Hydrogen bonds not only provide a structural scaffold to assemble donor and acceptor moieties of zinc porphyrin-quinone dyads but also control the photoinduced electron transfer to afford the long-lived charge-separated states

Article abstract of DOI:10.1021/jp050352y

A series of zinc porphyrin-quinone linked dyads [ZnP-CONH-Q, ZnP-NHCO-Q, and ZnP-n-Q (n = 3, 6, 10)] were designed and synthesized to investigate the effects of hydrogen bonds which can not only provide a structural scaffold to assemble donor and acceptor moieties but also control the photoinduced electron-transfer process. In the case of ZnP-CONH-Q and ZnP-NHCO-Q, the hydrogen bond between the N-H proton and the carbonyl oxygen of Q results in the change in the reduction potential of Q. The strong hydrogen bond between the N-H proton and the carbonyl oxygen of Q^.- in ZnP-CONH-Q^.-, ZnP-NHCO-Q^.-, and ZnP-n-Q^.- (n = 3, 6, 10) generated by the chemical reduction has been confirmed by the ESR spectra, which exhibit hyperfine coupling constants in agreement those predicted by the density functional calculations. In the case of ZnP-n-Q (n = 3, 6, 10), on the other hand, the hydrogen bond between two amide groups provides a structural scaffold to assemble the donor (ZnP) and the acceptor (Q) moiety together with the hydrogen bond between the N-H proton and the carbonyl oxygen of Q, leading to attainment of the charge-separated state with a long lifetime up to a microsecond.

Full text of DOI:10.1021/jp050352y

J. Phys. Chem. B 2005, 109, 7713-7723
7713
Hydrogen Bonds Not Only Provide a Structural Scaffold to Assemble Donor and Acceptor
Moieties of Zinc Porphyrin-Quinone Dyads but Also Control the Photoinduced Electron
Transfer to Afford the Long-Lived Charge-Separated States
Ken Okamoto and Shunichi Fukuzumi*
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, SORST, Japan
Science and Technology Agency, Suita, Osaka 565-0871, Japan
ReceiVed: January 20, 2005
A series of zinc porphyrin-quinone linked dyads [ZnP-CONH-Q, ZnP-NHCO-Q, and ZnP-n-Q
(n ) 3, 6, 10)] were designed and synthesized to investigate the effects of hydrogen bonds which can not
only provide a structural scaffold to assemble donor and acceptor moieties but also control the photoinduced
electron-transfer process. In the case of ZnP-CONH-Q and ZnP-NHCO-Q, the hydrogen bond between
the N-H proton and the carbonyl oxygen of Q results in the change in the reduction potential of Q. The
strong hydrogen bond between the N-H proton and the carbonyl oxygen of Q•- in ZnP-CONH-Q•-,
ZnP-NHCO-Q•-, and ZnP-n-Q•- (n ) 3, 6, 10) generated by the chemical reduction has been confirmed
by the ESR spectra, which exhibit hyperfine coupling constants in agreement those predicted by the density
functional calculations. In the case of ZnP-n-Q (n ) 3, 6, 10), on the other hand, the hydrogen bond between
two amide groups provides a structural scaffold to assemble the donor (ZnP) and the acceptor (Q) moiety
together with the hydrogen bond between the N-H proton and the carbonyl oxygen of Q, leading to attainment
of the charge-separated state with a long lifetime up to a microsecond.
Introduction
charge-separated state of porphyrin-quinone dyads are also fast
and the lifetimes are mostly on the order of picoseconds or sub-
nanoseconds in solution,17-22 in contrast with the long-lived
charge-separated state in the natural photosynthesis.1-3
We report herein the first systematic study to clarify the
effects of hydrogen bonds which can not only provide a
structural scaffold to assemble the donor and acceptor moieties
In the bacterial photosynthetic reaction center (bRC) from
Rhodobacter (Rb) sphaeroides, an electron is transferred from
the singlet excited state of (BChl)2 via bacteriochlorophyll
(BChl) and bacteriopheophytin (Bphe) to the primary quinone
(QA) and finally to the secondary quinone (QB).1 Although QA
and QB are virtually identical, the difference in the hydrogen
bonds with the amino acids makes it possible to transfer an
electron from QA to QB.1-3 Hydrogen bonds are formed between
QA and the amide groups of D2 Phe261 and D2 His214, whereas
QB is hydrogen-bonded with D1 Ser264, D1 His215, and the amide
group of D1 Phe265 in photosystem II (PS II).2b In this case, the
hydrogen bonds can not only provide a structural scaffold but
also control the direction of electron transfer (ET).1-3 Thus, the
hydrogen bond has emerged as the paramount synthon for self-
assembly to investigate ET dynamics in supramolecular electron
donor (D) and acceptor (A) ensembles.4-10 However, there has
so far been no systematic study to elucidate the effects of
hydrogen bonds on control of both the structure and redox
reactivity of D-A ensembles.
On the other hand, extensive efforts have so far been devoted
to elucidating the distance and orientation dependence of ET
from electron donors (D) to acceptors (A) in a variety of D-A
linked systems.11-16 In particular, covalently and noncovalently
linked porphyrin-quinone dyads constitute one of the most
extensively investigated photosynthetic models, in which the
fast photoinduced electron transfer from the porphyrin singlet
excited state to the quinone occurs to produce the charge-
separated (CS) state, mimicking well the photosynthetic electron
transfer.17-24 However, the charge recombination rates of the
but also control the redox reactivity in
a series of
zinc porphyrin-quinone linked dyads [ZnP-CONH-Q,
ZnP-NHCO-Q, and ZnP-n-Q (n ) 3, 6, 10) shown in Chart
1]. In the case of ZnP-CONH-Q and ZnP-NHCO-Q, the
hydrogen bond between the N-H proton and the carbonyl
oxygen of Q results in the change in the redox potential of Q.
In the case of ZnP-n-Q (n ) 3, 6, 10), on the other hand, the
hydrogen bond between two amide groups provides a structural
scaffold to assemble the donor (ZnP) and the acceptor (Q)
moiety, leading to attainment of the charge-separated state with
a surprisingly long lifetime up to a microsecond.
Experimental Section
1
General Methods. H NMR and 13C NMR spectra were
measured on a JEOL JNM-AL300 NMR spectrometer. Fast
atom bombardment mass spectra (FAB-MS) were obtained on
a JEOL JMS-DX300 mass spectrometer. Matrix-assisted laser
desorption/ionization (MALDI) time-of-flight mass spectra
(TOF) were measured on a Kratos Compact MALDI I (Shi-
madzu). IR spectra of solid samples were recorded on a Thermo
Nicolet NEXUS 870 FT-IR instrument using 2 cm-1 standard
resolution at ambient temperature by KBr wafers containing 1%
of the sample. IR spectra of samples in CH2Cl2 were measured
using an ASI ReactIR 1000 spectrometer (PE-Sciex).
Materials. All solvents and chemicals were of reagent grade
quality, were obtained commercially, and were used without
* To whom correspondence should be addressed. E-mail: fukuzumi@
ap.chem.eng.osaka-u.ac.jp.
