Molecular organogel-forming porphyrin derivative with hydrophobic l-glutamide

Article abstract of DOI:10.1016/j.tetlet.2008.04.103

An l-glutamide-functionalized tetraphenylporphyrin derivative has been newly synthesized. This can dissolve in various organic solvents to form nanofibrillar aggregates with both right-handed and left-handed chiral stacking structures among the porphyrin

Full text of DOI:10.1016/j.tetlet.2008.04.103

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3987–3990
Molecular organogel-forming porphyrin derivative
with hydrophobic L-glutamide
Hirokuni Jintoku a, Takashi Sagawa b, Tsuyoshi Sawada a, Makoto Takafuji a,
Hiroshi Hachisako c, Hirotaka Ihara a,*
a Department of Applied Chemistry and Biochemistry, Kumamoto University, Kumamoto-shi, Kumamoto 860-8555, Japan
b Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan
c Department of Applied Chemistry, Sojo University, Kumamoto 860-8691, Japan
Received 7 March 2008; revised 14 April 2008; accepted 17 April 2008
Available online 22 April 2008
Abstract
An L-glutamide-functionalized tetraphenylporphyrin derivative has been newly synthesized. This can dissolve in various organic
solvents to form nanofibrillar aggregates with both right-handed and left-handed chiral stacking structures among the porphyrin rings.
It is also discussed on thermotropic and lyotropic phase transitions between gel and sol states.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
aqueous5 and organic6 media, and thus would induce chi-
rally-stacking interaction among the porphyrin rings to
develop to one-dimensional aggregates.
Molecular assemblies of porphyrin derivatives such as
bacteriochlorophylls are key components for light-harvest-
ing and energy migration in photosynthesis.1 Synthetic
porphyrins such as tetraazaporphyrin have been utilized
as essential compounds for plasma-display filters.2 There-
fore, highly ordered assemblies of synthetic porphyrins
such as molecular wires would be widely applicable and
promising for electronic devices as light-harvesting mole-
cular wires. For this purpose, many porphyrin-containing
derivatives have been reported. Some successes can be seen
in sugar-immobilized3 and polypeptide-supported4 por-
phyrins. In this article, we introduce a novel porphyrin
derivative (1) to form highly ordered structures by self-
assembling in organic media. The chemical structure can
be characterized by the fact that a long chain-alkylated
L-glutamide (2) is combined as a driving force for chirally
molecular ordering into tetraphenylporphine (3). 2 is a very
useful tool for promoting the intermolecular hydrogen
bonding to lead to superstructural fabrication both in
1 was prepared by coupling of 5-(4-carboxyphenyl)-
10,15,20-triphenyl-21H,23H-porphine (3) with N,N0-dido-
decyl L-glutamide (2) according to the previously reported
procedures6 with slight modification.
1 dissolved in various organic solvents such as metha-
nol, ethanol, acetonitrile, DMF, THF, benzene, pyridine,
and chloroform which were good solvents. However, when
a solvent system was adjusted by mixing with a poor
solvent such as cyclohexane, the 1 solution changed to a
*
Corresponding author. Tel.: +81 96 342 3661; fax: +81 96 342 3662.
E-mail address: ihara@kumamoto-u.ac.jp (H. Ihara).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.04.103

