Informative secondary chiroptics in binary molecular organogel systems for donor-acceptor energy transfer

Article abstract of DOI:10.1016/j.tetlet.2011.05.131

Pyrene-doped l-glutamide-based lipidic derivatives with different alkyl lengths (C_n-g-Pyr; n = 4, 8 and 12) were newly synthesized. All of the C_n-g-Pyr dissolved and showed thermotopically and lyotropically-induced excimer formations accompanied by induction of the positive Cotton effect in their CD spectra, indicating chirally ordered stacking. However, when C₄-g-Pyr and C₁₂-g-Pyr were mixed in a certain molar ratio, an unusual CD pattern from positive to negative ones was observed. In this study, energy transfer efficiency was investigated in a binary system of C_n-g-Pyr with C₁₂-g-TPP. The results revealed that simple modification of the alkyl length of C_n-g-Pyr enables enhancement of the energy transfer efficiency with C₁₂-g-TPP.

Full text of DOI:10.1016/j.tetlet.2011.05.131

Tetrahedron Letters 52 (2011) 4030–4035
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Informative secondary chiroptics in binary molecular organogel systems
for donor–acceptor energy transfer
Koji Miyamoto a, Hirokuni Jintoku a, Tsuyoshi Sawada a, Makoto Takafuji a, Takashi Sagawa b,
Hirotaka Ihara a,c,
⇑
a Department of Applied Chemistry and Biochemistry, Kumamoto University, Kumamoto 860-8555, Japan
b Institute of Advanced Energy, Kyoto University, Gokasho, Uji Kyoto 611-0011, Japan
c Kumamoto Institute for Photo-Electro Organics (Phenics), Kumamoto 862-0901, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Pyrene-doped L-glutamide-based lipidic derivatives with different alkyl lengths (Cn-g-Pyr; n = 4, 8
Received 15 April 2011
Revised 22 May 2011
Accepted 27 May 2011
Available online 2 June 2011
and 12) were newly synthesized. All of the Cn-g-Pyr dissolved and showed thermotopically and
lyotropically-induced excimer formations accompanied by induction of the positive Cotton effect in their
CD spectra, indicating chirally ordered stacking. However, when C4-g-Pyr and C12-g-Pyr were mixed in a
certain molar ratio, an unusual CD pattern from positive to negative ones was observed. In this study,
energy transfer efficiency was investigated in a binary system of Cn-g-Pyr with C12-g-TPP. The results
revealed that simple modification of the alkyl length of Cn-g-Pyr enables enhancement of the energy
transfer efficiency with C12-g-TPP.
Keywords:
Organogel
Energy transfer
FRET
Ó 2011 Elsevier Ltd. All rights reserved.
Chiroptics
Excimer
One-dimensionally nano-assembled materials have become
attractive as molecular electronic devices for mimicking natural
photosynthesis1 in terms of light-harvesting and energy migration2
when molecular donors and accepters can be integrated into their
nanostructures. There are several advantages to the use of these
tempted to develop a method enabling the use of a chiroptical
signal as an indicator for monitoring, controlling and optimizing
the molecular ordering state in a donor–accepter energy transfer
system. To accomplish this, we selected L-glutamic acid-based
self-assembling tool (g)6–8 and newly synthesized g-functionalized
pyrene derivatives with different alkyl lengths as lipophilic moie-
ties10,11 (Cn-g-Pyr; n = 4, 8 and 12) (Scheme 1). We found that
simple modification of the alkyl length induced drastic chiroptical
changes in their aggregation states and enabled enhancement of
the energy transfer efficiency with a porphyrin derivative in a
donor–acceptor binary assembled system.
