Synthesis, electrochemistry, and spectroelectrochemistry of a metalloporphyrin-viologen donor-acceptor diad

Article abstract of DOI:10.1039/b003738i

A new donor-acceptor (D-A) molecule, zinc 5-{N-[4-(l '-benzyl-4, 4'-bipyridinium-l-ylmethyl)benzyl]-4-pyridinio}-10, 15, 20-triphenylporphyrinate tris(hexafluorophosphate) has been synthesized. The diad and its precursors have fully been characterised by 'H and C NMR spectroscopy, mass spectrometry, UV/Visible spectroscopy and cyclic voltammetry. /;; situ UV/Visible and EPR measurements show that the site of the first and second electrochemical reductions is the benzyl viologen component of the molecule, whilst the third electron reduction process is associated with the porphyrin moiety. The first reduction process gave rise to an EPR signal due to the benzyl viologen radical, whilst the second caused the molecule to become diamagnetic. The third of these processes gave rise to a new EPR signal, which was found to correspond to the metalloporphyrin radical. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b003738i

View Article Online / Journal Homepage / Table of Contents for this issue
Synthesis, electrochemistry, and spectroelectrochemistry of a
metalloporphyrin–viologen donor–acceptor diad
Matthew T. Barton,a Natalie M. Rowley,*a Peter R. Ashton,a Christopher J. Jones,a
Neil Spencer,a Malcolm S. Tolleya and Lesley J. Yellowleesb
a School of Chemistry, The University of Birmingham, Edgbaston, Birmingham,
UK B15 2TT
b Department of Chemistry, University of Edinburgh, Edinburgh, UK EH9 3JJ
Received 10th May 2000, Accepted 10th July 2000
Published on the Web 24th August 2000
A new donor–acceptor (D–A) molecule, zinc 5-{N-[4-(1Ј-benzyl-4,4Ј-bipyridinium-1-ylmethyl)benzyl]-4-pyridinio}-
10,15,20-triphenylporphyrinate tris(hexafluorophosphate) has been synthesized. The diad and its precursors have
fully been characterised by 1H and 13C NMR spectroscopy, mass spectrometry, UV/Visible spectroscopy and cyclic
voltammetry. In situ UV/Visible and EPR measurements show that the site of the first and second electrochemical
reductions is the benzyl viologen component of the molecule, whilst the third electron reduction process is associated
with the porphyrin moiety. The first reduction process gave rise to an EPR signal due to the benzyl viologen radical,
whilst the second caused the molecule to become diamagnetic. The third of these processes gave rise to a new EPR
signal, which was found to correspond to the metalloporphyrin radical.
There has been much interest in artificial models of photo-
induced electron transfer, and covalently linked porphyrin–
viologen molecules have featured heavily in this work.1,2
Systems have been extensively studied photochemically, elec-
trochemically and spectroelectrochemically, but we are not
aware of detailed in situ EPR studies, which is the primary
focus of this paper. Wrighton and co-workers2 have previously
demonstrated that pulsed laser excitation (354.7 nm, 10 ns
pulse) of a pyridyltritolylporphyrin chromophore covalently
linked to a dibenzyl viologen, Bz2V2ϩ, electron acceptor
(porphyrin–viologen, P–V2ϩ) in acetonitrile led to intra-
molecular electron transfer quenching of the porphyrin singlet
would behave in a similar manner to those of Yonemoto et al.3
In the first step towards our goal we have recently reported
the synthesis, electrochemistry and spectroelectrochemistry of
a new donor–acceptor (D–A) molecule, 5-{N-[4-(1Ј-benzyl-
4,4Ј-bipyridinium-1-ylmethyl)benzyl]-4-pyridinio}-10,15,20-tri-
phenylporphyrin tris(hexafluorophosphate).7
In this paper we report the synthesis, characterisation, elec-
trochemistry and spectroelectrochemistry of a new, but related
metalloporphyrin-based donor–acceptor compound and its
precursors. The target molecule comprises a metalloporphyrin,
covalently attached to a dibenzyl viologen acceptor unit, the
acceptor end of which is sufficiently small to be occluded by
large pore zeolites, such as zeolite Y. Although the diad
described here is closely related to that studied previously by
Wrighton and co-workers,2 it is a new compound and has been
characterised by more detailed spectroelectrochemical studies
and low temperature in situ EPR studies.
