Photoresponsive J-aggregation behavior of a novel azobenzene-phthalocyanine dyad and its third-order optical nonlinearity

Article abstract of DOI:10.1021/jp710461p

The photoresponsive J-aggregation behaviors of a novel azobenzene- substituted zinc phthalocyanine (azo-ZnPc dyad) were studied by UV/vis, fluorescence, and ¹H NMR spectroscopy. Upon illumination with 365 nm UV light, the trans-cis isomerization of azobenzene can efficiently reduce the steric hindrance around the peripheral oxygen atom of azo-ZnPc, shortening the possible distance between two phthalocyanine molecules and, consequently, greatly improving the tendency of J-aggregation of azo-ZnPc dyad. The third-order optical nonlinearities of the photoresponsive J-aggregates (before and after illumination) were measured by a Z-scan technique at 532 nm with a pulse duration of 25 ps. The Z-scan spectra revealed that all the samples possessed large positive nonlinear refraction and positive nonlinear absorption, exhibiting a self-focusing effect and reverse saturable absorption, respectively. The second molecular hyperpolarizabilities of the dyad in two conditions were measured to be 3.87 ?? 10^-30 and 4.82 ?? 10^-30 esu, respectively. AU the results suggest that the azo-ZnPc dyad has potential in the field of nonlinear optics applications. ? 2008 American Chemical Society.

Full text of DOI:10.1021/jp710461p

J. Phys. Chem. B 2008, 112, 7387–7394
7387
Photoresponsive J-Aggregation Behavior of a Novel Azobenzene-Phthalocyanine Dyad and
Its Third-Order Optical Nonlinearity
Zihui Chen,† Cheng Zhong,† Zhi Zhang,† Zhongyu Li,*,†,‡ Lihong Niu,† Yuejing Bin,† and
Fushi Zhang*,†
The Key Lab of Organic Photoelectrons & Molecular Engineering of Ministry of Education, Department of
Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, and Department of Chemical
Engineering, Jilin Institute of Chemical Technology, Jilin 132022, People’s Republic of China
ReceiVed: October 30, 2007; ReVised Manuscript ReceiVed: March 2, 2008
The photoresponsive J-aggregation behaviors of a novel azobenzene-substituted zinc phthalocyanine (azo-
1
ZnPc dyad) were studied by UV/vis, fluorescence, and H NMR spectroscopy. Upon illumination with 365
nm UV light, the trans-cis isomerization of azobenzene can efficiently reduce the steric hindrance around
the peripheral oxygen atom of azo-ZnPc, shortening the possible distance between two phthalocyanine molecules
and, consequently, greatly improving the tendency of J-aggregation of azo-ZnPc dyad. The third-order optical
nonlinearities of the photoresponsive J-aggregates (before and after illumination) were measured by a Z-scan
technique at 532 nm with a pulse duration of 25 ps. The Z-scan spectra revealed that all the samples possessed
large positive nonlinear refraction and positive nonlinear absorption, exhibiting a self-focusing effect and
reverse saturable absorption, respectively. The second molecular hyperpolarizabilities of the dyad in two
conditions were measured to be 3.87 × 10-30 and 4.82 × 10-30 esu, respectively. All the results suggest that
the azo-ZnPc dyad has potential in the field of nonlinear optics applications.
Introduction
tions. Sessler et al.19 have reported that cytidine-phthalocyanina-
tozinc undergoes self-assembly to form a J-aggregate even at
low concentrations in dichloromethane-toluene mixed solvents
as a result of both π-π interactions and cytidine macrocycle
interactions. Recently, Kobuke and co-workers20 have reported
a methodology for supramolecular organization of imidazolyl-
appended phthalocyanines. The complementary coordination of
imidazolyl group from one phthalocyanine molecule to the
central Zn2+ or Mg2+ ion in a second phthalocyanine molecule
causes a “slipped cofacial dimer”, with extremely large stability
constants of 1011-1012 M-1. Quite recently, Watarai and co-
workers21 have observed that two-dimensional high-ordered
J-aggregates of thioether-substituted phthalocyanine and sub-
phthalocyanine derivatives were formed at the toluene/water
interface upon addition of Pd(II) into the system. Only a year
ago, our group has provided another facile strategy to construct
phthalocyanine J-aggregates based on Zn2+-O coordination.22
According to the J-aggregates formation mechanism of R-aryl/
alkoxy-substituted zinc phthalocyanines,22 the oxygen-linked
substitution in phthalocyanine rings would affect the formation
of J-aggregates. The size of substitution determines the distance
between two phthalocyanines due to steric hindrance. At the
same time, the electronic properties of substitution will affect
the polarity of adjacent oxygen atom. Therefore, peripheral
substituents could influence the interaction between Zn-O
atoms, which could also influence the optical nonlinearity of
phthalocyanine.
