Photophysical and electrochemical properties of monoporphyrin rare earth liquid crystalline materials

Article abstract of DOI:10.1016/j.jpcs.2007.01.018

The synthesis and characterization of [5-(p-alkacyloxy ) phenyl-10,15,20-tri-phenyl] porphyrin (APTPP) and its lanthanide complexes(lanthanide ions: Ho(III), Dy(III), Er(III), Yb(III)) are reported. They form hexagonal columnar discotic columnar (Col_h) liquid crystals over an extended domain of temperature. Luminescence spectra of the compounds are discussed. Quantum yields of Q band are in the region 0.004570-0.05847. The electrochemical property is studied by cyclic voltammetry. The synthesized APTPP and its lanthanide complexes exhibit two one-electron reversible redox reactions and three redox reactions, respectively. The photovoltaic properties and charge-transfer process of the liquid crystalline compounds are investigated by surface photovoltage spectroscopy (SPS) and electric-field-induced surface photovoltaic spectroscopy (EFISPS) techniques, and the bands are analogous with the ultraviolet (Uv) -visible absorption spectra, which reveal that all compounds are P-type semiconductors. All of compounds are nonelectrolytes.

Full text of DOI:10.1016/j.jpcs.2007.01.018

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 68 (2007) 541–548
www.elsevier.com/locate/jpcs
Photophysical and electrochemical properties of monoporphyrin rare
earth liquid crystalline materials
Ã
Miao Yu , Guo-Jun Chen, Guo-Fa Liu
College of Chemistry, Jilin University, Changchun 130023, People’s Republic of China
Received 11 July 2006; received in revised form 8 November 2006; accepted 11 January 2007
Abstract
The synthesis and characterization of [5-(p-alkacyloxy ) phenyl-10,15,20-tri-phenyl] porphyrin (APTPP) and its lanthanide
complexes(lanthanide ions: Ho(III), Dy(III), Er(III), Yb(III)) are reported. They form hexagonal columnar discotic columnar (Colh)
liquid crystals over an extended domain of temperature. Luminescence spectra of the compounds are discussed. Quantum yields of Q
band are in the region 0.004570–0.05847. The electrochemical property is studied by cyclic voltammetry. The synthesized APTPP and its
lanthanide complexes exhibit two one-electron reversible redox reactions and three redox reactions, respectively. The photovoltaic
properties and charge-transfer process of the liquid crystalline compounds are investigated by surface photovoltage spectroscopy (SPS)
and electric-field-induced surface photovoltaic spectroscopy (EFISPS) techniques, and the bands are analogous with the ultraviolet (Uv)
–visible absorption spectra, which reveal that all compounds are P-type semiconductors. All of compounds are nonelectrolytes.
r 2007 Elsevier Ltd. All rights reserved.
Keyword: A. Organometallic compounds; A. Semiconductors; B. Chemical synthesis; C. Differential scanning calorimetry (DSC); C. X-ray diffraction
1. Introduction
porphyrins in electron-transfer processes. Their photo-
stability, high visible extinction coefficients, and low
processing costs make them attractive candidates for
electrophotographic, photovoltaic, and photoelectrochem-
ical applications [6–10]. In this paper, we introduce two
series of disk-shaped lanthanide porphyrin LCs. Their
liquid crystalline phases are determined by X-ray diffrac-
tion at small angles region, and differential scanning
calorimetry (DSC). We comprehensively study electroche-
mical, photophysical and semiconductor properties of the
compounds by cyclic voltammetry, luminescence spectro-
scopy and surface photovoltaic spectroscopy. The structure
of the compound is shown in Figs. 1 and 2.
Liquid crystal (LC) displays are known in which a single
(liquid) crystal, 5 mm thick, covering an area of ca. 500 cm2,
is formed by simple capillary-filling of the isotropic
material between two sheets of conducting glass (coated
with the appropriate aligning agents) and cooling into the
liquid crystalline phase [1]. LCs are one of the few classes of
organic materials to have entered the field of sophisticated
electronic applications, in particular as displays. This is due
to the fact that they can be orientated in applied electric
and magnetic fields [2]. Metallomesogens provide a number
of new shapes and structures from which to test the limits
of LC stability. Representative examples of these materials
have been found for all major classes of rod- and disk-
shaped LCs [3–5].