excited state within the laser pulsewidth to reduce the linked
ϩ
Bz2V2ϩ to Bz2Vϩ . Transient Bz2V was detected directly by
ؒ
ؒ
resonance Raman spectroscopy. Confirmation of the assign-
ment of the transient Bz2Vϩ came from comparison of the
ؒ
spectra with that of an authentic sample of Bz2Vϩ , and of
ؒ
electrochemically reduced P–V2ϩ, which was spectroscopically
confirmed to form P–Vϩ . Fluorescence lifetime determinations
ؒ
Results and discussion
Synthesis
for P–V2ϩ and P gave a rate constant for intramolecular electron
transfer, ket = 8 × 107 sϪ1, consistent with the ability to observe
electron transfer within the laser pulsewidth. More recently,
Yonemoto et al.3 have shown that donor–acceptor complexes
of the type [(2,2Ј-bipyridine)2Ru(4-CH3-2,2Ј-bipyridine-4Ј)–
(CH2)n–(4,4Ј-bipyridinium-CH3)]4ϩ (n = 2–5, 7, 8) can be
exchanged onto the surface of large-pore zeolites (Y, L and
mordenite). From solid state CP-MAS spectra of 13C-labelled
compounds, it was established that these diads occupy surface
sites, in which the acceptor end is occluded by the zeolite chan-
nels, while the size-excluded donor end is exposed. It was also
found that the back electron transfer reaction is approximately
105 times slower for diads on the zeolite surface than in solution.
We have previously synthesized a series of peripherally metal-
lated porphyrins and reported their photochemical proper-
ties.4–6 Following on from our earlier work, we wished to
develop new systems which give rise to longer lifetimes of the
photoinduced charge-separated states, with a view to using
the phototransferred electron to carry out a further chemical
reduction. In order to achieve this, we constructed diad systems
capable of occlusion by zeolite Y, in the hope that our systems
Porphyrin 1 was prepared by an Adler-type condensation of
pyrrole, benzaldehyde and pyridine-4-carbaldehyde.7,8 Com-
pound 2 was prepared in high yield by treatment of 1 in chloro-
form with zinc acetate in methanol.9 Compound 3 was prepared
by quaternisation of the pyridyl moiety of 2 with an excess of
α,αЈ-dibromo-p-xylene. The synthesis of compound 4 has pre-
viously been described.7 Further reaction of 3 with 4, followed
by counter-ion exchange, afforded the desired diad 5. 1H NMR
spectroscopy, liquid secondary ion mass spectrometry (LSIMS)
and elemental analyses confirm the identity of all of the
reported compounds.
1H NMR Spectra
1
In general, the H NMR spectra of compounds 1–5 were well
resolved and could easily be assigned. Large shifts were seen
due to the presence of quaternised nitrogen atoms and the ring-
current effect of the porphyrin macrocycle. These generally
shifted signals to very high frequency, except for internal
3170
J. Chem. Soc., Dalton Trans., 2000, 3170–3175
This journal is © The Royal Society of Chemistry 2000
DOI: 10.1039/b003738i

View Article Online
porphyrin NH protons which were shifted to low frequency by
the ring current.
was also studied in acetonitrile solution. In general, porphyrins
are expected to undergo two reduction and two oxidation
processes, each consistent with a one-electron transfer reaction.
Compound 17 shows two chemically reversible reductions in
the expected range and one irreversible oxidation, all associated
with the porphyrin macrocycle. Presumably the second oxid-
ation expected for a porphyrin is masked by solvent breakdown
(i.e. beyond around ϩ1.50 V).
The zincated analogue of 1, compound 2, exhibits two reduc-
tions (Ϫ1.26 and Ϫ1.55 V vs. SCE). The first is chemically
reversible, whilst the second is irreversible leading to the form-
ation of a new product wave at Ϫ0.40 V. Compound 2 also has
three irreversible oxidation processes (ϩ0.94, ϩ1.24 and ϩ1.52
V). All of the waves are at more negative potential when com-
pared to those of the non-zincated compound, an effect which
has been observed previously when inserting zinc into a por-
phyrin macrocycle.10 Two oxidation processes are expected for
a porphyrin, and it is possible that the additional third oxid-
ation process which is observed here is due to isoporphyrin
formation.11
Compound 3 exhibits three oxidation processes, at similar
potentials to those of 2 and assigned as above for 2. It also
exhibits two reduction processes. That at Ϫ0.84 V is irreversible
and probably associated with the pyridinium porphyrin entity,
being similar in potential to the third reduction potential
observed for compound 5 (see below). Compound 4 exhibits
two reduction processes at Ϫ0.76 and Ϫ1.47 V.
