Optics and Photonics Journal, 2011, 1, 70-74
doi:10.4236/opj.2011.12011 Published Online June 2011 (http://www.SciRP.org/journal/opj/)
Copyright © 2011 SciRes. OPJ
Nonlinear Refraction of Peripheral-Substituted Zinc
Phthalocyanines Investigated by Nanosecond and
Picosecond Z Scans
Yunjing Li1, Timothy M. Pritchett2, Jiandong Huang3, Meirong Ke3, Wenfang Sun1*
1Department of Chemistry and Biochemistry, North Dakota St at e University, Fargo, USA
2U. S. Army Researc h L a boratory, AMSRD-ARL-SE-EM, 2800 Powder Mill Road, Adelphi, Maryland, USA
3College of Chemistry and Chemical Engineering, Institute of Research on Functional Materials, Fuzhou University,
Fuzhou, China
E-mail: Wenfang.Sun@ndsu.edu
Received April 6, 2011; revised May 9, 2011; accepted May 19, 2011
The singlet and triplet excited-state refraction cross-sections of dimethyl sulfoxide (DMSO) solutions of ten
zinc phthalocyanine derivatives with mono- or tetra-peripheral substituents at 532 nm were obtained by si-
multaneous fitting of closed-aperture Z scans with both nanosecond and picosecond pulse widths. Self-fo-
cusing of both nanosecond and picosecond laser pulses was observed in all complexes at 532-nm wavelength.
The complexes with substituents at all of the four α-positions exhibit relatively larger refraction cross-sec-
tions than the other complexes. The wavelength dependence of the singlet refraction cross-section of a rep-
resentative complex was observed to be non-monotonic in the range of 470 - 550 nm.
Keywords: Nonlinear Refraction, Zinc Phthalocyanine, Z Scan, Excited-State Refraction Cross-Section,
Wavelength Dispersion
1. Introduction
In recent years, there has been a growing interest in ma-
terials with a large and fast nonlinear optical response fo r
optical device applications [1-3]. Among the various
organic materials investigated, metallophthalocyanines
(MPcs) have attracted considerable attention because
their structures can be easily modified without affecting
their stability or altering their processability features
[4-6]. Most recently, the photophysical properties of ten
new zinc phthalocyanine derivatives with mono or
tetraperipheral substituents (structures shown in Figure 1)
were studied [7]. All complexes were found to exhibit
reverse saturable abso rption of nanosecond pulses at 532
nm and of picosecond pulses over a broad visible spec-
tral range. The singlet and triplet excited- state absorption
cross-sections were obtained by fitting nanosecond and
picosecond open-aperture Z-scan data using a five-level
model. It is found that the complexes with tetra substitu-
ents at the α-positions exhibit larger ratios of triplet ex-
cited-state absorption to ground-state absorption cross-
sections (σT/σg) than the other complexes; and the ratio of
singlet excited-state absorption cross-section to ground-
state absorption cross-section decreases from 470 nm to
550 nm [7]. This study is quite intriguing; however, no
information was obtained on the nonlinear refraction of
these complexes from the open-aperture Z-scan study.
In order to gain an understanding of how the peripheral
substituents influence the nonlinear refractive properties of
these complexes, nanosecond and picosecond closed-ap-
erture Z-scan measurements were carried ou t in th is stud y.
Using the previously measured values of excited-state
absorption cross-sections, values of the singlet and the
triplet excited-state refraction cross-sections were deter-
mined from the closed-aperture Z-scan data; the results are
reported in this paper. In addition, the wavelength de-
pendence of the singlet excited-state refraction cross-sec-
tion over a range extending from 470 nm to 550 nm was
studied using picosecond closed-aperture Z scans.
