Unusual spectral and electrochemical properties of azobenzene-substituted porphyrins

Article abstract of DOI:10.1016/j.electacta.2011.11.061

A series of azobenzene-substituted porphyrin derivatives, MPA_n (M = H₂, Zn; n = 1, 2, 4), has been prepared. These compounds were characterized by spectral and electrochemical methods. The Soret and Q bands of ZnPA_n were r

Full text of DOI:10.1016/j.electacta.2011.11.061

Electrochimica Acta 62 (2012) 51–62
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Electrochimica Acta
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Unusual spectral and electrochemical properties of azobenzene-substituted
porphyrins
Kuo Yuan Chiua, Yi-Jung Tua, Chia-Jung Leea, Te-Fang Yanga,∗ ∗ ∗, Long-Li Laia, Ito Chaob,∗∗
,
Yuhlong Oliver Sua,c,∗
a Department of Applied Chemistry, National Chi Nan University, 1 University Rd., Puli, Nantou County 545, Taiwan
b Institute of Chemistry, Academia Sinica, 128, Academia Road Sec. 2, Nankang, Taipei 115, Taiwan
c Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Rd., Taichung 402, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of azobenzene-substituted porphyrin derivatives, MPAn (M = H2, Zn; n = 1, 2, 4), has been pre-
pared. These compounds were characterized by spectral and electrochemical methods. The Soret and Q
bands of ZnPAn were red-shifted along with the increase of Q-band intensity ratio (ε˛/εˇ) as the sub-
stitution of azobenzene increased. Addition of imidazole towards ZnPAn caused a significant increase
in Q band intensity ratio. A noteworthy characteristic is that the Q band intensity ratio was reversed
(>1.0) by imidazole ligation for ZnPA4. Additionally, cyclic voltammetry of H2PAn exhibited anodic shift
of the first oxidation potential but cathodic shift for the second oxidation potential for porphyrin free
bases as azobenzene number increased. According to DFT calculation, the unusual features in UV–vis
spectra and redox potentials arised mainly due to the intramolecular ␲-conjugation between porphyrin
and azobenzene.
Received 4 October 2011
Received in revised form
15 November 2011
Accepted 17 November 2011
Available online 14 December 2011
Keywords:
Azobenzene–porphyrin
Cyclic voltammetry
Spectroelectrochemistry
DFT calculation
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Azobenzene, one of the most important dyes, behaves in theory
as an electron-withdrawing group [9] but there is no investigation
The design and synthesis of molecular devices have been the
foci of intense research in recent years. Porphyrins and metallopor-
phyrins are commonly chosen as a basic component in electronic
materials because their outstanding electrochemical and spectral
properties have an amazing range for modulation via the alter-
ation of the central metal and the peripheral substituents [1–4].
Moreover, the interaction between porphyrin and substituents
influences the efficiency of energy or electron transfer so that the
selection of substituents has become an important issue. A great
deal of recent research has been devoted to the investigation of
chromophore-linked metalloporphyrin dyads [5–8] with the hope
for the construction of artificial redox and photosynthetic systems.
With this objective, porphyrins are commonly functionalized with
a number of redox-active chromophores, such as fullerenes [5],
quinones [6], ferrocenes [7], and other porphyrins [8].
using this group as an electron acceptor in molecular electronics
since the original feature of azobenzene can be changed by the
intramolecular electronic communication between azo group and
adjacent chromophores. In contrast, azobenzenes are widely used
as photoresponsive linkers [10,11] between chromophores since
the E/Z photoisomerization of azobenzenes has been shown to
be a useful way to regulate molecular motion in numerous pho-
toswitchable devices [12,13]. To our knowledge, there are some
reports on the physical properties of the azobenzene-substituted
porphyrins [9,14,15], in which the fluorescence quenching have
been observed, whereas other fundamental characteristics for
these compounds have not been thoroughly studied.
We report herein the characterization of a new class of modi-
fied porphyrins, the azobenzene-linked free base porphyrins and
the corresponding zinc complexes (Fig. 1). Azobenzene is cova-
lently connected to the meso-positions of the porphine ring to
form mono-, di- and tetra-substituted porphyrins. The spectral
and electrochemical characteristics of the azobenzene–porphyrins
have been studied using UV–vis absorption and cyclic voltammetry
(CV). These results are expected to provide more comprehensive
information on their chemical properties. The chemical structure
of the porphyrin complexes and their abbreviations are shown in
Fig. 1.
∗
Corresponding author. Tel.: +886 49 2910960x4150.
Corresponding author Tel: +886 2 27898530.
Corresponding author Tel: +886 49 2910960x4946.
E-mail addresses: tfyang@ncnu.edu.tw (T.-F. Yang), ichao@chem.sinica.edu.tw (I.
∗∗
∗ ∗ ∗
Chao), yosu@ncnu.edu.tw (Y.O. Su).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.11.061