The electrochemistry of compound 5 was investigated by
cyclic voltammetry in both dichloromethane and acetonitrile.
The cyclic voltammograms were generally similar, showing two
oxidation and four reduction processes in each case. However,
the compound was poorly soluble in dichloromethane, and the
second reduction process appeared to be associated with some
electrode coating. This problem was not found with acetonitrile
as solvent, and so the interpretation of the electrochemical data
is based on our observations with acetonitrile solutions.
In acetonitrile, two oxidation processes are observed. The
first is assigned to the porphyrin entity. It is at a potential
13C NMR Spectra
The spectrum of compound 5 has partially been assigned
by means of 2-dimensional NMR [heteronuclear single quan-
tum coherence (HSQC)] and PENDANT15 experiments. The
assignment of quaternary carbon environments is incomplete
(peaks listed in curly brackets in Experimental section).
Electronic absorption spectra
The electronic spectral data for compounds 1–5 are shown in
Table 1. In dichloromethane the porphyrin-based compounds
show absorption spectra characteristic of the porphyrin moiety.
Compound 1,7 an unmetallated porphyrin, shows the expected
five major absorptions: the intense B or Soret band at 415 nm,
and four Q bands in the range 500–650 nm which decrease in
intensity with increasing wavelength. The electronic spectra of
the metallated porphyrin compounds show the characteristic
Soret band and only two Q bands, due to increased sym-
metry. Compound 3 shows an additional absorption at 233 nm,
corresponding to the xylene moiety. The absorption due to the
acceptor component (266 nm for compound 4) combines with
the xylene absorption to give a broader band at 256 nm when
present in 5.
There is small perturbation in the positions of the Q bands of
compound 5, compared to those of the individual components,
indicating the possibility of a small amount of ground state
electronic interaction between the components of the diad,
when assembled.
Cyclic voltammetry
The electrochemical properties of compounds 1–5 were exam-
ined by cyclic voltammetry in dry dichloromethane using a
platinum bead working electrode and [n-Bu4N][BF4] as support-
ing electrolyte (Table 2). The electrochemistry of compound 5
J. Chem. Soc., Dalton Trans., 2000, 3170–3175
3171

View Article Online
Table 1 Electronic spectral data in dichloromethane for compounds 1–5
Compound
B λa(ε)b
Qy(1,0) λa(ε)c
Qy(0,0) λa(ε)d
Qx(1,0) λa(ε)d
Qx(0,0) λa(ε)d
λa(ε)c
17
2
415 (2.4)
416 (2.4)
414 (1.5)
513 (1.7)
547 (6.5)
604 (4.3)f
600 (6.0)f
587 (4.6)
644 (2.7)
561 (1.4)e
556 (8.7)d,e
3
233 (2.3)
266 (5.2)
256 (6.8)
47
5
415 (2.2)
559 (1.7)e
621 (1.5)c,f
a In nm. b In L molϪ1 cmϪ1 × 105. c In L molϪ1 cmϪ1 × 104. d In L molϪ1 cmϪ1 × 103. e Q(1,0). f Q(0,0).
Table 2 Electrochemical data obtained for compounds 1–5a
Compound
Solvent
Oxidation, Ef/V
ϩ1.16b
Reduction, Ef/V (∆Ep/mV)
17
2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeCN
Ϫ1.05 (100)
Ϫ1.26 (130)
Ϫ0.84b
Ϫ0.76 (109)
Ϫ0.18 (71)
Ϫ0.32 (72)
Ϫ1.40 (100)
ϩ0.94b
ϩ1.01b
ϩ1.24b
ϩ1.52b
ϩ1.46b
Ϫ1.55b
Ϫ1.72b
3
ϩ1.25b
47
5
Ϫ1.47 (160)
ϩ0.97b
ϩ0.96b
ϩ1.46b
ϩ1.21b
Ϫ0.67b,c
Ϫ0.88b
Ϫ0.95 (56)
Ϫ1.62b
Ϫ1.78b
5
Ϫ0.73 (61)
a All measurements were made by cyclic voltammetry, in the solvent specified, containing 0.2 mol dmϪ3 [n-Bu4N][BF4] at a platinum bead working
electrode. The scan rate was 200 mV sϪ1. All potentials are vs. SCE. The ferrocenium–ferrocene couple was used as an internal reference
(Ef = 0.55 0.01 V in dichloromethane, 0.46 0.01 V in acetonitrile). Values in parentheses are peak–peak separations for the reversible processes
(∆Ep for the ferrocenium–ferrocene couple under the same conditions was about 90 30 mV in dichloromethane and 70 10 mV in acetonitrile).
