Porphyrins as ITO photosensitizers: Substituents control photo-induced electron transfer direction

Article abstract of DOI:10.1039/c2jm34118b

Porphyrins have attracted much attention as dyes for photovoltaic applications due to their remarkable light harvesting properties and tunability of electronic behaviour. The photophysical and photochemical properties of porphyrins are influenced by electron-donating or electron-withdrawing substituents that can be attached at the perimeter of the porphyrin macrocycle. The current work shows that changing the porphyrin peripheral substituents can affect the direction of interfacial charge transfer at the interface of porphyrin and Indium tin oxide (ITO), a degenerate n-type semiconductor that is commonly used as a transparent conductive electrode in organic optoelectronic devices. Soret-band excitation resulted in electron injection from the molecular layer to the ITO in all porphyrin derivatives studied, suggesting that electron injection to ITO is faster than relaxation from the porphyrin upper excited state to the lower one. However, the direction of photo-induced electron transfer in the 500-650 nm spectral range (Q-bands excitation in porphyrins) was found to depend on the peripheral substituents. This is highly relevant for photovoltaic devices, as the solar spectrum peaks in this spectral range. The charge transfer behaviour was shown to depend on the composition of the interfacial adsorbed monolayer. Therefore, it is proposed that porphyrin derivatives can be used for modulating photo-induced interfacial transport at ITO/organic layer interfaces in a predefined, controllable way.

Full text of DOI:10.1039/c2jm34118b

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PAPER
Porphyrins as ITO photosensitizers: substituents control photo-induced
electron transfer direction†
Yulia Furmansky, Hela Sasson, Paul Liddell, Devens Gust, Nurit Ashkenasy and Iris Visoly-Fisher*ab
ab
ab
c
c
db
Received 25th June 2012, Accepted 6th August 2012
DOI: 10.1039/c2jm34118b
Porphyrins have attracted much attention as dyes for photovoltaic applications due to their remarkable
light harvesting properties and tunability of electronic behaviour. The photophysical and
photochemical properties of porphyrins are influenced by electron-donating or electron-withdrawing
substituents that can be attached at the perimeter of the porphyrin macrocycle. The current work shows
that changing the porphyrin peripheral substituents can affect the direction of interfacial charge
transfer at the interface of porphyrin and Indium tin oxide (ITO), a degenerate n-type semiconductor
that is commonly used as a transparent conductive electrode in organic optoelectronic devices. Soret-
band excitation resulted in electron injection from the molecular layer to the ITO in all porphyrin
derivatives studied, suggesting that electron injection to ITO is faster than relaxation from the
porphyrin upper excited state to the lower one. However, the direction of photo-induced electron
transfer in the 500–650 nm spectral range (Q-bands excitation in porphyrins) was found to depend on
the peripheral substituents. This is highly relevant for photovoltaic devices, as the solar spectrum peaks
in this spectral range. The charge transfer behaviour was shown to depend on the composition of the
interfacial adsorbed monolayer. Therefore, it is proposed that porphyrin derivatives can be used for
modulating photo-induced interfacial transport at ITO/organic layer interfaces in a predefined,
controllable way.
12,13
Introduction
density.
2
Recently, interfacial modification of TiO /polymer
interfaces in OPV devices by adsorption of photo-active dyes,
including Ru-based, indoline-based and phthalocyanine-based
Using organic materials as components of optoelectronic devices
is aimed at exploiting their structural and mechanical flexibility,
low weight and low cost, while reducing the environmental
dyes, was shown to improve the energy conversion efficiencies of
14–17
the devices.
Herein we describe a study of charge injection at
1–3
impact of production and operation. Among the many factors
limiting the performance of organic photovoltaic (OPV) devices,
the efficiency of charge carrier injection at the interfaces between
the interface between Indium tin oxide (ITO) and different
porphyrin derivatives aimed at understanding the effect of
substituents on such charge injection. Such understanding will
allow both optimizing the use of porphyrins in molecular OPV
devices, as well as evaluating the potential of porphyrins as
photo-active interfacial modifiers in ITO/polymer devices.
Porphyrins have attracted much attention as dyes for photo-
4
the photoactive layer and the electrodes is a central issue.
Modification of an inorganic electrode’s surface by attachment
of organic molecules is a promising way to improve the efficiency
by modulating the interfacial energetics at the heterojunction,
utilizing the molecular structural versatility for fine tuning of the
voltaic applications due to their remarkable light harvesting
18–21
5–10
interface energetics.
An adsorbed molecular monolayer can
properties and tunability of electronic behaviour.
