Mesomorphic and photoconductive properties of a mesogenic long-chain tetraphenylporphyrin nickel(II) complex

Article abstract of DOI:10.1039/b009089l

A tetraphenylporphinatonickel-based mesogenic compound 5,10,15,20-tetrakis(4-n-pentadecylphenyl)porphinatonickel(II) (C₁₅TPPNi) was synthesised and its mesomorphic and photoconductive properties were investigated. The photoconductive properties were compared with those of the metal-free compound. All measurements were carried out for symmetrical ITO/C₁₅TPPNi/ITO type cells and steady-state currents in the constant temperature conditions were detected. A photocurrent rectification was seen in either the crystal phase or lamellar mesophase of C₁₅TPPNi. The metal-free compound with pentadecyl chains showed a strong dependence of photocurrent rectification on mesomorphic phases: the rectification is shown in the cell of crystal and low-temperature lamellar phases and it disappears in the high-temperature lamellar mesophase. This phenomenon was interpreted in terms of the various mechanisms of charged carrier generation with the phase transition, based on the results of the action spectra and the illumination light intensity dependence of the photocurrent. The improvement of the contact for electric conduction at the interface upon heating was found, which is a special feature of this mesomorphic phase in the ITO/C₁₅TPPNi/ITO cell in comparison with the metal-free compound.

Full text of DOI:10.1039/b009089l

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Mesomorphic and photoconductive properties of a mesogenic long-
chain tetraphenylporphyrin nickel(II) complex
Hirosato Monobe, Shoji Mima, Takushi Sugino and Yo Shimizu*
Department of Organic Materials, Osaka National Research Institute, AIST, METI, 1-8-31
Midorigaoka, Ikeda, Osaka 563-8577, Japan. E-mail: yo@onri.go.jp
Received 13th November 2000, Accepted 26th January 2001
First published as an Advance Article on the web 12th March 2001
A tetraphenylporphinatonickel-based mesogenic compound 5,10,15,20-tetrakis(4-n-
pentadecylphenyl)porphinatonickel(II) (C15TPPNi) was synthesised and its mesomorphic and photoconductive
properties were investigated. The photoconductive properties were compared with those of the metal-free
compound. All measurements were carried out for symmetrical ITO/C15TPPNi/ITO type cells and steady-state
currents in the constant temperature conditions were detected. A photocurrent rectification was seen in either
the crystal phase or lamellar mesophase of C15TPPNi. The metal-free compound with pentadecyl chains
showed a strong dependence of photocurrent rectification on mesomorphic phases: the rectification is shown in
the cell of crystal and low-temperature lamellar phases and it disappears in the high-temperature lamellar
mesophase. This phenomenon was interpreted in terms of the various mechanisms of charged carrier generation
with the phase transition, based on the results of the action spectra and the illumination light intensity
dependence of the photocurrent. The improvement of the contact for electric conduction at the interface upon
heating was found, which is a special feature of this mesomorphic phase in the ITO/C15TPPNi/ITO cell in
comparison with the metal-free compound.
cated situation of discotic lamellar mesophases is showing a few
1. Introduction
interesting features for their potential in charged carrier
migration. The lamellar mesophases exhibited by a series of
long-chain metallotetraphenylporphyrins tend to show very
high viscosity, indicating the high molecular order of aggrega-
tions which was evidenced by several reflection peaks in the
wide angle region of their X-ray diffraction patterns. This fact,
in other words, indicates that the lamellar mesophases of
tetraphenylporphyrin mesogens may have some effective
structures appropriate for charge migration and/or electronic
hopping in their mesomorphic structure.
In 1986, the first report about the photocurrent behaviour
was published for an ITO sandwich-type cell into which a
long-chain tetraphenylporphyrin without any metal ions in the
central core, the metal-free derivative, was injected.13 The
phase transition dependence of the photocurrent on the
positive electrode illumination clearly showed a marked
increase of photocurrent at the crystal–lamellar phase
transition, while at the phase transition between two lamellar
mesophases the photocurrent stepwise decreases. Further-
more, the V–I characteristics of photocurrent exhibit remark-
able rectification behaviour for both crystal and low-
temperature lamellar phases, though it disappears at the
phase transition between the low- and high-temperature
lamellar mesophases, which is the first report of an
intermesophase transition dependence of photocurrent recti-
fication for a cell using a mesophase material. Several attempts
have been made to explain this phenomenon in terms of
various mechanisms of photocarrier generation in the cell on
the phase transitions.13–17
The physicochemical properties of metalloporphyrins have
been extensively studied during the last three decades from a
biomimetic viewpoint.1,2 The porphyrin skeleton has an
extended p-conjugation system with 24 p-electrons leading to
a wide range of wavelengths for light absorption and p-type
properties as an electronic system. The analogous chlorophyll,
in particular, is well-known to play an important role in
photosynthetic processes and some metalloporphyrins show a
remarkable photovoltaic effect in cells with the appropriate
structure.3–6 These indicate that metalloporphyrins have some
potentiality for electronic charge transportation and excited
energy migration.7
However, these properties derived from the electronic state
of a molecule are subject to the metal species inserted in the
central core. Sometimes they are likely to show a drastic change
of electronic properties in their molecular aggregations. Thus, a
well-designed molecular architecture of porphyrins could
realise a variety of high performance functional properties if
the self-assembled structure is well controlled.8,9
On the other hand, mesogenic compounds are one of the
most interesting categories of self-assembled systems from
these points of view. In particular, discotic liquid crystals are
strongly expected to show high order anisotropic properties for
electronic charge carrier transportation as well as energy
migration because of their characteristic structures such as
columnar mesophases.6,10,11 Several disc-shaped mesogens
derived from a porphyrin skeleton have been reported so
far.7 However, as for the mesomorphic behaviour of tetra-
phenylporphyrin and its metal complexes, only a few reports
have been published to reveal the occurrence of unfamiliar
mesophases such as the so-called ‘discotic lamellar mesophase’,
which have layered structures formed by disc-shaped molecules
and are just like smectic mesophases of rod-like molecules.12
The details of these lamellar mesophases have not been clarified
yet, though some variations of molecular arrangements for
discotic lamellar phases are expected. However, this compli-
In this work, a new long-chain metallotetraphenylporphyrin,
5,10,15,20-tetrakis(4-n-pentadecylphenyl)porphinatonickel(II),
abbreviated as C15TPPNi, was synthesised and its meso-
morphic phase transition behaviour was investigated. This
complex is interesting in terms of photoconductivity because its
molecular arrangement is different from that of the metal-free
compound probably due to the non-coplanarity of the
tetraphenylporphyrin skeleton arising from the short Ni–N
DOI: 10.1039/b009089l
J. Mater. Chem., 2001, 11, 1383–1392
This journal is # The Royal Society of Chemistry 2001
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attached to the alkyl chain), 7.78 (d, 2H, meta), 9.95 (s, 1H,
CHO). IR (KBr) nmax/cm21 1680 (CLO str.).
