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

Article abstract of DOI:10.1039/b002589p

A new donor-acceptor (D-A) molecule, 5,10,15,20-[N-benzyl-N'-(4-benzyl- 4,4'-bipyridinium-4-pyridyl)]triphenylporphyrin tris(hexafluorophosphate), 4, has been synthesised. The diad, 4, and its precursors, have been fully 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 electrochemical reduction is the benzylviologen component of the molecule. The second reduction wave from cyclic voltammetry was shown by in-situ EPR to comprise two unresolved one- electron processes, and this was confirmed by chronocoulometry. The first of these two reduction processes rendered the diad diamagnetic, as was shown by the disappearance of the signal due to the benzylviologen radical. The second gave rise to the appearance of a new EPR signal, which was found to correspond to the porphyrin radical. We believe this to be the first reported resolved spectrum of a monoreduced porphyrin radical.

Full text of DOI:10.1039/b002589p

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Synthesis, electrochemistry and spectroelectrochemistry of a
porphyrin–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, University of Birmingham, Edgbaston, Birmingham, UK B15 2T T
E-mail: N.M.Rowley=bham.ac.uk
b Department of Chemistry, University of Edinburgh, Edinburgh, UK EH9 3JJ
Received (in Cambridge, UK) 31st March 2000, Accepted 12th May 2000
Published on the Web 15th June 2000
A new donorÈacceptor (DÈA) molecule, 5,10,15,20-[N-benzyl-N@-(4-benzyl-4,4@-bipyridinium-4-pyridyl)]-
triphenylporphyrin tris(hexaÑuorophosphate), 4, has been synthesised. The diad, 4, and its precursors, have been
fully 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 Ðrst electrochemical reduction is
the benzylviologen component of the molecule. The second reduction wave from cyclic voltammetry was shown by
in-situ EPR to comprise two unresolved one-electron processes, and this was conÐrmed by chronocoulometry. The
Ðrst of these two reduction processes rendered the diad diamagnetic, as was shown by the disappearance of the
signal due to the benzylviologen radical. The second gave rise to the appearance of a new EPR signal, which was
found to correspond to the porphyrin radical. We believe this to be the Ðrst reported resolved spectrum of a
monoreduced porphyrin radical.
We have previously synthesised a series of peripherally-
metallated porphyrins and reported their photochemical
properties.1h3 It was found that these complexes undergo
rapid electron transfer from the excited singlet state of the
porphyrin to the [Mo(TpMe,Me)(NO)Cl] acceptor moiety
[where TpMe,Me is hydrotris(3,5-dimethylpyrazolyl)borate].
These are very rare examples of electron transfer to external,
but covalently-linked, transition metal based redox centres.
Wrighton et al.4 have previously demonstrated intramole-
cular electron transfer in a covalently linked porphyrinÈ
viologen diad by resonance Raman spectroscopy. They
reported an electron transfer rate of ca. 8 ] 107 s~1È this
relatively small value being consistent with the low driving
force (*G0 ca. [0.4 eV) and the large distance between donor
and acceptor. More recently, Yonemoto et al.5 have shown
the synthesis, characterisation, electrochemistry and spectro-
electrochemistry of a new porphyrin-based donorÈacceptor
compound and its precursors. The target molecule comprises
a porphyrin, covalently attached to a dibenzylviologen accep-
tor unit, the acceptor end of which is sufficiently small to be
occluded by large pore zeolites, such as zeolite Y.
Results and discussion
Synthesis and characterisation of compounds 1–4
Synthesis. Compounds 1 to 4 are illustrated in Fig. 1.
Porphyrin 1 was prepared by an Adler-type condensation of
pyrrole, benzaldehyde and pyridine-4-carbaldehyde.6 The
molar ratio of reactants (1 : 1.18 benzaldehyde to pyridine-4-
carbaldehyde) was found to give the highest yield, after puriÐ-
cation by column chromatography, of the mono-substituted
porphyrin. Compound 2 was prepared by mono-substitution
of 4,4@-bipyridyl. The reaction was performed in toluene, by
slow addition of benzyl bromide to an excess of 4,4@-bipyridyl.