2. Experimental section
Porphyrin thin films have received considerable attention
because of their interesting excited-state chemistry and
catalytic behavior and because of Nature’s ubiquitous of
2.1. Apparatus and measurements
Elementary analysis was measured by a Pekin-Elmer
240C auto elementary analyzer. Ultraviolet (Uv) –visible
spectra were recorded on a shimadzu Uv-240 spectro-
photometer in the range 350–700 nm using chloroform
Ã
Corresponding author. Tel.: +86 13504329737;
fax: +86 0431 5168439.
E-mail address: yumiao@jlu.edu.cn (M. Yu).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.01.018

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Fig. 1. Porphyrin liquid crystalline compounds: (a) [5-(p-alkacyloxy)phenyl-10,15,20-tri-phenyl] porphyrin, n ¼ 8, 10 correspond to 10L, 12L,
respectively; (b) [5-(p-alkacyloxy) phenyl-10,15,20-tri-phenyl] porphyrin lanthanide complex, n ¼ 8, Ln ¼ Er, Ho, Dy corresponding to 10ErOH,
10HoOH, 10DyOH, respectively; n ¼ 10, Ln ¼ Yb, Er, Ho corresponding to 12YbOH, 12ErOH, 12HoOH, respectively.
at room temperature were determined by cyclic voltam-
metry using a three-electrode system under deaerated
conditions and a CHI 6000A electrochemical analyzer.
3.0
2.5
2.2. Synthesis of the compounds
2.0
2.2.1. 12L
[5-(p-hydroxy)phenyl-10,15,20-tri-phenyl]porphyrin was
prepared by the literature method [11]. This product
(2.00 g) was dissolved in 150 ml of benzene and dodecan-
noyl chloride (2.0 ml) in 20 ml benzene was added dropwise
to the above solution within 2 h at 701. The reaction
solution was refluxed for 2 h and cooling to room
temperature. Benzene was removed under reduced pres-
sure. Then 150 ml distilled water was added to the reaction
mixture and extracted four times with 150 ml chloroform.
The chloroform solution was applied to chromatography
aluminum oxide column. The first band containing 12L
was collected, the elution solution concentrated and the
product dried under vacuum. 10L was prepared by the
similar method.
1.5
1.0
0.5
0.0
400
500
600
700
Wavelength(mm)
Fig. 2. Uv/visible spectra of 12L (—) and 12HoOH (yy) in a solution of
CHCl3.
as solvent. IR spectra were recorded on a Nicolet 5PC-FT-
IR spectrometer using KBr pellets in the region
400–4000 cmꢀ1. The 1H NMR spectra were recorded in
deuterated chloroform using a Varian-Unity-400 NMR
spectrometer and employing tetramethylsilane as internal
reference. Molar conductances of 10ꢀ3 mol dmꢀ3 chloro-
form solution at 251 were measured on a DDX-111A
conductometer. differential scanning calorimetry (DSC)
was recorded on a NETESCH DSC 204. The optical
microscopical properties were observed by a XinTian XP1
(CCD: TOTA-500II) polarized light microscope equipped
with a variable temperature stage (Linkam TMS 94). X-ray
diffraction was recorded by a Shimadzu XRD-6000.
