Spectroscopic properties and cyclic voltammetry on a series of meso-tetra(p-alkylamidophenyl)porphyrin liquid crystals and their Mn complexes

Article abstract of DOI:10.1016/j.molstruc.2008.01.014

A series of meso-tetra(p-alkylamidophenyl)porphyrin ligands and their manganese(III) complexes are reported in this paper. The mesomorphism was investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM) and the results

Full text of DOI:10.1016/j.molstruc.2008.01.014

Available online at www.sciencedirect.com
Journal of Molecular Structure 889 (2008) 28–34
www.elsevier.com/locate/molstruc
Spectroscopic properties and cyclic voltammetry on a series
of meso-tetra(p-alkylamidophenyl)porphyrin liquid crystals
and their Mn complexes
Erjun Sun a, Yuhua Shi a, Ping Zhang a, Mi Zhou b, Yihua Zhang a,
Xuexin Tang a, Tongshun Shi a,*
a College of Chemistry, Jilin University, Jiefang Road 2519, Changchun 130023, PR China
b College of Physics, Jilin University, Jiefang Road 2519, Changchun 130023, PR China
Received 12 September 2007; received in revised form 2 January 2008; accepted 9 January 2008
Available online 17 January 2008
Abstract
A series of meso-tetra(p-alkylamidophenyl)porphyrin ligands and their manganese(III) complexes are reported in this paper. The mes-
omorphism was investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM) and the results show
that only the porphyrin ligands with long side chains show liquid crystalline behavior, and they exhibit a high phase transition temper-
ature and a broad mesophase temperature span. Furthermore, we investigated the properties of the compounds by means of UVꢀvis
spectra, infrared spectra, Resonance Raman spectra, fluorescence spectra, thermal analysis and cyclic voltammetry. These studies indi-
cate that the length of side chains has little effect on the properties of porphyrin compounds. According to thermal studies, the decom-
position of porphyrin ligand and Mn complex is a continuous process.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Porphyrins; Mn complexes; Liquid crystal; Cyclic voltammetry; Thermal analysis
1. Introduction
[7]. Since then, many mesogenic porphyrins have been
synthesized and their mesomorphic properties were inves-
tigated [8–12]. But up to now, most of the side chains of
TPP derivative liquid crystals are alkyl, alkoxy, or acyl-
oxy group, those bearing the alkylamido group as side
chains porphyrin liquid crystals have been reported little
[13]. In this paper, a series of 5,10,15,20-tetra(p-alkyl-
amidophenyl)porphyrin ligands and their Mn complexes
were synthesized and their liquid crystal properties were
studied firstly (the structures are shown in Fig. 1). The
mesomorphism studies show that only the ligands with
long chains of more than 12 carbon atoms have liquid
crystalline behavior, while the Mn complexes are not
liquid crystals. Furthermore, we have investigated these
compounds by Resonance Raman spectra, cyclic voltam-
metry, thermal studies and fluorescence spectroscopy.
These studies will provide wider ground for choice and
application of the materials.
Porphyrins and metalloporphyrins have received
extraordinary attention in recent years for using these com-
pounds in photodynamic therapy [1], optoelectronic
devices [2], sensors [3], molecular logic devices [4], and arti-
ficial solar energy harvesting and storage schemes [5]. The
porphyrin skeleton has an extended p-conjugation system
[6], leading to a wide range visible light absorptions and
p-type properties as an electronic system. Thus, the control
of orientation of porphyrin chromophores can play a very
important role.
Porphyrin liquid crystal is an important member of
the liquid crystals family. The first liquid crystalline por-
phyrin derivative was reported by Goodby et al. in 1980
*
Corresponding author. Tel./fax: +86 431 85168898.
