Photoinduced Electron Transfer between Mono-6-p-nitrobenzoyl-β-cyclodextrin and Adamantanamine-C n-porphyrins

Article abstract of DOI:10.1021/jp0347419

A series of monotailed porphyrins, zinc 5-{4-[ω-(1-adamantaneamino)alkyloxy]phenyl}-10,15,20-triphenyl porphyrinate (ZnPC_nA, n = 4, 5, 6), were synthesized in which the porphyrin moiety was connected to 1-adamantanamine via a flexible hydrocarbon chain. It was found that photoinduced electron transfer could occur between these porphyrin compounds and mono-6-p-nitrobenzoyl-β-cyclodextrin (NBCD) in aqueous solution. Detailed steady-state and time-resolved fluorescence measurements revealed two pathways of electron transfer, i.e., the electron transfer between the free donor and free acceptor in solution (dynamic quenching), and the electron transfer between the donor and acceptor bound in a supramolecular complex (static quenching). In these two pathways, the static quenching was found to be highly efficient and dominant in the presence of NBCD. The remarkably large electron-transfer rate (k_SET, ca. 1.0 × 10⁹ s^-1) of the static quenching was found to be very close to that of a covalently linked porphyrin-nitrobenzene dyad.

Full text of DOI:10.1021/jp0347419

J. Phys. Chem. B 2003, 107, 14087-14093
14087
Photoinduced Electron Transfer between Mono-6-p-nitrobenzoyl-â-cyclodextrin and
Adamantanamine-Cn-porphyrins
Yong-Hui Wang,† Man-Zhou Zhu,† Xiao-Yuan Ding,† Jian-Ping Ye,‡ Lei Liu,*,†,§ and
Qing-Xiang Guo*,†,
Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, China,
Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Science, Beijing 10080, China,
and National Laboratory of Applied Organic Chemistry, Lanzhou UniVersity, Lanzhou 730000, China
ReceiVed: March 23, 2003; In Final Form: August 21, 2003
A series of monotailed porphyrins, zinc 5-{4-[ω-(1-adamantaneamino)alkyloxy]phenyl}-10,15,20-triphenyl
porphyrinate (ZnPCnA, n ) 4, 5, 6), were synthesized in which the porphyrin moiety was connected to
1-adamantanamine via a flexible hydrocarbon chain. It was found that photoinduced electron transfer could
occur between these porphyrin compounds and mono-6-p-nitrobenzoyl-â-cyclodextrin (NBCD) in aqueous
solution. Detailed steady-state and time-resolved fluorescence measurements revealed two pathways of electron
transfer, i.e., the electron transfer between the free donor and free acceptor in solution (dynamic quenching),
and the electron transfer between the donor and acceptor bound in a supramolecular complex (static quenching).
In these two pathways, the static quenching was found to be highly efficient and dominant in the presence of
NBCD. The remarkably large electron-transfer rate (kSET, ca. 1.0 × 109 s-1) of the static quenching was
found to be very close to that of a covalently linked porphyrin-nitrobenzene dyad.
1. Introduction
Cyclodextrin (CD) is one of the most important water-soluble
host molecules. It is a cyclic oligosaccharide with six (R), seven
Photosynthesis has evolved over time to prompt ultrafast
photoinduced electron transfer from the electronically excited
chlorophylls to quinone receptors.1 Model studies of this process
with synthetically derived photosynthetic mimics not only can
improve our understanding of the detailed mechanism of
biological photosynthesis,2 but also may enable us to construct
artificial systems for the conversion of solar energy into
chemical potential.3
The simplest model for the photoinduced electron transfer is
composed of an electron donor and acceptor covalently linked
to each other.4 Studies on these donor-acceptor dyads have
revealed interesting dependency of the electron-transfer rate on
the donor-acceptor distance, the donor-acceptor orientation,
the nature of the spacer, and the nature of the solvent.4
Nevertheless, these systems cannot fully mimic the biological
electron transfer, because in the biological systems the donor
and acceptor are held together by proteins without any covalent
linkage.
A better model for the photoinduced electron transfer is
composed of an electron donor and acceptor connected via
noncovalent interactions. Although such noncovalent interactions
as hydrogen bonding,5 π-stacking,6 and metal-ligand coordina-
tion7 can be used to construct the supramolecular photoinduced
electron-transfer system, the most ideal approach to mimic the
biological processes is to build the donor-acceptor dyad in
aqueous solution by means of hydrophobic interactions. Thus,
the aqueous photoinduced electron transfer in peptide,8 nucleic
acid,9 micelle,10 and certain water-soluble host-guest systems
have been intensively studied.