Porphyrin-
affect semiconductor surface electronic properties by changing
type compounds are frequently found in nature as part of the
photosynthetic mechanism. The photophysical and photochem-
ical properties of porphyrins are influenced by electron-donating
or electron-withdrawing substituents that can be attached at the
perimeter of the porphyrin macrocycle at the meso- or beta-
5,11
the surface electron affinity
or/and the net surface-charge
a
Department of Chemistry, Ben Gurion University of the Negev, Be’er
Sheva, Israel. E-mail: irisvf@bgu.ac.il; Fax: +972-8-6472943; Tel:
+
b
972-8-6472716
22
positions. For example, pyridyl meso-substituents were found
to decrease the reduction potential of the porphyrin resulting in
23
Ilse Katz Institute for Nanoscale Science and Technology, Ben Gurion
University of the Negev, Be’er Sheva, Israel
c
Department of Chemistry and Biochemistry, Arizona State University,
Tempe, AZ, USA
n-type conductance in solid porphyrin films. The effect of
different substituents has been widely studied in porphyrin-based
24,25
d
Department of Materials Engineering, Ben Gurion University of the
Negev, Be’er Sheva, Israel
dye sensitized solar cells.
2
Charge injection into TiO elec-
trodes was found to be strongly influenced by the nature of the
† Electronic supplementary information available: Fig. S1–S4 and Tables
S1–S3. See DOI: 10.1039/c2jm34118b
26
binding
group,
attachment
of
donoric–acceptoric
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2
5,27
substituents
and the nature of spacer between the porphyrin
anchoring group, while the phenyl-4-carboxy-(2,4-dinitrophenyl
hydrazine) group in DNH-porphyrin has an increased electron-
donating nature. The other two chemically adsorbed species are
porphyrin–fullerene dyads (PF-dyad and NPF-dyad), where C60
is a strong electron acceptor. Such dyads showed quantitative
28
macrocycle and anchoring functionality. Dye-sensitized solar
cells using porphyrins with an electron-donating meso-substit-
uent and an electron-withdrawing anchoring group at the
opposite meso-position have achieved record energy conversion
24,25
efficiencies.
ITO is a degenerate n-type semiconductor with a
yields of charge-separated species and long lifetimes of the
18,40
29,30
large bandgap (3.75–4.1 eV)
which is commonly used as a
charge-separated states when measured in solution.
In both
transparent conductive oxide electrode in OPV devices due to its
electronic conductivity and optical transparency. Tin oxide
nanoparticles were also studied as an alternative or addition to
PF-dyad and NPF-dyad the fullerene moiety is attached at the
5-meso-position via a conjugated phenyl spacer. In the NPF-dyad
the mesityl group at the 15-meso-position was replaced by a
41
the more commonly used TiO electrode in dye sensitized solar
2
pyridine ring as an electron-withdrawing substituent. DME-
and TMP-porphyrins were used for the formation of physically
adsorbed multilayers on ITO substrates modified by monolayers
of chemically adsorbed porphyrin derivatives (see below).
UV-Vis spectra of the adsorbed monolayers on ITO show the
Soret band to be somewhat broader and red-shifted (1–2 nm)
compared to that in solution (Fig. 1a compared to Fig. S1 and
Table S1 in the ESI†). The lack of larger shifts rules out aggre-
gation in the adsorbed films, probably due to the presence of
18
cells due to their improved conductivity and electron mobility.
Surface and interface electronic properties of ITO can be
significantly altered by the adsorption of a molecular mono-
29,31
layer,
and such treatments were used to improve the perfor-
mance of organic PV and electro-optical devices utilizing ITO or
32–34
ZnO as the transparent electrode.
Surface photovoltage spectroscopy (SPS) is a contactless, non-
destructive and sensitive tool for measuring illumination-induced
electronic potential changes and charge redistribution at semi-
42,43
bulky substituents.
The adsorption density in monolayers
37
conductor surfaces and heterojunctions. SPS was previously
was estimated from the Soret absorption peak height and
extinction coefficients measured in solution, and was found to be
employed to characterize the photovoltage induced at various
35–39
ꢂ10 ꢂ2
metal oxide/porphyrin film interfaces.
Though porphyrins
0.4 ꢀ 0.2 ꢁ 10
mol cm in all monolayers. In multilayers
are expected to donate electrons from their excited state to metal
oxide electrode, in certain porphyrins an opposite direction of
charge transfer was found upon excitation at the porphyrin Q-
(10–100 nm thick), the Soret absorption peaks of all porphyrins
except for TMP-porphyrin further broadened and red shifted by
10–15 nm relative to those in solution, indicating interactions
3
7–39
band absorption.
Specifically, such behaviour was observed
with the polar substrate and/or partial edge-on-edge J-type
44,45
37
at a porphyrin-C60 dyad/TiO
2
interface. The direction and
aggregation (Fig. 1b and Table S1†).
Absorption spectra of
magnitude of photovoltage was found to depend also on the
central substituent (metal or hydrogen atoms) of the porphyrin
the porphyrin derivatives in solution show a Soret band
absorption peak near 420 nm and four Q-bands at 517, 553, 593
and 650 nm (Fig. S1†), in accordance with typical values reported
35,37
molecule.
