The photochemical and electrochemical properties of chiral porphyrin dimer and self-aggregate nanorods of cobalt(II) porphyrin dimer

Article abstract of DOI:10.1016/j.ica.2009.10.015

In this paper, the novel chiral porphyrin dimer ligand and its cobalt(II) porphyrin dimer were synthesized by using a glutamate bridging group. The FT-IR and Raman spectra of the chiral porphyrin dimer were investigated. Furthermore, the photochemical and electrochemical properties of dimer were studied. In addition, we prepared the nanorods of the cobalt(II) porphyrin dimer using liquid-solid-solution (LSS) technologies. The shape and dimension of the spontaneous aggregates of cobalt(II) porphyrin dimer were characterized by the transmission electron microscopy (TEM). The results show the diameter and shape of the aggregates can be controlled by refining the stocked solution temperature.

Full text of DOI:10.1016/j.ica.2009.10.015

Inorganica Chimica Acta 363 (2010) 317–323
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
The photochemical and electrochemical properties of chiral porphyrin dimer
and self-aggregate nanorods of cobalt(II) porphyrin dimer
a,
c
Ximing Guo a,b,c, , Bin Guo , Tongshun Shi
*
*
a School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
b The Academy of Science and Technology, Harbin Institute of Technology, Heilongjiang 150001, People’s Republic of China
c College of Chemistry of Jilin University, Chang Chun 130023, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, the novel chiral porphyrin dimer ligand and its cobalt(II) porphyrin dimer were synthesized
by using a glutamate bridging group. The FT-IR and Raman spectra of the chiral porphyrin dimer were
investigated. Furthermore, the photochemical and electrochemical properties of dimer were studied. In
addition, we prepared the nanorods of the cobalt(II) porphyrin dimer using liquid–solid-solution (LSS)
technologies. The shape and dimension of the spontaneous aggregates of cobalt(II) porphyrin dimer were
characterized by the transmission electron microscopy (TEM). The results show the diameter and shape
of the aggregates can be controlled by refining the stocked solution temperature.
Received 15 September 2009
Accepted 22 October 2009
Available online 28 October 2009
Keywords:
Chiral porphyrin dimer
Nanorods
Luminescence
Electrochemistry
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
possibility of modulating the structure of the final aggregates by
tuning the properties of porphyrin monomer or exterior conditions
Chemistry as a significant bridge between the bioscience and
material sciences, its many disciplines allow explanation of how
biomolecules self-assemble to form functional nanostructures
and provide the tools to synthesize analogous structures in nonbi-
ological environments. The fabrication of well-defined nanoscale
objects is essential for the understanding and development of po-
tential applied materials [1–3]. In this respect, porphyrins consti-
tute an important class of synthetic and natural compounds
because they take vital role in bioscience and advanced materials.
Recently, the self-aggregate porphyrins with well-defined shapes
and dimensions are of great applications in magnetic properties,
photo-catalytic, electronics, photonics and light-energy conver-
sion. For example, in artificial light-harvesting systems [4–6], it is
expected that the porphyrin nanorods will have unique photonic
properties not obtainable by other analogous inorganic and organic
nanoparticles, larger-scaled materials containing the macrocycle,
or by the molecules themselves. However, the aggregate morphol-
ogies of porphyrin completely depend on the exterior conditions
(pH, ionic strength, solvent, temperature and so on), which is
advantageous for aggregate porphyrin to prepare the different
functional materials. A challenging aspect in these systems is the
to obtain new materials. The amphiphilic porphyrin aggregates in
aqueous solution [7,8] have demonstrated the form of fibers, rib-
bons and tubules. In communication [9], we have also reported
the changes of UV–Vis and CD spectral with the various aggregate
size and formation of the chiral cobalt(II) porphyrin dimer [10–12].
But the vibrational spectrum, photochemical and electrochemical
properties of the chiral porphyrin dimers have not been investi-
gated. In this study, we qualitatively assign the FT-IR and Raman
bands of the novel chiral porphyrin dimers. The photochemical
and electrochemical properties of the novel chiral porphyrin di-
mers are investigated. In addition, the nanorods of cobalt(II) por-
phyrin dimer are prepared by the liquid–solid-solution (LSS)
technologies. The shape and dimensions of the cobalt(II) porphyrin
dimer aggregates are characterized by the Transmission electron
microscopy (TEM). The results show that the diameter and shape
of the aggregates were dependent on the temperature of the
solution.