(â), or eight (γ) glucose units. The internal wall of CD is
hydrophobic, whereas the two rims of CD are hydrophilic. As
a result, CD can form inclusion complexes with many organic
compounds in aqueous solution.11 This binding property of CD
has been successfully used in the construction of artificial
enzymes,12 drug delivery systems,13 and molecular machines.14
The binding property of CD is also expected to be useful in the
assembly of supramolecular photoinduced electron-transfer
systems.
It should be mentioned that the effects of the addition of
native CDs on the photoinduced electron-transfer reactions in
water have been studied by many groups.15 However, much less
effort has been devoted to the use of CDs in constructing
photoinduced electron-transfer systems.16 Recently, De Cola et
al. synethsized metal-coordinated CDs and studied their pho-
toinduced electron transfer with viologens.17 Park et al. syn-
thesized naphthalene-substituted â-CD and studied its photo-
induced electron transfer with adamantylmethyl viologen.18 We
synthesized a CD electron acceptor, p-nitrobenzoyl-â-cyclo-
dextrin (NBCD), and studied its photoinduced electron transfer
with naphthalene derivatives.19
In agreement with what were found by De Cola and Park in
their systems, we observed very fast and efficient photoinduced
electron transfer in our NBCD-naphthalene system.19 Since no
chemical bond is available between NBCD and naphthalene,
this electron transfer must occur through space but not through
bond. Nevertheless, in our NBCD-naphthalene system the
electron-donating moiety of the electron donor (i.e. the naph-
thalene ring) is directly included in the cavity of the electron
acceptor. It remains interesting to know whether we can use an
electron donor whose electron-donating moiety and binding
moiety are separated from each other, and whether the distance
between the electron-donating moiety and binding moiety affects
the velocity and efficiency of the electron transfer.
* To whom correspondence should be addressed.
† University of Science and Technology of China.
‡ Chinese Academy of Science.
§ Current address: Department of Chemistry, Columbia University, New
York, NY 10027. E-mail: leiliu@chem.columbia.edu.
Lanzhou University. E-mail: qxguo@ustc.edu.cn.
10.1021/jp0347419 CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/22/2003

14088 J. Phys. Chem. B, Vol. 107, No. 50, 2003
Wang et al.
black solid was collected by filtration and washed with methanol
several times. The black solid was then dissolved in 500 mL of
chloroform-methanol (7:3 v/v). Zinc acetate (60 g, 0.328 mol)
in 300 mL of methanol was added. The solution was refluxed
for 3 h. After the solvent was evaporated, the residue was
extracted by chloroform. The organic layer was concentrated
and purified by using flash chromatograph over silica gel column
(eluent: chloroform). The second fraction was collected and
concentrated. This method of purification was repeated once
more with chloroform as eluent to obtain purple solid porphyrin.
1
Yield 2.47 g (3.56%). H NMR (CDCl3) δ 8.92-8.84 (8 H,
m), 8.20 (6 H, m), 7.95 (2 H, d), 7.76-7.50 (9 H, m), 7.10 (2
H, d), 5.03 (1 H, s).
B. Zinc 5-(4-(ω-Bromoalkyloxy)phenyl)-10,15,20-triphen-
ylporphyrinate (ZnPCnBr). A typical procedure for the syntheses
of ZnPCnBr is as follows (e.g. n ) 4): ZnTPPOH (0.5 g 0.72
mmol), 1,4-dibromobutane (2.0 g, 9.26 mmol), and potassium
carbonate (2.0 g, 14.5 mmol) were suspended in 50 mL of dry
N,N-dimethylformamide and stirred for 48 h at room temper-
ature. The solvent was evaporated under reduced pressure. The
residue was dissolved in chloroform and washed in water. The
organic layer was dried over anhydrous sodium sulfate and
evaporated to remove the solvent. The residue was purified with
flash chromatography over silica gel column (eluent: chloro-
form). The first fraction was collected and concentrated. This
method of purification was repeated two times and purple solid
was obtained.
Figure 1. Electron acceptor and electron donors in the supramolecular
systems and reference compounds (ZnTPPMO, ZnTPPNO2).
Therefore, in the present study we synthesized a series of
monotailed zinc tetraphenylporphyrins in which the zinc por-
phyrin moiety was connected to 1-adamantanamine via a flexible
hydrocarbon chain (Figure 1). We chose metalloporphyrins as
the electron donor because of their good biological relevance.
We chose 1-admantanamine as the binding site because of its
strong complexation with â-CD.20 We were interested in
knowing whether photoinduced electron transfer could occur
efficiently between the zinc tetraphenylporphyrins and NBCD.
We were also interested in knowing the effect of the length of
the hydrocarbon linker on the velocity and efficiency of the
electron-transfer reaction.