E-mail address: sunerjun1974@hotmail.com (T. Shi).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.01.014

E. Sun et al. / Journal of Molecular Structure 889 (2008) 28–34
29
Usample ¼ UTPPZnðF sample=F TPPZnÞðATPPZn=Asample
Þ
Where Fsample and FTPPZn are measured fluorescence inte-
gral areas (under the fluorescence spectra) of the sample
and the reference TPPZn, respectively. Asample and ATPPZn
are the absorbance of the sample and the reference. Usample
and UTPPZn (UTPPZn = 0.033 [15]) are the quantum yields of
the sample and the reference TPPZn at same excitation
wavelength. The effect of solvent is neglect.
All reagents and solvents were of the commercial
reagent grade and were used without further purification
˚
except DMF was predried over activated 4 A molecular
sieve and vacuum distilled from calcium hydride (CaH2)
prior to use. The dry CH3CN was obtained by redistillation
from CaH2.
2.2. Preparation of porphyrin ligands and Mn complexes
2.2.1. Synthesis of 14L
Fig. 1. The structures of porphyrin ligands and their Mn complexes.
14L was prepared by the reaction of the 5,10,15,20-tetra
(4-aminophenyl)porphyrin (TAPPH2) which was synthe-
sized in our laboratory according to the method of Kruper
[16] with myristyl chloride. TAPPH2 (1.00 g) was dissolved
in freshly distilled CHCl3 (150 ml), and triethylamine
(2 ml) was added. A solution of myristyl chloride (2.8 ml)
in CHCl3 (10 ml) was added drop wise to the above solution
within 2 h with stirring at 70 °C. The reaction solution then
was refluxed for 8 h and cooled to room temperature. Then
200 ml distilled water was added to the reaction mixture
and extracted three times by equal volumes of freshly dis-
tilled chloroform. The chloroform solution was concen-
trated and applied to a silica gel column, and the column
was eluted with CHCl3AC2H5OH (9:1, V/V). The first band
was collected and condensed. The precipitated product was
crystallized from chloroform and ethanol and was obtained
as purple solid. Yield: 38.0%. 1H NMR (DMSO-d6, 500 Hz):
d 8.863 (s, 8H, pyrrole ring), 10.308 (s, 4H, ANHACOA),
8.050–8.137 (m, 16H, benzene ring), 1.713–1.742 (t, 8H,
ACOACH2A), 1.271–1.419 (m, 88H, alkyl CH2), 0.829–
0.856 (t, 12H, alkyl CH3), ꢀ2.898 (s, 2H, pyrrole N–H). Ele-
mental analysis: calc. for C100H138N8O4: C 79.21, H 9.17, N
7.39; Found: C 79.29, H 9.11, N 7.34.
2. Experimental
2.1. Materials and methods
UVꢀvis spectra were collected on a Shimadzu UV-240
spectrometerin the region of350ꢀ700 nm using DMF assol-
vent. Infrared spectra were recorded on a Nicolet 5PC-FT-
IR spectrometer using KBr pellets in the region of
4000ꢀ400 cmꢀ1. Raman spectra were recorded with a Reni-
shawinvia Raman spectrophotometer equipped with an inte-
gral microscopy. Radiation of 514.5 nm was obtained from
an Ar+ laser. Redox potentials of the compounds
(10ꢀ3 mol/L) in dried DMF containing 0.1 mol/L TBAP as
supporting electrolyte were determined at room temperature
by cyclic voltammetry with a CHI 660A electrochemical ana-
lyzer using a three-electrode system under deaerated condi-
tions. Platinum button and platinum wire were served as
working and counter electrodes, respectively. The reversibil-
ity of the electrochemical processes was evaluated by stan-
dard procedures, and all potentials were recorded against
an Ag/Ag+ reference electrode (0.01 mol/L AgNO3 in aceto-
nitrile (CH3CN) solution). Thermal analysis were recorded
on TGA and DTA apparatus (sample: 3ꢀ4 mg, heating rate:
10 °C/min, and atmosphere: static air). The differential scan-
ning calorimetry (DSC) was performed with a Perkin-Elmer
7 series thermal analysis system at a scan rate of 10 °C/min
under nitrogen flow. Optical microscopical properties were
observed by a XinTian XP1 (CCD: TOTA-500 II) polarized