2. Experimental
2.1. Materials and methods
5-Hydroxyl-10,15,20-triphenylporphyrin(MHTPP) was synthe-
sized as in literature [13]. Methylene dichloride and chloroform
(being further purified by the traditional methods before using)
were purchased from Tiantai Company. Pyrrole was from Aldrich
(A.R grade), anhydrous ethanol, aluminum oxide (200–300 mesh)
* Corresponding authors. Address: School of Materials Science and Engineering,
Harbin Institute of Technology, Harbin 150001, People’s Republic of China. Tel./fax:
+86 451 8640 1179.
E-mail addresses: ximing.guo@yahoo.com.cn, xi_ming_guo@126.com (X.m. Guo),
bguo@hit.edu.cn (B. Guo).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.10.015

318
X.m. Guo et al. / Inorganica Chimica Acta 363 (2010) 317–323
silica gel G were from the Chemical Reagent Company of Shanghai
Instrument. Element analyses were carried out with a Perkin–El-
mer 240C auto element analyzer; electronic absorption spectra
were measured on Shimadzu UV-2400 spectrophotometer; FAB
mass spectrum was recorded with a JEOL AX-505 spectrometer:
FT-IR spectrum was measured on an impact Nicolet 5PC FT-IR
spectrophotometer with KBr disks. Raman spectra were recorded
with a Jobin Yvon Raman spectrophotometer equipped with an
integral microscopy. Radiation of 457.9 nm was obtained from an
Ar+ laser. The fluorescence measurements were performed in
1 cm quartz cell using Shimadzu RF-5301 spectrofluorimeter at
excitation wavelength of 400 nm and scan range from 500 nm to
800 nm. The nanocomposites were characterized by transmission
electron microscope (JEOL 2010 microscope operated at 120 kV);
the cyclic voltammetry was carried out in dried DMF (0.1 mol/L
TBAP as a supporting electrolyte) by CHI660A electrochemical ana-
lyzer. A three-electrode system was used: Platinum wires as work-
ing and counter electrodes; Ag/Ag+ as reference electrode
(0.01 mol/L AgNO3 in acetonitrile solution). All solvent/supporting
electrolyte solutions were deaerated using high-purity Ar. The
reversibility of the electrochemical processes was evaluated by
standard procedures.
0.83 (t, 2H), 1.06 (m, 1H), 1.17–1.14 (t, 2H), 6.50 (s, 2H, NH),
7.42–7.45 (m, 4H, m-C6H4OH), 7.53–7.59 (m, 2H, m-C6H4OH),
7.60–7.67 (2H, m-C6H4OH), 7.74–7.69 (t, 2H, m-C6H5), 7.75–7.93
(m, 18H, p, m, o-C6H5), 7.95–8.12 (d, 2H, m-C6H5), 8.14 (s, 6H, m-
C6H5), 8.24–8.37 (m, 2H, p-C6H5), 8.38–8.49 (d, 2H pyrrole b-H),
8.57–8.51 (d, 2H pyrrole, b-H), 8.57–8.91 (t, 12H pyrrole b-H);
UV–Vis [DMF, kmax/nm (
e
 105)]: 420(8.5) 515(0.13) 552(0.08)
589(0.04) 649(0.05).
2.3. Synthesis of cobalt(II) porphyrin dimer
A mixture of porphyrin dimer ligand (160 mg, 0.12 mmol) and
hydrous cobalt chloride (0.485 g) in DMF (30 mL) was heated at
80 °C for 1.5 h with stirring under nitrogen atmosphere. After cool-
ing to room temperature, the mixture was added to 100 mL chloro-
form. The solution was washed three times with the double
distilled water, after removing excess cobalt chloride and DMF,
the solution was dried with anhydrous sodium sulfate overnight,
concentrated and loaded on aluminum oxide column chromatogra-
phy with the mixture of acetone and chloroform as elute, The pur-
ple–red crude product was gave and further purified on Al2O3
column. The product was recrystallized from chloroform using
absolute methanol and dried on vacuum, the structure of cobalt(II)
porphyrin dimer was displayed in Scheme 1, Gained 121.3 mg, Ele-
mental Anal. Calc. for C93H61N9O4Co2: C, 75.15; H, 4.14; N, 8.48.
