Self-assembly studies of allosteric photosynthetic antenna model systems

Article abstract of DOI:10.1039/b510968j

In this paper allosteric interactions are employed to enhance electron transfer in a pseudorotaxane system. Binding of an axial ligand (tbpy) to an electron donating mono-cavity appended zinc porphyrin (ZnP) increases the complexation strength to an elect

Full text of DOI:10.1039/b510968j

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www.rsc.org/njc | New Journal of Chemistry
Self-assembly studies of allosteric photosynthetic antenna model systems
a
a
a
b
Paul J. Thomassen, Jantien Foekema, Ribera Jordana i Lluch, Pall Thordarson,
c
a
a
Johannes A. A. W. Elemans, Roeland J. M. Nolte* and Alan E. Rowan*
Received (in Durham, UK) 1st August 2005, Accepted 25th November 2005
First published as an Advance Article on the web 20th December 2005
DOI: 10.1039/b510968j
In this paper allosteric interactions are employed to enhance electron transfer in a pseudo-
rotaxane system. Binding of an axial ligand (tbpy) to an electron donating mono-cavity appended
zinc porphyrin (ZnP) increases the complexation strength to an electron accepting mono-
substituted bipyridine, resulting in a more pronounced electron transfer process after excitation of
the zinc porphyrin. Binding constants are presented for this host and different mono-substituted
bipyridine guests.
Introduction
Recently our group has developed several porphyrin-based
8
host–guest systems that display unique allosteric effects. One
One of the most fundamental steps in photosynthesis is photo-
induced charge separation, which takes place in the photo-
1
synthetic reaction centre. Here, complex arrays of chromo-
of these systems was based on a monocavity-appended zinc
porphyrin host ZnP that has a strong affinity for both violo-
gen guests (V) and axial ligands and exhibits a positive
2
phores trap solar energy and convert it into chemical energy.
8b,9
heterotropic binding behaviour.
The study of electron and energy transfer within synthetic
chromophoric arrays has therefore been of great biological
relevance and hence a large number of covalently-linked
porphyrin-containing donor–acceptor systems have been
synthesised and their photodynamic properties have been
We have demonstrated that these allosteric interactions can
be employed in the self-assembly of host–guest systems to
increase the percentage ratio of the host–guest complex in
solution relative to their separate, uncomplexed components, a
form of allosteric magnification.
3
,4
studied in great detail. In Nature however, the chromo-
phores are all non-covalently attached to a protein scaffold,
which holds them at the correct separation and orientation for
the occurrence of fast, unidirectional and efficient electron
transfer (ET). This aspect of the photosynthetic antenna has
As a continuation of this research, we present here two new
model antenna systems, which are constructed with the help of
allosteric interactions. Fig. 1a shows the molecular compo-
nents used in this study: ZnP and axial ligand 4-tert-butylpyr-
0
idine (tbpy), non-methylated-4,4 -bipyridine appended Au-
5
also been extensively mimicked.
porphyrin (AuP-bipy), and its analogue with four non-methy-
0
In contrast to the above, the use of cooperative and
allosteric interactions for the self-assembly of photo-active
complexes has so far been largely unexplored, although it
has been shown that the exploitation of cooperative coordina-
tion reactions between chromophores has major advantages
over employing hydrogen-bonding or standard non-coopera-
lated-4,4 -bipyridines covalently attached to the Au-porphyrin
(AuP-(bipy) ). Self-assembly of these components in solution
4
results in an equilibrium mixture containing the pseudo-rotax-
anes (AuP-bipy)-ZnP and (AuP-(bipy) )-(ZnP) (Fig. 1b). This
4
4
equilibrium can be driven to the full complex by the addition
of a ligand (tbpy) that can be axially bound to zinc, resulting in
6
tive coordination chemistry.
pseudo-rotaxanes (AuP-bipy)-ZnP-tbpy and (AuP-(bipy)
4
)-
A special case of cooperativity, in which a binding event in a
multivalent host at one site causes a discrete, reversible
alteration in the structure of the host at a remote binding site,
(
ZnP) -(tbpy)
4
4
.
7
Results and discussion
is called allosterism. Allosteric interactions can be both
positive and negative, and can act between a host and identical
guests (homotropic), or between a host and different guests
Model studies on mono-substituted bipyridines
(
heterotropic).
In earlier work it has been demonstrated that ZnP binds
5
viologen derivatives very strongly in its cavity (K > 10
ass
ꢀ
1
M
) and that the bulky ligand tbpy coordinates simulta-
a
neously to the porphyrin zinc ion at the outside of the cavity
1
(Fig. 2).