Excitation and emission spectra at room temperature in
the region 300–800 nm using 10ꢀ5 mol dmꢀ3 chloroform
solution were measured by FS920 steady-state fluorescence
spectrometer. The SPS and EFISPS were recorded with
Asokid junction photovoltaic cell ITO/sample/ITO (ITO:
idium tin oxide) using light source-monchromator-lock-in
diction technique. Redox potentials of the porphyrins in
DMF containing 0.1 M TBAP as a supporting electrolyte
2.2.2. 12YbOH
A mixture of 12L (300 mg, 0.4 mmol) and hydration
YbCl3 ꢁ 6H2O (600 mg, 1.5 mmol) were heated in an
imidazole (10.0 g) melt at 2101 under the protection and
stirring of a dry nitrogen stream for 2 h. The extent of the
reaction was followed by measuring the Uv–visible spectra
of the reaction solution at 10 min intervals. After cooling
the reaction mixture to 1001, 150 ml distilled water was
added and the solution filtered, washed several times with
distilled water in a separating funnel, and finally the
product was dried under vacuum. The crude product was
dissolved in chloroform (200 ml). The solution was shaken
with 100 ml 0.1% aqueous AgNO3 and the chloroform
layer was separated. The mixture of chloroform (30 ml) and
methanol (20 ml) were added to the separated chloroform.
The mixture was again shaken with 100 ml 0.1% aqueous
AgNO3. This procedure was repeated until no more AgCl
precipitated. The chloroform solution was concentrated
and applied to chromatography aluminum oxide column.
The first band containing a small amount ligand, 12L, was

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543
eluted by chloroform. The second band eluted by chloro-
form containing 20% methanol, obtained the solution of
the complex. The product was crystallized by concentrated
the solution and finally product was dried under vacuum.
The title compound was obtained as a purple solid. Other
complexes (12ErOH, 12HoOH, 10ErOH, 10HoOH,
10DyOH) were prepared by the similar method.
disappear, since the hydrogen atoms of the porphyrin core
are replaced by the lanthanide ion to form Ln–N bond.
Existence of the Ln–OH bend vibration band in the region
1065–1068 cmꢀ1 shows that the hydrogen atoms of
hydroxyl group is bonded to lanthanide ion. Assignments
of other absorption bands are also presented in Table 2.
3.3. Molar conductances
3. Results and discussion
The molar conductance values of the ligand 10L and its
complexes 10ErOH, 10HoOH, 10DyOH are at 0.135, 0.522,
0.036 and 0.032 Oꢀ1 cm2 molꢀ1, respectively. Those of the
ligand 12L and its complexes 12YbOH, 12ErOH, 12HoOH
are at 0.125, 0.036, 0.051 and 0.042 Oꢀ1 cm2 molꢀ1, respec-
tively. It indicates that the ligands and all complexes show
nonelectrolytic behavior [12].
3.1. Composition of the compounds
The elemental analyses for carbon, hydrogen, nitrogen,
empirical formulae, yields and the decomposition tempera-
tures are given in Table 1.
3.2. IR spectra
3.4. 1H NMR spectra
The main spectral band frequencies of the porphyrin
compounds are presented in Table 2. The spectral bands at
966 and 3319 cmꢀ1 in the ligands porphyrin are assigned to
the N–H bending and stretching vibrations of the
porphyrin core. The two vibration bands of complexes
The 1H NMR chemical shift values in deuterated
chloroform (d, ppm) for the ligand 10L and complexes
10ErOH are collected in Table 3. Compared with the
ligand 10L, the signal of the complex 10ErOH disappears
Table 1
Characterization data of compounds
Compounds
Emperical formula
C (%)
N (%)
H (%)
Yields (%)
Dec. temp. (1)
10L
12L
C54N4H48O2
C56N4H52O2
82.58 (82.62)
82.66 (82.72)
67.05(67.08)
67.16 (67.22)
67.40 (67.43)
67.16 (67.20)
67.65 (67.61)
67.69 (67.74)
7.16 (7.14)
6.90 (6.89)
5.81 (5.80)
5.82 (5.81)
5.81 (5.83)
5.58 (5.60)
5.60 (5.63)
5.64 (5.65)
6.14 (6.16)
6.46 (6.45)
4.87 (4.89)
4.90 (4.88)
4.90 (4.89)
5.12 (5.10)
5.11 (5.13)
5.15 (5.14)
70
68
83
84
88
82
84
86
4400
4400
4200
4200
4200
4200
4200
4200
10ErOH
10HoOH
10DyOH
12YbOH
12ErOH
12HoOH
ErC54N4H47O3
HoC54N4H47O3
DyC54N4H47O3
YbC56N4H51O3
ErC56N4H51O3
HoC56N4H51O3
Theoretical values in parenthesis.