10.1021/jp050352y CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/06/2005

7714 J. Phys. Chem. B, Vol. 109, No. 16, 2005
Okamoto and Fukuzumi
CHART 1
further purification unless otherwise noted. Benzonitrile (PhCN)
was purchased from Aldrich Co., USA, and Tokyo Kasei
Organic Chemicals, Japan, and purified by successive distillation
over P2O5.25 Tris(2,2′-bipyridyl)ruthenium(III) hexafluorophos-
phate [Ru(bpy)3(PF6)3] was prepared according to the litera-
ture.26 Thin-layer chromatography (TLC) and flash column
chromatography were performed with Art. 5554 DC-Alufolien
Kieselgel 60 F254 (Merck), and Fujisilicia BW300, respectively.
The synthetic procedures of synthetic precursors [H2P-
CONH-(MeO)2Ph, H2P-CONMe-(MeO)2Ph, H2P-NHCO-
(MeO)2Ph, H2P-n-COOH (n ) 3, 6, 10), and H2P-n-
(MeO)2Ph (n ) 3, 6, 10)] and reference compounds (ZnP-
NHCO-ref, ZnP-CONH-ref) are similar to those in our previous
reports.27 The synthetic details and characterizations are de-
scribed in Supporting Information (S1-S7).
ZnP-CONH-Q. A dichloromethane solution of boron tri-
bromide (BBr3; 0.26 mmol) was added to a dichloromethane
solution (30 mL) of H2P-CONH-(MeO)2Ph (2) (140 mg,
0.13 mmol) at 0 °C under argon. After the solution was stirred
at 0 °C for 3 h, BBr3 was quenched by adding water and
saturated aqueous sodium hydrogen carbonate. The organic
phase was dried over sodium sulfate and filtrated. After removal
of solvent, the residue was purified by column chromatography
(SiO2, chloroform). The purified compound (H2P-CONH-
(HO)2Ph) was further stirred under Ar with PbO2 (250 mg) in
dichloromethane (30 mL) at room temperature for 2 h. PbO2
was removed carefully by suction filtration. After removal of
solvent, the residue was purified by column chromatography
(SiO2, chloroform). The H2P-CONH-Q was refluxed in
dichloromethane with a methanolic solution of zinc acetate
(Zn(OAc)2). After water was added to the solution, the organic
phase was dried over sodium sulfate and filtrated. After removal
of solvent, the column chromatography on silica gel with
chloroform as the eluent and subsequent recrystallization from
chloroform and hexane gave ZnP-CONH-Q as a deep red-
purple solid. (45 mg, 0.039 mmol, 30%). 1H NMR (300 MHz,
CDCl3): δ 9.17 (s, 1H(NH)), 9.02 (d, 1H(Ph), J ) 5 Hz), 9.01
(s, 8H(â)), 8.87 (d, 1H(Ph), J ) 5 Hz), 8.41 (d, 1H(Ph), J )
8 Hz), 8.27 (d, 1H(Ph), J ) 8 Hz), 8.09 (m, 6H(Ar)), 7.94
(m, 1H(Q)), 7.80 (m, 3H(Ar)), 6.90 (m, 2H(Q)), 1.53 (s, 54H-
(tBu)). 13C NMR (300 MHz, CDCl3): δ 188 (Q, CdO), 183
(Q, CdO), 166 (amide, CdO), 148 (ring, C-N). UV-vis
(CHCl3, nm): 426, 553, 597. TOF-MS: m/z 1163.9. FAB-MS
(FAB+): m/z 1164.4. HR-MS (FAB+): m/z 1163.5621 (calcd
for C75H81N5O3Zn, 1163.5644). FT-IR (KBr): 1701 (amide,
CdO), 1668 (CdO), 1651 (CdO), 1593 (alkene), 1493 (N-
H). FT-IR (CH2Cl2): 1701 (amide, CdO), 1669 (CdO), 1652
(CdO), 1593 (alkene), 1498 (N-H). TLC (SiO2): Rf ) 0.70 in
CHCl3.
ZnP-CONMe-Q. The same synthetic procedures as those
from H2P-CONH-(MeO)2Ph to ZnP-CONH-Q were
employed for preparation of ZnP-CONMe-Q using
H2P-CONMe-(MeO)2Q as the precursor. The column chro-
matography for purification of H2P-CONMe-Q(OH)2 was
performed using chloroform:MeOH ) 50:1 in silica gel. The
column chromatography on silica gel with chloroform as the
eluent and subsequent recrystallization from chloroform and
hexane gave ZnP-CONMe-Q as a deep red-purple solid
1
(131 mg, 0.11 mmol, 61%). H NMR (300 MHz, CDCl3):
δ 9.00 (s, 8H(â)), 8.75 (d, 1H(Ph), J ) 8 Hz), 8.22 (d, 1H(Ph),
J ) 8 Hz), 8.07 (m, 6H(Ar)), 7.87 (d, 2H(Ph), J ) 8 Hz), 7.78
(m, 1H(Q)), 7.77 (m, 3H(Ar)), 6.83 (m, 2H(Q)), 3.60
(s, 3H(N-Me)) 1.53 (s, 54H(tBu)). 13C NMR (300 MHz,
CDCl3): δ 187 (Q, CdO), 182 (Q, CdO), 172 (amide, CdO),
150.6 (ring, C-N). UV-vis (CHCl3, nm): 425, 552, 596.
TOF-MS: m/z 1177.3. FAB-MS (FAB+): m/z 1178.5.
HR-MS (FAB+): m/z 1177.5796 (calcd for C76H83N5O3Zn,
1177.5801). FT-IR (KBr): 1678 (amide, CdO), 1675 (Q,
CdO), 1653 (Q, CdO), 1592 (alkene), 1338 (N-Me). FT-IR
(in CH2Cl2): 1682 (amide, CdO), 1680 (Q, CdO), 1653 (Q,
CdO), 1592 (alkene), 1340 (N-Me). TLC: Rf ) 0.50 in CHCl3.
ZnP-NHCO-Q. This compound was prepared in the same
manner as ZnP-CONH-Q. The residue was purified by column
chromatography (SiO2, chloroform:ethyl acetate ) 9:1). ZnP-
NHCO-Q was obtained as a deep red-purple solid (80 mg,
0.069 mmol, 38%). 1H NMR (300 MHz, CDCl3): δ 11.5
(s, 1H(NH)), 9.00 (s, 8H(â)), 8.28 (d, 2H(Ph), J ) 8 Hz), 8.09
(m, 6H(Ar)), 8.03 (d, 2H(Ph), J ) 8 Hz), 7.78 (s, 3H(Ar)), 7.53

Hydrogen Bonds in Zinc Porphyrin-Quinone Dyads
J. Phys. Chem. B, Vol. 109, No. 16, 2005 7715
(s, 1H(Q)), 6.97 (s, 2H(Q)), 1.53 (s, 54H(tBu)). 13C NMR
(300 MHz, CDCl3): δ 186 (Q, CdO), 182 (Q, CdO), 163
(amide, CdO), 148 (ring, C-N). UV-vis (CHCl3, nm): 425,
552, 595. TOF-MS: m/z 1164.7. FAB-MS (FAB+): m/z
1164.6. HR-MS (FAB+): m/z 1163.5647 (calcd for C75H81N5O3-
Zn, 1163.5644). FT-IR (KBr): 1697 (amide, CdO), 1680
(CdO), 1659 (CdO), 1591 (alkene), 1530 (N-H). FT-IR
(CH2Cl2): 1693 (amide, CdO), 1678 (CdO), 1663 (CdO),
1592 (alkene), 1536 (N-H). TLC (SiO2): Rf ) 0.26 in CHCl3.
