Coordination control of intramolecular electron transfer in boronate ester-bridged donor-acceptor molecules

Article abstract of DOI:10.1039/a802473a

Acid-base reaction at the bridge in a donor-acceptor molecule can influence intramolecular electron-transfer reactions; this has been demonstrated in boronate ester bridged zinc porphyrin-diimide dyads in which intramolecular electron transfer reaction has been completely suppressed by coordination of F^- on the bridge boron.

Full text of DOI:10.1039/a802473a

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Coordination control of intramolecular electron transfer in boronate
ester-bridged donor–acceptor molecules
Hideo Shiratori,a Takeshi Ohno,b Koichi Nozaki,b Iwao Yamazaki,c Yoshinobu Nishimurac and Atsuhiro
Osuka*a†
a Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto, 606-8502, Japan
b Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, 560-8531, Japan
c Department of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628, Japan
Acid–base reaction at the bridge in a donor–acceptor
molecule can influence intramolecular electron-transfer
reactions; this has been demonstrated in boronate ester
bridged zinc porphyrin–diimide dyads in which intra-
molecular electron transfer reaction has been completely
suppressed by coordination of F2 on the bridge boron.
boronate ester-bridged models, 1b and 1c, undergo essentially
the same ET reactions as those of 2b and 2c; the relative
fluorescence intensities of 1b and 1c to 1a are 0.70 and 0.36, and
the fluorescence lifetimes of 1b and 1c are 1.18 ns and 0.57 ns,
respectively, which are both shorter than the lifetime (1.40 ns)
of 1a. Transient absorption spectroscopy also revealed the
formation of the ion-pair state, and the lifetimes of the ion-pair
state are 40 ± 5 ns for 1b (not shown) and 13 ± 1 ns (60%) and
290 ± 10 ns (40%) for 1c [Fig. 1(a), solid line].
The ET dynamics were also examined in the presence of
tetra-n-butylammonium fluoride (TBAF). In the 11B NMR
spectra taken in benzene, a broad boron signal of 1a appearing
at around 10 ppm with respect to a trimethylborate internal
standard was shifted to 214 ppm upon an addition of 1 equiv.
of TBAF, indicating the complete coordination of F2 on the
Intramolecular electron transfer (ET) is the subject of many
studies that are aimed at elucidating the role of the various
parameters which govern the ET rate.1 Among these, the
chemical identity and nature of the bridge that connects donor
and acceptor are important, since ET reactions can be facilitated
by appropriate molecular orbitals provided by the bridge.2
When a donor–acceptor molecule undergoes ET with a rate
tunable by external physical and/or chemical input, it may
constitute a switching ET molecular system. We report here
intramolecular photoinduced electron transfer in boronate ester
bridged zinc porphyrin–pyromellitimide (ZP–PI, 1b) and zinc
porphyrin–1,8:4,5-naphthalenetetracarboxylic diimide (ZP–NI,
1c) (Scheme 1) in which the Lewis acidic boronate ester may
enable alteration of the ET rate upon base coordination. This has
indeed been demonstrated by addition of fluoride anion (F2)
which suppresses the ET.
Porphyrin 1a was prepared according to the method reported
by Toi et al.3 Attempted hydrolysis of 1a was unsuccessful in
our hands under both basic3 and acidic conditions.4 Thus,
boronic acid 3 was prepared by the cross-condensation of
4-formylphenylboronic acid 4 and 3,5-di-tert-butylbenzalde-
hyde 5 with dipyrrylmethane 6 followed by oxidation with
p-chloranil under mild conditions (0 °C, 30 min) in 35% yield.
Boron-bridged dyads 1b and 1c were prepared by refluxing a
toluene solution of 3 in the presence of diol 7 or 8 with 84% and
90% isolated yields, respectively.
C6H13
C6H13
But
N
N
N
N
Zn
R
But
C6H13
C6H13
O
1a ; R1=R2=Me
1b ; R1=H, R2=PI
1c ; R1=H, R2=NI
R1
R2
R =
B
O
2a ; R1=R2=Me
2b ; R1=H, R2=PI
2c ; R1=H, R2=NI
R1
O
R = HC
R2
O
3
R = B(OH)2
But
HO
HO
In benzene, the fluorescence of 1ZP* in 2b and 2c is
quenched by the attached PI or NI; the relative fluorescence
intensities of 2b and 2c to 2a are 0.89 and 0.36, respectively,
and the fluorescence decays of 2b and 2c measured by the time-
correlated single photon counting technique have been found to
obey a single exponential function with lifetimes of 1.21 ns and
0.49 ns, respectively, which are shorter than the lifetime (1.39
ns) of 2a. The observed fluorescence quenching suggests charge
separation (CS) between 1ZP* and PI or NI. The ET quenching
has been confirmed by picosecond time resolved transient
absorption spectroscopy. The transient absorption spectra taken
for excitation with a 17 ps laser pulse at 532 nm revealed the
appearance of a 715 nm absorption band due to PI2 for 2b (not
shown) and of 480 nm and 610 nm bands due to NI2 for 2c [Fig.
1(b)],5 respectively, clearly indicating ion-pair formation.
Charge recombination kinetics of the ion pair to the ground state
in 2b measured by monitoring the 715 nm absorbance change
follows a single exponential decay with t = 84 ± 6 ns, while the
similar kinetic trace at 480 nm in 2c follows a biphasic decay
with t1 = 77 ± 16 ns (66%) and t2 = 380 ± 100 ns (34%).6 The
CHO
B
CHO
But
4
5
C6H13
C6H13
HO
HO
PI
N
H
N
H
6
7
HO
HO
O
NI
B
O
8
9
O
O
O
O
PI = N
N
C6H13
NI = N
O
N
O
C6H13
O
O
Scheme 1 Structures of models studied
Chem. Commun., 1998
1539