52
K.Y. Chiu et al. / Electrochimica Acta 62 (2012) 51–62
In a sealed tube with potassium carbonate (1.0 mmol), 4-
(4-hydoxyphenylazo)-benzaldehyde (1.0 mmol), DMF (6 mL) and
1-bromohexane (1.2 mmol) were added subsequently. The mix-
ture was stirred for 20 h at 110–120 ◦C, aqueous sodium hydroxide
solution (NaOH: 0.10 g, H2O: 50 mL) was added. The mix-
ture was extracted with CH2Cl2. The combined extracts were
dried (MgSO4), the solvent was removed at reduced pressure.
The residue was chromatographed on silica gel to give 4-(4-
hexyloxyphenylazo)benzaldehyde (yield = 53%).
2.1.2. H2PA1
A
solution
of
4-(4-hexyloxyphenylazo)benzaldehyde
(0.5 mmol), mesitylaldehyde (1.5 mmol) and pyrrole (2 mmol)
in 200 mL CHCl3 (7.5% EtOH) was purged with N2 for 10 min, then
BF3•OEt2 (24 mmol) was then added. The solution was stirred for
1 h at room temperature then DDQ (1.6 mmol) was added. The
mixture was stirred for an addition 1 h and then the solvent was
removed. Column chromatography afforded the porphyrin as the
second moving band (yield = 13%). UV/vis (CH2Cl2) ꢀmax/nm: 352,
420, 516, 551, 592, 648; HRMS (FAB+): m/z 945.5226 (M+); 1H NMR
(CDCl3/ppm): ı 8.815 (2H, d), 8.677 (2H, d), 8.616 (4H, s), 8.313 (2H,
d), 8.223 (2H, d), 8.063 (2H, d), 7.256 (6H, s), 7.078 (2H, d), 2.603
(13H, s), 1.835 (27H, d), −2.571 (2H, s). 13C NMR(CDCl3/ppm):
161.9, 152.2, 147.1, 144.2, 139.4, 138.3, 138.1, 137.7, 135.2, 127.7,
125.0, 120.8, 118.3, 118.17, 117.9, 114.8, 68.4, 31.6, 29.7, 29.2, 25.7,
22.6, 21.8, 21.7, 21.5, 14.1. Anal. Calc’d for C65H64N6O:C, 82.59; H,
6.82; N, 8.89; O, 1.69. Found:C, 82.55; H, 6.82; N, 8.88; O, 1.65.
Fig. 1. Chemical structure of metalloporphyrins containing azobenzene and
azobenzene derivative; M = H2, Zn.
2. Experimental
2.1.3. ZnPA1
2.1. Synthesis
ZnPA1 was obtained by metallation of H2PA1 with excess zinc
acetate under nitrogen in DMF for 2 h at 150 ◦C. Completion of
the reaction was checked by the shift of the Q bands spectropho-
tometrically. The solution was evaporated and further purified
by column chromatography on silica gel with CH2Cl2. yield = 88%.
UV/vis (CH2Cl2) ꢀmax/nm: 352, 421, 550, 589; HRMS (FAB+): m/z
1007.4373 (M+); 1H NMR(CDCl3/ppm): ı 8.896 (2H, d), 8.751 (2H,
d), 8.688 (4H, s), 8.335 (2H, d), 8.225 (2H, d), 8.063 (2H, d), 7.254
(6H, s), 7.079 (2H, d), 2.606 (13H, s), 1.838 (27H, d).
In order to aid the solubility of porphyrins, the long-chain
alkoxyl moiety was used on the end of the azobenzene substituent.
Porphyrin free bases were prepared following Lindsey’s meth-
ods [16,17]. Metallation was achieved by heating the porphyrin
free base with excess zinc acetate in DMF for 2 h under nitro-
gen [18]. Completion of the reaction was checked by the shift of
the Q bands spectrophotometrically. The solution was evaporated
and further purified by column chromatography on silica gel with
CH2Cl2. 4-Methoxylazobenzene (CH3OAz) was prepared according
to a literature method [19]. All the porphyrins were characterized
by UV–vis, FAB+–HRMS, 1H and 13C NMR spectroscopy and ele-
mental analysis. The formation constant of five-coordinated zinc
porphyrins were calculated by plot of log [(Ai − Aobs)/(Aobs − Af)] vs.
log [HIm]. Azobenzene exists possibly in E and Z forms. The E iso-
mer is more stable and can be switched to Z form by exciting with
a UV light source [14]. In order to minimize the change, the synthe-
sis and electrochemical experiments were performed in the dark.
We presumed that all of our azobenzene–porphyrins were in the E
form. Even if small amount of Z-azobenzene moiety was generated
during our experiments, it would thermally relax to the E form via
isomerization.
2.1.4. H2PA2
Pyrrole (250 mmol) and mesitaldehyde (10 mmol) were added
to a dry 250-mL round-bottomed flask and degassed with a
stream of N2 for 10 min. BF3•OEt2 (1 mmol) was then added,
and the solution was stirred under N2 at room temperature for
1 h. CH2Cl2 (50 mL) was then added, and the organic phase was
washed with diluted water (500 mL) and dried (Na2SO4). The sol-
vent was removed under vacuum to give a black oil and then
cyclohexanes (100 mL) were added to precipitate a yellow solid
that was filtered giving 5-mesityldipyrromethane (yield = 45%). A
solution of 4-(4-hexyloxyphenylazo)benzaldehyde (4 mmol) and
5-mesityldipyrromethane (4 mmol) in 1000 mL CHCl3 (7.5% EtOH)
was purged with N2 for 20 min, then BF3•OEt2 (100 ␮L) was then
added. The solution was stirred for 1 h at room temperature then
DDQ (6 mmol) was added. The mixture was stirred for an addition
1 h and then the solvent was removed. The solid was washed by
methanol and give H2PA2 (yield = 30%). UV/vis (CH2Cl2) ꢀmax/nm:
352, 422, 517, 554, 593, 649; HRMS (FAB+): m/z 1107.6010 (M+);
1H NMR (CDCl3/ppm): ı 8.846 (4H, d), 8.704 (4H, d), 8.346 (4H, d),
8.237 (4H, d), 8.066 (4H, d), 7.256 (4H, d), 7.081 (4H, d), 2.609 (18H,
s), 1.833 (26H, d), −2.606 (2H, s). 13C NMR (CDCl3/ppm): 162.0,
152.2, 147.1, 144.1, 139.4, 138.3, 137.8, 135.3, 127.8, 125.0, 120.8,
118.6, 114.8, 68.4, 31.6, 29.7, 29.2, 25.7, 22.6, 21.6, 21.5, 14.1. Anal.
Calc’d for C74H74N8O2: C, 80.26; H, 6.74; N, 10.12; O, 2.89. Found:
C, 80.27; H, 6.73; N, 10.12; O, 2.79.
2.1.1. 4-(4-Hexyloxyphenylazo)benzaldehyde [20]
4-Aminobenzoaldehyde (50.0 mmol) was added to fluoroboric
acid (50 wt% in H2O, 50 mL) and then sodium nitrite (60.0 mmol)
was added slowly at 0 ◦C. The resulting mixture was kept at 0 ◦C
and stirred for 1 h; the precipitated diazonium salt was filtered off
and dried.
Phenol (10 mmol) in aqueous potassium hydroxide (KOH:
0.62 g, H2O: 5 mL) was added to the salt (11.0 mmol) in
EtOH, the solution was stirred for 3 h at room temperature.
Hydrochloric acid (10%, 50 mL) was added and the solid was
filtered off and chromatographed on silica gel to give 4-(4-
hydoxyphenylazo)benzaldehyde.