ZnPC4Br: Yield 0.48 g (80.5%). 1H NMR (CDCl3) δ 8.98-
8.93 (8 H, m), 8.22-8.20 (6 H, m), 8.10 (2 H, d), 7.76-7.70
(9 H, m), 7.23 (2 H, d), 4.25 (2 H, t), 3.60 (2 H, t), 2.24-2.16-
(4 H, m).
ZnPC5Br: Yield 0.36 g (59.4%). 1H NMR (CDCl3) δ 8.95-
8.84 (8 H, m), 8.21 (6 H, m), 8.05 (2 H, d), 7.77-7.50 (9 H,
m), 7.21 (2 H, d), 4.21 (2 H, t), 3.59 (2 H, t), 2.10-2.04 (4 H,
m), 1.84-1.76 (2 H, m).
2. Experimental Section
ZnPC6Br: Yield 0.46 g (74.7%). 1H NMR (CDCl3) δ 8.99-
8.93 (8 H, m), 8.22-8.20 (6 H, m), 8.10 (2 H, d), 7.76-7.71
(9 H, m), 7.24 (2 H, d), 4.24 (2 H, t), 3.49 (2 H, t), 2.03-1.96
(4 H, m), 1.64-1.73 (4 H, m).
2.1. Materials. NBCD was synthesized following the reported
procedure.19 Pyrrole, benzaldehyde, 1,4-dibromobutane, 1,5-
dibromopentane, 1,6-dibromohexane, 4-hydroxybenzaldehyde,
4-nitrobenzaldehyde, 1-adamantanamine, and zinc acetate were
obtained commercially and used without further purification.
Deionized water was used throughout the experiments.
C. Zinc 5-(4-(ω-(1-Adamantaneamino)alkyloxy)phenyl)-
10,15,20-triphenylporphyrinate (ZnPCnA). A typical procedure
for the syntheses of ZnPCnA is as follows (e.g. n ) 4): A 100-
mL sample of an ethanol/chloroform (1:1 v/v) solution of
ZnPC4Br (0.3 g, 0.36 mmol) and 1-adamantanamine (0.1 g, 6.62
mmol) in autoclave was heated at 100 °C for 10 h. After the
solvent was removed, the residue was extracted with 5% Na2-
CO3 and chloroform. The organic layer was purified over a silica
gel column with chloroform/methanol (15:1, v/v). Purple solid
was obtained.
2.2. Instruments. 1H NMR spectra were recorded on a Bruker
DMX-500 NMR spectrometer. Absorption spectra were re-
corded on a Shimadzu UV-2100 spectrometer. Fluorescence
emission spectra were measured with a Hitachi MP850 spec-
trometer. Fluorescence lifetime was determined with a Horiba
NBES-1100 single photon counting instrument. Both the steady-
state and time-resolved fluorescence were measured in degassed
aqueous solution at room temperature. MALDI-TOF mass
spectra were recorded with a BIFLEX III instrument.
1
ZnPC4A: Yield 0.23 g (71.2%). H NMR (CDCl3)δ 8.97-
8.90 (8 H, m), 8.20-8.16 (6 H, m), 8.11 (2 H, d), 7.75-7.65
(9 H, m), 7.20 (2 H, d), 4.32 (2 H, m), 3.20 (2 H, m), 2.54 (2
H, m), 2.25 (9 H, m), 2.12 (2 H, m), 1.81-1.73 (4 H, m), 1.52
(2 H, m), 1.26 (1 H, s). IR (KBr) 3421, 3050, 2902, 2847, 1596,
2.3. Electrochemical Measurements. Cyclic voltammetry
measurements were carried out with a CHI 620A Electrochemi-
cal Analyzer. A platinum microsphere was used as the working
electrode and the counterelectrode was platinum wire; a Fc+/
Fc (Fc ) ferrocene) couple was used as the reference electrode.
CH3CN (spectrophotometric grade) was used as the solvent and
0.1 M Bu4NClO4 was used as the supporting electrolyte.
2.4. Syntheses of Reagents. A. Zinc 5-(4-Hydroxyphenyl)-
10,15,20-triphenylporphyrinate (ZnTPPOH). Freshly distilled
pyrrol (26.8 g, 0.4 mol) was added to a solution of 4-hydroxy-
benzaldehyde (12.2 g, 0.1 mol) and benzaldehyde (31.8 g, 0.3
mol) in 500 mL of propionic acid. The mixture was refluxed
for 4 h. After most of the propionic acid was removed by
distillation under vacuum, 500 mL of methanol was added. The
1507, 1484, 1438, 1338, 1244, 1172, 1067, 991, 794, 701 cm-1
.