However, to the best of our knowledge, no
45,46
systematic study of the effect of porphyrin peripheral substitu-
ents on photo-induced charge transfer at interfaces with metal
oxide electrodes has been performed hitherto. Here we report the
investigation of the effect of porphyrin substituents on photo-
induced charge transfer direction at porphyrin/ITO hetero-
junctions. SPS characterization shows that modifying the
porphyrin macrocycle’s periphery strongly affects the direction
of photo-generated interfacial charge transfer, due to changes in
energy level alignment and/or changes in the orientation of the
molecules at the interface. Our results show that an interfacial
porphyrin monolayer can tailor the electronic properties of the
interface between photoactive organic films and ITO, controlling
interfacial photo-induced charge transfer.
for free-base porphyrins.
Negligible differences in peak
position and shape among the various porphyrins point to weak
electronic interactions between the porphyrins and the attached
45,47
substituents in the ground state.
Cyclic voltammetry was carried out to determine the redox
potentials of the chemically adsorbing porphyrins. Cathodic
shifts of the first oxidation and reduction potentials in DNH-
porphyrin compared to DC-porphyrin result from the electron-
donating nature of the attached 2,4-dinitrophenyl hydrazine
group. The relative anodic shifts of the redox potentials of the
NPF-dyad compared to PF-dyad confirm the electron-with-
drawing nature of the pyridine ring (Table S2†). The small shifts
are another indication of the weak electronic interactions
between the porphyrin and its substituents, as noted from the
absorption spectra. In porphyrin–fullerene dyads, an additional
transition indicates the reduction of the fullerene moiety. The
molecular highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energies, at the
electron energy scale, were estimated from the porphyrins’
Results and discussion
Absorption, adsorption and cyclic voltammetry
The schematic molecular structures of the six differently
substituted free-base tetraphenylporphyrins used in this study
are presented in Scheme 1. The substituents were chosen in order
to study the effect of electron donating or accepting substituents.
Four types of porphyrins modified with carboxylic acid func-
tionalities were chemically adsorbed on ITO surfaces: DC-
porphyrin and DNH-porphyrin, both modified with a benzoic
acid group at the 5-meso-position, differ by their 15-meso-posi-
tion substituents: the second benzoic acid in DC-porphyrin can
act both as an electron accepting group and as a second
oxidation and reduction potentials, respectively, using
a
+
48
conversion value of ꢂ5.39 eV relative to the Fe/Fe scale. The
estimated energy levels of the molecular orbitals are depicted in
Scheme 2, together with the energies of the porphyrin excited
states, calculated using excitation energies obtained from the
25,49,50
absorption spectra,
and the measured Fermi level energies
of bare ITO and ITO modified by thick multilayers of the four
porphyrin derivatives. The latter were determined using the
Kelvin probe technique.
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Scheme 1 Schematic structures of porphyrin derivatives used in this study.
Surface photovoltage spectroscopy (SPS)
the molecular layer thickness was previously noted when multi-
layers were illuminated through the porphyrin/air surface, a
phenomenon related to depletion layers developed in thick
porphyrin layers creating inner fields in opposite directions at the
SPS was used to study photo-induced charge transfer processes
13,37,51
at the porphyrin/ITO interfaces.
Fig. 2 shows the surface
photovoltage (SPV) as a function of the illuminating wavelength
obtained from ITO samples modified by multilayer films of
various porphyrin derivatives. SPV is the difference in the
measured contact potential difference (CPD ¼ sample work
function ꢂ probe work function) in the dark and under illumi-
nation, i.e., SPV ¼ ꢂ(CPDlight ꢂ CPDdark). A decrease in CPD
upon illumination (increased SPV) indicates a decrease in the
surface work function, corresponding to positive charging of the
surface, and vice versa. The samples were illuminated through
the transparent substrate. This configuration enhanced the
measurement sensitivity to the interface of interest, i.e., the
35,36,38,39
ITO/porphyrin and porphyrin/air interfaces.
When very
thin films (about one monolayer thick) were measured in SPS,
photo-induced charge transfer between porphyrins and the metal
37–39
oxide electrode was noted.
We assume that illuminating
multilayers/ITO interfaces through the transparent substrate
results in differences in surface potential, measured at the
porphyrin layer surface, due to photo-induced charge transfer
between porphyrins and the ITO.
A change in SPV is noted under illumination at wavelength
near the absorption peaks, with the SPV peaking near the
absorption peak positions (Fig. 2 and S2†). A decrease in the
CPD (increased SPV) was observed for all porphyrin multilayer
interfaces upon illumination at wavelengths approaching 435 nm
35,36,38,39,52
porphyrin–ITO interface.