J-aggregates of dye molecules are attractive from a spectro-
scopic viewpoint, because the systems are expected to show
the crossover between the macroscopic properties of bulk
materials and microscopic ones of isolated molecules. One of
the linear spectroscopic properties of J-aggregates is a sharp
excitonic absorption peak called the J-band, red-shifted from
the monomer band.1,2 The experimental and theoretical works
have shown that molecular J-aggregates are self-organized,
quasi-one-dimensional systems of cyanine dyes that have large
third-order susceptibility in the J-band as well as fast response
times.3–6 Enhancement of the nonlinear optical properties is the
result of the exciton delocalization over the aggregates.7
Over the past two decades, phthalocyanines have been
extensively studied as an important class of third-order nonlinear
optical materials because of their extensively delocalized two-
dimensional 18-electron system, their structural flexibility, their
exceptionally high thermal and chemical stability, and their
potential for use in photonic applications such as optical switches
and optical limiting devices.8–13 Owing to the extended π system,
it is well-known that these macrocyclic compounds exhibit a
high aggregation tendency, forming dimeric and oligomeric
species in solutions.14–16 It has been shown that this molecular
association greatly influences the intrinsic nature of macrocycles
including their spectroscopic, photophysical, electrochemical,
and nonlinear optical properties.17 Therefore, much effort has
been put into assembling J-aggregates. For example, Li and Ng18
have demonstrated that phthalocyanine-nucleobase conjugates
such as phthalocyanine-adenine derivatives in dichloromethane
can form J-aggregates due to a variety of intermolecular interac-
It is well-known that photochromic compounds undergo
reversible isomerization under the illumination of suitable lights,
resulting in photoinduced changes in their molecular geometry
and electronic delocalization distributions.23 The incorporation
of photochromic compounds and other functional materials has
led to numerous light-responsive systems such as nano/molecular
machines,24 photocontrolled delivery systems,25 etc. Among
various photochromic compounds, azobenzene, which featured
* Corresponding authors: tel +86 10 62782596; fax +86 10 62770304;
e-mail zhongyuli@mail.tsinghua.edu.cn (Z.L.), zhangfs@mail.tsinghua.edu.cn
(F.Z.).
† Tsinghua University.
‡ Jilin Institute of Chemical Technology.
10.1021/jp710461p CCC: $40.75
2008 American Chemical Society
Published on Web 05/31/2008

7388 J. Phys. Chem. B, Vol. 112, No. 25, 2008
Chen et al.
SCHEME 1: Synthetic Route to Azo-ZnPc
its photoinduced trans-cis isomerization, has been most widely
used because of the large photoinduced changes in its geometry
and physical properties.26
characterized by NMR, IR, UV-vis, MS, and elemental
analysis.
Synthesis of 3-(Phenylazophenoxy)phthalonitrile (1). 4-Phe-
nylazophenol (1.00 g) and 3-nitrophthalonitrile (0.82 g) were
dissolved in N,N-dimethylformamide (DMF; 10 mL). After the
mixture was stirred for 15 min at room temperature, finely
grounded dry potassium carbonate powder (1.4 g) was added.
The mixture was kept stirring for 24 h at room temperature
and poured into 60 mL of ice-water. After precipitation was
complete, the solid was filtered off and washed with water.