3988
H. Jintoku et al. / Tetrahedron Letters 49 (2008) 3987–3990
gel state. A typical example can be obtained as follows:
when 1 (0.5 mM) dissolved in a mixture of cyclohexane
and THF (20:1) at 60 °C to a solution state and cooled
down to 20 °C, a sol-to-gel transition happened immedi-
ately as shown in Figure 1a. A TEM observation in the cast
film from the 1 cyclohexane–THF (20:1) gel showed well-
developed fibrous aggregates with the minimum thickness
of around 3 nm (Fig. 1b and c) although some parts were
larger as twisted ribbon-like and bundled aggregates.7
The minimum thickness is nearly close to the molecular
length (3.75 nm) of 1 estimated by HyperChem-MM2.
Therefore, it is preliminarily concluded that the 1 aggre-
gates are based on molecular layered structures similar to
aqueous lipid membranes and the gel formation can be
brought about through the three-dimensional network for-
mation with the 1-nanofibrillar aggregates. This phenome-
non can be recognized as molecular gelation.8
The Soret band of a porphyrin moiety (3) is very useful
for evaluating the dispersion state of 1 in organic media.
Figure 2a includes a UV–visible spectrum in a cyclohex-
ane–THF (20:1) mixture containing 0.1 mM of 1 at
10 °C. The kmax at 417 nm corresponds to that in the Soret
band when 3 is in a monomeric dispersion state. However,
a drastic spectral change was observed by slight change of
the concentration of 1. For example, when the concentra-
tion increased from 0.1 mM to 0.2 mM, the solution not
only changed to a fragile gel but also the Soret band split
into two peaks with kmaxs of 399 and 428 nm. A similar
split pattern was reported in a protoporphyrin IX deriva-
tive with bis(glycosamides) in an aqueous solution9 and a
bis(imidazolyl)porphyrinato–cobalt(III) complex in chlo-
roform10 although their detailed mechanisms are different
from our observation.
a
417 nm
0.5
0.4
0.3
0.2
0.1
0
0.1 mM
399 nm
428 nm
0.2 mM
b
420 nm
10
5
0.1 mM
0.2 mM
0
-5
433 nm
-10
399 nm
-15
300
350
400
450
500
Wavelength (nm)
Fig. 2. UV–visible (a) and CD (b) spectra of 1 in a cyclohexane–THF
(20:1) mixture at 10 °C.
explained only by the induction of chiral ordering of the
achiral porphyrin moiety through the formation of highly
ordered structures with the chiral L-glutamide moiety. Sim-
ilar induction of chirality was observed by isoquinoline5-
and pyrene11-containing L-glutamide derivatives in organic
media. On the other hand, the concentration dependency
of [h]max (Fig. 3) showed a distinct bending point at
The further important information was obtained by CD
spectroscopy. As shown in Figure 2b, an extremely strong
Cotton effect was observed in 0.2 mM. The maximum
molecular ellipticity ([h]max) reaches 1.5 ꢀ 106 deg cm2
dmolꢁ1 at 420 nm and ꢁ1.45 ꢀ 106 deg cm2 dmolꢁ1 at
398 nm, and the CD pattern was almost constant in the
concentration above 0.2 mM. Since a porphyrin moiety is
achiral, the strong CD strength with the kmax shift can be
6
1.5
1.0
cac
ε
5
4
[θ]
3
2
0.5
0
1
0
-6
-5
Log [1]
-4
-3
Fig. 1. Temperature-dependent sol-to-gel transition of 1 (0.5 mM) in a
cyclohexane–THF (20:1) mixture (a) and TEM images of 1 aggregates in
the cast film prepared from 0.2 mM (b and c). Stained by 2.0 wt %
ammonium molybdate after casting and drying.
Fig. 3. Concentration-dependent molecular coefficient (e) (a) and mole-
cular ellipticity [h] (b) at 417 nm on Log[1] in a cyclohexane–THF (20:1)
mixed system at 10 °C.