Photovoltage dependence on
Fig. 1 Absorption spectra of porphyrin (a) monolayers and (b) multi-
layers on ITO substrates. The monolayer spectra were derived by sub-
tracting the reference spectra of bare ITO from that of monolayer-
modified samples. Due to inaccuracies resulting from interferences in
ITO, some monolayer spectra show negative absorption where the
absorption is very small.
Fig. 2 SPV spectra of (a) DC- and DNH-porphyrin, and (b) PF- and
NPF-dyad multilayers on ITO, showing the surface photovoltage as a
function of illuminating wavelength. The arrows indicate the positions of
the corresponding absorption peaks.
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(
Fig. 2), i.e., approaching the Soret excitation energy. Such SPV
increase indicates that Soret band absorption in all porphyrin
derivatives resulted in electron injection from the molecular layer
into the ITO. Upon Soret band excitation electrons are excited
from the HOMO to the Soret-related level (process 1 in
Scheme 3) and injected to the ITO’s conduction band (process 2
in Scheme 3) due to the higher relative position of the Soret band
energy level with respect to the conduction band minimum of
ITO. Thus, electron injection from the Soret excited singlet state
occurs faster than relaxation of this state to the lower excited
state (Q-band). The ITO’s conduction band edge level was
deduced from the ITO’s Fermi level energy which was deter-
mined experimentally (see Scheme 2). As a degenerate n-type
semiconductor, the Fermi level of ITO is close to (slightly higher
Scheme 3 Schematic diagrams of (a) DC- and (b) DNH-porphyrin/ITO
interface electronic levels. Arrows indicate electron excitation or transfer
direction. Electron injection to ITO as a result of Soret band excitation is
shown by processes 1 and 2. Positive/negative charging of the molecular
layer following Q-bands excitation (process 3) is shown by process 4 in
(
a)/(b), respectively.
29,31
edge, which is related to differences in the ITO’s surface band
bending. ITO is known to have many electron energy surface
states resulting in a significant surface band-bending of up to
than) the conduction band minimum in the bulk.
Four shoulders/peaks are seen in the SPV spectra between 500
and 650 nm (Fig. 2a), in agreement with the Q-band transitions
5
3
1
3
eV. The SPV measured at wavelengths shorter than ca.
(
Fig. 1). Although injection of excited electrons from both
70 nm is related to super-bandgap absorption in ITO, leading to
porphyrins into the ITO’s conduction band upon Q-band exci-
tation was expected, from the Q-bands’ energy level alignment
with respect to the ITO energy levels (Scheme 2), such behaviour
is not always noted. A decrease in CPD (increased SPV) upon
absorption in the Q-bands of DNH-porphyrin films indicates
photo-induced electron injection from the molecular layer into
the ITO electrode as expected. However, for the DC-porphyrin,
CPD increases (decreased SPV) upon Q-band absorption, indi-
cating a light-induced negative charging of the molecular layer,
i.e., electron injection from ITO to the molecular layer (Fig. 2a).
Similarly, the SPV spectra of NPF-dyad/ITO samples show a
decrease in CPD (increased SPV) at Q-band absorption, while
those of PF-dyad/ITO samples show an increase in CPD
(
partial) ‘‘flattening’’ of the surface band banding (Fig. S3†). An
increase in SPV, as seen in this case, is typical of n-type semi-
conductors and indicates a significant surface band bending.
Upon DNH-porphyrin adsorption, reduced ITO band bending is
noted by a smaller increase in SPV (Fig. 3 and S3†). The
reduced interface band bending of the ITO upon adsorption of
DNH-porphyrin will promote electron injection to the ITO upon
excitation of the Q-bands (processes 3 and 4 in Scheme 3b).
However, no such decrease in interfacial band bending is seen
in DC-porphyrin/ITO samples; the larger band bending at the
DC-porphyrin/ITO interface (compared to DNH-porphyrin/ITO
interface) introduces a barrier to this process, since the conduc-
tion band edge at the surface is higher in energy than the Q-band
related levels of the porphyrin (Scheme 3a). The excitation of
electrons is then followed by the transfer of electrons from
occupied ITO surface states into the emptied HOMO states of
the excited porphyrin (processes 3 and 4 in Scheme 3a), resulting
in negative charging of the molecular layer. We note that
the latter process is feasible also in the former case of DNH-
porphyrin but is unlikely to result in a net negative charge in the
molecular layer due to electron injection from Q-band related
levels to the ITO’s conduction band. Explaining the difference in
electron transfer directions upon Q-band excitations between the
two porphyrins by a barrier induced by interfacial band bending
assumes band bending of about 1.1 eV. 1.1 eV is the measured
(
decreased SPV) at Q-band absorption (Fig. 2b). It should be
noted that the Q-band transitions are not as clearly resolved in
the dyad SPV spectra, probably due to intramolecular photo-
induced charge transfer between the porphyrin and fullerene
occurring at a faster rate than electron transfer at the ITO
37
interface. Indeed, differential SPV spectra show the coincidence
of wavelengths corresponding to transitions in the dyads’ SPV
with Q-band absorption peaks (Fig. S2†).