ZnP-3-Q. This compound was prepared in the same manner
as ZnP-CONH-Q using H2P-3-(MeO)2Ph as the precursor.
The column chromatography for purification was performed
using chloroform:MeOH ) 95:5 in silica gel. Removal of
solvent gave ZnP-3-Q as a deep red-purple solid. (100 mg,
0.080 mmol, 73%). 1H NMR (300 MHz, CDCl3): δ 9.00
(s, 8H(â)), 8.95 (m, 2H(NH)), 8.19 (d, 2H(Ph), J ) 8 Hz), 8.09
(m, 6H(Ar)), 7.92 (d, 2H(Ph), J ) 8 Hz), 7.78 (m, 3H(Ar)),
7.71 (m, 1H(Q)), 6.79 (m, 2H(Q)), 2.72 (t, 2H(-CH2-
CONHQ), J ) 7 Hz), 2.67 (t, 2H(NHCO-CH2-), J ) 7 Hz),
1.53 (s, 54H(tBu)), 1.25 (m, 2H(-CH2-)). UV-vis (CHCl3,
nm): 425, 552, 596. TOF-MS: m/z 1248.9. FAB-MS (FAB+):
m/z 1250.6. FT-IR (KBr): 1722 (amide, CdO), 1703 (amide,
CdO), 1666 (Q, CdO), 1658 (Q, CdO), 1592 (alkene), 1525
(N-H), 1497 (N-H). FT-IR (in CH2Cl2): 1725 (amide,
CdO), 1677 (amide, CdO), 1661 (Q, CdO), 1637 (Q, CdO),
1592 (alkene), 1535 (N-H), 1494 (N-H). TLC: Rf ) 0.30 in
CHCl3.
cell was used with a platinum working electrode (surface area
of 0.3 mm2) and a platinum wire as the counter electrode. The
Pt working electrode (BAS) was routinely polished with BAS
polishing alumina suspension and rinsed with acetone before
use. The measured potentials were recorded with respect to the
Ag/AgNO3 (0.01 M) reference electrode. All potentials
(vs Ag/Ag+) were converted to values vs SCE by adding
0.29 V.28 All electrochemical measurements were carried out
under an atmospheric pressure of Ar.
ESR Measurements. The radical anions, ZnP-CONH-Q•-,
ZnP-CONMe-Q•-, ZnP-NHCO-Q•-, and ZnP-n-Q•-, were
produced by the chemical reduction with tetramethylsemi-
quinone radical anion (1.0 × 10-5 M), which was obtained by
the reaction between hydroquinone and p-benzoquinone in the
presence of tetramethylammonium hydroxide.29 The radical
cation, ZnP•+-CONH-Q, was generated by the chemical
oxidation with Ru(bpy)33+. A solution containing the radical
anion or the radical cation was transferred to an ESR tube under
an atmospheric pressure of Ar. The ESR spectra were recorded
on a JEOL X-band spectrometer (JES-RE1XE) with a quartz
ESR tube (1.2 mm i.d.). The ESR spectra were measured under
nonsaturating microwave power conditions. The magnitude of
modulation was chosen to optimize the resolution and the signal-
to-noise (S/N) ratio of the observed spectra. The g values were
calibrated with an Mn2+ marker, and the hyperfine coupling
(hfc) constants were determined by computer simulation using
Calleo ESR Version 1.2 program coded by Calleo Scientific
on an Apple Macintosh personal computer. For the detection
of the charge-separated state, a quartz ESR tube (internal
diameter, 4.5 mm) containing a deaerated PhCN solution of
ZnP-10-Q dyad (5.0 × 10-5 M) was irradiated in the cavity
of the ESR spectrometer with the focused light of a 1000-W
high-pressure Hg lamp (Ushio-USH1005D) through an aqueous
filter at low temperatures. The internal diameter of the ESR
tube is 4.5 mm, which is small enough to fill the ESR cavity
but large enough to obtain good signal-to-noise ratios during
the ESR measurements under photoirradiation. The ESR spectra
in frozen PhCN were measured under nonsaturating microwave
power conditions at 173 K with a JEOL X-band spectrometer
(JESRE1XE) using an attached variable-temperature apparatus.
ZnP-6-Q. This compound was also prepared in the same
manner as ZnP-3-Q. The column chromatography for purifica-
tion was performed using chloroform in silica gel. Removal of
solvent gave ZnP-6-Q as a deep red-purple solid (45 mg,
0.035 mmol, 29%). 1H NMR (300 MHz, CDCl3): δ 8.92
(s, 8H(â)), 8.88 (s, 1H(NH)), 8.87 (s, 1H(NH)), 8.22 (d,
2H(Ph), J ) 8 Hz), 8.01 (m, 6H(Ar)), 7.92 (m, 1H(Q)),
7.82 (d, 2H(Ph), J ) 8 Hz), 7.71 (m, 3H(Ar)), 6.67 (m,
2H(Q)), 2.48 (m, 2H(NHCO-CH2-)), 2.37 (m, 2H(-CH2-
CONHQ)), 1.48 (s, 54H(tBu)), 1.25 (m, 8H(-CH2-)). UV-
vis (PhCN, nm): 431, 560, 601. TOF-MS: m/z 1291.0. FAB-
MS (FAB+): m/z 1290.5. FT-IR (KBr): 1717 (amide, CdO),
1699 (amide, CdO), 1667 (Q, CdO), 1653 (Q, CdO), 1592
(alkene), 1524 (N-H), 1498 (N-H). FT-IR (in CH2Cl2): 1727
(amide, CdO), 1713 (amide, CdO), 1672 (Q, CdO), 1654 (Q,
CdO), 1590 (alkene), 1512 (N-H), 1501 (N-H). TLC: Rf )
0.50 in CHCl3.
Fluorescence Quenching. Fluorescence measurements of
ZnP-n-Q, ZnP-NHCO-ref, and ZnP-CONH-ref were car-
ried out on a SHIMADZU spectrofluorophotometer (RF-5000).
The excitation wavelength of ZnP was set at λ ) 560 nm in
MeCN, PhCN, CH2Cl2, tetrahydrofuran (THF), and toluene.
Typically, CH2Cl2 and THF solutions were deaerated by five
freeze-pump-thaw cycles. MeCN, PhCN, and toluene solu-
tions were deaerated by argon purging for 15 min prior to the
measurements. Time-resolved fluorescence spectra were mea-
sured by a Photon Technology International GL-3300 with a
Photon Technology International GL-302, nitrogen laser/pumped
dye laser system (50 ps pulse duration), equipped with a four-
channel digital delay/pulse generator (Stanford Research System
Inc. DG535) and a motor driver (Photon Technology MD-5020).
ZnP-10-Q. This compound was also prepared in the same
manner as ZnP-3-Q. The column chromatography on silica
gel with chloroform gave ZnP-10-Q as a deep red-purple solid.
1
(40 mg, 0.030 mmol, 18%). H NMR (300 MHz, CDCl3):
δ 9.00 (m, 8H(â)), 8.98 (s, 1H(NH)), 8.97 (s, 1H(NH)), 8.18
(d, 2H(Ph), J ) 8 Hz), 8.08 (m, 6H(Ar)), 7.97 (m, 1H(Q)),
7.91 (d, 2H(Ph), J ) 8 Hz), 7.78 (m, 3H(Ar)), 6.75 (m,
2H(Q)), 2.42 (t, 2H(NHCO-CH2-), J ) 7 Hz), 2.28 (t,
2H(NHCO-CH2-), J ) 7 Hz), 1.52 (s, 54H(tBu)), 1.25 (m,
16H(-CH2-)). UV-vis (CHCl3, nm): 426, 559, 600.
