Electron transfer in a hydrogen-bonded assembly consisting of porphyrin-diimide

Article abstract of DOI:10.1039/a803541e

Electron-transfer reactions in an H-bonded assembly composed of a diacylaminopyridine bearing zinc porphyrin and 1,8:4,5-naphthalenetetracarboxylic diimide(K_a = 2.8 × 10² dm³ mol^-1) in benzene occur with k_CS = 4.1 × 10¹⁰ s^-1 and k_CR = 3.7 × 10⁹ s^-1, while the corresponding covalently linked model with a comparable distance and energy gap undergoes electron transfer with k_CS = 9.9 × 10¹⁰ s^-1 and k_CR = 6.7 × 10⁸ s^-1.

Full text of DOI:10.1039/a803541e

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Electron transfer in a hydrogen-bonded assembly consisting of
porphyrin–diimide
Atsuhiro Osuka,*a Ryusho Yoneshima,a Hideo Shiratori,a Tadashi Okada,*b Seiji Taniguchib and Noboru
Matagac
a Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
b Department of Chemistry, Graduate School of Engineering Science and Research Center for Materials Science at Extreme
Conditions, Osaka University, Toyonaka 560-8531, Japan
c Institute for Laser Technology, Utsubo Honmachi, Nishi-ku, Osaka 550-0004, Japan
Electron-transfer reactions in an H-bonded assembly com-
posed of a diacylaminopyridine bearing zinc porphyrin and
1,8:4,5-naphthalenetetracarboxylic diimide (Ka = 2.8 3 102
dm3 mol21) in benzene occur with kCS = 4.1 3 1010 s21 and
kCR = 3.7 3 109 s21, while the corresponding covalently
linked model with a comparable distance and energy gap
undergoes electron transfer with kCS = 9.9 3 1010 s21 and
tylbenzaldehyde,
aldehyde
5
and
4,4A-diethyl-
3,3A-dimethyldipyrrylmethane gave porphyrin 6 in 62% yield
(on the basis of 5), which was converted to a 2,6-diaminopyr-
idyl-substituted porphyrin 7 in 67% yield by hydrolysis under
acidic conditions (HBr–phenol–acetic acid). Porphyrin 9 was
prepared by the reaction of 7 with hexanoyl chloride and
subsequent insertion of Zn.‡ Diimides 10 and 11 were prepared
by the cross-condensation of 1,8:4,5-naphthalenete-
tracarboxylic dianhydride with urea and 2-aminopropane-
1,3-diol followed by silylation with tert-butyldimethylsilyl
chloride in ca. 30 and 13% yields, respectively. The siloxyl
substituents were introduced in order to increase the solubilities
of these diimides in non-polar organic solvents.
kCR = 6.7 3 108 s21
.
In recent years there has been a considerable upsurge in the
development of hydrogen (H)-bonded assemblies that can
undergo efficient electron-transfer (ET) reactions,1–6 since most
biological electron transfers take place in non-covalently
assembled arrays of several chromophores where the overall
electronic coupling between the donor and acceptor seems to
depend on the electronic interaction at a non-covalent H-bonded
motif. Although several synthetic models have been presented
to display ET within H-bonded assemblies, key aspects of ET
dynamics still remain unclarified. In particular, ion-pair states
have been only scarcely identified by transient absorption
spectroscopy, leaving behavior of photogenerated ion pairs
across H-bonds unknown except for a limited number of
examples.3,6,7
1
To confirm the complexation via H-bonding, the H NMR
spectra of 9 in the absence and presence of increasing amounts
of 10 were examined in C6D6, CDCl3 and CD2Cl2. Typical
chemical shift changes upon complexation of 9 and 10 in CDCl3
are summarized in Scheme 2. Most prominently the imide Ha
proton and the amide Hb protons were downfield shifted, while
the other protons were essentially unperturbed. The fact that no
particular upfield shift was observed for the aromatic protons of
10 precludes the coordination of 10 to the zinc centre in 9.
Similar shifts were observed in C6D6 and CD2Cl2, while no such
shifts were detected in [2H8]THF or [2H6]DMF. The observed
chemical shift changes are consistent with a complexation by
Here, we study an H-bonded assembly of a photoexcitable
electron-donor zinc porphyrin
9 and electron-acceptor
1
1,8:4,5-naphthalenetetracarboxylic diimide 10 and ET dy-
namics within the assembly. Porphyrin 9 bears a meso-
substituted 2,6-diacylaminopyridyl group that is expected to
form a strong H bond to the diimide 10.8 The radical anion of 10
possesses sharp and characteristic absorption bands that have
proven to be particularly useful for analysis of electron-transfer
kinetics.9 Aldehyde 5 was prepared in six steps from citrazinic
acid (Scheme 1). The cross-condensation of 3,5-di-tert-bu-
three-points H-bonding as depicted in Scheme 2. From the H
NMR titration, the binding constants were determined using a
standard non-linear least-square curve fitting method: 2.8 3 102
dm3 mol21 in benzene, 1.0 3 102 dm3 mol21 in CDCl3 and 79
dm3 mol21 in CD2Cl2. Further, the continuous variation method
(Job’s plot) was used to determine the complexation stoichio-
metry as 1:1.
Steady-state fluorescence studies carried with a benzene
solution of 9 (1.0 3 1025 m)) in the presence of 10 (5.0 3 1023
CO2H
CO2H
CHO
C6H13
C6H13
But
But
R
N
R
i
ii, iii
6 M = H2, R = NHTS
7 M = H2, R = NH2
8 M = H2, R = NHCOC6H13
9 M = Zn, R = NHCOC6H13
N
N
N
N
M
HO
N
1
OH
Cl
O
N
2
Cl
Cl
N
3
Cl
C6H13
C6H13
iv, v
vi
O
N
O
O
10 R1 = H, R2 = CH(CH2OTBDMS)2
11 R1 = R2 = CH(CH2OTBDMS)2
12 R1 = D, R2 = CH(CH2OTBDMS)2
O
R1
N
O
R2
CHO
C6H13
C6H13
O
N
O
O
TsHN
N
4
NHTs
TsHN
N
5
NHTs
N
N
N
Me
Zn
CH2
N
O
C6H13
Scheme 1 Reagents and conditions: i, POCl3, 160 °C, 78%; ii, BF3·OEt2,
NaBH4, 74%; iii, activated MnO2, 66%; iv, neopentyl glycol, p-TsOH,
99%; v, toluene-p-sulfamide, Cu powder, 190 °C, 63%; vi, 10% H2SO4,
TFA, 76%
N
C6H13
C6H13
13
Chem. Commun., 1998
1567