Recrystallization from EtOH gave 1.21 g of needlelike yellow
crystals with a yield of 80%. mp 161-162 °C; MS (M + Na+)
347.2; 1H NMR (300 MHz, DMSO-d6; ppm) 7.37-7.43 (6, 2H),
7.45-7.49 (q, 1H), 7.54-7.62 (m, 3H), 7.82-7.92 (m, 4H),
7.96-8.02 (6, 2H); 13C NMR (300 MHz, DMSO-d6; ppm)
106.7, 113.7, 116.1, 116.7, 120.9, 123.1, 123.9, 125.4, 129.6,
130.0, 132.2, 136.7, 149.7, 152.4, 157.3, 159.4. Elemental
analysis calcd for C20H12N4O: C, 74.06; H, 3.73; N, 17.27.
Found: C 74.06, H 3.63, N 17.02. IR (cm-1) 3087, 2230, 1584,
1572, 1493, 1450, 1095, 930, 801, 727.
In the present study, a novel R-aryloxy-substituted zinc
phthalocyanine (azobenzene-substituted zinc phthalocyanine,
hereafter, abbreviated as azo-ZnPc dyad) was prepared. This
molecule was expected to form J-aggregates through Zn-O
coordination. Four azobenzene units were introduced as pe-
ripheral substituents to examine whether the trans-cis isomer-
ization of azobenzene units could exert apparent influences upon
the J-aggregation behaviors and optical properties of this
1
material. UV/vis, fluorescence, and H NMR spectroscopies
were applied to demonstrate that azo-ZnPc formed photore-
sponsive J-aggregates in noncoordinating solvents. Third-order
optical nonlinearities of the photoresponsive J-aggregates of the
azo-ZnPc dyad were measured by a picosecond Z-scan technique.
Experimental Section
Materials and Devices. 4-Phenylazophenol was purchased
from Alfa Aesar and used without further purification. The other
reagents used were commercial products from Beijing Chemical
Reagent Co. Solvents for spectroscopy were refined by standard
methods, while others were used as received.
Synthesis of Azo-ZnPc. A mixture of 3-(phenylazophenox-
y)phthalonitrile (1.00 g), zinc acetate (0.12 g), and 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU; 0.15 g) in 2-ethoxyeth-
anol (30 mL) was refluxed for 2 h under argon flux. The
resulting green suspension was cooled to ambient temperature
and poured into 100 mL of ice-water. The precipitate was
filtered off. Finally, pure azo-ZnPc (52 mg) was obtained by
chromatography on silica gel with commercially available
chloroform as the eluent. MALDI-TOF: calcd 1362.7, found
Absorption spectra were measured on a HP8452A spectro-
photometer. IR spectra were recorded on a Nicolet Avatar 360
FT-IR spectrometer made in America. NMR spectra were
recorded on a JEOL JNM-ECA300 spectrometer with tetram-
ethylsilane (TMS) as internal reference. Mass spectrometry was
obtained on a Bruker LC-MS/MS (Esquire-LC) 1100 series;
TOF-MS spectra were measured with BEFLEX III. A Carlo-
Erba-1106 element analyzer was used for elemental analyses.
Fluorescence spectra were recorded on a Hitachi F-4500
fluorescence spectrophotometer. Melting points were determined
with a XT4A microscopic melting point apparatus produced by
Beijing Keyi photoelectronic corporation and were used without
further calibration. Photoillumination was carried out with a
high-pressure mercury lamp (mejiro precision, SHG-200; 1000
W, made in Japan) with suitable filters. Transmission electron
microscopy (TEM) was performed on a model H-800 electron
microscope operating at acceleration voltage of 100 kV. To
prepare TEM samples for the images of the aggregation, a drop
of the 8 × 10-6 M chloroform solution of azo-ZnPc before or
after UV light illumination was deposited onto a microgrid,
which had been coated with carbon. The excess solution was
blotted away with a strip of filter paper.
1
1362.5. H NMR (300 MHz, DMSO-d6; ppm) 7.2-7.6 (m,
23H), 7.6-8.2 (m, 22H), 8.4-8.5 (t, 1H), 8.5-9.0 (q, 1H, J3
) 0.71, J5 ) 0.025), 9.1-9.2 (q, 1H). Elemental analysis calcd
for C80H48N16O4Zn: C, 70.51; H, 3.55; N, 16.45. Found: C,
70.21; H, 3.36; N, 16.56. IR (cm-1) 3060, 1577, 1480, 1413,
1278, 1115, 930, 835, 742, 685.
