Synthesis and properties of a meso- tris-ferrocene appended zinc(ii) porphyrin and a critical evaluation of its dye sensitised solar cell (DSSC) performance

Article abstract of DOI:10.1039/c4ra03105a

A zinc(ii) porphyrin derivative, F3P, was prepared containing a single ferrocene group appended at three of the meso positions. The final meso position contains a benzoic acid unit, and was designed to be an anchoring moiety to a semiconductor surface. The cyclic voltammogram for the porphyrin derivative reveals that the three ferrocene units are oxidised and reduced at E _1/2 = +0.04 V vs. Fc⁺/Fc. Other redox responses are porphyrin-based at +1.1 V vs. Fc⁺/Fc (irreversible) and -1.87 V and -2.20 V vs. Fc⁺/Fc (both quasi-reversible). The room temperature Moessbauer spectrum for F3P comprises a doublet (δ Fe 0.44 mm s ^-1; ΔE_Q 2.35 mm s^-1; peak width 0.27 mm s^-1). The data are consistent with the ferrocene groups non-interacting with the porphyrin moiety in the ground state. Excitation of the compound in THF with an ultra-short laser pulse produces in less than 1 ps the charge separated state which comprises Fc⁺-porp^- and decays back to the ground state in 24 ps. Roughly the same behaviour is observed for the dye absorbed on TiO₂, the only slight difference is the appearance of a long-lived component in the decay records. The electron for the porphyrin radical anion does not inject into the conduction band of the TiO ₂, only the porphyrin excited state participates in any electron injection process. Partly because of this factor the performance of the dye attached to TiO₂ in a DSSC device is rather limited (J_SC = 0.068 mA cm^-2, V_OC = 283 mV, FF = 0.42, η = 0.008%). This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03105a

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Synthesis and properties of a meso- tris–ferrocene
appended zinc(II) porphyrin and a critical evaluation
of its dye sensitised solar cell (DSSC) performance†
Cite this: RSC Adv., 2014, 4, 22733
a
b
b
D. Sirbu, C. Turta, A. C. Benniston, F. Abou-Chahine,c H. Lemmetyinen,c
*
c
*
d
d
*
*
N. V. Tkachenko, C. Wood and E. Gibson
A zinc(II) porphyrin derivative, F3P, was prepared containing a single ferrocene group appended at three of
the meso positions. The final meso position contains a benzoic acid unit, and was designed to be an
anchoring moiety to a semiconductor surface. The cyclic voltammogram for the porphyrin derivative
reveals that the three ferrocene units are oxidised and reduced at E1/2 ¼ +0.04 V vs. Fc+/Fc. Other redox
responses are porphyrin-based at +1.1 V vs. Fc+/Fc (irreversible) and ꢀ1.87 V and ꢀ2.20 V vs. Fc+/Fc
(both quasi-reversible). The room temperature Mossbauer spectrum for F3P comprises a doublet (d Fe
¨
0.44 mm sꢀ1; DEQ 2.35 mm sꢀ1; peak width 0.27 mm sꢀ1). The data are consistent with the ferrocene
groups non-interacting with the porphyrin moiety in the ground state. Excitation of the compound in
THF with an ultra-short laser pulse produces in less than 1 ps the charge separated state which
comprises Fc+c–porpꢀc and decays back to the ground state in 24 ps. Roughly the same behaviour is
observed for the dye absorbed on TiO2, the only slight difference is the appearance of a long-lived
component in the decay records. The electron for the porphyrin radical anion does not inject into the
conduction band of the TiO2, only the porphyrin excited state participates in any electron injection
process. Partly because of this factor the performance of the dye attached to TiO2 in a DSSC device is
rather limited (JSC ¼ 0.068 mA cmꢀ2, VOC ¼ 283 mV, FF ¼ 0.42, h ¼ 0.008%).
Received 7th April 2014
Accepted 12th May 2014
DOI: 10.1039/c4ra03105a
www.rsc.org/advances
PDT sensitisers8 and dye sensitised solar cells (DSSC).9 In regard
to the latter case the direct conversion of sunlight to electricity
Introduction
is of major relevance because of its potential application to low-
power technologies and the large-scale manufacture of devices
by simple wet chemistry processing.10 The basic working of
DSSC relies on the injection of an excited state electron into the
conduction band of an appropriate n-type semiconductor.11 The
classic material used is TiO2. The hole le behind on the
sensitizer dye is plugged by a redox shuttle and the circuit is
completed by the electron from the conduction band reducing
via a secondary electrode the oxidised redox relay.12 Numerous
reports are available discussing the different processes in DSSC
and trying to identify the “bottle-neck” which is limiting the
efficiency of manufactured devices.13 There is one argument
that charge must be shied away from the semiconductor
surface rapidly to generate long-lived charge-separated
species.14 The analogy with natural photosynthetic reaction
centre complexes is clear,15 but like in Nature the required
assemblies to perform such task become more intricate. A
challenge is to identify rudimentary molecular systems that may
perform secondary charge shi.16 The most basic porphyrin-
based molecular system is exemplied in Fig. 1 incorporating
meso ferrocene groups. The cartoon represents the basic
working of the system following excitation of the porphyrin
moiety, where process 1 is oxidation of a ferrocene by the
Porphyrins and comparable derivatives are ubiquitous in nature
and constitute the molecular cornerstone of several light-driven
processes which help maintain life on Earth.1 The good cross-
section capture of visible photons for the chromophores,
coupled to their excellent redox excited-state behaviour are but
two reasons for the continued interest in porphyrin derivatives.2
Mimicry systems for articial photosynthesis applications is
certainly one major research area in which porphyrin deriva-
tives have a special place.3 The manufacture of molecular
dyads,4 triads and tetrads5,6 for propagating efficient, direc-
tional and long-lived charge separation continues to be of
intense interest. Coupled to such fundamental studies is the
application of porphyrin derivatives in areas such as sensors,7
aInstitute of Chemistry, Academy of Sciences of Moldova, 3, Academiei str., Chisinau,
MD-2028, Republic of Moldova
bMolecular Photonics Laboratory, School of Chemistry, Newcastle University,
Newcastle-upon-Tyne, NE1 7RU, UK
cDepartment of Chemistry and Engineering, Tampere University, Tampere, Finland
dSchool of Chemistry, The University of Nottingham, Nottingham, NG7 2RD, UK
† Electronic supplementary information (ESI) available: Additional molecular
orbital calculations, cyclic voltammogram and time-resolved spectra. See DOI:
10.1039/c4ra03105a
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Fig.