Counter-ion exchange then gave the desired organic-soluble
product. Compound 3 was prepared by quaternisation of the
pyridyl moiety of compound 1. A large excess of a,a@-
dibromo-p-xylene was added to increase the yield of the
monosubsitited a,a@-dibromo-p-xylene product. Quatern-
isation of 2 on reaction with 3 gave compound 4, again using
a large excess of 2. Counter-ion exchange then gave the
desired organic-soluble product. 1H NMR spectroscopy,
LSIMS and elemental analyses conÐrm the identity of all of
the reported compounds.
that donorÈacceptor complexes of the type [(2,2@-bipy) RuM4-
2
CH -2,2@-bipy-4@)(CH ) (4,4@-H bipy-CH )]4` (n \ 2È5, 7, 8)
3
2 n
2
3
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 channels, 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.
As outlined above, we have already successfully synthesised
donorÈacceptor systems which undergo photoinduced elec-
tron transfer, generating charge-separated states with lifetimes
of the order of several hundred picoseconds. We now wish to
develop novel systems which give rise to longer lifetimes of the
photoinduced charge-separated states. In order to achieve
this, we have constructed diad systems capable of occlusion by
zeolite Y, in the hope that our systems will behave in a similar
manner to those of Yonemoto et al.5 In this paper we report
1H NMR Spectra. In general, the solution 1H NMR spectra
of compounds 1È4 were well-resolved and easily assigned.
Large shifts were seen due to the presence of quaternised
nitrogen atoms and the ring-current e†ect of the porphyrin
macrocycle. These generally shifted signals to very high fre-
quency, except for internal porphyrin NH protons which were
shifted to low frequency by the ring current.
¤ Electronic supplementary information (ESI) available: UV/Visible
spectra of 1È4 and cyclic voltammogram of 4. See http://www.rsc.org/
suppdata/nj/b0/b002589p/
13C NMR Spectra. The spectrum of 4 has been partially
DOI: 10.1039/b002589p
New J. Chem., 2000, 24, 555È560
555
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2000

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Fig. 1 Structures of compounds 1 to 4.
assigned by means of 2-dimensional NMR (HSQC) and
PENDANT experiments. The assignment of quaternary
carbon environments is incomplete (peaks listed in curly
brackets in Experimental section).
Compound 1 shows two reversible reductions and one irre-
versible oxidation, at the expected values for a porphyrin. Pre-
sumably the second oxidation expected for
a free-base
porphyrin is masked by solvent breakdown (i.e. beyond
around ]1.50 V). Compound 2 exhibits two reversible
reductions at the potentials expected for a viologen-type
moiety. Compound 3 exhibits a single reversible reduction at
[0.70 V, which is attributed to the Ðrst reduction of the pyri-
dinium porphyrin moiety, being similar in potential to the
second reduction potential observed in 4 (vide infra). No
second reduction potential of the pyridinium porphyrin entity
was observed, possibly as a result of being masked by solvent
breakdown. The two oxidation processes expected for a
porphyrin entity could not be observed.
Electronic absorption spectra. The electronic spectral data
for compounds 1È4 are shown in Table 1. In dichloromethane,
the porphyrin-based compounds show absorption spectra
characteristic of the porphyrin group, with Ðve major absorp-
tions: the intense B or Soret band at around 420 nm, and four
Q bands in the range 500È650 nm which decrease in intensity
with increasing wavelength. Compound 3 shows an additional
absorption at 235 nm, corresponding to addition of the xylene
moiety. The absorption due to the acceptor component (266
nm in 2) combines with this xylene absorption to give a
broader band at 257 nm when present in 4.
In general, the spectrum of 4 is a simple addition of the
spectra of its components, indicating that there is little or no
ground state electronic interaction between the components of
the diad, when assembled. This is encouraging with respect to
using these compounds as photoactive devices, since it implies
that electron transfer should not take place until the molecule
is photochemically promoted to its excited state.
On assembly of 2 and 3 to make compound 4, the waves of
the di†erent components add to give the observed Ðve redox
processes. Two oxidation processes are observed. The Ðrst
oxidation is assigned to the porphyrin entity. It is at a less
positive potential compared to 1 and is irreversible. The
second oxidation is presumably due to the decomposition
product of the Ðrst oxidation process. The Ðrst reduction
([0.18 V) is reversible and is assigned to the acceptor moiety.
Chronocoulometry at [0.30
V conÐrms that this Ðrst
reduction process involves one electron. The reversible process
at [0.70 V has a large current and coulometric experiments
conÐrm that this process involves two electrons. Further
experiments (vide infra) conÐrm that the reduction process at
[0.70 V consists of two one-electron processes which occur at
very similar potentials and cannot be resolved under these
Cyclic voltammetry. The electrochemistry of compounds
1È4 was investigated by cyclic voltammetry in a 0.2 mol dm~3
solution of [n-Bu N][BF ] in dry dichloromethane (Table 2).