C15TPPH2 was synthesised by the method of Adler et al.19
with slight modifications. 4-n-Pentadecylbenzaldehyde
(12.47 g, 45.4 mmol) and the equimolar pyrrole (3.05 g,
45.4 mmol) were mixed in 120 ml of propionic acid and the
mixture was refluxed for 30 minutes. After cooling to room
temperature, the black solution was filtered to give dark violet
solids. Washing with acetone gave clear violet crystals with
metallic lustre. The chlorine derivative obtained as a by-
product of the cyclization reaction was oxidised to the
corresponding tetraphenylporphyrin according to the litera-
ture.21,22 The crude product was dissolved in 50 ml of ethanol-
free chloroform and 2,3-dichloro-5,6-dicyano-4-benzoquinone
(DDQ) (1.12 g, 4.92 mmol) in 90 ml of benzene was added. The
solution was refluxed for 3 hours. Purification of the product
was carried out twice by column chromatography [(i) neutral
activated alumina, Merck active I using chloroform as eluent,
(ii) neutral alumina, Merck active II–III using benzene as
eluent], followed by recrystallisation from benzene–acetone
(1 : 9). 1.4 g of violet crystals with metallic lustre were obtained
(yield: 8.5%). 1H-NMR (500 MHz, CDCl3, d in ppm): 22.72 (s,
2H, NH), 0.90 (t, 12H, CH3), 1.33 (m, 80H, (CH2)10CH3), 1.48
(quintet, 8H, C6H4CH2CH2CH2CH2), 1.56 (quintet, 8H,
C6H4CH2CH2CH2), 1.91 (quintet, 8H, C6H4CH2CH2), 2.94
(t, 8H, C6H4CH2), 7.54 (d, 8H, meta to the phenyl carbon
attached to the meso position of the porphyrin ring), 8.12 (d,
8H, ortho to the phenyl carbon attached to the meso position of
the porphyrin ring), 8.86 (s, 8H, b position of the pyrrole). IR
Scheme 1 Chemical structure of C15TPPNi.
bond.10 Furthermore, some photoconduction properties were
studied for a symmetrical ITO/C15TPPNi/ITO cell.
2. Experimental
2.1. Compounds
Synthesis and purification of a mesogenic nickel tetraphenyl-
porphyrin complex were carried out according to a well-known
method with slight modifications. Formylation of n-penta-
decylbenzene18 and the following cyclization reaction of the
resultant n-pentadecylbenzaldehyde with pyrrole19 gave the
metal-free tetraphenylporphyrin derivative, 5,10,15,20-tetra-
kis(4-n-pentadecylphenyl)porphyrin (C15TPPH2). The nickel(II)
complex was synthesised from C15TPPH2 and a larger molar
excess of NiCl2.20 The details are described as follows.
(KBr)
n
max/cm21
:
3545 (NH str.). lmax (emax)/nm
(l mol21 cm21) (benzene): 421 (566000), 484 (4600), 517
(21000), 552 (11600), 593 (6100), 649 (5600).
The metal complex C15TPPNi was synthesised according to
the literature.19 The corresponding metal-free porphyrin
C15TPPH2 (0.50 g, 0.344 mmol) and a larger molar excess of
NiCl2 (0.407 g, 3.16 mmol) were mixed in 68 ml of N,N-
dimethylformamide and the solution was refluxed for 6 hours.
After cooling at room temperature and then in a refrigerator
overnight, the solution was filtrated to give a solid and washing
with acetone gave red-violet crystals with metallic lustre.
Purification of the product was carried out by column
chromatography (neutral activated alumina, Merck active I
using chloroform as eluent) and recrystallised from benzene–
acetone (1 : 9) solution, followed by Soxhlet extraction with
methanol for purification of the ionic species. 0.394 g of red-
violet crystals with metallic lustre was obtained (yield: 72%).
1H-NMR (500 MHz, CDCl3, d in ppm): 0.87 (t, 12H, CH3),
1.27 (m, 80H, (CH2)10CH3), 1.44 (quintet, 8H,
Scheme 2 Synthesis procedure of C15TPPNi.