1 (Top) simple cartoon showing potential electron transfer
pathways following excitation of the zinc(II) porphyrin. (1) Charge
separation involving oxidation of a ferrocene; (2) charge recombina-
tion; (3) electron injection into conduction band of TiO2 from the
zinc–porphyrin excited state; (4) electron injection into conduction
band of TiO2 from the porphyrin radical anion. (Bottom) tris–ferrocene
zinc(II) porphyrin, F3P, containing the carboxylic acid anchoring unit.
excited state and process 2 is charge recombination. It could be
argued that if charge injection from the porphyrin excited state
(process 3) competed with process 1 then the system would
operate. Alternatively, if electron injection from the porphyrin
anion (process 4) was fast and efficient then the energy wasting
recombination process 2 would become less signicant. We are,
of course, not considering the recombination process involving
back electron transfer from the conduction band of the TiO2.
Evidently, knowledge of the intramolecular chromophore elec-
tron transfer events is critical. Prior work by Nadtochenko and
co-workers suggested for the free-base tetraferrocenyl porphyrin
that processes 1 and 2 occurred in around 200 fs and 17 ps,
respectively.17 There are no reports on the zinc(II) analogue. In
this work we consider the zinc(II) tris–ferrocenyl porphyrin F3P
and elucidation of its properties especially probing the rates for
processes 1 to 4. The dye attaches to TiO2 but performance in a
DSSC is limited.
Scheme 1 Reagents and conditions: (i) DCM, TFA, DDQ, RT; (ii)
Zn(OAc)2$2H2O, MeOH; (iii) NaOH, MeOH–H2O–THF, reflux.
aldehyde–pyrrole coupling reactions (e.g., 2 + 2, 4 + 1). However,
following very careful column chromatography on silica gel, 4
was isolated in 14% yield as a purple solid. The alternative
strategy involved the use of the preformed 5-ferrocenyldipyrro-
methane 3 (ref. 20) and its condensation with 2 and ferroce-
necarboxaldehyde. Despite what may appear to be a better
method the nal yield was only slightly improved to 18%,
probably because of the scrambling reactions. 1H NMR spectra
recorded for samples of 4 prepared by both methods were
identical. The nal two reactions of incorporation of zinc(II) into
the porphyrin ring and ester hydrolysis worked in high yields to
afford the desired compound F3P. Full characterisation of F3P
Results and discussion
Synthesis
13
by 1H-, C- and DEPT-135ꢁ NMR spectra, as well as two-
Preparation of symmetrical porphyrin derivatives is relatively
undemanding requiring only a suitable aldehyde to couple with
pyrrole in the presence of acid.18 Complexity is introduced when
the requirement is to have non-identical meso aryl groups, and
have them proximal or distal to each other in the case of bis-aryl
derivatives. Various methods to control the spatial location of
aryl-meso groups were introduced by Lindsey and co-workers.19
Since we only required the presence of a single aryl carboxylate
group, the simplest approach was to incorporate three ferrocene
groups. Two strategies were attempted for the preparation of
F3P via the free-base porphyrin and are illustrated in Scheme 1.
dimensional homo-(1H/1H COSY-45ꢁ) and heteronuclear
(1H/13C HSQC and HMBC) correlation spectra corroborated the
structure. The FTIR spectrum for F3P displayed a strong stretch
at 1690 cmꢀ1 in agreement with presence of the carboxylic acid.
The MALDI mass spectrum for F3P contained a cluster of peaks
at m/z ¼ 1044 and a theoretical pattern which agreed with the
observed data. There was an additional peak at m/z ¼ 1061
which can be assigned to a deprotonated water adduct.
Molecular orbital calculations
Certainly the most uncomplicated method is the condensation Previous work by Nemykin and co-workers21 determined the
of 2 with three equivalents of ferrocenecarboxaldehyde in the frontier molecular orbital picture for free-base tetra-ferrocene
presence of pyrrole and trace acid. The drawback with this porphyrin derivatives. Part of this study was to rationalize the
approach is the number of side-products formed by competing observation that as the number of ferrocene groups in
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Table 1 Comparison of selected bond lengths calculated by DFT for
F3Pa
Average Zn–N
bond length/A
Average Fe–C
bond length/A
˚
˚
Method
B3LYP 3-21G*
1.98
2.03
2.06
2.02
2.05 (free); 2.04 (meso)
2.06 (free); 2.06 (meso)
2.13 (free); 2.13 (meso)
2.03 (free); 2.04 (meso)
B3LYP 6-31G(d)
B3LYP LanL2DZ
B3PW91 6-31(d)
a
Calculated using the programme Gaussian 03.
meso(ferrocenyl) containing porphyrins increased there are
subtle shis in both the Q- and B-bands in their absorption
spectra. Part of the reason is introduction of HOMOs associated
with the ferrocene units into the overall MO picture, and
potential charge transfer contributions to the absorption
envelope. Considering these previous ndings we felt it was
prudent to map out a clear MO picture for F3P. For a typical
calculation the ground-state structure of F3P was modelled
using a semi-empirical (AM1) method. The obtained structure
was then rened further using DFT (B3LYP) and the 3-21G* and
6-31G(d) basis sets. Prior work by Slota et al.22 suggested that
B3LYP with the LanL2DZ basis set is more conducive to col-
lecting accurate structures for large metalloporphyrin mole-
cules. The alternative employed by Nemykin et al.23 was the
Perdew nonlocal correlation function (B3PW91). Hence, calcu-
lations were re-run under more complex basis sets for
comparison purposes (Table 1). There is a denite increase in
the Zn–N bond lengths (ca. 2%–4%) by changing the basis set
from 3-21G* to 6-31G(d) to LanL2DZ and to B3PW91 6-31(d);
values for the latter three methods are more consistent with
Fig. 3 Selected molecular orbitals for F3P calculated by DFT (B3PW91)
and using a 6-31G(d) basis set.