Found: C, 75.41; H, 4.72; N, 8.24%. 1H NMR (300 MHz, DMSO), d:
12.48 (br, 16H, pyrrole-H), 10.3 (s, 4H, m-C6H4O), 8.57 (m, 18H,
m, p-C6H5), 8.19 (s, 4H, o-C6H4O), 7.95–7.88 (t, 12H, o-C6H5),2.89
(s, 2H, CH2), 2.73 (s, 2H, CH2) 1.22–1.14 (d, 2H NH2), 0.85 (br, s,
2.2. Synthesis of porphyrin dimer ligand
To a 250-mL three-necked round-bottomed flask containing
thionyl chloride (15 mL, 0.21 mmol) were added glutamic acid
(amidogen was protected) (14.70 mg, 0.1 mmol), then the solution
was stirring for 2 h at the atmosphere bath. Followed by adding the
5-hydroxyl-10,15,20-triphenylporphyrin (631 mg, 1 mmol) dis-
solved in anhydrous benzene (120 mL) to the reaction solution,
the mixture was stirred for 4 h in the boiled benzene, after cooling
to room temperature. The mixture was washed with 100 mL 0.5 M
sodium carbonate solution to remove of excess thionyl chloride
and glutamic acid. Then the solution was washed three times with
the double distilled water and dried with anhydrous sodium sul-
fate overnight. After removing of the solvent, the solid was dis-
solved in the chloroform and loaded on aluminum oxide column,
the products were eluted by the different ratios of acetone and
chloroform and collected main band. According to literature’s re-
port [13,14] amidogen was deprotected, the crude products was
purified with silica gel column and concentrated. The product
was further recrystallized from chloroform using absolute metha-
nol and dried on vacuum, the structure of porphyrin dimer was
displayed in Scheme 1, Gained 88.3 mg, Yield, 12.8%, Mass [FAB],
m/z: calc. 1372.4, found 1374.5 [M+1]; Elemental Anal. Calc. for
C93H65N9O4: calc. C, 81.38; H, 4.77; N, 9.18. Found: C, 81.41; H,
4.76; N, 9.24%. 1H NMR (DMSO-d6, 333 k): d:-2.93 (d, 4H), 0.85–
1H, CH); UV–Vis [DMF, kmax/nm (
e
 105)]: 409 (6.4), 536 (0.04).
2.4. Preparation of nanorods
The procedure to prepare nanorods of cobalt(II) porphyrin di-
mer was initiated by dissolving 14.86 mg porphyrin dimer in
10 mL DMF, which was then equally divided into five. Each was
vigorously stirred under different designated temperatures (a-
273 K, b-298 K, c-308 K d-318 K and e-328 K). As followed, the so-
dium chloride aqueous solution with 0.01 mol/L was added slowly
to each of the five, and then the system was treated for 12 h. After
adding 6 mL CHCl3 to each of the five stocked solution, which was
divided into two phases, respectively: a and b retain purple–red in
the below solution and colorless in the above solution; c, d and e
also retain purple–red in the below solution and light red in the
above solution. For c, d and e, the suspension appeared in the
above solution, which suggested that the metalloporphyrin was
dispersed into the mixture of DMF and water. This was explained
by chloroform volatilization or the solubility of aggregate decline
in chloroform with the solution temperature increase. A spontane-
ous phase-separation process then occurred because of the aggre-
gate metalloporphyrin and the incompatibility between the
hydrophobic surfaces and their hydrophilic surroundings, such
the metalloporphyrin nanocrystals can be easily collected in the
chloroform at the bottom of container.
3. Result and discussion
3.1. The spectral properties of chiral porphyrin dimer
3.1.1. UV–Vis spectra
Typical porphyrin absorption bands include an intense near-UV
band (Soret) and two visible bands (Q): Q (0,0) represents excita-
tion from the lowest vibrational level of the ground singlet elec-
tronic state to the lowest vibrational level of the first excited
singlet electronic state, and Q (1,0) bands have one quantum of
Scheme 1. The structure of porphyrin dimer.