TOF-MS: m/z 1347.0. FAB-MS (FAB+): m/z 1346.7. FT-IR
(KBr): 1720 (amide, CdO), 1700 (amide, CdO), 1661 (Q,
CdO), 1652 (Q, CdO), 1592 (alkene), 1540 (N-H), 1508
(N-H). FT-IR (in CH2Cl2): 1732 (amide, CdO), 1718 (amide,
CdO), 1685 (Q, CdO), 1661 (Q, CdO), 1592 (alkene), 1508
(N-H), 1500 (N-H). TLC: Rf ) 0.55 in CHCl3.
The excitation wavelength was 431 nm using POPOP (Wako
Pure Chemical Ind. Ltd., Japan) as a dye. The fluorescence
lifetimes of ZnP-CONH-ref were determined as τ ) 2.04 ns
in MeCN, τ ) 2.04 ns in PhCN, τ ) 1.88 ns in CH2Cl2,
τ ) 2.10 ns in THF, and τ ) 2.15 ns in toluene by an
exponential curve fit of the fluorescence decay using a
microcomputer. The fluorescence lifetimes of ZnP-NHCO-ref
were also determined as τ ) 1.77 ns in MeCN, τ ) 1.91 ns in
PhCN, τ ) 1.81 ns in CH2Cl2, τ ) 2.13 ns in THF, and
τ ) 2.06 ns in toluene. All samples were excited at 431 nm
Electrochemical Measurements. Electrochemical measure-
ments were performed on a BAS 100W electrochemical analyzer
in deaerated PhCN containing 0.1 M Bu4NPF6 (TBAPF6) as
supporting electrolyte at 298 K. A conventional three-electrode

7716 J. Phys. Chem. B, Vol. 109, No. 16, 2005
Okamoto and Fukuzumi
TABLE 1: Edge-to-Edge Distances (Ree) and Center-to-Center Distances (Rcc), Redox Potentials, and Driving Forces of Electron
Transfer (-∆GET, -∆GBET
)
Eox,d V
Ered,d V
dyad
Ree,a Å
Ree,a,b
Å
Ree,a,c
Å
Rcc,a Å
ZnP•+/ZnP
Q/Q•-
-∆GET,e eV
-∆GBET, eV
ZnP-CONH-Q
3.78
(3.83)
12.66
(12.59)
0.78
0.77
0.77
-0.36
-0.46
-0.17
0.91
1.14
ZnP-CONMe-Q
ZnP-NHCO-Q
3.78
(3.78)
12.66
(12.64)
0.82
1.11
1.23
0.94
3.81
(3.80)
12.66
(12.85)
Fc-NHCO-Q
Fc-NMeCO-Q
ZnP-3-Q
0.48
-0.15
-0.36
-0.40
(Fc+/Fc)
0.52
(Fc+/Fc)
0.79
10.74
15.31
21.47
8.26
(8.18)
12.20b
0.86
0.88
0.89
1.19
1.17
1.16
(12.54)b
ZnP-6-Q
8.21
(4.26)
(4.29)
12.51b
0.79
0.79
-0.38
-0.37
(12.44)c
ZnP-10-Q
8.27
10.81b
(10.82)c
a Estimated from optimized structure (B3LYP/6-31G(d) or BLYP/6-31G(d)). The values in parantheses are those of Q•-
.
b Type A structure.
1
c Type B structure. d vs SCE in the presence of 0.1 M TBAPF6 in PhCN. e Photoinduced ET from ZnP* to Q in PhCN.
with a repetition rate of 10 Hz (pulse width, 3-4 ns), and the
fluorescence signal was analyzed after passing through a
monochromator set at the peak emission of the corresponding
sample.
The NMR parameters for TMS were calculated and used as the
reference compound in order to compare isotropic shielding with
the experimentally observed chemical shifts. A natural popula-
tion analysis36 (NPA) was used to ascertain the location of the
“extra” electron density in the radical anion species; geometries
were optimized using BLYP at the 6-31G(d) level. The energy
difference between the optimized structures of the A type and
B type in the radical anion (Ph-6-Q•-, Ph-10-Q•-) was
evaluated on the basis of BLYP/6-31G calculations with the
ROHF formalism.
Time-Resolved Absorption Measurements. Nanosecond
transient absorption measurements were carried out using a
Panther OPO pumped by a Nd:YAG laser (Continuum,
SLII-10, 4-6 ns full width at half-maximum (fwhm)) at
428-435 nm with the power of 10 mJ as an excitation source.
The solution was deoxygenated by argon purging for 10 min
prior to the measurements. The quantum yields of the charge-
separated states of ZnP-n-Q (n ) 3, 6, 10) were determined
using a comparative method.30 In particular, the strong zinc
tetraphenylporphyrin triplet-triplet absorption (470 nm )
74 000 M-1 cm-1; ΦT ) 0.88)30 served as a probe to determine
the quantum yields for formation of the CS state in PhCN.
Theoretical Calculations. Density-functional theory (DFT)
calculations were performed on a COMPAQ DS20E computer.
Geometry optimizations were carried out using the BeckeLYP
and 6-31G(d) basis set with unrestricted Hatree-Fock (UHF)
formalism, Becke3LYP functional, and 6-31G(d) basis set31 or
BeckeLYP and 6-31G(d) basis set31 with the restricted Hatree-
Fock (RHF) formalism as implemented in the Gaussian 98
program.32 Subsequently, the harmonic vibrational frequencies
were calculated, where calculated frequencies have been scaled
to avoid systematic errors.33,34 The 13C magnetic shielding
tensors (ø) of DFT-optimized structures of Ph-CONH-Q,
Ph-CONMe-Q, and Ph-NHCO-Q compounds, and tetrameth-
ylsilane (TMS) as reference, were obtained following the gauge-
including atomic orbital (GIAO) approach at the DFT level.35
Results and Discussion
Synthesis. The synthesis and characterizations of ZnP-
CONH-Q, ZnP-CONMe-Q, ZnP-NHCO-Q, and ZnP-n-Q
(n ) 3, 6, 10) dyads, and ZnP-CONH-ref and ZnP-NHCO-
ref are described in detail in Experimental Section and Sup-
porting Information S1-S7.