The electrochemistry of 4 was also studied in an acetonitrile
solution.
4
4
Table 1 Electronic spectral data in dichloromethane for compounds 1 to 4
Q (0,0)
x
B
ja(e)b
Q (1,0)
y
Q (0,0)
y
Q (1.0)
ja(e)d
x
ja(e)c
ja(e)c
ja(e)d
ja(e)c
1
2
3
4
415(2.4)
513(1.7)
547(6.5)d
587(4.6)
644(2.7)
266(5.2)
235(1.6)b
257(3.9)
414(1.4)
416(8.3)c
516(1.2)
518(1.2)
570(1.0)
565(1.1)
585(8.7)
588(8.6)
653(4.9)
651(5.8)
a In nm. b e in mol~1 l cm~1 ] 105. c e in mol~1 l cm~1 ] 104. d e in mol~1 l cm~1 ] 103.
556
New J. Chem., 2000, 24, 555È560

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Table 2 Electrochemical data of compounds 1È4a
Oxidation
Reduction
Solvent
CH Cl
E (*E )
E (*E )
E (*E )
E (*E )
E (*E )
f
p
f
p
f
p
f
p
f
p
1
2
3
4
4
]1.16b
[1.05(100)
[0.76(109)
[0.70(56)
[0.18(63)
[0.30(102)
[1.40(100)
[1.47(160)
2
CH Cl
2
2
2
2
2
CH Cl
2
CH Cl
]0.95b
]0.66b
]1.35b
]0.97b
[0.70(150)
[0.78(78)
[1.63b
[1.70b
2
MeCN
a All measurements were made by cyclic voltammetry, in the solvent speciÐed, containing 0.2 mol dm~3 [n-Bu N][BF ] at a platinum bead
4
4
working electrode. The scan rate was 200 mV s~1. All potentials are vs. SCE. The ferroceniumÈferrocene couple was used as an internal reference
(0.55 ^ 0.01 V). Values in parentheses are peakÈpeak separation in mV for the reversible processes. (*E for the ferroceniumÈferrocene couple
p
under the same conditions was about 120 mV). b Denotes an irreversible process.
conditions. The irreversible reduction at [1.63
V
is
EPR Spectral studies. The site of the Ðrst reduction process
of 4 was conÐrmed as the viologen by using in-situ EPR spec-
troscopy in a 1.0 M solution of [n-Bu N][BF ] in DMF at
porphyrin-based and leads to the formation of two product
waves on the return sweep (at [0.96 V and [1.16 V).
Cyclic voltammetry measurements were also recorded for 4
in acetonitrile. A shift, when compared to dichloromethane
solutions, of ca. 100 mV to more negative potential was seen
for the reduction processes, and of ca. 300 mV to less positive
potential for the oxidation processes. This is probably due to
the change in solvent, since the ferrocene couple was also
shifted.
4
4
233 K. When the controlled potential is set to [0.61 V a
signal develops at g \ 2.012 which is indicative of formation
of a radical anion species (Fig. 3). The signal shows around 27
resolved lines. Simulations (Fig. 3) demonstrate that this is
attributed to coupling with three N atoms (we infer then that
these are the acceptor N atoms, aN \ 4.08 G), six H atoms
(aH \ 1.41 G) and a further six H atoms (aH \ 1.00 G). Con-
sideration of the structure of 4 would suggest that the redox
site at [0.30 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 couplings observed in the EPR spectrum are
due to the methylene protons (2 per nitrogen) and six are aro-
matic protons (again 2 per nitrogen) adjacent to N, although a
detailed assignment of EPR coupling to speciÐc H atoms
requires further study. This demonstrates that the dibenzyl-
viologen moiety is the Ðrst site of reduction, and therefore the
most likely receptor for the transferred electron in the pho-
toinduced electron transfer process.
Spectroelectrochemistry. In-situ electronic absorption
spectra of 4 were recorded from a 0.1 mol dm~3 solution of
[n-Bu N][BF ] in dry acetonitrile at 253 K. From cyclic volt-
4
4
ammetry, two reversible reduction processes are seen in ace-
tonitrile, at [0.30 V and [0.78 V, and were investigated. The
irreversible process at [1.70 V was not investigated.
