, 40, 8783-8789. (f) Roszak, A.
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4938. (d) Kuramochi, Y.; Satake, A.; Kobuke, Y. J. Am. Chem. Soc. 2004,
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Ko, D. M.; Kim, D.; Ito, F.; Ishibashi, Y.; Khan, S. R.; Nagasawa, Y.;
Miyasaka, H.; Ikeda, C.; Takahashi, R.; Ogawa, K.; Satake, A.; Kobuke,
Y. Chem.sEur. J. 2005, 11, 3753-3761.
* To whom correspondence should be addressed. Phone: +81-942-35-9404.
Fax: +81-942-35-9400.
† IMCE, Kyushu University.
‡ Kurume National College of Technology.
§ Interdisciplinary Graduate School of Engineering Sciences, Kyushu Uni-
versity.
| AIST.
(1) Deisenhofer, J.; Michel, H. Angew. Chem., Int. Ed. Engl. 1989, 28,
829-847.
(2) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34,
40-48. (b) Choi, M.-S.; Yamazaki, T.; Yamazaki, I.; Aida, T. Angew.
Chem., Int. Ed. 2004, 43, 150-158. (c) Imahori, H. J. Chem. Phys. B 2004,
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10.1021/jo060768n CCC: $33.50 © 2006 American Chemical Society
Published on Web 06/22/2006
5752
J. Org. Chem. 2006, 71, 5752-5760

Self-Assembled Fluorescent Hexaazatriphenylenes
aggregates composed of π-conjugated components is used as a
new type of light-harvesting antenna as well as an energy-
transfer reagent.8-10 Shinkai and co-workers reported that an
efficient energy transfer takes place from a one-dimensional
aggregate composed of an unsubstituted perylenediimide donor
to a substituted perylenediimide acceptor incorporated into the
aggregate.8 Meijer and co-workers found that an aggregate
composed of oligo(p-phenylenevinylene)s with different repeat-
ing units provides an attractive energy-transfer medium.9 In the
self-assembled systems, the acceptor molecules, which possess
structural similarity with the donor molecules, are efficiently
incorporated into the well-defined columnar-type aggregates
composed of donor molecules without destabilization of the self-
assembled ordering. Thus, study of the one-dimensional ag-
gregates composed of the π-conjugated components is a new
attractive task for chemists to develop an artificial light-
harvesting system.
tion of HAT derivatives with six alkyl/alkoxy-chain-containing
biphenyl groups and their self-assembling nature both in solution
and in the film state. Finally, the light-harvesting effect of the
HAT-based aggregate system is discussed.
Results and Discussion
Preparation. HATs with six alkyl/alkoxy-chain-containing
biphenyl groups (6a, 3′,4′-dioctyloxybiphenyl-4-yl; 6b, 4′-
octyloxybiphenyl-4-yl; 6c, 4′-octylbiphenyl-4-yl; 6d, 4′-((S)-
3,7-dimethyloctyloxy)biphenyl-4-yl) were prepared by conden-
sation reactions of the corresponding diaryl diketones 4a-d with
hexaaminobenzene 512 (Scheme 1). The key synthetic interme-
diate diaryl diketones 4a-d were derived from commercially
available 4,4′-dibromobenzil (3) by Suzuki coupling reactions
with the corresponding arylboronic acids 2a-d in the presence
of a palladium(0) catalyst. The boronic acids 2a-d were
obtained from the corresponding bromides 1a-d by treatment
with butyllithium followed with trimethyl borate. In the case
of 2b, the corresponding free boronic acid 2b′ was used without
protection with 2,2-dimethyl-1,3-propanediol. HATs 6a-d were
characterized on the basis of a spectroscopic method, MALDI-
TOF mass spectrometry, and elemental analysis.
HATs 6a-d can dissolve in common organic solvents such
as benzene, toluene, dichloromethane, chloroform, and THF.
Also, nonpolar solvents (hexane, octane, and cyclohexane) are
effective to dissolve 6a-d except for 6b in hexane and octane.
A transparent film of 6a-d can be prepared by a conventional
spin-coating technique.
UV-Vis Spectroscopy. π-Stacking interaction constructing
the one-dimensional aggregates was confirmed by means of
UV-vis spectroscopy. In polar chloroform solutions, 6a-d
provides two absorption bands around 410 and 360 nm (Figure
1a, Table 1, and Supporting Information). The two bands are
assigned to the 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. The
spectral pattern with an intense 0-0 transition band and a
shoulder-like 0-1 transition band changed scarcely depending
on the concentration and temperature, indicating that 6a-d
dissolved molecularly in polar chloroform (Figure 1a and
Supporting Information).
Recently, we reported that fluorescent hexaazatriphenylenes
(HATs) with six aromatic groups can be self-assembled one-
dimensionally in the concentrated solution state to form orga-
nogels as well as in the bulk state to form columnar liquid
crystals.11 This finding led us to design and prepare a new type
of light-harvesting system based on the HAT-based one-
dimensional aggregates. As a result of structural screening in
the precedent work, one can find that a combination between a
central HAT core and six peripheral biphenyl functional groups
plays an important role in constructing the one-dimensional
aggregates.11 However, detailed analysis of the HAT-based
aggregates was limited because of the high temperature (>300
°C) in the columnar liquid crystal formation and high concentra-
tion in the organogel formation. When the aggregate is formed
in a homogeneous organic solution at moderate or dilute
concentration, the aggregation behavior would be analyzed
conveniently by spectroscopic methods such as UV-vis,
1
fluorescence, and H NMR. The desired aggregate would be
obtained by introducing alkyl chains into the HAT derivative.
