Diarylurea-linked zinc porphyrin dimer as a dual-mode artificial receptor: Supramolecular control of complexation-facilitated photoinduced electron transfer

Article abstract of DOI:10.1021/ja0294717

A novel porphyrinic receptor 1 in which two zinc porphyrins are bridged by two diarylurea linkers was developed for recognition of a viologen derivative (hexyl viologen, HV). The electronic absorption spectra as well as the ¹H NMR experiments revealed that the HV molecule was bound to the cleft in 1 mainly through carbonyl dipole-charge interactions to afford a 1:1 complex. From the steady-state fluorescence spectroscopic study, the photoinduced electron transfer (PET) from 1 to HV was extremely facilitated by the receptor-substrate complexation. The receptor 1 also formed a 1:1 complex with 1,4-diazabicyclo[2.2.2]octane (DABCO) through two Zn-N coordination interactions, and, using DABCO as an inhibitor, we suppressed the PET reaction via the substrate exchange. Copyright

Full text of DOI:10.1021/ja0294717

Published on Web 03/18/2003
Diarylurea-Linked Zinc Porphyrin Dimer as a Dual-Mode Artificial Receptor:
Supramolecular Control of Complexation-Facilitated Photoinduced Electron
Transfer
Shigeyuki Yagi,* Masayuki Ezoe, Isamu Yonekura, Toru Takagishi, and Hiroyuki Nakazumi
Department of Applied Materials Science, Graduate School of Engineering, Osaka Prefecture UniVersity,
1
-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan
Received November 26, 2002; E-mail: yagi@ams.osakafu-u.ac.jp
Well-defined electron donor-acceptor arrays in the photosyn-
thetic reaction center ensure highly efficient photoinduced electron
1
transfer (PET) and long-lived charge separation. Such natural
photochemical functions have been receiving much attention in
terms of the development of molecular-scale photonic devices aimed
2
at light-energy conversion, and, thus, the construction of non-
covalent assemblies of those components with photochemical and
electrochemical properties suitable for PET has been one of the
3
important subjects in supramolecular chemistry. From this view-
point, porphyrin-based artificial receptors for redox active substrates
have been widely studied because of the structural analogy to the
Figure 1. (a) Electronic absorption spectra of 1-3, and (b) absorption
spectral changes of 1 upon addition of increasing amounts of HV (0, 0.30,
photosynthetic pigments, where recognition of the acceptors such
as quinone,3
b,c,4
viologen,5 and fullerene6 has been intensively
0
.70, 0.90, 1.2, 1.5, 2.5, and 5.0 µM; [1] ) 1.5 µM) in CHCl3/DMSO (10/
investigated. In the next stage, it is a challenging theme to devise
molecular photonic systems with on-off controllable triggers, for
example, the control of PET by supramolecular regulatory stimuli.
Here we report the complexation-facilitated PET system consisting
of a novel diarylurea-linked zinc porphyrin dimer 1 and a viologen
derivative (hexyl viologen perchlorate, HV). We also report the
inhibitory control of the PET using an alternative substrate.
1, v/v) at 293 K.
Table 1. Binding Constants of 1-3 for HV As Determined by
Electronic Absorption (Kabs) and Fluorescence (Kem) Spectroscopy
1 a
K
em/M-1 a
compound
K
abs/M-
1
2
3
546 000b
>1 000 000
3890
b
b
b
4300
1100b
2300
a
In CHCl3/DMSO (10/1, v/v) at 293 K. b Errors are within 10%.
The shoulder in the monomer absorption region implies that the
excitonic interaction is partially canceled due to the structural
flexibility. On the other hand, the dimer 1 shows the narrow blue-
shifted band, and, thus, bridging two porphyrin moieties by two
diarylurea linkers yielded a tightly fixed cofacial structure.
The compounds 1-3 exhibited different binding abilities for HV.
Adding increasing amounts of HV to solutions of 1-3 led to red
shifts of the Soret absorption accompanying saturation behaviors
with an isosbestic point due to the receptor-substrate complexation
(
Figure 1b). The stoichiometry for each complex was confirmed
8
as 1:1 by either the molar ratio method or Job’s analysis. As
summarized in Table 1, the binding constant Kabs of 1‚HV was
9
1
much larger than those of 2‚HV and 3‚HV. In the H NMR spectra
a
of the 2‚HV system, the signals assigned to the HV’s protons H ,
b
c
H , and H exhibited large complexation-induced shifts to the
upfield due to ring-current anisotropy from the porphyrin π-planes,
indicating that HV was bound to the cleft consisting of the two
porphyrin moieties.10 Taking it into consideration that ZnTTolP
did not form a complex with HV and that the increase of the
diarylurea unit from 2 to 1 led to the increase of affinity to HV,
charge-dipole interactions between HV and the two carbonyl groups
in 1 directed into the inner of the cleft play an important role in
the formation of the stable complex,12 in addition to the electrostatic
interaction between HV and well-organized porphyrin moieties.