During the Ðrst reduction Fig. 2, when the potential is set to
[0.61 V, no signiÐcant change is seen in the Soret band or
the four Q bands. However, the band at 260 nm decreases
dramatically in intensity and a new, broad absorption is seen
at around 890 nm together with a shoulder at 300 nm. This
Ðrst reduction process is therefore assigned to the acceptor
component of the molecule as the characteristic absorption
bands of the porphyrin moiety remain relatively unaltered fol-
lowing bulk reduction at [0.61 V. The reduction process is
reversible under these conditions since the original spectrum is
recovered by returning the potential to ]0.20 V.
On application of a potential of [1.40 V, the EPR signal
decays as the system becomes diamagnetic. Thus the second
electron enters the same orbital as the Ðrst on the acceptor
part of the compound giving rise to a diamagnetic species. We
would suggest that the diamagnetic species generated with the
Upon controlled potential reduction at [1.40 V the Soret
band collapses and broadens and the Q bands also collapse.
New absorptions develop at around 380 nm and 450 nm.
These spectral changes indicate that
a porphyrin-based
reduction takes place at this potential and under these condi-
tions the process is irreversible, since the original spectrum
cannot be regenerated in this solvent and at this temperature
by setting the potential back to ]0.20 V. Later evidence (vide
infra) shows that two electrons are transferred during the
process at [0.78 V.
Fig. 3 In-situ EPR spectrum recorded during reduction of 4 to 4~ at
[0.61 V in DMF at 233 K and simulated EPR spectrum. Simulation
of 4~ used parameters in text with a Lorentzian linewidth of 1.0 G.
Fig. 2 Electronic spectra recorded during reduction of 4 to 4~.
New J. Chem., 2000, 24, 555È560
557

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second reducing electron is a genuine diamagnetic species,
where the second reduction electron has entered the same
orbital as the Ðrst reduction electron, and is therefore not a
coupled biradical. The evidence for this is the potential
separation of the two reduction processes by 520 mV, which is
typical of the potential di†erence for two electrons pairing up
in the same molecular orbital. Thus the Ðrst reduction elec-
tron enters an antibonding molecular orbital based on the
viologen part of the molecule, and the second reduction elec-
tron enters the same orbital, spin pairs, and therefore the mol-
ecule is diamagnetic. A new signal subsequently develops as a
third electron is added to the molecule, indicating that the
process seen at [0.78 V in cyclic voltammetry in acetonitrile
consists of two unresolved one-electron processes (Fig. 4). The
third reduction electron enters a molecular orbital on the
porphyrin part of the molecule, hence the species once again
becomes paramagnetic. After the third electron is added, the
EPR signal has 13 lines and is centred at g \ 2.012. In this
case, simulations (Fig. 4) show that the signal shape may be
modelled by coupling to four equivalent N atoms (presumably
porphyrin-based, aN \ 2.70 G) and one further N atom
(presumably pyridinium, aN \ 5.40 G). Reversal of the applied
potential to [0.61 V shows collapse of the signal at g \ 2.012
to give a featureless EPR spectrum and then growth of the
Ðrst signal at g \ 2.012. The EPR experiments show that at
233 K compound 4 is stable in redox states 4~, 42~ and 43~.
As far as we are aware this is the Ðrst reported resolved spec-
trum of the reduced porphyrin radical. Previous studies report
only a broad unresolved signal for the monoreduced porphy-
rin radical.7,8 We were surprised that this was the Ðrst
example we could Ðnd in the literature of a resolved EPR
spectrum of a monoreduced porphyrin radical. Previous
reports show a broad signal with a peak width of around 12
G, whereas our spectrum has a peak width of 1 G, which
explains why we observe coupling to the nitrogen centres,
whereas earlier spectra were unresolved.