4-n-Pentadecylbenzaldehyde was prepared with a slight
modification of a formylation method by Reiche et al.18 n-
Pentadecylbenzene (12.25 g, 42.5 mmol, Tokyokasei) was
dissolved into 60 ml of dried dichloromethane and the solution
was cooled on ice. TiCl4 (16.77 g, 88.4 mmol) was added into
the solution and 1,1-dichloromethyl methyl ether (4.70 g,
40.9 mmol) was added dropwise with vigorous stirring below
10 ‡C. The solution was stirred for 30 minutes on an ice bath
and for 45 minutes at room temperature. The reaction mixture
was poured into 500 ml of ice-cold water and stirred for more
than 30 minutes. The yellow organic layer was separated from
the aqueous layer and was washed by distilled water three times
to be dried on anhydrous Na2SO4. Evaporation of solvent gave
an orange oily liquid. The crude product was purified by
column chromatography with silica gel using hexane at first,
then a hexane–benzene (4 : 1) mixture, as eluents. The para-
derivative was treated with activated carbon in methanol
solution and finally recrystallised from hexane to give 6.95 g
C6H4CH2CH2CH2CH2),
1.53
(quintet,
8H,
C6H4CH2CH2CH2), 1.86 (quintet, 8H, C6H4CH2CH2), 2.88
(t, 8H, C6H4CH2), 7.47 (d, 8H, meta to the phenyl carbon
attached to the meso position of the porphyrin ring), 7.90 (d,
8H, ortho to the phenyl carbon attached to the meso position of
the porphyrin ring), 8.75 (s, 8H, b position of the pyrrole). lmax
(emax)/nm (l mol21 cm21) (benzene): 420 (263000), 530 (19700).
Elemental analysis of C104H148N4Ni (1512.95): found (calcu-
lated): C 82.58 (82.56), H 10.14 (9.86), N 3.57 (3.70%).
2.2. General measurements
FT-IR spectra were measured using a Perkin Elmer Para-
gon1000 spectrometer. UV-visible absorption spectra were
recorded using a Shimadzu UV-2500PC spectrophotometer.
1H-NMR measurements were carried out using a JEOL JNM-
A50N FT-NMR spectrometer.
1
(25.3 mmol) of 4-n-pentadecylbenzaldehyde (yield: 51%). H-
2.3. Measurements of mesophase behaviour
NMR (500 MHz, CDCl3, d in ppm): 0.87 (t, 3H, CH3), 1.24 (m,
24H, (CH2)12CH3), 1.63 (quintet, 2H, C6H4CH2CH2), 2.67 (t,
2H, C6H4CH2), 7.32 (d, 2H, ortho to the phenyl carbon
The phase transition behaviour was studied using a differential
scanning calorimeter (DSC) (TA Instruments, 2920 MDSC)
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Fig. 2 Micrographic texture of a C15TPPNi cell at 80 ‡C.
obtain a homogeneous monodomain film by modifying the
cooling process. The electrical contact to the electrodes was
attained using an electroconductive silver paste (Fujikurakasei,
D-500) and copper wires. A 500 W xenon lamp (Ushio, UXL-
500D) was used as the light source and the light was
monochromated by a scanning monochrometer (Jasco, CT-
10 and MS-25) to illuminate the sample cell in a temperature
controllable cryostat under an inert atmosphere (Ar gas). The
V–I characteristics of dark- and photo-currents were measured
using a digital electrometer (Advantest, R8340) as a closed
circuit under electric bias. The illumination light intensity was
fixed to 0.34 mW cm22 at 420 nm and 680 nm for all
measurements. The temperature conditions were controlled
to within 0.2 K using a temperature controller (Chino, SU10)
and
a cryostat (Oxford, Optistat). The positive carrier
mobilities were measured by the TOF (time-of-flight)
method. A Nd : YAG laser was used for pulsed light irradiation
and the light wavelength was selected using an optical
parametric oscillator (HOYA Continium, SURELITE II-10
and SURELITE OPO). The sample cell was set up in the
cryostat and externally biased by a stabilised DC power supply.
The transient photocurrent was detected by a digital oscillo-
scope (Iwatsu, DS9244) with a preamplifier (EG&G, 5185).
Fig. 1 Schematic diagram of the circuit and configuration of
sandwich type ITO/C15TPPNi/ITO cell used for dark- and photo-
current measurements.
a
3. Results and discussion
with a mechanical cooling attachment and the texture was
observed using a polarising microscope (Olympus, BH-2)
equipped with a hot stage (Mettler, FP80HT). Powder X-ray
diffraction studies were carried out by using a Rigaku
Geigerflex X-ray diffractometer (CuKa) equipped with a
handmade heating apparatus.
3.1. Mesophase behaviour
Fig. 3a and b show DSC thermograms of C15TPPNi at heating
and cooling rates of 5 and 1 ‡C min21, respectively. On the first
heating run (bottom), two endothermic peaks at 70 ‡C and
122 ‡C were detected. The clearing point was recognised at
122 ‡C by microscopic observation of the textures. On the
second heating run, peaks appeared at 40 ‡C, 51 ‡C and 68 ‡C in
Fig. 3a, and peaks at 42 ‡C and 67 ‡C, and a shoulder at 51 ‡C
were detected in Fig. 3b. With the decrease of the exothermic
peak at 54 ‡C on the second cooling run, the endothermic peaks
at 40 ‡C and 51 ‡C were decreased and the crystal–mesophase
transition peak at 68 ‡C was increased on the second heating
run. These were probably induced by the formation of a
metastable crystalline state in the cooling process at the
different cooling rate. The thermodynamic parameters accom-
panying these transitions are listed in Table 1.
