Synthesis, electrochemical, and photophysical studies of hexadecachlorinatedphthalocyaninato zinc(II)

Article abstract of DOI:10.1016/j.dyepig.2011.03.028

Synthesis of a new symmetrical 1,4,8,11,15,18,22,25-octahexyloxy-2,3,9,10, 16,17,23,24-octa-(3,5-dichlorophenyl)phthalocyaninato zinc(II), ZnPc, has been described and characterized by ¹H NMR, ¹³C NMR, MS, UV-Vis, and IR spectrometry. The newly prepared ZnPc is soluble in organic solvents and is not aggregated in solution. The photophysical properties were studied by steady-state absorption and emission, cyclic voltammetry, and nanosecond transient absorption techniques. The prepared ZnPc absorbs and emits at longer wavelengths compared to that of reported phthalocyanine derivatives. The electron-donating properties of the ZnPc have been examined by mixing it with the electron-accepting dicyanoperylene-3,4,9,10-bis(dicarboximide), PDICN₂. The recorded nanosecond transient spectra in the visible/near-IR region showed clearly the electron-transfer from the triplet-excited state of the ZnPc to PDICN₂ with a rate of 3.40 × 10⁸ M^-1 s^-1. Light absorption in a wide section of the solar spectrum, favorable redox properties, and the electron-transfer properties suggest usefulness of the ZnPc in light-energy harvesting and developing optoelectronic devices.

Full text of DOI:10.1016/j.dyepig.2011.03.028

Dyes and Pigments 91 (2011) 231e236
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, electrochemical, and photophysical studies
of hexadecachlorinatedphthalocyaninato zinc(II)
,
a
a
a
a
Shaya Y. Al-Raqa a , Mouslim Messali , Saleem Al-Refae , Badr S. Ghanem , Ziad Moussa ,
*
Saleh Ahmed a, Mohamed E. El-Khouly b, Shunichi Fukuzumi b
,
**
a Department of Chemistry, Faculty of Science, Taibah University, P.O. Box 30002, Al-Madinah Al- Munawrah, Kingdom of Saudi Arabia
b Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of a new symmetrical 1,4,8,11,15,18,22,25-octahexyloxy-2,3,9,10,16,17,23,24-octa-(3,5-dichlor-
ophenyl)phthalocyaninato zinc(II), ZnPc, has been described and characterized by 1H NMR, 13C NMR, MS,
UVeVis, and IR spectrometry. The newly prepared ZnPc is soluble in organic solvents and is not
aggregated in solution. The photophysical properties were studied by steady-state absorption and
emission, cyclic voltammetry, and nanosecond transient absorption techniques. The prepared ZnPc
absorbs and emits at longer wavelengths compared to that of reported phthalocyanine derivatives. The
electron-donating properties of the ZnPc have been examined by mixing it with the electron-accepting
dicyanoperylene-3,4,9,10-bis(dicarboximide), PDICN2. The recorded nanosecond transient spectra in the
visible/near-IR region showed clearly the electron-transfer from the triplet-excited state of the ZnPc to
Received 21 January 2011
Received in revised form
18 March 2011
Accepted 22 March 2011
Available online 2 April 2011
Keywords:
Zinc phthalocyanine
Electron-transfer
Laser photolysis
PDICN2 with a rate of 3.40 ꢀ 108 Mꢁ1
s
ꢁ1. Light absorption in a wide section of the solar spectrum,
favorable redox properties, and the electron-transfer properties suggest usefulness of the ZnPc in light-
energy harvesting and developing optoelectronic devices.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
applications. However, the solubility of Pcs can be improved by
introducing different solubility-enhancing substituents such as
Phthalocyanines (Pc) have demonstrated several interesting
properties such as high chemical and thermal stability, rich redox
alkyl, alkoxy, phenoxy and macrocyclic groups and/or a central
metal [19]. These substituents enhance the solubility of phthalo-
cyanines and modify their aggregation in solution and other
physical properties [20]. Recently, we reported that zinc phthalo-
cyanine derivatives functionalized with substituents such as phe-
noxy and alkyloxy groups in the peripheral and non-peripheral
positions on the benzenoid rings were moderately soluble in
common organic solvents. The compounds are potential photo-
sensitizers [21,22] and increased interest has been shown in the
synthesis of hexadecasubstituted metal phthalocyanines [22e25].