light microscope, equipped with a variable temperature stage
(Linkam TMS 94). The fluorescence spectra were recorded
with a SPEX Fluorolog-2T2 spectrofluorometer (450-W
xenon lamp as the excitation source) and obtained from
10ꢀ5 mol/L DMF solution in 1 ꢁ 1 ꢁ 5 cm fluorescence cells
at room temperature. The quantum yields of porphyrins
were estimated from the emission and absorption spectra
by a comparative method using the following equation [14]:
2.2.2. Synthesis of 14Mn
The complex 14Mn was prepared by the reaction of 14L
(0.20 g) with MnCl2ꢂꢂꢂ4H2O (2.00 g) in CHCl3 (20 ml) and
DMF (20 ml) mixture at 70 °C under the protection of
nitrogen stream for about 3 h. The extent of the reaction
was monitored by measuring the UV–vis spectrum of the
reaction solution. The reaction mixture then was chro-
matographed
on
silica
gel
column
(eluent:
CHCl3AC2H5OH 9:1) and crystallized from chloroform
and ethanol. Yield: 80.3%. Elemental analysis: calc. for
C100H136N8O4MnCl: C 74.85, H 8.54, N 6.98; Found: C
74.77, H 8.61, N 6.95.
The preparation methods and results of other ligands
and Mn complexes are similar to those mentioned above.

30
E. Sun et al. / Journal of Molecular Structure 889 (2008) 28–34
3. Results and discussion
3.1. Liquid crystals
The crystalline phase is characterized by DSC and the
results show that only the ligands of 12L, 14L, 16L and
18L are liquid crystals while the 8L, 10L and all the
Mn complexes are not. The calorimetric data of porphy-
rin liquid crystals are shown in Fig. 2 and the transition
temperatures and enthalpies of the porphyrin ligands are
given in Table 1. From Fig. 2 and Table 1, we can see
that each of the ligands shows one or two mesophases.
The porphyrin ligands show a high transition temperature
and a broad mesophase temperature span. The lowest
temperature for the transition from crystal to liquid–crys-
tal phase is 115 °C (18L) and the highest is 182 °C (12L).
The broadest mesophase temperature span is 160 °C (16L
Fig. 3. Optical texture of 16L, cooling from the isotropic state.
and 18L) and the narrowest is 78 °C (12L). With increas-
ing the side chain length, the mesophase temperature span
increased and the transition temperature decreased. The
optical texture was observed by viewing the birefringence
of the samples between crossed polarizers in a polarizing
microscope. Fig. 3 shows the birefringence texture micro-
graph of 16L.
Unlike porphyrin ligands, the Mn complexes show
none of the liquid crystal behaviour. As we know, most
discotic liquid crystals have a molecular structure that
consists of a flat central core with several flexible chains
placed around the outside edge of the core. But there is
non-coplanarity of Mn complexes because of the axis
coordination. This structure is unfavorable for the
whole macromolecule to pack closely to form liquid
crystal. On the other hand, the self-assemble of the
metal porphyrins may be another reason. The amide
N of the side chains may coordinate to the metal ion
of the metal porphyrin which forms complicated multi-
porphyrin aggregation.
Fig. 2. Differential scanning calorimetry of 12L, 14L, 16L and 18L. Black,
solid phase; white, mesophase; gray, isotropic.
Table 1
Calorimetric data for compounds 12L, 14L, 16L and 18L
3.2. UVꢀvis spectra
Compound T/ °C (DH/(KJ molꢀ1))a
DT/ °Cb
12L
Heating
Cooling
Fig. 4 shows the absorption spectra of 8L and 8Mn. As
we know, the UVꢀvis absorption bands of the porphyrins
are due to the electronic transitions from the ground state
(S0) to the two lowest singlet excited states S1 and S2 [17].
The S0 ? S1 transition gives rise to the weak Q bands in
visible region while S0 ? S2 transition produces the strong
Soret band in near UV region. In this work, the absorption
bands of the 8L appear at 425, 520, 555, 595 and 650 nm.