The alkyl chains can maintain a subtle balance between HAT-
HAT interaction and HAT-solvent interaction to provide the
one-dimensional aggregate. In this paper, we report the prepara-
(5) (a) Tsuda, A.; Nakamura, T.; Sakamoto, S.; Yamaguchi, K.; Osuka,
A. Angew. Chem., Int. Ed. 2002, 41, 2817-2821. (b) Hwang, I.-W.;
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Sutton L. R. Org. Lett. 2000, 2, 2435-2438.
In contrast, nonpolar hexane and cyclohexane solutions of
6a-d show different spectral patterns with an intense 0-1
transition band and a shoulder-like 0-0 transition band (Figure
1b, Table 1, and Supporting Information). The trend is very
similar to those of π-stacked aggregates with an H-type parallel
stacking mode,13 which is rationalized by the molecular exciton
model.14 In 6c, a concentration- and temperature-dependent
UV-vis spectral change was observable in the hexane and
octane solutions (Figure 1b and Supporting Information). The
absorbance of the 0-0 band around 380 nm increased with
decreasing concentration and increasing temperature. The result-
ing spectra were very similar to those observed in the chloroform
(7) (a) Drain, C. M.; Nifiatis, F.; Vasenko, A.; Batteas, J. D. Angew.
Chem., Int. Ed. 1998, 37, 2344-2347. (b) Schugar, H. J. Angew. Chem.,
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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.
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Chem., Int. Ed. 2003, 42, 332-335. (c) Sugiyasu, K.; Fujita, N.; Takeuchi,
M.; Yamada, S.; Shinkai, S. Org. Biomol. Chem. 2003, 895-899. (d)
Guerzo, A. D.; Olive, A. G. L.; Reichwagen, J.; Hoph, H.; Desvergne, J.-
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Bull. Chem. Soc. Jpn. 1989, 62, 3127-3131. (b) Mataka, S.; Shimojyo,
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Weiss, E. A.; Wasielewski, M. R. J. Am.Chem. Soc. 2002, 124, 9582-
9590. (b) Wang, W.; Han, J. J.; Wang, L.-Q.; Li, L.-S.; Shaw, W. J.; Li, A.
D. Q. Nano Lett. 2004, 3, 455-458.
(11) Ishi-i, T.; Hirayama, T.; Murakami, K.; Tashiro, H.; Thiemann, T.;
Kubo, K.; Mori, A.; Yamasaki, S.; Akao, T.; Tsuboyama, A.; Mukaide, T.;
Ueno, K.; Mataka, S. Langmuir 2005, 21, 1261-1268.
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1965, 11, 371-392.
J. Org. Chem, Vol. 71, No. 15, 2006 5753

Ishi-i et al.
SCHEME 1. Preparation of HATs Having Alkyl Side Chains, 6a-d
solutions. The phenomena are attributed to the dynamic
exchange between the monomer and the aggregate species.
In hexane, a comparison between the concentration depen-
dence of 6a, 6c, and 6d indicates the order of aggregate stability
to be 6a > 6d > 6c (Figure 1a and Supporting Information). In
cyclohexane, the concentration dependence of 6b is smaller than
that of 6a (Supporting Information). As a summary of the
foregoing results, one can conclude that the order of aggregate
stability is 6b > 6a > 6d > 6c. Compared to alkyl-chain-
containing HAT 6c, alkoxy-chain-containing HATs 6a, 6b, and
6d provide superior aggregative nature, which would be
attributed to the expansion of the π-system including oxygen
atoms. Among the alkoxy-chain-containing HATs, the low
stability of 6d would be ascribed to unfavored packing of the
chiral alkyl groups arising from the branched structure.15
Compared to 6b with six octyloxy groups, 6a with twelve
octyloxy groups seems to show more favorable solvation to
decrease the aggregative nature.
The chirality in 6d is reflected in CD spectra in hexane
(Supporting Information). Under the aggregated conditions of
0.1-1.0 mM hexane solution at 20 °C, Cotton effects are
observable around the absorption maxima, although the mag-
nitude is small. In contrast, such Cotton effects could not be
observed in polar chloroform solution. The results suggest that
the aggregate of 6d takes a helix stacking mode to make a left-
handed or right-handed helix.11
The aggregate structure formed in nonpolar solvent can be
preserved in the film state. For example, the spin-coating film
of 6b obtained from a cyclohexane solution provides an intense
0-1 band around 360 nm in the UV-vis spectrum (Figure 2).
The spectral pattern is very similar to that of the aggregated 6b
in the cyclohexane solution. The result shows that in the film
state the 6b molecules are self-assembled to preserve the
H-aggregate stacking mode created in the nonpolar cyclohexane
solution. In contrast, another spin-coating film obtained from a
polar chloroform solution, in which 6b dissolves molecularly,
provides a nonaggregated spectral pattern with an intense 0-0
band around 400 nm (Figure 2). Similar aggregation behavior
in the film state was observed in 6a, 6c, and 6d.