Third-Order Optical Nonlinearity Measurement. Third-
order optical nonlinearity of azophthalocyanine was determined
by standard Z-scan technique.27 This technique, known for its
simplicity and sensitivity, relies on the distortions induced in
the spatial and temporal profile of the input beam on passing
through the sample. It is used widely in material characterization
because it provides not only the magnitudes but also the sign
of real and imaginary parts of third-order nonlinear susceptibility
(ꢀ(3)). For the present study, a Nd:YAG laser (model PL2143B,
EKSPLA) with a 25 ps pulse width at 532 nm was used as the
light source. The laser beam (TEM0,0) was focused onto the
sample with a 150 mm focal length lens, leading to a measured
Azo-ZnPc dyad was synthesized through a rather facile route
as shown in Scheme 1. All products were unambiguously

Novel Azobenzene-Phthalocyanine Dyad
J. Phys. Chem. B, Vol. 112, No. 25, 2008 7389
beam waist of 25 µm and the pulse energy of 3.0 µJ at the
focus. The on-axis transmitted beam energy, the reference beam
energy, and their ratios were measured with an energy ratiometer
(Rm Laser 6600 Probe Corp.) simultaneously. In order to reduce
the possible thermal accumulation effect, the laser repetition
rate was set to 1 Hz. For open aperture, all the transmitted power
was collected and focused onto the detector by use of another
lens (120 mm). The Z-scan measurements were performed with
samples dissolved in chloroform with a concentration of 2.8 ×
10-5 M and placed in a standard 1-mm quartz cuvette.
Results and Discussion
C4h Symmetry Configuration of Azo-ZnPc. Scheme 1
shows the synthetic route to azo-ZnPc. Nucleophilic substitution
reaction between 4-phenylazophenol and 3-nitrophthalonitrile
formed the 3-(phenylazophenoxy)phthalonitrile (1) in 80% yield;
subsequently, 1 reacted with anhydrous zinc acetate through a
template-oriented reaction in 2-ethoxyethanol to produce target
compound azo-ZnPc. It is worth noting that the phthalocyanine
product is obtained as a mixture of four regioisomers of C4h,
D2h, C2V, and Cs symmetry with similar physical and chemical
properties, and only the C4h symmetrical structure is shown in
Scheme 1. While it is theoretically possible to separate these
isomers due to their different geometries, it has been ac-
complished only for very specific phthalocyanines by use of
specially designed HPLC columns.28 Often, even in these cases,
the best results probably give only an enriched isomeric fraction.
Previous studies revealed that with appropriately large substit-
uents or under mild reaction conditions, the 1,8,15,22-isomer
with C4h symmetry is selectively29 or even exclusively28,30
produced in preference to the statistical mixture of isomers. Now
that the azo-ZnPc possesses large substituents, the C4h sym-
metrical product shown in Scheme 1 may be the dominant
isomer.
Figure 1. Absorption spectral changes of (a) azo-ZnPc and (b)
compound 1 in chloroform under the irradiation of 365 nm UV light
(c ) 1.19 × 10-5 M).
UV-Vis and Fluorescence Spectra. UV-Vis spectra of
compound 1 and azo-ZnPc before and after UV light illumina-
tion in chloroform are illustrated in Figure 1. Typical UV-vis
spectra of phthalocyanines feature two distinct absorption bands,
namely B-band and Q-band, that can be readily interpreted by
use of Gouterman’s highly simplified four orbital model. For
azo-ZnPc, the B-band overlaps with the π-π* transition
absorption band of azobenzene units at 300-400 nm. However,
in addition to the normal Q-band (698 nm), an unusual red-
shifted sharp band at 740 nm was observed. Upon illumination
with 365 nm UV light, azobenzene units underwent photoi-
somerization from trans to cis isomers. The decreased and blue-
shifted absorption peak at 340 nm, and the increased absorption
peak at 400-500 nm, which is similar to spectral changes of
compound 1 under UV illumination, are persuasive evidence
of such photoisomerization. Accompanying such photoreaction,
the absorption band at 698 nm decreased sharply and slightly
red-shifted to 710 nm. Meanwhile, the absorption band at 740
nm increased from 0.2 to 0.6 after illumination for 2 min, which
is a 3-fold increase.