K.Y. Chiu et al. / Electrochimica Acta 62 (2012) 51–62
53
Table 1
Absorption data and molar extinction coefficients ε (104 M−1 cm−1) in parentheses of azobenzene-substituted porphyrins in CH2Cl2.
Azo
Porphyrin
B band
Q band
CH3OAzb
H2TMP
H2PA1
H2PA2
H2PA4
ZnTMP
ZnPA1
ZnPA2
ZnPA4
(HIm)ZnTMP
(HIm)ZnPA1
(HIm)ZnPA2
(HIm)ZnPA4
347 (2.7),433 (0.1)a
418 (40)
420 (34)
422 (29)
430 (30)
421 (67)
421 (31)
423 (17)
431 (32)
431 (69)
432 (36)
434 (18)
441 (31)
514 (1.5), 547 (0.4), 591 (0.6), 647 (0.3)
516 (1.9), 551 (0.8), 592 (0.6), 648 (0.6)
517 (1.9), 554 (1.4), 593 (0.7), 649 (1.1)
520 (2.2), 558 (1.7), 593 (0.7), 651 (0.8), 690 (0.5)
550 (2.2), 587 (0.09)
550 (1.7), 589 (0.3)
550 (1.1), 592 (0.3)
552 (2.7), 593 (1.2)
567 (2.0), 606 (0.9)
352 (3.2)
352 (4.7)
352 (7.9)
352 (2.1)
352 (2.4)
352 (7.6)
b
352 (2.4)
352 (2.8)
352 (9.1)
567 (2.5), 609 (1.4)
567 (1.8), 614 (1.3)
570 (2.6), 614 (2.8)
a
Shoulder.
ZnPA2 was slightly soluble in CH2Cl2.
b
2.1.5. ZnPA2
The auxiliary electrode is a platinum wire and the reference elec-
trode is a homemade Ag/AgCl, KCl(sat’d) reference electrode. The
spectroelectrochemical cell was composed of a 1 mm cuvette, a
platinum gauze thin layer as working electrode, a platinum wire as
auxiliary electrode, and an Ag/AgCl, KCl(sat’d) reference electrode.
Yield = 90%. UV/vis (CH2Cl2) ꢀmax/nm: 352, 423, 550, 592; HRMS
(FAB+): m/z 1169.5156 (M+); 1H NMR (CDCl3/ppm): ı 8.934 (4H, d),
8.787 (4H, d), 8.361 (4H, d), 8.237 (4H, d), 8.067 (4H, d), 7.257 (4H,
d), 7.081 (4H, d), 2.612 (18H, s), 1.833 (26H, d).
3. Theoretical calculations
2.1.6. H2PA4
A solution of 4-(4-hexyloxyphenylazo)benzaldehyde (2 mmol)
and pyrrole (2 mmol) in 200 mL CHCl3 (7.5% EtOH) was purged with
N2 for 10 min, then BF3•OEt2 (24 mmol) was then added. The solu-
tion was stirred for 1 h at room temperature then DDQ (1.6 mmol)
was added. The mixture was stirred for an addition 1hr and then
the solvent was removed. Column chromatography afforded the
porphyrin as the first moving band (yield = 40%). UV/Vis(CH2Cl2)
ꢀmax/nm: 352, 430, 520, 558, 593, 651, 690; HRMS (FAB+): m/z
1431.7605 (M+); 1H NMR (CDCl3/ppm): ı 8.959 (8H, s), 8.376 (8H,
d), 8.280 (8H, d), 8.079 (8H, d), 7.099 (8H, d), 4.103 (8H, t), 1.871
(8H, m), 1.523, (36H, m) −2.677 (2H, s). 13C NMR (CDCl3/ppm):
162.0, 152.3, 147.1, 144.1, 135.4, 125.0, 120.9, 119.7, 114.8, 68.4,
31.6, 29.2, 25.7, 22.6, 14.1. Anal. Calc’d for C92H94N12O4: C, 77.17;
H, 6.62; N, 11.74; O, 4.47. Found: C, 77.20; H, 6.64; N, 11.77; O, 4.45.
All calculations have been carried out using density functional
theory (DFT) method in the Gaussian program [21]. The geomter-
ies were optimized using the B3LYP functional with 6-31G(d) basis
set for C, H, N, O atoms and LANL2DZ for Zn atom since this theory
level has been successfully applied in the electronic structures of
porphyrins and metalloporphyrins [22,23]. For the reason of com-
putational efficiency, we compute a series of model compounds by
replacing the bulky OC6H13 group with the OCH3 group, labeled as
MPAn–OCH3 and (HIm)MPAn–OCH3. All fully optimized geometries
were confirmed to be local minima by frequency analysis. Molec-
ular orbitals (MO) of the optimized structures were obtained with
the program Gauss View (version 4.1.2) [24].
4. Results and discussion
2.1.7. ZnPA4
4.1. Absorption spectra studies
Yield = 90%. UV/vis (CH2Cl2) ꢀmax/nm: 352, 433, 556, 599; HRMS
(FAB+): m/z 1510.1109 (M+); 1H NMR (CDCl3/ppm): ı 9.046 (8H, s),
8.378 (8H, d), 8.273 (8H, d), 8.079 (8H, d), 7.090 (8H, d), 4.101 (8H,
t), 1.551 (44H, m).
The UV–vis absorption spectral data of azobenzene-containing
porphyrins (H2PAn; n = 1, 2, 4) and their zinc complexes (ZnPAn;
n = 1, 2, 4) are summarized in Table 1. The absorption spectra of
H2PAn and ZnPAn in CH2Cl2 are basically given by the superimposi-
tion of the spectra of the individual porphyrin and azobenzene. The
meso-azobenzenes do not markedly affect the absorption charac-
teristic of prophyrins; the Soret and Q bands of porphyrins are just
shifted bathochromically by a few nanometers upon the substitu-
tion of each azobenzene. In comparison with the UV–vis absorption
spectra of H2TMP and ZnTMP, both the Soret and Q band regions are
shifted to longer wavelengths with the increasing substituent num-
ber of azobenzene. In particular, H2PA4 display a 12 nm red shift in
comparison with H2TMP in the Soret band. A slight increase in the
relative intensity of the Q˛ band occurs as the number of azoben-
zene increases. The ratio of Q-band intensity (ε˛/εˇ) increases in
the order ZnPA1 < ZnPA2 < ZnPA4.
2.1.8. CH3OAz [19]
Anal. Calc’d for C13H12N2O: C, 73.56; H, 5.70; N, 13.20; O, 7.54.
Found: C, 73.59; H, 5.70; N, 13.25; O, 7.45.
2.2. Instrument
Electrochemistry was performed with a CHI Model 700 series
electroanalytical workstation. Absorption spectra were measured
with a HP-8453 UV/vis spectrophotometer. 1H NMR spectra were
obtained with a Varian Unity Inova 300 WB spectrometer.
2.3. Electrodes
The UV–vis spectra for ZnPA1, ZnPA2, and ZnPA4 are also
changed upon addition of imidazole [25]. The original Q bands
decreased in absorbance and new bands appeared at longer wave-
length. In addition, the intensity ratio of Q˛ and Qˇ band (␧˛/␧ˇ)
is dramatically enhanced from 0.18–0.44 for four-coordinate
ZnPAn to 0.56–1.07 for five-coordinate (HIm)ZnPAn (Table 2).
Cyclic voltammetry was conducted with the use of
a
three-electrode cell in which a BAS glassy carbon electrode
(area = 0.07 cm2) was used as working electrode. The glassy carbon
electrode was polished with 0.05 ␮m alumina on Buehler felt pads
and was ultrasonicated for 2 min to remove the alumina residue.