The results of powder X-ray diffraction measurements for
the mesophase at 100 ‡C and for the crystal at 30 ‡C are shown
in Fig. 4a and b, respectively. In the X-ray diffraction pattern
of the mesophase on the wide angle region, the broad halo
around 2h ~20‡ derived from the molten alkyl chains indicates
this phase to be mesomorphic as shown in Fig. 4a. However, a
few sharp reflections were also detected in the wide angle region
which could be derived from the existence of a columnar
structure in the smectic-like layer. Four peaks appear in the low
2.4. Measurements of photoconductivity
All measurements were carried out to obtain steady-state
currents in the constant temperature conditions. Dark- and
photo-currents were measured using the apparatus illustrated
in Fig. 1. The sample cells were prepared using ITO-coated
glass electrodes separated by 25 mm thick polyimide film or
glass bead spacers with 2 mm diameter in parallel and fixed with
silicon cement resulting in 29.4 and 4.2 mm thick cells,
respectively. The effective area of the electrodes was adjusted
to 1 cm2 by etching of the ITO thin film. The cell thickness was
evaluated by interferometry using a UV-visible spectrophot-
ometer. C15TPPNi was injected into the cell space by capillary
action in the isotropic phase (ca. 130 ‡C) and the cell was cooled
to room temperature very slowly (ca. 1 ‡C min21). Microscopic
observation of the cell revealed that the thin film of C15TPPNi
was formed to have polydomain structures. Thus, all
measurements in this work were for a polydomain film as
shown in Fig. 2. The lamellar mesophase is highly viscous as
suggested by X-ray diffraction studies and it was impossible to
˚
˚
˚
angle region and these spacings are 34.0 A, 17.0 A, 11.3 A, and
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Fig. 3 DSC thermograms of C15TPPNi. Heating and cooling rates were (a) 5 ‡C min21 and (b) 1 ‡C min21, respectively.
˚
8.1 A. The ratio of these spacings is 1 : 1/2 : 1/3 : 1/4, character-
istic of a so-called ‘lamellar’ structure. Another weak peak
˚
appears in the low angle region at the spacing of 22.6 A; the
ratio between this peak and first one is 1 : 1/d2, characteristic of
a tetragonal arrangement of Ni ions which is centred in the
porphyrin ring. On the other hand, three peaks appear in the
3.2. Photocurrent action spectra
Fig. 5 shows the absorption spectra of an ITO/C15TPPNi/ITO
cell and a benzene solution. There were no significant
differences between the spectral patterns. The absorption of
the cell at wavelengths smaller than 340 nm is due to the
absorption of ITO electrodes. Also, no changes of the band
maxima were observed through the crystal and mesophase in
the Soret band near 420 nm and the Q band around 530 nm.
Fig. 6 and 7 show the photocurrent action spectra for
positive and negative electrode illumination for 29.4 and
˚
low to wide angle region and these spacings are 9.50 A, 4.55 A
˚
˚
and 3.16 A. This suggests that molecular stacks of columns
exist in the layer. The results indicate that this phase is like a
crystal but is still a lamellar type mesophase, MLC. There are
slight differences in the X-ray diffraction patterns between
Fig. 4a and b. Several additional peaks appear in the crystal
phase at 30 ‡C as shown in Fig. 4b. However, the peaks
corresponding to a layered structure remained as in the
mesophase. This means the structural change on the MLC to
crystal phase transition is rather small.
C15TPPNi shows one mesophase, a MLC phase, between
the crystal and the isotropic phases, while the corresponding
metal-free compound shows two lamellar-type mesophases, a
ML and a MLC phase. The lamellar phase which was exhibited
by C15TPPNi is the low-temperature lamellar mesophase in
the metal-free compound which is close to the crystal phase.
The stability of the Ni complex is lower than that of the
metal-free compound probably due to the non-coplanarity of
nickel porphyrin induced by a twist of the square planar
geometry.1,9
Table 1 Thermodynamic quantities for the phase transitions of
C15TPPNi
T/ ‡C (DH/kJ mol21
)
Sample
Mode
Cr0–MLC
MLC–I
Recrystallisation 1st heating 70 (104)
from solution
122 (23)
Cr2–Cr1
Cr1–MLC
MLC–I
Fig. 4 X-Ray diffraction patterns (powder) of C15TPPNi (non-
oriented) at (a) 105 ‡C in the mesophase and (b) 30 ‡C in the crystalline
phase.
Melt-grown
40
(19)
68 (47) 122 (24)
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Fig. 5 Absorption spectra of a C15TPPNi film sandwiched between
glass plates (2.8 mm thick) and a benzene solution.
4.2 mm thick cells, respectively. These were measured at room
temperature (crystalline state), the lower temperature region
(80 ‡C) and the higher temperature region (110 ‡C) of the MLC
phase.
For the positive electrode illumination of the crystalline cell
with 29.4 mm thickness, the spectral pattern in the longer
wavelength region does not coincide with the absorption
spectrum, showing an antibatic relation, and in the shorter
wavelength region corresponding to the Soret band the
photocurrent is very small. This indicates that most of the
hole carriers generated near the interface are trapped and/or
recombined during the transportation. Thus, the photocurrent
observed is derived from the carriers generated in the bulk
region. This could explain the antibatic relation between the
Fig. 7 Action spectra for the positive (solid line) and negative (dotted
line, relieved in white) electrode illumination of a 4.2 mm thick ITO/
C
15TPPNi/ITO cell for (a) the crystal (30 ‡C; $), (b) the lower
temperature region of MLC (80 ‡C; Y) and (c) the higher temperature
region of MLC (110 ‡C; &) phases. Bias: 8000 V cm21 (actual applied
voltage 3.33 V), light intensity: 0.34 mW cm22
.
action and absorption spectra in the longer wavelength region
where the compound has a lower absorption coefficiency. In
addition, the spectral pattern for the negative electrode
illumination of the crystalline cell was similar to that for the
positive one. In the lower and higher temperature regions of the
MLC phase, the same patterns were seen except for the
enhanced photocurrent in the crystal phase.
