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0
Chemistry Letters Vol.37, No.1 (2008)
Fluorescence Resonance Energy Transfer in Host–Guest Inclusion Complexes
of Cyclodextrin–Porphyrin Composite in Aqueous Solution
ꢀ
Kiyotada Hosokawa, Yutaka Miura, Takayuki Kiba, Toyoji Kakuchi, and Shin-ichiro Sato
Division of Biotechnology and Macromolecular Chemistry, Graduate School of Engineering,
Hokkaido University, Sapporo 060-8628
(Received August 13, 2007; CL-070864; E-mail: s-sato@eng.hokudai.ac.jp)
A composite of zinc tetraphenylporphyrin linked with ꢀ-cy-
concentration. Guest molecules, AN (Sigma-Aldrich), MA (Na-
calai tesque), DMA (Tokyo kasei), or naphthalene (NA; Sigma-
Aldrich), were dissolved in ethanol. Four hundred microliters of
the guest ethanol solution (ca. 10 mol L ) was added to the
aqueous ZnP–ꢀ-CD solution. The solution was stirred for 12 h,
and insoluble matter was removed by filtration. The host/guest
stoichiometry was determined to be ca. 0.8 from their absorption
spectra. The binding constant K was determined from the
fluorescence titration measurements. The titration curves fitted
well with the formation of a 1:1 complex and yielded binding
clodextrin was prepared as a convenient scaffold for a self-as-
sembled energy-transfer complex. The energy-transfer proper-
ties of the host–guest complexes were investigated for several
guest molecules (naphthalene, anthracene, 2-methylanthracene,
and 9,10-dimethylanthracene) by means of fluorescence and
fluorescence-excitation spectroscopies.
ꢁ4
ꢁ1
Fluorescence resonance energy transfer (FRET) of a linked
donor–acceptor (DA) composite has been widely investigated
7
ꢁ1
constants: K ¼ 2:95 ꢂ 10 M
for ZnP–ꢀ-CD/AN, K ¼
1
6
ꢁ1
6
ꢁ1
because of its relevance to biomimetic photosynthesis. The por-
phyrin-based DA composites, in which porphyrins are covalent-
ly linked to either an energy donor or acceptor, are one of the
4:86 ꢂ 10 M for ZnP–ꢀ-CD/MA, K ¼ 1:04 ꢂ 10 M for
ZnP–ꢀ-CD/DMA.
Steady-state absorption, fluorescence, and excitation spectra
at room temperature were measured with a U-3010 spectropho-
tometer (Hitachi) and an F-4500 fluorescence spectrometer
(Hitachi), respectively.
Figure 1 shows absorption and emission spectra of ZnP–ꢀ-
CD in ethanol/water (4:96) mixed solvent. The Soret, Q(0,1),
and Q(0,0) absorption bands located at 433, 558, and 602 nm,
respectively. The Soret band was broadened owing to aggrega-
tion. The rise of background in the UV region is the Rayleigh
scattering from aggregates. The broadening of the Soret band
was not observed for ZnP–ꢀ-CD in ethanol.
2
,3
most important categories in the linked FRET system. On
the other hand, self-assembled supramolecular systems have
been receiving great attention in many fields of material sci-
4
ence. Composites that include cyclodextrins (CDs) are a very
useful scaffold for these supramolecular systems. The photoreac-
tion properties and relaxation dynamics of excited guest mole-
cules encapsulated in CDs have been investigated by several
5
workers. Recently, Kano et al. demonstrated the usefulness of
the porphyrin–CD composite as a scaffold for heteroporphyrin
arrays.6
In the present study, we synthesized a zinc tetraphenylpor-
phyrin linked with ꢀ-CD (ZnP–ꢀ-CD), which was designed as
a scaffold for the self-assembled energy-transfer complex. The
energy-transfer properties of the host–guest complexes, in which
the relative donor–acceptor geometry would be almost the same,
were investigated for several guest molecules (naphthalene
Figure 2 displays emission spectra of ZnP–ꢀ-CD in the pres-
ence and in the absence of the donor in ethanol/water (4:96)
mixed solvent at room temperature. Each emission spectrum
of AN, MA, DMA, and NA complexes was measured with the
excitation of each donor absorption wavelength, at which the
ZnP absorption was negligibly small; the reference spectra (thin
line) were obtained for solution containing only ZnP–ꢀ-CD in
order to show that no ZnP emission was observed in the absence
of donor. The excitation of AN, MA, and DMA complexes gave
ZnP emissions at 610 and 650 nm, whereas no ZnP emission was
observed for ZnP–ꢀ-CD/NA complexes.
(
NA), anthracene (AN), 2-methylanthracene (MA), and 9,10-di-
methylanthracene (DMA)) by means of fluorescence and fluo-
rescence excitation spectroscopies. The FRET efficiency of the
self-assembled DA complexes is discussed herein on the basis
of the F o¨ rster mechanism.
The freebase composite (H2P–ꢀ-CD) was prepared by a
condensation reaction of H2PCOOH with 6-amino-ꢀ-CD in
dry DMF/THF (1:1) mixed solvent containing a small amount
of 1,3-dicyclohexylcarbodiimide, 1-hydroxybenzotriazole hy-
drate, and triethylamine. H2PCOOH was obtained by dealkyla-
tion of 5-(4-methoxycarbonylphenyl)-10,15,20-triphenyl-21H,
0
0
.14
.12
ZnP–β-CD
0.10
0.08
2
3H-porphine (Tokyo Chemical).
H2P–ꢀ-CD was allowed to react with zinc acetate to yield
0
0
.06
.04
10
the ZnP–ꢀ-CD by stirring in a chloroform/methanol (5:1) mixed
solvent for 4 days. The formation of the ZnP–ꢀ-CD was con-
firmed by the fast atom bombardment (FAB) mass spectrometry
7
0.02
0.00
1
300
400
500
Wavelength/nm
600
700
(
3
m=z ¼ 1837:7) and H NMR spectroscopy (ꢁ, DMSO, 400MHz;
.34–5.53 (CD), 7.78 (phenyl), 8.16 (phenyl), 8.76 (ꢀ-pyrrole)).
Self-assembled DA inclusion complexes were prepared as
follows. The ZnP–ꢀ-CD was dissolved in water at a saturated
Figure 1. Absorption (solid line) and emission (dotted line)
spectra of ZnP–ꢀ-CD in ethanol/water (4:96) mixed solvent.
Copyright Ó 2008 The Chemical Society of Japan
Hosokawa, Kiyotada