modied depending on the method and a representative struc-
ture is shown in Fig. 2. Consistent with previous calculations is
the observed “bowing” of the porphyrin ring. And the ferrocene
units, because of unfavourable steric interactions, are twisted
relative to the plane of the porphyrin ring. The barrier to rotation
of the ferrocene group around the connector meso axis has been
previously estimated to be in the order of 0.9 kcal molꢀ1, and so
in solution ferrocene internal rotation is very likely.24
From the four calculations the two most consistent molec-
ular orbital diagrams were obtained using B3PW91 6-31(d),
B3LYP 6-31G(d) and B3LYP LanL2DZ; in the latter the basis set
core electron are frozen (48 for F3P). The frontier molecular
orbital picture calculated by B3LYP 3-21G* was slightly different
to the other two cases. We have taken the MO picture from the
B3PW91 6-31(d) calculation (Fig. 3) to be a more comprehensive
picture, and collected in ESI† are MO pictures from the other
three calculations. The HOMO is predominantly localised on
the three ferrocene moieties, whereas the HOMOꢀ1 is exclu-
sively porphyrin based. The MOs HOMOꢀ2 to HOMOꢀ6 are
entirely ferrocene centred. In turns out that the LUMO and
LUMO+1 are degenerate and porphyrin-based p* orbitals in
agreement with Gouterman's four orbital model.25 Both orbitals
are well separated in energy from the LUMO+2 which resides on
the meso benzoic acid group. Very crudely the HOMO–LUMO
gap represents ferrocene-to-porphyrin charge transfer, the
energy difference being 2.59 eV (479 nm).
˚
crystallographically determined bond lengths (Zn–N 2.04 A). It
is very noticeable that the average Fe–C (ferrocene) bond
lengths calculated using B3LYP (LanL2DZ) are longer than
corresponding values returned from the other methods. The
average crystallographically determined Fe–C bond length for
˚
ferrocene is 2.05 A. The molecular picture for F3P is only slightly
Electrochemistry and absorption spectroscopy
The redox behaviour for F3P was measured by cyclic voltam-
metry in dry THF using 0.2 M TBATFB as background electro-
lyte. DCM afforded rather poor results probably because of
solubility problems. The cyclic voltammogram (ESI†) reveals
redox processes associated with both the porphyrin and ferro-
cene groups. On the oxidation side of the CV a clear quasi-
reversible wave is observed at +0.04 V vs. Fc+/Fc, which is
associated with redox at the ferrocene sites. No splitting of the
Fig. 2 Energy-minimised computer calculated structure for F3P using
DFT (B3PW91) and a 6-31G(d) basis set.
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Fig. 4 Room temperature UV-vis absorption spectrum for F3P in
chloroform.
Fig. 5 Room temperature 57Fe-Mossbauer spectrum for F3P showing
¨
the characteristic doublet pattern.
wave is seen but the peak's separation difference of 180 mV
would suggest that there are a series of overlapping waves. At a
more positive potential an irreversible wave is observed at +1.1 V
vs. Fc+/Fc which is porphyrin-based oxidation. Upon scanning
to negative potentials two quasi-reversible one-electron waves
are seen at ꢀ1.87 V and ꢀ2.20 V vs. Fc+/Fc and are again asso-
ciated with redox at the porphyrin site. The overall redox
behaviour for F3P is consistent with previous work carried out
on ferrocenyl–porphyrin derivatives.21,26 It is worth noting that
the computer calculated difference in energy between oxidation
of a ferrocene (HOMO) and porphyrin (HOMOꢀ1) is only 380
mV. The much larger value found electrochemically (ca. 1 V) is
rationalised, in part, by the large electrostatic repulsion created
by a tri-positive charge so close to the porphyrin core.
The room temperature electronic absorption spectrum for
F3P in chloroform is shown in Fig. 4. The spectrum comprises a
strong Soret B-band at 431 nm (1.6 ꢂ 105 Mꢀ1 cmꢀ1) typical of a
divalent metalloporphyrin and a much weaker Q-band at 661
nm (1.7 ꢂ 104 Mꢀ1 cmꢀ1). The weak band at 310 nm (2.1 ꢂ 104
Mꢀ1cmꢀ1) may be assigned to the lesser discussed porphyrin N-
band. The Soret band also contains a shoulder in the region of
ꢃ490 nm which is ascribed by Nemykin et al. to predominantly
ferrocene-to-porphyrin charge transfer. Certainly this interpre-
tation is consistent with the computer calculated energy-gap
discussed previously. The absorption in the visible spectrum of
the porphyrin is improved in the region between Soret and Q
bands compared to the tetra-phenyl porphyrin analogue.27
Table 2 Experimental Mossbauer parameters for F3P and reference
¨
compounds at room temperaturea
Compound
d, mm sꢀ1
DEQ, mm sꢀ1
G, mm sꢀ1
ZnTFP
F3P
Fc
0.45
0.44
0.44
2.55
2.35
2.35
Na
0.27
Na
a
Reference compounds as discussed in the text taken from ref. 28
and 29.
are essentially identical. It is worth noting that no ferrocenium
assignable signals are observed, which would be present if
partial oxidation had occurred. At least in the ground state and
solid state the compound F3P behaves as if it comprises isolated
ferrocene and porphyrin subunits.
Dye sensitised solar cell performance
Since the Iꢀ/I3ꢀ electrolyte couple does not have enough driving
force to reduce the ferrocenium, photovoltaic experiments were
conducted in a cell using Co(II/III)tris(4,40-di-methoxy-bipyridine)
perchlorate (E0 ¼ ꢀ0.26 V vs. Fc+/Fc). The electrolyte consisted of
0.1 M Co(II/III)tris(4,40-di-methoxy-bipyridine) perchlorate, 0.1 M
4-tert-butylpyridine, 0.2 M, tert-butyl ammonium perchlorate
and 0.015 M NOPF6 in propylene carbonate as solvent. The plot
of photocurrent density versus voltage (under standard AM 1.5
global sunlight at 1000 W mꢀ2 and a temperature of 298 K) for
F3P is shown in Fig. 6. The experiment was repeated in darkness
and proved the photovoltaic nature of the effect. The short
circuit current density was found to be 0.068 mA cmꢀ2, the open
circuit voltage was 283 mV, the ll factor was 0.42 and the power
conversion efficiency (h) was evaluated to be 0.0081%. Each of
these values are extremely low and F3P can be considered a poor
sensitiser for a DSSC.