Hydrogen Bond to Stabilize Q•- of ZnP-Q Dyads. The
cyclic voltammograms of ZnP-CONH-Q exhibited two re-
versible one-electron redox couples of two redox-active moieties
at 0.78 V for the ZnP•+/ZnP couple and -0.36 V for the Q/Q•-
couple (vs SCE), as shown in Table 1, where similar one-
electron redox potentials of the ZnP•+/ZnP and Q/Q•-couples
are obtained for ZnP-n-Q (n ) 3, 6, 10). When the amide
proton of ZnP-CONH-Q is replaced by the methyl group in
ZnP-CONMe-Q, the one-electron reduction potential of Q
(E0 vs SCE ) -0.46 V) is negatively shifted as compared
red
with the E0red value of Q in ZnP-CONH-Q, whereas the one-
electron oxidation potential (E0ox vs SCE ) 0.77 V) of the ZnP
moiety in ZnP-CONMe-Q is virtually the same as the E0
ox

Hydrogen Bonds in Zinc Porphyrin-Quinone Dyads
J. Phys. Chem. B, Vol. 109, No. 16, 2005 7717
Figure 1. ESR spectra and the computer simulation spectra of (a) ZnP-CONH-Q•- and (b) ZnP-CONMe-Q•- (1.0 × 10-5 M) in deaerated
CH2Cl2 at 263 K, and the optimized structures of (a) Ph-CONH-Q•-, (b) Ph-CONMe-Q•-, and (c) Ph-NHCO-Q•- by Gaussian 98 with UBLYP/
6-31G(d) basis set [the values (G) in parentheses are provided by Gaussian 98 with UBLYP/6-31G(d) basis set].
value (0.78 V) of ZnP-CONH-Q. Similarly the replacement
Section). Since the E0red value of p-benzoquinone (-0.50 V vs
of the amide proton by the methyl group results in a negative
SCE)37 is more negative that the E0 values of the Q moiety
red
shift of the E0 value of Q in the ferrocene-quinone dyad:
of the dyads, the electron transfer is thermodynamically favor-
able to produce Q•- in the dyads. The ESR spectra of ZnP-
CONH-Q•-, ZnP-CONMe-Q•-, and ZnP-NHCO-Q•- are
shown in Figure 1 together with the computer simulation spectra.
The g value is determined as g ) 2.0039, 2.0041, and 2.0048,
respectively. The g value (2.0041) of ZnP-CONMe-Q•- is
larger than the value (2.0039) in ZnP-CONH-Q•-. This
indicates that more spin is localized on the oxygen atom which
red
Fc-NMeCO-Q (-0.38 V) as compared with the E0 values
red
of Fc-NHCO-Q (-0.15 V) and ZnP-NHCO-Q (-0.17 V),
where the ZnP moiety is replaced by Fc. Such a negative shift
of the E0red value of Q by the replacement of the amide proton
by the methyl group indicates that Q•- is stabilized by formation
of a hydrogen bond between the amide proton and the carbonyl
oxygen of Q•-. The larger potential shift (0.19 eV) in ZnP-
NHCO-Q by the replacement of the amide proton by the methyl
group than that (0.10 eV) in ZnP-CONH-Q suggests that the
hydrogen bond in ZnP-NHCO-Q•- is stronger than that in
ZnP-CONH-Q•-.
has
a large spin-orbit coupling constant in ZnP-
CONMe-Q•- as compared with that in ZnP-CONH-Q•-.
The hyperfine coupling constants (hfc) determined by the
computer simulation are shown in Figure 1. The hfc values of
three protons of the Q•- moiety in ZnP-CONH-Q•- (3.80,
2.10, and 1.62 G) and in ZnP-NHCO-Q•- (4.30, 1.95, and
1.80 G) are highly inequivalent, whereas those in ZnP-
CONMe-Q•- (2.62, 2.55, and 2.40 G) are rather equivalent.38
The hfc values can be largely reproduced by the Gaussian 98
The hydrogen-bonded Q•- species are detected by ESR.
ZnP-CONH-Q•-, ZnP-CONMe-Q•-, and ZnP-NHCO-Q•-
were produced by the electron-transfer reduction of ZnP-
CONH-Q, ZnP-CONMe-Q, and ZnP-NHCO-Q by semi-
quinone radical anion in CH2Cl2 at 298 K (see Experimental

7718 J. Phys. Chem. B, Vol. 109, No. 16, 2005
Okamoto and Fukuzumi
Figure 2. Optimized structures and selected bond lengths of (a) Ph-CONH-Q, (b) Ph-CONMe-Q, (c) Ph-NHCO-Q, (d) Ph-3-Q,
(e) Ph-6-Q (type A), (f) Ph-6-Q (type B), (g) Ph-10-Q (type A), and (h) Ph-10-Q (type B) calculated by a DFT method with Gaussian 98
[B3LYP/6-31G(d) basis set for a-c and BLYP/6-31G(d) basis set for d-h].
calculation with the UBLYP/6-31G(d) basis set as indicated by
the calculated hfc values in parentheses in Figure 1.38 In these
calculations, the porphyrin moiety is replaced by the phenyl
group, since the porphyrin moiety is expected to have no
significant effect on the structure and spin densities of the Q
moiety. The comparison of the observed hfc values with the
calculated values indicates that the formation of hydrogen bond
in ZnP-CONH-Q•- and ZnP-NHCO-Q•- results in the less
spin densities on the carbons adjacent to the carbonyl group of
the Q•- moiety of which forms the hydrogen bond with the
NH proton as compared with those adjacent to the other carbonyl
group. This indicates that the negative charge is more localized
on the carbonyl oxygen which forms the hydrogen bond with
the N-H proton due to the electrostatic stabilization, when the
spin is more localized on the carbons adjacent to the other
carbonyl group. The DFT calculations also support the charge
localization due to the formation of the hydrogen bond: the
negative charge density of the hydrogen bonded carbonyl
oxygen (-0.216 in ZnP-CONH-Q•- and -0.222 in ZnP-
NHCO-Q•-) is significantly larger than that of the other
carbonyl oxygen (-0.171 in ZnP-CONH-Q•- and -0.143 in
ZnP-NHCO-Q•-), whereas the difference in the narrative
charge densities between the two carbonyl oxygens in ZnP-
CONMe-Q•- becomes smaller (-0.199 and -0.168).
distance, the C-O bond length of the hydrogen bonded carbonyl
group in Ph-NHCO-Q•- (1.291 Å) becomes longer as
compared with the distance in Ph-CONH-Q•- (1.285 Å)
because of the weakening of the C-O bond by the stronger
hydrogen bonding with the amide proton in Ph-NHCO-Q•-
relative to Ph-CONH-Q•-. This is consistent with the larger
potential shift (0.19 eV) in ZnP-NHCO-Q by the replacement
of the amide proton by the methyl group than that (0.10 eV) in
ZnP-CONH-Q due to the stronger hydrogen bond in ZnP-
NHCO-Q•- than in ZnP-CONH-Q•- (vide supra). The
distance in Ph-CONH-Q•- (1.285 Å) also becomes the longer
C-O bond length in the position of Q1 (see Chart 1) in Ph-
CONMe-Q•- (1.282 Å) due to formation of hydrogen bond in
Ph-CONH-Q•-.
Formation of the much weaker hydrogen bond in the neutral
state (Q) than in the reduced state (Q•-) was examined on the
basis of wavenumbers of CdO stretching vibration in the
position of Q1 in Chart 1 and the 13C chemical shifts of the
CdO of Q in the 13C NMR spectra. The wavenumbers for
CdO in the position of Q1 (Chart 1) in KBr as well as in CH2-
Cl2 solution (see Experimental Section) become larger in the
following sequence: ZnP-NHCO-Q (1659 cm-1 in KBr,
1663 cm-1 in CH2Cl2), ZnP-CONH-Q (1668 cm-1 in KBr,
1669 cm-1 in CH2Cl2), and ZnP-CONMe-Q (1675 cm-1 in
KBr, 1680 cm-1 in CH2Cl2); see Supporting Information S10.