Element Anal. Calcd for C58H51N5OZn: C, 77.51; H, 5.68; N,
7.80. Found: C, 77.87; H, 5.74; N, 7.78. MALDI MS (m/z)
898 (exact mass 897.3).
ZnPC5A: Yield 0.22 g (67.1%). 1H NMR (CDCl3) δ 8.96-
8.87 (8 H, m), 8.20-8.14 (6 H, m), 8.08 (2 H, d), 7.74-7.65
(9 H, m), 7.26 (2 H, d), 4.25 (2 H, m), 2.96 (2 H, m), 2.38-
1.93 (15 H, m), 1.85-1.52 (6 H, m), 1.25 (1 H, s). IR (KBr)
3431, 3050, 2911, 2848, 1599, 1506, 1480, 1444, 1335, 1241,
1171, 1067, 996, 797, 749, 707 cm-1. Element Anal. Calcd for

Electron Transfer between Cyclodextrins and Porphyrins
J. Phys. Chem. B, Vol. 107, No. 50, 2003 14089
TABLE 1: Redox Potentialsa (mV) of the Porphyrins in
CH3CN and ∆GPET (mV) between the Porphyrins and
NBCD Calculated with the Rehm-Weller Equation
-•
b
c
compd
P+• P2+ -NO2
λ
em (nm) E00
∆GPET
ZnPC4A
ZnPC5A
ZnPC6A
ZnTPPMO 410 698
NBCD
425 705
431 710
438 723
658
669
659
645
1884
1853
1881
1922
-418
-381
-402
-571
-1041
a The potentials are referenced to Fc+/Fc at a scan rate of 100 mV/s
in 1 × 10-3 M solutions of the compounds in 0.1 M Bu4NClO4-CH3CN
b
at 298 K. E00 (meV) ) 12398/λem (Å). Strictly E00 should be estimated
by the overlap between absorption and emission spectra. The E00 value
calculated by using the previous equation is therefore slightly under-
estimated. c ∆GPET (mV) was in fact the value in CH3CN. The
corresponding ∆GPET value in aqueous solution should be more negative
than ∆GPET in CH3CN due to the better solvation of the charge-separated
species in water.
Figure 3. Fluorescence intensity of ZnPC4A in aqueous solution (4.0
µM) with aqueous solutions of NBCD (bottom), â-CD (upper), and
NBCD + â-CD (middle). n ) [host]/[guest], host ) NBCD, â-CD, or
NBCD + â-CD, guest ) ZnPC4A.
Figure 2. Cyclic voltammogram of ZnPC4A in acetonitrile.
C59H53N5OZn: C, 77.63; H, 5.81; N, 7.68. Found: C, 77.91;
H, 5.77; N, 7.54. MALDI MS (m/z) 912 (exact mass 911.4).
ZnPC6A: Yield 0.25 g (75.1%). 1H NMR (CDCl3) δ 8.96-
8.87 (8 H, m), 8.20-8.16 (6 H, m), 8.10 (2 H, d), 7.73-7.67
(9 H, m), 7.24 (2 H, d), 4.23 (2 H, m), 2.95 (2 H, m), 2.30-
1.93 (15 H, m), 1.74-1.56 (8 H, m), 1.25 (1 H, s). IR (KBr)
3419, 3050, 2902, 2847, 1596, 1507, 1483, 1483, 1439, 1337,
1243, 1172, 1065, 991, 794, 751, 699 cm-1. Element Anal.
Calcd for C60H55N5OZn: C, 77.75; H, 5.94; N, 7.56. Found:
C, 77.39; H, 6.05; N, 7.49. MALDI MS (m/z) 926 (exact mass
925.4).
Figure 4. Two pathways of electron-transfer reactions between the
excited ZnPCnA and NBCD.
of the emission (λem). With use of the redox potentials and
excitation energy, the ∆GPET values are calculated to be negative
for all the porphyrin compounds. Therefore, photoinduced
electron transfer may take place between ZnPCnA (or ZnT-
PPMO) and NBCD.
3.2. Steady-State Fluorescence. The interaction between
NBCD and ZnPCnA in aqueous solution is studied with use of
the steady-state fluorescence. When an aqueous solution of
ZnPCnA is titrated with NBCD, the fluorescence intensity from
the porphyrin moiety decreases sharply until it is completely
quenched (Figure 3). In comparison, when ZnPCnA is titrated
with â-CD, the fluorescence intensity from the porphyrin moiety
remains constant.
Since the energy of the excited singlet porphyrin derivatives
is much lower than that of NBCD, energy transfer cannot take
place from an excited porphyrin species to NBCD. Therefore,
the only plausible mechanism for the fluorescence quenching
is that photoinduced electron transfer occurs between ZnPCnA
and NBCD.