Compared with the ultraviolet characteristic absorption
peak of TPP [18], the ultraviolet absorption peaks of por-
phyrin ligands show red shift. It is possibly because the
para-alkylamidoyl group on the phenyl group of the
meso-position of porphyrin ring is a donating electronic
group, it enables the electronic density on the phenyl ring
to strengthen, thus the phenyl ring conjugates with porphy-
rin macrocycle to a certain degree, this kind of conjugation
action causes electron transition energy of the porphyrin
Cc 182 (10.52) Dd 270 (41.96) Ie
I 252 (ꢀ50.52) D 174 (ꢀ13.53) C
88
78
14L
Heating
Cooling
C 163 (25.36) D1 177 (10.80) D2 274 (57.31) I
111
91
I 246 (ꢀ54.47) D2 173 (ꢀ23.72) D1 155 (ꢀ15.64)
C
16L
Heating
Cooling
C 121 (3.66) D1 156 (77.59) D2 281 (77.36) I
I 265 (ꢀ46.14) D2 148 (ꢀ43.71) D1 112 (ꢀ3.64) C 153
160
18L
Heating
Cooling
a
C 115 (4.20) D1 143 (67.91) D2 275 (57.09) I
I 251 (ꢀ53.99) D2 131 (ꢀ57.94) D1 106 (ꢀ4.44) C 145
160
Heating rate 10 °C minꢀ1, cooling rate 10 °C minꢀ1
Temperature range of liquid crystal.
Crystal.
Discotic mesophase.
Isotropic liquid.
.
b
c
d
e

E. Sun et al. / Journal of Molecular Structure 889 (2008) 28–34
31
Fig. 5. The IR spectra of 8L and 8Mn.
Fig. 4. The UV–vis spectra of 8L and 8Mn.
macrocycle to reduce, making its ultraviolet absorption
peak red shift. The 8Mn complex has an intense Soret band
at 470 nm, with additional bands at 400, 425, 520, 570,
615 nm and the spectra patterns are similar to those of
the five-coordinated Mn porphyrin complexes [19]. After
the metal ion coordinated to porphyrin, the number of Q
band decreases and the absorption frequencies shift
because of the increasing of molecular symmetry. The spec-
tra shapes and peak positions of other porphyrin ligands
and Mn complexes are similar to those we mentioned
above which shows that the length of side chains has little
effect on the absorption spectra.
3.4. Resonance Raman spectra
The resonance Raman spectra of porphyrin ligands and
Mn complexes using 514.5 nm excitation are shown in Figs.
6 and 7. The Resonance Raman spectra of tetraphenylpor-
phyrin derivatives have been studied extensively [21–23].
Thus, the assignments of Raman bands of porphyrin
ligands and Mn complexes are discussed briefly here. The
wavenumber positions of vibration bands in the high-fre-
quency region (900ꢀ1600 cmꢀ1) are sensitive to the core
size, axial ligation and electron density of the central metal
ion. In this region, the 1552 cmꢀ1 band of porphyrin
ligands is assigned to CbꢀCb stretch m2, which is up-shifted
to 1570 cmꢀ1 in Mn complexes. The band at about
1493 cmꢀ1 of porphyrin ligands is assigned to the vibration
3.3. Infrared spectra
The IR bands at 3313ꢀ3318 cmꢀ1, 966ꢀ968 cmꢀ1 of
porphyrin ligands are due to the NAH stretching and
bending vibration of the porphyrin core, but they disap-
pear in the Mn complexes because the hydrogen atom in
the NAH bonding is replaced by metal ion [20]. In addi-
tion, a new band appears at about 1011 cmꢀ1 in the IR
spectrum of Mn complexes, which is the characteristic of
porphyrin metallation. The bands at about 3300 cmꢀ1 of
Mn complexes are assigned to NꢀH stretching vibration
of amide group on the side chains, but in porphyrin
ligands, they are combined with the NAH stretching of
the porphyrin core and cannot been distinguished. The
bands of the compounds at about 2850 and 2920 cmꢀ1
are assigned to CAH stretching vibration. The bands in
the range of 1658ꢀ1666 cmꢀ1 are assigned to C@O stretch-
ing vibration (amide I) and the bands in the range of
1519ꢀ1535 cmꢀ1 are assigned to NAH in-plane bending
(amide II). The bands at 717ꢀ721 cmꢀ1 are assigned to
the methylene in-plane rocking vibration of straight alkyl
chain including over four carbons. The IR spectra of 8L
and 8Mn are shown in Fig. 5.