57
¨
Fe Mossbauer spectroscopy
¨
The room temperature Mossbauer spectrum for F3P is shown in
Fig. 5 with pertinent parameters presented in Table 2. Also
included are data for the reference compound zinc(II) tetra-
ferrocenylporphyrin (ZnTFP).28 The isomer shi d and large
quadrupole splitting DEQ for the clear doublet are typical for
low-spin iron (d6) ferrocene derivatives. The comparison of F3P
and unsubstituted ferrocene29 data shows that the porphyrin
core has no major inuence on the electron density at the iron
nucleus, despite the close proximity of the ferrocene and
Pump-probe spectroscopy
porphyrin groups. The presence of only one doublet and the The excited-state property for F3P was elucidated using femto-
narrow line width, G, indicates that the three ferrocene moieties second pump-probe spectroscopy in MeCN and THF. The latter
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Fig. 6 Example of an I–V curve (B ¼ dark, C ¼ light) for a F3P based
dye sensitized solar cell using Co(II) (4,40-dimethoxy-2,20-bipyridine)3
(ClO4)2 0.1 M, 4-tert-butylpyridine 0.2 M, tert-butyl ammonium
perchlorate 0.1 M and NOPF6 0.015 M in propylene carbonate. VOC
¼
0.28 V and JSC ¼ 0.068 mA cmꢀ2
.
solvent was found to be much better and avoided unnecessary
precipitation of the material during data collection. The
compound was excited with a 70 fs laser pulse at 480 nm which
almost exclusively populates the S2 (Soret) state for the
porphyrin. Time resolved transient absorption spectra with
compensated group velocity dispersion are presented in Fig. 7,
along with decay proles at a few selected wavelengths. The
measurements were also tted globally to three and four expo-
nential models. Although a four exponential t gave roughly a
15% improvement of the sigma value it mainly resulted in a very
short-lived component (80 fs) which arises most probably from
“non-exponential” fast thermal relaxation and will not be dis-
cussed further.
There is an almost instantaneous bleach in the region
around 660 nm corresponding to the Q-bands of the porphyrin.
To the low energy side of this bleach is a broad prole which is
assigned to the zinc–porphyrin radical anion.30 It is not possible
to identify the ferrocenium ion because of its low molar
absorption coefficient, but it must be produced as part of the
charge transfer process to account for the anion. The results
from a global t of the spectroscopic data are presented in
Fig. 8, and shows the decay component spectra (curves with
symbols) and time-resolved transient spectra right aer the
excitation (at 0 ps) and aer fast relaxation with 0.19 ps time
constant (at 1 ps). An interesting feature is that initial bleaching
of the Q-band at 670 nm recovers by roughly half with a time
constant of 0.19 ps, but then the bleaching increases by one
third with a time constant 2.5 ps before complete recovery with
a time constant of 24 ps. The interpretation of the results is not
straightforward, though it is noticeable that the 2.5 ps compo-
nent does not affect the spectrum much outside the Q-band
region. Tentatively this component can be attributed to
conformation changes associated with the charge separated
(CS) state. The fast time constant is presumably formation of
the CS state. The S2 to S1 conversion in zinc tetraphenylpor-
Fig. 7 Selected transient absorption spectra recorded after excitation
of F3P in THF with a 70 fs laser pulse delivered at 480 nm (top).
Transient absorption decay profiles of F3P in THF at selected wave-
lengths (bottom). Symbols are measured data and lines are fit curves.
There are reports of energy and electron transfer from the S2
state of zinc porphyrin adducts.31 Considering the rate for
charge separation we cannot discount that the process is
competing with internal conversion. The charge recombination
process though slower by comparison is still extremely rapid
and reforms the ground state in s ¼ 24 ps. This measured value
is very similar to that reported by Nadtochenko and co-workers
for a free-base porphyrin analogue.17 The presence of the zinc(II)
ion appears to have no real affect on the charge recombination
process.
Fig. 8 Decay component spectra (lines with symbols) and time-
phine is, depending on the solvent, on the order of 1–3.5 ps. resolved spectra at 0 and 1 ps delay time for F3P in THF.
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possible reason for this is the rather strong coupling between
the electron and hole due to the short distance between the
donor and acceptor.33 This interpretation would mean that the
CS state is not a “real” radical ion pair, but rather an electron
density redistribution. It is noted that the driving force is very
favourable for anion injection and estimated from electro-
chemistry data (Ered ¼ ꢀ1.5 V vs. NHE) and the conduction band
energy for TiO2 (ꢀ0.5 V vs. NHE) to be around 1.0 eV. There is
the possibility that the reduction potential for the porphyrin is
altered by its attachment to the TiO2 and the driving force is
much smaller.
Considering that direct electron injection from the
porphyrin excited state (process 3, Fig. 1) occurs, it is very
probable that hole transfer to the appended ferrocene (process
1, Fig. 1) also takes place to generate the ferrocenium ion. Given
that the Co2+/Co3+ couple for the cobalt bipyridyl- and terpyr-
idyl-based electrolytes have redox potentials of ꢀ0.21 V and
ꢀ0.32 V vs. Fc+/Fc,34 then again it is feasible that the redox
shuttle process should proceed in a DSSC incorporating F3P.
Other contributing factors to the very low photocurrent are
twofold. One evident problem is the close proximity of the
ferrocene groups to the semi-conductor surface which will
facilitate charge recombination. Even the presence of a blocking
layer on the TiO2 surface may not be sufficient to prevent back
electron transfer. There is the additional argument that ferro-
cenium/ferrocene may not be conducive for coupling to the
cobalt(II/III) electrolyte redox relay because of slow electron
exchange kinetics.35
Fig. 9 Decay component spectra obtained from global fit of transient
absorption data for F3P on TiO2.
The behaviour for F3P attached to TiO2 was also measured
and compared to a control compound containing phenyl groups
instead of the ferrocene moieties (ZnTPP). Time-resolved tran-
sient absorption spectra are presented in ESI,† and except for a
relatively weak long-lived component the results are similar to
those obtained in THF solution. The rather good agreement
between the transient absorption responses for F3P in solution
and attached to TiO2 indicates that charge separation (CS) in
the molecule is not affected by the presence of TiO2 surface.