b Irreversible process. c This reduction appeared to be associated with some other process, possibly electrode coating.
comparable with that of compound 2 and is irreversible. The
second oxidation is therefore presumably due to the decom-
position product of the first oxidation process. The first reduc-
tion appears at Ϫ0.32 V in acetonitrile. This reversible process
has been assigned by other methods to the acceptor moiety of
the molecule (see below). The solvent dependence of the first
reduction process indicates that the dibenzyl viologen moiety
must interact with the solvent. The second reduction process is
also reversible in acetonitrile (Ϫ0.73 V) and is assigned to the
dibenzyl viologen moiety. In situ EPR experiments demon-
strate that the first two reduction processes are closely asso-
ciated with each other (see below). The third (reversible) and
fourth (irreversible) reduction processes are both assigned to
porphyrin based electron transfer.
Fig. 1 Electronic spectra recorded during reduction of compound 5 to
؊ showing the collapse of the Soret band.
For the analogous non-zincated diad7 it was found that the
second and third reduction processes were at very similar
potentials, and unresolved by cyclic voltammetry under equiv-
alent conditions. It has already been noted (see above) that
adding zinc to a porphyrin ring causes a negative shift in all
electrochemical processes associated with the macrocycle.
When comparing the zincated and free-base diads, both the first
and second dibenzyl viologen reductions appear at similar
potentials, whereas the first and second porphyrin reductions
are seen at different potentials. Thus two waves may be resolved
for the zincated diad which were unresolved for the free-base
diad.
5
Spectroelectrochemistry
In situ electronic absorption spectra of compound 5 were
recorded in a 0.1 mol dmϪ3 solution of [n-Bu4N][BF4] in dry
acetonitrile at 253 K. From cyclic voltammetry, three reversible
reduction processes are seen, at Ϫ0.32, Ϫ0.73 and Ϫ0.95 V,
and these were further investigated. The irreversible reduction
process at Ϫ1.78 V was not investigated.
During the first reduction (Fig. 1), when the potential is set
to Ϫ0.50 V, small changes are seen in the Soret band and the
two Q bands of the porphyrin moiety. The bands shift to
higher wavelength (by around 30 nm) and there is some
collapse in intensity, notably in the Soret band. The band at
260 nm decreases, a shoulder develops at 320 nm, and a
new, broad absorption is seen at around 890 nm (Fig. 2). This
Fig. 2 Electronic spectra recorded during reduction of compound 5
to 5؊.
first reduction process is therefore assigned to the acceptor
component of the molecule, and is reversible under these con-
ditions since the original spectrum is recovered by returning
the potential to ϩ0.20 V. Since the porphyrin bands also
change slightly at this potential, it would appear that there is
also some degree of porphyrin involvement in the reduction. In
contrast, it was previously found7 that the analogous free-base
diad did not exhibit any significant amount of porphyrin
involvement in the equivalent first reduction.
3172
J. Chem. Soc., Dalton Trans., 2000, 3170–3175

View Article Online
Fig. 3 In situ EPR spectrum recorded during reduction of compound
5 to 5؊ at Ϫ0.50 V in DMF at 233 K and simulated EPR spectrum.
Simulation of 5Ϫ used parameters in the text with a Lorentzian line-
width of 1.1 G.
Fig. 4 In situ EPR spectrum recorded during reduction of compound
52Ϫ to 53؊ at Ϫ1.52 V in DMF at 233 K and simulated EPR spectrum.
Simulation of 53Ϫ used parameters in the text with a Lorentzian line-
width of 2.7 G.
Upon controlled potential reduction at Ϫ0.85 V there is little
further change to the porphyrin bands, indicating that there is
little porphyrin involvement in the site of the second reduction
process. This is to be expected since this process is assigned
to a dibenzyl viologen reduction. When the potential is set to
Ϫ1.52 V there is a dramatic change in the UV/Visible spec-
trum of compound 5. A strong new absorption develops at
around 480 nm and the characteristic Q bands are almost totally
bleached. A broad absorbance is seen centred on 1000 nm. This
process is irreversible under these conditions, since the original
spectrum cannot be reproduced by setting the potential back to
ϩ0.2 V, and is assigned as a porphyrin-based reduction.