X.m. Guo et al. / Inorganica Chimica Acta 363 (2010) 317–323
319
vibration in the first excited singlet electronic state. In metallopor-
phyrins with square symmetry, these bands are due to degenerate
excited state with x and y polarization. In the free base porphyrin
the Qx (0,0) and Qy (0,0) bands are no longer degenerate due to
the proton axis. The absorption bands of free base porphyrin dimer
ligands appear at 420,515, 552, 589 and 649 nm; the absorption
bands of cobalt(II) porphyrin dimer appear at 409 nm and
536 nm (Fig. 1). Fig. 1 shows that the Soret-band and Q bands of
the dimer red-shifted compared with those of MHTPP, the Soret-
band red-shifted from 418 to 420 nm. The four Q bands of MHTPP
appeared at 515, 550, 590 and 650 nm; the four Q absorbance
bands of the dimer ligand appeared at 515, 552, 589, and
649 nm, respectively (Fig. 1), these changes can be attributed to
the interaction between amidogen of amino acid and N–H porphy-
rin ring. The interaction causes either a linear action with respect
to the direction of Qx-, Bx-transition dipole moments or, in the
opposite case, to the direction of the Qy-, By-transition dipole mo-
ments. Both the types of interaction within the dimer ligand enable
the generation of parallel and perpendicular interaction simulta-
neously because the transition dipole moments of Qx-, Bx- and
Qy-, By-type transitions have parallel and perpendicular orienta-
tions. The transition dipole moments of the Qx- and Bx-bands lay
along the axis connecting the two inner hydrogen atoms while
the Qy- and By-transitions belong to the transition dipole moments,
which are perpendicular to the former axis [13].
Fig. 2. FT-IR spectra of the porphyrin dimer ligand and cobalt(II) porphyrin dimer.
vibration of the amino acid. The bands in the range of 1519–
1535 cmÀ1 are assigned to N–H in-plane bending (amidogen II)
[13]. The bands at 1265 and 1216 cmÀ1 was ascribed to the C–O
stretching vibration in the porphyrin dimer ligand, however, the
bands of cobalt porphyrin complexes was displayed 1259 and
1205 cmÀ1 which can be attributed to the enhancement of both
the dihedral angel of the two porphyrin chromophores planes by
metalation and reduction of the distance of porphyrin ring intradi-
mer. The bands at 1471 cmÀ1, 1170 cmÀ1, 1072 cmÀ1,798 cmÀ1
were ascribed to the skeleton vibration of porphyrin dimer ligand,
the skeleton vibration model of cobalt porphyrin complexes ap-
peared at the 1349 cmÀ1, 1170 cmÀ1, 1072 cmÀ1, 796 cmÀ1 [11].
The bands at 717–721 cmÀ1 are assigned to the methylene in-
plane rocking vibration of straight alkyl chain.
3.1.2. FT-IR spectra
FT-IR spectra of porphyrin dimer ligand and cobalt(II) porphyrin
dimer are shown in Fig. 2. The FT-IR bands at 3500 cmÀ1 and
3429 cmÀ1 of porphyrin dimer ligand are assigned to N–H stretch-
ing vibration of radical amino acid and the band of cobalt(II)
porphyrin dimer at 3440 cmÀ1 was observed. The C–H stretching
vibration of porphyrin dimers appears at 2850 cmÀ1 and
2920 cmÀ1. The bands at 3313, 964 cmÀ1 are attributed to the
N–H stretching and bending vibration of the porphyrin ligand core.
In cobalt(II) porphyrin dimer, these bands disappear and a new
band appears at about 1002 cmÀ1, which is the characteristic of
metalloporphyrin. The C@O stretching vibration band of the por-
phyrin dimer ligand is weak due to the intramolecular hydrogen-
bonding, however, The vibrational bands of cobalt(II) porphyrin di-
mer appears at 1741 and 1704 cmÀ1.The band of the porphyrin di-
mer appears 1654 cmÀ1 which was ascribed to the N–H bending
3.1.3. Raman spectra
The Raman spectra of porphyrin dimer ligand and cobalt(II) por-
phyrin dimer using 457.9 nm excitation are displayed in Fig. 3,
those Raman bands exhibited a sizable frequency difference be-
tween porphyrin dimer ligand and cobalt(II) porphyrin dimer.