Experimental
General details
All reagents were purchased from Aldrich and were used
without further puriÐcation. Reaction solvents were deoxy-
genated and dried by standard methods before use. Com-
pounds were puriÐed by column chromatography (column
lengths ca. 0.3 m) using Kiesel gel 60 (Merck 5554; 70È230
mesh). 1H NMR spectra were recorded using a Bruker AC300
(300 MHz) or Bruker AMX400 (400 MHz) spectrometer. 13C
NMR spectra were recorded using a Bruker DRX500 (125
MHz) or Bruker AMX400 (100 MHz) spectrometer and
assignments were made using
PENDANT and HSQC experiments.
a combination of both
Low resolution liquid secondary ion mass spectra (LSIMS)
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 seconds per decade. Absorption
spectra were obtained using a Shimadzu UV-240 spectropho-
tometer. 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
package, with ca. 10~3 mol dm~3 solutions under dry N in
dry dichloromethane or acetonitrile. A Pt bead working elec-
2
trode was used, with 0.2 mol dm~3 [n-Bu N][BF ] as sup-
4
4
porting electrolyte, and a scan rate of 200 mV s~1. Potentials
were recorded vs. a saturated calomel reference electrode, and
ferrocene (E \ 0.55 ^ 0.01 V in dichloromethane) was added
f
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. Electro-
generation potentials of the reduction processes were at more
negative values than the E of the couple.
f
The optically transparent electrode cell (O.T.E.) for use in
UV/vis/near-IR spectrometers has been described previously.9
EPR spectra were recorded on an X-band Bruker
ER200DSCR spectrometer, using the in-situ cell described pre-
viously.10 EPR simulations are performed using WINEPR
SimFonia.11
Microanalyses were performed by the University of North
London.
Syntheses
5,10,15,20-(4-Pyridyl)triphenylporphyrin, 1. Pyridine-4-car-
baldehyde (8.0 ml, 68.0 mmol) and benzaldehyde (6.0 ml, 57.6
mmol) were added to propionic acid (450 ml) in a 2 l three-
neck round-bottom Ñask. The Ñask was equipped with two
condensers and an overhead stirrer. The mixture was stirred
and brought to reÑux. Freshly-distilled pyrrole (8.8 ml, 127.0
mmol) was added dropwise over a period of 45 minutes and
then the resulting mixture was stirred under reÑux for 1 hour.
The mixture was then left to cool for 18 hours. The purple
mixture of products was separated from the black solution by
vacuum Ðltration.
The products were separated using column chromatog-
raphy. The Ðrst band (tetraphenylporphyrin) was eluted using
dichloromethane (3 l). The desired product was collected as
the second band, eluting with 1% methanol in dichloro-
methane (5 l). The solvent was removed in vacuo to give a
purple solid. Yield: 0.83 g (4.25%). 1H NMR (300 MHz,
CDCl cf. Chart 1 for ring and atom labelling): d 9.04 (2H, d,
Fig. 4 In-situ EPR spectrum recorded during reduction of 42~ to
43~ at [1.40 V in DMF at 233 K and simulated EPR spectrum.
Simulation of 43~ used parameters in text with a Lorentzian linewidth
of 1.0 G.
3
3J \ 5.1 Hz, pyridine H and H ), 8.86 (4H, A B , J \ 4.8
a
a{
2 2 AB
Hz, *d \ 0.10 ppm, pyrrole protons of rings A and B), 8.88
AB
(4H, s, pyrrole protons of rings C and D), 8.22È8.24 (6H, m,
558
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ortho protons of Ph in Ph -porphH ), 8.19 (2H, d, 3J \ 5.5
under vacuum and residual solid NH Cl was removed by
washing with water. The product was dissolved in dichloro-
3
2
4
Hz, pyridine H and H ), 7.76È7.78 (9H, m, meta and para
b{
protons of Ph in Ph -porphH ), [2.79 (2H, s, central NH of
b
methane and dried over MgSO . The pure product was
3
2
4
porphyrin ring). Found: C, 84.0; H, 4.9; N, 11.3. Calc. for
obtained as a red-purple solid. Yield: 0.11 g (71.3%). 1H
C
H
N : C, 83.9; H, 4.8; N, 11.4%. LSIMS: m/z 616
NMR (400 MHz, d -acetone, see Chart 1 for ring and atom
43 29
5
6
([M]`).
labelling): d 9.64 (2H, d, 3J \ 6.5 Hz, H and H of porphyrin
a
a{
pyridine ring), 9.52 (2H, unresolved, H and H of 4,4@-bipyri-
e
e{
dine), 9.49 (2H, d, 3J \ 5.3 Hz, H and H of 4,4@-bipyridine)
a{
9.12 (2H, d, 3J \ 6.6 Hz, H and H of porphyrin pyridine
b{
ring), 9.00 (2H, d, 3J \ 4.7 Hz, H and H of 4,4@-bipyridine),
v{
N-Benzyl-4-pyridinium-4º-pyridine hexaÑuorophosphate, 2.