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such as DMF and triethylamine but such solvent effects are not
observed for 2b and 2c.
(a)
1c
At the present stage, it seems difficult to identify the main
reason for this CS inhibition by F2 coordination. The energy
levels of the ion-pair states, geometrical parameters, and
electronic coupling should be changed upon F2 coordination.
Since the addition of TBAF had almost no effect on the redox
potentials of these models,8 the energy levels of the ion-pair
states do not change significantly, at least in polar DMF
solution, but are more difficult to estimate in nonpolar benzene
solution. Since the center-to-center distance between ZP and the
boron is estimated to be longer than that between the boron and
the diimide,9 the negative charge at the boron may raise the
energy level of the ion-pair state, thereby decreasing the kCS
value. A change from a neutral trigonal boron to a tetrahedral
‘ate’ anion induces a slight shortening of the donor–acceptor
distance by 0.4 Å as well as changes in the molecular orbitals of
the bridge that will alter the electronic coupling between ZP and
the diimide acceptor, relevant for CS. The former structural
change seems to be unimportant since comparable kCS rates
were observed for the structural analogues, 2b and 2c. The latter
effect, which may be evaluated by considering the molecular
orbitals of the phenylboronate 9, causes a considerable increase
in the LUMO energy of the bridge upon F2 coordination,10
suggesting the decreased electronic coupling due to the high
LUMO energy to be critical in the observed CS inhibition.
Coordination on a neutral bridging boron may provide a
convenient method of controlling the intramolecular ET. The
generality of this method will be tested in other ET systems and
also in triplet–triplet energy transfer systems. Studies in this
direction are ongoing and will be reported soon elsewhere.
This work was partly supported by a Grant-in-Aid for
Scientific Research (No. 09440217) from the Ministry of
Education, Science, Sports and Culture of Japan and by CREST
(Core Research for Evolutional Science and Technology) of
Japan Science and Technology Corporation (JST).
–
1c + F
(b)
A
2c
–
2c + F
400
500
600
700
800
wavelength / nm
Fig. 1 Transient absorption spectra of 1c (a) and 2c (b) in benzene, at a delay
time of 6 ns. Solid lines: in the absence of F2; dotted lines: in the presence
of 1.2 equiv. F2.
1.0
0.8
1b
1c
2b
2c
0.6
0.4
0.2
0.0
Notes and References
† E-mail: osuka@kuchem.kyoto-u.ac.jp
1 M. R. Wasielewski, Chem. Rev., 1992, 92, 435, and references
therein.
2 H. M. McConnell, J. Chem. Phys., 1961, 35, 508; S. Lasson, J. Am.
Chem. Soc., 1981, 103, 4034; M. N. Paddon-Row, Acc. Chem. Res.,
1994, 27, 18, and references therein.
0
1
2
3
4
5
F– / equiv.
Fig. 2 Change of fluorescence intensities of diimide-linked models upon
addition of F2. Data for 1b,c and 2b,c were recorded relative to 1a and 2a,
respectively. Concentrations of models were 1 µm.
3 H. Toi, Y. Nagai, Y. Aoyama, H. Kawabe, K. Aizawa and H. Ogoshi,
Chem. Lett., 1993, 1043.