The difference in electron transfer direction between DC- and
DNH-porphyrins is explained by different alignment of the
Q-band energy levels with respect to the ITO’s conduction band
Scheme 2 Electron energy level diagram of the four surface modifying
porphyrin derivatives deduced from cyclic voltammetry, absorption and
Kelvin probe measurements. Excited state energies are marked as dashed
lines. The work functions of the multilayer samples are represented in
green, while that of bare ITO is marked in brown.
Fig. 3 SPV of different monolayer/ITO samples upon illumination with
a high intensity xenon-mercury lamp (290 W), which is proportional to
the band bending at the molecular layer/ITO interface.
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energy difference between the ITO’s work function (zconduc-
tion band edge in the bulk) and the lowest Q-band level in
porphyrins (see Scheme 2). Such band bending is reasonable in
significance for various optoelectronic devices. We have there-
fore studied the effect of the different porphyrin derivatives as
interfacial modifying monolayers between multilayers of pho-
toactive species and ITO. Multilayer films of DME-porphyrin, of
average thickness 40 ꢀ 5 nm as estimated from AFM topography
imaging, were spin cast on ITO electrodes premodified by
chemically adsorbed monolayers of the four porphyrin deriva-
tives. The DME-porphyrin lacks the carboxylic side groups,
preventing its chemical coupling to the surface, hence it is not
expected to replace the interfacial monolayer.
35,53
view of previously published values.
We note that explaining
the differences in SPS upon Q-band illumination by opposite
35
inner fields in the molecular multilayers are ruled out by the
almost similar Fermi level alignment is all porphyrin derivatives
(
Scheme 2).
As noted above, the two dyads also showed pronounced
differences in their SPV spectra at the Q-band absorption range
Fig. 2b). However, no significant differences in interfacial band
(
Fig. 4 presents the SPV spectra of those samples, compared to
that of the DME-porphyrin multilayer on un-modified ITO. The
introduction of interfacial modifying layers of porphyrin dyes
was found to influence the interfacial charge transfer. The
direction of the change was similar to the behaviour of the
porphyrins in homogeneous multilayers (Fig. 2a and b): the
introduction of DNH-porphyrin (Fig. 4a) and NPF-dyad
(Fig. 4b) monolayers modified the interfacial charge transfer at
the Q-band range in favour of charge injection from the photo-
active molecular layer to the ITO electrode. DC-porphyrin
interfacial monolayers had a negligible effect on the interfacial
charge transport (Fig. 4a), however, a PF-dyad monolayer
increased the charge injection from ITO to the photoactive layer
upon Q-band excitation (Fig. 4b). Similar results were obtained
with another non-adsorbing porphyrin derivative (TMP-
porphyrin) as a photoactive multilayer film (Fig. S4†). The results
suggest that porphyrin monolayers can act as modifying inter-
facial layers between photo-active layers and ITO electrodes in a
predictable manner. Although light absorption and charge
generation occur largely in the photoactive multilayer film, the
bending were noted between ITO modified with PF- and NPF-
dyads (Fig. 3 and S3†), therefore explaining the differences in the
electron transfer direction upon Q-band absorption in both
dyads by an interfacial barrier is not feasible. In addition, the
dyads are known to create a long-lived charge separated state
18
under illumination, therefore it is expected that their charge
injection to/from ITO will be controlled by different mechanisms
than those active in the porphyrins. Indeed, a fast transfer of
electrons from the excited porphyrin states to the fullerene may
diminish the transfer of such electrons to the ITO. A possible
mechanism for negative surface charging upon Q-band illumi-
nation in the case of PF-dyad is that following photo-excitation
and electron transfer to the fullerene acceptor, the porphyrin
radical cations accept electrons from ITO surface states, forming
a negatively charged layer, in accordance with previously pub-
37,54,55
lished mechanisms.
The opposite behavior of NPF-dyad
may result from its binding mode: pyridine rings are known to
form hydrogen bonds between the nitrogen atom and hydroxyl
41,56,57
protons on metal oxide surfaces.
In addition, coordinative
bonding may form between the electron-withdrawing pyridine
and Lewis acid sites on the ITO surface, as was demonstrated for
58
TiO2. We propose that this configuration, where the porphyrin
moiety is located in close proximity to the ITO surface, leads to
higher probability of electron injection to the ITO electrode upon
Q-band excitation, which competes with electron transfer to the
fullerene moiety. We note that explaining the changes in SPV
measured following illumination by the formation of molecular
dipoles within the dyads is unlikely as our molecular multilayers
are largely disordered.