Department of Organic Chemistry, Institute for Molecules and
Materials, Radboud University Nijmegen, Toernooiveld 1, 6525 ED
Nijmegen, The Netherlands. E-mail: R.Nolte@science.ru.nl;
Fax: (þ31)24-3653393; Tel: (þ31)24-3652143
0
To establish whether mono-substituted bipyridines would
also display strong binding inside ZnP, three different model
compounds of this class were synthesised, M-bipy, P-bipy and
T-bipy (Fig. 3).
b
c
Australian Key Centre for Microscopy and Microanalysis/School of
Chemistry, The University of Sydney, Madsen Building, NSW 2006
Sydney, Australia
Department of Molecular Materials, Institute for Molecules and
Materials, Radboud University Nijmegen, Toernooiveld 1, 6525 ED
Nijmegen, The Netherlands. E-mail: A.Rowan@science.ru.nl;
Fax: (þ31)24-3653393; Tel: (þ31)24-3652323
The simplest of these compounds, M-bipy, is complexed in
ꢀ3
the cavity of ZnP at a concentration of 10
M in CDCl3/
1
CD CN = 1 : 1 (v/v), according to H NMR measurements.
3
1
48 | New J. Chem., 2006, 30, 148–155
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Fig. 3 Mono-substituted bipyridines.
host–guest complex and a rapid exchange between the free
species and the complex. This complexation–decomplexation
process is too rapid to be frozen out using NMR and hence a
broad average signal is observed. The proton signals of M-bipy
were not detectable, seemingly due to coalescence of the
signals of the bound and unbound species.
A fluorescence titration, in which M-bipy was added to ZnP,
revealed a quenching of the fluorescence of the zinc porphyrin
at 605 nm (excitation 426 nm) of 6% after the addition of 1
equivalent and of 97% after the addition of 100 equivalents of
the guest (Fig. 5, Table 2). From UV-Vis titrations, a binding
4
ꢀ1
constant K
a
= 7.5 ꢂ 10 M was calculated. At a ratio of
ZnP/M-bipy = 1 : 1, this value indicates an occupation of the
ꢀ
6
Fig. 1 (a) Molecules used in the construction of antenna systems. (b)
PM3 optimised structures of (AuP-bipy)-ZnP, (AuP-bipy)-ZnP-tbpy
host cavity of 7% at a concentration of 10 M and of 95% at
a ratio of 1 : 100, which corresponds well with the observed
quenching of 6 and 97% at both titration points. The observed
fluorescence behaviour confirms the conclusion by Milgrom
et al. that mono-substituted bipyridines, just like viologens,
are effective quenchers of Zn porphyrin fluorescence due to
fast ET from the porphyrin to the mono-alkylated bipyridi-
nium unit, which is tentatively attributed to a weak ligation to
4 4
and MMFF optimised structures of (AuP-(bipy) )-(ZnP) and
(
AuP-(bipy) )-(ZnP) -(tbpy)
4
4
4
.
Both the crown ether proton signals of ZnP and the proton
signals of the aromatic side-walls are severely broadened and
shifted upfield by 0.3 ppm (Table 1, for the proton numbering
see Fig. 4). The broadening is caused by the asymmetry of the
1
1
the zinc ion.
In agreement with observations by Milgrom et al. no
evidence for axial binding was found. A 1 : 1 mixture of
M-bipy and Zn tetrakis-meso-p-toluyl-porphyrin (ZnTTP)
ꢀ3
3
showed no complexation at 10 M concentration in CDCl /
1
CD CN = 1 : 1 (v/v), according to H NMR. This indicates
3
that binding inside the cavity of ZnP is the predominant
binding mode. A fluorescence titration in which M-bipy was
added to ZnTTP gave 13% quenching of fluorescence after the
addition of 500 eq of M-bipy (Table 2). This may be attributed
to a solvent effect, given the fact that addition of M-bipy
results in an increase in the ionic strength and therefore
activity of the solution, possibly leading to a different associa-
tion constant for the binding of acetonitrile to the zinc ion.
Alternatively and perhaps more plausibly, a binding mode
parallel to the porphyrin plane through p–p interactions, in
contrast to axial binding, could be responsible for the observed
quenching. However, this low value for the observed quench-
ing does highlight the ineffectiveness of mono-substituted
bipyridines to quench the excited state of the zinc porphyrin,
when axial binding is the only binding mode available.
Fig. 2 Schematic representation summarizing the possible complexes
of ZnP with tbpy and dimethyl-viologen, together with the relevant
binding constants.
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a
Table 1 Induced chemical shifts (Dd, ppm) of proton signals upon binding of mono-substituted bipyridine units in the cavity of ZnP
0
0
a
b
a
b
g
1a
6.20
2a
3.52
2b
3.23
3a
4.17
3b
4.07
b
Ref.