materials over rigid p-conjugated polymer systems including good
dispersity, diverseness of chemical tuning and supramolecular
functionalization. Conversely, chiral molecules such as amino acids
and sugars comprise some of the most useful base materials for
one-dimensional fabrication of nanoscale suprastructures.3 For
example, a polymer of amino acids folds spontaneously into
one-dimensional rigid structures such as
a
-helix and 31-helix.4
The solubility (dispersity) of Cn-g-Pyr was evaluated in a 1 mM
dispersion. As summarized in Table 1, all Cn-g-Pyr showed good
and wide solubility in various organic solvents at 60 °C, with the
exception of n-hexane. Conversely, apparent mass gelation behav-
iours were observed in some organic solvents such as benzene, tol-
uene and cyclohexane at 10 °C. This phenomenon can be classified
into ‘‘low-molecular mass gelation’’ or ‘‘molecular gelation’’.13 TEM
observations showed that well-developed nano-fibrillar aggregates
were mainly included in the cast film with Cn-g-Pyr (Fig. S1),
which indicated that low-molecular mass gelation can be induced
through three-dimensional network formation with fibrillar
aggregates.
Furthermore, amino acid-derived amphiphilic and lipophilic mole-
cules often create nanoscale helical, tubular and fibrillar aggregates
through molecular ordering in aqueous5,6 and organic media.7–9
The similarities between these systems are due to peptide bonds
working as the main driving force for the creation of suprastruc-
tures with unique chiral signals in their aggregation morphology
and chiroptical properties.
In this study, we investigated chiroptical information in amino
acid-derived donors and acceptors as model compounds expected
to be used in molecular electronic devices. Specifically, we at-
The finding that N2,N3-dialkyl
alkyl groups (C4-g) have similar organogelation ability to a C12-g
L-glutamide moieties with shorter
⇑
Corresponding author. Tel./fax: +81 96 342 3661.
E-mail address: ihara@kumamoto-u.ac.jp (H. Ihara).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.05.131

K. Miyamoto et al. / Tetrahedron Letters 52 (2011) 4030–4035
4031
Scheme 1.
Table 1
Solubility of Cn-g-Pyr in various organic solvents at 10 and 60 °C
Solvents
er12
10 °C
60 °C
C4-g-Pyr
C8-g-Pyr
C12-g-Pyr
C4-g-Pyr
C8-g-Pyr
C12-g-Pyr
n-Hexane
Cyclohexane
Benzene
1.88
I
I
I
I
I
I
2.02
2.27
2.38
4.81
6.02
7.58
G
G
G
S
G
S
S
S
S
P
G
G
G
S
G
S
S
S
S
P
G
G
G
S
G
S
P
P
P
P
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
Toluene
Chloroform
Ethyl acetate
THF
Acetone
Ethanol
20.5
24.5
32.6
35.9
Methanol
Acetonitrile
Concentration: 1.0 mM, S: solution, I: insoluble, P: precipitation, G: gelation.
moieties is important because it indicates that any long-chain alkyl
group can work as a non-conductive moiety when used in a semi-
conducting organic thin film; thus, a shorter alkyl length such as a
C4 group would be more desirable for this purpose. In addition, this
finding suggested that the g moiety is essential for gelation in this
system, while the alkyl length affects the solubility or lipophilicity.
The aggregation states of Cn-g-Pyr in organic media can be esti-
mated by their chiroptical properties, which are determined by CD
and UV spectroscopy because a pyrene moiety is attached to a g
values at [h]max. However, similar behaviours could be confirmed
in other assembling solvents, such as toluene and ethyl acetate.
These findings suggest that all Cn-g-Pyr can form chirally ordered
secondary structures in an assembling solvent.
An unusual chirality change was observed in binary systems
containing the proper combination of C4-g-Pyr and C12-g-Pyr. As
shown in Figure 2a, a 2-to-1 mixed system of C4-g-Pyr and C12
-
g-Pyr in benzene provided a strong and negative Cotton effect
(indicating S-chirality), which was opposite to the signal produced
by their single component systems (indicating R-chirality). The
remarkable changes were not observed in the cases of C12-g-Pyr/
C8-g-Pyr and C8-g-Pyr/C4-g-Pyr. A detailed investigation of this
phenomenon revealed several important characteristics. First,
[h]max in the 2-to-1 binary system was observed at 362 nm, which
is slightly red-shifted from its location at 358 and 355 nm in the
single component systems of C4- and C12-g-Pyr, respectively. These
findings indicated that J-type (head-to-tail) orientation
(Fig. S2)10,16 among the pyrene moieties can be promoted by mix-
ing. Second, the chirality inversion in the binary system could only
be observed in the narrow condition of a 2:1 mixing ratio of C4-
and C12-g-Pyr (Fig. 2b). Third, the 2:1 binary system showed a tem-
perature-dependent transition in the CD intensity that was similar
to the system composed of C12-g-Pyr alone, while no significant
transition was observed in C4-g-Pyr alone (Fig. 3). These findings
demonstrate that C4-g-Pyr is compatible with C12-g-Pyr in mixed
systems and that phase transition behaviour is directly influenced
by the components of C12-g-Pyr.