Fig. 6. Raman spectra of porphyrin ligands in 900–1700 cmꢀ1 region with
excitation wavelength at 514.5 nm.

32
E. Sun et al. / Journal of Molecular Structure 889 (2008) 28–34
Fig. 8. Fluorescence spectra of the 8L, 10L, 12L, 14L, 16L, 18L and TPP.
Fig. 7. Raman spectra of Mn complexes in 200–1700 cmꢀ1 region with
excitation wavelength at 514.5 nm.
plexes. The fluorescence of Soret band is attributed to the
transition from the second excited singlet state S2 to the
ground state S0, which is much weaker than that of the
S1 ? S0 transition of the Q band emission. Its quantum
yield is so low that the fluorescence becomes unobservable
in this work. Fluorescence of S1 consists of two bands, Q
(0ꢀ1) and Q (0ꢀ2). From Fig. 8, it can be seen that Q
(0ꢀ1) and Q (0ꢀ2) fluorescent bands of porphyrin ligands
are in the regions of 655ꢀ657 and 717ꢀ720 nm, respec-
tively. Compared with the fluorescent bands at 653 and
715 nm of TPP, the emission peaks of porphyrin ligands
have shifted to the red region. This shows that the porphy-
rin macrocycle conjugation is affected by the electronic
donating groups. The conjugation is enhanced when the
amide groups linked on the phenyl group.
of phenyl ring, and it has little shift to 1491 cmꢀ1 in com-
plexes, which confirmed that the metal ion has little effect
on the phenyl at meso positions. The band at about 1457
cmꢀ1 of porphyrin ligands is assigned to CaꢀCm stretch
m3 and it does not appear in Mn complexes. Raman bands
in 1300ꢀ1450 cmꢀ1 are due to the out-of-phase coupled
(CaꢀCb)/(CaꢀN) stretching modes. The 1358 and
1329 cmꢀ1 bands of porphyrin ligands are assigned to the
m4 and m12 modes, respectively. The m4 mode of Mn com-
plexes appears at 1370 cmꢀ1, while its m12 band shifts to
1338 cmꢀ1. The 1242 cmꢀ1 band of porphyrin ligands
and the 1232 cmꢀ1 band of Mn complexes are attributed
to Cmꢀph stretch m1. The band at 1001 cmꢀ1 of porphyrin
ligands is assigned to the vibration of pyrrole breathing and
phenyl stretching (m15), which does not shift in Mn com-
plexes. The band at about 964 cmꢀ1 of porphyrin ligands
is assigned to pyrrole breathing m6, but it disappeared in
Mn complexes because the hydrogen atom in the NAH
bonding is replaced by metal ion. The 401 and 365 cmꢀ1
band of Mn complexes are assigned to the vibration of
NAMn. As we can see from Figs. 6 and 7, the band posi-
tion and relative intensity of porphyrin ligands and Mn
complexes with different length of side chains are almost
identical, which showed that there is no structural change
of porphyrin skeletons because the porphyrins have similar
para-substituted aromatic group on the methylene bridge
of porphyrins.
The fluorescence intensities of Mn complexes are much
weaker than that of the porphyrin ligands. The result is
same with the reference and confirmed that the transitional
metal on centre of porphyrin ring quench the fluorescence
of porphyrin [25].