2. Experimental
The Z-scan experimental setup was described previously
[7]. A Quantel Brilliant Nd: YAG laser operating at its
Copyright © 2011 SciRes. OPJ
Figure 1. Structures of ZnPc derivatives 1 - 10.
second-harmonic output (4.1 ns, 10 Hz) and a picosec-
ond EKSPLA PG 401 optical parametric generator (OPG)
pumped by the third-harmonic output of an EKSPLA PL
2143A Nd:YAG laser (21 ps, 10 Hz) were used for the
closed-aperture Z-scan experiments. The HWe–2M beam
waist was measured to be 32
m for the nanosecond Z
scans and 40
m at 532 nm for the picosecond Z scans
by knife edge. The sample solutions were placed in a
2-mm quartz cuvette for the ps measurements and in a
1-mm cuvette for the ns measurements, which were less
than the Rayleigh ranges to fulfill the thin sample ap-
proximation condition. A small aperture (S = 0.22 for the
ns measurement and 0.35 for the ps measurement at 532
nm, and S = 0.34 – 0.42 for the ps measurements from
470 nm to 570 nm) was placed before the detector at the
far field for the closed aperture experiments.
In order to abstract the nonlinear refraction cross- sec-
tion, a five-level model described previously [7] was
used to fit the open-aperture and the closed-aperture
curves. First, the values of the excited-state absorption
cross-sections for the singlet and trip let excited states (σS
and σT, respectively) were obtained by simultaneously
fitting nanoseco nd and picoseco nd open-ap erture Z scans
at 532 nm, in which the effects of excited-state refraction
are absent, using the five-band model that employed in-
dependently measured values of the excited-state life-
times and of the triplet yield. The values of σS and σT
obtained from the open-aperture Z scans, together with
the measured values of the excited-state lifetimes and of
the triplet yield, were then reinserted in a generalized
five-band model that includes the effects of both ex-
cited-state refraction and excited-state absorption.
The effects of excited-state refraction are described
mathematically by the nonlinear phase
computed at
each point on the exit face of the sample from the fol-
lowing equation:
() ()rr
. (1)
Where ()r
and ()r
are the refraction cross-sec-
tions of th e singlet excited state and the lowest-lying trip-
let excited state, respectively. The number densities of
molecules in these states, denoted by nS, and nT respec-
tively, are functions of both the position (r, z) in the sam-
ple and the time t; they are computed as described in [7].
For each ZnPc derivative, a single pair of refractive
cross-section values (()r
) was chosen to simulta-
neously fit both the picosecond and the nanosecond
closed-aperture Z scans. In fitting closed-aperture Z scans
at wavelengths other than 532 nm, the value of ()r
assumed to be the same as that obtained from the simul-
taneous fitting of the corresponding 532-nm Z scans.
3. Results and Discussion
The electronic absorption spectra of complexes 1 - 10 in
dimethyl sulfoxide (DMSO) have been reported previ-
ously [7]. Figure 2 shows the absorption spectrum of
complex 5, which is fairly typical of the spectra of all ten
complexes. As noted in [7], all complexes possess a fairly
broad “transparency window” between the B-bands and
Q-bands (425 nm and 575 nm, respectively), and all
complexes exhibit strong reverse saturable absorption for
both nanosecond a nd pi cosecon d l aser pul ses at 532 nm .
Figure 3 shows the closed-aperture Z-scan curves for
complex 6, which are representative of all ten ZnPc de-
rivatives. The valley-peak feature is indicative of a posi-
tive refraction nonlinearity (self-focusing). The profile is
strongly asymmetric about a normalized transmittance of
unity, above which the peak is almost completely
Figure 2. UV-vis absorption spectrum of complex 5 in
Copyright © 2011 SciRes. OPJ
Figure 3. Normalized nanosecond and picosecond closed-
aperture Z-scan data (squares) and fitting curves (solid
lines) for complex 6 in DMSO at 532 nm.
suppressed. The asymmetry arises from the presence of
an absorptiv e nonlinearity (reverse saturable absorption).
Our previously published open-aperture Z-scan results
demonstrate that all of the ZnPc derivatives display sig-
nificant reverse saturable absorption [7].
For compounds possess both nonlinear absorption and
nonlinear refraction, a simple division of the closed-ap-
erture Z-scan curve by that obtained with the aperture
removed (open-aperture) does not necessarily provide a
good approximation of the curve that would be obtained
with a closed-aperture Z scan of a material having no
nonlinear absorption [8,9]. For this reason, the two-step
procedure described in the experimental section was em-
ployed to obtain the fitting curves shown by solid lines in
Figure 3. The solid curves in Figure 3 represent the best
fit of the experimental Z-scan data for complex 6 and
correspond to the values of ()r
= (5.0 ± 1.0)×10–17
cm2 and ()r
= (1.0 ± 0.5) × 10–17 cm2.