54
K.Y. Chiu et al. / Electrochimica Acta 62 (2012) 51–62
Table 2
some ␲-electron on meso-azobenzene, which expands the extent
of ␲-conjugation of HOMO (Fig. 3). Because more extended ␲-
conjugation will raise the energy of HOMO, the energy-stabilization
effect of electron-withdrawing azobenzene on HOMO is canceled.
Consequently, the energy of HOMO increases with increasing
number of azobenzene. In contrast to the HOMO, the extent of
␲-conjugation of HOMO-1 always remains unchanged so that
the energy of HOMO-1 follows the stabilizing effect of electron-
withdrawing azobenzene. Because the two opposite behaviors of
HOMO and HOMO-1 cause the increased energy gap and Q˛ inten-
sity, it can be found in Table 2 that the Q-band intensity ratio slightly
increases as the extent of substitution of azobenzene increases.
In the presence of axial imidazole, both the HOMO and HOMO-1
of (HIm)ZnPAn–OCH3 are greatly destabilized by electron donated
from imidazole to porphyrin ring through central Zn and the con-
formational distortion of porphyrin ring. For (HIm)ZnPA1–OCH3,
the HOMO are raised from −4.95 eV to −4.57 eV, and the HOMO-
1 are increased from −5.15 eV to −4.85 eV in comparison with
ZnPA1–OCH3. On the a2u-like HOMO, there is ␲-electron dis-
tributed on the central N atoms, but this is not the case for the
a1u-like HOMO-1 (Fig. 3). Therefore, the electron-donating effect
of imidazole is more prominent in HOMO than in HOMO-1 and
an increase in the energy gap between HOMO and HOMO-1 is
observed for (HIm)ZnPAn–OCH3 when compared to ZnPAn–OCH3
in Fig. 3. For ZnPA1–OCH3 and (HIm)ZnPA1–OCH3, the energy gap
increases from 0.20 eV to 0.28 eV. Similar changes in the energy
gap of HOMO and HOMO-1 are also observed for ZnPA2–OCH3
and (HIm)ZnPA2–OCH3 (0.22 → 0.30 eV), and ZnPA4–OCH3 and
(HIm)ZnPA4–OCH3 (0.28 → 0.33 eV).
The ratio of Q-band intensity (ε˛/εˇ) for zinc porphyrins, and the formation con-
stants (Kf) for imidazole ligation to zinc porphyrins.
a
b
Compound
ε˛/εˇ
Kf
ε˛/εˇ
ZnTMP
ZnPA1
ZnPA2
ZnPA4
0.13
0.18
0.27
0.44
1.9 × 104
1.1 × 105
2.0 × 105
3.2 × 105
0.48
0.56
0.75
1.07
a
Four-coordinate zinc porphyrin.
Four-coordinate zinc porphyrin.
b
The Q-band intensity ratios for imidazole ligation increase in the
order (HIm)ZnPA1 < (HIm)ZnPA2 < (HIm)ZnPA4. The most notewor-
thy feature is that the absorbance ratio of Q˛ at 614 nm and Qˇ
at 570 nm bands is larger than 1 for (HIm)ZnPA4 (Fig. 2). To our
knowledge, such intensity inversion of the Q bands is unusual in
meso-substituted porphyrins since the Q-bands usually follow the
order of relative intensities Qˇ > Q˛.
It has been reported that the intensity of Qˇ bands is mainly
derived from vibronic coupling with the intense Soret band, so
the Qˇ intensity is not easily affected by peripheral substituents
and ligands [26]. This indicates that the variation in Q-band inten-
sity ratios mainly resulted from the intensity of Q˛ band. A larger
Q-band ratio originates from a larger Q˛ band. Based on the Gouter-
man four-orbital model, the intensity of Q˛ band is dependent on
the energy gap between porphyrin a1u and a2u orbitals [26,27]. If
the two porphyrin orbitals, a2u and a1u, are nearly degenerative,
the Q˛ band will be small, and vice versa. It is also the reason why
four-coordinate porphyrins usually exhibit very small Q-band ratio.
Therefore, the energy gap between HOMO and HOMO-1 can be
used to reflect the Q band intensity ratio. The orbital energies of
HOMO and HOMO-1 of the model compounds, ZnPAn–OCH3 and
(HIm)ZnPAn–OCH3, are displayed in Fig. 3. As can be seen, the
HOMO and HOMO-1 are a2u and a1u-like MO, respectively, and the
trend of energy gap between HOMO and HOMO-1 is in agreement
with experimental trend for the Q band intensity ratio.
We first scrutinize the MOs of ZnPAn–OCH3 to comprehend
the influence of meso-azobenzene substituents. As the number of
azobenzene increases, the energy level of HOMO-1 shifts slightly
to a more negative value from −5.15 eV to −5.17 eV. Conversely,
the a2u-like HOMO shifts positively from −4.95 eV to −4.89 eV. The
reverse behavior seen for a2u-like HOMO can be understood from
the inspection of the MO patterns in Fig. 3. Although the HOMO
and HOMO-1, respectively, remain as a2u and a1u-like orbitals
with the increasing number of azobenzene, a2u-like HOMO shows
In the above discussion, we have presented that the relative
energies of HOMO and HOMO-1 of azobenzene-substituted por-
phyrins can be increased with the substitution of azobenzene
and ligation of imidazole and are associated with the increase in
Q-band intensity ratio. Since (HIm)ZnPA4–OCH3 has four meso-
azobenzenes and one axial imidazole, the energy of HOMO is much
more destabilized than HOMO-1. Not surprisingly, the unusual high
ratio in the Q band (ε˛/εˇ > 1) is then observed.
4.2. Electrochemical studies of 4-methoxylazobenzene
CH3OAz (4-methoxylazobenzene) exhibited one redox couple
at Epc = −1.52 V (Fig. 4(a)) in CH2Cl2. The reduction potential for
CH3OAz was not affected by 1-methylimidazole (MeIm), indicating
no specific interaction between CH3OAz and MeIm. On the other
hand, the current of the original redox couple for CH3OAz gradually
decreased and a new reduction wave at Epc = −1.38 V appeared as
the original peak when imidazole (HIm) was added to the solution
•
1.6
(Fig. 4). Therefore, we presume that CH3OAz− would quickly form
hydrogen bonding with HIm after CH3OAz was reduced [28].
552
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
614
[HIm]=
0.00
570
4.3. Electrochemcial and spectroelectrochemical studies of
free-base porphyrins
6.56E-06
1.29E-05
1.90E-05
2.96E-05
4.29E-05
5.52E-05
7.74E-05
1.65E-04
1.00E-02
4.3.1. Cyclic voltammetry
593
Fig. 5(a) shows the cyclic voltammogram of H2PA1 in CH2Cl2. In
the oxidation part, two reversible redox couples could be observed
at E1/2 = +1.05 and +1.43 V. In the reduction part, an overlap-
ping wave at Epc = −1.38 V and one reversible redox couple at
E1/2 = −1.66 V could be observed.
Upon addition of HIm, an irreversible reduction wave split
away from the overlapping wave and the peak potential occurs at
Epc = −1.27 V, more anodically than the original reduction reaction
(Fig. 6A). The first irreversible reduction wave was thus attributed
to the hydrogen bond formation of imidazole with the azobenzene
anion radical (Scheme 1) and the second and third redox cou-
ples were due to porphyrin ring reduction. Furthermore, the third
500
550
600
650
700
750
Wavelength (nm)
Fig. 2. Absorption spectral change of ZnPA4 (4 × 10−5 M) in CH2Cl2 in the presence
of various concentration of imidazole.

K.Y. Chiu et al. / Electrochimica Acta 62 (2012) 51–62
55
Fig. 3. MO diagrams of four-coordinate ZnPAn–OCH3 and five-coordinate (HIm)ZnPAn–OCH3 calculated by a DFT method at the B3LYP/6-31G(d) level.
reduction potential started shifting anodically when more than 1.0
equivalent of HIm was added. The third reduction potential shifted
by about +0.1 V as [HIm] reaches two equivalents of [H2PA1], indi-
cating that the extra HIm forms hydrogen bonding with the doubly
reduced state of porphyrin ring (Scheme 2) [28]. As the concentra-
tion of HIm increased, the current of the oxidation wave at ∼0 V
also increased (Fig. 6(a)–(e)). It suggested that the reduced form
H2PA1 would undergo chemical reaction with HIm to produce a
new compound which was oxidized at ∼0 V.
anodically (Scheme 3). However, the shift (about +0.07 V) in E1/2 for
the H2PA1−2/−3 wave in the presence of HIm is smaller than that in
CH2Cl2 even though three reductions in DMF are resolvable.
There are two special features in the redox chemistry of free
bases H2PA1, H2PA2, and H2PA4 in CH2Cl2 (Fig. 5). Firstly, the
azobenzene moieties were reduced at a potential close to that of the
porphyrin ring. The current ratio for azobenzene moieties versus
porphyrin reduction were about 2.0 and 4.0 for H2PA2 and H2PA4,
respectively. It appears that H2PA2 and H2PA4 could undergo mul-
tiple electron reduction in a step. Secondly, the potential of the
first oxidation wave is slightly more positive as the number of
meso-azobenzene increases, in line with the electron-withdrawing
nature of the azo group. The second oxidation, however, exhibits
a more negative potential as the number of meso-azobenzene
To probe the solvent effect [29], the redox behaviors are mea-
sured for H2PA1 under the same experimental conditions in DMF
(Fig. 6B). DMF is a polar solvent and can stabilize the reduced por-
phyrin in the electrochemical experiment. In DMF, the reductions
of H2PA1 and azobenzene were resolved as compared with those in
CH2Cl2. The reduction of H2PA1 exhibited three resolvable reduc-
tion waves, namely one irreversible reduction wave at Epc = −1.29 V
and two redox couples at E1/2 = −0.99 and −1.51 V, respectively.
Upon addition of HIm, the current of the second reduction wave
decreased and the reduction peak potential moved to −1.06 V.
The first and third redox couples are thus assigned to porphyrin
ring and the second wave is azobenzene reduction. The effect of
imidazole towards the third redox couple of H2PA1 is similar to
that in CH2Cl2. When imidazole concentration was higher than
one equivalent of H2PA1, the third redox couple shifted potential
increases (Table 3). The difference in oxidation potential (ꢁEox
)
thus becomes smaller concurrently.
In order to interpret these unusual tendencies, we continued
to seek help from DFT calculations. We examined the MO for all
charge states, and found the patterns of the highest occupied MO
(HOMO) of the neutral state and ␣-HOMO of the cation radical
state to be very different (Fig. 7). For the neutral H2PAn–OCH3,
the dihedral angle between azobenzene and porphyrin ring (ꢂ1)
is in the range of 64.9–66.6◦ (Table 4). The two phenyl rings of
meso-azobenzene is about coplanar (ꢂ2 ꢂ3 < 5◦) and the mesityl