As a continuation of our ongoing project, we report herein the
synthesis of hexadecasubstituted phthalocyaninato zinc(II) (ZnPc) by
the substitution of phthalocyanine with hexyloxy groups in non-
peripheral positions and chlorinated phenyl groups in peripheral
positions, ZnPc 5 (Scheme 1). The presence of bulky substituents in
non-peripheral positions leadsto a distorted Pc conformation [26,27]
with a concomitant modification of the Q-band [28,29]. In this paper,
we report the photophysical behaviour of ZnPc 5 and compare the
results with the planar zinc tetra-tert-butylphthalocyanine, ZnTBPc,
(Fig. S1 in the Supporting Information). The electron-transfer (ET)
process in a combination of ZnPc 5 with the electron-accepting
chemistry, delocalized 18
p-electron system suitable for electron-
transfer processes, ability to form complexes with various metal
ions, and semi-conducting properties when appropriately self-
assembled via pep stacking interactions [1,2]. These unique prop-
erties render them suitable for a number of industrial and medical
applications, namely, nonlinear optical devices, electroluminescent
displays, photovoltaic cells, low-dimensional conductors, chemical
sensors, photodynamic therapy, catalytic activity and biomimetic
model systems of the primary processes of natural photosynthesis
[3e10].
The properties of phthalocyanines have often been modulated
by means of peripheral substituents or axial coordination to the
metal center [11e18]. Non-substituted phthalocyanines are spar-
ingly soluble in most organic solvents, hence limiting their
* Corresponding author. Tel.: þ966507738708; fax: þ96648454770.
** Corresponding author.
E-mail addresses: sraqa@taibahu.edu.sa (S.Y. Al-Raqa), fukuzumi@chem.eng.
osaka-u.ac.jp (S. Fukuzumi).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.03.028

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S.Y. Al-Raqa et al. / Dyes and Pigments 91 (2011) 231e236
Scheme 1. Synthesis of ZnPc 5.
dicyanoperylene-3,4,9,10-bis(dicarboximide), PDICN2, has been
examined and compared with ZnTBPc/PDICN2 system to reveal the
effect of ring conformation of ZnPc on the rates of the forward
electron-transfer and back electron-transfer processes. The reason
for choosing such combination of ZnPc/PDICN2 is that: (1) the
combination of the ZnPc/PDICN2 affords broad absorption bands
which can cover about half the useful solar light spectrum, and (2)
the phthalocyanine/perylenediimide systems have unique
morphology in dispersed heterojunction devices; therefore, both
photocurrent generation and charge transport usually function well
[30e32] Although there have been several examples of phthalocy-
anine/perylenediimide ensembles in the literature [33e37], studies
of the bimolecular ET process of the phthalocyanine/per-
ylenediimide blends are rare [38] in the literature.
purification unless otherwise stated. IR spectra were recorded on
a Shimadzu Fourier Transform Infrared Spectrometer FTIR-400
using KBr disks. 1H NMR spectra were recorded on a Bruker
AVANCE 400 MHz and were obtained using the deuterated
solvents specified. Microanalyses were performed using a Vario
Elemental analyzer CHNS.
Electrochemical measurements were performed on an ALS630B
electrochemical analyzer in deaerated benzonitrile (PhCN) con-
taining tetra-n-butylammonium hexafluorophosphate (TBAPF6;
0.10 M) as supporting electrolyte at 298 K. A conventional three-
electrode cell was used with platinum working electrode (surface
area 0.3 mm2) and a platinum wire as counter electrode. The Pt
working electrode was polished after each run with BAS polishing
alumina suspension and rinsed with acetone before use. The
measured potentials were recorded versus Ag/AgNO3 reference
electrode. All electrochemical measurements were carried out
under an atmospheric pressure of argon.
2. Experimental
2.1. General
Fluorescence lifetimes were measured by a Photon Technology
International model GL-3300 nitrogen laser with a Photon Tech-
nology International model GL-302 dye laser, equipped with a four-
channel digital delay/pulse generator (Stanford Research System
Inc. DG535) and a motor driver (Photon Technology International
MD-5020).