The chemical shifts for CdO carbons of the 13C NMR spectra
in CDCl3 (see S11) become larger in the following sequence:
ZnP-CONMe-Q (182.1), ZnP-CONH-Q (183.0), and ZnP-
NHCO-Q (185.6) in accordance with weakening of the CdO
(Q1) bond strength as indicated in the IR spectra. The DFT
calculations of Ph-CONH-Q, Ph-CONMe-Q, and Ph-
NHCO-Q, with Gaussian 98 (B3LYP/6-31G(d) basis set),
reproduce well the experimental wavenumbers and chemical
The optimized geometries of Ph-CONH-Q•-, Ph-CONMe-
Q•-, and Ph-NHCO-Q•- are also shown in Figure 1. The O-H
distances between the oxygen atom of Q•- and the amide
hydrogen in Figure 1 are 1.97 Å in Ph-CONH-Q•- and
1.70 Å in Ph-NHCO-Q•-, which are much shorter than the
calculated distances in the neutral state: Ph-CONH-Q,
2.10 Å; Ph-NHCO-Q, 1.87 Å, respectively (Figure 2a,c) by a
DFT method with Gaussian 98 (B3LYP/6-31G(d) basis set). In
accordance with such a decrease in the O-H hydrogen bond

Hydrogen Bonds in Zinc Porphyrin-Quinone Dyads
J. Phys. Chem. B, Vol. 109, No. 16, 2005 7719
Figure 3. ESR spectra and the computer simulation spectra of (a) ZnP-3-Q•-, (b) ZnP-6-Q•-, and (c) ZnP-10-Q•- (1.0 × 10-5 M) in deaerated
CH2Cl2 at 258 K and optimized structures of (a) Ph-3-Q•-, (b) Ph-6-Q•-, and (c) Ph-10-Q•- by Gaussian 98 with UBLYP/6-31G(d) basis set
[the values (G) in parentheses are provided by Gaussian 98 with UBLYP/6-31G(d) basis set].
shifts in 13C NMR (S10, S11). The estimated distance of CdO
(Q1) bond becomes longer in the same order as IR spectra in
KBr and CH2Cl2 solution: Ph-NHCO-Q (1.232 Å), Ph-
CONH-Q (1.228 Å), and Ph-CONMe-Q (1.225 Å) as shown
in Figure 2a-c.
Hydrogen Bond Providing a Structural Scaffold To
Assemble the Donor and Acceptor Moieties in ZnP-n-Q
Dyads. The optimized structures of ZnP-n-Q (n ) 3, 6, 10)
are calculated using Gaussian 98 with BLYP/6-31G(d) basis
set as shown in Figure 2d-h.39 Ph-6-Q and Ph-10-Q afford
two kinds of optimized structures: one is the same type as Ph-
3-Q (type A); the other is the hydrogen-bonded structure of
amide hydrogen with carbonyl oxygen in the Q•- moiety (type
B). The type A of Ph-6-Q and Ph-10-Q is 12.5 and 13.2 kJ
mol-1 more stable than the type B. The O-H distance between
the oxygen atom of Q and the amide proton shown in Figure
2d-h is 2.10 Å in the type A of Ph-n-Q irrespective of the
difference in the number of methylene spacer (n ) 3, 6, 10),
which is the same as that (2.10 Å) in Ph-CONH-Q. The other
O-H distance between two amide groups is 2.13 Å for Ph-
3-Q, 2.09 Å for Ph-6-Q, and 2.16 Å for Ph-10-Q. The
observed wavenumbers for CdO and N-H bonds both in KBr
and in CH2Cl2 for CdO and N-H bonds (S12) are virtually
the same irrespective of the difference in methylene numbers

7720 J. Phys. Chem. B, Vol. 109, No. 16, 2005
Okamoto and Fukuzumi
in accordance with the optimized structures in Figure 2. The
two weak interactions (hydrogen bonds) between CdO bonds
and N-H bonds may be formed.
The ESR spectra of ZnP-3-Q•-, ZnP-6-Q•-, and ZnP-
10-Q•- (1.0 × 10-5 M) are shown in Figure 3 together with the
computer simulation spectra. The g values are determined as
2.0039, 2.0038, and 2.0037, respectively. At first glance, the
hyperfine structure of ZnP-3-Q•- (five main lines) is quite
different from those of ZnP-6-Q•- and ZnP-10-Q•- (eight
main lines). The hyperfine coupling constants (hfc) are deter-
mined by the computer simulation as shown in Figure 3. The
largest hfc value in the Q•- moiety of ZnP-3-Q•- (3.36 G) is
significantly smaller than that of ZnP-6-Q•- (4.70 G) and
ZnP-10-Q•- (4.30 G). This indicates that the hydrogen-bonded
structure of ZnP-3-Q•- is different from that of ZnP-6-Q•-
and ZnP-10-Q•- as supported by the DFT calculations. The
most stable structure of Ph-3-Q•- calculated by a DFT method
with Gaussian 98 (UBLYP/6-31G(d) basis set) contains two
hydrogen bonds between amide protons and carbonyl oxygens,
as shown in Figure 3 where the observed hfc values are well-
reproduced by the calculated values using the optimized
structure. The O-H distances between the oxygen atom of Q•-
and the amide hydrogen in Figure 3 are 1.87 and 1.95 Å in
Ph-3-Q•-, which are much shorter than the calculated distances
in the neutral form: Ph-3-Q (2.13 and 2.10 Å) in Figure 2. In
contrast to the case of the neutral form of Ph-6-Q and Ph-
10-Q (type A in Figure 2), the most stable structure of Ph-6-
Q•- and Ph-10-Q•- is type B in which the amide proton
adjacent to the Ph moiety forms the hydrogen bond with
carbonyl oxygen of the Q•- moiety. The observed hfc values in
Figure 3 can be well-reproduced by those in parentheses in
Figure 3 calculated using the optimized structures of the type
B in Ph-6-Q•- and Ph-10-Q•-. The energies of the type B
structure of Ph-6-Q•- and Ph-10-Q•- are by 9.6 and 5.4 kJ
mol-1 more stable than those of type A, respectively. The O-H
distances in Ph-6-Q•- (1.80 Å) and Ph-10-Q•- (1.82 Å) are
much shorter than those in the neutral state, Ph-6-Q (2.12 Å)
and Ph-10-Q (2.25 Å).
Photoinduced Electron Transfer in ZnP-Q Dyads. The
fluorescence intensities of ZnP-CONH-Q and ZnP-n-Q
(n ) 3, 6, 10) in PhCN were much smaller than that of the
reference compound without Q (ZnP-CONH-ref). The oc-
currence of photoinduced ET was confirmed by observation of
the ESR spectrum of ZnP•+-10-Q•-, which was generated by
photoirradiation of a deaerated PhCN solution of ZnP-10-Q
at 173 K, as shown in Figure 4a. The ESR spectrum of
ZnP•+-10-Q•- (g ) 2.0028 in Figure 4a) consists of the
superposition of the ESR signals of ZnP•+-CONH-Q
(g ) 2.0023 in Figure 4b) and ZnP-CONH-Q•- (g ) 2.0043
in Figure 4c), produced by the chemical oxidation and reduction,
respectively.