Fluorescence Spectroscopy. The aggregation of 6a-d is
reflected in steady-state fluorescence spectra. In chloroform, the
emission band around 460 nm in 6c changed scarcely depending
on the concentration (Figure 3b). In contrast, in hexane solutions
of 6c, the emission band is shifted bathochromically from 434
to 471 nm with increasing concentration (from 0.001 to 1.0
mM), according to facilitated aggregation (Figure 3a and Table
1). The emission band (471 nm) in a 1.0 mM hexane solution
of 6c is observed at longer wavelength than that (461 nm) in
1.0 mM chloroform solution; nevertheless, the polarity of hexane
is lower than that of chloroform. The bathochromic shift
observed in hexane solutions would be attributed to a stabiliza-
tion effect on the excited state in the aggregates.16 Probably,
(15) Kastler, M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; Mu¨llen, K. J.
Am. Chem. Soc. 2005, 127, 4286-4296.
(16) Ikeda, M.; Takeuchi, M.; Shinkai, S. Chem. Commun. 2003, 1354-
1355.
5754 J. Org. Chem., Vol. 71, No. 15, 2006

Self-Assembled Fluorescent Hexaazatriphenylenes
FIGURE 2. UV-vis spectra of the spin-coating films of 6b obtained
from chloroform and cyclohexane solutions (5 mM).
FIGURE 1. UV-vis spectra of 6c in (a) chloroform and (b) hexane
at 20 °C at 0.001, 0.01, 0.1, and 1.0 mM.
TABLE 1. UV-Vis and Fluorescence Spectral Data for 6a, 6b, 6c,
and 6d at 20 °C
concn
(mM)
λFL
compd
solvent
hexane
λ
max (nm) (ꢀ (M-1 cm-1))
(nm)
6a
1.0
0.1
0.01
1.0
0.1
0.01
1.0
0.01
1.0
0.1
0.01
1.0
0.01
1.0
0.1
0.01
0.001
1.0
0.01
1.0
356 (sh, 71000), 386 (sh, 65000)
356 (71000), 386 (sh, 67000)
356 (71000), 386 (sh, 69000)
363 (sh, 85000), 388 (87000)
363 (sh, 85000), 390 (90000)
363 (sh, 85000), 401 (98000)
370 (sh, 87000), 409 (100000)
370 (sh, 87000), 409 (100000)
349 (78000), 385 (sh, 71000)
354 (79000), 386 (78000)
497
492
487
cyclohexane
chloroform
504
504
482
472
462
479
479
471
464
456
434
461
461
481
477
471
480
480
6b
6c
cyclohexane
357 (81000), 391 (86000)
chloroform
hexane
362 (sh, 90000), 406 (110000)
362 (sh, 93000), 406 (110000)
344 (78000), 378 (sh, 68000)
345 (79000), 381 (sh, 73000)
344 (79000), 385 (86000)
345 (79000), 390 (92000)
chloroform
hexane
362 (sh, 90000), 406 (110000)
362 (sh, 93000), 406 (110000)
347 (82000), 382 (sh, 71000)
347 (82000), 382 (sh, 73000)
347 (82000), 387 (79000)
363 (sh, 73000), 406 (90000)
363 (sh, 73000), 406 (90000)
6d
0.1
0.01
1.0
FIGURE 3. Steady-state fluorescence spectra of 6c at 20 °C in (a)
hexane (λex ) 391 nm) and (b) chloroform (λex ) 403 nm) at 0.001,
0.01, 0.1, and 1.0 mM.
chloroform
0.01
the stabilization effect is ascribed to the migration of the
photogenerated exciton or the excimer formation in the ag-
gregate structures.17 Similar concentration-dependent emission
behavior was observable in 6a, 6b, and 6d (Table 1 and
Supporting Information). The order of the concentration-
dependent fluorescence spectral change in 6a-d is in accordance
with that of the UV-vis spectral change, indicating again the
order of aggregate stability to be 6b > 6a > 6d > 6c.
In chloroform solutions, 6a-d provide moderate fluorescence
quantum yields (ΦFL) of 0.54-0.60. The ΦFL values are reduced
J. Org. Chem, Vol. 71, No. 15, 2006 5755

Ishi-i et al.
TABLE 2. Fluorescence Quantum Yields (ΦFL) for 6a, 6b, 6c, and
6da
ΦFL
compd
hexane
0.39
cyclohexane
chloroform
6a
6b
6c
6d
0.54
0.47
0.32
0.46
0.60
0.60
0.54
0.60
0.21
0.36
a Relative to quinine bisulfate (0.55).
in nonpolar hexane (0.21-0.39) and cyclohexane (0.32-0.54)
solutions (Table 2); nevertheless, the polarity of hexane and
cyclohexane is smaller than that of chloroform. The lowering
emission ability is also supportive of the aggregation of 6a-d
in the nonpolar solvents.
The stabilization of the excited state in the aggregates of
6a-d would be supported by time-resolved fluorescence spectra.
For a dilute 0.001 mM hexane solution of 6c, the decay curve
at 487 nm was fitted to a double exponential with lifetimes of
0.36 ns (89%) and 1.60 ns (11%). The shorter lifetime with
89% population suggests that the monomer species of 6c exists
mainly at the dilute 0.001 mM concentration. At a high
concentration of 1.0 mM, the populations of the two lifetimes
are inverted to be 14% (0.57 ns) and 86% (2.22 ns), according
to facilitated aggregation.18,19 The difference in both the shorter
and longer lifetimes (0.36 ns versus 0.57 ns for the short lifetime
and 1.60 ns versus 2.22 ns for the long lifetime) is likely due
to the error introduced by the assumption that decay is fitted
with only two distinct lifetimes. Theoretically, the decay is more
complicated and is fitted to a multiple exponential, because the
aggregates have size and length distributions.17,18 However, it
can be emphasized that the aggregates obtained at higher
concentration include more long-lived species compared to those
obtained at lower concentration, indicating that the aggregation
is facilitated with increasing concentration. Similarly, fluores-
cence decays of 6a in hexane and 6b in cyclohexane were fitted
with a double exponential, and a long-lived species was given
at high concentration, although the trend of lifetime populations
was not straightforward such as that of 6c arising from the
analysis error (Supporting Information).