Dicyanoperylene-3,4,9,10-bis(dicarboximide) were prepared
according to the literature [39]. Zinc tetra-tert-butylph-
thalocyanine (ZnTBPc, 99%) was purchased from Aldrich. All
solvents were of analytical grade and were used without further

S.Y. Al-Raqa et al. / Dyes and Pigments 91 (2011) 231e236
233
Steady-state absorption spectra were recorded on a Shimadzu
UVe3100 PC spectrometer or a Hewlett Packard 8453 diode array
spectrophotometer at room temperature. Fluorescence measure-
ments were carried out on a Shimadzu RF-5300PC spectro-
fluorophotometer. Phosphorescence spectra were obtained by an
(KBr): vmax cmꢁ1 806, 1176 (CeOeC), 1303, 1369, 1465, 1589 (C]C),
2233 (C^N), 1562, 2869, 2931, C32H32Cl4N2O2: C, 62.15; H, 5.22; N,
4.53%. Found: C, 62.1; H, 5.3; N, 4.53. 1H NMR (400 MHz, CDCl3):
0.84 (6H, t, 2CH3), 1.09e1.26 (12, m, eCH2CH2(CH2)3CH3), 1.5e1.55
(4H, quin, eCH2CH2(CH2)3CH3), 3.8 (4H, t, OCH2), 6.9 (4H, s, AreH),
7.31 (2H, t, AreH). 13C NMR (400 MHz, CDCl3): 13.97 (CH3), 22.46
(CH2), 25.26 (CH2), 29.73 (CH2), 31.30 (CH2), 76.35 (OCH2), 112.48
(C), 128.73 (2 ꢀ CH), 128.45 (CH), 132.24 (C), 134.99 (C), 135.63 (C),
139.57 (CeCl), 156.17 (CN) ppm.
SPEX fluorolog s3-spectrophotometer. For nanosecond transient
absorption measurements, deaerated solutions of the compounds
were excited by a Panther OPO equipped with a Nd:YAG laser
(Continuum, SLII-10, 4e6 ns fwhm) with a power of 10e15 mJ per
pulse. The photochemical reactions were monitored by continuous
exposure to a xenon lamp (150 W) as a probe light and a photo-
multiplier Hamamatsu 2949 tube as a detector. Solutions were
deoxygenated by argon purging for 15 min prior to the measure-
ments. For the 1O2 phosphorescence measurement, an O2-satu-
rated benzonitrile solution containing the ZnPc sample in a quartz
cell (optical path length 10 mm) was excited at 355 nm using
2.5. Synthesis of 1,4,8,11,15,18,22,25-octahexyloxy-
2,3,9,10,16,17,23,24-octa-(3,5-dichlorophenyl)phthalocyaninato
zinc(II), (ZnPc 5)
A mixture of 4, zinc acetate dihydrate (25 mg, 0.11 mmol) and
three drops of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in
1-pentanol (5 mL) was heated at reflux for 24 h with stirring under
nitrogen. The reaction mixture was cooled to room temperature
and the solvent was rotary evaporated under reduced pressure to
obtain a green deposit, which was dissolved in methanol (5 mL)
and the solution was filtered to remove the solid impurities. The
solvent was rotary evaporated under reduced pressure and the
remaining residue was purified by column chromatography (silica
gel, eluent: n-hexane/ethyl acetate 10:0.2 mL). Complex 5 was
precipitated from methanol as green powder. Yield (6.8 mg, 15%). IR
(KBr): vmax cmꢁ1 804, 1170 (CeCl), 2862, 2928 (CH2), 2951 (AreCH).
C128H130Cl16N8O8Zn: C, 60.5; H, 5.16; N, 4.41%. Found: C, 60.96; H,
5.69; N, 4.11. MALDI-MS: isotopic cluster at 2540 [Mþ]. 1H NMR
(400 MHz, CDCl3): 0.88 (24H, t, 8 eCH3), 1.12e1.29 (64H, m), 4.75
(16H, br, 8 OCH2), 7.34 (16H, br, AreH), 7.37 (8H, t, AreH). 13C NMR
(400 MHz, CDCl3): 14.2 (CH3), 22.3 (CH2), 26.2 (CH2), 30.20 (CH2),
32.0 (CH2), 77.1 (OCH2), 127.1 (2 CH), 130.1 (CH), 130.9 (C), 134.2 (C),
136.1 (C), 139.6 (C), 150.8 (C), 152.2 (C) ppm.
a
Cosmo System LVU-200S spectrometer. A photomultiplier
(Hamamatsu Photonics, R5509- 72) was used to detect emission in
the near infrared region (band path 1 mm).