From the fluorescence quenching in Figure 5, the rate constant
(kET) of ET from the singlet excited state of the ZnP moiety to
the Q moiety in ZnP-CONH-Q is determined as 1.5 × 1010
s-1 in PhCN by the comparison of the intensity of ZnP-
CONH-ref (τ ) 2.04 ns in MeCN, τ ) 2.04 ns in PhCN,
τ ) 1.88 ns in CH2Cl2, τ ) 2.10 ns in THF, and τ ) 2.15 ns
in toluene determined from fluorescence decay) using eq 1. The
kET values in
Figure 4. ESR spectra of (a) ZnP•+-10-Q•-, (b) ZnP•+-CONH-Q,
and (c) ZnP-CONH-Q•- (1.0 × 10-5 M), produced by the photo-
irradiation in deaerated PhCN at 173 K.
Figure 5. Fluorescence spectra of ZnP-CONH-Q, ZnP-3-Q,
ZnP-6-Q, ZnP-10-Q, and ZnP-CONH-ref (1.0 × 10-5 M) in
deaerated PhCN at 298 K.
larger than those of ZnP-CONMe-Q. Such an enhancement
of the photoinduced electron-transfer results from the larger
driving force (-∆GET ) 0.91 eV) for ZnP-CONH-Q) than
that (-∆GET ) 0.82 eV) for ZnP-CONMe-Q due to the
hydrogen bond (vide supra).
The rate constants (kET) of photoinduced electron transfer
from 1ZnP* to Q in ZnP-n-Q (n ) 3, 6, 10) are also determined
in several solvents by the comparison of the fluorescence decay
of ZnP-NHCO-ref (see Experimental Section) using eq 2.40
-1
kET ) (τZnP-n-Q
)
-1 - (τZnP-NHCO-ref
)
(2)
The representative fluorescence decay curves of ZnP-n-Q
(n ) 3, 6, 10) in toluene are shown in Figure 6 as compared
with the curve of ZnP-NHCO-ref. The kET values of ZnP-
n-Q (n ) 3, 6, 10) in several solvents are summarized in
Table 2.
The plots of logarithm of the ET rate constants (log kET) vs
the methylene spacer numbers in ZnP-linked-Q dyads in several
solvents are shown in Figure 7. The kET values for ZnP-3-Q,
ZnP-6-Q, and ZnP-10-Q in several solvents are approxi-
mately the same irrespective of the difference in the number of
the methylene spacer, whereas the kET values for ZnP-
CONH-Q are much faster than those of ZnP-3-Q, ZnP-6-
-1
kET ) (τ
) ([I0/I] - 1)
(1)
ZnP-CONH-ref
with MeCN, CH2Cl2, THF, and toluene were also determined
as summarized in Table 2 (see Experimental Section). The kET
values of ZnP-CONH-Q in MeCN and PhCN are significantly

Hydrogen Bonds in Zinc Porphyrin-Quinone Dyads
J. Phys. Chem. B, Vol. 109, No. 16, 2005 7721
TABLE 2: Rate Constants of Photoinduced ET (kET) in MeCN, PhCN, CH2Cl2, THF, and Toluene
kET, s-1
solvent
ꢀr
ZnP-CONH-Q
ZnP-CONMe-Q
ZnP-NHCO-Q
ZnP-3-Q
ZnP-6-Q
ZnP-10-Q
MeCN
PhCN
37.5
25.2
4.9 × 1010 a
4.5 × 109 a
4.7 × 108 b
8.4 × 107 b
(1.7 × 108 a
3.7 × 108 b
1.1 × 108 b
(1.8 × 108 a
3.1 × 108 b
(2.4 × 108 a
1.5 × 1010 a
2.0 × 109 a
6.3 × 108 a
1.4 × 108 b
)
)
)
CH2Cl2
THF
toluene
8.93
7.58
2.38
2.9 × 1010 a
1.4 × 1010 a
3.1 × 1010 a
6.6 × 108 b
3.6 × 108 b
6.9 × 108 b
3.3 × 108 b
1.6 × 108 b
2.3 × 108 b
1.3 × 108 b
1.1 × 108 b
1.8 × 108 b
a kET ) τ(ZnP-CONH-ref)-1[(φFL0/φFL) - 1]. kET ) τ(ZnP-3,6,10-Q
)
- τ(ZnP-NHCO-ref) .
b
-1
-1
Figure 6. Fluorescence decay curves of ZnP-3-Q, ZnP-6-Q, ZnP-
10-Q, and ZnP-NHCO-ref (1.0 × 10-5 M) in deaerated toluene at
298 K.
Figure 8. (a) Time-resolved absorption spectra observed at 140 ns
(b) and 6 µs (O) after laser pulse excitation (428 nm) of a deaerated
MeCN solution of ZnP-3-Q (5.0 × 10-6 M) at 298 K. (b) First-order
plots of absorbance at 480 nm after laser excitation (laser power, 3.5,
2.4, and 1.5 mJ, respectively) at 435 nm.
Back-Electron Transfer in ZnP-Q Dyads. The back-
electron transfer (BET) processes in ZnP-Q dyads have also
been investigated using nanosecond laser flash photolysis. The
representative resolved-transient spectra of a deaerated MeCN
solution of ZnP-3-Q excited at the ZnP moiety at 428 nm are
shown in Figure 8a. The formation of the charge-separated state
(ZnP•+-3-Q•-) is clearly observed as indicated by the appear-
ance of the absorption bands at 470 and 630 nm due to ZnP•+,44
and also the band at 450 nm due to Q•-.45 The absorption band
corresponding to the CS state disappears by the BET process.
The absorption decay at 630 nm obeys first-order kinetics as
shown in Figure 8b, where the constant kBET value (1.0 ×
106 s-1) is obtained irrespective of the difference in the laser
power. It should be noted that this is the first example of the
long-lived CS state of porphyrin-quinone linked systems.17-24
In the case of ZnP-CONH-Q, ZnP-CONMe-Q, and ZnP-
NHCO-Q, the CS states were not detected due to the fast BET
process. The kBET values of ZnP-n-Q (n ) 3, 6, 10) in MeCN
and PhCN are summarized in Table 3 together with the quantum
yield of CS (ΦCS) determined by the comparative method using
zinc tetraphenylporphyrin (see Experimental Section). The
Figure 7. Plots of rate constants of photoinduced ET (log kET, solid
line) and BET (log kBET, broken line) vs numbers of methylene chains
in ZnP-CONH-Q, ZnP-3-Q, ZnP-6-Q, and ZnP-10-Q in MeCN,
PhCN, CH2Cl2, THF, and toluene.
Q, and ZnP-10-Q. Such a difference in the kET values is
ascribed to the difference in the edge-to-edge distance (Ree)
between ZnP and Q (see Table 1), since the Ree value of ZnP-
CONH-Q (3.78 Å) is much shorter than those of ZnP-3-Q
(8.26 Å), ZnP-6-Q (8.21 Å), and ZnP-10-Q (8.27 Å), which
are virtually the same (see Table 1).41 Thus, the hydrogen-bond
formation between two amide groups provides control of a
structural scaffold even in polar solvent (MeCN), enabling the
efficient photoinduced ET.42,43

7722 J. Phys. Chem. B, Vol. 109, No. 16, 2005
Okamoto and Fukuzumi
TABLE 3: Rate Constants of BET (kBET) in MeCN and
PhCN, and Quantum Yields of CS (ΦCS) in PhCN
process, leading to attainment of the charge-separated state with
a long lifetime up to a microsecond.
solvent
ZnP-3-Q
ZnP-6-Q
ZnP-10-Q
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research Priority Area (Grant No.