3.6. Thermal studies
Thermogravimetric analyses (TGA) and differential
thermal analysis (DTA) curves of 12L and 12Mn are
shown in Figs. 9 and 10, respectively. DTA curve of the
ligand has two endothermic peaks at 240 and 276 °C and
five exothermic peaks at 356, 400, 420, 478 and 520 °C,
respectively. The endothermic peak at 240 °C corresponds
to a 7.2% weight loss of TGA curve, which is the loss of
crystalline water [26]. The endothermic peak at 276 °C of
DTA curve maybe caused by the phase transition of the
compound because there is no weight loss on the TGA
curve. In order to confirm our conclusion, we have applied
differential scanning calorimetry (DSC) to 12L. When the
second heating started from room temperature to 300 °C,
3.5. Fluorescence spectra
The emission spectra of porphyrin ligands and TPP in
DMF obtained at an excitation wavelength of 420 nm are
shown in Fig. 8, and the peak values of their emission spec-
tral bands are given in Table 2. There are fluorescence of
the S2 (Soret band) and the S1 (Q band) in porphyrin com-

E. Sun et al. / Journal of Molecular Structure 889 (2008) 28–34
33
Table 2
The emission spectra data of porphyrin ligands and TPP
Compounds
Solvent
kex (nm)
Q (0ꢀ1)
Q (0ꢀ2)
Uf
8L
DMF
DMF
DMF
DMF
DMF
DMF
DMF
420
420
420
420
420
420
420
655
656
657
657
657
656
653
720
717
720
721
718
717
715
0.043
0.025
0.023
0.019
0.045
0.057
0.110a
10L
12L
14L
16L
18L
TPP
a
Uf value of TPP is obtained from Ref. [24].
the DSC picture showed only one peak at about 270 °C
and the peak at 240 °C disappeared (not shown in this arti-
cle) because the crystalline water had lost during the first
heating. The exothermic peaks are corresponding to the
decomposition of the ligand. The TGA curves of the ligand
have several weight-loss processes. Decomposition began
at 300 °C and finished completely at 800 °C. This is a con-
tinuous decomposition process; the four small exothermic
peaks are the loss of chains of porphyrin ring and the big
exothermic peak at 520 °C corresponds to collapse of the
porphyrin skeleton. When heated to 800 °C, the porphyrin
ligand was completely decomposed and the TGA curve
became smooth. For Mn complex, the DTA curve has
one small endothermic peak at 232 °C and five exothermic
peaks at 293, 380, 401, 470 and 525 °C, respectively. The
endothermic peak corresponds to 1.1% weight loss of
TGA curve, which is the lost of one crystalline water.
The exothermic peak at 293 °C corresponds to a weight
of 3.7%, in good agreement with the calculated 3.6% weight
loss of the lost of coordinate chlorion. The decomposition
of Mn complex is a continuous process too. It was decom-
posed completely in the range of 340ꢀ600 °C, involved the
loss of side chains and the collapse of the whole porphyrin
framework. After 600 °C, TGA curve became smooth and
weight loss did not occur again, leaving about 7% residue
of oxidate.
Fig. 10. TGA (a) and DTA (b) pictures of 12Mn.
3.7. Electrochemistry
Cyclic voltammograms illustrating the reduction of
TPP, 8L and 8Mn in DMF, containing 0.1 mol/L TBAP
as supporting electrolyte are shown in Fig. 11, and a sum-
mary of the half-wave potentials of porphyrin ligands and
Mn complexes is listed in Table 3. The cyclic voltammo-
grams of TPP and 8L are similar in shape and yield well-
defined current–voltage curves for a one-electron transfer.
Reactions at the electrode surface correspond to the reduc-
tion of the neutral porphyrin ring; TPP is reduced at ꢀ1.27
and ꢀ1.73 V while 8L is reduced at ꢀ1.49 and ꢀ1.94 V.
The first reduction of TPP and 8L corresponds to the for-
mation of p anion radical and the second reduction leads to
the formation of porphyrin dianion. The potentials of 8L
shift cathodically along the potential axis for about
220 mV compared to TPP. This can be explained by the
Fig. 11. Cyclic voltammograms of TPP, 8L, 8Mn in DMF + 0.1 mol/l
TBAP.