At Ϫ1.52 V a new signal develops (Fig. 4) as a third electron
is added to the molecule. This EPR signal has 13 lines and is
centred at g = 2.011. In this case, simulations (Fig. 4) show that
the signal shape may be modelled by coupling of the unpaired
electron to four equivalent N atoms (presumably porphyrin-
based, aN = 2.65 G), one further N atom (presumably pyridin-
ium, aN = 5.40 G) and two H atoms (exact identity undefined,
aH = 2.7 G). Reversal of the applied potential to Ϫ0.61 V shows
collapse of the signal at g = 2.011 to give a featureless EPR
spectrum and then growth of the first signal at g = 2.012. The
EPR experiments show that at 233 K compound 5 is stable in
redox states 5؊, 52؊ and 53؊
.
EPR Spectral studies
Conclusion
The site of the first reduction process of compound 5 was con-
firmed using in situ EPR spectroscopy in a 1.0 mol dmϪ3 solu-
tion of [n-Bu4N][BF4] in DMF at 233 K. When the controlled
potential is set to Ϫ0.50 V a signal develops at g = 2.012 which
is indicative of the formation of a radical species (Fig. 3). The
signal shows around 27 resolved lines. Simulations (Fig. 3)
demonstrate that this is attributed to coupling of the unpaired
electron with three N atoms (we infer then that these are the
acceptor N atoms, aN = 4.01 G), six H atoms (aH = 1.30 G)
and a further six H atoms (aH = 1.00 G). Consideration of the
structure of 5 would suggest that the redox process at Ϫ0.32 V
should be attributed to the acceptor part of the compound
which has three equivalent nitrogen sites (the porphyrin ring
having four equivalent nitrogen sites). It is likely that six H
atoms observed in the EPR spectrum are methylene protons (2
per nitrogen) and six are aromatic protons (again 2 per nitro-
gen) adjacent to N, although we cannot discount all twelve H
atoms being aromatic protons, and hence a detailed assignment
of EPR coupling to specific H atoms requires further study. This
demonstrates that the dibenzyl viologen moiety is the first site of
reduction, and therefore the most likely receptor for the trans-
ferred electron in the photoinduced electron transfer process.
On application of a potential of Ϫ0.85 V the EPR signal
decays as the system becomes diamagnetic. Since the two
electrochemically added electrons are pairing up in this way they
must be strongly coupled on the molecule. Therefore this second
reduction is also assigned to the acceptor end of the molecule.
We have described the synthesis and characterisation of a new
potentially photoactive donor–acceptor diad, compound 5, and
its precursors. In situ UV/Visible and EPR measurements have
shown that the site of the first and second electrochemical
reductions is the dibenzyl viologen component of the molecule,
and that the third reduction process is associated with the
porphyrin entity. It appears, however, from in situ UV/Visible
measurements, that there is a small degree of porphyrin
involvement in the first electrochemical reduction process.
These findings are in contrast to those for the analogous free
base diad,7 where it was found by in situ UV/Visible measure-
ments that the site of the first electrochemical reduction was
purely the benzyl viologen component. In addition, the second
reduction wave from cyclic voltammetry of the free base anal-
ogue was shown by in situ EPR to comprise two unresolved
one-electron processes. The first reduction process rendered the
diad diamagnetic, as was shown by the disappearance of the
signal due to the benzyl viologen radical. The second gave rise
to the appearance of a new EPR signal, which was found to
correspond to the porphyrin radical. For the zinc analogue,
reported here, the second and third reduction processes are
resolved.
These are significant steps towards our ultimate aim of
assembling zeolite-occluded diads in order to attempt the
photogeneration of long-lived charge separated states. The
results presented here suggest that the photochemistry of the
J. Chem. Soc., Dalton Trans., 2000, 3170–3175
3173

View Article Online
free base and zincated diads will be different, due to apparent
differences in the ground-state electronic interactions between
the donor and acceptor components of the diads.