FIGURE 4. 1H NMR spectra of 6c in octane-d18 at 0.005, 0.01, 0.05,
0.1, 0.5, and 1.0 mM: (a) at 20 °C and (b) at 100 °C.
1
1H NMR Spectroscopy. In the H NMR spectra, a line-
broadening effect arising from the π-stacked aggregation was
observed in 6a-c in octane-d18 and cyclohexane-d12. The
chemical shifts and the shape of the broad resonances are
dependent on the concentration and temperature, indicating the
dynamic exchange between the monomer and aggregate species.
Typical examples of 6c in octane-d18 at 20 and 100 °C are shown
in Figure 4. At high concentration (∼1.0 mM) the proton signals
appeared in a higher magnetic field compared to those at dilute
concentration (∼0.0005 mM). The higher magnetic field shift
is ascribed to the π-stacking among the HAT core moieties.11
The chemical shifts for the ArOCH2 proton for 6b and the
ArCH2 proton for 6c were plotted against the concentration
(Figure 5). For 6a, the chemical shifts for the ArOCH2 protons
could not be checked because of the overlap for the two types
of ArOCH2 proton signals. The concentration dependence at
20 °C can be analyzed by an infinite association model,20 by
which the self-association constant (K) was estimated (Table
3). In cyclohexane-d12 at 20 °C, the K value (74300 M-1) of
6b is larger by 1 order of magnitude than that (5750 M-1) of
6c, indicating the superior aggregative nature of 6b. In 6c, the
K value (10500 M-1) in octane-d18 is 2 times larger than that
(5750 M-1) in cyclohexane-d12.
In octane-d18 solutions of 6c, the K values were obtained at
different temperatures in the range of 20-100 °C (Figure 6 and
Table 3). The thermodynamic parameters for the self-association
were estimated by the van’t Hoff plot. The obtained ∆H° (-29.3
kJ mol-1) and ∆S° (-22.8 J mol-1 K-1) values seem to indicate
an entropically driven self-association. Probably, the 6a-d
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Nuckolls, C. J. Am. Chem. Soc. 2004, 126, 5234-5242.
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J.; Park, S. Y. Chem. Commun. 2004, 70-71.
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5756 J. Org. Chem., Vol. 71, No. 15, 2006

Self-Assembled Fluorescent Hexaazatriphenylenes
TABLE 3. Association Constants (K)a and Thermodynamic Parametersb for Self-Aggregation of 6b and 6c
K (M-1
)
∆H°
∆S°
compd
solvent
20 °C
40 °C
60 °C
80 °C
100 °C
(kJ mol-1
)
(J mol-1 K-1
)
6b
6c
6c
cyclohexane-d12
cyclohexane-d12
octane-d18
74300
5750
10500
5400
2400
1400
840
-29.3
-22.8
a Determined on the basis of the infinite model by nonlinear least-squares fitting of chemical shift data.20 The experimental uncertainty of the K value
amounts to about (5-10%. All the correlation coefficients were >0.99. b Determined by the van’t Hoff plot. The correlation coefficient was 0.999.
CHART 1. Molecular Structure of 7 and 8 as a
Light-Harvesting Acceptor and Donor, Respectively
FIGURE 5. Chemical shift vs concentration for 6b (ArOCH2 proton)
in cyclohexane-d12 and 6c (ArCH2 proton) in cyclohexane-d12 and in
octane-d18 at 20 °C.
of 6. In addition, 7 also acts as an electron acceptor to generate
charge separation when HAT derivative 811 with six electron-
rich triphenylamino (TPA) moieties is used as an electron-donor
FIGURE 6. Chemical shift vs concentration for 6c in octane-d18 at
20, 40, 60, 80, and 100 °C.
molecule (Chart 1). Here, we selected HAT 6b as a light-
harvesting antenna as well as an energy-transfer reagent, because
molecules are self-assembled mainly by π-stacking interactions
in the central HAT core in nonpolar solvent.
6b has the most superior aggregative nature among 6a-d.
A film sample of 6b doped with 5 mol % 7 (100:5 mixture
Light Harvesting. As described above, fluorescent HAT
of 6b/7) was prepared by spin coating from a cyclohexane
derivatives 6a-d form one-dimensional aggregates both in
solution and in the film state. To apply these fluorescent
aggregates to a light-harvesting antenna, we selected HAT
solution, and the energy transfer from the 6b-based aggregate
to 7 incorporated into the aggregate was examined. The spin-
coating samples provide a clear overlap of the fluorescence
derivative 7 with six perylenediimide (PDI) moieties as an
spectrum of 6b and the absorption spectrum of 7 (Supporting
energy-acceptor molecule (Chart 1). In a precedent study, we
have reported that 7 is self-assembled both in solution and in
the film state to form a highly stabilized dimer aggregate, in
Information). In the precedent artificial light-harvesting systems
based on the self-assembled one-dimensional aggregates, quan-
titative analysis of the energy transfer was difficult due to
which an efficient energy transfer occurs from the HAT core
significant overlap between donor and acceptor emissions,
to the PDI moiety.21 These results provide the valuable
information that an efficient light-harvesting system can be
created when the dimer of 7 is incorporated into the aggregate
(21) Ishi-i, T.; Murakami, K.; Imai, Y.; Mataka, S. Org. Lett. 2005, 7,
3175-3178.