In the longer wavelength region, the spectral pattern for a
29.4 mm thick cell shows an antibatic relation to the absorption
spectrum of an ITO/C15TPPNi/ITO cell, though a symbatic
relation can be seen for a 4.2 mm thick cell in the shorter
wavelength region. This result strongly indicates that the
photoconductive phenomena of the cell are related to the
bimolecular recombination process and the charged carrier
generation process near the electrode interface as well as in the
bulk region.
The relation of illumination light intensity I0 to current
density j is expressed as shown in eqn. (1);23 this enabled us to
speculate that the carrier generation process contributed to the
observed photocurrent, where e is the electronic charge, m the
effective carrier mobility, E the electric field, n the average
concentration of charge carriers, d the film thickness, g the
photocarrier quantum yield, I0 the incident photon flux per unit
area, a the absorption coefficient, and c the recombination
coefficient in the model. According to this equation, a symbatic
relation between j and a is predicted for small values of ad and
an antibatic one for large values of ad.
j~nemE~2em=d(gI0=ac)0:5½1{ exp ({ad=2)ŠE
As for the action spectra of the 4.2 mm thick cell, complex
spectral patterns were observed as shown in Fig. 7. While the
spectra for the negative electrode illumination were almost
antibatically related to the absorption spectra in all phases,
(1)
Fig. 6 Action spectra for the positive (solid line) and negative (dotted
line, relieved in white) electrode illumination of a 29.4 mm thick ITO/
C15TPPNi/ITO cell for (a) the crystal (30 ‡C; $), (b) the lower
temperature region of MLC (80 ‡C; Y) and (c) the higher temperature
region of MLC (110 ‡C; &) phases. Bias: 8000 V cm21 (actual applied
voltage 23.52 V), light intensity: 0.34 mW cm22
.
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Fig. 8 A schematic representation of the proposed charged carrier generation mechanisms in a sandwich type ITO/C15TPPNi/ITO cell.
those for the positive electrode illumination look like over-
lapped symbatic and antibatic spectra in the crystal and in the
lower temperature region of the MLC phase. In the higher-
temperature region of the MLC phase, the spectral patterns for
both polarities of illumination look antibatic. These results
indicate that an extrinsic mechanism contributes to the
observed photocurrent for the positive electrode illumination
in the crystal and the lower temperature region of the MLC
phase, in addition to an intrinsic one. In the higher-temperature
region of the MLC phase, however, it could be inferred that
only intrinsic mechanism contributes to the observed photo-
current. In addition, the spectra for the negative electrode
illumination show some complexity in which the patterns
consist of mainly antibatic with some symbatic ones. In
particular, a maximum of the photocurrent was observed
between minima and maxima in the absorption spectrum,
indicating that the illuminated light from the negative side
might reach the interface on the counter electrode to generate
carriers in an extrinsic way in the thinner cell.
This indicates that the observed photocurrent arises from (i)
an extrinsic process performed at the interface between an ITO
electrode and a mesogenic long-chain metallotetraphenylpor-
phyrin and (ii) an intrinsic process working in the bulk region
as shown in Fig. 8. This carrier generation situation is similar
to the metal-free one.15,16 In fact, Tanimura et al. reported that
an electronic injection occurs from the exited tetraphenylpor-
phyrin (non-peripheral alkyl chains) to the ITO electrode.24
This injection phenomenon could act as a generation process of
hole carriers in this system. Thus, it is reasonably thought that
such an electronic injection similarly occurs at the interface
between a mesogenic metallotetraphenylporphyrin and an ITO
electrode, when the long-chain metallotetraphenylporphyrin is
raised to the excited state by light absorption.
Fig. 9 Light intensity dependence of photocurrents for a 4.2 mm thick
ITO/C15TPPNi/ITO cell under 8000 V cm21 electric bias and 420 nm
light illumination for the positive ($) and negative (&) electrodes at
room temperature.
j!I0n
(2)
These values are crucial for the photocarrier generation process
and the carrier recombination in transit. The relation among
the carrier generation, recombination and light exponent n is
summarised in Table 2 for low intensity measurements.25–27
Actually the values of light exponent were obtained by log-log
plots of photocurrent vs. illumination light intensity, in which
all plots exhibited well-defined straight lines to give the value n
within the intensity region of the measurements as shown in
Fig. 9. As for the case of the present cell, extrinsic and intrinsic
processes are generating one carrier and two carriers by one
photon absorption, respectively.
Table 3 shows the film thickness and the light wavelength
dependence of light exponent values experimentally obtained at
room temperature for both positive and negative electrode
illumination. The light exponent values for positive electrode
illumination appeared between 0.5 and 1, in contrast to almost
unity of the exponent values for the negative electrode
illumination. These results indicate that in the positive
electrode illumination, the charge carrier contributing to the
observed photocurrent is ‘holes’ generated by both extrinsic
3.3. Light intensity dependence of photocurrent
On the other hand, it is well known that the current density j
observed is related to the illumination light intensity I0 as
shown in eqn. (2) and the value of light exponent n varies
depending on the charge carrier generation process.