4 M. Takeuchi, Y. Chin, T. Imada and S. Shinkai, Chem. Commun., 1996,
1867.
boron atom. Similar shifts were observed for 1b and 1c. Fig. 2
shows the effects of TBAF addition on the fluorescence
intensities of 1b,c and 2b,c. It is evident that the fluorescence
intensity of 1c increased with increasing amount of TBAF and
returned to the original unquenched level with ca. 1 equiv. of
TBAF, while the fluorescence intensity of 2c remained
quenched to a constant level up to 5 equiv. of TBAF added.
Essentially the same trend was observed for 1b and 2b. Upon
addition of 5 equiv. TBAF, the short fluorescence lifetimes of
free 1b and 1c were restored to the unquenched levels (1.56 and
1.57 ns, respectively), which are almost the same as that (1.58
ns) of the acceptor-free reference compound 1a under the same
conditions. In contrast to the ET reactions of 2b and 2c, which
are rather independent of TBAF [Fig. 1(b)], the transient
absorption spectra of 1b and 1c in the presence of 1.2 equiv.
TBAF are practically the same as those of the acceptor-free 1c
with no indication of ion-pair formation [Fig. 1(a), dotted line].7
Therefore it is concluded that the CS reactions in 1b and 1c are
completely blocked by F2 coordination. Since restoring the
initial ET activities of 1b and 1c after the addition of F2 is
important for constructing a real ON/OFF system, the addition
of a large amount of 9 to solutions of F2-complexed 1b and 1c
was examined but did not lead to the recovery of the ET
activities. Finally, note that the CS reactions in 1b and 1c are
also completely blocked in solvents with high donor numbers
5 A. Osuka, S. Nakajima, K. Maruyama, N. Mataga, T. Asahi, I.
Yamazaki, Y. Nishimura, T. Ohno and K. Nozaki, J. Am. Chem. Soc.,
1993, 115, 4577; A. Osuka, R.-P. Zhang, K. Maruyama, N. Mataga, Y.
Tanaka and T. Okada, Chem. Phys. Lett., 1993, 215, 179.
6 This biphasic decay behavior may be accounted for by considering the
singlet–triplet intersystem crossing within the ion-pair since the rate of
charge recombination is comparable to the rate of the singlet–triplet
intersystem crossing of the ion-pair state. W. Udo, Y. Sakaguchi, H.
Hayashi, G. Nohya, R. Yoneshima, S. Nakajima and A. Osuka, J. Phys.
Chem., 1995, 99, 13930.
7 The transient absorption spectra indicate that the S1 state of the ZP does
not undergo any ET and is converted to the T1 state absorbing at 455
nm.
8 The one-electron redox potentials of the donor and the acceptor moieties
have been measured in DMF by cyclic voltammetry: Eox(ZP) = 0.22,
E
red(PI) = 21.24 and Eox(NI) = 20.98 V for 1a–c, and Eox(ZP) =
0.22, Ered(PI) = 21.18 and Ered(NI) = 20.94 V for 2a–c vs. ferrocene–
ferrocenium ion.
9 Estimated distance between the ZP and the boron is 9.4 Å and those
between the boron and the diimide are 7.6 and 7.8 Å in 1b and 1c,
respectively.
10 MO calculations were performed using MacSpartan Plus (ab initio,
3-21G basis set). Calculated LUMO energies are 3.4, 8.2, and 4.1 eV for
neutral 9, F2-coordinated 9, and an acetal reference bridge, re-
spectively.
Received in Cambridge, UK, 1st April 1998; 8/02473A
1540
Chem. Commun., 1998