4,4@-Bipyridyl (3.75 g, 24 mmol, 2.0 equiv.) was dissolved in
warm toluene (100 ml) and heated with stirring to 80 ¡C.
Benzyl bromide (2.00 g, 12 mmol, 1.0 equiv.) was dissolved in
toluene (20 ml) and added to the mixture over a period of 3
hours. The reaction was then reÑuxed with stirring for 18
hours. The resulting yellow precipitate was Ðltered hot under
vacuum and left to air-dry. The residue was dissolved in warm
deionised water (40 ml) and a saturated solution of NH PF
a
b
v
8.94 (2H, d, 3J \ 4.7 Hz, H and H of 4,4@-bipyridine), 8.87È
b
b{
8.91 (4H, m, pyrrole protons of rings A and B), 8.81È8.83 (4H,
m, pyrrole protons of rings C and D), 8.22È8.26 (6H, m, ortho
protons of Ph in Ph -porphH ), 8.00 (4H, A B , J \ 8.2
3
2
2 2 AB
Hz, *d \ 0.12 ppm, phenyl ring E), 7.81È7.88 (9H, m, meta
AB
and para protons of Ph in Ph -porphH ), 7.64È7.67 (2H, m,
3
2
4
6
ortho protons of phenyl ring F), 7.48È7.50 (3H, m, meta and
para protons of phenyl ring F), 6.45 (2H, s, H and H ), 6.30
was added until no further precipitation was observed. The
o†-white precipitate was Ðltered o† under vacuum, washed
with deionised water and dried in a vacuum desiccator to give
the pure product. Yield: 2.15 g (45.4%). 1H NMR (300 MHz,
c
c{
(2H, s, H and H ), 6.17 (2H, s, H and H ), [2.74 (2H, s,
d
d{
e
e{
central NH of porphyrin ring). 13C NMR (125 MHz,
CD CO): d 65.3 (C[H ] ), 65.8 (C[H ] ), 66.5 (C[H ] ),
d -acetone cf. Chart 1 for ring and atom labelling): d 9.35 (2H,
3
c 2
d 2
e 2
6
M113.5, 123.1, 124.2N, 128.6 and 128.7 (meta and para CH of Ph
d, 3J \ 7.0 Hz, H and H ), 8.86 (2H, d, 3J \ 5.2 Hz, H and
a
a{
e
in Ph -porphH ), 129.2 and 129.3 (CH of pyrrole rings C and
H ), 8.68 (2H, d, 3J \ 6.6 Hz, H and H ), 7.99 (2H, d,
3
2
e{
b
b{
D), 129.9 (meta and para CH of Ph in Ph -porphH ), 131.0
(CH of phenyl ring F), 131.3 (CH of porphyrin pyridine ring),
131.8 (CH of phenyl ring F), 132.1 and 132.5 (CH of phenyl
3J \ 5.9 Hz, H and H ), 7.65È7.68 (2H, m, phenyl ring ortho
3
2
v
v{
positions), 7.49È7.52 (3H, m, phenyl ring meta and para
b
positions), 6.11 (2H, s, CH ). Found: C, 52.3; H, 3.7; N, 7.0.
2
ring E), M133.4, 134.2N, 134.4 and 135.1 (CH of pyrrole rings A
Calc. for C
m/z 247 ([M [ PF ]`).
H
N PF : C, 52.1; H, 3.9; N, 7.1%. LSIMS:
17 15
2
6
and B), 136.0 (ortho CH of Ph in Ph -porphH ), M136.3, 142.9,
3
2
6
143.0N, 144.5, 147.2 and 147.4 (CH of porphyrin pyridine ring
a
and CH ), M152.0, 152.3, 161.7N. Found: C, 58.4; H, 3.9;
a, e
5,10,15,20-[N-(4-Bromomethylbenzyl)-4-pyridyl]triphenyl-
porphyrin hexaÑuorophosphate 3. Compound 1, (0.33 g,
5.33 ] 10~4 mol, 1.0 equiv.) and a,a@-dibromo-p-xylene (1.67
g, 6.67 ] 10~3 mol, 12.5 equiv.) were dissolved in dry THF (70
ml) and heated to reÑux 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 NH PF
N, 6.9. Calc. for C
H
N P F
:
C, 58.3; H, 3.7; N,
68 52 7 3 18
7.0%. LSIMS: m/z 1402 ([M]`), 1256 ([M [ PF ]`), 1111
6
([M [ 2PF ]`).