Table 3 Film thickness dependence of light exponents in the positive
and negative electrode illumination for a symmetrical ITO/C15TPPNi/
ITO cell
Table 2 Light exponents depending on the charged carrier generation
and transit processes of photoconduction for low intensity illumination
Light exponent
Light exponent
Recombination
Electrode illuminated
Film thickness/mm
Wavelength/nm
positive
negative
Carrier number by one photon
limited
negligible
4.2
420
680
680
0.62
0.89
1.0
1.0
1.0
1.0
1
2
0.5
1.0
1.0
2.0
29.8
1388
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Fig. 11 Applied electric field dependence of photocurrents for ITO/
C15TPPNi/ITO cells (a) 29.4 mm and (b) 4.2 mm thick for the crystal
Fig. 10 Applied electric field dependence of dark-current for ITO/
15TPPNi/ITO cells (a) 29.4 mm and (b) 4.2 mm thickness for the crystal
C
(30 ‡C; $), lower temperature region of MLC (80 ‡C; Y) and higher
temperature region of MLC (110 ‡C; &) under 420 nm (dotted line,
relieved in white) and 680 nm (solid line) light illumination with an
(30 ‡C; $), lower temperature region of MLC (80 ‡C; Y) and higher
temperature region of MLC (110 ‡C; &) phases of C15TPPNi.
intensity of 0.34 mW cm22
.
and intrinsic processes, though in the negative electrode
illumination the carrier is generated only by the intrinsic
process.
3.4. Applied electric field dependence of dark- and photo-
currents
Table 4 shows the phase dependence of light exponent values
experimentally obtained for both positive and negative
electrode illumination of a 4.2 mm thick cell. The light exponent
values for the positive electrode illumination appeared between
0.5 and 1, and the values increase as the temperature is
elevated. This implied that the contribution of the extrinsic
process for the observed photocurrent depends on the phase
and it decreased with the crystal to MLC phase transition as
well as the temperature increase in the MLC phase.
Fig. 10 shows the dark-current vs. applied electric field
characteristics of the ITO/C15TPPNi/ITO cells; (a) 29.4 mm
and (b) 4.2 mm thick cells for the crystal, the low temperature
region of MLC (80 ‡C) and the higher temperature region of
MLC (110 ‡C) phases for C15TPPNi. Symmetrical features are
clearly shown in the bias electric field of ¡20000 V cm21. The
conductivity s was calculated to be y10215 O21 cm21 for the
crystal and the low temperature region of MLC phases and
y10213 O21 cm21 for the higher temperature region of the
MLC phase. The conductivity of C15TPPNi is comparable with
that of the free base, C15TPPH2, in any phases.
Fig. 11 shows the photocurrent vs. applied electric field
characteristics of the cells at room temperature, 80 ‡C and
110 ‡C. Marked rectification of the photocurrent is apparent
for both the crystal and MLc phase 4.2 mm thick cells, the
positive electrode illumination causing a larger photocurrent
which increases with the bias voltage, though symmetrical
behaviour is observed for the MLC phase 29.4 mm thick cell.
The wavelength effect on the photocurrent is remarkable: see
Fig. 11(b). In the case of illuminating light wavelength of
420 nm, corresponding to the Soret band absorption max-
imum, the photocurrent rectification is more remarkable than
that at 680 nm which is a tail part of the Q-band absorption.
Table 4 Light exponents depending on the phase transitions of
C15TPPNi in a 4.2 mm thick ITO/C15TPPNi/ITO cell under 420 nm
light illumination
Light exponent
Electrode illuminated
positive
Phase
negative
Crystal (30 ‡C)
MLC (80 ‡C)
ML (110 ‡C)
0.62
0.89
1.0
1.0
1.0
1.0
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Fig. 12 Schematic representation of the carrier generation mechanism
dependence of the observed photocurrent to the electrodes.
This phenomenon probably relates to the penetration depth of
the illuminated light. The results indicate that the photo-
conductive phenomena of the cell are related to the bimolecular
recombination process, the photocarrier generation process
near the mesogen and electrode interface as well as in the bulk
region and where the carriers transport toward the counter
electrode (Fig. 12). It is a similar process to the case of metal-
free C15TPPH2 in the MLC phase.
Thus, the photocurrent rectification observed in the V–I
(photocurrent) characteristics of the 4.2 mm thick cell, which
could be seen for the crystal and MLC phases, is mainly caused
by the carriers generated by the extrinsic process of hole
generation by an electron injection from the excited porphyrin
to the ITO electrode. The extrinsic process occurs only for the
positive electrode, leading to the rectification, and probably
electron injection from the excited long-chain tetraphenylpor-
phyrin to the ITO electrode is promoted by an applied electric
field. Similar rectification behaviour was observed for the same
type of cell using the analogous metal-free, Cu(II), and Zn(II)
complexes having long chains.13–16,28 For the same type of
symmetrical cell using a mesomorphic Zn(II) porphyrin (octa-
substituted), a similar mechanism of charged carrier generation
was proposed in relation to the photovoltaic properties.4
It was revealed that a series of phase transitions for this
compound do not cause any drastic changes to the lifetime of
the singlet excited state of C15TPPH2, which varies within the
range 8.5–10 ns through the crystal to ML phase transitions,
though it drops to 7 ns at the clearing point.29 In the case of
phthalocyanine solid films, it is known that their photocurrent
behaviour is significantly sensitive to the molecular order of the
films on the electrode surface. A crystal to MLC phase
Fig. 13 Temperature dependence of (a) dark-current under
4000 V cm21 and (b) photocurrent at 420 nm light illumination
under 4000 V cm21 electric bias, for the positive ($) and negative
(&) electrode illumination of a 4.2 mm thick ITO/C15TPPNi/ITO cell.
transition gives rise to changes not only of the molecular
order, but also of the state of molecular fluctuation. The change
of molecular order caused by this phase transition is inferred to
be a change of the molecular stacking manner based on the
XRD results. It would be reasonable to think that the
molecular stacking is crucial for charge carrier hopping and
the hopping probability is quite sensitive to the relative
configuration of molecules, a manner of stacking as for the
columnar structure. On the other hand, the change of
fluctuational modes and degrees caused by mesomorphic
phase transitions could affect the charge carrier hopping
rate.30 For the present cell using a mesomorphic compound,
both changes of molecular ordering and molecular dynamics
should be considered for the observed phenomena.