J. Org. Chem, Vol. 71, No. 15, 2006 5757

Ishi-i et al.
FIGURE 8. Fluorescent images under UV light (365 nm) for 6a/7/8
mixtures with 100:0:0, 100:5:0, 100:5:5, and 0:0:5 molar ratios (from
left to right) in the spin-coating film.
Conclusion
In conclusion, we have demonstrated that hexaazatri-
phenylenes with six alkyl- and alkoxy-chain-containing biphenyl
groups can be self-assembled to form one-dimensional ag-
gregates both in solution and in the film state. The aggregation
of the hexaazatriphenylene molecules is mainly stabilized by
the π-stacking of the central aromatic core. The aggregation
ability highly depends on the nature of the alkyl and alkoxy
chains at the peripheral positions. Fluorescence spectroscopy
is a useful method to study the aggregation. According to
facilitated aggregation, the emission band shifts to a longer
wavelength region together with reduction of the fluorescence
quantum yield as well as an increase of the fluorescence lifetime.
The self-assembled fluorescent hexaazatriphenylene can be
developed to an artificial light-harvesting antenna as well as an
energy-transfer reagent. When a hexaazatriphenylene derivative
having an energy- and electron-accepting perylenediimide
moiety is doped with the fluorescent hexaazatriphenylene
aggregate, an efficient energy transfer takes place from the
aggregate to the acceptor moiety. Further, by the combination
with the hexaazatriphenylene derivative having an electron-
donating triphenylamino moiety, an electron transfer can be
achieved between the donor and acceptor moieties in the self-
assembled aggregate system. We believe that the present study
will provide valuable new information not only for creation of
one-dimensional architectures but also for artificial light-
harvesting systems.
FIGURE 7. Fluorescence spectra excited at the HAT chromophore
(395 nm) of 6a/7/8 mixtures with 100:0:0 (black line), 100:5:0 (red
line), 100:5:5 (blue line), and 0:0:5 (green line) molar ratios in the
spin-coating film (from cyclohexane solution) at 20 °C. The fluores-
cence spectrum of the 100:5:0 mixture excited at the PDI chromophore
(at 517 nm) is indicated by a dotted red line.
because the donor and acceptor components have similar
structural features such as perylenediimides with different
substituents and oligo(p-phenylenevinylene)s with different
repeating units.8,9 On the other hand, the present system has a
versatile advantage of good separation between the donor and
acceptor emissions, because of the structural difference of the
donor HAT and the acceptor PDI moieties.
In the presence of 7, the emission from 6b around 510 nm
was quenched significantly even upon excitation of the HAT
chromophore (Figure 7). In addition, the PDI emission around
650 nm upon indirect excitation of the HAT chromophore is
enhanced by a factor of 4 compared to that observed upon direct
excitation of the PDI chromophore, indicating the light-
harvesting effect of the 6b-based aggregate (Figure 7). A good
energy transfer efficiency of ca. 80% can be estimated on the
basis of a comparison between absorption and excitation spectra
(Supporting Information). When the spin-coating film was
prepared from polar chloroform solutions including molecularly
dissolved 6b, the light-harvesting effect was reduced signifi-
cantly (Supporting Information), suggesting the importance of
the 6b-based aggregate as a good light-harvesting antenna as
well as an energy transfer reagent.
Experimental Section
Spectroscopic Measurement. UV-vis spectra were measured
in a 0.01 cm width quartz cell (1.0 mM), a 0.1 cm cell (0.1 mM),
a 1.0 cm cell (0.01 mM), and a 10.0 cm cell (0.001 mM).
Fluorescence spectra were measured in a 1.0 cm width quartz cell.
Time-resolved fluorescence spectra were measured by a single-
photon-counting method with a nitrogen-filled nanosecond flash
lamp (pulse width of 1 ns, pulse repetition rate of 40 Hz).
Preparation of Spin-Coating Films. Film samples for the
measurements of UV-vis and fluorescence spectroscopy were
prepared by drop casting and subsequent spin coating (2000 rpm,
30 s) from cyclohexane or chloroform solutions on a quartz cell
(12.5 × 12.5 × 45 mm). The 100:0:0 molar ratio sample was
obtained from a 5.0 mM cyclohexane solution (50 µL) of 6b, the
100:5:0 molar ratio sample from a cyclohexane solution (50 µL)
including 5.0 mM 6b and 0.25 mM 7, the 100:5:5 molar ratio
sample from a cyclohexane solution (50 µL) including 5.0 mM
6b, 0.25 mM 7, and 0.25 mM 8, and the 0:5:0 molar ratio sample
from a 0.25 mM chloroform solution (50 µL) of 7.