Illumination at the 500–650 nm spectral range, where
porphyrin Q-band excitation occurs, is at the peak of the solar
spectrum. Therefore, photoinduced electron transfer at this
range is highly relevant for photovoltaic energy conversion. In
porphyrin-sensitized solar cells, which typically utilize electrons
injected from the porphyrins to the TiO electrode, the opposite
2
direction of electron transfer, as seen at DC-porphyrin/ITO
interface, can be detrimental for the device function. Further-
more, wavelength-dependent directions of photo-induced dipoles
can be utilized in wavelength-sensitive photosensors and artificial
color vision. Understanding the details of photo-induced elec-
tron injection can help tailor the device function with the
molecular structure.
Porphyrins and dyads for ITO interfacial modification
Fig. 4 SPV spectra of DME-porphyrin multilayers adsorbed on ITO
modified with (a) DC- and DNH-porphyrin monolayers, and (b) PF- and
NPF-dyad monolayers. The arrows indicate the positions of the corre-
sponding absorption peaks in the DME-porphyrin absorption spectrum.
Our results suggest that the first monolayer adsorbed on the
surface of the ITO may determine the photo-response of multi-
layers adsorbed on the surface. Such behaviour may be of
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interfacial chemistry, determined by the chemically adsorbed
photoactive monolayer, significantly affects the photo-induced
charge transfer efficiency at the interface.
compounds and molecular structures was established by TLC,
1
mass- and H-NMR-spectroscopies. The NMR spectra were
recorded using a Bruker Avance DMX500 spectrometer. Mass
spectra were measured by a Bruker Daltonics Reflex IV Matrix-
Assisted Laser Desorption Ionization Time-Of-Flight (MALDI-
TOF) instrument.
Summary and conclusions
This work has shown that changing the porphyrin peripheral
substituents can affect the direction of interfacial charge transfer
at porphyrin/ITO interfaces. Soret-band excitation resulted in
electron injection from the molecular layer to the ITO in all
porphyrin derivatives, suggesting that electron injection is faster
than relaxation from the porphyrin upper excited state to the
lower one. Adsorption of DNH-porphyrin was shown to induce a
smaller ITO surface band bending, which favoured the injection
of Q-band excited electron from the porphyrin layer to the ITO
conduction band. In contrast, DC-porphyrin adsorption resulted
in electron transfer from the ITO to the molecular layer upon
Q-band excitation, while the opposite direction of electron
transfer was blocked by a barrier formed by the relatively large
band bending at the ITO surface. Similar differences in the
direction of electron injection following Q-band excitation were
noted in porphyrin–fullerene dyads differing by a pyridine end
group. This difference was explained by the dyads’ different
adsorption configurations on the ITO surface. The pyridine ring
attached to the porphyrin moiety in NPF-dyads is proposed to
act as an electron-accepting anchoring group, enhancing photo-
induced charge injection into the ITO conduction band at the
expense of intramolecular electron transfer to the fullerene
moiety. This photo-induced charge injection in the 500–650 nm
spectral range (Q-band excitation in porphyrins) is highly
relevant for photovoltaic devices, as the solar spectrum peaks in
this range.
DME-porphyrin
A solution of 5-mesityldipyrromethane (264 mg, 1 mmol) and
methyl 4-formylbenzoate (164 mg, 1 mmol) in 300 ml CH Cl
2
2
was degassed by bubbling with nitrogen for 30 min. The reaction
mixture was shielded from ambient light and treated with tri-
60
fluoric acid (150 ml) at room temperature. After stirring for 40
min under a N atmosphere, p-chloranil (490 mg, 2 mmol) was
2
added and the reaction mixture was stirred for an additional 1 h.
Triethylamine (1 ml) was added, and the solvent was evaporated.
The resulting solid crude was purified by silica gel chromatog-
ꢃ
raphy (ethyl acetate/petroleum ether (b.p. 60–80 C) ¼ 0/1 /
1
/0). The resulting residue was washed with acetonitrile, filtered
and dried under vacuum, producing a pure DME-porphyrin
1
compound (135 mg, 0.165 mmol, 33% yield). H NMR
(
(
(
200 MHz, CDCl ) p.p.m.: d 1.84 (s, 12H), 2.64 (s, 6H), 4.12
3
s, 6H), 7.21 (s, 4H), 8.29–8.46 (dd, 8H), 8.70–8.76 (m, 8H). MS
MALDI-TOF) 815.0, calc. 815.2.
DC-porphyrin
A mixture of DME-porphyrin (135 mg, 0.165 mmol) in 100 ml of
dioxane and 1 N NaOH (10 ml) was refluxed for 6 h. After
cooling, acidification with 1 N HCl and subsequent filtration
produced DC-porphyrin as a vivid reddish purple powder (124
1
mg, 0.156 mmol, 95% yield). H NMR (500 MHz, d -DMSO)
6
The charge transfer behaviour was shown to depend on the
chemical nature of the first monolayer adsorbed on the surface.