In low-frequency region, the Raman spectra of porphyrin
dimer ligand are significantly different from those of porphyrin
dimer ligand. For porphyrin dimer ligand, the only weak Raman
Fig. 1. UV–Vis spectra of the porphyrin dimer ligand and cobalt(II) porphyrin
dimer.
Fig. 3. Raman spectra of the porphyrin dimer ligand and cobalt(II) porphyrin dimer.

320
X.m. Guo et al. / Inorganica Chimica Acta 363 (2010) 317–323
band in low-frequency region was observed at 330 cmÀ1. This band
was assigned to the m8 mode. The m8 mode consists of an in-plane
translational motion of the pyrrole rings, which can be described
as a uniform breathing of the whole porphine ring accompanied
by an in-plane C CmC deformation in the pyrrole rings [15]. While
a
a
a very strong band and two weak bands were observed for cobal-
t(II) porphyrin dimer (at 396 cmÀ1 and 253 cmÀ1w, 334 cmÀ1
,
w
vs
respectively). The differences of Raman spectra in low-frequency
region suggest that the structures or vibrational dynamics espe-
cially around the C CmC bond-angles are altered by the metal
a
a
ions. In the 900–1650 cmÀ1 high-frequency region of the Raman
spectra of porphyrin dimer ligand and cobalt(II) porphyrin dimer,
bands generally arise from the totally symmetric vibrational
modes such as C Cm, C Cb, CbCb, pyrrole quarter-ring, and pyrrole
a
a
half-ring stretching. The wavenumber positions of Raman bands
in the high-frequency region are sensitive to the core size, axial
ligation and electron density of the central metal ion [16–18]. In
this region, the band at 1550 cmÀ1 of porphyrin dimer ligand is as-
Fig. 4. The fluorescence spectra of the porphyrin dimer ligand and cobalt(II)
porphyrin dimer.
signed to CbCb stretch
m
2 mode, which is up-shifted to 1565 cmÀ1 in
2 mode is observed with enhanced
cobalt(II) porphyrin dimer. The
m
intensity in cobalt(II) porphyrin dimer. In fact, it is one of the most
intense bands in the high-frequency region. The band at 1498 cmÀ1
of porphyrin dimer ligand is assigned to the vibration of phenyl
ring; the band at about 1455 and 1453 cmÀ1 of porphyrin dimer li-
by metal ion, the fluorescence quenching is a well-known phenom-
enon [13]. Fig. 4 shows the fluorescence emission spectra of the
porphyrin dimer ligand and cobalt complex in DMF. The absor-
bance of two compounds is almost coincident at vicinity of excita-
tion wavelength, so we can directly compare their fluorescence
intensities in experimental conditions. It can be seen from Fig. 4
that Q(0À1) and Q(0À2) emission bands of them are in the regions
of 654 and 719 nm, respectively. However, the fluorescent inten-
sity of the cobalt(II) porphyrin dimer (654 nm) is only 5.3% of that
of porphyrin dimer ligand, which indicates that the metal ions in-
ner metalloporphyrin efficiently quenches the fluorescence inten-
sities of porphyrin moiety. This result has been reasonably
explained by the classic Marcus theory, for such a derivative, the
photoinduced intramolecular electron transfer from the singlet ex-
cited state inner free base porphyrin to one low-spin cobalt(II) [14].
The electron transfer is the primary de-excitation path for the ex-
cited state and the electron transfer rates depend on the angle and
distance of donor–acceptor in the classic Marcus theory. For non-
adiabatic electron transfer, fluorescence can become the dominant
decay pathway for the excited free base porphyrin. The S1 ? S0 (Q
band) quantum yield depends on the relative rates of the radiative
process S1 ? S0 and two radiationless processes S1ꢀ ? S0 and
S1ꢀ ? Tn. Thus the excited state S1 is primarily deactivated by
radiationless decay. That the spin forbidden process S1ꢀ ? Tn is
predominant for radiationless deactivation of the excited state S1.