The binding of HV was accompanied by fluorescence quenching
of the porphyrin receptors. The fluorescence emission spectral
11
We previously reported the construction of cofacial porphyrin
arrays using a diarylurea linkage (e.g., the single-linked dimer 2).7
The double-linked dimer 1 was synthesized from 5,10-bis(3-amino-
4-methylphenyl)-15,20-bis(4-methylphenyl)porphyrin in a manner
similar to the preparation of 2. In Figure 1a are shown electronic
absorption spectra of 1-3 in the Soret region. The absorption band
of 2 exhibits a blue shift by 3 nm as compared with that of 3, the
weak excitonic interaction between the porphyrin chromophores
indicating that a cofacial conformer of 2 is predominantly formed.
4068
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J. AM. CHEM. SOC. 2003, 125, 4068-4069
10.1021/ja0294717 CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
cence emission was suppressed by complexation with HV, the
addition of DABCO led to recovery of the emission, indicating
DABCO competes with HV to kick it away from the cleft of 1
Scheme 1).14 Indeed, in the H NMR spectra, the HV’s signals
1
(
returned to their original positions upon addition of DABCO,
whereas DABCO’s signal appeared at -4.76 ppm.
In summary, we developed a novel dual-mode porphyrinic
receptor 1 that binds HV and DABCO in different manners. The
PET from 1 to HV was facilitated by complexation and suppressed
by substrate exchange with DABCO, and, thus, the supramolecular
control of the PET was achieved.
Figure 2. (a) Fluorescence emission spectra of 1 (1.5 µM) in the presence
of varying concentrations of HV (a-i; 0, 0.30, 0.60, 0.90, 1.2, 1.5, 2.5,
Acknowledgment. This work was supported by a grant from
the Iketani Science and Technology Foundation.
3
.0, 6.0 µM). (b) The fluorescence quenching profiles for 1‚HV, 2‚HV,
and 3‚HV systems monitored at 608 nm. The emission spectra were obtained
in CHCl3/DMSO (10/1, v/v) at 293 K. λex ) 562 nm.
Supporting Information Available: Synthetic procedure for 1
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 3. Fluorescence emission spectra of (a) 1, (b) 1 + HV (3.0 µM),
(
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1
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changes of 1 upon addition of HV are shown in Figure 2. As the
concentration of HV increased, the emission intensity of 1 was
reduced due to PET from the porphyrin to HV (Figure 2a).13 The
spectral changes finally reached a plateau, indicating that the
fluorescence quenching was induced by the complexation (Figure
(
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(
(
2b). Although 2 and 3 also exhibited fluorescence quenching upon
8) The stoichiometry was determined for 1‚HV by the molar ratio method
addition of HV, the quenching efficiency varied in the receptors.
The binding constant Kem for each receptor obtained from the
fluorescence spectral changes validly corresponded to Kabs (Table
using fluorescence emission spectroscopy and for 2‚HV and 3‚HV by
1
Job’s method using H NMR.
(
9) The binding constants were determined by the least-squares analyses of
the absorbance changes according to a 1:1 complexation model.
a
b
c
(
10) The complexation-induced shift of the protons H , H , and H is 0.92,
.34, and 1.64 ppm upfield, respectively, in the 1:1 mixture of 2 and HV
([2] ) [HV] ) 1.0 mM in CDCl /DMSO-d (10/1, v/v) at 293 K). In
1), and, therefore, the PET was exclusively facilitated by the
1
complexation. It is interesting that the increase in magnitude of
the binding constant led to the increase of the fluorescence
quenching efficiency. Although the details are not clear at this point,
the well-defined structure of 1 affording tight fixation of HV should
give rise to correct donor-acceptor orientation.
3
6
1
‚HV, the receptor-substrate association and dissociation was slow enough
to afford broadening signals of HV’s protons, although the upfield shifts
of H -H were observed.
a
c
(
(
(
11) ZnTTolP: [5,10,15,20-tetrakis(p-tolyl)porphyrinato]zinc(II).
12) Ong, W.; Gomez-Kaifer, M.; Kaifer, A. E. Org. Lett. 2002, 4, 1791.
13) PET from porphyrins to viologens have been well known: (a) Akins, D.
L.; Zhu, H.-R.; Guo, C. J. Phys. Chem. 1993, 97, 8681. (b) Hayashi, T.;
Takimura, T.; Ogoshi, H. J. Am. Chem. Soc. 1995, 117, 11606. (c)
Kaganer, E.; Joselevich, E.; Willner, I.; Chen, Z.; Gunter, M. J.; Gayness,
T. P.; Johnson, M. R. J. Phys. Chem. B 1998, 102, 1159.
The diarylurea linker also affords enough space between two
porphyrin rings to bind 1,4-diazabicyclo[2.2.2]octane (DABCO)
7
through two Zn-N coordination interactions. Thus, the inhibitory
14) The Kabs for 1‚DABCO was determined as 695 000 M- (error 8.4%).
1
(
control of the PET was examined by addition of DABCO to a
solution containing 1 and HV (Figure 3). Although the fluores-
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