unit with
a chiral centre. When Cn-g-Pyr was dissolved at
0.25 mM in chloroform the UV spectrum showed a normal pattern
with kmaxs at 345, 329, and 316 nm, which were assigned to the
absorption bands based on a pyrene moiety. Moreover, no specific
Cotton effect was observed in the CD spectrum, with the molar
ellipticity ([h]max) around the absorption band of the pyrene moi-
ety being around 1 Â 103 deg cmÀ2 dmolÀ1 at temperatures be-
tween 10 and 60 °C. Conversely, the use of benzene, which has
been identified as an assembling solvent, led to remarkable spec-
tral changes in both the UV and CD spectra while the concentration
of measurement was less than the critical gelation concentration
(cgc: 0.30 mM in benzene). As shown in Figure 1c, the UV spectra
of C12-g-Pyr benzene solution showed remarkably reduced absor-
bance at 345, 329, and 316 nm when the temperature decreased
from 60 to 10 °C. Furthermore, this UV spectral change was accom-
panied by the appearance of a shoulder around 355 nm. In CD
spectra, induction of a positive Cotton effect showing R-chirality
was indicated with extremely large ellipticity around the shoulder
absorption at 10 °C (Fig. 1f). This is understandable based on the
induction of secondary chirality14,15 in response to intermolecular
chiral stacking of the pyrene moiety. The detailed CD patterns of
C4-g-Pyr and C8-g-Pyr showed slightly different kmax and intensity
Donor–accepter systems have attracted considerable attention
as energy transfer models. These can be roughly classified into
covalent17 and non-covalent18 systems; therefore, the present
study can contribute to their use as non-covalent systems when

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K. Miyamoto et al. / Tetrahedron Letters 52 (2011) 4030–4035
Figure 1. Temperature-dependent UV (a–c) and CD (d–f) spectral changes of Cn-g-Pyr (0.25 mM) in benzene at 10 °C (red) and 60 °C (blue).
Figure 2. CD spectra of C4-g-Pyr (blue), C12-g-Pyr (black) and a 2-to-1 mixture of C4-g-Pyr and C12-g-Pyr (red) in benzene at 10 °C. The total concentrations of pyrene were
adjusted as 0.25 mM. (b) Dependencies of the molar ratio of C4-g-Pyr on [h]max (open circles) and kmax (solid circle) in the binary system of C4-g-Pyr and C12-g-Pyr in benzene
at 10 °C.

K. Miyamoto et al. / Tetrahedron Letters 52 (2011) 4030–4035
4033
(Fig. 4a) and C8-g-Pyr (Fig. 4b), although the fluorescence intensity
differed. Excimer formation is an important functionality of Cn-g-
Pyr because efficient crossover between an emission band of a
monomeric pyrene and an absorption band of a tetraphenylpor-
phyrin derivative as a donor component is expected as a possible
pathway to a Förster resonance energy transfer (FRET) system.20
Indeed, the excimeric emission band of Cn-g-Pyr overlapped with
not only the Soret absorption band, but also the Q and Qb bands
a
of tetraphenylporphyrin (Fig. S3).