56
K.Y. Chiu et al. / Electrochimica Acta 62 (2012) 51–62
Fig. 5. Cyclic voltammograms of 1.0 x 10−3 M (a) H2PA1; (b) H2PA2; (c) H2PA4 in
CH2Cl2 containing 0.1 M TBAP. Scan rate: 0.1 V/s.
reduction of dihedral angle ꢂ1 upon first oxidation afford stronger
␲-conjugation between porphyrin and meso-azobenzenes. There-
fore, the ␣-HOMO of the cation radical state, which is related to the
second oxidation, shows ␲-electron delocalization over the whole
molecule. The extent of electron delocalization in ␣-HOMO is very
different as the number of azobenzene increases. As more extended
␲-conjugation will raise the energy of HOMO, it is not surprising
that the orbital energies of ␣-HOMO of cation radical H2PA1–OCH3,
H2PA2–OCH3, and H2PA4–OCH3 are rising (Fig. 7), thus counter-
acting the energy-lowering effect of the electron-withdrawing azo
group and affording a more negative potential for second oxidation
as the number of meso-azobenzene increases. The lack of significant
change in the extent of ␲-electron delocalization among HOMOs
of neutral H2PAn–OCH3 explains why the electron-withdrawing
effect of the azo group can still be observed for the first oxidation.
Fig. 4. Cyclic voltammograms of 1.0 × 10−3 M CH3OAz in CH2Cl2 containing 0.1 M
TBAP and various concentrations of HIm. [HIm] = (a) 0.00; (b) 0.50; (c) 1.00; (d) 1.50;
and (e) 2.0 × 10−3 M. Scan rate: 0.1 V/s.
groups are perpendicular to porphyrin ring (ꢂ4 ≈ 90◦). The HOMO
of the neutral state, which is related to the first oxidation, closely
resembles Gouterman’s a2u orbital and there is only a glimmer of
␲-electron delocalized on meso-azobenzene (Fig. 7). This situation
remains no matter how many meso-azobenzene are present. The
dihedral angle ꢂ1 decrease from 64.9◦ to 66.6◦ at neutral state to
54.1–58.5◦ at cation state. At the cation radical state, the dihedral
angles ꢂ2, ꢂ3, and ꢂ4 are not significantly changed (Table 4). The
Table 3
Half-wave potentials (V vs. Ag/AgCl) of free-base porphyrins in CH2Cl2/TBAP (Fc 0/+ = +0.56 V).
Oxidationa
Reductiona
E2ox
E1ox
ꢁE
E1red
E2red
E3red
ꢁEred
c
ꢁEox
CH3OAz
H2TMP
H2PA1
−1.44
0.44
0.38
0.32
0.20
+1.48
+1.43
+1.40
+1.29
+1.04
+1.05
+1.08
+1.09
2.37
2.43
2.36
2.24
−1.33
−1.68
−1.60
−1.64
−1.57
0.35
0.22
0.36
0.42
−1.38b,d
−1.28b
−1.15
−1.38b,d
−1.42b
−1.27
e
H2PA2
e
H2PA4
ꢁEOX = E2ox − E1ox (difference between the first and second oxidation potential), ꢁE = E1ox − E1red, ꢁE1red = E1red − E3red
.
a
b
c
Peak potential.
The reduction of azobenzene moiety.
The overlapping wave.
The E1/2 was obtained from differential pulse voltammetry.
d
e
Scheme 1.

K.Y. Chiu et al. / Electrochimica Acta 62 (2012) 51–62
57
Table 5
The LUMO and LUMO+1 energy levels of H2PAn.
Compound
H2PA1
MO
Energy (eV)
LUMO
LUMO+1
−2.26373
−2.18074
Scheme 2.
H2PA2
H2PA4
LUMO
LUMO+1
−2.32251
−2.18482
LUMO
LUMO+1
−2.32441
−2.28686
Fig. 8(d) shows the absorption spectra of H2PA4 in CH2Cl2 at
various applied potentials. In the potential range of Eappl. = +0.00
to +1.18 V, the equilibrium spectra of H2PA4 under various poten-
tials were obtained. As the potential was moved anodically, the
characteristic absorption of azobenzene substituent at 352 nm,
and the Soret band (430 nm) gradually decreased while a new
peak at 475 nm grew. The Q band absorbance at 520 nm gradually
decreased and a new peak at 689 nm grew concurrently.
Scheme 3.
The LUMO and LUMO+1 energy levels of H2PAn were obtained by
the DFT calculation and the values were listed in Table 5. The energy
of the LUMO and LUMO+1 decreased with increasing number of
azobenzene moiety. It indicated that the azobenzene substituent
was an electron-withdrawing group.
Fig. 9 was the absorption spectra of H2TMP and H2PAn of which
the reduction potentials (Eappl.) were applied at their respective
onset potentials. When Eappl. = −1.33 V (Fig. 9(a)), the Soret band
(418 nm) and one Q band (514 nm) of H2TMP decreased and the
other three Q band (547, 595 and 652 nm) increased, correspond-
ing to the anion radical formation. The spectral changes of H2PAn
were similar to that of H2TMP. When each Eappl. was at onset reduc-
tion potential, the characteristic absorbance of azobenzene moiety
(352 nm), Soret band and first two Q bands decreased significantly,
and the other Q bands increased. Noteworthily, the azobenzene
peak and Soret band decreased concurrently. We used the DFT cal-
culation results (Fig. 7) to explain the phenomena. In the cation
radical state, stronger ␲-conjugation was observed between por-
phyrin and meso-azobenzenes. Conjugation was also applied to in
the anion radical, and both the azobenzene peak and Soret band
decreased in absorbance at the same time indicated that the charge
delocalized between the porphyrin ring and azobenzene moiety.
4.3.2. Spectroelectrochemistry
Fig. 8(a) shows the absorption spectra of H2TMP in CH2Cl2 at
various applied potentials. When Eappl. = +0.85 to +1.21 V, the Soret
band (418 nm) and three of Q bands (514, 547 and 591 nm) of
•
H2TMP decreased. The corresponding H2TMP+ exhibit a strong
absorption peak at 437 nm and the other Q band (634 nm) increased
in absorbance.
In the potential range of Eappl. = +0.00 to +1.15 V, the spec-
tra of H2PA1 under various potentials were obtained (Fig. 8(b)).
As the potential shifted anodically, the characteristic absorp-
tion of azobenzene substituent at 352 nm and the Soret band
(420 nm) gradually decreased while a new peak at 446 nm grew.
The absorbance at 516 nm gradually decreased and a new peak at
647 nm grew concurrently.
Fig. 8(c) was the spectroelectrochemical results of H2PA2 and
the pattern of spectral change was similar to H2PA1. In the potential
range of Eappl. = +0.00 to +1.20 V, the equilibrium spectra of H2PA2
under various potentials were obtained. As the potential shifted
anodically, the characteristic absorption of azobenzene substituent
at 352 nm and the Soret band (422 nm) gradually decreased while
a new peak at 453 nm grew. The Q band absorbance at 517 and
554 nm gradually decreased and a new peak at 658 nm grew con-
currently.
4.4. Electrochemcial and spectroelectrochemical studies of zinc
porphyrins
4.4.1. Cyclic voltammetry
The electrochemical results of ZnPAn are similar to that of
H2PAn. Fig. 10(a) shows the cyclic voltammogram of ZnPA1
in CH2Cl2. Two reversible redox couples could be observed at
E1/2 = +0.79 and +1.11 V in the oxidation part. An overlapping wave
at Epc = −1.43 V (a shoulder at Epc = −1.34 V) and one reversible
redox couples could be observed at E1/2 = −1.59 V in the reduction
part. The current ratios for azobenzene moieties versus zinc por-
phyrin reduction were also about 2.0 and 4.0 for ZnPA2 and ZnPA4,
respectively. The results were the similar to H2PAn. The peak cur-
rent of ZnPA2 was not as clear as the others because of its low
solubility in CH2Cl2. The peak potentials of ZnPAn were summarized
in Table 6.
Table 4
The dihedral angles (ꢂ) of optimized H2PAn–OCH3 (n = 0, 1, 2, 4) at the neutral and
cation radical state.
With the imidazole coordinating onto a zinc porphyrin, the first
oxidation wave shifts cathodically but the second one shifts anodi-
cally. The first reduction wave of zinc porphyrin shifts cathodically
[30]. The formation constant of imidazole with zinc porphyrin is
about 104–105. Upon addition of MeIm (Fig. 11), the first oxida-
tion wave shifted cathodically by +0.10 V. The second one, however,
shifted anodically and was irreversible [30]. The first overlapping
irreversible reduction wave became a single wave (the shoulder
reduction wave shifted cathodically) and the reduction wave at
Epc = −1.43 V did not shift (the peak current increased). The second
reduction wave (E1/2 = −1.59 V) remained unchanged. The bind-
ing constant between the zinc porphyrin and MeIm is ∼104 M−1
[30,33]. MeIm coordinated to the anions of zinc porphyrin with
H2TMP
Neutral state
H2PA1–OCH3
H2PA2–OCH3
H2PA4–OCH3
ꢂ1
ꢂ2
ꢂ3
ꢂ4
–
–
65.1◦
0.1◦
0.0◦
90◦
64.9◦
0.1◦
66.6◦
1.1◦
0.7◦
–
–
0.1◦
90◦
89.0◦
Cation radical state
ꢂ1
ꢂ2
ꢂ3
ꢂ4
–
–
54.1◦
2.8◦
54.9◦
2.6◦
58.5◦
1.5◦
0.6◦
–
–
1.0◦
0.9◦
90◦
88.7◦
86.3◦