8.83
8.27
8.74
7.71
4.56
c
c
c
c
c
M-bipy
P-bipy
T-bipy
—
ꢀ5.07
—
ꢀ3.95
—
ꢀ2.97
—
ꢀ3.83
—
ꢀ1.43
ꢀ0.3
ꢀ0.3
ꢀ0.3
ꢀ0.3
ꢀ0.3
ꢀ0.17
ꢀ0.16
ꢀ0.14
ꢀ0.12
ꢀ0.10
ꢀ0.42
ꢀ0.85
ꢀ0.36
0.14
0.15
0.01
ꢀ0.23
ꢀ0.23
ꢀ0.30
ꢀ0.30
0.09
d
e
d
e
d
e
d
d
e
d
e
d
e
d
e
ꢀ5.14
ꢀ4.00
ꢀ2.98
ꢀ3.84
ꢀ1.44
ꢀ
0.39
ꢀ0.02
AuP-bipy
ꢀ5.37
ꢀ4.14
ꢀ2.97
ꢀ2.94
ꢀ3.76
ꢀ1.42
ꢀ1.41
ꢀ0.06
ꢀ0.06
0.23
f
ꢀ0.32
0.01
—
ꢀ0.03
AuP-(bipy)
4
—
—
—
0.03
ꢀ0.08
0.24
a
ꢀ3
CDCl
3
/CD
3
CN = 1 : 1 (v/v). [ZnP] = [guest] = 10 M. T = 298 K. A shift with a negative value represents an upfield shift and vice
b
versa. Proton signals of ZnP or the respective guests in the uncomplexed form. Broad peaks. Proton signals from the front side of the ZnP
c
d
e
f
2
host. Proton signals from the back side of the ZnP host. The signal can not be distinguished from the H O signal.
To investigate the effect of the presence of an alkyl chain
1
spacer, compound P-bipy was synthesised. H NMR measure-
(AQM) to 84%, which again corresponds well with the
calculated 86% complexation at this concentration.
ꢀ3
ments at a concentration of 10 M in CDCl /CD CN = 1 : 1
3
In a reverse approach, the addition of 500 eq of tbpy to a
1 : 1 mixture of ZnP and P-bipy in CHCl /CH CN = 1 : 1 (v/v),
3
(
v/v) again showed that the guest was bound into the cavity of
3
3
the ZnP host, since large upfield shifts were observed for the
signals of the bipyridine protons (Table 1). Apparently the
alkyl chain spacer slows down the exchange process between
bound and unbound species, allowing for the detection of clear
mono-substituted bipyridine proton signals. From the ob-
served chemical shifts one can conclude that the P-bipy guest
is bound asymmetrically in the ZnP host. The porphyrin is
shifted towards the alkyl chain (the induced chemical shift of
resulted in an AQM of the initial fluorescence by an additional
46%. By using the respective binding constants, an increase
in population of the host–guest complex from 16% to 32%
can be calculated. The addition of 500 eq tbpy to a solution
of exclusively ZnP resulted in a fluorescence quenching of
only 10%.
As a third reference compound, T-bipy was investigated,
which has a 3,5-di-tert-butyl-benzene stopper group which is
bulky enough that ZnP can only bind on the bipyridine unit
0
the a-protons is bigger than that of the a -protons), which is in
agreement with the known affinity of porphyrin clips for alkyl
1
from one side (Fig. 1). H NMR experiments at a concentra-
ꢀ3
1
0b
chains.
tion of 10 M in CDCl /CD CN = 1 : 1 (v/v) showed a
3
3
5
From UV-Vis titrations a binding constant K = 2.2 ꢂ 10
similar binding geometry as the one found for P-bipy (Table
1). Again, the zinc porphyrin is complexed asymmetrically
above the bipyridine unit and shifted towards the alkyl chain.
Furthermore, the crown ether signals of ZnP are split up into
two sets of signals, indicating a ‘front’- and a ‘back’-side. The
front side we have defined as the side of the ZnP host where
the alkyl chain protrudes from the cavity. Also the signal of
the b-pyrrolic protons not above the cavity is split into two
signals for both sides because of the loss of symmetry in the
host upon binding of an unsymmetric guest.
a
ꢀ
1
M
was determined. The slower complexation–decomplexa-
tion process is seemingly the result of this increase in binding
constant when compared to M-bipy. The addition of 10 eq of
P-bipy to ZnP resulted in the quenching of the fluorescence
emission of 66%, which corresponds perfectly with the calcu-
ꢀ6
lated 67% occupation of the host at a concentration of 10
Table 2).