Another signicant nding is electron injection from the
porphyrin anion to the TiO2 does not take place in the lifetime
of the CS state (29 ps). It is very noticeable (Fig. 9) that the minor
long-lived component has clear bleaching of the porphyrin Q-
band and probably comes from direct injection of electron from
the photo-excited porphyrin into the TiO2. This is the only
indication of possible electron injection and its low intensity is
in good agreement with the low efficiency of the DSSC.
The decay component spectra for ZnTPP attached to TiO2 are
very different to those shown in Fig. 9 (see ESI†). The fast
component, 0.13 ps, can be attributed to electron injection into
the TiO2 because of the broad negative band around 670 nm,
which is assigned to the porphyrin cation. Further re-shaping of
the cation band takes place with a time constant of 3.1 ps and is
in qualitative agreement with other similar systems. Recombi-
nation is also a “multi-exponential” process with lifetimes in
the range from few tens of picoseconds to nanoseconds.
Conclusions
In the preparation of the trisubstituted ferrocene-based
porphyrin derivative it would appear that the “one-pot” stoi-
chiometry-correct reaction works as well as the preorganised
method starting from a ferrocene–dipyrromethane precursor.
The only advantage of the second method is the slightly easier
purication of the nal product. The nal zinc(II) compound is
an analogue of meso-ferrocenyl porphyrins recently prepared by
Rochford et al.36 for evaluation as sensitizers in DSCC. Although
the group published no measurements our studies would
suggest that performance will be poor especially for derivatives
where ferrocenes are close to the anchoring carboxylic acid
group. However, it will be interesting to see if an improved
response is observed for compounds where the ferrocene is
trans to the carboxylic acid anchoring unit. The ferrocenium
produced would be away from the semiconductor surface and
we could expect charge recombination to be much slower.
Evaluation of excited state deactivation for DSSC application
The excited state spectroscopic data for F3P attached to TiO2 Despite this our results clearly show that only direct photo-
shows unequivocally that the dye is rather poor at injecting injection from the porphyrin excited state is relevant for the
electrons into the conduction band of the semiconductor TiO2. outlined ferrocene–porphyrin derivatives. Hence in the design
Generally electron injection into TiO2 is extremely fast and in of porphyrin-based systems for DSSC applications strongly
the order of 300 fs.32 For the control compound the excited state coupled donor–acceptor pairs should be avoided.
porphyrin injects electrons in around 130 fs. Assuming this
latter value is the same for F3P the maximum yield of electrons
Experimental
into the TiO2 conduction band is only 60%. It is clear that
injection of an electron from the photo-generated porphyrin 1H-, C- and DEPT-135ꢁ NMR spectra, as well as two-dimen-
13
anion (Fig. 1) into the TiO2 conduction does not take place. One sional homo-(1H/1H COSY-45ꢁ) and heteronuclear (1H/13C
22738 | RSC Adv., 2014, 4, 22733–22742
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HSQC and HMBC) correlation spectra were recorded with were prepared with an area of 0.25 cm2 by screen printing two
Bruker – Avance-III 400 MHz and Jeol ECS-400 MHz. Chemical layers of transparent colloidal TiO2 paste (Solaronix, Ti-Nano-
shis for 1H- and 13C-NMR spectra are referenced relative to xide T/SP) followed by one layer of a light-scattering TiO2 paste
TMS or the residual protiated solvent. IR spectra were recorded (Solaronix, Ti-Nanoxide R/SP) and heated at 450 ꢁC in an air
with
a Perkin Elmer Spectrum 100 FT-IR spectrometer. atmosphere in an oven (Nabertherm LE 14/11/B150) for 30
Elemental analysis was performed on an Elemental Vario EL3 minutes between deposition steps. The conducting glass
machine. Atomic absorption spectra were measured using an substrates were post-treated by immersion in a 40 mM aqueous
AAS-3 machine. Electronic absorption spectra were recorded TiCl4 solution at 70 ꢁC for 25 minutes and then washed with
using a Hitachi U3310 spectrophotometer. The 57Fe-Mossbauer water. The electrodes were gradually heated in an oven
¨
ꢁ
spectrum was acquired at room temperature (RT) using a (Nabertherm LE 14/11/B150) at 450 C in an air atmosphere.
conventional spectrometer in the constant-acceleration mode
TiO2 lms were immersed in a dye bath containing 0.28 mM
(MS4, Edina, USA) equipped with a 57Co source (3.7 GBq) in a F3P and 0.60 mM chenodeoxycholic acid in DCM and le at RT
rhodium matrix. Isomer shis are given relative to a-Fe at RT. for 18 h. The lms were then rinsed in ethanol to remove excess
¨
The spectrum was tted using the Mossbauer tting program dye and dried at RT. Solar cells were assembled with a thermally
(Edina). platinized counter electrode (Pilkington TEC8, sheet resistance
Cyclic voltammetry experiments were performed using a 8 U per square) using a 30 mm thick thermoplastic frame
fully automated HCH Instruments Electrochemical Analyzer (Dyesol, Surlyn 1702). The electrolyte solution, containing either
and a three electrode set-up consisting of a glassy carbon 1-propyl-3-methylimidazolium iodide 0.8 M, tert-butyl pyridine
working electrode, a platinum wire counter electrode and an Ag/ 0.3 M, iodine 0.1 M, and guanidinium thiocyanate 0.05 M in
AgCl reference electrode. Ferrocene was used as an internal methoxypropionitrile (data not shown), or Co(4,4-dimethoxy-
standard. All studies were performed in deoxygenated DCM or 2,20-bipyridine)3 (ClO4)2 0.1 M, 4-tert-butylpyridine 0.2 M, tert-
THF containing TBATFB (0.2 M) as background electrolyte. butyl ammonium perchlorate 0.1 M, NOPF6 0.015 M, in
Redox potentials were reproducible to within ꢄ15 mV.
propylene carbonate, was introduced through the pre-drilled
Femto- to pico-second time-resolved absorption spectra were hole in the counter electrode, which was sealed aerwards with
collected using a pump-probe apparatus consisting of a Libra F- Surlyn and a glass cover slide.
1K (Coherent Inc.) generator producing 100 fs pulses with 1 mJ
energy at 800 nm with a repetition rate of 1 kHz, and a Topas-C
(Light Conversion Ltd.) OPA providing pump pulses in a wide
spectrum range (UV-visible-near IR). The measurements part for
the pump-probe method was a ExciPro (CDP Inc., Russia)
equipped with two array detectors coupled to a spectrometer
(CDP2022i) for probe detection in the visible (300–1000 nm) and
near IR (900–1700 nm) ranges. A white light continuum was
used as the probe and was generated in a sapphire plate, and
the overall time resolution of the instrument was 100–200 fs
depending on the sample type and wavelength range.