We propose that modified porphyrin derivatives can be used for
the modification of interfacial charge transfer at ITO/photo-
active organic layer interfaces in a predefined, controllable way.
The relationships between porphyrin substituents and interfacial
optoelectronic properties presented in this work could be utilized
for fabrication of improved organic optoelectronic devices.
p.p.m.: d 1.75 (s, 12H), 2.56 (s, 6H), 7.34 (s, 4H), 8.35 (m, 8H),
8
.63–8.81 (dd, 8H); MS (MALDI-TOF) 787.4, calc. 786.9.
DNH-porphyrin
2
,4-Dinitrophenyl hydrazine synthesis: 2,4-dinitrochlorobenzene
(
1.50 g, 7.4 mmol) was dissolved in 50 ml of ethanol and the
resulting solution was added dropwise to a solution of 5 ml of
hydrazine monohydrate in 50 ml ethanol for 30 min. The reac-
tion mixture was refluxed with stirring for 3 h. Brine was added
and the crystalline solid was filtered and dried (1.34 g, 6.8 mmol,
Experimental
Materials
ꢃ
1% yield): m.p. 198–202 ; H NMR (200 MHz, CDCl ) p.p.m.:
3
1
9
d 4.0 (s, 2H), 7.83–7.88 (d, 1H), 8.28–8.29 (d, 1H), 9.10–9.12
Solvents were purchased from Bio-Lab, Fluka, D-chem and
Acros Organics Chemicals (all ‘‘absolute grade’’, HPLC
analyzed) and were used without further purification. 5-Mesi-
tyldipyrromethane (95%) was purchased from Frontier Scien-
tific, USA. 4-Formylbenzoate (99%), p-chloranil, 2,4-
dinitrochlorobenzene ($98%), hydrazine monohydrate (98%)
and pyridine (99.8%) were purchased from Sigma-Aldrich. 3,4,5-
Trimethoxy benzylamine (98%) was purchased from D-chem,
Israel. All chemicals were used as received without further
purification.
(
d, 1H), 9.44 (s, 1H).
DC-porphyrin (124 mg, 0.156 mmol) was added to a solution of
2
ml thionyl chloride in 50 ml toluene and refluxed for 6 h. After
cooling, the solvent was evaporated and the solid crude (bis(acyl
chloride)-porphyrin) was dissolved in 20 ml of toluene and used
in the next stage without purification. The solution of activated
porphyrin (10 ml, 0.08 mmol) was added to a solution of
2
,4-dinitrophenyl hydrazine (10 mg, 0.05 mmol) in 2 ml of dry
pyridine. The reaction mixture was stirred for 5 h at room
temperature and then poured into 1 N HCl solution (100 ml).
The product was extracted with ethyl acetate (100 ml), washed
Synthesis
2 4
with water several times and dried over anhydrous Na SO . The
Synthesis of different meso-substituted porphyrins was carried
resulting solid crude product was purified by silica gel chroma-
59
out based on previously reported procedures. The purity of
tography (ethyl acetate/petroleum ether ¼ 0/1 / 1/0). A mixture
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 20334–20341 | 20339

View Article Online
of the two compounds with one or two 2,4-dinitrophenyl
hydrazine substituent was obtained (TLC silica, ethyl aceta-
te : petroleum ether ¼ 1 : 1) and was further separated by addi-
tional silica gel chromatography (ethyl acetate/petroleum ether ¼
respectively, at 2000 rpm. All samples were stored in nitrogen
ambient in the dark after preparation and between
measurements.
0
/1 / 1/0). The desired compound was obtained in 15% yield
Surface photovoltage spectroscopy measurements
1
(
(
(
11.6 mg): H NMR (500 MHz, d6) p.p.m.: d ꢂ2.51 (s, 2H), 1.85
s, 12H), 2.63 (s, 6H), 7.365 (s, 4H), 7.82–7.84 (d, 1H), 8.38–8.40
m, 1H), 8.44–8.52 (m, 8H), 8.76–8.88 (dd, 8H), 9.13 (d, 1H),
In SPS, the surface photovoltage is measured as a function of the
illuminating wavelength, providing information on the energetics
of the surface or interface under study. Light induced charge
transfer between the ITO substrate electrode and an adsorbed
molecular layer results in dipole formation which is expressed in
the contact potential difference (CPD) measured between the
ITO surface and a Kelvin probe (CPD ¼ sample work function ꢂ
probe work function). The changes in CPD were measured by the
Kelvin probe technique using a reference gold electrode (Besocke
Deltha Phi, Germany). The measurements were conducted at
room temperature and in a nitrogen-rich ambient environment
(ꢄ14% humidity). The samples were placed in a fully darkened
Faraday cage and the CPD was followed until it reached a
constant value (ꢀ1 mV) before illumination was turned on. For
SPS, the CPD signal was monitored as a function of photon
energy. The illumination was provided by a 350 W xenon–
mercury light source (Oriel Inc., USA) and passed through a
double monochromator, model MS257 (Oriel Inc., USA). The
1
0.26 (s, 1H), 10.70 (s, 1H); MS (MALDI-TOF) 966.9, calc. 967.