Figure 2. Absorption spectral change of azo-ZnPc in THF (c ) 1.19
× 10-5 M) under 365 nm UV light irradiation.
in chloroform to UV reduction. To test the latter assumption,
the spectra of azo-ZnPc in other solvents were investigated.
Figure 2 shows the UV-vis spectra of azo-ZnPc in tetrahy-
drofuran (THF). In contrast to Figure 1 (azo-ZnPc in chloro-
form), azo-ZnPc in THF showed only one typical Q-absorption
band at 690 nm. The longer wavelength absorption band at 740
nm was not shown any more. Upon illumination with UV light,
azobenzene moieties could also isomerized to cis from the trans
conformation. However, unlike in chloroform, the isomerization
of azobenzene shows negligible influence upon the Q-band of
azo-ZnPc. Provided the longer wavelength absorption in Figure
1 arises from the excitonic interaction between the Pc and
azobenzene units, why was this phenomenon not observed in
THF? Extensive investigations of the UV-vis spectra of azo-
ZnPc in various solvents showed that in coordinating solvents
There is the possibility that the longer wavelength band could
be from either photodecomposed species or a split component
of the Q-band by excitonic interaction between Pc and azoben-
zene chromophores. Slota and Dyrda31 had investigated in detail
the photostability of various metal phthalocyanines (MPc) in
organic solvents. They found that, regardless of the solvents,
MPc (M ) Li, Mg, Fe, Zn, and Pb) decomposed only under
prolonged illumination of UV light without any change of λmax
.
Azo-ZnPc was proved extremely stable under UV light. Thus
it is unreasonable to attribute the spectral changes of azo-ZnPc

7390 J. Phys. Chem. B, Vol. 112, No. 25, 2008
Chen et al.
Figure 3. Fluorescence spectral changes of azo-ZnPc in chloroform
(c ) 1.19 × 10-5 M) upon irradiation with 365 nm UV light.
such as dimethyl sulfoxide (DMSO), ketone, pyridine, etc., azo-
ZnPc behaved as it did in THF; while in noncoordinating
solvents such as toluene, dichloromethane, etc., the spectral
changes of azo-ZnPc were similar to those in chloroform. This
means that the UV-vis spectral properties of azo-ZnPc are
solvent-dependent.
J-Aggregation Behavior of Azo-ZnPc Dyad. Huang et al.22
have pointed out that R-aryloxy-substituted zinc phthalocya-
nine could form J-type aggregates in noncoordinating solvents
through the complementary coordination of the peripheral
oxygen atom of one phthalocyanine to the central Zn2+ in
another phthalocyanine. Such J-aggregates showed a UV-vis
spectrum similar at long wavelengths to that of azo-ZnPc in
chloroform. If the molecule structure of azo-ZnPc is taken into
account, it is rational to deduce that the unusual spectra of azo-
ZnPc in chloroform also occur because of the formation of
phthalocyanine J-aggregate through Zn-O coordination.
Several attempts were conducted to verify the above assump-
tion. It was well proved that J-aggregates of organic dyes were
fluorescent whereas H-aggregates were not.32 As shown in
Figure 3, with the irradiation of UV light, the fluorescence
spectra of azo-ZnPc in chloroform changed drastically. The
intensity of fluorescence emission at 706 nm kept decreasing
during the UV illumination process, and at the same time, a
new peak at 743 nm came into sights, with three isobestic points
at 739, 755, and 812 nm. Generally a one-step equilibrium
mechanism between monomer and the aggregate (nPc T Pcn)
was assumed. Therefore, the fluorescence peaks at 706 and 743
nm can be attributed to the monomer and the J-aggregate of
azo-ZnPc, respectively. Both Figures 1 and 3 indicate the same
UV light enhancement from the J-aggregation. In other words,
for azo-ZnPc, there is a strong tendency to form J-aggregates
when the azobenzene unit is in the cis configuration.
