Chirally self-assembled porphyrin nanowires assisted by L-glutamide-derived lipid for excitation energy transfer

Article abstract of DOI:10.1039/b818358a

An L-glutamide-functionalized tetraphenylporphyrin (1) has been newly synthesized and its self-assembling behavior in organic solvents is reported. 1 forms nanofibrous assemblies in a dilute solution to show specific induced circular dichroism at the abso

Full text of DOI:10.1039/b818358a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Chirally self-assembled porphyrin nanowires assisted by L-glutamide-derived
lipid for excitation energy transfer†
a
b
a
a
Hirokuni Jintoku, Takashi Sagawa, Makoto Takafuji and Hirotaka Ihara*
Received 20th October 2008, Accepted 3rd March 2009
First published as an Advance Article on the web 2nd April 2009
DOI: 10.1039/b818358a
An L-glutamide-functionalized tetraphenylporphyrin (1) has been newly synthesized and its
self-assembling behavior in organic solvents is reported. 1 forms nanofibrous assemblies in a dilute
solution to show specific induced circular dichroism at the absorption band around the porphyrin
moiety. It enables us to observe the meso-stable and stepwise H- and J-like hierarchical aggregations.
Energy transfer studies in a mixed system with a pyrene-functionalized L-glutamide reveal the
promotion of energy transfer from the pyrene excimer to the porphyrin moiety.
Introduction
Molecular assemblies of porphyrins and their derivatives are
attractive as molecular electronic devices for mimicking nat-
1
ural photosynthesis in terms of light-harvesting and energy
2
migration. In this context, we have previously reported on a mixed
system composed of L-glutamide derivatives functionalized with
protoporphyrin IX (2) and pyrene (3) for energy transfer as shown
3
in Scheme 1. Spectroscopic studies suggested that 3 could form
highly-ordered molecular gels in both benzene and cyclohexane
to promote the formation of co-facial chiral aggregates with 2
through their amide hydrogen bonding. Energy transfer studies in
the mixed assemblies provided evidence for steady-state singlet–
singlet energy transfer from the pyrene excimer to the free base
porphyrin moiety. However, the protoporphyrin IX with four long
chain alkyl groups (2) failed to gelate any of the solvents tried
because of low solubility. For the sake of making a highly oriented
molecular gel system with porphyrin as a light-harvesting group,
we report herein a novel fabrication of a porphyrin combined with
an L-glutamide-derived lipid with two long chain alkyl groups as
a lipophilic part and tetraphenylporphyrin as a functional head
group (1).
Results and discussion
Solubility and gelation properties
Various kinds of organic solvents such as acetonitrile, methanol,
ethanol, DMF, THF, chloroform and benzene were good solvents
to dissolve 1 as shown in Table 1, while 2 was less soluble in any
solvent. However, it was observed that addition of a poor solvent
a
Department of Applied Chemistry & Biochemistry, Kumamoto University,
2
-39-1 Kurokami, Kumamoto 860-8555, Japan. E-mail: ihara@kumamoto-
u.ac.jp; Fax: (+81) 96 3423662; Tel: (+81) 96 3423661
b
Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-
Scheme 1 L-Glutamide derivatives with a porphyrin (1 or 2) or pyrene
3) head group as self-assembling dyes.
0
011, Japan. E-mail: t-sagawa@iae.kyoto-u.ac.jp; Fax: (+81) 774 383508;
(
Tel: (+81) 774 384580
†
Electronic supplementary information (ESI) available: Fig. S1: Resolu-
tion of CD spectrum; Fig. S2: Polarized optical micrograph observation;
Fig. S3: Temperature dependent fluorescence spectral change of 3; Fig. S4:
such as cyclohexane converted a solution to a gel state. In this
study, we mainly examined the association behaviors of 1 in a
cyclohexane–THF (20 : 1) mixed system.
◦
Concentration dependent fluorescence spectral change at 60 C; Fig. S5:
Concentration dependency of emission intensities at 450 nm. See DOI:
1
0.1039/b818358a
2
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a
Table 1 Solubility of 1
◦
◦
◦
solvent
water
0 C
20 C
60 C
–
–
S
I
S
S
–
S
S
S
S
I
I
I
dimethyl sulfoxide (DMSO)
N,N-dimethyl formamide (DMF)
acetonitrile
pyridine
methanol
S
S
S
S
S
S
S
S
S
S
I
S
S
S
S
S
S
S
S
S
S
–
S
S
I
1
,4-dioxane
ethanol
tetrahydrofuran (THF)
ethyl acetate
chloroform
ether
benzene
toluene
–
S
I
S
S
I
n-hexane
cyclohexane
–
I
I
a
[1]: 1.0 mM, S: soluble, I: insoluble.