Two pathways of electron transfer can take place between
the excited ZnPCnA and NBCD. The first is dynamic quenching,
which corresponds to the bimolecular electron transfer between
NBCD and free excited ZnPCnA in solution. The second is static
quenching, which refers to the intrasupramolecular electron
transfer between NBCD and ZnPCnA included in the NBCD
cavity23 (Figure 4).
3. Results and Discussion
3.1. Electrochemistry. Using the Rehm-Weller equation (eq
1),21 we can calculate the free energy change of a photoinduced
electron-transfer reaction.
∆GPET ) e[ED /D - EA/A-•] - E
-
+•
00
e2
e2
1
+
1
r-
1
1
-
+
-
(1)
(
)(
)
4πꢀsꢀ0Rcc 8πꢀ0 r
ꢀref ꢀs
+•
-•
(ED /D and EA/A are the redox potentials of the electron donors
22
and acceptor, respectively. E00 is the energy of the excited
state from which electron transfer occurs. Rcc is the center-to-
center distance of the positive and negative charges in the
charge-separated state. r+ and r- are the radii of the positive
and negative ions. ꢀs is the relative permittivity of the solvent.
ꢀ0 is the vacuum permittivity.) If ∆GPET < 0, electron transfer
can occur between the photoexcited electron donor and ground-
state electron acceptor.
The redox potentials of ZnPCnA and NBCD are measured
by using cyclovoltammetry (Table 1). A typical cyclic volta-
mmogram of ZnPC4A is shown in Figure 2. The excitation
energy of ZnPCnA (E00) is estimated by using the wavelength

14090 J. Phys. Chem. B, Vol. 107, No. 50, 2003
Wang et al.
Figure 6. The fluorescence quenching of ZnPC4A by NBCD and other
electron acceptors in aqueous solution ([ZnPC4A] ) 4.0 µM).
Figure 5. Plot of I0/I against [NBCD] for the fluorescence quenching
of ZnPC4A (A, O) and ZnTPPMO (B, b) in water at room temperature
(I0 is the fluorescence intensity in the absence of NBCD; I is the
fluorescence intensity in the presence of NBCD).
intensity is high because of the insignificant dynamic quenching.
The other is the inclusion complex between ZnPC4A and NBCD,
whose fluorescence intensity is very low because of the highly
efficient static quenching. Using this two-state model11 and the
I0/I-[NBCD] plots, we can easily calculate the binding constants
between ZnPCnA and NBCD, which are 2.6 × 107, 1.2 × 107,
and 1.3 × 107 M-1 for n ) 4, 5, and 6, respectively.
3.3. Stern-Volmer Equation. We plot in Figure 5 I0/I
against the concentration of NBCD observed in the titration
experiment. Herein, I0 is the fluorescence intensity of the
porphyrin compound (4.0 µM) in the absence of NBCD. I is
the fluorescence intensity in the presence of the quencher,
NBCD.
According to the Stern-Volmer equation, when there is only
dynamic quenching in a fluorescence system we should have
the following equation.24
It is worthy to note that the binding constants between
ZnPCnA and NBCD are much larger than the binding constants
(∼105 M-1) determined previously for the complexes between
other adamantine derivatives and â-CD.11 We assume that the
charge-transfer interaction between the electron donor and
acceptor, in addition to the hydrophobic interactions, may
contribute to the extraordinary stability of the ZnPCnA-NBCD
complexes.
I
0 ) 1 + τ0kq[Q]
(2)
I
3.5. Control Experiments. To confirm the intrasupra-
molecular quenching mechanism, some control experiments
have been conducted. In the first control experiment, fluores-
cence titration is performed with ethyl p-nitrobenzoate (ETNB),
cyclohexyl p-nitrobenzoate (CHNB), bornyl p-nitrobenzoate
(BONB), or cholesteryl p-nitrobenzoate (CSNB) as the quench-
er. It is found that when 1.0 equiv of a p-nitrobenzoyl ester is
added to ZnPC4A, the fluorescence intensity of ZnPC4A
decreases by 0.38% for ETNB, 0.29% for CHNB, 0.55% for
BONB, and 0.36% for CSNB. In comparison, when 1.0 equiv
of NBCD is added to ZnPC4A, the fluorescence intensity of
porphyrin decreases by 95.5% (Figure 6). These observations
indicate that without a binding site, only the dynamic quenching
occurs between ZnPC4A and a p-nitrobenzoyl ester.
Herein, kq is the bimolecular quenching constant. τ0 is the
fluorophore lifetime in the absence of quencher. A good example
for eq 2 is the quenching of ZnTPPMO fluorescence by NBCD,
for which a straight line is obtained between I0/I and [NBCD]25
(Figure 5B). It should be noted that Figure 5B also shows that
the dynamic quenching of the porphyrin fluorescence by NBCD
is not significant.