Figure 4. Absorption spectral changes of azo-ZnPc (a, before UV light
irradiation; b, after UV light irradiation for 3 min, c ) 1.19 × 10-5
M) in chloroform .
aggregates through Zn-O coordination in chloroform, compared
1
with monomer, the J-aggregates would show several H NMR
signal changes that could be predicted. As shown in Figure 5a,
the hydrogen atoms in azo-ZnPc are labeled with arabic
numbers. The coordination of Zn-O would give the strongest
influence on the environments of hydrogen atoms 3 and 4,
therefore affecting the chemical shifts and the shape of corre-
sponding resonances. Hydrogen atoms 2 and 5 would be less
influenced, and the influence on hydrogen atoms 1, 6, 7, and 8
would be the least. Since Zn-O coordination destroyed the C2V
symmetry of azobenzene units, the corresponding resonance
would split and make the spectrum more complex. The 1H NMR
of azo-ZnPc illustrated in Figure 5 precisely matched all above
predictions. Figure 5c is the 1H NMR spectrum of azo-ZnPc in
DMSO-d6. In DMSO, azo-ZnPc existed as monomers, as shown
in the UV-vis spectrum. Therefore, it is safe to assume that
Figure 5c represented the 1H NMR spectrum of azo-ZnPc
monomers. The corresponding resonance signal of each hydro-
1
gen atom in azo-ZnPc could be identified by analysis of H
Another distinct behavior of R-aryloxy-substituted zinc ph-
thalocyanine J-aggregate is that the addition of methanol could
breaks the J-aggregate,22 and as a consequence, the regular
Q-band shape is restored. Figure 4 shows the spectral change
of initial and UV-illuminated solutions of azo-ZnPc in chloro-
form with addition of methanol. As methanol was titrated,
absorption at 740 nm of both solutions decreased gradually and
finally disappeared completely with the increase in absorption
at 698 nm. This experiment together with the fluorescence
spectra can roughly explain the unusual red-shifted peak at 740
nm of Figure 1, which can be attributed to J-aggregates.
However, other more persuasive evidence is still required.
1H NMR is a powerful tool to monitor molecular structure
changes. If it is assumed that azo-ZnPc indeed formed J-
NMR spectra in Figure 5a-c. The peaks for H1 were at δ )
8.10-8.20 ppm; the quartet signals at 7.92-8.02 ppm belonged
to H2; the peaks of H3, H7, and H8 were located at 7.40-7.54
ppm; the peaks for H4 were observed at 7.26-7.38 ppm; and
the signals of H5 and H6 appeared at 7.66-7.88 ppm. In CDCl3,
the position of each hydrogen peak was changed to some extent,
compared with those in DMSO. As shown in Figure 5d,e, the
peaks of H2 downshifted to 7.88-8.08 ppm and the peaks of
H5 downshifted to 7.76-7.94 ppm. The signal of H3 was
upshifted. Besides those position shifts, the shapes of the peaks
at 7.40-7.54 and 7.26-7.38 ppm changed as well. With UV
light illumination, these changes became more apparent. How-
ever, it is worth noting that the peaks of H1 remain unchanged
in Figure 5c-e, which was consistent with theoretical prediction

Novel Azobenzene-Phthalocyanine Dyad
J. Phys. Chem. B, Vol. 112, No. 25, 2008 7391
hindrance around oxygen atoms is apparently decreased. The
trans-cis isomerization of azobenzene units shortened the
possible distance between two macrocycles and, consequently,
enhanced the aggregation ability of azo-ZnPc.
TEM studies provided direct evidence for evaluating the
different aggregate abilities between trans- and cis-azo-ZnPc.
Figure 7a shows the TEM image of a sample prepared from a
chloroform solution of azo-ZnPc without UV light irradiation
(trans-azo-ZnPc). Azo-ZnPc was uniformly deposited on the
microgrid. However, samples prepared from UV light-il-
luminated solution (cis-azo-ZnPc) exhibited strikingly different
TEM images. In Figure 7b, a lot of nanowires can be observed,
which were absent in Figure 7a. This phenomenon is quite
similar to our previous work.22b Thus it is reasonable to attribute
the nanowires to the strong J-aggregation of cis-azo-ZnPc.
Third-Order Optical Nonlinearity. Figure 8 shows the
normalized transmission without aperture at 532 nm (open
aperture, OA) as a function of distance along the lens axis.
Measured data of azo-ZnPc are shown (2) before and (0) after
irradiation. Each point corresponds to the average of 5 pulses.