Zinc 5-[N-(4-bromomethylbenzyl)-4-pyridinio]-10,15,20-tri-
phenylporphyrinate hexafluorophosphate, 3. Compound 2 (0.34
g, 0.5 mmol, 1.0 equivalent) and α,αЈ-dibromo-p-xylene (1.65 g,
6.25 mmol, 12.5 equivalents) were dissolved in dry THF (100
ml) and heated to reflux under nitrogen for 6 hours. The reac-
tion mixture was then left to cool with stirring for 18 hours. The
resulting solution was evaporated to half of its original volume
in vacuo, and then a saturated solution of NH4PF6 was added
until no further precipitation was seen. The mixture was filtered
under vacuum and allowed to air-dry. The crude product was
separated from residual starting materials by column chrom-
atography, eluting with 1% methanol in dichloromethane (2.5
L). The solvent was removed to afford 3 as a purple solid.
Experimental
General details
All reagents were purchased from Aldrich and used without
further purification. Reaction solvents were deoxygenated and
dried by standard methods before use. Compounds were puri-
fied by column chromatography (column lengths ca. 0.3 m)
using Kiesel gel 60 (Merck 5554; 70–230 mesh). 1H NMR
spectra were recorded using Bruker AC300 (300 MHz) or
AMX400 (400 MHz) spectrometers, 13C NMR spectra using
Bruker DRX500 (125 MHz) or AMX400 (100 MHz) spectro-
meters and assignments made using a combination of both
PENDANT and HSQC experiments.
Low resolution LSIMS spectra were obtained from a VG
Zabspec mass spectrometer utilising a m-nitrobenzyl alcohol
matrix and scanning in the positive ion mode at a speed of
5 s per decade. Absorption spectra were obtained using a
Shimadzu UV-240 spectrophotometer. Solvent background
corrections were made in all cases.
Cyclic voltammetry was carried out using an EG & G
model 362 potentiostat and the Condecon 310 software pack-
age, with ca. 10Ϫ3 mol dmϪ3 solutions under dry N2 in dry
dichloromethane or acetonitrile. A platinum bead working
electrode was used, with 0.2 mol dmϪ3 [n-Bu4N][BF4] as sup-
porting electrolyte, and a scan rate of 200 mV sϪ1. Potentials
were recorded vs. a saturated calomel reference electrode, and
ferrocene (Ef = 0.55 0.01 V in dichloromethane) was added as
an internal standard. The data obtained were reproducible, the
experimental error being 10 mV.
All spectroelectrochemical studies were carried out using
an Autolab System containing a PSTAT20 potentiostat, using
General Purpose Electrochemical System (GPES) Version 4.5
software. Positive feedback ohmic compensation was applied
for all cyclic voltammograms recorded. All solutions were
purged with Ar for 20 minutes prior to study. Electrogeneration
potentials of the reduction processes were at more negative
values than the Ef of the couple.
1
Yield: 0.20 g (38.9%). H NMR (300 MHz, CD3CN): δ 9.03
3
(2 H, d, J = 6.3, pyridine ring Ha and HaЈ), 8.93 (4 H, A2B2,
JAB = 4.8, ∆δAB = 0.08 ppm, pyrrole protons of rings A and B),
8.84 (4 H, A2B2, JAB = 4.8, ∆δAB = 0.02 ppm, pyrrole rings C and
D), 8.77 (2 H, d, 3J = 6.6, pyridine ring Hb and HbЈ), 8.18–8.22
(6 H, m, ortho protons of Ph in porphyrin), 7.75–7.83 (9 H, m,
meta and para protons of Ph in porphyrin), 7.69 (4 H, A2B2,
JAB = 8.5 Hz, ∆δAB = 0.04 ppm, phenyl ring E), 5.97 (2 H, s, Hc
and HcЈ) and 4.69 (2 H, s, Hd and HdЈ). Found: C, 60.8; H, 3.5;
N, 7.0. Calc. for C51H35BrF6N5PZn: C, 60.8; H, 3.5; N, 7.0%.
LSIMS: m/z 862 ([M Ϫ PF6]ϩ), 782 ([M Ϫ Br Ϫ PF6]ϩ) and 677
([M Ϫ (CH2C6H4CH2Br) Ϫ PF6]ϩ).