TEM observation of 1 aggregates in the cast film from a
Fig. 2 UV–visible spectra (a) and molecular coefficient (e) at 417 nm
cyclohexane–THF (20 : 1) mixture showed well-developed fibrous
networks (Fig. 1). The minimum diameters of these fibers are
around 4 nanometers although some parts are twisted and
bundled. Their lengths are at least in the order of hundreds
(
b), CD spectra (c) and molecular ellipticity (q) at 417 nm (d) of 1 in
◦
a cyclohexane–THF (20 : 1) mixture at 10 C. The arrows indicate the
concentration increase from 0.1 mM (red line) to 0.75 mM (blue line).
of micrometers. The geometric dimensions of 1 are 3.75 ¥
versus log [1] (Fig. 2d) shows that the [q]417 value is very small
in concentrations below 0.1 mM and almost constant in the
concentrations above 0.2 mM. Therefore, Fig. 2b and 2d indicate
3
1.75 ¥ 0.44 nm estimated with HyperChem-MM2 calculations.
Therefore, we estimate that the minimum sizes of the fibrous
aggregates of 1 are based on a molecular-layered structure.
that the critical aggregation concentration (cac) of 1 is around
◦
0
.1 mM in a cyclohexane–THF (20 : 1) mixed system at 10 C.
After warming a cyclohexane–THF (20:1) mixture containing
◦
0.2 mM of 1 at over 60 C for 20 min, a UV–vis spectral change
◦
of 1 at 10 C was recorded at every 2 min interval for 20 min as
shown in Fig. 3a. Two isosbestic points were observed at 405 and
4
26 nm and the split Soret band appeared. This result is ascribed
to the equilibrium between the solution state of 1 (at 417 nm)
and the gel state of aggregated 1 (at 398 nm and 428 nm). A
◦
time-course of CD spectral change of 1 at 10 C revealed complex
Cotton effects (Fig. 3b). CD spectra showed hierarchical spectral
changes. Positive and negative Cotton bands were observed at
around 395 nm and 425 nm at the initial stage (from 0 min to
Fig. 1 Temperature-dependent sol-to-gel transition of 1 (0.5 mM) in a
cyclohexane–THF (20 : 1) mixture and TEM image of 1 aggregates in the
cast film prepared by 0.2 mM. Stained by 2.0 wt% ammonium molybdate.
1
0 min in Fig. 3b), and a new negative Cotton band was observed
at 430 nm at the later stage (from 10 min to 20 min in Fig. 3b).
Detailed investigation of the CD spectrum as shown in Fig. 3
confirms that the CD pattern centered at 398 nm can be assigned
to hypsochromically shifted H-like aggregates and this is also
Photophysical properties
The Soret band (417 nm) of 1 showed a characteristic split by
about 30 nm (398 nm and 428 nm) when the concentration was
increased from 0.10 mM to 0.75 mM as shown in Fig. 2a. The
plots of the molecular coefficient at 417 nm (e417) versus log [1]
(
Fig. 2b) showed that a drastic spectral change was observed by
a slight change of the concentration of 1. Therefore, the Soret
band at 417 nm in Fig. 2a may correspond to the monomeric
form or weakly aggregated species like micelles of 1. A similar
split pattern was reported in a protoporphyrin IX derivative with
4
bis(glycosamides) in an aqueous solution and a bis(imidazolyl)-
5
porphyrinato-cobalt(III) complex in chloroform although their
detailed mechanisms are different from our observation. Also,
further important information was obtained by CD spectroscopy.
A strong Cotton effect was observed at 375–460 nm on increasing
the concentration of 1 as shown in Fig. 2c. The plot of [q]417
Fig. 3 Time-course of UV–visible (a) and CD (b) spectral change of 1
(0.25 mM) in a cyclohexane–THF (20 : 1) mixture at 10 C. The arrows
indicate a time increase from 0 min (red line) to 20 min (blue line).