2.2. Synthesis of 4,5-dibromo-3.6-dihydroxyphthalonitrile, (1)
To
a stirred solution of 3,6-dihydroxyphthalonitrile (5 g,
31.25 mmol) in tert-butanol (30 mL) at 45 ꢂC was added N-bro-
mosuccinimide, NBS (10.5 g, 64.5 mmol) portion-wise over 25 min.
Stirring was continued for 2 h and then more NBS (10.5 g,
64.5 mmol) was added over 25 min. After a further 2 h, the reaction
mixture was cooled to room temperature and poured into an
aqueous solution of sodium metabisulfite (10 g in 60 mL, excess) at
0 ꢂC with vigorous stirring. The mixture was stirred for 10 min and
the precipitate was filtered and washed with cold water (50 mL).
The product was dried under vacuum at 60 ꢂC for 24 h to yield
4,5-dibromo-3.6-dihydroxyphthalonitrile as off-white powder
(7.8 g, 79%), m.p. 248 ꢂC, IR (KBr): vmax cmꢁ1 2251, 3288 (br).
2.3. Synthesis of 4,5-dibromo-3.6-dihexyloxyphthalonitrile, (2)
3. Results and discussion
A mixture of 4,5-dibromo-3.6-dihydroxyphthalonitrile 1 (2.50 g,
7.86 mmol), triphenylphosphine (4.95 g, 18.9 mmol), and hexanol
(1.5 g, 14.7 mmol) was dissolved in dry tetrahydrofuran (80 mL) and
cooled to 0 ꢂC. A solution of diisopropyl azodicarboxylate (4.35 g,
21.5 mmol) in tetrahydrofuran (30 mL) was added drop-wise over
30 min. The solution was slowly warmed to room temperature and
stirred for an additional 10 h. The solvent was then removed under
reduced pressure to give a dark oil, which was dissolved in diethyl
ether (20 mL). The solution was filtered to remove undissolved
triphenylphosphine oxide. The solvent was evaporated and the
remaining residue was purified by column chromatography (silica
gel, eluent: n-hexane/tetrahydrofuran 10:1). The product was
recrystallized from cyclohexane to obtain 4,5-dibromo-3.6-dihex-
yloxyphthalonitrile. Yield (2.72 g, 76%) as white powder. m.p.
39e41 ꢂC. C20H26Br2N2O2: C, 49.4; H, 5.39; N, 5.76%. Found: C, 50;
H, 5.56; N, 5.95. 1H NMR (400 MHz, CDCl3): 0.91 (6H, t), 1.33e1.38
(8H, m), 1.48e1.56 (4H, m), 1.86e1.93 (4H, m), 4.2 (4H, t) ppm. 13C
NMR (400 MHz, CDCl3): 13.98, 22.5, 25.29, 29.92, 31.43, 109.21,
112.36, 129.6, 156.39 ppm.
3.1. Synthesis
Compound 4,5-dibromo-3.6-dihydroxyphthalonitrile
1 was
synthesized according to published procedure [40]. There are two
bromine substituents and two phenolic groups that can be used to
introduce two different substituents at the 3.6- and 4,5-positions of
the phthalonitrile. The Mitsunobu reaction was utilized to rapidly
alkylate the two alcohol groups at low temperature to obtain
4,5-bromo-3,6-dihexyloxyphthalonitrile, 2. This reaction is used to
dehydrate alcohols and phenols using triphenylphosphine and
diisopropyl azodicarboxylate (DIAD) [41]. Subsequently, Suzuki
reaction was applied to introduce the two substituted phenyl
groups at the 4- and 5-positions in the phthalonitrile derivative.
Compound 4 was prepared by the reaction of 4,5-dibromo-
3,6-dihexyloxyphthalonitrile 2 with 3,5-dichlorodphenylboronic
acid 3, Scheme 1. Compound 4 was purified chromatographically to
I
give 37% yield. The H NMR spectrum showed all the substituents
and ring protons in their respective regions. For compound 4 the
aromatic region displayed one triplet of triplet at 7.31 ppm and one
doublet peak at 6.9 ppm which can be attributed to the six aromatic
protons for each compound. IR spectra clearly indicated the
formation of compound 4 with the appearance of nitrile stretching
2.4. Synthesis of 4,5-bis(3.5-dichlorophenyl)-3,6-bis(hexyloxy)
phthalonitrile, (4)
frequency at 2233 cmꢁ1
.