When the UV-Vis titration of ZnP with P-bipy was carried
out in the presence of 500 eq tbpy, the K -value increased to
M
(
a
5
ꢀ1
8b
6
.8 ꢂ 10 M as a result of allosteric effects. In this case, the
UV-Vis titrations in which T-bipy was added to ZnP showed
quenching of the fluorescence of ZnP after addition of 10 eq
P-bipy had risen, due to ‘allosteric quenching magnification’
an increase in binding constant, compared to P-bipy, to K =
a
5
ꢀ1
3.6 ꢂ 10 M . As can be seen from Table 2, the total zinc
porphyrin fluorescence quenching by this guest is also a little
higher than the quenching by P-bipy.
Fig. 5 Fluorescence quenching of ZnP by M-bipy, P-bipy and P-bipy
3 3
in the presence of 500 eq tbpy. CHCl /CH CN = 1 : 1 (v/v). T = 298
K. Excitation wavelength = 426 nm. Emission wavelength = 605 nm.
ꢀ6
Fig. 4 Proton numbering of ZnP and mono-substituted bipyridine
[ZnP] = 10 M.E ZnP þ M-bipy; ’ ZnP þ P-bipy; TT ZnP/tbpy þ
P-bipy; m ZnP þ T-bipy.
molecules.
1
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a
) of Zn porphyrin (P) fluorescence and calculated association constants K
Table 2 Quenching (Q
the absence and presence of substrates R
F
a
and free energies DG to various ligands L in
b
ꢀ1
ꢀ1
c
P
L
R
Q
F
(1 eq) (%)
Q
F
(10 eq) (%)
Total Q
F
(%)
K
ass /M
DG/kJ mol
Q
F
4
ZnP
ZnP
ZnP
ZnP
ZnP
ZnP
ZnTTP
ZnP
ZnP
ZnP
ZnP
ZnP
ZnP
a
M-bipy
P-bipy
P-bipy
T-bipy
tbpy
6
35
66
84
74
97 (100 eq)
7.5 ꢂ 10
2.2 ꢂ 10
6.8 ꢂ 10
3.6 ꢂ 10
ꢀ28
ꢀ30
ꢀ33
ꢀ32
ꢀ15
ꢀ19
ꢀ22
ꢀ31
ꢀ34
5
5
5
19
36
22
tbpy
1.9
2.0
2
10 (500 eq)
46 (500 eq)
13 (500 eq)
4 ꢂ 10
d
3
tbpy
P-bipy
1.9 ꢂ 10
7.6 ꢂ 10
3
M-bipy
AuP-bipy
AuP-bipy
tbpy
AuP-(bipy)
AuP-(bipy)
tbpy
3
75
90
5
17
34
3 ꢂ 10
5
tbpy
AuP-bipy
8 ꢂ 10
d
14 (500 eq)
e
e
e
e
5
4
20
18
88
83
7 ꢂ 10
ꢀ33
ꢀ32
5
4
tbpy
AuP-(bipy)
4 ꢂ 10
0.90
d
4
10
ꢀ6
3 3
Excitation wavelength = 426 nm. Emission wavelength = 560 nm. T = 298 K. CHCl /CH CN = 1 :1 (v/v). [ZnP] = [ZnTTP] = 10
b
M. Association constants obtained from fluorescence data have been compared with the reported Kass-values determined from UV-Vis data.
They were found to be similar and are therefore not mentioned. In the calculation of the association constants it was assumed that the quantum
c
yield of the host and of the host–guest complexes are the same. Allosteric quenching magnification factor: (observed percentage zinc porphyrin
quenching by 1 eq of guest in the absence of tbpy)/(observed percentage zinc porphyrin quenching by 1 eq of guest in the presence of 500 eq
d
tbpy). Observed percentage of the quenching of zinc porphyrin fluorescence relative to the fluorescence in the absence of tbpy. Equivalents of
e
mono-substituted bipyridine guest (corresponds to 4 ꢂ [AuP-(bipy)
4
]).
Gold porphyrin antenna systems
for the back side of ZnP (see Fig. 7). As before, the front side
of the host molecule has been defined as the side from which
the alkyl chain protrudes.
Numerous covalent free base and zinc porphyrin-viologen
arrays have been studied in the literature, and it was found
that attached viologens are efficient quenchers of both the
The addition of AuP-bipy to a solution of ZnP in CHCl
3
/
1
2
CH CN = 1 : 1 (v/v) results in quenching of the zinc
3
excited singlet and triplet state of the central porphyrin.
A
through-solvent ET process from the central porphyrin to
1
2c,d
viologen was cautiously suggested.
a process occurs in our system as well.