Table 2
Infrared spectral frequencies (cmꢀ1) of compounds
Compounds
10 L
O–H
C–H
N–H (pyrrole)
CQO
C–C (benzol ing)
1597
C–H (pyrrole)
1471
Ln–OH
C–O
N–H (pyrrole)
966
2926
2855
2920
2851
2922
2852
2922
2851
2922
2853
2922
2852
2922
2850
2922
2850
3319
3319
1761
1199
1199
1199
1199
1198
1198
1198
1198
12 L
1765
1763
1763
1762
1762
1760
1762
1597
1596
1596
1596
1597
1598
1597
1466
1468
1466
1464
1465
1466
1465
966
10ErOH
10HoOH
10DyOH
12YbOH
12ErOH
12HoOH
3424
3420
3422
3430
3429
3431
1068
1068
1067
1065
1065
1066

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at ꢀ2.74 ppm, since the hydrogen atom in the N–H bond is
replaced by the rare earth ion and the signal at 0.20 ppm
appears. The latter being due to hydroxyl group coordi-
nated to the rare earth ion. The disappearance of the signal
at ꢀ2.74 ppm and the appearance of the signal peak at
0.20 ppm indicate that the porphyrin ligand and hydroxyl
group are coordinated to the lanthanide ion.
vibrations of the porphyrin core. When the lanthanide
complexes formed, the two bands disappeared. Which
indicate the hydrogen atoms of the porphyrin core are
replaced by the lanthanide ion to form Ln–N bond.
Existence of the Ln–OH bend vibration band in the region
1065–1068 cmꢀ1 shows that the hydrogen atoms of
hydroxyl group is bonded to lanthanide ion. The elemental
analytical experimental data for carbon, hydrogen, nitro-
gen consistent with theoretical values. The molar con-
ductance values of the complexes are lower. They are all
nonelectrolyte.
3.5. Uv–visible spectra
The maximum absorption values (lmax) of the ligand and
complexes are shown in Table 4. Characteristic Q and B
(Soret) bands of porphyrins and metal porphyrins in visible
and near-ultra violet ranges are assigned as the transitions
from ground state (S0) to the lowest excited singlet (S1) and
second lowest excited singlet state (S2), respectively. The
absorption spectra of all complexes are very similar.
Compared with the ligands, the number of the absorption
bands of the complexes decrease, it is attributed to
symmetry increase of the complexes. Soret bands exhibit
small shifts to longer wavelengths. The absorption bands in
the Q-band region are designed as I, II, III, IV from red to
blue. These are similar with the results of literatures [13,14].
The IR spectral bands at 966 and 3319 cmꢀ1 in the ligand
porphyrin are assigned to the N–H bending and stretching
It may be concluded from the results mentioned above
that (a) porphyrin ligand is coordinated to a lanthanide ion
in a tetradentate fashion and (b) the coordination number
of central earth ion is 5. The lanthanide ion would be
outside of the porphyrin molecular plane.
3.6. Luminescence spectra
Table 5 gives the emission spectral data of the ligands
and complexes. The emission spectra of ligand 12L and
complex 12HoOH are shown in Fig. 3.