H. Jintoku et al. / Tetrahedron Letters 49 (2008) 3987–3990
3989
0.1 mM. The [h]417 value is very small in the concentration
below 0.1 mM and almost constant in the concentration
above 0.2 mM. Therefore, it is estimated that the critical
aggregation concentration (cac) of 1 is around 0.1 mM in
a cyclohexane–THF (20:1) mixed system at 10 °C.
The detailed investigation of the CD pattern enables us
to clarify the chirally ordered structures of the 1 aggregate.
Firstly, there is almost no CD signal around the absorption
band on a porphyrin moiety in the concentrations below
cac. The induction of CD is only observed in the concentra-
tion above cac. Secondly, the complicated CD pattern in
Figure 2b can be segmented into two kinds of CD spectra
as shown in Figure 4a and b. The two kmaxs of the split
UV–visible spectrum (Fig. 2a) agree with those in the CD
spectra, respectively. Thirdly, a thermotropic sol-to-gel
transition was also observed. For example, the cyclo-
hexane–THF (20:1) mixed system containing 0.25 mM of
1 showed a remarkable temperature-dependent spectral
change in both their UV–visible and CD spectroscopies.12
In this case, two isosbestic points were observed at
405 nm and 426 nm in this temperature dependency. These
observations support the following proposed structures on
the chiral 1 aggregates: the split pattern in Figure 2a can be
explained by two species composed of right-handed
(R-chiral) H-like aggregates with a hypsochromical shift
and left-handed (S-chiral) J-like aggregates with a batho-
chromical shift according to the Simonyi’s explanation.13
An FT-IR spectrum of the 1 aggregates in a cyclo-
hexane–THF (20:1) mixture showed typical amide I
absorptions corresponding to the parallel and antiparallel
hydrogen bonding14,15 at 1637 and 1678 cmꢁ1, respectively.
This spectrum is similar to that in a solid state with an
absorption peak at 1639 cmꢁ1 but different from that in a
chloroform solution state with an absorption peak at
1652 cmꢁ1. The schematic illustration shown in Figure 5
satisfies this assumption. Figure 5 shows that fibrous
networks were consisted of chirally ordered structure.
One-dimensional aggregation of 1 can be derived from a
long-range face-to-face stacking (H-like aggregation) and
an edge-to-edge interaction (J-like aggregation) can be
almost simultaneously formed to lead to a ribbon-like
morphology.7
Fig. 5. Schematic proposal on the ordered structure in the 1 aggregates.
In conclusion, an L-glutamide-functionalized tetra-
phenyl-porphyrin derivative 1 can form a molecular gel
in organic media through three-dimensional network for-
mation with chirally self-assembled nanofibrillar aggre-
gates. It should be also emphasized that a unique chiral
microenvironment is produced by both R-chiral H-aggre-
gation and S-chiral J-aggregation among the porphyrin
moieties. In addition, these phenomena are reversible
(viz. switchable) by changing the temperature, solvent-
combination, concentration, and so on. Further investiga-
tion on singlet–singlet energy migration using a 1 system
will be reported later.
2. Experimental
1H NMR (400 MHz) spectra were recorded in CDCl3,
SiMe4 as an internal standard with JNM-EX400 (JEOL).
IR spectra were measured in a KBr method with FT/IR-
4100 (JASCO). TEM images were observed with JEM-
2000X (JEOL). UV–visible and CD spectra were measured
with UV–vis Spectrophotometer V-560 (JASCO) and
Spectropolarimeter J725 (JASCO), respectively.
A solution of 1 N NaOH 3.75 mL was added dropwise
to 5-(4-methoxycarbonylphenyl)-10,15,20-triphenyl-21H,
23H-porphine (0.25 g, 0.4 mmol) in DMF (100 mL) at
room temperature for 30 min. After the reaction mixture
was stirred at room temperature for 3 h, it was acidified
with aqueous 2.0 wt % HCl to pH 2.0. The precipitates
were filtered and dried in vacuo, to give 5-(4-carboxy-
phenyl)-10,15,20-triphenyl-21H,23H-porphine (0.23 g, 92%).
0.40 g (0.6 mmol) of 5-(4-carboxyphenyl)-10,15,20-tri-
phenyl-21H,23H-porphine and 0.35 g (0.7 mmol) of N,N0-
didodecyl-L-glutamide, which was obtained by the
previously reported procedure6 with slight modification,
were dissolved in THF (150 mL) in the presence of triethyl-
amine (1.2 mL, 0.9 mmol) and diethyl cyanophosphate
(1.5 mL, 0.9 mmol). After being stirred for 1 day at room
temperature, the solution was concentrated in vacuo.
Fig. 4. Resolution of CD spectrum of the 0.2 mM gel in a cyclohexane–
THF (20:1) mixture at 10 °C.