1-Benzyl-4-(4-pyridyl)pyridiniumhexafluorophosphate, 4. The
synthesis and characterisation of this compound have been
described previously.7
Compound 5 (Fig. 5). Compounds 3 (0.10 g, 1.06 × 10Ϫ4 mol,
1.0 equivalent) and 4 (0.55 g, 1.4 × 10Ϫ3 mol, 14.0 equivalents)
were dissolved in dry acetonitrile (100 ml). The mixture was
stirred under reflux for 12 hours under an atmosphere of
nitrogen and then left to cool. The volume of the resulting
solution was reduced to half under vacuum, then a saturated
solution of NH4PF6 was added until no further precipitation
was seen. The crude product was collected by vacuum filtra-
tion and purified by column chromatography (methanol,
The optically transparent electrode cell (O.T.E.) for use in
UV/vis/near-IR spectrometers has been described previously.12
EPR spectra were recorded on an X-band Bruker ER200D-
SCR spectrometer, using the in situ cell described previously.13
EPR simulations were performed using WINEPR SimFonia.14
Microanalyses were performed by the University of North
London.
Preparations
5,10,15-Triphenyl-20-(4-pyridyl)porphyrin, 1. The synthesis
and characterisation of this compound has been described
previously.7
Zinc 5,10,15-triphenyl-20-(4-pyridyl)porphyrinate, 2. Com-
pound 1 (0.50 g, 0.81 mmol) was dissolved in chloroform (100
ml) and heated to reflux. Zinc acetate (0.15 g, 0.81 mmol) in
methanol (2 ml) was added and the solution heated under reflux
for 30 minutes. The resulting solution was evaporated to dry-
ness in vacuo and purified by column chromatography, eluting
with dichloromethane. The solvent was removed to give com-
pound 2 as a red-purple solid. Yield: 0.41 g (73.6%). 1H NMR
(300 MHz, DMSO): δ 8.94 (2 H, d, 3J = 5.9 Hz, pyridine Ha and
HaЈ), 8.78–8.82 (8 H, m, pyrrole protons of rings A, B, C and D),
8.18–8.21 (8 H, overlapping signals, ortho protons of Ph and Hb,
HbЈ of pyridine), 7.77–7.82 (9 H, m, meta and para protons of
Ph). Found: C, 76.2, H, 3.9; N, 10.3. Calc. for C43H27N5Zn: C,
76.1; H, 4.0; N, 10.3%. LSIMS: m/z 678 ([M]ϩ).
Fig. 5 Atom labelling in diad 5.
3174
J. Chem. Soc., Dalton Trans., 2000, 3170–3175

View Article Online
NH4ϩClϪ
[2.0 mol dmϪ3], nitromethane; ratio of 7:2:1).
References
(aq)
The desired product was collected as the major coloured band.
1 For example: A. Harriman, G. Porter and A. Wilowska, J. Chem.
Soc., Faraday Trans. 2, 1984, 191; G. Blondeel, D. De Keukeleire,
A. Harriman and L. R. Milgrom, Chem. Phys. Lett., 1985, 118, 77;
I. Okura and H. Hosono, Inorg. Chim. Acta, 1991, 189, 145 and
references therein.
2 R. J. McMahon, R. K. Forcé, H. H. Patterson and M. S. Wrighton,
J. Am. Chem. Soc., 1988, 110, 2670; R. K. Forcé, R. J. McMahon,
J. Yu and M. S. Wrighton, Spectrochim. Acta, Part A, 1989, 45, 23
and references therein.
3 E. H. Yonemoto, Y. I. Kim, R. H. Schmehl, J. O. Wallin, B. A.
Shoulders, B. R. Richardson, J. F. Haw and T. E. Mallouk, J. Am.
Chem. Soc., 1994, 116, 10557.
4 N. M. Rowley, S. S. Kurek, M. W. George, P. D. Beer, S. M. Hubig,
C. J. Jones, J. M. Kelly and J. A. McCleverty, J. Chem. Soc., Chem.
Commun., 1992, 497.
5 N. M. Rowley, S. S. Kurek, J.-D. Foulon, T. A. Hamor, C. J. Jones,
J. A. McCleverty, S. M. Hubig, E. J. L. McInnes, N. N. Payne and
L. J. Yellowlees, Inorg. Chem., 1995, 34, 4414.
6 N. M. Rowley, S. S. Kurek, P. R. Ashton, T. A. Hamor, C. J. Jones,
N. Spencer, J. A. McCleverty, G. S. Beddard, T. M. Feehan,
N. T. H. White, E. J. L. McInnes, N. N. Payne and L. J. Yellowlees,
Inorg. Chem., 1996, 35, 5526.
7 M. T. Barton, N. M. Rowley, P. R. Ashton, C. J. Jones, N. Spencer,
M. S. Tolley and L. J. Yellowlees, New J. Chem., 2000, 24, 555.
8 A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour
and L. Korsakoff, J. Org. Chem., 1967, 32, 476.