The fluorescence intensity of cobalt complex largely declined
which was attributed to enhancing the S1ꢀ ? Tn radiationless pro-
cess due to cobalt(II) [20].
gand is assigned to C Cm stretch m3 mode and these bands do not
a
appear in cobalt(II) porphyrin dimer. The m2 mode arises mainly
from the symmetric C Cm stretching, which is mixed with the CbCb
a
stretching of the same symmetry. The m3 mode has a noticeable
component of the symmetric stretching motion of C Cm bonds.
a
On the other hand, the m2 mode has a major contribution from
the in-phase CbCb stretching along with a smaller contribution
from the C Cm stretching. Thus, mixing of these stretching motions
a
causes nearly complete cancellation of the intensity for the m3
mode, while the intensity of the m2 mode is increased in
metallo-tetraphenylporphines under D4h point group symmetry
[19]. Raman bands in 1300–1450 cmÀ1 are due to the out-of-phase
coupled C Cb/C N stretching modes. The 1358 and 1325 cmÀ1
a
a
bands of porphyrin ligand are assigned to the m4 and m12 modes,
respectively. The m4 mode of cobalt(II) porphyrin dimer appears
at 1369 cmÀ1 while its m12 band shifts to 1311 cmÀ1
, . The
1239 cmÀ1 band of porphyrin dimer ligand and the 1237 cmÀ1
band of cobalt(II) porphyrin dimer are attributed to Cm-ph stretch-
ing
to the vibration of pyrrole Cb–H stretching
m
1 mode. The band at 1080 cmÀ1 of porphyrin ligand is assigned
m
9 mode, which does not
shift in cobalt(II) porphyrin dimer. The band at 999 cmÀ1 of
porphyrin ligand is assigned to the vibration of pyrrole breathing
and phenyl stretching m15 mode, which is up-shifted to
1009 cmÀ1 in cobalt(II) porphyrin dimer. The band at about
960 cmÀ1 of porphyrin ligand is assigned to pyrrole breathing m6
mode, but it disappeared in cobalt(II) porphyrin dimer because
the hydrogen atom in the N–H bonding is replaced by metal ion.
3.1.5. Electrochemistry
The cyclic voltammetry of porphyrin dimer ligand and cobalt(II)
porphyrin dimer is performed to evaluate their redox potentials. In
the tetraphenylporphyrin complex, the cobalt has a stable two va-
lence oxidation state in air. So two metal redox reactions can take
place, specifically an oxidation: Co(II) ? Co(III) and a reduction:
Co(II) ? Co(I). In addition, two ring reductions and one ring oxida-
tion is observed (Fig. 5). The first electroreduction occur À0.448 V
corresponds to the Co(III)/Co(II) electrode reaction. The half wave
potentials at E1/2 = À0.22 V was negative-shift relative to that of
TPPCo. The change of E1/2 for Co(III) ? Co(II) of cobalt complex
was attributed to the Co(III) axial interaction with the amido of
amino acid. A second reduction occurs at À1.29 V, which was as-
signed to Co(II)/Co(I) [21–26]. The oxidation of Co(II)/Co(I) is faint
which can be attributed to forming intramolecular or intermolecu-
lar polymer due to the Co(I) bearing strong nucleophility [27]. For
3.1.4. Luminescence
The optical properties of porphyrin can be related to its molec-
ular skeleton. The nucleus of porphyrin consists of four pyrrole
rings linked by methine-bridge: the corresponding macrocycle is
fully conjugated plane, containing an 18-electron aromatic
p-sys-
tem. The extended aromatic system is responsible for the high mo-
lar absorbance. Porphyrins hold the S2 (B band) emission band and
the S1 (Q band) emission band. The S2 fluorescence band is attrib-
uted 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(0À1) and Q(0À2). When
the hydrogen atoms in the center of porphyrin ring are replaced

X.m. Guo et al. / Inorganica Chimica Acta 363 (2010) 317–323
321
tion) and other interaction (metal–metal, intermolecular electro-
static et al). For cobalt(II) porphyrin dimer, the aggregate can be
adopted a head-to-tail (J-aggregate), face-to-face (H-aggregate) or
nonspecific aggregate arrangement in different experimental con-
ditions. When the solution was placed at lower temperature (sam-
ple a, 273 K and sample b, 298 K), The H-aggregate or nonspecific
aggregate is dominant in aggregate morphologies of cobalt(II) por-
phyrin dimer. However, as temperature increase, the J-aggregate
take vital role in the aggregate morphologies of cobalt(II) porphy-
rin dimer because the J-aggregate is stable in thermodynamics
while H-aggregate is stable in dynamics. The aggregate morpholo-
gies transform from H- to J-aggregate with the stocked solution
temperature increase. Thus, the aggregate species grow single
which leads to forming homogeneous and ordered chiral nanorods
of cobalt(II) porphyrin dimer [29]. Fig. 6 shows the transmission
electron microscope (TEM) images of self-aggregate porphyrin di-
mer at different temperature. From Fig. 6 we can see that the
aggregate shape of porphyrin dimer changes from disordered
stacking to ordered aggregates; the aggregate configuration of di-
mer transform from fractal, big brick up to rod-like particles; the
diameter of aggregates changes from micron to nanometer, and
the diameter size distribution of the nanorods changes from inho-
mogeneity to homogeneity (Fig. 5e) with solution temperature
increase.