1390
J. Mater. Chem., 2001, 11, 1383–1392

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j~nemE
(3)
According to the result of TOF studies as shown in Fig. 14, the
positive carrier mobility is kept in the order of
1024 cm2 V21 s21 in the whole temperature range of the MLC
phase and hole conduction is dominant for the observed
photocurrents. We could not evaluate the carrier mobility for
negative charge because of the undetectable level of the
transient photocurrent observed. The cell used for the mobility
measurements was not a mono-domain film of the mesogen.
The lamellar mesophase of C15TPPNi has high viscosity and
consequently it was difficult to obtain uniform alignment in the
film as a large domain. Thus, the real mobility is, at least, faster
than 1024 cm2 V21 s21. It is also reported that the hole
mobility is temperature independent in the mesomorphic
phase in the case of triphenylene derivative discotic crystals
with hexagonal arrays and the corresponding metal-free
C15TPPH2.31,32 In our case, positive carrier mobility was
observed to be independent of the temperature. According to
the recent works by Funahashi et al., domain boundaries of
conventional rod-like liquid crystalline systems essentially do
not affect the charge migration efficiency and this is proposed
to be a characteristic property of charge transportation in
liquid crystals.33,34 This implies that the surprising tempera-
ture-independent mobility change is a characteristic of the
lamellar mesophase. The evaluated carrier mobility of the
lamellar mesophase (1024 cm2 V21 s21) was smaller than that
in the hexagonal columnar phase for an alkylthiotriphenylene
derivative (1023 cm2 V21 s21) and that in more ordered
Fig. 14 (a) Temperature dependence of positive carrier mobility of
15TPPNi. (b) A photocurrent transient in the MLC (80 ‡C) phase for
positive electrode illumination. Cell thickness: 4.2 mm, electric field:
C
1.26105 V cm21, wavelength of pulse: 420 nm.
3.5. Temperature dependence of dark- and photo-currents and a
positive carrier mobility
structures
such
as
the
helical
columnar
phase
(1021 cm2 V21 s21).11,35
Fig. 13a and b show the dark- and photo-currents as a function
of temperature for a 4.2 mm thick ITO/C15TPPNi/ITO cell,
respectively. The applied bias was 4000 V cm21 (1.68 V), the
illumination light wavelength was 420 nm and the light
intensity was 0.34 mW cm22. The dark-current temperature
dependence of the MLC and isotropic phases obeyed the
Arrhenius’ equation in the low temperature range. The values
of the activation energy for the conduction in each phase were
calculated to be 0.69 eV and 0.72 eV for the MLC and isotropic
phases, respectively. In the higher temperature region of the
MLC phase, the dark-current was drastically increased. This
could be explained by a pre-transition effect. It seems to be due
to the additional contribution of ionic species to the current,
probably a small amount of impurities or the generated hole
molecules which could be transferable with the increase of
molecular motion near the phase transition.
Therefore, this increase of photocurent could be mainly due
to an increase of the effective number of charge carriers because
the carrier mobility is independent of the temperature. This
increase of photocurrent could be owing to the improvement of
carrier transportation at the the domain–domain (in the bulk
region) and the electrode–sample interfaces in the conduction
process. In addition, the ionic conduction might contribute to
the steady-state photocurrent with increasing temperature. The
contact of a mesogenic material and a solid electrode which
provides electric and ionic conduction pathways at the
interfaces may have been improved by the molecular motion
on heating (wetting). Although the fluctuations of the
columnar structure are disadvantageous for electronic con-
duction, the advantage of molecular contact at the interfaces
seems to be dominant for electric conduction in this ITO/
C15TPPNi/ITO system.
On the other hand, the photocurrent shows different
behaviour from the dark-current. It behaves in a different
manner between the positive and negative electrode illumina-
tions. In positive electrode illumination, the photocurrent
drastically increases upon heating the MLC phase while there is
a gradual increase in the crystal and MLC phases for negative
electrode illumination, followed by an undetectable level of
photocurrent at the MLC to isotropic phase transition for both
cases. In the MLC phase, the electronic conduction such as
charge hopping is probably dominant as the charge migration
process, rather than ionic conduction. Because of the high
viscosity of the lamellar mesophase, it could not be thought
that the charged porphyrin molecules themselves are able to
easily migrate. Although there still remains the contribution of
the ionic conduction for observed photocurrent as a increase of
the temperature. We could expect two reasons that the hole
conduction is dominant in ITO/C15TPPNi/ITO cells. Firstly,
the intrinsic defects are mainly p-type impurities, and secondly,
the level of the work function of ITO is less than that for the
metal-free system.
4. Conclusion
The action spectra and the applied electric field dependence of
the photocurrent indicated that hole generation at the interface
of the positive electrode is essential for the photocurrent
rectification behaviour observed in the crystal and lamellar
(MLC) phases of C15TPPNi. These phenomena may be related
to the molecular order near the mesogen–electrode interface. In
the case of the metal-free analogue, the photocurrent rectifica-
tion was observed in the crystal and low-temperature lamellar
phases and it disappears at the low-temperature lamellar (MLC
)
to high-temperature lamellar (ML) phase transition. This
suggests that the lamellar mesophase which was exhibited in
the C15TPP Ni complex is the low-temperature lamellar phase
in the metal-free compound which is close to the crystal phase.
This is also supported by the X-ray diffraction results.
The dark- and photo-currents were increased on heating and
it might be that electronic conduction rather than ionic
conduction is dominant because the molecule is too large for
ionic conduction in this highly viscous mesophase. Since the
positive carrier mobility was retained in the MLC phase, the
The current density j is proportionally dependent on the
charged carrier number n, mobility m and electric field strength
E as shown in eqn. (3).