9 J.-H. Fuhrhop and K. M. Smith, Laboratory Methods in Porphyrin
and Metalloporphyrin Research, Elsevier, Oxford, 1976.
10 D. G. Davis, in The Porphyrins, ed. D. Dolphin, Academic Press,
New York, 1978, vol. 5, ch. 4.
ϩ
Solvent was removed under vacuum and residual solid NH4
-
ClϪ removed by washing with water. The product was dissolved
in dichloromethane and dried over MgSO4. The pure product
1
was obtained as a red-purple solid. Yield: 0.10 g (63.2%). H
NMR (400 MHz, d6-acetone): δ 9.56 (2 H, d, J = 6.6, Ha and
3
3
HaЈ of porphyrin pyridine ring), 9.51 (2 H, d, J = 6.8, Hε and
HεЈ of 4,4Ј-bipyridine), 9.46 (2 H, d, 3J = 7.0, Hα and HαЈ of 4,4Ј-
3
bipyridine), 9.03 (2 H, d, J = 6.5, Hb and HbЈ of porphyrin
pyridine ring), 8.97 (4 H, A2B2, JAB = 4.8, ∆δAB = 0.05 ppm,
pyrrole protons of rings A and B), 8.88 (4 H, A2B2, JAB = 4.8,
∆δAB = 0.02 ppm, pyrrole protons of rings C and D), 8.77–8.81
(4 H, m, Hχ,HχЈ and Hβ,HβЈ of 4,4Ј-bipyridine), 8.19–8.21 (6 H,
m, ortho protons of Ph in porphyrin), 7.99 (4 H, A2B2, JAB = 8.5
Hz, ∆δAB = 0.11 ppm, phenyl ring E), 7.78–7.93 (9 H, m, meta
and para protons of Ph in porphyrin), 7.64–7.65 (2 H, m, ortho
protons of phenyl ring F), 7.48–7.49 (3 H, m, meta and para
protons of phenyl ring F), 6.40 (2 H, s, Hc and HcЈ), 6.29 (2 H, s,
Hd and HdЈ) and 6.14 (2 H, s, He and HeЈ). 13C NMR (100 MHz,
CD3CO): δ 64.3 (C[Hc]2), 65.1 (C[Hd]2), 65.7 (C[He]2), {112.3,
121.7, 122.9}, 126.4, 126.5, 127.6 and 127.7 (meta and para CH
of Ph in porphyrin), 129.2 and 129.4 (CH of phenyl ring F),
129.2 and 130.1 (CH of pyrrole rings A and B), 130.4 and
130.8 (CH of phenyl ring E), 131.8 and 132.2 (CH of pyrrole
rings C and D), 132.9 and 135.1 (CH of pyrrole rings A and B),
133.5 (CHb of porphyrin pyridine ring), 134.2 (ortho CH of
Ph in porphyrin), 134.8, 135.1, 142.5, 142.7, 142.8, 146.0,
148.0, 149.9, 150.1, 150.7 and 161.6. Found: C, 55.8; H, 3.4;
N, 6.5. Calc. for C68H50F18N7P3Zn: C, 55.7; H, 3.4; N, 6.7%.
LSIMS: m/z 1318 ([M Ϫ PF6]ϩ), 1174 ([M Ϫ 2PF6]ϩ) and 677
([2]ϩ).
11 M. Veyrat, R. Ramasseul, I. Turowska-Tyrk, W. R. Scheidt,
M. Autret, K. M. Kadish and J.-C. Marchon, Inorg. Chem., 1999,
38, 1772.
12 D. Collison, F. E. Mabbs, E. J. L. McInnes, K. J. Taylor, A. J. Welch
and L. J. Yellowlees, J. Chem. Soc., Dalton Trans., 1996, 329.
13 E. J. L. McInnes, R. D. Farley, C. C. Rowlands, A. J. Welch, L.
Rovatti and L. J. Yellowlees, J. Chem. Soc., Dalton Trans., 1999, 4203.
14 WINEPR SimFonia, version 1.25, Bruker Analytische Messtechnik
GmbH, 1996.
Acknowledgements
We thank the EPSRC (UK) for financial support.
15 J. Homer and M. Perry, J. Chem. Soc., Chem. Commun., 1994, 373.
J. Chem. Soc., Dalton Trans., 2000, 3170–3175
3175