The enhanced HAT emission in the 100:5 mixture of 6b/7 is
reduced to one-third, when 5 mol % 8 is newly doped as an
electron donor (Figure 7). The result suggests that in the 100:
5:5 mixture of 6b/7/8, an electron transfer takes place from the
excited state of the PDI moiety in 7, generating an ion pair of
the PDI anion radical in 7 and the TPA cation radical in 8.22,23
The energy transfer and subsequent electron transfer phenomena
were easily visualized by a change in the emission color from
brilliant green HAT emission to brilliant red PDI emission and
after very weak PDI emission (Figure 8).
(22) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.;
Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119-1122.
(23) Sandanayaka, S. D. A.; Matsukawa, K.; Ishi-i, T.; Mataka, S.; Araki,
Y.; Ito, O. J. Phys. Chem. B 2004, 108, 19995-20004.
General Procedure for Preparation of 2,2-Dimethyl-1,3-
propanediol 3,4-Bis(octyloxy)phenylboronate (2a). To a solution
5758 J. Org. Chem., Vol. 71, No. 15, 2006

Self-Assembled Fluorescent Hexaazatriphenylenes
of 1a (921 mg, 2.3 mmol) in dry THF (10 mL) was added dropwise
1.58 M n-butyllithium/hexane solution (1.7 mL, 2.7 mmol) at -78
°C for 15 min under an argon atmosphere. After the mixture was
stirred at -78 °C for 0.5 h, trimethyl borate (340 µL, 3.0 mmol)
was added dropwise for 15 min to the mixture. The mixture was
stirred at -78 °C for 1 h and warmed to room temperature. After
the reaction mixture was quenched with aqueous 1.2 M hydrochloric
acid solution (to pH 5-7), it was extracted with diethyl ether. The
combined organic layers were washed with brine, dried over
anhydrous magnesium sulfate, and evaporated in vacuo to dryness.
The residue (760 mg) and 2,2-dimethyl-1,3-propanediol (208 mg,
2.0 mmol) were dissolved in dichloromethane (20 mL), and the
resulting solution was stirred at room temperature for 2 h under an
argon atmosphere. After the evaporation, the residue was purified
by silica gel column chromatography (KANTO 60N) eluting with
dichloromethane/hexane (2:3, v/v) to give 2a in 40% yield (411
mg, 0.92 mmol) as a colorless oil: IR (KBr, cm-1) 2927, 1598,
4.00 (t, J ) 6.6 Hz, 4 H), 7.13 (d, J ) 8.3 Hz, 4 H), 7.57 (d, J )
8.3 Hz, 4 H), 7.70 (d, J ) 8.3 Hz, 4 H, ArH), 8.03 (d, J ) 8.3 Hz,
4 H, ArH); FAB-MS (positive, NBA) m/z 619 [(M + 1)+]. Anal.
Calcd for C42H50O4: C, 81.51; H, 8.14. Found: C, 81.45; H, 8.13.
Data for bis(4′-octylbiphenyl-4-yl)ethanedione (4c): yellow
powder; mp 122-124 °C; IR (KBr, cm-1) 2921, 2851, 1660 (νCd
O), 1601, 1180, 881; 1H NMR (CDCl3) δ 0.88 (t, J ) 6.9 Hz, 6 H),
1.18-1.32 (m, 20 H), 1.65 (quint, J ) 7.9 Hz, 4 H), 2.66 (t, J )
7.9 Hz, 4 H), 7.29 (d, J ) 8.6 Hz, 4 H), 7.56 (d, J ) 8.6 Hz, 4 H),
7.73 (d, J ) 8.6 Hz, 4 H), 8.06 (d, J ) 8.6 Hz, 4 H); FAB-MS
(positive, NBA) m/z 587 [(M + 1)+]. Anal. Calcd for C42H50O2:
C, 85.96; H, 8.59. Found: C, 85.92; H, 8.58.
Data for bis{4′-[(S)-3,7-dimethyloctyloxy]biphenyl-4-yl}-
ethanedione (4d): white powder; mp 102-104 °C; IR (KBr, cm-1
)
2925, 1677, 1660 (νCdO), 1600, 1249, 1184, 828; 1H NMR (CDCl3)
δ 0.87 (d, J ) 6.6 Hz, 12 H), 0.96 (d, J ) 6.3 Hz, 6 H), 1.15-1.87
(m, 20 H), 4.05 (t, J ) 6.6 Hz, 4 H), 6.99 (d, J ) 8.6 Hz, 4 H),
7.58 (d, J ) 8.6 Hz, 4 H), 7.70 (d, J ) 8.3 Hz, 4 H), 8.04 (d, J )
8.3 Hz, 4 H); FAB-MS (positive, NBA) m/z 675 [(M + 1)+]. Anal.
Calcd for C46H58O4: C, 81.86; H, 8.66. Found: C, 81.68; H, 8.64.
General Procedure for Preparation of 2,3,6,7,10,11-Hexakis-
{3′,4′-dioctyloxybiphenyl-4-yl}-1,4,5,8,9,12-hexaazatriphenyl-
ene (6a). A mixture of hexaaminobenzene trihydrochloride (5) (13
mg, 0.047 mmol) and 4a (123 mg, 0.14 mmol) in acetic acid (5
mL) was heated at the refluxing temperature for 17 h under an
argon atmosphere. After the reaction mixture was cooled to room
temperature, it was poured into water and extracted with chloroform.
The combined organic layers were washed with saturated aqueous
sodium hydrogen carbonate solution and with brine, dried over
anhydrous magnesium sulfate, and evaporated in vacuo to dryness.