The solid line is the theoretical fit. The OA curves exhibit the
normalized valleys, indicating the presence of reverse saturable
absorption with a positive coefficient ꢁ. The nonlinear absorption
coefficient ꢁ (meters per watt) can be obtained from a best fit
performed on the experimental data of the OA measurement:27
Figure 5. 1H NMR spectra of (a) 4-phenylazophenol in CDCl3, (b)
compound 1 in DMSO-d6, (c) azo-ZnPc in DMSO-d6, (d) azo-ZnPc in
CDCl3, and (e) azo-ZnPc in CDCl3 after 15 min of UV light irradiation.
[-ꢁI0Leff ⁄ (1 + z2/z02)]m
∞
TOA
)
(1)
∑
(m + 1)3⁄2
m)0
based on Zn-O coordination between two azo-ZnPc molecules.
These results strongly supported the assumption of the comple-
mentary coordination structure of the peripheral oxygen atom
of one azo-ZnPc molecule to the central Zn2+ of another azo-
ZnPc molecule in chloroform. Since the J-aggregate of azo-
ZnPc is the inevitable result of such Zn-O coordination, the
1H NMR experiments provided another powerful line of
evidence for the formation of Zn-O coordination-based J-
aggregates of azo-ZnPc in chloroform.
where TOA is the normalized transmittance for OA, Leff ) [1 -
exp(-RL)/R] is the effective thickness of the sample (L denotes
its thickness), R is the linear absorption coefficient of sample,
I0 is the on-axis illumination at the focus, z is the sample
2
position, z0 ) πω0 /λ is the Rayleigh range, ω0 is the beam
waist at local point (z ) 0), and λ is the laser wavelength.
Figure 9 shows the normalized transmission for closed
aperture (CA) of Z-scan. The large valley-to-peak configurations
of CA curves suggest that the refractive index changes are
positive, exhibiting a strong self-focusing effect. To extract the
nonlinear refractive index n2 from the Z-scan, pure nonlinear
refraction curves were obtained from the division of CA data
by OA data. Then the normalized transmittance T(z) is given
by27
Distance and steric hindrance play important roles in deter-
mining the reaction or configuration of a molecule. For phtha-
locyanines, the size of the peripheral substituents is essential
for their aggregation ability. The introduction of bulky dendritic
fragments has been exploited to generate nonaggregated ph-
thalocyanines.33 Recently, we reported a novel family of
R-aryloxy-substituted zinc phthalocyanines that are called
phthalocyanine-diarylethene dyads.34 Because of the big pe-
ripheral substituents, the phthalocyanine-diarylethene dyads do
not form J-aggregates at all, while R-aryloxy-substituted zinc
phthalocyanines with small peripheral substituents strongly favor
J-aggregation. As mentioned above, azo-ZnPc with azobenzene
units in the cis configuration (cis-azo-ZnPc) has a larger
tendency to form J-aggregates than those with azobenzene in
the trans configuration (trans-azo-ZnPc). The difference in
aggregation ability may be explained by the large steric
crowding changes around oxygen atom as a result of the UV
light-induced trans-cis isomerization of azobenzene moieties.
Figure 6 shows the optimized structure of azo-ZnPc by the
semiempirical PM3 method. As we can see from Figure 6a,b,
for trans-azo-ZnPc, the plane of trans-azobenzene is perpen-
dicular to that of the phthalocyanine ring. Substituents at 1 and
15 positions are located below the plane of phthalocyanine ring,
while substituents at 8 and 22 positions are above it. Four
azobenzene endowed zinc phthalocyanine with the ability to
alter the steric hindrance around oxygen. After the UV light-
induced trans-cis isomerization, as shown in Figure 6c,d, steric
4x
T(z) ) 1 -
∆ꢂ0
(2)
(x2 + 9)(x2 + 1)
and
∆Tp-v ) 0.406(1 - S)0.25|∆ꢂ0| for |∆ꢂ0| e π
∆ꢂ0λ
(3)
(4)
n2 )
2πI0Leff
where ∆ꢂ0 is the on-axis phase shift at the focus and ∆Tp-v is
the difference of transmittance between the normalized peak
and valley. The linear transmittance of far-field aperture, S, is
defined as the ratio of the pulse energy passing through the
aperture to the total energy.