Fig. 9. TGA (a) and DTA (b) pictures of 12L.

34
E. Sun et al. / Journal of Molecular Structure 889 (2008) 28–34
Table 3
of para-alkylamideyl group on the phenyl group has little
effect on porphyrin properties. The UVꢀvis spectra, infra-
red spectra, Resonance Raman spectra, fluorescence spec-
tra and cyclic voltammograms are almost identical.
According to thermal studies, porphyrin ligand and Mn
complex are stable up to nearly 300 °C, and the decompo-
sition of the compounds is a continuous process. The fluo-
rescent emission peaks of porphyrin ligands are red shift
compared to TPP. The quantum yields are in the range
of 0.019ꢀ0.057. The potentials of porphyrin ligands shift
cathodically compared to TPP due to the electron-donating
group at para position of the four phenyl groups. The cyc-
lic voltammetry of Mn complexes is different from the por-
phyrin ligand, which shows not only the redox of the
porphyrin ring, but also the redox of the metal ion Mn
(III)/Mn (II).
Cyclic voltammetry data of porphyrin ligands and Mn complexes
Compounds
Ered1/V
Ered2/V
TPP
8L
10L
12L
14L
ꢀ1.27
ꢀ1.49
ꢀ1.51
ꢀ1.50
ꢀ1.50
ꢀ1.51
ꢀ0.65
ꢀ0.66
ꢀ0.67
ꢀ0.67
ꢀ0.67
ꢀ0.66
ꢀ1.73
ꢀ1.94
ꢀ1.96
ꢀ1.96
ꢀ1.96
—
ꢀ1.77
ꢀ1.78
ꢀ1.78
ꢀ1.77
ꢀ1.78
ꢀ1.76
16L
8Mn
10Mn
12Mn
14Mn
16Mn
18Mn
attachment of an electron-donating group at para position
of the four phenyl groups. The half-wave potentials would
shift in a manner predicted by the Hammett linear free
energy relationship DE1/2 = 4rq [27], where r is the total
polar substituent constant which is dependent on the kind
and position of the four substituents, and q is the reaction
constant which is given in volts and expresses the suscepti-
bility of the electrode reaction to the total polar effect of the
substituents. Its value depends on the kind of electroactive
group and the composition of the solvent and supporting
electrolyte, as well as the temperature.
References
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4151.
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Bocian, D. Holten, J.S. Lindsey, J. Am. Chem. Soc. 122 (2000) 7579.
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[4] F. Remacle, S. Speiser, R.D. Levine, J. Phys. Chem. B 105 (2001)
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[5] P.G. Van Patten, A.P. Shreve, J.S. Lindsey, R.J. Donohoe, J. Phys.
Chem. B 102 (1998) 4209.
The cyclic voltammogram of 8Mn obtained in DMF is
different from the ligand. Two reduction waves are
obtained, the first at ꢀ0.65 V corresponds to the reaction
Mn (III)/Mn (II), and the second occurs at ꢀ1.77 V and
can be assigned as formation of the anion radical [28].
The peak separation of the reduction Mn (III)/Mn (II) is
much larger than that of the reduction of porphyrin ring.
This phenomenon could be due to the coordination of
Mn porphyrin. The removal of the coordination ion during
reduction should result a lower rate constant. As listed in
Table 3, there are similar results for the samples whose cen-
tral ions are identical except 16L and 18L because they are
not soluble well in DMF. Comparing the cyclic voltammet-
ric results of porphyrin ligands and Mn complexes, it is
found that the redox potentials are almost identical when
the length of side chains increased. This is probably due
to the substituent constants of the alkylamido groups are
almost identical and the size of porphyrin has little effect
on the redox potentials.
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The above experimental results indicate that only the
ligands with long chains of more than 12 carbon atoms
are mesogeic while the Mn complexes are not. The length