4,5-bis(3.5-dichlorophenyl)-3,6-bis(hexyloxy)phthalonitrile
4
The target ZnPc 5 presented in Scheme 1 was synthesized by
cyclotetramerization of 4 following the phthalocyanine synthetic
procedure reported previously [22]. The reaction of phthalonitrile 4
with zinc acetate dihydrate in DBU under reflux for 24 h afforded 5
in 15% yield.
was prepared from 3,5-dichlorophenylboronic acid 3 (260 mg,
1.64 mmol). Purification of the product was achieved by column
chromatography (silica gel, eluent: petroleum ether/ethyl acetate
100:2.5 v/v). 37% yield (101 mg) as white powder. m.p. 190 ꢂC. IR

234
S.Y. Al-Raqa et al. / Dyes and Pigments 91 (2011) 231e236
The structure of ZnPc 5 was elucidated by spectroscopic and
analytical tools. For example, the IR spectrum showed the disap-
pearance of the sharp C^N stretching frequency at 2233 cmꢁ1
,
consistent with the cyclotetramerization of 4. 1H NMR spectrum of
5 showed one triplet signal at 7.37 ppm attributed to the 8 CH-
aromatic protons, and one broad signal at 7.34 ppm attributed to
another 16 aromatic protons. The triplet at 0.88 ppm integrated for
24 protons assigned to the terminal methyl groups. The 16 meth-
yleneoxy proton resonances at 4.75 ppm appear as broad signal.
Further elucidation of 5 was proved by mass spectrometry, which
showed a molecular ion-peak at 2540 m/e.
3.2. Steady-state absorption and emission studies
As shown in Fig. 1, the toluene solution of ZnPc shows absorp-
tion around 361 nm attributed to the phthalocyanine Soret band,
whereas the strong absorption at 748 nm with two vibronic
shoulders at 668 and 713 nm was attributed to phthalocyanine
Fig. 2. Transient absorption spectra of ZnPc (0.1 mM) by nanosecond-laser excitation
in deaerated benzonitrile; lex ¼ 430 nm. Inset: Time profile at 630 nm.
Q-bands that arise from
around 410 nm is common in alkoxy-substituted phthlalocyanines,
which is attributed to ne * transitions [42,43]. On the other hand,
pep* transitions. The weak absorption
p
3.3. Photoinduced electron-transfer reaction of ZnPc 5 with PDICN2
in benzonitrile
the absorption spectrum of the extensively studied ZnTBPc
exhibited absorptions of the Soret and Q-bands at 347 and 677 nm,
respectively. From this comparison, the Q-band of the examined
ZnPc 5 exhibit red shift by approximately 82 nm compared to that
of the reference ZnTBPc. The red shift of the absorption ZnPc 5 can
be rationalized by the higher conjugation of ZnPc 5. It is worth
mentioning that the observed shift is consistent with the trend
Upon photoexcitation of ZnPc in deaerated benzonitrile, the
nanosecond transient spectrum of ZnPc 5 at 3
ms exhibited
absorption in the range of 400e850 nm with a maximum at 630 nm
(Fig. 2), which corresponds to the triplet-excited state of ZnPc
(3ZnPc*). The 3ZnPc* decayed within 200
ms with a rate constant
detected upon distortion from planarity of the 18
p-electron
1.7 ꢀ 104 sꢁ1, from which the lifetime of the 3ZnPc* was found be as
system of porphyrins [44], taking into account that the presence of
bulky substituents in non-peripheral positions leads to a distorted
Pc conformation [26,27] with a concomitant modification of the
Q-band [28,29]. It was found that addition of ferric perchlorate to
a benzonitrile solution containing ZnPc 5 resulted in a remarkable
spectral change in the range of 800e1000 nm. The radical cation of
ZnPc (ZnPcꢃþ) was clearly observed at 810 nm, which is blue-shifted
by approximately 40 nm compared to that of the ZnTBPc radical
cation (ZnTBPcꢃþ) (Fig. S3 in the Supporting Information).
Steady-state fluorescence spectroscopy was used to probe the
excited state properties. Upon 640 nm light excitation, the fluo-
rescence emission spectrum of ZnPc exhibited a band at 753 nm
(Fig. 1, right) which was red-shifted by approximately 59 nm
compared with ZnTBPc. The fluorescence lifetime of the single-
t-excited state of ZnPc exhibited mono-exponential decay with
a lifetime w2.50 ns (Fig. S4), which is slightly shorter compared
with ZnTBPc [45].