However, it is clear from Figure 5A that the plot of I0/I against
[NBCD] does not fit eq 2 for ZnPC4A. In this system, I0/I
increases sharply when a tiny amount of NBCD is added. Then
the increase of I0/I slows down slightly as [NBCD] increases.
Once [NBCD] is larger than 15 µM, I0/I reaches its maximum
and remains constant despite the further increase of [NBCD].
This I0/I-[NBCD] curve clearly indicates that in addition to
the dynamic quenching, the static quenching (i.e. the intra-
supramolcular photoinduced electron transfer) should also exist
in the ZnPC4A-NBCD system.
It should be mentioned that the fluorescence property of
ZnPC4A in the presence of NBCD (>15 µM) is similar to that
of the reference compound ZnTPPNO2. In ZnTPPNO2, a donor
(porphyrin) moiety is connected covalently to an acceptor (p-
nitrophenyl) moiety. As a result, the fluorescence of the
porphyrin moiety of ZnTPPNO2 is almost fully quenched via
an intramolecular electron transfer. The fluorescence of ZnT-
PPNO2 is also independent of the addition of an external
quencher (e.g. NBCD).
3.4. Binding Constants. The remarkable quenching of the
porphyrin fluorescence by NBCD in the ZnPCnA system is
mainly caused by a highly efficient intrasupramolecular static
quenching. There are two distinct species in the ZnPCnA-
NBCD solution. One is free ZnPCnA, whose fluorescence
3.6. Competition Experiments. â-CD alone does not exert
any quenching effect on ZnPC4A (Figure 3). However, in a
competition experiment with an equal amount of â-CD and
NBCD (i.e. [â-CD]:[NBCD] ) 1:1), the IF value cannot be
quenched to the same extent as that observed in the titration
with NBCD alone (Figure 3). In fact, the final IF value is 28.4
for the â-CD-NBCD titration experiment, whereas the final IF
value for the NBCD titration experiment is 12.8. Since the
original IF value of the pure ZnPC4A solution is 208.9, we can
calculate that in the ZnPC4A-â-CD-NBCD system ([â-CD]
) [NBCD] . [ZnPC4A]), 8.0% of total ZnPC4A should stay
in â-CD whereas 92.0% of total ZnPC4A should stay in NBCD.
This means that the binding constant of the ZnPC4A-NBCD
complex should be about 11 times larger than that of ZnPC4A-
â-CD. As the binding constant of ZnPC4A-NBCD is 2.6 ×
107 M-1, the binding constant of ZnPC4A-â-CD should be 2.2
× 106 M-1
.

Electron Transfer between Cyclodextrins and Porphyrins
J. Phys. Chem. B, Vol. 107, No. 50, 2003 14091
TABLE 2: Intrasupramolecular Electron-Transfer Rates
q
(kSET) and Fluorescence Quenching Quantum Yield (ΦF ) of
the ZnPCnA-NBCD Systemsa in Aqueous Solution
compd
τS/ns
τL/ns
kSET/109 s-1 ΦFq (%)d
ZnPC4A
ZnPC5A
ZnPC6A
0.71 ( 0.05 1.85 ( 0.07 0.87 ( 0.12
0.52 ( 0.04 1.81 ( 0.06 1.37 ( 0.16
0.67 ( 0.06 1.73 ( 0.07 0.91 ( 0.14
61.7
71.3
61.2
1.14 ( 0.11c
b
ZnTPPNO2 0.69 ( 0.05
ZnTPPb
3.21 ( 0.09
a [NBCD] ) [porphyrin] ) 4.0 µM. b ZnTPP ) zinc 5,10,15,20-
tetraphenyl porphyrinate. τL for ZnTPP was measured in the absence
of NBCD. c Calculated by using the following: kSET ) 1/τS(ZnTPPNO
)
2
- 1/τL(ZnTPP)
is small, we let τ0 equal τL.
.
d Because τL is not sensitive to [NBCD] when [NBCD]
Figure 7. Fluorescence intensity changes of ZnPC4A with addition of
methanol in aqueous solution in the presence of NBCD ([ZnPC4A] )
[NBCD] ) 4.0 µM).
The binding constant for ZnPC4A-NBCD is larger than that
for ZnPC4A-â-CD, possibly because of the charge-transfer
interaction between the electron donor and acceptor. The binding
constant for ZnPC4A-â-CD is about four times larger than that
for the complex between â-CD and 1-adamantanecarboxylic acid
(4.8 × 105 M-1),26 possibly because the hydrocarbon linker in
ZnPC4A also contributes to the binding. In any case, the â-CD-
NBCD titration experiment indicates that â-CD can inhibit the
quenching of NBCD to some extent.