3990
H. Jintoku et al. / Tetrahedron Letters 49 (2008) 3987–3990
Mizoguchi, T.; Kakitani, Y.; Koyama, Y.; Akutsu, H. Proc. Natl.
Acad. Sci. U.S.A. 2007, 104, 790–795.
2. Davies, C.; Petty, D. M.; Willis, M. R. Synth. Met. 1989, 28, 775–780.
3. Tamaru, S.; Nakamura, M.; Takeuchi, M.; Shinkai, S. Org. Lett.
2001, 3, 3631–3634.
4. Hasobe, T.; Kamat, P. V.; Troiani, V.; Solladie, N.; Ahn, T. K.; Kim,
S. K.; Kim, D.; Kongkanand, A.; Kuwabata, S.; Fukuzumi, S. J.
Phys. Chem. B 2005, 109, 19–23.
5. (a) Yamada, K.; Ihara, H.; Ide, T.; Fukumoto, T.; Hirayama, C.
Chem. Lett. 1984, 1713–1716; (b) Ihara, H.; Hachisako, H.; Hiray-
ama, C.; Yamada, K. Liq. Cryst. 1987, 2, 215–221.
6. (a) Ihara, H.; Hachisako, H.; Hirayama, C.; Yamada, K. J. Chem.
Soc., Chem. Commun. 1992, 1244–1245; (b) Ihara, H.; Yoshitake, M.;
Takafuji, M.; Yamada, T.; Sagawa, T.; Hirayama, C.; Hachisako, H.
Liq. Cryst. 1999, 26, 1021–1027.
7. Supplementary data regarding the ribbon-like and bundled
aggregates.
8. (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159; (b) Van
Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263–
2266; (c) Ihara, H.; Takafuji, M.; Sakurai, T. Encycl. Nanosci.
Nanotechnol. 2004, 9, 473–495; (d) Shimizu, T.; Masuda, M.;
Minamikawa, H. Chem. Rev. 2005, 105, 1401–1443; (e) Oda, R.
Materials with Self-assembled Fibrillar Networks 2006, 577–612; (f)
Ariga, K.; Nakanishi, T.; Hill, J. P. Curr. Opin. Colloid Interface Sci.
2007, 12, 106–120.
The residue was redissolved in chloroform, and the organic
layer was washed three times with 0.2 N HCl and 0.2 N
NaOH, then washed with water, and dried with sodium
sulfate. The solution was concentrated in vacuo, and the
residue was reprecipitated from chloroform–n-hexane and
then dried in vacuo to give a purple solid: 0.50 g (80%).
Mp 229.5–233.0 °C. IR (KBr): 3316, 2925, 2853, 1639,
1
1542 cmꢁ1. H NMR (400 MHz, CDCl3): d ꢁ2.78 (s, 2H,
–NH), 0.80–0.87 (m, 6H, –CH3), 1.17–1.35 (br, 40H, –
(CH2)10–), 2.32 (q, 2H, J = 5.2 Hz, –CH2–), 2.45–2.75
(br, 2H, –CH2CO–), 3.30–3.38 (m, 4H, –NCH2–), 4.74 (q,
1H, J = 6.0 Hz, –CH–), 6.03–6.06 (br, 1H, NH), 7.04–
7.07 (br, 1H, NH), 7.73–7.79 (m, 9H, ArH), 8.20–8.33
(m, 10H, ArH), 8.42–8.44 (br, 1H, NH), 8.79–8.87 (m,
8H, ArH). Anal. Calcd for C74H87N7O3: C, 79.01; H,
7.81; N, 8.72. Found: C, 78.88; H, 8.09; N, 8.61.
Acknowledgment
This work was supported in part by a Grant-in-Aid for
Scientific Research from the Ministry of Education,
Science, Sports, and Culture of Japan.
9. Fuhrhop, J.-H.; Demoulin, C.; Boettcher, C.; Koning, J.; Siggel, U. J.
Am. Chem. Soc. 1992, 114, 4159–4165.
10. Ikeda, C.; Fujiwara, E.; Satake, A.; Kobuke, Y. Chem. Commun.
2003, 616–617.
Supplementary data
11. (a) Ihara, H.; Sakurai, T.; Yamada, T.; Hashimoto, T.; Takafuji, M.;
Sagawa, T.; Hachisako, H. Langmuir 2002, 18, 7120–7228; (b)
Takafuji, M.; Ishiodori, A.; Yamada, T.; Sakurai, T.; Ihara, H.
Chem. Commun. 2004, 1122–1123.
12. Supplementary data regarding the temperature-dependent spectral
change in both their UV–visible and CD spectroscopies.
13. Symonyi, M.; Bikadi, Z.; Zsila, F.; Deli, J. Chirality 2003, 15, 680–
698.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2008.
04.103.
References and notes
1. (a) McDermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaite-
Lawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaaca, N. W. Nature
1995, 374, 517–521; (b) Melkozernov, A. N.; Barber, J.; Blankenship,
R. E. Biochemistry 2006, 45, 331–345; (c) Egawa, A.; Fujiwara, T.;
14. Yamada, N.; Ariga, K.; Naito, M.; Matsubara, K.; Koyama, E. J.
Am. Chem. Soc. 1998, 120, 12192–12199.
15. Suzuki, M.; Setoguchi, C.; Shirai, H.; Hanabusa, K. Chem. Eur. J.
2007, 13, 8193–8200.