Fig. 5. Cyclic voltammograms of porphyrin dimer ligand and cobalt(II) porphyrin
dimer in DMF + 0.1 mol/L TBAP.
the ring redox of porphyin dimer ligand, the first ring reduction of
the ligand occurs at À1.58 V and the second reduction has not been
observed; in the cobalt complex, the first and second reduction of
porphyrin ring occurs at À1.56 V and À1.93 V, respectively. This
was corresponds to the formation of
p anion radical and the second
reduction leads to the formation of porphyrin dianion. However,
the oxidation peak is broad which can be attributed to intramolec-
ular charge-transfer between porphyrin and amino acid or porphy-
rin and porphyrin [28]. The reduction peak twice higher than that
of oxidation peak which can be ascribed to two-electron reduction
and one-electron oxidation in cobalt porphyrin dimer. An irrevers-
ible ring oxidation peak occurs at 0.56 V and 0.60 V corresponds to
the electronoxydation of ligand and cobalt complex, respectively.
3.2.1. The UV–Vis properties of self-assembly nanorods
The UV–Vis of cobalt(II) porphyrin dimer demonstrated great
differences with the change of the stocked solution temperature.
The exciton theory developed by Kasha predicts the occurrence
of hypsochromic or bathochromic shifts for the relevant absorption
bands, in the case of H-aggregate or J-aggregate interactions
respectively. The UV–Vis of the cobalt(II) porphyrin dimer with
the changes of the temperature was displayed in Fig. 7. We find
that the shoulder peak appears at 435 nm, the variational tendency
to absorbance at 415 nm and 435 nm of dimer have been inserted
into Fig. 7 with the changes of the temperature, the absorbance
intensities of 415 nm decline and that of 435 nm rise with the
stocked solution temperatures increase, which can be attributed
to forming aggregates of cobalt(II) porphyrin dimer. However, the
absorbance values of 415 nm and 435 nm have an abrupt change
when the stocked solution is between 338 K and 343 K, which
3.2. Spectral properties of self-assembly nanorods of cobalt(II)
porphyrin dimer under different conditions
General speaking, The primary reaction in the preparation of
metalloporphyrin nanocrystals through LSS involved the
p–p
stacking, hydrophilic and hydrophobic grounds interaction at
the interfaces of metalloporphyrin (solid), chloroform–liquid phase
(liquid) and water–DMF (N,N-dimethylformamide) solutions (solu-
Fig. 6. TEM images of porphyrin nanorods at the different temperature, (a) 273 K, (b) 298 K, (c) 308 K, (d) 318 K and (e) 328 K.

322
X.m. Guo et al. / Inorganica Chimica Acta 363 (2010) 317–323
Fig. 8. The comparison of face–face and end–end couplings of H-and J-aggregated
chromophores.
Fig. 7. The changes of UV–Vis spectra of cobalt(II) porphyrin dimer in water–DMF
with temperature varying from 303 K to 383 K, the interval of temperature is 5°.