We presume that such
The insertion of gold into the central porphyrin enables
circumvention of the problem of overlapping absorption
spectra that is often encountered in donor–acceptor free-base
porphyrin and zinc porphyrin systems, enabling one porphyrin
1
3
type to be selectively excited. The use of gold porphyrins as
electron acceptors for electron-donating photo-excited zinc
1
porphyrins has been well described in the literature. More
4
recently the precise process of electron transfer has been
1
4d
questioned.
We have synthesised the porphyrin-bipyridine arrays ana-
1
2a,b
logous to the procedure described in the literature.
Gold
was inserted into the free base compounds by refluxing them in
1
5
the presence of KAuCl and NaOAc in acetic acid.
4
The UV-Vis absorption spectrum of a 1 : 1 mixture of ZnP
and AuP-bipy in CHCl /CH CN = 1 : 1 (v/v) at a concentra-
3
3
ꢀ6
tion of 10 M is almost a superposition of the UV-Vis spectra
of the respective pure compounds (Fig. 6). However, the Soret
band of ZnP is observed to be red-shifted by 3 nm, because at
this concentration a percentage of the bipyridine units of AuP-
bipy is complexed inside ZnP. The absence of large shifts or
appearance of new peaks proves that there is no ground-state
electronic interaction between the two components.
Fig. 6 UV-Vis absorption and fluorescence emission spectra of ZnP
and AuP-bipy and of a 1 : 1 mixture in the absence of and in the
1
The H NMR spectrum of a 1 : 1 mixture of AuP-bipy and
presence of 500 eq tbpy. CHCl
Excitation (ZnP) = 426 nm; excitation (AuP-bipy) = 411 nm. ZnP
UV-Vis): 426, 545 nm; (fluor) = 605 nm. AuP-bipy (UV-Vis): 411, 521
3 3
/CH CN = 1 : 1 (v/v). T = 298 K.
ZnP in CDCl /CD CN = 1 : 1 (v/v) shows a binding mode
3
3
similar to ZnP and T-bipy. Large upfield shifts for the
bipyridine protons are observed and again a split between
signals from the b-pyrrolic protons not above the cavity and
the crown ether protons into a set for the front side and one
(
ꢀ6
nm; no fluorescence emission detected. [ZnP] = [AuP-bipy] = 10 M.
ZnP (red); ZnP þ AuP-bipy (blue); ZnP þ AuP-bipy þ tbpy (purple);
AuP-bipy (green).
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Fig. 8 Fluorescence quenching for ZnP by AuP-bipy and AuP-(bipy)
in the absence of and in the presence of 500 eq tbpy. CHCl /CH CN =
: 1 (v/v). T = 298 K. Excitation = 426 nm. Emission = 605 nm.
4
3
3
1
ꢀ
6
[
ZnP] = 10 M.
~
ZnP þ AuP-bipy; TT ZnP/tbpy þ AuP-bipy;
; ’ ZnP/tbpy þ AuP-(bipy)
m ZnP þ AuP-(bipy)
4
4
.
1
Fig. 7 H NMR spectra (400.15 MHz, CDCl
98 K). (A) b-pyrrolic region of ZnP. (B) Aromatic region of
AuP-bipy. (C) Crown ether region of ZnP. (D) Partial spectrum of
AuP-bipy)-ZnP.
3 3
/CD CN = 1 : 1 (v/v),
5
ꢀ1
2
ZnP and AuP-(bipy)
, K
a
= 7 ꢂ 10 M , is also a little higher
4
than the binding constant between ZnP and AuP-bipy (K
a
=
5
ꢀ1
(
3
ꢂ 10 M ).
Repetition of this titration in the presence of 500 eq tbpy in
CHCl /CH CN = 1 : 1 (v/v) did not result in an increase in
fluorescence quenching. At a host–guest ratio of 1 : 1 (AuP-
bipy) /ZnP = 1 : 4), a fluorescence quenching of 18% was
porphyrin fluorescence at 605 nm, after excitation at 426 nm.
This quenching increases from 17% at a 1 : 1 host–guest ratio
to 75% at a ratio of 1 : 10 (Fig. 8, Table 2). Repetition of this
titration in the presence of 500 eq of tbpy, resulted in 34% and
3
3
(
4
observed and after the addition of 2.5 eq of AuP-(bipy) the
4
9
0% quenching, respectively, at these ratios (Fig. 8, Table 2),
total quenching of ZnP fluorescence amounted to 83%. This is
in both cases a little lower than observed for the titration
experiment in the absence of tbpy, demonstrating a negative
cooperative effect and an AQM factor of 0.9 (Fig. 8, Table 2).
The binding constant that was calculated from these titration
experiments is also slightly lower than that for the AuP-bipy/
demonstrating an AQM factor of 2.