◦
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Table 2 Absorption frequencies of 1 under various conditions
frequencies / cm-
1
amide I
amide II
a (solid)
1639
1542
1551
1558, 1523
b (cyclohexane/THF (20 : 1)) 1676, 1637
c (chloroform) 1652
assigned to right-handed (R-) chirality, while another centered
at 428 nm is assigned to the bathochromically shifted J-like
aggregates and left-handed (S-) chirality† according to Simonyi’s
6
explanation. These results indicate that the dispersed sol state of 1
turned gradually into H-like aggregates with R-chirality, then the
H-like aggregates started to form J-like aggregates with S-chiral
orientation and completely developed to keep a steady state.
An FT-IR spectrum of the 1 aggregates in a cyclohexane–THF
(
20 : 1) mixture showed typical amide I absorptions corresponding
7,8
to parallel and antiparallel hydrogen bonding at 1637 and
1
in a solid state with an absorption peak at 1639 cm but different
from that in a chloroform solution state with an absorption peak
at 1652 cm- (Fig. 4).
Fig. 5 Schematic proposal for the ordered structure in the 1 aggregate.
-
1
678 cm , respectively (Table 2). This spectrum is similar to that
-
1
energy-donating compound for energy transfer to 1 because the
emission band of a pyrene excimer can overlap partially with an
absorption band of 1.
1
A fluorescence spectrum of 3 was examined in a mixed system of
cyclohexane and THF (20 : 1). The excimeric emission band of 3
◦
was observed at 450 nm as a peak top (10 C), while a monomeric
emission band could be detected with peak tops at 378, 398 and
◦
4
18 nm (60 C).† According to our previous work, these results
indicate that a pyrene moiety can form an excimeric state induced
◦
by highly oriented assembling at 10 C.
As shown in a UV–vis spectrum of 1 (0.01 mM) and a
fluorescence spectrum of 3 (0.05 mM), partial spectral overlapping
can be realized between excimeric emission and UV–vis spectral
◦
bands at 10 C (Fig. 6). This result indicates the possibility of
energy transfer from the pyrene excimer to the porphyrin moiety.
Fig. 4 FT-IR spectra of 1 under various conditions. Solid state (a),
cyclohexane–THF (20 : 1) solution (b), and chloroform solution (c) at
◦
2
5 C.
The schematic illustration shown in Fig. 5 satisfies this as-
sumption. Fig. 5 shows that fibrous networks can consist of
structures chirally ordered to become nanofibrillar aggregates.
One-dimensional morphology 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. Polarized micrograph
observation of the cast film of 1 also revealed that the quasi-solid
state of 1 was a highly oriented liquid crystalline phase.†
Fig. 6 UV–vis spectrum of monomeric 1 (a) and excimeric emission
spectrum of 3 (b) in a cyclohexane–THF (20 : 1) mixture at 10 C.
◦
Excitation wavelength was 350 nm.
Energy transfer system
We have reported that the pyrene-functionalized L-glutamide
As shown in Fig. 7a, an increase in the emission intensity at
(
3) undergoes a temperature-induced phase transition between
653 nm, which was assigned to the emission band of 1, was
◦
monomeric and aggregated states in benzene and that the ex-
cimeric emission of a pyrene moiety can be observed only in the
aggregated states. Therefore, in this study, 3 was selected as an
observed by addition of 3 at 10 C. No similar increase was
◦
detected at 60 C.† It was also confirmed that 5-(4-carboxyphenyl)-
3,9
10,15,20-triphenyl-21H,23H-porphine (MCTPP) having no
2
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the 1 and 3 mixed system is brought about through (1) aggregation
of 3, (2) incorporation of 1 into the aggregates of 3 and (3) energy
transfer from the pyrene excimer to the porphyrin moiety.
Sol-gel matrices have been sometimes utilized to retard struc-
tural changes in the macromolecules of proteins such as the
cooperative behavior of the rearrangement of the packing of the
10
subunits, quaternary conformational changes, and so on. Such
a matrix-assisted assembly in sol-gel states allows the molecules
to diffuse slowly and enables us to measure the meso-stable
structure of the molecular assemblies even by the steady-state
11
observation. In this regard, our new system of porphyrin 1 and
pyrene 3 in combination with an L-glutamide part might enable
the observation of effective energy transfer from 3 to 1.