Equations 3 and 4 are valid for nonresonant nonlinearity. Our
samples have about 4% absorption at 532 nm, which is the
excitation wavelength for the Z-scan experiment. Therefore,
population of the first excited state of studied azobenzene-
phthalocyanine also contributes to the nonlinear refractive index.
Although Sheik-Bahae’s theory must be used for nonresonant
nonlinearity, we can still use the theory to calculate the effective

7392 J. Phys. Chem. B, Vol. 112, No. 25, 2008
Chen et al.
Figure 6. Computer-optimized structure of azo-ZnPc by semiemphirical PM3: (a) top view of trans-azo-ZnPc;(b) side view of trans-azo-ZnPc; (c)
top view of cis-azo-ZnPc; (d) side view of cis-azo-ZnPc.
Figure 8. Normalized transmission without aperture at 532 nm (open
aperture, OA) as a function of distance along the lens axis. Measured
data of azo-ZnPc are shown (2) before and (0) after irradiation. Each
point corresponds to the average of 5 pulses. The solid line is the
theoretical fit.
Figure 7. TEM images of azo-ZnPc. The samples were prepared from
a chloroform solution of azo-ZnPc: (a) before UV light irradiation and
(b) irradiated by UV light for 2 min. [azo-ZnPc] ) 8 × 10-6 M.
nonlinear refractive index since the absorption of the studied
samples are small at 532 nm.
Accordingly, the real and imaginary parts of ꢀ(3) and the
molecular second hyperpolarizability γ of sample can also be
calculated by35
Figure 9. Normalized transmission for the closed aperture (CA) of
Z-scan.
cn20
120π2
c2n20
240π2ω
ꢀ(3)
NcL
Reꢀ(3) (esu) )
Imꢀ(3) (esu) )
n2 (m2/W)
ꢁ (m/W)
(5)
order nonlinear susceptibility (ꢀ(3), electrostatic units) and the
molecular second hyperpolarizability (γ, electrostatic units) are
calculated and listed in Table 1.
(6)
(7)
From Table 1, it is found that the studied azobenzene-
phthalocyanine dyad shows large second-order molecular hy-
perpolarizabilities, on the order of 10-30 esu, both before and
after UV illumination. The value of γ before irradiation is 1.25
times larger than that after illumination. This enhancement can
be attributed to the increased degree of J-aggregation of the
azo-ZnPc dyad after UV light irradiation.
γ )
where n0 is the linear refractive index of sample, ω is the angular
frequency of the light-field, Nc is the molecular number density
per cubic centimeter, and L is the local-field correction factor,
which may be approximated by [(n0 +2)/3]4.
Shirk et al.36 have reported the γ values of scandium,
yttrium, and several lanthanide bis(phthalocyanines) that were
studied by the degenerate four-wave mixing (DFWM)
2
The nonlinear absorption coefficient (ꢁ, meters per watt), the
nonlinear refraction coefficient (n2, meters2 per watt), the third-

Novel Azobenzene-Phthalocyanine Dyad
J. Phys. Chem. B, Vol. 112, No. 25, 2008 7393
TABLE 1: Values of Nonlinear Absorption and Refraction
Coefficients, Third-Order Nonlinear Susceptibility, and
Molecular Second Hyperpolarizability of Azo-ZnPc
mental Research Program (2007CB808000), and Postdoctoral
Science Foundation of China (Grant 20060390056).
physical values
before irradiation
after irradiation
References and Notes
n2/10-19 m2/W
4.26
1.48
2.26
3.87
5.30
4.03
2.82
4.82
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Conclusion
A novel azobenzene-substituted zinc phthalocyanine (azo-
ZnPc dyad) was synthesized, and its photoresponsive J-aggre-
gation behavior was studied by UV/vis, fluorescence, and H
1
NMR spectroscopies. Third-order optical nonlinearities of the
photoresponsive J-aggregates of the azo-ZnPc dyad (before and
after irradiation conditions) were measured by a Z-scan tech-
nique at 532 nm with pulse duration of 25 ps. The third-order
nonlinear refraction coefficient, nonlinear absorption coefficient,
and molecular second hyperpolarizability values were obtained.
All the results suggested that the studied azo-ZnPc dyad may
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Acknowledgment. This work was supported by National
Natural Science General Foundation of China (Grants 20333080,
20572059, 20502013, and 20773077), National Key Funda-

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