The residue was separated by silica gel chromatography (WAKO
C300) eluting with chloroform/hexane (1:2, v/v) and by GPC
(polystyrene JAIGEL-1H and 2H) eluting with chloroform to give
6a in 35% yield (44 mg, 0.016 mmol) as a glassy brown solid: mp
> 250 °C dec; IR (KBr, cm-1) 2924, 2854, 1604, 1500, 1365, 1250,
1
1476, 1420, 1317 (νB-O), 1261, 1233, 1138; H NMR (CDCl3) δ
0.88 (t, J ) 6.6 Hz, 6 H), 1.02 (s, 6 H), 1.28-1.54 (m, 20 H), 1.81
(quint, J ) 6.6 Hz, 4 H), 3.75 (s, 4 H), 4.01 (t, J ) 6.6 Hz, 2 H),
4.02 (t, J ) 6.6 Hz, 2 H), 6.86 (d, J ) 7.9 Hz, 1 H), 7.30 (d, J )
2.3 Hz, 1 H), 7.36 (dd, J ) 2.3, 7.9 Hz, 1 H); FAB-MS (positive,
NBA) m/z 446 (M+); HR-MS (EI, positive) m/z calcd for C27H47O4B
446.3567, found 446.3571 (M+).
1
Data for 4-octyloxyphenylboronic acid (2b′): white solid; H
NMR (CDCl3) δ 0.89 (t, J ) 6.6 Hz, 3 H), 1.25-1.55 (m, 10 H),
1.72-1.88 (m, 2 H), 4,05 (t, J ) 6.6 Hz, 2 H), 7.00 (d, J ) 7.5
Hz, 2 H), 8.14 (d, J ) 7.5 Hz, 2 H).
Data for 2,2-dimethyl-1,3-propanediol 4-octylphenylboronate
(2c): white needles; mp 44-46 °C; IR (KBr, cm-1) 2925, 1611,
1
1480, 1419, 1379, 1307 (νB-O), 1248, 1131; H NMR (CDCl3) δ
0.87 (t, J ) 6.3 Hz, 3 H), 1.02 (s, 6 H), 1.26-1.28 (m, 10 H), 1.60
(quint, J ) 7.3 Hz, 2 H), 2.60 (t, J ) 7.3 Hz, 2 H), 3.76 (s, 4 H),
7.17 (d, J ) 7.9 Hz, 2 H), 7.71 (d, J ) 7.9 Hz, 2 H); FAB-MS
(positive, NBA) m/z 302 (M+). Anal. Calcd for C19H31BO2: C,
75.50; H, 10.34. Found: C, 75.59; H, 10.30.
1
1142; H NMR (CDCl3) δ 0.88 (t, J ) 6.6 Hz, 36 H), 1.29-1.55
(m, 120 H), 1.85 (quint, J ) 6.6 Hz, 24 H), 4.06 (t, J ) 6.6 Hz, 12
H), 4.08 (t, J ) 6.6 Hz, 12 H), 6.96 (d, J ) 8.3 Hz, 6 H), 7.21-
7.26 (m, 12 H), 7.65 (d, J ) 8.6 Hz, 12 H), 8.00 (d, J ) 8.6 Hz,
12 H); MALDI-TOF-MS (positive, dithranol) m/z 2685.00 [M+,
calcd for C180H246N6O12 2684.89]. Anal. Calcd for C180H246N6O12:
C, 80.49; H, 9.23; N, 3.13. Found: C, 80.26; H, 9.11; N 3.13.
Data for 2,3,6,7,10,11-hexakis(4′-octyloxybiphenyl-4-yl)-
1,4,5,8,9,12-hexaazatriphenylene (6b): glassy yellow solid; mp
> 250 °C dec; IR (KBr, cm-1) 2924, 2854, 1605, 1497, 1363, 1247,
Data for 2,2-dimethyl-1,3-propanediol 4-{(S)-3,7-dimethyloctyl-
oxy}phenylboronate (2d): colorless oil; IR (KBr, cm-1); 2959,
1602, 1568, 1344, 1318 (νB-O), 1173, 1132, 1019; 1H NMR
(CDCl3) δ 0.86 (d, J ) 6.6 Hz, 6 H), 0.93 (d, J ) 6.4 Hz, 3 H),
1.00 (s, 6 H), 1.13-1.83 (m, 10 H), 3.74 (s, 4 H), 4.00 (t, J ) 6.4
Hz, 2 H), 6.86 (d, J ) 8.7 Hz, 2 H), 7.71 (d, J ) 8.7 Hz, 2 H);
FAB-MS (positive, NBA) m/z 346 (M+); HR-MS (EI, positive)
m/z calcd for C21H35O3B 346.2683, found 346.2681 (M+).