3.7. Methanol Co-solvation. When methanol is added to the
aqueous ZnPC4A-NBCD solution, the fluorescence intensity
of the system increases sharply, until the concentration (v/v) of
methanol reaches 20% (Figure 7). After the concentration of
methanol reaches 20%, addition of more methanol into the
system does not significantly change the fluorescence intensity.
Clearly, the role of methanol here is to destroy the hydrophobic
interaction between NBCD and ZnPC4A. As less ZnPC4A is
included in NBCD, less static quenching can take place. When
the concentration of methanol is higher than 20%, no complex
between NBCD and ZnPC4A can be formed in the solution and
therefore the fluorescence quenching under this condition should
be completely bimolecular in nature.
Figure 8. The change of the magnitude of the short-lived (AS) and
long-lived (AL) component of ZnPC4A with increasing concentration
of NBCD ([ZnPC4A] ) 4.0 µM).
of the ZnPCnA-NBCD systems (τS ) 0.71, 0.52, 0.67 ns for n
) 4, 5, 6). Therefore, the fluorescence quenching rate of a
ZnPCnA-NBCD complex is close to that of ZnTPPNO2. On
the other hand, the lifetime of ZnTPP fluorescence by itself is
3.21 ns. This value is larger than τL of ZnPCnA (i.e. the lifetime
of ZnPCnA by itself) possibly due to the structure difference.
It is also found that the magnitude of the short-lived
component (AS) increases sharply with increasing concentration
of NBCD, whereas the magnitude of the long-lived component
(AL) decreases sharply at the same time (see Figure 8). This
behavior is consistent with the two-state model described in
the above discussion.
We are interested in knowing if co-solvation can affect the
time-resolved fluorescence. Therefore, we add methanol to an
aqueous solution of 1:1 ZnPC4A-NBCD. Interestingly, when
the concentration of methanol increases from 0 to 20% (v/v),
the fractional contribution of the short-lived component de-
creases sharply from 90.7% to 0. For the 20% methanol solution,
the decay profile is monoexponential with a lifetime about 1.59
ns. When the concentration of methanol increases from 20% to
100%, the decay profile is always monoexponential. This
observation is consistent with the fact that in a 20-100%
methanol solution the fluorescence decay of the ZnPC4A-
ETNB system is monoexponential.
3.8. Time-Resolved Fluorescence. The photoinduced elec-
tron transfer between NBCD and ZnPCnA is also studied by
using the time-resolved fluorescence. From these experiments,
it is found that the decay of the fluorescence intensity of ZnPCnA
in water is monoexponential in the absence of NBCD. In
comparison, the decay of the fluorescence intensity of ZnPCnA
in the presence of NBCD obeys a double exponential function.
F(t) ) AL exp(-t/τL) + AS exp(-t/τS)
(3)
Equation 3 means that there are two components in the decay,
i.e., a long-lived one (L) and a short-lived one (S). The long-
lived component can be attributed to the fluorescence decay of
uncomplexed ZnPCnA, whereas the short-lived component
should come from the fluorescence decay of ZnPCnA complexed
with NBCD.
3.9. Electron-Transfer Rates. We can calculate the rate
constant of the photoinduced electron transfer using the fol-
lowing equation.
It is found that the lifetime (τL or τS) of the long-lived
component or short-lived component is fairly insensitive to the
concentration of NBCD in the range 0 e [NBCD] e [ZnPCnA].
The insensitivity of τS indicates that only one type of ZnPCnA-
NBCD complex exists in the solution. The insensitivity of τL
indicates that the bimolecular dynamic quenching between
NBCD and free ZnPCnA molecules is not significant in the
presence of a low concentration of NBCD (e4.0 µM).27
Interestingly, the lifetime of ZnTPPNO2 fluorescence is
measured to be 0.69 ns (see Table 2). This value is close to τS
kSET ) 1/τS - 1/τL
(4)
It is found that kSET values for the ZnPCnA-NBCD systems
are 0.87 × 109, 1.37 × 109, and 0.91 × 109 s-1 for n ) 4, 5,
and 6. These rates are close to the intramolecular electron-
transfer rate of ZnTPPNO2 (1.14 × 109 s-1). Therefore, the
flexible carbon chain and the cyclodextrin-adamantane inter-
action do not significantly slow the electron transfer between
the porphyrin moiety and nitrobenzene moiety.

14092 J. Phys. Chem. B, Vol. 107, No. 50, 2003
Wang et al.