J. Mater. Chem., 2001, 11, 1383–1392
1391

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11 N. Boden, R. J. Bushby and J. Clements, J. Chem. Phys., 1993, 98,
5920.
12 D. Adam, P. Schumacher, J. Simmerer, L. Haussling,
K. Siemensmeyer, K. H. Etzbach, H. Ringsdorf and D. Haarer,
Nature, 1994, 371, 141.
13 Y. Shimizu, A. Ishikawa and S. Kusabayashi, Chem. Lett., 1986,
1041.
14 Y. Shimizu, A. Ishikawa, S. Kusabayashi, M. Miya and A. Nagata,
J. Chem. Soc., Chem. Commun., 1993, 656.
photocurrent increase arose from a increase of the number of
carriers which was due to the better charge transportation at
the interfaces. The contact of a mesogenic material and a solid
electrode which provides electric and ionic conduction path-
ways at the interfaces was improved by the molecular motion
on heating (wetting). We conclude that the improvement of the
matching for conduction at the interface on heating is a special
feature of this MLC mesomorphic phase.
15 Y. Shimizu, T. Fuchita, T. Higashiyama and T. Sugino, Mol.
Cryst. Liq. Cryst., 1999, 331, 575.
16 Y. Shimizu, T. Higashiyama and T. Fuchita, Thin Solid Films,
1998, 331, 279.
Acknowledgements
17 Y. Shimizu, H. Monobe, S. Mima, T. Higashiyama, T. Fuchita
and T. Sugino, Mater. Res. Soc. Symp. Proc., 1999, 559, 211.
18 A. Reiche, H. Gross and E. Ho¨ft, Chem. Ber., 1966, 93, 88.
19 A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour
and L. Korskaoff, J. Org. Chem., 1967, 32, 476.
20 A. D. Adler, F. R. Longo, F. Kampas and J. Kim, J. Inorg. Nucl.
Chem., 1970, 32, 2443.
The authors acknowledge the Agency of Industrial Science and
Technology (AIST), Ministry of International Trade and
Industry of Japan (MITI) for the financial support under the
national R&D project, ‘‘Harmonised Molecular Materials’’
operated in Industrial Science and Technology Frontier
Program of AIST-MITI.
21 K. Rousseau and D. Dolphin, Tetrahedron Lett., 1974, 48, 4251.
22 G. H. Barnet, M. F. Hudson and K. M. Smith, J. Chem. Soc.,
Perkin Trans. 1, 1975, 1401.
23 G. Giro, J. Kalinowski, P. D. Marco, V. Fattori and G. Marconi,
Chem. Phys. Lett., 1993, 211, 580.
References
1
The Porphyrins, ed. D. Dolphin, Academic Press, New York, 1978,
vol. I–VII.
24 K. Tanimura, T. Kawai and T. Sakata, J. Chem. Phys., 1979, 45,
2639.
2
The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, San Diego, 2000.
F. J. Kampas, K. Yamashita and J. Fajer, Nature, 1980, 284, 40.
B. A. Gregg, M. A. Fox and A. J. Bard, J. Phys. Chem., 1990, 94,
1586.
25 R. F. Chaiken and D. R. Kearns, J. Chem. Phys., 1966, 45, 3966.
26 G. R. Johnston and L. E. Lyons, Aust. J. Chem., 1970, 23, 1571.
27 G. R. Johnston and L. E. Lyons, Chem. Phys. Lett., 1968, 2, 489.
28 Y. Shimizu and H. Nakayama, unpublished data.
29 Y. Shimizu and T. Higashiyama, Proc. SPIE, Int. Soc. Opt. Eng.,
1998, 3319, 177.
30 J. Simmerer, B. Glu¨sen and W. Paulus, Adv. Mater., 1996, 8, 815.
31 D. Adam, T. Closs, D. Funhoff, D. Haarer, H. Ringsdorf,
P. Schuhmacher and K. Siemensmeyer, Phys. Rev. Lett., 1993, 70,
457.
32 H. Monobe, S. Mima and Y. Shimizu, Chem. Lett., 2000, 1004.
33 M. Funahashi and J. Hanna, Appl. Phys. Lett., 1998, 73, 3733.
34 M. Funahashi and J. Hanna, Appl. Phys. Lett., 2000, 76, 2574.
35 A. M. van de Craats, J. M. Warman, M. P. de Haas, D. Adam,
J. Simmerer, D. Haarer and P. Schuhmacher, Adv. Mater., 1996, 8,
823.
3
4
5
T. Saito, S. Wada, T. Kobayashi, S. Suzuki and T. Iwayanagi,
J. Phys. Chem., 1993, 97, 8026.
Z. D. Popvoic, Chem. Phys., 1984, 86, 311.
P. G. Schouten, J. M. Warman, M. P. de Haas, M. A. Fox and H.-
L. Pan, Nature, 1991, 353, 736.
6
7
8
9
Metallomesogens—Synthesis, Properties and Applications, ed.
J. L. Serrano, Wiley-VCH, Weinheim, 1996.
N. Boden and B. Movaghar, in Handbook of Liquid Crystals, eds.
D. Demus, J. W. Goodby, G. W. Gray, H. W. Spiess and V. Vill,
Wiley-VCH, Weinheim, 1998, vol. 2B, ch. IX.
10 Y. Shimizu, M. Miya, A. Nagata, K. Ohta, I. Yamamoto and
S. Kusabayashi, Liq. Cryst., 1993, 14, 735.
1392
J. Mater. Chem., 2001, 11, 1383–1392