General Procedure for Preparation of Bis{3′,4′-dioctyloxy-
biphenyl-4-yl)ethanedione (4a). To a mixture of 3 (301 mg, 0.82
mmol) and tetrakis(triphenylphosphine)palladium(0) (60 mg, 0.05
mmol, 3 mol %) in benzene (32 mL) were added 2a (822 mg, 1.80
mmol), ethanol (8 mL), and aqueous 2 M sodium carbonate solution
(16 mL) at 60 °C under an argon atmosphere, and the resulting
mixture was heated at 80 °C for 18 h. The reaction mixture was
poured into water and extracted with chloroform. The organic layer
was washed with brine, dried over anhydrous magnesium sulfate,
and evaporated in vacuo to dryness. The residue was purified by
silica gel column chromatography (WAKO C300) eluting with
chloroform/hexane (1:1, v/v) to give 4a in 90% yield (644 mg,
1
822; H NMR (CDCl3) δ 0.90 (t, J ) 6.6 Hz, 18 H), 1.30-1.50
(m, 60 H), 1.82 (quint, J ) 6.6 Hz, 12 H), 4.00 (t, J ) 6.6 Hz, 12
H), 6.97 (d, J ) 8.6 Hz, 12 H), 7.60 (d, J ) 8.6 Hz, 12 H), 7.62
(d, J ) 8.6 Hz, 12 H), 7.96 (d, J ) 8.6 Hz, 12 H); MALDI-TOF-
MS (positive, dithranol) m/z 1916.13 [M+, calcd for C132H150N6O6
1916.16]. Anal. Calcd for C132H150N6O6: C, 82.72; H, 7.89; N,
4.38. Found: C, 82.64; H, 7.85; N, 4.35.
Data for 2,3,6,7,10,11-hexakis(4′-octylbiphenyl-4-yl)-1,4,5,8,9,-
12-hexaazatriphenylene (6c): glassy yellow solid; mp > 250 °C
dec; IR (KBr, cm-1) 2923, 2851, 1605, 1496, 1363, 1232, 1192,
1
1137, 1005, 815; H NMR (CDCl3) δ 0.89 (t, J ) 6.4 Hz, 18 H),
0.74 mmol) as an orange powder: mp 108-111 °C; IR (KBr, cm-1
)
1.29-1.66 (m, 72 H), 2.66 (t, J ) 6.9 Hz, 12 H), 7.28 (d, J ) 8.3
Hz, 12 H), 7.60 (d, J ) 8.3 Hz, 12 H), 7.69 (d, J ) 8.6 Hz, 12 H),
8.01 (d, J ) 8.6 Hz, 12 H); MALDI-TOF-MS (positive, dithranol)
m/z 1819.57 (M+, calcd for C132H150N6 1820.20). Anal. Calcd for
2922, 1668 (νCdO), 1597, 1523, 1257, 1205, 1176, 1141, 835, 813;
1H NMR (CDCl3) δ 0.88 (t, J ) 6.6 Hz, 12 H), 1.29-1.55 (m, 40
H), 1.85 (quint, J ) 6.6 Hz, 8 H), 4.05 (t, J ) 6.6 Hz, 4 H), 4.06
(t, J ) 6.6 Hz, 4 H), 6.96 (d, J ) 8.3 Hz, 2 H), 7.15 (d, J ) 2.3
Hz, 2 H), 7.18 (dd, J ) 2.3, 8.3 Hz, 2 H), 7.70 (d, J ) 8.6 Hz, 4
H), 8.03 (d, J ) 8.6 Hz, 4 H); FAB-MS (positive, NBA) m/z 875
[(M + 1)+]. Anal. Calcd for C58H82O6: C, 79.59; H, 9.44. Found:
C, 79.59; H, 9.46.
Data for bis(4′-octyloxybiphenyl-4-yl)ethanedione (4b): yellow
powder; mp 157-158 °C; IR (KBr, cm-1) 2954, 2921, 2856, 1666
(νCdO), 1598, 1189, 827, 817; 1H NMR (CDCl3) δ 0.89 (t, J ) 6.6
Hz, 6 H), 1.20-1.55 (m, 20 H), 1.81 (quint, J ) 6.6 Hz, 4 H),
C
132H150N6: C, 87.08; H, 8.30; N, 4.62. Found: C, 87.11; H, 8.36;
N, 4.68.
Data for 2,3,6,7,10,11-hexakis{4′-[(S)-3,7-dimethyloctyloxy]-
biphenyl-4-yl}-1,4,5,8,9, 12-hexaazatriphenylene (6d): glassy
yellow solid; mp > 250 °C dec; IR (KBr, cm-1) 2925, 1605, 1524,
1497, 1364, 1247, 1194, 822; 1H NMR (CDCl3) δ 0.88 (d, J ) 6.6
Hz, 36 H), 0.97 (t, J ) 6.4 Hz, 18 H), 1.12-1.90 (m, 80 H), 4.05
(t, J ) 6.6 Hz, 12 H), 6.99 (d, J ) 8.7 Hz, 12 H), 7.61 (d, J ) 8.7
Hz, 12 H), 7.65 (d, J ) 8.6 Hz, 12 H), 8.00 (d, J ) 8.6 Hz, 12 H);
J. Org. Chem, Vol. 71, No. 15, 2006 5759

Ishi-i et al.
MALDI-TOF-MS (positive, dithranol) m/z 2084.94 (M+, calcd for
144H174N6O6 2084.35). Anal. Calcd for C144H174N6O6: C, 82.95;
Researchers with Fixed-term Appointments, Special Coordina-
tion Funds for Promoting Science and Technology.
C
H, 8.41; N, 4.03. Found: C, 82.79; H, 8.40; N 4.10.
Supporting Information Available: 1H NMR, UV-vis, excita-
tion, and fluorescence spectra, van’t Hoff plot for self-association,
fluorescence lifetimes, and general experimental procedures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. We thank Professor Dr. Shigeori Tak-
enaka (Kyushu Institute of Technology) and Dr. Keiichi Otsuka
(Kyushu University) for the measurement of MALDI-TOF-MS.
This work was partially supported by Supporting Young
JO060768N
5760 J. Org. Chem., Vol. 71, No. 15, 2006