16205020) from the Ministry of Education, Culture Sports,
Science and Technology, Japan.
k
BET, s-1
MeCN
PhCN
PhCN
1.0 × 106
1.6 × 106
0.36a
1.1 × 107
9.8 × 106
0.29a
1.3 × 107
1.0 × 107
0.36a
ΦCS
(0.26)b
(0.28)b
(0.33)b
a Determined by the comparative method. b Determined by the
fluorescence quenching.
Supporting Information Available: Synthetic details and
characterizations of precursors of ZnP-linked-Q dyads
(S1-S7), comparison of calculation methods of the hfc values
of ZnP-CONH-Q•-, ZnP-CONMe-Q•-, ZnP-NHCO-Q•-,
and ZnP-3-Q•- (S8) and that of ZnP-n-Q•- (n ) 6, 10) (S9),
experimental and calculated wavenumbers of N-H, CdO bond
(S10), experimental and calculated chemical shifts of 13C NMR
in ZnP-CONH-Q, ZnP-CONMe-Q, and ZnP-NHCO-Q
(S11), experimental wavenumbers of N-H, CdO bond in
ZnP-n-Q (n ) 3, 6, 10) (S12), and decay profiles of ZnP•+-n-
Q•- (n ) 3, 6, 10) at 480 nm in MeCN (S13) (PDF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
(1 - I/I0) values (0.26-0.33 in parentheses in Table 3) obtained
from fluorescence quenching agree with the ΦCS values
(0.29-0.36) determined from the comparative method.
The plots of logarithm of the BET rate constants (log kBET
)
vs the number of methylene spacer of ZnP-n-Q in several
solvents are also shown in Figure 7. The kBET values of
ZnP•+-n-Q•- (n ) 6, 10) in MeCN and PhCN are much faster
than those in ZnP•+-3-Q•- (see S13). In contrast to the BET
process, the kET values of ZnP-n-Q are virtually the same
irrespective of the number of methylene spacer of ZnP-n-Q
(Figure 7). Such a difference between the ET and BET processes
may result from the difference in the hydrogen-bonded structures
between the neutral sate (ZnP-n-Q) and the CS state (ZnP•+-
n-Q•-) (vide supra). The formation of hydrogen bond in the
type B structure of ZnP•+-n-Q•- (n ) 6, 10 in Figure 3) affords
the closer distance between ZnP and Q (4.26 Å in ZnP•+-6-
Q•- and 4.29 Å in ZnP•+-10-Q•-) as compared with that in
ZnP•+-3-Q•- (8.18 Å), leading to the faster BET.
References and Notes
(1) (a) The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J.
R., Eds.; Academic Press: San Diego, CA, 1993. (b) Anoxygenic Photo-
synthetic Bacteria; Blankenship, R. E., Madigan, M. T., Bauer, C. E., Eds.;
Kluwer: Dordrecht, The Netherlands, 1995.
(2) (a) Ferreira, K. N.; Iverson, T. M.; Maghlaoui, K.; Barber, J.; Iwata,
S. Science 2004, 303, 1831. (b) Remy, A.; Gerwert, K. Nat. Struct. Biol.
2003, 10, 637. (c) Lubitz, W.; Lendzian, F.; Bittl, R. Acc. Chem. Res. 2002,
35, 313.
(3) (a) Breton, J.; Boullais, C.; Burie, J.-R.; Nabedryk, E.; Mioskowski,
C. Biochemistry 1994, 33, 14378. (b) Diner, B. A.; Force, D. A.; Randall,
D. W.; Britt, R. D. Biochemistry 1998, 37, 17931. (c) Ishikita, H.; Knapp,
E.-W. J. Am. Chem. Soc. 2004, 126, 8059.
The kET and kBET values are evaluated using the Marcus
equation for intramolecular ET (eq 3),46 where V is the electronic
coupling matrix element. The λ and V values are
kET ) (4π3/h2λkBT)1/2V2 exp[-(∆GET + λ)2/4λkBT] (3)
(4) Chang, C. J.; Brown, J. D. K.; Chang, M. C. Y.; Baker, E. A.;
Nocera, D. G. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, Germany, 2001; Vol. 3, pp 409-461.
(5) (a) Fredericks, J. R.; Hamilton, A. D. In ComprehensiVe Supra-
molecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D.,
Vo¨gtle, F., Lehn, J.-M., Eds.; Pergamon: Oxford, U.K., 1996; Vol. 9, pp
565-594. (b) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996,
35, 1154.
evaluated from the kET values and -∆GET values of
ZnP-CONH-Q (kET ) 1.5 × 1010 s-1, -∆GET ) 0.91 eV),
and those of ZnP-NHCO-Q (kET ) 6.3 × 108 s-1, -∆GET
)
,
1.11 eV) in PhCN using eq 3 as 0.57 eV and 17 cm-1
respectively. The predicted kBET values are determined as
4.0 × 108 s-1 in ZnP-CONH-Q and 1.0 × 1010 s-1 in
ZnP-NHCO-Q using 0.57 eV and 17 cm-1. This is consistent
with no observation of CS states in the nanosecond laser flash
photolysis measurements. In the same manner, the λ and V
values are determined from the kET and kBET values with -∆GET
and -∆GBET values in PhCN as 0.63 eV and 0.8 cm-1 in
ZnP-3-Q, where the Ree distance (8.26 Å) in the neutral state
is virtually the same as that (8.18 Å) in the radical anion state.
The reorganization energy (λ ) 0.63 eV) in ZnP-3-Q is larger
than that in ZnP-CONH-Q and ZnP-NHCO-Q (λ )
0.57 eV) due to an increase in the solvent reorganization energy,
whereas the V value decreases with an increase in the Ree
distance. Both the ET and BET processes in ZnP-3-Q
(Ree ) 8.28 Å) are much slower than those in ZnP-CONH-Q
and ZnP-NHCO-Q (Ree ) 3.78-3.81 Å) due to the longer
Ree distance. In the case of ZnP-6-Q and ZnP-10-Q, however,
the structure is changed from type A before the ET process to
type B after the ET process. The Ree distances of the type B
structure of the CS states (ZnP•+-6-Q•-, ZnP•+-10-Q•-)
become significantly shorter than the Ree distance in ZnP•+-
3-Q•-, affording the shorter CS lifetimes in ZnP-6-Q and
ZnP-10-Q.
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(38) The hfc values were also calculated using the B3LYP and ADF
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1
(40) The ET processes from ZnP* to Q in ZnP-CONH-Q in several
solvents are too fast to be determined by the fluorescence decay.
(41) The Rcc distances were estimated by the structures, which were
provided by adding the optimized structure of ZnP calculated by the ADF
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(42) The kET value is known to decrease with increasing the distance
0
(Ree), obeying kET ) kET exp(-âRee).43 From the distance dependence of
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solvents using the Ree values of ZnP-CONH-Q and ZnP-n-Q (n ) 3, 6,
10) in Table 1, the â values were determined by the slope as 1.14 Å-1 in
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1.08 Å-1 in toluene. The â values between 1.01 and 1.15 Å-1 agree with
those reported previously for saturated hydrocarbon spacers (0.8-
14,43a-c
1.4 Å-1
)
or protein (1.0-1.4 Å-1).43d,e
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