UV-Vis titration experiments confirmed that these results
were the consequence of an increase in binding constant from
5
ꢀ1
5
ꢀ1
K = 3 ꢂ 10 M to K = 8 ꢂ 10 M in the presence of the
a
a
axial ligand tbpy. In energy terms this corresponds to a DDG of
ꢀ
1
only ꢀ3 kJ mol , which does however demonstrate the first
example of an allosteric effect being employed in the self-
assembly of synthetic photo-active systems.
5
ꢀ1
ZnP/tbpy-system, namely K
a
= 4 ꢂ 10 M
.
The reverse approach of adding tbpy to a 4 : 1 mixture of
ZnP and AuP-(bipy)
Fig. 9). This value is equal to the quenching observed for the
addition of tbpy to ZnP in the absence of any guest molecules
Table 2). We ascribe this observed inability of tbpy to
4
resulted in only 10% extra quenching
The reverse approach, where tbpy was added to a 1 : 1
(
3 3
mixture of ZnP and AuP-bipy in CHCl /CH CN = 1 : 1 (v/v)
resulted in an increase in quenching of 14% (Fig. 9).
Before fluorescence quenching experiments were attempted
on the gold porphyrin with four attached guest sites AuP-
(
allosterically magnify the amount of host–guest complex in
solution to steric hindrance around the central porphyrin,
which increases in the order ((AuP-bipy)-ZnP) o ((AuP-
(
bipy)
4
, it was first tested whether four zinc porphyrin clips
1
could be complexed to this molecule. H NMR experiments of
bipy)-ZnP-tbpy) o ((AuP-(bipy)
ZnP) -(tbpy) ) (Fig. 2).
4
)-(ZnP)
4
) o ((AuP-(bipy)
4
)-
ꢀ
3
a mixture of 4 : 1 ZnP/AuP-(bipy) at a concentration of 10
4
(
4
4
M in CDCl /CD CN = 1 : 1 (v/v) did not show the presence
3
3
of uncomplexed bipyridinium units, indicating a complete
formation of the 4 : 1 host–guest complex. Unfortunately,
the crown ether proton signals of ZnP were significantly
broadened, rendering assignment impossible. Therefore the
proton signals of the bipyridine guest were scarcely visible.
From the peaks that could be interpreted, a similar binding
geometry as for the complexes of ZnP with P-bipy was
deduced (Table 1).
The quenching behaviour of compound AuP-(bipy)4 is
interesting. Addition of 0.25 eq of AuP-(bipy) to ZnP results
4
in a quenching of the fluorescence of the latter porphyrin of
2
0%, similar as in the case of the complex of ZnP with AuP-
Fig. 9 Fluorescence quenching for ZnP by AuP-bipy and AuP-(bipy)
during addition of 500 eq tbpy. CHCl /CH CN = 1 : 1 (v/v). T = 298
K. Excitation = 426 nm. Emission = 605 nm. [ZnP] = [AuP-bipy] =
4
bipy at a host–guest ratio of 1 : 1 (Fig. 8, Table 2). The total
quenching of ZnP fluorescence after the addition of 2.5 eq of
3
3
ꢀ6
ꢀ5
AuP-(bipy)
4
adds up to 88%, a little higher than for the ZnP-
10 M. [AuP-(bipy) ] = 2.5 ꢂ 10 M. ’ ZnP/AuP-bipy þ tbpy;E
4
AuP-bipy complex. The calculated binding constant between
ZnP/AuP-(bipy)
4
þ tbpy.
1
52 | New J. Chem., 2006, 30, 148–155
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1
a Perkin Elmer luminescence Ls50B spectrometer. H NMR
spectra were obtained on a Varian Unity Inova 400. CHCl
was distilled over CaCl and CH CN was distilled over CaH
3
2
2
3
before use. ZnP and M-bipy were synthesised according to
a,16
literature procedures.
9
0
(
5-(4,4 -Bipyridinium)pentoxy)benzene hexafluorophosphate
P-bipy)
(
To dibromopentane (14.5 ml, 106 mmol) and K CO (3 g) in
3
2
DMF (70 ml) was added phenol (1 g, 10.6 mmol), after which
the mixture was refluxed under a N atmosphere for 3 h. After
2
cooling and filtration all liquids were distilled off under
reduced pressure. The remaining solid was dissolved in diethyl
ether (20 ml) and this solution was added drop-wise to
methanol (200 ml). The resulting precipitate was filtered off,
washed with methanol and purified by column chromatogra-
Fig. 10 Schematic representation of possible ET processes in [2]-
pseudo-rotaxane (AuP-bipy)-ZnP.