TMP-porphyrin
DC-porphyrin (124 mg, 0.156 mmol) was added to a solution of
2
ml thionyl chloride in 50 ml toluene and refluxed for 6 h. After
cooling, the solvent was evaporated and the solid crude (bis(acyl
chloride)-porphyrin) was dissolved in 20 ml of toluene and used
in the next stage without purification. The solution of activated
porphyrin (10 ml, 0.08 mmol) was added to a solution of 3,4,5-
trimethoxy benzylamine (40 mg, 0.2 mmol) in 2 ml of dry pyri-
dine. The reaction mixture was stirred for 5 h at room temper-
ature and then poured into 1 N HCl solution (100 ml). The
product was extracted with ethyl acetate (100 ml), washed with
water several times and dried over anhydrous Na SO . The
2
4
resulting solid crude product was partially purified by silica gel
chromatography (ethyl acetate/petroleum ether ¼ 0/1 / 1/0).
The resulting mixture was further separated by additional silica
gel chromatography (ethyl acetate/petroleum ether ¼ 0/1 / 1/0).
The desired compound was obtained at 22% yield (20.2 mg):
ꢂ2
illumination intensity was in the range 10–50 mW cm , with
wavelength intervals of 1 nm and duration of 2 s per interval. The
samples were illuminated through the transparent substrate.
1
H-NMR (500 MHz, d6) p.p.m.; MS (MALDI-TOF) 1144.7,
Complementary molecular layer characterization
calc. 1145.
UV/VIS absorption measurements were carried out using a
JASCO V-570 UV/VIS/NIR spectrophotometer. Topographical
imaging of surface morphology was obtained by AFM (Solver-
PF- and NPF-dyads
The porphyrin–fullerene dyads were synthesized by Prof. Devens
Pro, NTMDT, Russia) in semi-contact mode, using the
ꢂ1
61
Gust group via previously reported synthetic procedures.
noncontact tip Multi75Al-G (3 N m
– 75 kHz; Budget
Sensors). Cyclic voltammetry measurements were performed
using a three-electrode potentiostat (VersaSTAT4, Princeton
Applied Research). The measurements were carried out in dried,
distilled dichloromethane solution containing 0.10 M tetra-n-
butylammonium hexafluorophosphate supporting electrolyte
using a glassy carbon working electrode (CH Instruments, Inc.),
Sample preparation
Pure mono- and multi-layers of porphyrin derivatives were
prepared on ITO thin films (80–100 nm thick) with nominal sheet
ꢂ1
resistance 20–25 U sq , deposited on SiO
2
passivated float glass
substrates (Delta Technologies). The substrates were cleaned
using freshly prepared piranha solution (4 : 2 vol ratio concen-
trated H SO and 30 vol% H O ) for 40 min followed by washing
2
electrode area 0.07 cm , a platinum wire as a counter electrode,
+
and a Ag/Ag quasi-reference electrode, with a sweep rate of
2
4
2
2
ꢂ1
0
.1 V s . The measurements were performed at room temper-
with deionized water (18.2 MU cm) and drying under nitrogen
stream (Caution: Piranha is a very strong oxidant and reacts
violently with many organic materials). Cleaned substrates were
immersed in dye solution overnight (12–24 h) for adsorption via
carboxylic acid substituents. PF- and NPF-dyads were adsorbed
from chloroform solutions (0.5 mM); DNH-porphyrin from a
toluene solution (0.5 mM); and DC-porphyrin from a tetrahy-
drofuran (THF) solution (2 mM). Multilayer samples of the
chemically adsorbing porphyrin derivatives were prepared by
removal of electrodes from the dye solution and subsequent
solvent evaporation under a weak nitrogen stream without
washing. Monolayer samples were prepared by removal of the
substrates from the dye solutions followed by thorough rinsing in
their appropriate solvents. Physically adsorbed multilayer
samples were prepared by spin casting from a 13 mM solution of
DME-porphyrin or TMP-porphyrin in THF and chloroform,
2
ature under N . The potentials were reported vs. the ferrocene/
+
ferrocenium (Fe/Fe ) internal reference couple, whose redox
+
potential vs. Ag/Ag was 0.2 V.
Acknowledgements
This research was supported by a BSF grant #2008340. The
authors thank Dr A. Aizikovich for help with the synthesis. YF
thanks the Jacques Lewiner Chemistry Department Scholarship
and BGU Internal Interdisciplinary Research fellowship for
financial support. This work was supported in part by a grant
from the U. S. Department of Energy (DE-FG02-03ER15393).
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