Computer calculations
Computational calculations were performed using a 32-bit
version of Gaussian 03 (ref. 37) on a quadruple-core Intel Xeon
system with 4 GB RAM. The calculations were run in parallel,
fully utilising the multi-core processor. Energy minimisation
calculations were monitored using Molden and run in parallel
with frequency calculations to ensure optimised geometries
represented local minima.
Synthesis
Time-resolved transient absorption data were manipulated
using the freely available soware package, Dect. In a typical All chemicals were purchased from commercial sources and
analysis the whole collection of differential absorption spectra used as received unless otherwise stated. Basic solvents for
was inspected over the full timescale, and decay kinetics were synthesis were dried using literature methods. Solvents for
obtained in the whole measured wavelength range with 5 nm spectroscopic investigations were of the highest purity
step by averaging spectra in 5 nm intervals at each delay time. available.
Lifetimes were obtained by global t of the whole data set. Best
Preparation of methyl-4-formylbenzoate dimethyl acetal (1).
ts were judged by the usual methods of remaining residuals Concentrated H2SO4 (3.2 mL, 60 mmol, 1.5 eq.) was slowly
and sigma value. Current–voltage (I–V) characteristics were added to a solution of 4-formylbenzoic acid (6 g, 40 mmol) in
measured under 1000 W mꢀ2 and 100 W mꢀ2 AM 1.5G illumi- MeOH–CHCl3 (130 mL/30 mL), and the resulting mixture was
nation using a Newport solar simulator (model 91160) and a reuxed for 4 h. During the reaction water was removed by
Keithley 2400 source per meter.
azeotropic distillation. The resulting reaction mixture was le
to cool to room temperature, poured into water (100 mL) and
extracted with CHCl3 (100 mL). The separated organic layer was
washed with aqueous NaHCO3 (10%, 2 ꢂ 50 mL) and distilled
Solar cell preparation
Fluorine-doped tin oxide (FTO) glass substrates (Pilkington, water (2 ꢂ 50 mL). The separated organic layer was dried over
TEC15, sheet resistance 15 U per square) were cleaned in an anhydrous Na2SO4, ltered and the solvent removed in vacuo to
ultrasonic bath overnight using (in order) ethanol, water and give 1 (6 g, 29 mmol, 71% yield). 1H-NMR (400 MHz, CDCl3,
ethanol. The conducting glass substrates were pretreated by TMS, d): 8.04 (d, J ¼ 8.2 Hz, 2H, o-Ph), 7.53 (d, J ¼ 8.2 Hz, 2H, m-
ꢁ
immersion in a 40 mM aqueous TiCl4 solution at 70 C for 25 Ph), 5.44 (s, 1H, formyl), 3.91 (s, 3H, methyl ester), 3.32 (s, 6H,
minutes and then washed with water. Mesoporous TiO2 lms dimethyl acetal).
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When TLC analysis indicated that all ferrocenecarboxaldehyde
was consumed (1.5 h) DDQ (0.68 g, 3.0 mmol, 1 eq.) was added
and the mixture reuxed for another 1.5 h. Aer this period the
reaction mixture was deposited on silica-gel and chromato-
graphed using CH2Cl2–MeOH–(Et)3N (100 : 4 : 1) mixture as
eluent. The obtained crude product was puried by two
consecutive column chromatography on silica-gel using
toluene–(Et)3N (50 : 1) and toluene–petroleum ether (40–60 ꢁC)–
(Et)3N (15 : 10 : 1) mixtures as eluent. The major green-brown
fraction was collected, solvent reduced in vacuo and the
precipitate recrystallized from toluene–hexane mixture to give 4
(0.41 g, 0.41 mmol, 14% with respect to 2) as a purple micro-
crystalline powder. 1H-NMR (400 MHz, CDCl3, TMS, d): 9.76 (ap
q, J ¼ 4.8 Hz, 4H, b-pyrrole–Fc–Fc), 9.66 (d, J ¼ 4.8 Hz, 2H, b-
pyrrole–Ph–Fc), 8.53 (d, J ¼ 4.8 Hz, 2H, b-pyrrole–Ph–Fc), 8.43
(d, J ¼ 8.1 Hz, 2H, m-Ph), 8.23 (d, J ¼ 8.1 Hz, 2H, o-Ph), 5.40 (ap t,
J ¼ 1.7 Hz, 6H, a-Cp1,2,3), 4.80 (ap t, J ¼ 1.7 Hz, 2H, b-Cp2), 4.77
(ap t, J ¼ 1.7 Hz, 4H, b-Cp1,3), 4.12 (s, 3H, methyl ester), 4.02 (s,
15H, CpH1,2,3), ꢀ1.12 (s, 2H, NH). 13C NMR (101 MHz, CDCl3, d):
167.51 (C-carboxylic), 147.15 (i-Ph), 146.0–147.0 (a-pyrrole–Ph–
Fc, vbr), 145–147 (a-pyrrole–Fc–Fc, vbr), 143.5–144.5 (a-pyrrole–
Ph–Fc, vbr), 134.48 (o-Ph), 131.47 (b-pyrrole–Ph–Fc), 131.18 (b-
pyrrole–Fc–Fc), 130.55 (b-pyrrole–Fc–Fc), 129.76 (b-pyrrole–Ph–
Fc), 129.57 (p-Ph), 128.15 (m-Ph), 118.53 (meso-C–Fc2), 118.23
(meso-C–Ph), 118.00 (meso-C–Fc1,3), 89.80 (Ci-Cp2), 89.16 (Ci–
Cp1,3), 77.60 (a-Cp2), 77.11 (a-Cp1,3), 70.60 (CpH), 69.29 (b-
Cp1,3), 69.22 (b-Cp2), 52.54 (C-methyl ester).