58
K.Y. Chiu et al. / Electrochimica Acta 62 (2012) 51–62
Fig. 6. Cyclic voltammograms of 1.0 × 10−3 M H2PA1 in (A) CH2Cl2 and (B) DMF containing 0.1 M TBAP and various concentrations of HIm. [HIm] = (a) 0.00; (b) 0.50; (c) 1.00;
(d) 1.50; and (e) 2.00 × 10−3 M. Scan rate: 0.1 V/s.
Fig. 7. MO diagrams of the H2PAn–OMe (n = 0, 1, 2, 4) for neutral and cation radical states.
Table 6
Half-wave potentials (V vs. Ag/AgCl) of zinc porphyrins in CH2Cl2/TBAP (Fc 0/+ = +0.56 V).
Oxiadtiona
Reductiona
E2ox
E1ox
ꢁE
E1red
E2red
E2red
ꢁEred
c
ꢁEox
CH3OAz
ZnTMP
ZnPA1
ZnPA2
ZnPA4
−1.44
−1.48
−1.34b
−1.31b
−1.24b
0.32
0.33
0.31
0.29
+1.11
+1.14
+1.14
+1.16
+0.79
+0.81
+0.83
+0.87
2.27
2.24
2.14
2.11
−1.43b
−1.42b
−1.35
−1.59
−1.58
−1.62
0.25
0.27
0.38
ꢁEOX = E2ox − E1ox (difference between the first and second oxidation potential), ꢁE = E1ox − E1red, ꢁE1red = E1red − E3red
.
a
b
c
Peak potential.
The reduction of azobenzene moiety.

K.Y. Chiu et al. / Electrochimica Acta 62 (2012) 51–62
59
1.15V
1.09V
1.06V
1.03v
0.97V
0.88V
0.80V
0.75V
0.00V
1.21V
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
420
446
1.12V
1.06V
1.03V
1.00V
0.97V
0.94V
0.85V
1.0
0.8
0.6
0.4
0.2
0.0
418
437
(a)
(b)
352
634
591
647
516
514
200
400
600
800
1000
200
400
600
800
1000
Wavelength (nm)
Wavelength (nm)
1.18V
1.15V
1.12V
1.09V
1.06V
1.00V
0.91V
0.00V
422
1.20V
1.17V
1.11V
1.08V
1.05V
0.99V
0.90V
0.80V
0.00V
430
475
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
(c)
(d)
453
689
352
658
352
517
554
520
200
400
600
800
1000
200
400
600
800
1000
Wavelength (nm)
Wavelength (nm)
Fig. 8. Spectral changes of (a) H2TMP 5.0 × 10−5 M and (b) H2PA1 2.5 × 10−5 M; (c) H2PA2 2.5 × 10−5 M; (d) H2PA4 2.5 × 10−5 M in CH2Cl2 containing 0.1 M TBAP at various
applied potentials.
0.00 V
-1.33 V 1 min
2 min
3 min
4 min
420
1.0
0.8
0.6
0.4
0.2
0.0
0.00 V
-1.15 V 1 min
2 min
(a) 1.0
0.8
(b)
418
3 min
4 min
516
514
5 min
6 min
0.6
654
650
551
550
547
7 min
652
595
599
600
8 min
0.4
352
0.2
500
700
500 550 600 650 700
0.0
200
400
600
800
1000
200
400
600
800
1000
Wavelength (nm)
Wavelength (nm)
422
1.0
0.8
0.6
0.4
0.2
0.0
(d)
0.00 V
-1.05 V 1 min
2 min
430
0.00 V
(c) 1.0
0.8
-1.10 V 1 min
2 min
3 min
3 min
4 min
4 min
5 min
5 min
517
554
6 min
520
6 min
0.6
558
7 min
7 min
8 min
9 min
654
598
593
9 min
651
0.4
352
352
500
550
600
650
700
0.2
500
550
600
650
700
0.0
200
400
600
800
1000
200
400
600
800
1000
Wavelength (nm)
Wavelength (nm)
Fig. 9. Spectral changes of 2.5 × 10−5 M (a) H2TMP; (b) H2PA1; (c) H2PA2; and (d) H2PA4 in CH2Cl2 containing 0.1 M TBAP at onset reduction potentials.