2 2
phy (silica; n-hexane/CH Cl = 7 : 3 (v/v)) and the product
was dried in vacuo. A portion of the resulting white solid
0
(
2.32 g, 9.55 mmol) was dissolved in DMF (50 ml) and 4,4 -
bipyridine was added (5.9 gr, 38.2 mmol). This mixture was
stirred under a N2 atmosphere at 90 1C for 72 h. After
concentration of the solution under reduced pressure, it was
added to a saturated NH PF (aq) solution (200 ml). The
To investigate the effect of the gold ion in the central
4
porphyrin of AuP-bipy and AuP-(bipy) , we have compared
the reduction potentials of the mono-substituted bipyridinium
unit with the reduction potential of gold(III) porphyrins.
Milgrom et al. determined a reduction potential in DMF for
4
6
precipitate was filtered off, washed with water and purified by
column chromatography (silica; CH Cl /MeOH/MeNO
: 1 : 1) and the product was dried in vacuo. Yield: 70% of
2
2
2
=
1
/0 11
ZnP-(bipy)
4
of ꢀ1.77 V vs. Fc
.
Gold porphyrins usually
1/0
6
have a reduction potential around ꢀ1.2 V vs. Fc , depending
a white solid.
1
1
4
on the solvent and the substituents on the porphyrin. One
would expect that also in this solvent system the reduction
potential of a gold(III) porphyrin is significantly less negative
than that of appended mono-substituted bipyridine. This
would mean that the gold porphyrin is the better candidate
for electron acceptance. However, since electron transfer (ET)
to the mono-substituted bipyridinium guest as well as ET to
the gold porphyrin has to occur ‘through space’, the rate of ET
is proportional to exp(ꢀ2R/L) (R = donor–acceptor separa-
tion; L = Van der Waals radius). The PM3 model of (AuP-
bipy)-ZnP gives an indication of the distance between the zinc
ion and the positively charged nitrogen atom and between the
3 3
H NMR (CDCl /CD CN = 1 : 1 (v/v), 300.13 MHz, 25
3
1
2
C): d = 8.79 (d, 2H, PyH-2,6, J = 6.3 Hz), 8.73 (d, 2H, PyH-
3 3
0
0
,6 , J = 6.9 Hz), 8.26 (d, 2H, PyH-3,5, J = 5.1 Hz), 7.71
0 0
3
d, 2H, PyH-3 ,5 , J = 6.3 Hz), 7.24 (t, 2H, ArH-3,5, J = 6.6
3
(
3
Hz), 6.86 (m, 3H, ArH-2,4,6), 4.56 (t, 2H, N–CH , J = 7.6
2
3
Hz), 3.97 (t, 2H, O–CH , J = 7.9 Hz), 2.07 (m, 2H, N–C–
2
CH ), 1.85 (m, 2H, O–C–CH ), 1.57 (m, 2H, C–CH –C).
2
2
2
1
6
] .
FAB-MS: m/z = 319 [P-bipy ꢀ PF
0
3
,5-Di-tert-butyl-1-(5-(4,4 -bipyridinium)pentoxy)benzene
hexafluorophosphate (T-bipy)
This compound was synthesised analogous to P-bipy, but 3,5-
di-tert-buylphenol (2.2 g, 10.6 mmol) was used instead of
˚
˚
zinc ion and the gold ion, namely 5 A and 20 A, respectively.
In view of these distances, a direct ET from the zinc porphyrin
to the gold porphyrin will be negligible compared to ET with
the mono-substituted bipyridine as electron acceptor. How-
ever, a successive charge shift from the mono-substituted
bipyridine unit to the gold porphyrin should be possible and
is probable (Fig. 10). The positively charged nitrogen atom
phenol.
1
3 3
H NMR (CDCl /CD CN = 1 : 1 (v/v), 300.13 MHz,
3
2
5 1C): d = 8.85 (d, 2H, PyH-2,6, J = 6.3 Hz), 8.73 (d, 2H,
3 3
PyH-2 ,6 , J = 6.9 Hz), 8.26 (d, 2H, PyH-3,5, J = 5.1 Hz),
0
0
0 0
3
7
.71 (d, 2H, PyH-3 ,5 , J = 6.3 Hz), 6.99 (s, 1H, ArH-4), 6.69
3
(
(
(
s, 2H, ArH-2,6), 4.58 (t, 2H, N–CH
2
,
J = 7.6 Hz), 3.98
, J = 7.9 Hz), 2.10 (m, 2H, N–C–CH ), 1.85
m, 2H, O–C–CH ), 1.60 (m, 2H, C–CH –C), 1.26 (s, 18H,
˚
and the central porphyrin gold ion are only 18 A apart, and
3
t, 2H, O–CH
2
2
the studies on zinc tetraviologen complexes which were men-
tioned earlier, have already shown that ET across this distance
is possible. Emission spectra of the [2]-pseudo-rotaxane
2
2
1
CH ). FAB-MS: m/z = 431 [T-bipy ꢀ 2PF ] .