The near 651 nm bands of two ligands disappear. Other
spectral bands undergo only a small change. There are
fluorescence of the S2 (B, Soret band) and S1 (Q band) in
porphyrin complexes. Fluorescence of the B (Soret) band is
attributed to transition from the second excited singlet
state S2 to ground state S0, S2-S0. The Soret fluorescence
Table 3
1H NMR spectral bands of ligand and complex
Compounds
10L
Proton
numbers
Position of the
protons
d (ppm)
Table 5
Emission spectral data of ligands and compounds
8
19
2
Pyrrole, ring
Phenyl, ring
–OOC(CH2)–
–(CH2)n–
–CH3
8.85
Compounds
Peak values (nm)
Quantum yields (Ff)
7.24, 7.50, 7.78, 8.22
2.72–2.76
1.25–1.90
0.88–0.93
ꢀ2.74
Q(0–0)
Q(0–1)
Q(0–2)
16
3
10L
12L
601
601
605
604
604
608
601
605
651
652
653
653
653
655
653
652
717
717
714
709
716
713
714
715
0.02201
0.05847
0.01661
0.004570
0.01306
0.01075
0.008707
0.05605
2
N–H
10ErOH
10HoOH
10DyOH
12YbOH
12ErOH
12HoOH
10ErOH
8
19
2
Pyrrole, ring
Phenyl, ring
–OOC(CH2)–
–(CH2)n–
–CH3
8.41
7.99, 7.63, 7.27
2.40
1.33
0.90
0.20
16
3
1
–OH
Table 4
Uv–visible spectral data of compounds
Compounds
lmax (nm)
B (Soret)
I
II
III
IV
10L
12L
417 (2.8 ꢂ 105)
418 (2.5 ꢂ 105)
423 (2.5 ꢂ 105)
421 (2.9 ꢂ 105)
418 (3.0 ꢂ 105)
422 (2.7 ꢂ 105)
421 (1.9 ꢂ 105)
516 (1.6 ꢂ 104)
515 (9.6 ꢂ 103)
519 (1.2 ꢂ 103)
516 (4.0 ꢂ 104)
516 (1.9 ꢂ 104)
515 (8.1 ꢂ 103)
515 (6.7 ꢂ 103)
550 (7.6 ꢂ 103)
550 (4.9 ꢂ 103)
552 (8.7 ꢂ 104)
552 (1.6 ꢂ 105)
552 (2.6 ꢂ 104)
552 (1.8 ꢂ 104)
590 (4.9 ꢂ 103)
590 (2.7 ꢂ 103)
590 (2.3 ꢂ 104)
591 (4.2 ꢂ 104)
591 (1.2 ꢂ 104)
590 (8.0 ꢂ 103)
591 (7.0 ꢂ 103)
647 (3.9 ꢂ 103)
647 (1.9 ꢂ 103)
10ErOH
10HoOH
10DyOH
12YbOH
12HoOH
552 (1.4 ꢂ 104)
Molar extinction coefficient in parenthesis.

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is about two orders of magnitude weaker than the S1-S0
of Q band emission. Its quantum yield is so low that
sometimes fluorescence becomes unobservable. This fluor-
escence emission does not occur yet at room temperature in
our experimental excited wavelength, 415 nm. Q (0–0)
fluorescence bands of the complexes are in the region
601–608 nm. Q (0–1) fluorescence bands of the complexes
are in the region 651–655 nm and Q (0–2) bands
709–717 nm. They are mirror symmetric to the absorption
spectra. Quantum yields (Ff) of Q band for the complexes
are in the range 0.0045–0.056 and ligands 0.022–0.058. The
S1-S0 quantum yield depends on the relative rates of the
radiative process S1-S0 and two radiationless processes
24000
18000
12000
6000
0
S1
S0 and S1
Compared the complexes with their ligands, quantum
Tn.
yields of complexes are much less than ligands. The
fluorescence quantum yields of our complexes are much
less than 0.20. Thus, the excited state of the complexes S1 is
primarily deactivated by radiationless decay. Therefore,
spin-forbidden process S1
Tn is the predominant
route for radiationless deactivation of S1 in the complexes.
The mentioned above quantum yields (Ff) were calcu-
lated by the following equation:
500
600
700
800
Wavelength(nm)
Ff ¼ Ffs n2 As If =ðn2s A IfsÞ,
Fig. 3. Emission spectrum of 12L (—) and 12HoOH (yy).
a
b
40
a
external electric field
100
80
a : 0.50V d : -0.25V
b : 0.25V e : -0.50V
c : 0.15V f : -1.0V
b
60
30
20
10
0
40
c
20
0
-20
-40
-60
-80
-100
d
e
f
300
400
500
600
700
300
400
500
600
700
Wavelength(nm)
Wavelength(nm)
c
d
60
50
40
40
30
20
10
0
30
20
10
0
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength(nm)
Wavelength(nm)
Fig. 4. SPS and EFISPS of 12L and 12HoOH. (a) EFISPS of 12HoOH. (b) SPS of 12HoOH. (c) EFISPS of 12L. (d) SPS of 12L.