Conclusions
It has been described that the porphyrin-functionalized L-
glutamide derivative 1 can form a molecular gel in organic
media through three-dimensional network formation with self-
assembled nanofibrillar aggregates. Particularly, 1 formed a unique
chiral assembly with both R-chiral H-like and S-chiral J-like
aggregations in a hydrophobic environment. These phenomena are
reversible with temperature, solvent-combination, concentration,
and so on. In this paper, a mixed system of 1 with the pyrene-
functionalized L-glutamide derivative 3 has been also investigated
as an energy transfer model.
Fig. 7 Fluorescence spectra of mixtures containing 0.01 mM of 1 and
0
–0.05 mM of 3 (a), and 0.01 mM of MCTPP and 0–0.05 mM of 3 (b) in a
Experimental
◦
cyclohexane–THF (20 : 1) mixture at 10 C. The arrows indicate increasing
concentrations of 3 from 0 mM (red line) to 0.05 mM (blue line). Excitation
wavelength was 350 nm.
1
H NMR (400 MHz) spectra were recorded in CDCl
3
, SiMe as
4
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). The gel was cast
in a carbon-coated copper grid and stained by 2.0 wt% ammonium
molybdate after casting and dried by a vacuum pump under
reduced pressure. The accelerating voltage of the TEM was 80 kV
and the beam current was 40 A. MALDI TOF-MS spectra were
recorded on a Voyager RP (PerSeptiv Biosystem). UV–visible, CD
and fluorescence spectra were measured with V-560 (JASCO), J725
L-glutamide moiety showed no significant effect under the same
conditions (Fig. 7b).
The concentration dependence of 3’s emission (Fig. 8) shows
that a distinct increase in the emission intensity at 653 nm (I653
)
can be seen at concentrations between 0.025 mM and 0.05 mM. On
the other hand, the I653 of MCTPP was not enhanced by addition
of 3 but rather decreased at concentrations above the cac of 3.†
In the absence of 3, the I653 per molecule was almost constant
in 0–0.1 mM of 1 but decreased at concentrations above 0.1 mM.
This is attributed to quenching due to the aggregation of 1 (the cac
was estimated to be around 0.1 mM by both the UV–vis and CD
methods). These results indicate that the enhancement of I653 in
(
JASCO) and FP-6500 (JASCO) respectively.
solution of NaOH 3.75 mL was added
dropwise to 5-(4-methoxycarbonylphenyl)-10,15,20-triphenyl-
1H,23H-porphine (0.25 g, 0.4 mmol) in DMF (100 mL) at
A
1
N
2
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-carboxyphenyl)-10,15,20-triphenyl-21H,23H-
porphine (0.23 g, 92%) as a purple solid. 0.40 g (0.6 mmol) of 5-(4-
carboxyphenyl)-10,15,20-triphenyl-21H,23H-porphine and 0.35 g
(
0.7 mmol) of N,N¢-didodecyl-L-glutamide, which was obtained
12
by the previously reported procedure with slight modification,
were dissolved in THF (150 mL) in the presence of triethy-
lamine (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. 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
Fig. 8 Emission intensities at 653 nm of a mixture of 3 and 0.01 mM of
with sodium sulfate. The solution was concentrated in vacuo, and
1
(᭹) or 3 and 0.01 mM of MCTPP (᭺).
the residue was reprecipitated from chloroform/n-hexane and
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29.5–233.0 C. (Found: C, 78.88; H, 8.09; N, 8.61. C74
H
87
N
7
O
3
-
1
requires C, 79.01; H, 7.81; N, 8.72%) nmax(KBr)/cm 3316, 2925,
2
3
2
0
853, 1639, 1542; d
.80–0.87 (6H, m, -CH
H
(400 MHz; CDCl
), 1.17–1.35 (40H, br, -(CH
-), 2.45–2.75 (2H, br, -CH
-), 4.74 (1H, quintet, J = 6.0 Hz, -CH-),
.03–6.06 (1H, br, NH), 7.04–7.07 (1H, br, NH), 7.73–7.79 (9H, m,
3
; Me
4
Si) -2.78 (2H, s, -NH),
10-), 2.32
CO-),
3
2
)
3
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3
2H, quintet, J = 5.2 Hz, -CH
2
2
.30–3.38 (4H, m, -NCH
2
7
6
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6
N. Yamada, K. Ariga, M. Naito, K. Matsubara and E. Koyama, J. Am.
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