Preparation of Zn(II)-5,10,15-trisferrocenyl-20-(methyl-4-
benzoate) porphyrin (5). A solution of Zn(OAc)2$2H2O (220 mg,
1.0 mmol, 10 eq.) in MeOH (8 mL) was added to a solution of
free base 4 (100 mg, 0.1 mmol) in benzene (50 mL) with stirring
at RT. When TLC analysis indicated that all starting material
was consumed (2 h) the reaction mixture was washed with
distilled water (3 ꢂ 50 mL), the solution dried over anhydrous
Na2SO4, ltered and solvent removed in vacuo to give 5 (90 mg,
85 mmol, 85%) as a purple microcrystalline powder. 1H-NMR
(400 MHz, CDCl3, TMS, d): 10.01 (ap q, J ¼ 4.8 Hz, 4H, b-pyrrole–
Fc–Fc), 9.92 (d, J ¼ 4.7 Hz, 2H, b-pyrrole–Ph–Fc), 8.65 (d, J ¼ 4.7
Hz, 2H, b-pyrrole–Ph–Fc), 8.41 (d, J ¼ 8.4 Hz, 2H, m-Ph), 8.24 (d,
J ¼ 8.4 Hz, 2H, o-Ph), 5.45 (ap t, J ¼ 1.8 Hz, 2H, a-Cp2), 5.43 (ap t,
J ¼ 1.9 Hz, 4H, a-Cp1,3), 4.81 (ap t, J ¼ 1.8 Hz, 2H, b-Cp2), 4.78
(ap t, J ¼ 1.8 Hz, 4H, b-Cp1,3), 4.10 (s, 3H, methyl ester), 4.091 (s,
10H, CpH1,3), 4.090 (s, 5H, CpH2). 13C NMR (101 MHz, CDCl3,
d): 167.58 (C-carboxylic), 150.68 (a-pyrrole–Ph–Fc), 150.13 (a-
pyrrole–Fc–Fc), 149.76 (a-pyrrole–Fc–Fc), 148.31 (a-pyrrole–Ph–
Fc), 147.98 (i-Ph), 134.45 (o-Ph), 132.70 (b-pyrrole–Ph–Fc),
131.92 (b-pyrrole–Fc–Fc), 131.77 (b-pyrrole–Fc–Fc), 130.34 (b-
pyrrole–Ph–Fc), 129.44 (p-Ph), 128.01 (m-Ph), 119.46 (meso-C–
Fc2), 119.28 (meso-C–Ph), 118.81 (meso-C–Fc1,3), 90.70 (Ci–Cp2),
90.51 (Ci–Cp1,3), 77.72 (a-Cp2), 77.36 (a-Cp1,3), 70.65 (CpH),
68.90 (b-Cp1,2,3), 52.47 (C-methyl ester). Elemental analysis
calcd for C58H42Fe3N4O2Zn: C 65.72, H 3.99, N 5.29, found: C
66.60, H 4.25, N 5.15.
Preparation of methyl-4-formylbenzoate (2). A mixture of 1 (6
g, 28.6 mmol) and p-toluenesulfonic acid monohydrate (5.4 g,
28.4 mmol, 1 eq.) in acetone (150 mL) was stirred at RT. When
TLC analysis indicated that all the starting material was
consumed (ca. 20 h) the reaction mixture was poured into
CHCl3 (200 mL). The solution washed with aqueous NaOH
(10%, 3 ꢂ 100 mL) and distilled water (100 mL). The organic
layer was separated, dried over anhydrous Na2SO4, ltered and
the solvent removed in vacuo to give 2 (4.6 g, 28.0 mmol, 98%).
1H-NMR (400 MHz, CDCl3, TMS, d): 10.10 (s, 1H, formyl), 8.20
(d, J ¼ 8.1 Hz, 2H, o-Ph), 7.95 (d, J ¼ 8.1 Hz, 2H, m-Ph), 3.96 (s,
3H, methyl ester).
Preparation of 5-ferrocenyldipyrromethane (3). A solution of
ferrocenecarboxaldehyde (4 g, 18.7 mmol) dissolved in pyrrole
(50 mL, 0.75 mol, 40 eq.) was bubbled with Ar for 15 min. TFA
(0.14 mL, 1.82 mmol, 0.1 eq.) was added and the solution was
stirred at RT in an Ar atmosphere. Aer 15 min aqueous NaOH
(0.1 M, 23 mL) was added to stop the reaction. The mixture was
poured into water (40 mL) and extracted with ethyl acetate (60
mL). The collected organic layer was washed with distilled water
(2 ꢂ 60 mL), separated and dried over anhydrous Na2SO4,
ltered and solvent removed in vacuo. The obtained crude
product was puried by chromatography on silica-gel using a
petroleum ether (40–60 ꢁC)–ethyl acetate (9 : 1) mixture as
eluent, collecting the rst brown coloured band. The solvent
was removed in vacuo to give 3 (4.9 g, 14.8 mmol, 79%). 1H-NMR
(400 MHz, CDCl3, TMS, d): 7.87 (br s, 2H, NH), 6.64 (ddd, J ¼ 2.6,
0
2.6, 1.6 Hz, 2H, H2 + H2 –pyrrole), 6.16 (dd, J ¼ 5.9, 2.7 Hz, 2H,
0
0
H3 + H3 -pyrrole), 6.02 (tdd, J ¼ 2.3, 1.6, 0.7 Hz, 2H, H4 + H4 –
pyrrole), 5.20 (s, 1H, methane), 4.16 (ap t, 2H, a-Cp), 4.09 (s, 5H,
CpH), 4.08 (ap t, 2H, b-Cp). 13C NMR (101 MHz, CDCl3, d):
0
0
133.29 (C5 + C5 –pyrrole), 116.94 (C2 + C2 –pyrrole), 108.12 (C3 +
0
0
C3 –pyrrole), 106.67 (C4 + C4 –pyrrole), 90.33 (Cpipso), 69.03
(CpH), 68.16 (b-Cp), 67.69 (a-Cp), 38.26 (methane).