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546
where ns, As and Ifs are the refractive index, absorbance
and integrated intensity of standard sample at excited
wavelength, respectively. Meso-tetraphenylporphyrin Zinc,
ZnTPP was used as standard sample, Ffs ¼ 0.033 [15].
3.7. Surface photovoltaic spectroscopy (SPS) and electric
field induced surface photovoltaic spectroscopy (EFISPS)
The SPS and EFISPS of the compounds 12L and
12HoOH are shown in Fig. 4. Their photovoltaic spectral
bands are given in Table 6. It is found that the photovoltaic
action spectra follow the absorption spectra response well,
indicating that they are corresponding to similar electron
transition process. Porphyrin molecule is of the conjugate p
bonding system. The p-orbitals in porphyrins are analo-
gous to valance of organic semiconductor, and its p*-
orbital to the conduction band. Photogenerated charge
carriers in the p system are non-localized, their motion is
free in the valance band, while photo-generated electrons in
the conduction band. In this kind of system, the band to
band transition is characterized as a p–p* transition,
exhibiting chiefly Soret (B) (a2u(p)-eg(p*) transition) and
Q (a1u(p)-eg(p*) transition) bands. The surface photo-
voltaic (SPV) response band at 300–400 nm (P band) is
corresponding to the higher energy level transition process,
b2u(p)-eg(p*).
The photoexcitation and photogenerated charge transfer
processes of the compounds are shown in Fig. 5,
where EC, Ev and Ef are the minimum energy of the
conduction band, the maximum energy of the valence
band and the Fermi energy level, and Eg is the
forbidden bandwidth, respectively [16]. The signal detected
by surface photovoltage spectroscopy is equivalent to
the change in the surface potential barrier on illumina-
tion: dvs ¼ vs*ꢀvs0, where vs* and v0s are the surface
potential heights before and after illumination, respec-
Fig. 5. Photoexcitation and photogenerated charge transfer processes in
the compounds.
tively. As far as band-to-band transitions are concerned, a
positive response of the surface photovoltage (dvs40)
means that the sample is characterized as a P-type
semiconductor [16].
Compared the SPS and EFISPS of two compounds,
it can be found that there are two characterizations.
One is that the SPV response intensities of P band of
complex is larger than ligand, which indicated that the P
band of the complex exhibits higher photo-electric conver-
sion efficiency. On the other hand, the number of SPV
response peaks of Q band decrease when the complex
forms. This is the similar with behavior of Uv–visible
spectrum. Increasing a positive electric field on ITO
(indium tin oxide), photovoltaic response intensities of
the complex and ligand are all enhanced, which demon-
strates the external electric field is of the same sign
as the build-in field. On the contrary, when a negative
electric field is applied, photovoltaic response intensities
are all increased. It can be seen that the Q and Soret
bands of the ligand and complex exhibit a ‘‘simultaneous
response’’ with change of positive and negative electric
field, this indicates that they are corresponding to
an analogous transition characteristic, both belonging
to the p–p* transition. However, the variation rate of B
and Q bands with change of positive and negative
electric field is different. From above discussion, it can
be found that P band is different from B and Q bands.
P band is transition from the next highest occupied
molecular orbital (NHOMO) to the lowest unoccupied
molecular orbital (LUMO) transition, while B and Q
bands arise from coupling of the two transitions between
the highest occupied molecular orbital (HOMO) and
LUMO [17].