The variational tendency to absorbance at 415 nm and 435 nm of dimer have been
inserted into figure with the changes of the temperature.
can be isosbestic temperature of the transformation from H- to J-
aggregate. In communication [9], the UV–Vis of the cobalt(II) por-
phyrin dimer deposited in water–DMF for 12 h under different
temperatures has been analyzed as Refs. [30–33]. We speculated
this resulted from the following reasons: face–face assembly of co-
balt(II) porphyrin dimer results in the splitting of the excited state
because of the exciton-coupling between the two porphyrin mole-
cules. When the two porphyrins are stacked with little or no offset
(i.e., an ‘‘H” aggregate), electronic transition is allowed to the upper
state but not to the lower state, which results in the observed shift
of the absorption spectrum to a shorter wavelength. When two di-
mer molecules are assembled adjacent to one another in the minor
groove, a secondary coupling arises because of the end–end inter-
action between dimer molecules. This causes an additional split-
ting of the excited state, which is normally manifested in the
UV–Vis spectrum as a broadening or splitting of the absorption
band, finally, the absorption of the cobalt(II) porphyrin dimer
was red-shifted which was that the porphyrin dimer transforms
from H-aggregate to J-aggregate as the stocked solution tempera-
ture increases. The results can help us to well-understand the elec-
tronic coupling of the chromophors [34,35].
Fig. 9. Exciton-coupling model explain how spectral shifts relate to the aggregate
structure.
porphyrin ring intradimer when the aggregate morphologies of co-
balt(II) porphyrin dimer transform from H-aggregate to J-aggregate
[42–45]. As shown in Fig. 9, the transformation from H-aggregate
to J-aggregate decrease the face–face coupling, while enhance the
end–end coupling. The much larger intramolecular coupling is
reflective of the more extensive orbital overlap between two por-
phyrin rings in dimer compared with the end-to-end interactions
between adjacent dimers. So the shape and intensity of the CD
spectrum should be highly dependent on the aggregate formation
and the conformation of the chiral porphyrin dimer.
3.2.2. Circular dichroism (CD) specropolarimetry of self-aggregate
nanorods
The CD spectra of the cobalt(II) porphyrin dimer have been
measured and discussed under different temperature. The results
show a weak single CD band under lower temperature (273 K
and 298 K), a small bisignate curve under medium temperature
(308 K) and a stronger bisignate curve under higher temperature
(318 K and 328 K) [9]. The spectral shifts and splitting observed
in the CD spectra of the aggregate cobalt(II) porphyrin dimer,
which was rationalized by a simple exciton-coupling model, based
on work by Kasha et al. [36] and Davydov [37]. When the solution
was under lower or medium temperature, the H-aggregate or non-
aggregate of cobalt(II) porphyrin dimer is dominant in the stocked
solution. There was weak exciton-coupling or no exciton-coupling
in H-aggregate or non-aggregate of cobalt(II) porphyrin; (Fig. 8)
while the solution was under higher temperature, the J-aggregate
of cobalt(II) porphyrin dimer is dominant in the stocked solution,
the strong and splitting CD signal observed in the CD spectra
[38,39]. It is known that such an interaction depends on inter chro-
mophoric distance and twist, as well as the conformational rigidity
4. Conclusions
In this study, we successfully synthesized novel chiral porphy-
rin dimer ligand and its cobalt(II) porphyrin dimer by using a glu-
tamate bridging group. We assigned the FT-IR and Raman bands of
the chiral porphyrin dimers. Furthermore, the photochemical and
electrochemical properties of dimers were detailedly studied. In
addition, the nanorods of cobalt(II) porphyrin dimer were prepared
by adjusting the stocked solution temperature, according to our re-
sults, the different sized and functional nanoparticles of porphyrin
can be attained by refining the exterior conditions. The UV–Vis
spectra were always acting as the indicator of porphyrin aggre-
gates. However, for the chiral porphyrin, the changes of CD spectra
can help us to well-understand inter- and intramolecular elec-
tronic coupling and the aggregate formation of the chiral
porphyrin.
around the porphyrin
L-glutamic acid linkage [40,41]. The intramo-
lecular exciton-coupling was reinforced due to both the enhance-
ment of the dihedral angel and reduction of the distance of

X.m. Guo et al. / Inorganica Chimica Acta 363 (2010) 317–323
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