60
K.Y. Chiu et al. / Electrochimica Acta 62 (2012) 51–62
Fig. 10. Cyclic voltammograms of (a) ZnPA1 1.0 × 10−3 M; (b) ZnPA2 6.0 × 10−4 M;
(c) ZnPA4 1.0 × 10−3 M in CH2Cl2 containing 0.1 M TBAP. Scan rate: 0.1 V/s.
Fig. 12. Cyclic voltammetry of 1.0 x 10−3 M ZnPA1 in the presence of HIm in CH2Cl2
containing 0.1 M TBAP. [HIm] = (a) 0.00; (b) 0.50; (c) 1.00; (d) 1.50 × 10−3 M. Working
electrode: glassy carbon. Scan rate:0.1 V/s.
When the additive was HIm (Fig. 12), it not only coordinated
with zinc porphyrin but also formed hydrogen bonding with
reduced azobezene moiety. As HIm was added to the ZnPA1 solu-
tion stepwise, the first oxidation wave shifted cathodically and
the second one shifted anodically. There were two new waves at
Epc = −1.18 and −1.35 V in the reduction part. The anodic shift of
first zinc porphyrin ring reduction from −1.34 V to −1.18 V was due
to HIm coordinated on ZnPA1 and azobenzene moiety interacted
with HIm was Epc = −1.43 V to −1.35 V. The zinc porphyrin ring
reduction wave shifted cathodically (from E1/2 = −1.59 to −1.67 V).
4.4.2. Spectroelectrochemistry
Fig. 13 shows the spectroelectrochemical results of ZnTMP
and ZnPAn under various oxidation potentials. When the poten-
tial was moved anodically (Fig. 13(a)), the Soret band (421 nm)
of ZnTMP gradually decreased while a shoulder peak at 436 nm
grew. The Q band absorbance at 550 nm gradually decreased
and a broad band at 600–700 nm grew concurrently. As the
number of meso-azobenzene increases, the absorbance of azoben-
zene substituent at 352 nm increases and the Soret band of
ZnPAn is red-shifted (Table 1). In the first oxidation step of
each ZnPAn, the Soret band and Q band gradually decreased
Fig. 11. Cyclic voltammetry of 1.0 × 10−3 M ZnPA1 in the presence of MeIm in CH2Cl2
containing 0.1 M TBAP. [MeIm] = (a)0.00; (b)0.50; (c)1.00; and (d)1.50 × 10−3 M.
Working electrode: glassy carbon. Scan rate:0.1 V/s.
while the peak at 352 nm and
a broad band between 600
and 950 nm grew concurrently. The broad band was sug-
gested that the charge transfer between porohyrin ring and
azobenzene moiety [31–33]. The broad bands were intra-molecular
charge transfer bands. The results exhibited the typical metal por-
phyrin ring oxidized spectrum.
smaller formation constants (about 102–103 M−1) than the neu-
tral species [30]. It suggested that the MeIm was de-ligated from
(MeIm)ZnPA1 (Scheme 4) [30]. According to the results, we pre-
sume that the shoulder (Epc = −1.34 V) of the overlapping reduction
wave in Fig. 11(a) was attributed to the porphyrin ring reduc-
tion. The azobenzene moiety was reduced at Epc = −1.43 V, and
E1/2 = −1.59 V corresponds to the second reduction potential of por-
phyrin ring.
ZnTMP was used as model compound. When Eappl. = −1.48 V
(Fig. 14(a)), the Soret band (420 nm) gradually decreased while a
new peak at 435 nm grew. The absorbance at 550 nm gradually
decreased and two new peaks at 570 and 612 nm grew concur-
rently. The anion radical spectrum of ZnTMP was then obtained.
In order to further identify the reduction site on the ZnPAn, we
set the Eappl. at onset reduction potential and monitored the spec-
tral changes by spectroelectrochemical method. As E = −1.20 V was
applied (Fig. 14(b)), the Soret band (421 nm) and Q band (550 nm)
of ZnPA1 decreased while a shoulder peak at 435 nm and two new
peak between 600 and 700 nm increased in absorbance. But at this
potential, the characteristic peak (352 nm) of azobenzene moiety
has no significant change. In order to understand this phenomenon,
+ e-
+ e-
ZnPA1
+ MeIm
ZnPA1
2-
ZnPA1
+ e-, -MeIm
+ MeIm
(MeIm)ZnPA1
+ e-
(MeIm)ZnPA1
Scheme 4.

K.Y. Chiu et al. / Electrochimica Acta 62 (2012) 51–62
61
0.96V
0.90V
0.84V
0.81V
0.78V
0.72V
0.63V
0.00V
(a) 2.0
421
1.0
0.8
0.6
0.4
0.2
0.0
0.90V
0.85V
0.80V
0.77V
0.74V
0.69V
0.65V
0.60V
0.57V
0.53V
0.50V
0.40V
0.30V
(b)
1.8
421
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
436
550
352
550
200
400
600
800
1000
200
400
600
800
1000
Wavelength (nm)
Wavelength (nm)
0.99V
0.96V
0.90V
0.87V
0.84V
0.78V
0.69V
0.00V
1.01V
0.92V
0.86V
0.83V
0.80V
0.74V
0.65V
0.00V
431
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
(c)
(d)
423
352
470
552
352
550
683
869
200
400
600
800
1000
200
400
600
800
1000
Wavelength (nm)
Wavelength (nm)
Fig. 13. Spectral changes of (a) ZnTMP 5.0 × 10−5 M; (b) ZnPA1 1.88 × 10−5 M and (c) ZnPA2 2.5 × 10−5 M; (d) ZnPA4 2.5 × 10−5 M in CH2Cl2 containing 0.1 M TBAP at various
applied potentials.
421
420
435
0.00 V
-1.48 V 1 min
2 min
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
(b)1.0
0.8
0.00 V
-1.20 V 1 min
2 min
3 min
5 min
3 min
4 min
5 min
6 min
435
0.6
550
550
7 min
550
550
570
514
0.4
612
0.2
500
600
650
700
500
600
650
0.0
200
400
600
800
1000
200
400
600
800
1000
Wavelength (nm)
Wavelength (nm)
0.00 V
-1.16 V 1 min
2 min
(c) 1.0
0.8
(d) 1.0
0.00 V
-1.00 V 1 min
2min
431
3 min
3 min
0.8
0.6
0.4
0.2
0.0
4 min
4 min
423
5 min
5 min
6 min
550
6 min
0.6
552
8 min
7 min
9 min
650
593
623
592
0.4
690
0.2
500 550 600 650 700 750
500 550 600 650 700 750
0.0
200
400
600
800
1000
200
400
600
800
1000
Wavelength (nm)
Wavelength (nm)
Fig. 14. Spectral changes of (a) ZnTMP 2.5 × 10−5 M; (b) ZnPA1 2.5 × 10−5 M; (c) ZnPA2 1.5 × 10−5 M; (d) ZnPA4 2.5 × 10−5 M in CH2Cl2 containing 0.1 M TBAP at onset reduction
potentials.