Energy transfer was monitored and evaluated using the emis-
sion intensity at 650 nm based on the Q band of a porphyrin moi-
ety. As shown in Figure 5a, when g-free TPP (without a g unit) was
excited at 350 nm in the absence and presence of C4-g-Pyr, the
emission intensity decreased remarkably in the presence of
C4-g-Pyr. These findings can be explained by the fact that the exci-
tation energy is partially consumed by absorption at the aggre-
gated C4-g-Pyr, but that no significant energy transfer from the
C4-g-Pyr excimer to TPP occurs. Conversely, when the g-function-
alized TPP was replaced instead of g-free TPP, distinct emission
enhancement was observed as shown in Figure 5b and c. The com-
bination of C12-g-TPP with C4-g-Pyr showed a remarkably higher
Figure 3. Temperature dependencies of [h]k
la
and mixture of C4-g-Pyr and C12-g-Pyr (2:1). Concentrations are 0.25 mM.
values of Cn-g-Pyr (n = 4 and 12)
n
increase than that with
C12-g-Pyr. Similar behaviours were
observed in cyclohexane/chloroform system as same as in cyclo-
hexane/THF system. In addition, no similar increase in the Q band
emission was observed at 60 °C or in chloroform at 10 °C, promot-
ing an ordered-to-disordered transition. These results also indicate
that efficient energy transfer can only occur in their aggregated or-
dered states, and that these are enhanced by the proper adjust-
ment of alkyl chains of the Cn-g derivatives.
an acceptor porphyrin derivative is assembled with C12-g-Pyr as an
donor. In this study, C12-g-TPP19 containing a tetraphenylporphy-
rin moiety as a donor was selected based on its compatibility with
Cn-g-Pyr. Conversely, we found that the molecular ordering states
of Cn-g-Pyr could be controlled by choosing the alkyl length in the
binary system. These findings encouraged us to investigate the ef-
fects of the alkyl length, even in a donor–accepter binary system.
Therefore, we investigated the efficiency of energy transfer from
C4-g-Pyr or C12-g-Pyr excimers to C12-g-TPP in this study.
In conclusion, we described the effect of alkyl length in
L-glutamide-based lipidic compounds with a pyrene moiety (Cn-
g-Pyr; n = 4, 8 and 12) on the chiral aggregation state and energy
transfer efficiency. Cn-g-Pyr with shorter alkyl chains (e.g. n = 4)
were found to have sufficient organogelation ability through
nano-fibrillar aggregation. This aggregation was accompanied by
the excimer formation and secondary chirality induction based
on the molecular orientation. This is an advantageous property
because any long chain alkyl group can work as a non-conductive
moiety when used in a semiconducting organic thin film. However,
when the chiroptical signals were investigated in a binary system
of Cn-g-Pyr with different alkyl lengths, a proper combination
provided a drastic change in the CD patterns such as a positive-
to-negative transition. The results of the present study also
The fluorescence study to investigate the energy transfer was
conducted using a cyclohexane–THF mixed system instead of ben-
zene based on the energy transfer efficiency. As shown in Figure 4c,
C
12-g-Pyr exhibited lyotropically-induced excimeric emission
through excitation at 350 nm, which is on the absorption band of
the aggregated pyrene. A characteristic fluorescence spectrum
(kmax, 445 nm) assigned by an excimer was observed at concentra-
tions above 15
10 °C. On the other hand, the fluorescence spectra at concentra-
tions below 5 M were similar to those observed in chloroform,
lM in cyclohexane–THF (15:1) mixed systems at
l
indicating it was an excimeric emission with kmax values of 380,
398 and 418 nm. Similar results were obtained for C4-g-Pyr
revealed that
a binary system composed of C4-g-Pyr and
Figure 4. Concentration-dependent excimer emission of Cn-g-Pyr (n = 4 (a), 8 (b) and 12 (c)) in cyclohexane-THF (15:1) at 10 °C. Excitation wavelength: 350 nm.

4034
K. Miyamoto et al. / Tetrahedron Letters 52 (2011) 4030–4035
Figure 5. Fluorescence changes in the Q band of g-free TPP (a) and C12-g-TPP (5
lM) with addition of Cn-g-Pyr (n = 4 (b) and 12 (c)) in cyclohexane-THF (15:1) at 10 °C.
Excitation wavelength: 350 nm.
7. Ihara, H.; Yoshitake, M.; Takafuji, M.; Yamada, T.; Sagawa, T.; Hirayama, C. Liq.
Cryst. 1999, 26, 1021; Ihara, H.; Takafuji, M.; Sakurai, T.; Katsumoto, M.;
Ushijima, N.; Shirosaki, T.; Hachisako, H. Org. Biomol. Chem. 2003, 1, 3004.