6
4
6
was added until no further precipitation was seen. The
mixture was Ðltered under vacuum and allowed to air-dry.
The crude product was separated from the residual starting
materials by column chromatography, eluting with 1% meth-
anol in dichloromethane (2.5 l). The solvent was removed to
a†ord 3 as a purple solid. Yield: 0.14 g (28.2%). 1H NMR (300
MHz, CD CN cf. Chart 1 for ring and atom labelling): d 9.07
3
(2H, d, 3J \ 6.6 Hz, pyridine H and H ), 8.92 (4H, A B ,
a
a{
2 2
J
\ 4.8 Hz, *d \ 0.10 ppm, pyrrole protons of rings A
AB
AB
and B), 8.82È8.85 (4H, m, pyrrole protons of rings C and D),
8.77 (2H, d, 3J \ 6.6 Hz, pyridine H and H ), 8.17È8.21 (6H,
b
b{
m, ortho protons of Ph in Ph -porphH ), 7.98 (4H, A B ,
3
2
2 2
J
\ 7.4 Hz, *d \ 0.22 ppm, phenyl ring E), 7.78È7.82 (9H,
m, meta and para protons of Ph in Ph -porphH ), 6.00 (2H, s,
H and H ), 4.70 (2H, s, H and H ), [2.86 (2H, s, central
c{ d{
NH of porphyrin ring). Found: C, 64.8; H, 4.1; N, 7.6. Calc.
AB
AB
3
2
c
d
for C
H
N BrPF : C, 64.8; H, 4.0; N, 7.4%. LSIMS: m/z
51 37
5
6
800 ([M [ PF ]`), 616 ([M [ C H Br [ PF ]`).
6
8
8
6
5,10,15,20-[N-benzyl-Nº-(4-benzyl-4,4º-bipyridinium-4-
pyridyl)]triphenylporphyrin tris(hexaÑuorophosphate), 4. Com-
pound 3 (0.10 g, 1.06 ] 10~4 mol, 1.0 equiv.) and compound 2
(0.64 g, 1.63 ] 10~3 mol, 14.0 equiv.) were dissolved in dry
acetonitrile (50 ml). The mixture was stirred under reÑux 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 NH PF was
Chart 1 Atom labelling in diad 4.
Conclusions
In summary, we have described the design, synthesis and char-
acterisation of a new potentially photoactive donorÈacceptor
diad compound 4 and its precursors. In-situ UV/Visible and
EPR measurements have shown that the site of the Ðrst elec-
trochemical reduction is the benzylviologen component of the
molecule. The second reduction wave from cyclic voltammetry
4
6
added until no further precipitation was seen. The crude
product was collected by vacuum Ðltration and puriÐed by
column chromatography (methanol, NH Cl [2.0 M], nitro-
4
(aq)
methane; ratio of 7 : 2 : 1). The desired product was col-
lected as the major coloured band. Solvent was removed
New J. Chem., 2000, 24, 555È560
559

View Article Online
2
3
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.
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.
(a) R. J. McMahon, R. K. Force, H. H. Patterson and M. S.
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Am. Chem. Soc., 1994, 116, 10557.
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R. H. Felton and H. Linschitz, J. Am. Chem. Soc., 1966, 88, 1113.
M. Low, PhD thesis, University of Edinburgh, 1987.
D. Collison, F. E. Mabbs, E. J. L. McInnes, K. J. Taylor, A. J.
Welch and L. J. Yellowlees, J. Chem. Soc., Dalton T rans., 1996,
329.
was shown by in-situ EPR to comprise two unresolved one-
electron processes. The Ðrst of the two electron reduction pro-
cesses rendered the diad diamagnetic, as was shown by the
disappearance of the signal due to the benzylviologen radical.
The second of these processes gave rise to the appearance of a
new EPR signal, which was found to correspond to the
porphyrin radical. We believe this to be the Ðrst reported
resolved spectrum of a monoreduced porphyrin radical. This
is a signiÐcant step towards our ultimate aim of assembling
zeolite-occluded diads in order to attempt the photogenera-
tion of long-lived charge separated states.
4
5
6
Acknowledgements
7
8
9
We thank the EPSRC (UK) for Ðnancial support.
References
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560
New J. Chem., 2000, 24, 555È560