3
6
(
AuP-bipy)-ZnP recorded at 77 K revealed no phosphores-
0
5
-(4-(5-(4,4 -Bipyridinium)-1-pentoxy)phenyl)-10,15,20-tri
cence from the gold porphyrin after excitation of the zinc
porphyrin. Energy transfer to the gold porphyrin can therefore
be ruled out.
(
4-toluyl) gold(III) porphyrin dihexafluorophosphate (AuP-bipy)
The free base compound was synthesised according to a
1
literature procedure.
was dissolved in acetic acid (60 ml) and KAuCl
2a,b
This compound (100 mg, 96 mmol)
4
(363 mg, 0.96
Experimental
mmol) and NaOAc (31 mg, 378 mmol) were added. The
mixture was purged with N2 for 30 min and then refluxed
UV-Vis spectra were recorded on a Varian Cary 50 UV-Vis
spectrophotometer. Fluorescence titrations were performed on
under a N atmosphere and under exclusion of light for 7 days.
2
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After cooling, the solvent was evaporated and the remaining
solid was dissolved in CH Cl (40 ml). This solution was
washed with an aqueous 10% Na CO solution. After eva-
ing in a more pronounced electron transfer process after
excitation of the zinc porphyrin. The allosteric increase in
the association constant between the ZnP host and the mono-
substituted bipyridine guest in the presence of tbpy is evi-
denced by a more efficient quenching of the zinc porphyrin
fluorescence by the guest, relative to the same sample in the
absence of the axial ligand. Following electron transfer to the
bipyridine guest, charge-shift to an appended gold porphyrin
is tentatively suggested. Future studies are aimed at the
detailed investigation of the precise electron transfer processes
and the effect of allosterism upon it.
2
2
2
3
poration of the organic layer, the residue was dissolved in
DMF (5 ml) and this solution was added drop-wise, with
vigorous stirring, to a saturated aqueous NH PF solution.
4
6
The precipitate was filtered off, washed with water and metha-
nol and dried in vacuo. Purification by column chromatogra-
phy (silica, CH
2 2 3 3 2
Cl /CH OH/CH NO = 6 : 1 : 1) yielded the
desired product in 25% yield as an orange solid.
1
H NMR (CD
2 2
Cl , 400.15 MHz, 25 1C): d = 9.34 (bs, 8H,
3
b-pyrroleH), 8.97 (d, 2H, PyH-2,6, J = 6.5 Hz), 8.92 (d, 2H,
0
0
3
PyH-2 ,6 , J = 6.9 Hz), 8.40 (d, 2H, PyH-3,5, J = 6.7 Hz),
3
3
Acknowledgements
8
.10 (d, 2H, ArH-2,6 meta to O(CH ) -bipy, J = 7.7 Hz), 8.08
2 5
3
d, 6H, ArH-2,6, J = 7.5 Hz), 7.92 (d, 2H, PyH-3 ,5 , J =
0 0
3
(
We would like to thank Prof. dr ir R. A. J. Janssen and Dr
S. C. J. Meskers from the Technical University Eindhoven for
their assistance with the fluorescence measurements at 77 K.
The Council for the Chemical Sciences of the Netherlands
Organization for Scientific Research (CW-NWO) is acknowl-
edged for financial support to J.A.A.W.E. (Veni grant) and
A.E.R. (Vidi grant). P.T. thanks the University of Sydney for
a SESQUI Fellowship.
3
.8 Hz), 7.68 (d, 6H, ArH-3,5, J = 8.2 Hz), 7.37 (d, 2H, ArH-
5
3
3
3
,5 ortho to O(CH ) -bipy, J = 8.7 Hz), 4.75 (t, 8H, N–CH ,
2 5 2
3
J = 7.4 Hz), 4.40 (t, 8H, O–CH
), 2.38 (m, 2H, N–C–CH ), 2.20 (m, 2H, O–C–CH
m, 2H, C–CH –C). UV-Vis (CH Cl ): 415, 525. MALDI-
2
, J = 6.9 Hz), 2.73 (s, 9H,
CH
3
2
2
), 1.95
(
2
2
2
1
TOF: m/z = 1238 [AuP-bipy ꢀ PF
6
] .
0
5
,10,15,20-Tetrakis(4-(5-(4,4 -bipyridinium)-1-pentoxy)phenyl)
gold(III) porphyrin penta(hexafluorophosphate) (AuP-(bipy)
4
)
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2a,b
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