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characterize ET across an H-bond bridge.§¶ The transient
absorption studies revealed that the rates of charge separation
and recombination in 13 are 9.9 3 1010 and 6.7 3 108 s21
,
respectively. Thus it may be concluded that (1) electronic
coupling across an H-bond interface necessary for charge
separation is comparable to that across two C–C single bonds, in
line with recent results reported by Therien and coworkers,5 and
(2) an ion-pair across H-bonds is considerably shorter-lived in
comparison to a covalently linked ion-pair. The latter aspect that
can account for difficulty in detecting an ion-pair across
H-bonds may be understood in terms of stabilization of the ion
pair, thus the decrease in 2DG in the inverted region, by
H-bond reorganization which is accompanied by the charge
separation.7 The assembly 9–12 has been also studied by
transient absorption spectroscopy and found to undergo ET-
(–17)
O
O
H
(48)
H
Si
(7)
H
N
N
N
H
O
N
O
O
N
O
O
N
N
N
N
Zn
Ha
O
(953)
Si
Hb
(465)
H
(1)
H
H
(17)
H
(0)
H
H
(–39)
(17)
(–13)
H
(49)
H
(17)
Scheme 2 Complexation-induced (20%) changes in 1H NMR chemical
shifts (ppb) for 9–10 in CDCl3
reaction with kCS = 2.9 3 1010 s21 and kCR = 2.4 3 109 s21
,
revealing that the isotope effect kH/kD is 1.4 and 1.5 for charge
separation and recombination, respectively, roughly in line with
results reported by Nocera and coworkers.3a Our next target will
be the development of a H-bonded asembly of donor–acceptor
triads and tetrads which can realize long-lived charge separation
in spite of inherently shorter-lived ion-pairs across H-bonds.
Our attempts in this direction will be reported elsewhere.
This work was supported by Grant-in-Aids for Scientific
Research (No. 09440217 and 08874074) 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).
Notes and References
† E-mail: osuka@kuchem.kyoto-u.ac.jp
‡ All new compounds were fully characterized by 500 MHz 1H NMR and
FAB mass spectra.
§ The one-electron redox potentials of the donor and the acceptor moieties
have been measured in CHCl3 by cyclic voltammetry vs. ferrocene–
ferrocenium ion. The oxidation potential of 9 was 0.16 V and the reduction
potential of 10 was 21.05 V in the free form and 20.97 V in an H-bonded
assembly with 2,6-dihexanoylaminopyridine, while the oxidation and
reduction potentials of 13 were 0.11 and 21.06 V.
l / nm
Fig. 1 Transient absorption spectra of a benzene solution of 9 (2.0 3 1024
m) and 10 (1 3 1022 m) at lex = 532 nm. The dotted line shows the
spectrum at a 20 ps delay time for 9 alone.
¶ Estimated center-to-center distances are 14.7 and 12.8 Å for 9–10 and 13,
respectively.
m) revealed substantial fluorescence quenching (70%) com-
pared with the fluorescence of 9 alone. Such fluorescence
quenching was not observed in THF and DMF nor for a solution
of 9 and 11, indicating that a complexation via H-bonding was
crucial for fluorescence quenching, rather than diffusional
encounters.
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To provide further support for ET via H-bonding, time-
resolved transient absorption studies were carried out. Fig. 1
shows the picosecond transient absorption spectra of 9 alone
and in the presence of 10 at lex = 532 nm in benzene. Under the
conditions used, > 95% of 9 was complexed with 10 and the
fluorescence of 9 was completely quenched. The transient
spectrum of 9 alone (shown by dotted line at a delay time of 20
ps) is a typical transient absorption spectrum of a 5,15-diaryl-
octaalkyl zinc porphyrin, in which the absorption band at 458
nm is due to Sn / S1 absorption and the bleaching at 640 nm is
due to stimulated emission of the S1 state and these two bands
decay with t = 1.6 ns. The transient absorption spectra of the
complex 9–10 provided clear evidence for the formation of a
diimide anion radical (474 nm) and a zinc porphyrin cation
radical (655 nm), both of which were observed early at 20 ps
delay time and decayed with t = 270 ps. Thus, the rate of
charge recombination within the complex 9–10 is 3.7 3 109
s21. By analyzing the time profile at 458 nm where the main
absorbing species is the S1 state of the zinc porphyrin, the rate
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of charge separation has been determined to be 4.1 3 1010 s21
.
These ET rates across an H-bond were compared with those in
a related covalently linked model 13, which has a similar energy
diagram and a slightly shorter center-to-center distance, to
Received in Cambridge, UK, 12th May 1998; 8/03541E
1568
Chem. Commun., 1998