(c) Mikkelsen, K. V.; Ratner, M. A. Chem. ReV. 1987, 87, 113. (d) Closs,
G. L.; Miller, J. R. Science 1988, 240, 440. (e) Marcus, R. A. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1111.
It should be noted that the electron transfer between ZnPCnA
and NBCD must occur though space, because there is no bond
between ZnPCnA and NBCD. Compared to the electron transfer
in the systems held together by hydrogen bonds or metal
coordination, the present electron transfer is more challenging
because NBCD and ZnPCnA are held together by the hydro-
phobic interaction (a combination of weak van der Waals
interactions and solvent entropy effect).
The dependence of the electron transfer rate on the linker
length is intriguing. Although currently we cannot provide a
good explanation for the slightly higher rate observed for
ZnPC5A compared to those for ZnPC4A and ZnPC6A, we should
mention that in aqueous solution the hydrocarbon linker between
adamantane and porphyrin may be highly folded. Therefore, it
remains to clarify the actual distance between the p-nitrophenyl
moiety and porphyrin moiety in a ZnPCnA-NBCD complex.
In addition, the different conformations adopted by the Zn-
PCnA-NBCD complexes may also affect their individual
electron-transfer rates in a nonmonotonic way.28
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3.10. Quantum Yields. We can calculate the fluorescence
quenching quantum yield of the electron transfer using the
following equation.
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Am. Chem. Soc. 1998, 120, 10902. (b) Piotrowiak, P. Chem. Soc. ReV. 1999,
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Jonkheijm, P.; Chen, Z.; Wuerthner, F.; Meijer, E. W. J. Am. Chem. Soc.
2002, 124, 10252.
ΦFq ) kSET/[kSET + (1/τ0)]
(5)
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In eq 5 τ0 stands for the fluorescence lifetime of ZnPCnA in
the absence of NBCD in aqueous solution. According to Table
2, the quantum yields of the ZnPCnA-NBCD systems are
61.7%, 71.3%, and 61.2% for n ) 4, 5, and 6, respectively.
These values are smaller than the quantum yield found in the
NBCD-2-N,N-dimethylaminonaphthalene system (98.9%),
mainly because the fluorescence lifetime of the porphyrin
compound (∼1.8 ns) is much smaller than that of 2-N,N-
dimethylaminonaphthalene (24.9 ns).
4. Conclusion
A series of monotailed porphyrins were designed and
synthesized, in which the porphyrin moiety was connected to
1-adamantanamine via a flexible hydrocarbon chain. It was
found that photoinduced electron transfer could occur between
these porphyrin compounds and mono-6-p-nitrobenzoyl-â-
cyclodextrin in aqueous solution. Detailed steady-state and time-
resolved fluorescence experiments revealed two pathways of
electron transfer, i.e., the electron transfer between the free donor
and free acceptor in solution (dynamic quenching), and the
electron transfer between the donor and acceptor bound in a
supramolecular complex (static quenching). In these two
pathways, the static quenching was found to be highly efficient
and dominant in the presence of NBCD. The remarkably large
electron-transfer rate (kSET, ca. 1.0 × 109 s-1) of the static
quenching was also found to be very close to that of a covalently
linked porphyrin-nitrobenzene dyad.
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Acknowledgment. The work was supported by the NSFC
(No.20272057) and Ministry of Education of China.
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Electron Transfer between Cyclodextrins and Porphyrins
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concentration of the inclusion complex should be around 10-3-10-2 µM.
This means that less than 1% of ZnTPPMO is included in the NBCD cavity.
As a result, the quenching of ZnTPPMO fluorescence by NBCD should
predominantly be bimolecular dynamic quenching.
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ZnPC4A and NBCD.
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(23) On the basis of a theoretical study, we concluded that NBCD should
adopt a rim-covering conformation in aqueous solution. (Feng, Y.; Zhang,
H.-M.; Liu, L.; Wang, Y.-H.; Liang, Z.-X.; Guo, Q.-X. Chin. Chem. Lett.
2001, 12, 637.) The p-nitrobenzoyl moiety cannot enter the CD cavity
because of the geometry of the ester bond. Therefore, the CD cavity of
NBCD is open for the guest compound in the solution.
(24) For more details about the mechanism and application of the Stern-
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donor (D) and acceptor (A) can be expressed as folows: kDA )
2
(2π/p)|HDA| (4πλRT)-1/2 exp[-(∆G° + λ)2/4λRT]. In the ZnPCnA-NBCD
systems, HAD (electronic coupling matrix element) and ∆G° (reaction free
energy) should be roughly the same for different n. The reorganization
energy (λ) can be divided into two parts, λ ) λs + λi, including the solvent
(λs) and internal (λi) terms. The internal term should also be roughly the
same for different n. Therefore, the difference in the observed electron
transfer rate should rely on the solvent reorganization energy, which is
dependent on the donor-acceptor distance and donor-acceptor orientation.