59 m
s. The absorption band of the 3ZnPc* exhibited a red shift by
approximately 140 nm compared to that of the triplet-excited state
of ZnTBPc [24].
To examine the electron-donating effect of ZnPc, the nano-
second transient spectra were recorded for a mixture of ZnPc with
the electron-acceptor PDICN2 in benzonitrile (Fig. 3 and Figs. S5 and
S6). Photoexcitation of ZnPc (0.05 mM) in the presence of PDICN2
(0.00e0.08 mM) by a 430 nm laser source resulted in the charac-
teristic absorption band of the triplet-excited of ZnPc (3ZnPc*) at
630 nm. With the decay of the triplet-excited state of ZnPc, there
was a concomitant rise of the characteristic absorption band of the
PDICN2 radical anion (PDICN2 e) in the visible-near-IR region with
ꢃ
maxima 694, 773, and 935 nm [46,47]. The absorption band of
ꢃe
PDICN2 was confirmed by treating PDICN2 with tetrakis(dime-
thylamino)ethylene (TDAE) in benzonitrile (Fig. S7 in the
Supporting Information). In addition, the absorption band of
Fig. 1. Steady-state absorption spectra of ZnPc 5 and ZnTBPc in benzonitrile. (Right) Steady-state fluorescence spectra of ZnPc 5 and ZnTBPc in benzonitrile; lex ¼ 640 nm.

S.Y. Al-Raqa et al. / Dyes and Pigments 91 (2011) 231e236
235
Fig. 3. (Left) Nanosecond transient spectra of ZnPc (0.05 mM) in the presence of PDICN2 (0.08 mM) in deaerated benzonitrile, lex ¼ 430 nm. Inset: Rise-profile of PDCN2 e at 940 nm
(Right) Dependence of the decay-rates of the 3ZnPc* at 630 nm on concentration of PDICN2 in benzonitrile. Inset: Pseudo first-order plot.
ꢃ
the ZnPc radical cation (ZnPcꢃþ) was clearly observed at 810 nm.
the diffusion-control limit (kdiff) in benzonitrile [50]. The smaller ket
value of ZnPc/PDICN2 system is actually due to the steric hindrance
of the electron donors in ZnPc.
The decay of ZnPc* and the rise of ZnPcꢃþ/PDICN2 e seem to match,
which provides evidence for electron-transfer via 3ZnPc*. This was
supported by bubbling oxygen into the solutions where an inter-
molecular energy-transfer from the 3ZnPc* to oxygen emerged,
suppressing the electron-transfer process. These processes are
summarized in Scheme 2. The electron-transfer from the triplet-
excited stat of ZnPc to PDICN2 in polar benzonitrile solvent can be
rationalized as follows: polar solvent molecules rapidly surround
the radical ions once formed and consequently prevent the electron
return. The escape from the solvent cage is indeed a key feature of
electron-transfer in solution [48,49].
3
ꢃ
The electron-transfer process via the triplet-excited state of
ZnPc was supported from the viewpoint of thermodynamics of
electron-transfer process. The electrochemical measurements were
carried out using CV and DPV techniques at a glassy carbon elec-
trode in a solution of carefully dried benzonitrile containing 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6; 0.1 M) at
room temperature (Fig. 4). The measurements show the oxidation
potentials (Eox) of the ZnPc at 0.41 and 0.81 V vs Ag/AgNO3, which
comparable to that of ZnTBPc. On the other hand, the reduction
potentials (Ered) of PDICN2 were located at ꢁ0.58 and ꢁ0.98 V vs Ag/
AgNO3 (Fig. S8 in the Supporting Information). Based on the first Eox
of ZnPc, the first Eox of PDICN2, and the triplet energy of ZnPc, the
More details on the kinetics of the electron-transfer process are
shown in Fig. 3, where the rate constant of the electron-transfer
process (ket) was evaluated by monitoring the decay of 3ZnPc* and
-
ꢃ
the rise of PDICN2 as a function of PDICN2 concentration. The
decay profiles of 3ZnPc* obeyed first-order kinetics; each rate
constant is referred to (k1st). The linear concentration-dependence
of the observed k1st values gives the ket value, calculated as
free-energy change of the electron-transfer process ðꢁ
D
GTetÞ via the
triplet-excited state of ZnPc was estimated as 0.30 eV, corre-
sponding to 28.09 kJ molꢁ1 [51,52]. The negative ꢁ
D
GTet value via
the triplet-excited state of ZnPc suggests that the quenching
3.40 ꢀ 108 Mꢁ1
s
ꢁ1. This ket value is much smaller compared with
process is close to the diffusion-controlled limit (kdiff).