Table 6
Spectral bands of SPS and EFISPS of the compounds
Compounds External
field
voltage (V)
Maximum peaks (lmax/nm)
P
band
Soret (B) Q band
band
12L
0
0.25
349 424 516
348 424 516
346 424 517
344 423 517
340 422 521
556
556
557
558
557
596 658
595 651
597 670
597 660
599 656
0.50
3.8. LCs
ꢀ0.25
ꢀ0.50
Small-angle X-ray diffraction of 10L at room tempera-
ture is observed. The d spacing of the first three reflections
12HoOH
0
328 433 515
327 432 513
331 423 516
332 428 519
334 431 514
332 431 515
332 431 514
557
561
560
562
561
561
562
598
601
601
601
598
601
601
0.125
0.25
0.50
˚
˚
˚
are d100 ¼ 34.7 A, d110 ¼ 20.8 A, d200 ¼ 17.4 A. The ratios
of the d spacing are 1:1/O3:1/2, which corresponds to the
first three reflections of a hexagonal columnar structure.
We conclude that the mesophase structure is probably a
hexagonal columnar discotic columnar, Colh. Wide-angle
ꢀ0.25
ꢀ0.50
ꢀ1.0

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X-ray diffraction of the compounds gave a broad peak
(centered at 2y ¼ 201) due to the commonly seen liquid-like
packing of alkyl chains in the mesophase. The X-ray
diffraction results of other compounds are similar to
those of 10L. The liquid crystalline phase of the ligands
and complexes were characterized by differential scanning
calorimetry (DSC). The DSC results of the ligands
and complexes are summarized in Fig. 6. The transition
temperature and enthalpies of the various porphyrin
compounds are given in Table 7. Compared 12L with
its lanthanide complexes, the LC phase of the complexes
tends to room temperature, but enthalpy smaller than
the ligand.
HOMO and reduction reaction in LUMO. Cyclic voltam-
metric studies have been performed to evaluate the redox
portentials and determine the HOMO–LUMO energy
level. The electrochemical properties of two ligands
and 10HoOH were studied in 0.1 M tetrabutylammonium
perchlorate (TBAP) in DMF by cyclic voltammetry
(CV). Within the accessible potential window of the
solvent, two quasi-reversible redox processes of 10L, 12L
and 10HoOH have been observed, respectively. The
first and second redox potentials corresponding to the
porphyrin ring reaction of 10L and 12L are located
at E1/2 ¼ ꢀ1.712, ꢀ2.314 and ꢀ1.720, ꢀ2.189 V vs
Ag/Ag+, respectively [18,19]. The corresponding potentials
for the porphyrin ring reaction of 10HoOH are
located at E1/2 ¼ ꢀ2.201. The first redox potentials of
10HoOH concerning lanthanide metal ion are located
at E1/2 ¼ ꢀ1.384 V vs Ag/Ag+. Fig. 7 gives the cyclic
voltammogram of 10HoOH.
3.9. Cyclic voltammetry
Porphyrin ring has higher HOMO 3a2u(p) and lower
LUMO4eg(p*). It can occur to oxidation reaction in
Temperature(°C)
20 40 60 80 100 120 140 160 180
-80 -60 -40 -20
0
10L
12L
10HoOH
12YbOH
12ErOH
12HoOH
Fig. 6. Differential scanning calorimetry of ligands and compounds. Black ¼ solid phase, white ¼ mesophase, gray ¼ isotropic.
Table 7
Phase transition temperatures and enthalpy change of the porphyrin devirations
t/°C (ΔΗ/kJmol-1)
Compounds
17 9.0(16. 0)
142. 0(6.0)
55 .3(1 .4)
55. 3( 7.6)
28 .9 (0.3)
35. 0( 7.5)
C
C
C
C
LC
LC
LC
IL
IL
IL
10L
63 .3(67.4)
42. 0(0.2)
12L
10HoOH
12YbOH
12ErOH
20. 0(0.3)
LC
LC
IL
IL
-77. 0(3.2)
4. 4(1.2)
C
37. 9( 0.2)
C
LC
IL
12HoOH
C ¼ crystal, LC ¼ liquid crystal, IL ¼ isotropic liquid; Heating(
)cycles; Heating rate: 101/min.

ARTICLE IN PRESS
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548
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Acknowledgments
We would like thank the National Natural Science
Foundation of China for financial support of this work.