Preparation of 5,10,15-trisferrocenyl-20-(methyl-4-benzoate)
porphyrin (4)
(a) Synthesis from 5-ferrocenyldipyrromethane. A mixture of 2
(0.25 g, 1.5 mmol), ferrocenecarboxaldehyde (0.32 g, 1.5 mmol,
1 eq.), 3 (1.0 g, 3.0 mmol, 2 eq.) and TFA (0.09 mL, 1.2 mmol, 0.8
eq.) in CH2Cl2 (320 mL) was stirred at RT under an Ar atmo-
sphere. When TLC analysis indicated that all ferrocenecarbox-
aldehyde was consumed (ꢃ2 h), DDQ (0.34 g, 1.5 mmol, 1 eq.)
was added and the mixture was reuxed for another 1.5 h. Aer
this period the reaction mixture was deposited on silica-gel and
chromatographed using CH2Cl2–MeOH (100 : 3) mixture as
eluent. The obtained crude product was puried by column
chromatography separations on silica-gel using toluene–petro-
leum ether (40–60 ꢁC)–(Et)3N (15 : 10 : 1) mixture as eluent. The
major green-brown fraction was collected, solvent reduced in
vacuo and the precipitate was recrystallized from toluene–
hexane mixture to give 4 (0.27 g, 0.27 mmol 18%) as a purple
microcrystalline powder.
(b) Direct condensation of aldehydes. A mixture of 2 (0.49 g, 3
mmol), ferrocenecarboxaldehyde (2.12 g, 9.9 mmol, 3.3 eq.),
pyrrole (1 mL, 14.4 mmol, 4.8 eq.) and TFA (0.2 mL, 2.6 mmol,
0.87 eq.) in CH2Cl2 (250 mL) was stirred at RT in Ar atmosphere.
Preparation of Zn(II)-5,10,15-trisferrocenyl-20-(4-carboxy-
phenyl) porphyrin (F3P). NaOH (53 mg, 1.33 mmol, 20 eq.) in
MeOH (8 mL) and H2O (2 mL) was added to a solution of ester 5
(70 mg, 66 mmol) in THF (8 mL). The mixture was reuxed for
22740 | RSC Adv., 2014, 4, 22733–22742
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12 h under Ar atmosphere, than le for cooling over night at RT.
CHCl3 (70 mL) and water (70 mL) were added and mixture
acidied to pH ¼ 6 with 2.0 M HCl. The resulting green/brown
colored organic layer was separated, washed with water (3 ꢂ 50
mL), dried over anhydrous Na2SO4, ltered, solvent removed in
vacuo and the precipitate recrystallized from toluene–hexane
mixture to give 6 (51 mg, 49 mmol, 74%) as a purple micro-
crystalline powder. 1H-NMR (400 MHz, DMSO-d6, d): 9.86 (ap t, J
¼ 5.3 Hz, 4H, b-pyrrole–Fc–Fc), 9.77 (d, J ¼ 4.7 Hz, 2H, b-
pyrrole–Ph–Fc), 8.52 (d, J ¼ 4.7 Hz, 2H, b-pyrrole–Ph–Fc), 8.36
(d, J ¼ 8.0 Hz, 2H, m-Ph), 8.22 (d, J ¼ 8.0 Hz, 2H, o-Ph), 5.37 (s,
2H, a-Cp2), 5.34 (s, 4H, a-Cp1,3), 4.82 (s, 2H, b-Cp2), 4.79 (s, 4H,
b-Cp1,3), 4.13 (s, 5H, CpH2), 4.11 (s, 10H, CpH1,3). 13C NMR (101
MHz, DMSO-d6, d): 167.75 (C-carboxylic acid), 149.70 (a-pyrrole–
Ph–Fc), 149.23 (a-pyrrole–Fc–Fc), 148.94 (a-pyrrole–Fc–Fc),
147.46 (i-Ph), 147.33 (a-pyrrole–Ph–Fc), 134.20 (o-Ph), 132.11 (b-
pyrrole–Ph–Fc), 131.30 (b-pyrrole–Fc–Fc), 131.22 (b-pyrrole–Fc–
Fc), 129.82 (b-pyrrole–Ph–Fc), 127.65 (m-Ph), 118.79 (meso-C–
Ph), 118.04 (meso-C–Fc2), 117.53 (meso-C–Fc1,3), 90.50 (Ci–Cp2),
90.21 (Ci–Cp1,3), 77.19 (a-Cp2), 76.97 (a-Cp1,3), 70.35 (CpH),
68.58 (b-Cp1,3), 68.52 (b-Cp2). MS (MALDI-TOF, DCTB matrix, m/
z): [M]+ 1044.0, [M + O + H]+ 1061.0. HREI+ (m/z): [M]+
1044.0495, calcd for C57H40Fe3N4O2Zn: 1044.0486. Elemental
analysis calcd for C57H40Fe3N4O2Zn: C 65.46, H 3.85, N 5.36,
found: C 66.54, H 4.21, N 5.19. Atomic absorption calcd for
4 M. E. El-Khouly, O. Ito, P. M. Smith and F. D'Souza,
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S.-L. Wu, H.-P. Lu, H.-T. Yu, S.-H. Chuang, C.-L. Chiu,
C.-W. Lee, E. Wei-Guang Diau and C.-Y. Yeh, Energy
Environ. Sci., 2010, 3, 949; C.-W. Lee, H.-P. Lu, C.-M. Lan,
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C
57H40Fe3N4O2Zn: Fe 16.0, found: Fe 15.5. FTIR (cmꢀ1) 622 (m),
646 (w), 720 (s), 767 (sh), 794 (vs), 810 (sh), 821 (sh), 874 (w), 923
(m), 999 (s), 1030 (w), 1058 (m), 1079 (w), 1107 (m), 1176 (sh),
1191 (w), 1216 (w), 1236 (w), 1264 (w), 1291 (sh), 1312 (w), 1337
(w), 1353 (w), 1389 (w), 1410 (w), 1484 (m), 1565 (w), 1605 (m),
1689 (s), 1730 (sh), 2929 (vw), 3090 (br).
¨
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Acknowledgements
We thank Newcastle University and Nottingham University for 12 M. Wang, P. Chen, R. Humphry-Baker, S. M. Zakeeruddin
¨
nancial support, FP7-PEOPLE-2009-IRSES grant (246902) and and M. Gratzel, ChemPhysChem, 2009, 10, 290.
Academy of Finland grant (263486) for funding to cover the 13 S. A. Haque, E. Palomares, B. M. Cho, A. N. M. Green,
exchange of researchers. The EPSRC UK National Mass Spec-
trometry Facility at Swansea University is thanked for collecting
mass spectra of compounds.
N. Hirata, D. R. Klug and J. R. Durrant, J. Am. Chem. Soc.,
¨
2005, 127, 3456; M. Gratzel, J. Photochem. Photobiol., A,
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