ꢃe
In the long time-scale (2 ms), PDICN2 radical anion begins to
decay slowly after reaching the maximal absorbance (Fig. 5). The
decay time profile was fitted with second-order kinetics, suggesting
that bimolecular backward electron-transfer process between
þ
ꢃe
ZnPcꢃ and PDICN2 takes place. The rate constant of backward
electron-transfer (kbet) between ZnPcꢃþ and PDICN2 e was estimated
ꢃ
at 1.03 ꢀ 109 Mꢁ1 sꢁ1 from the slope of the second-order plot in the
ꢃe
form of kbet/3, where 3 is the molar extinction coefficient of PDICN2
þ
ꢃe
(3
¼ 1.81 ꢀ 104) [50]. The obtained kbet for ZnPcꢃ and PDICN2 is
smaller than the diffusion-control limit in benzonitrile [50].
In the case of the ZnTBPc/PDICN2 mixture system, the electron-
transfer process from the triplet-excited state of ZnTBPc to the
PDICN2 was confirmed by recording the absorption bands of the
radical ions (Figs. S9eS12 in Supporting Information). The ket and
Scheme 2. Electron-transfer process of ZnPc/PDICN2 system in benzonitrile.
Fig. 4. CV (left) and DPV (right) of ZnPc 5 in deaerated benzonitrile. Sweep rate: 50 mV sꢁ1
.

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e
Fig. 5. Decay of ZnPcꢃþ/PDICN2 on long time-scale produced under the same
ꢃ
conditions as described in Fig. 3. Inset: Second-order plot.
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kbet values were found to be 1.26
ꢀ
109 Mꢁ1 sꢁ1 and
1.71 ꢀ109 Mꢁ1 sꢁ1, respectively. The finding that the value of kbet of
ZnPc/PDICN2 is smaller than that of ZnTBPc/PDICN2 reflects the
effect of the bulky substituents of ZnPc in slowing the rate of ET via
the triplet-excited state. The steric effect probably decreases the
solvation of the radical ion in the pair because of shielding of the
polar solvent molecules, thus decreasing the kbet value of ZnPc/
PDICN2 system.
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4. Conclusion
The present work describes the synthesis and characterization
of a new symmetrical 1,4,8,11,15,18,22,25-octahexyloxy-2,3,9,10,16,
17,23,24-octa-(3,5-dichlorophenyl)phthalocyaninato zinc(II). The
synthesized complex possessed high solubility in various organic
solvents such as CH2Cl2, THF, acetone and ethyl acetate. The
presumably distorted ZnPc absorbed and emitted at longer wave-
lengths compared with the planar ZnTBPc. The bimolecular elec-
tron-transfer in ZnPc/PDICN2 system was confirmed by observing
þ
ꢃe
the transient absorption spectra of ZnPcꢃ and PDICN2 in the
visible and near-IR region. The observed kbet of the ZnPcꢃþ/PDICN2þ
ꢃe
was found to be considerably smaller than that of ZnTBPcꢃ
/
ꢃe
PDICN2 reflecting the effect of the bulky groups in slowing the
electron-transfer in ZnPc/PDICN2 system. The absorption in a wide
section of the solar spectrum, favorable redox properties, and the
electron-transfer properties suggest the usefulness of the ZnPc in
light energy-harvesting and developing optoelectronic devices.
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The author gratefully acknowledges the financial support from
Taibah University for the Faculty Research Grant (429/50). This work
was supported by a Global COE program, "the Global Education and
Research Centerfor Bio-EnvironmentalChemistry" fromtheMinistry
of Education, Culture, Sports, Science and Technology, Japan.
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Appendix. Supplementary data
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Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.dyepig.2011.03.028.
[52] The feasibility of the electron-transfer process from the triplet excited ZnPc to
the PDICN2 is controlled by the free-energy change ꢁ
GTet, which can be
References
D
expressed by the RehmeWeller relation:
D
GTet ¼ (Eox ꢁ Ered) ꢁ ET þ Ec; where
Eox is the first oxidation potential of the ZnPc, Ered is the first reduction
potential of PDICN2, ET is the triplet energy of ZnPc, and Ec is the Coulomb
energy term (approximately 0.06 eV in the polar benzonitrile).
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