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Simon, F. ois-X.; Schmutz, M.; Mesini, P. J.; Banerjee, A. Tetrahedron 2008, 64,
175.
C12-g-TPP showed the most effective enhancement of the emission
intensity based on the porphyrin moiety when compared with
mixed systems of C4-g-Pyr with g-free TPP and C12-g-Pyr with
C12-g-TPP. These findings indicate that the alkyl chain length not
only influences the molecular orientation among pyrene moieties,
but also provides a chance to form the preferable molecular fitting
for FRET.
10. Sagawa, T.; Fukugawa, S.; Yamada, T.; Ihara, H. Langmuir 2002, 18, 7223.
11. N2,N3-Dibutyl-N-[4-(1-pyrenylbutyroyl)]-
L
-glutamide (C4-g-Pyr): Triethylamine
(0.17 mL, 1.2 mmol) and diethyl-cyanophosphonate (0.17 mL, 1.1 mmol) were
added dropwise to the solution of N2,N3-dibutyl-
-glutamide (0.22 g,
Acknowledgment
L
0.86 mmol) and 1-pyrenylbutylic acid (0.297 g, 1.03 mmol) in chloroform
(100 mL) at 0 °C. The reaction solution was stirred for 2 h at 0 °C and then 12 h
at room temperature. The mixture solution was washed with aq NaOH solution
(0.2 N, 3 Â 50 mL), aq. HCl (0.1 N, 3 Â 50 mL), and water (50 mL). After the
organic layer was dried over Na2SO4, the filtrate was concentrated under
reduced pressure. The residue was recrystallized from ethanol to yield a
desired product as a yellow powder (0.27 g, 58%); mp 213–214 °C; FT-IR (KBr)
This work was supported in part by financial support of Japan
Society for the Promotion of Science Bilateral Joint Projects.
Supplementary data
m
3290, 3093, 3041, 2957, 2932, 2871, 1637, 1543 cmÀ1 1H NMR (400 MHz;
;
CDCl3) d 0.87–0.91 (6H, m, CH2CH3), 1.28–1.35 (4H, m, CH2CH3), 1.41–1.50 (4H,
m, NHCH2CH2), 1.90–2.10 (2H, m, CH2CH2Ar), 2.19–2.45 (2H, m, CH2CÃH),
2.19–2.26 (2H, m, C(@O)CH2(CH2)2Ar), 2.36–2.40 (2H, m, C(@O)CH2CH2CÃH),
3.15–3.36 (4H, m, CH2NH), 3.37–3.75 (2H, CH2Ar), 4.34 (1H, q, J = 4.9 Hz, CÃH),
5.91–5.92 (1H, br, NH), 6.77–6.79 (1H, br, NH), 7.02–7.04 (1H, br, NH), 7.85–
8.30 (9H, m, ArH); Anal. Calcd for C33H41N3O3 + 0.5H2O: C, 73.85; H, 7.89; N,
7.83. Found: C, 74.11; H, 7.74; N, 7.83.
Supplementary data (synthesis and characterization of pyrene-
derivatives. Experimental detail: TEM measurements, fluorescence
and UV–vis spectroscopies) associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2011.05.131.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
N2,N3-Dioctyl-N-[4-(1-pyrenylbutyroyl)]-
cyanophosphonate (0.30 mL, 2.0 mmol) were added dropwise to the solution
of N2,N3-dioctyl-
-glutamide (0.692 g, 1.08 mmol), 1-pyrenylbutylic acid
L-glutamide
(C8-g-Pyr):
diethyl-
L
References and notes
(0.486 g, 1.69 mmol) and triethylamine (0.30 mL, 2.2 mmol) in chloroform
(100 mL) at 0 °C. The reaction solution was stirred for 2 h at 0 °C and then 12 h
at room temperature. The mixture solution was washed with aq NaOH solution
(0.2 N, 3 Â 50 mL), aq HCl (0.1 N, 3 Â 50 mL), and water (50 mL). After the
organic layer was dried over Na2SO4, the filtrate was concentrated in vacuo.
The residue was recrystallized from ethanol to yield a desired product as a
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