62
K.Y. Chiu et al. / Electrochimica Acta 62 (2012) 51–62
the results of the conjugation between porphyrin ring and azoben-
zene moiety in the oxidation was used. Compared with the ꢁEox of
H2PAn (Table 3) and the ꢁEox of ZnPAn (Table 6), it was observed
that the ꢁEox of H2PAn decreased obviously with the increas-
ing number of azobenzene because of the stronger ␲-conjugation
between porphyrin ring and meso-azobenzenes (Fig. 7). When
the onset reduction potentials were applied, Soret band and
azobenzene peak (352 nm) decreased concurrently (Fig. 9). These
results revealed that the electron delocalization between por-
phyrin ring and meso-azobenzenes also could be obtained in the
reduction part. But the ꢁEox of ZnPAn has slightly decreased as
azobenzene number increased. It indicated that only a glimmer
of ␲-electron delocalized between meso-azobenzene and porpyrin
ring. The azobenzene peak (352 nm) was not significantly changed
at Eappl. = −1.20 V (Fig. 14). With the results, we could further con-
firm that the first reduction wave of ZnPA1 at Epc = −1.34 V was due
to the porphyrin ring reduction. When Eappl. was at onset reduc-
tion potential, the spectral change patterns of ZnPA2 and ZnPA4
were similar, the soret and Q band decreasing and azobenzene peak
having no change. According to the spectroelectrochemical results,
it is thus inferred that the first reduction sites of ZnPAn were at the
prophyrin ring.
References
[1] F. Li, S.I. Yang, Y. Ciringh, J. Seth, C.H. Martin III, D.L. Singh, D. Kim, R.R. Birge,
D.F. Bocian, D. Holten, J.S. Lindsey, J. Am. Chem. Soc. 120 (1998) 10001.
[2] D. Holten, D.F. Bocian, J.S. Lindsey, Acc. Chem. Res. 35 (2002) 57.
[3] B.M.J.M. Suijkerbuijk, R.J.M. Klein Gebbink, Angew. Chem. Int. Ed. 47 (2008)
7396.
[4] H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res. 42 (2009) 1809.
[5] S. Fukuzumi, K. Ohkubo, H. Imahori, J. Shao, Z. Ou, G. Zheng, Y.R. Chen, K. Pandey,
M. Fujitsuka, O. Ito, K.M. Kadish, J. Am. Chem. Soc. 123 (2001) 10676.
[6] A.N. Macpherson, P.A. Liddell, S. Lin, L. Noss, G.R. Seely, J.M. DeGraziano, A.L.
Moore, T.A. Moore, D. Gust, J. Am. Chem. Soc. 117 (1995) 7202.
[7] Y. Kashiwagi, H. Imahori, Y. Araki, O. Ito, K. Yamada, Y. Sakata, S. Fukuzumi, J.
Phys. Chem. A 107 (2003) 5515.
[8] J.K. Park, J. Chen, H.R. Lee, S.W. Park, H. Shinokubo, A. Osuka, D. Kim, J. Phys.
Chem. C 113 (2009) 21956.
[9] M. Autret, M. Le Plouzennec, C. Moinet, G. Simonneaux, J. Chem. Soc. Chem.
Commun. (1994) 1169.
[10] S. Tsuchiya, J. Am. Chem. Soc. 121 (1999) 48.
[11] T. Yamamura, A. Momotakeb, T. Arai, Tetrahedron Lett. 45 (2004) 9219.
[12] A. Aemissegger, V. Kräutler, W.F. van Gunsteren, D. Hilvert, J. Am. Chem. Soc.
127 (2005) 2929.
[13] I. Tomatsu, A. Hashidzume, A. Harada, J. Am. Chem. Soc. 128 (2006) 2226.
[14] C.A. Hunter, L.D. Sarson, Tetrahedron Lett. 37 (1996) 699.
[15] M.V. Peters, R. Goddard, S. Hecht, J. Org. Chem. 71 (2006) 7846.
[16] J.S. Lindsey, W. Wagner, J. Org. Chem. 54 (1989) 828.
[17] C.H. Lee, J.S. Lindsey, Tetrahedron 50 (1994) 11427.
[18] A.D. Adler, F.R. Longo, F. Kamas, J. Kim, J. Inorg. Nucl. Chem. 32 (1970)
2443.
[19] J. March, Advanced Organic Chemistry, 4th ed., Wiley, New York, 1992,
p. 1280.
[20] Y.C. Lin, K.L. Cheng, W.S. Tzeng, C.L. Su, L.J. Lee, L.Y. Wang, C.M. Yang, L.L. Lai,
Liq. Cryst. 33 (2006) 289.
5. Conclusions
This work displays several idiosyncratic features of absorption
spectra and electrochemistry for the azobenzene-containing por-
phyrins. (1) The Q-band intensity ratios (ε␣/εˇ) for ZnPAn slightly
increase with the increase number of meso-azobenzene. Upon addi-
tion of imidazole, the Q band ratio for (HIm)ZnPA4 is even larger
than 1. (2) The reduction of free base H2PAn to various charge
states would create a specific interaction with imidazole. (3) The
azobenzene moieties were reduced at a potential close to that of
the porphyrin ring, and it was observed that the current for the first
overlapping wave dramatically increase with the increase number
of meso-azobenzene. (4) The potential of the first oxidation wave of
H2PAn is slightly more positive but the second oxidation exhibits a
reverse shift as the number of meso-azobenzene increases. (5) The
first reduction wave of ZnPAn is at the zinc porphyrin ring reduction
and spectroelectrochemistry also reveal this results. Based on MO
analysis, points (1) and (4) have been rationalized. Further investi-
gations by replacing the azo group with imine or alkene links and
by variation of positions of the linkers will be undertaken in future
to see if those unusual features can also be observed.
[21] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheese-
man, J.A. Montgomery, T. Vreven Jr., K.N. Kudin, J.C. Burant, J.M. Millam, S.S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P.
Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Strat-
mann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich,
A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari,
J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Ste-
fanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith,
M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. John-
son, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, GAUSSIAN 03 Revision E. 01;,
Gaussian Inc., Wallingford, CT, 2004.
[22] R. Ma, P. Guo, H. Cui, X. Zhang, M.K. Nazeeruddin, M. Grätzel, J. Phys. Chem. A
113 (2009) 10119.
[23] Y. Nakamura, N. Aratani, H. Shinokubo, A. Takagi, T. Kawai, T. Matsumoto, Z.S.
Yoon, D.Y. Kim, T.K. Ahn, D. Kim, A. Muranaka, N. Kobayashi, A. Osuka, J. Am.
Chem. Soc. 128 (2006) 4119.
[24] R. Dennington II, T. Keith, J. Millam, K. Eppinnett, W.L. Hovell, R. Gilliland, Gauss
View, Version 4.1.2, Semichem, Inc., Shawnee Mission, KS, 2007.
[25] M. Nappa, J.S. Valentine, J. Am. Chem. Soc. 100 (1978) 5075.
[26] J.L. Retsek, C.J. Medforth, D.J. Nurco, S. Gentemann, V.S. Chirvony, K.M. Smith,
D. Holten, J. Phys. Chem. B 105 (2001) (2001) 6396.
[27] P.J. Spellane, M. Gouterman, A. Antipas, S. Kim, Y.C. Liu, Inorg. Chem. 19 (1980)
386.
[28] J. Bu, N.D. Lilienthal, J.E. Woods, C.E. Nohrden, K.T. Hoang, D. Truong, D.K. Smith,
J. Am. Chem. Soc. 127 (2005) 6423.
Acknowledgment
[29] F. Barrire, W.E. Geiger, J. Am. Chem. Soc. 128 (2006) 3980.
[30] C.L. Lin, M.Y. Fang, S.H. Cheng, J. Electroanal. Chem. 531 (2002) 155.
[31] J.C. Chang, C.J. Ma, G.H. Lee, S.M. Peng, C.Y. Yeh, Dalton Trans. (2005) 1504.
[32] C.W. Huang, K.Y. Chiu, S.H. Cheng, Dalton Trans. (2005) 2417.
[33] K.Y. Chiu, Y.O. Su, Chem. Commun. (2009) 2884.
This research is supported by the National Science Council of
the Republic of China (NSC 97-2113-M-260-005-MY3, NSC 99-
2811-M-260-006) and the National Center for High-performance
Computing for computer time and facilities.