Table 1 lists the absorption and refraction cross-sec-
tions of the singlet excited state and the triplet excited
state for the ten ZnPc derivatives in DMSO. The magni-
tudes of the excited-state refraction cross-sections are all
on the order of 10–17 - 10–16 cm2. The three
stituted complexes 8, 9 and 10 all possess much larger
triplet refractive cross-sections than either the mono sub-
stituted comple xes (1-5) or the
-tetrasubstituted co mplex
(6), which is similar to the trend displayed by the triplet
excited-state absorption cross-sections. This phenomenon
could be explained by the geometry of the
- and
-tetrasubstituted complexes. As shown in Figure 4, the
geometry-optimized structures for complexes 6 and 7
(obtained via B3LYP/3-21g level density functional the-
ory (DFT) calculation in vacuum using Gaussian 09) are
drastically different. The steric hindrance imposed by the
substituents at the α-positions makes the substituents in
complex 7 adopt a nearly perpendicular geometry to the
phthalocyanine ring. This makes complex 7 bulkier than
complex 6, which has the substituents at the
The bulkiness of complex 7 would reduce intermolecular
interactions, which prevents excitation quenching and
stabilizes the excited state. Consequently, the nonlineari-
ties of the complex are higher than those with larger ex-
tent of intermolecular aggregation [10]. The same effect
holds for complexes 8, 9 and 10. In contrast, the singlet
excited-state refraction cross-sections do not vary signifi-
cantly am ong these t en complexes.
The wavelength dependence of the excited-state refrac-
tion cross-section of complex 5 was studied at multiple
visible wavelengths; it was found to exhibit self-focusing
nonlinear refraction in the range of 470 nm to 570 nm.
Assuming ()r
values at other wavelengths equal to the
value obtained at 532 nm (6.0 × 10–17 cm2), the following
values of ()r
at wavelengths from 470 nm to 550 nm
were obtained by fitting the relevant ps Z scans of com-
plex 5: 4.0 × 10–17 cm2 (470 nm), 3.0 × 10–17 cm2 (500 nm)
and 5.0 × 10–17 cm2 (550 nm). The parameters used for
Figure 4. Geometry-optimized structures for complexes 6
(upper) and 7 (lower) via DF T calc ulation.
Copyright © 2011 SciRes. OPJ
Table 1. Best-fit excited-state parameters of ZnPc derivatives 1 - 10 in DMSO.
/10–18 cm2
/10–18 cm2
/10–18 cm2
/10–18 cm2
/10–18 cm2
40 ± 5
40 ± 5
45 ± 5
45 ± 8
45 ± 5
40 ± 6
30 ± 5
18 ± 4
18 ± 2
30 ± 5
180 ± 20
105 ± 20
70 ± 15
110 ± 20
60 ± 15
45 ± 10
170 ± 20
180 ± 25
400 ± 40
280 ± 30
40 ± 10
40 ± 5
35 ± 5
50 ± 10
60 ± 10
50 ± 10
50 ± 10
40 ± 10
25 ± 5
40 ± 10
150 ± 20
80 ± 10
100 ± 20
60 ± 10
60 ± 10
10 ± 5
60 ± 5
160 ± 20
250 ± 50
300 ± 50
*From Reference 7; #This work.
Table 2. Best-fit excited-state parameters of ZnPc derivative 5 in DMSO at various wavelengths.
/10–19 cm2
/10–18 cm2
/10–18 cm2
S(r) #
/10–18 cm2
/10–18 cm2
40 ± 8
32 ± 6
45 ± 5
40 ± 10
60 ± 15
60 ± 15
60 ± 15
60 ± 15
40 ± 20
30 ± 10
60 ± 10
50 ± 10
60 ± 10
60 ± 10
60 ± 10
60 ± 10
*From [7]; #This work.
the fitting are summar ized in Table 2. No significant dis-
persion of the nonlinear refraction cross-section was ob-
served for 5 in the spect ral regi on st udi ed.
Linear absorption by the sample creates a thermal lens
whose rise time is given by 0/
wc, where 0
w is the
radius of the focal spot and
c is the speed of sound in
the solvent used (in the present case, 1493 m/s). How-
ever, since these rise times (26.8 ns for the picosecond Z
scans and 21.4 ns for the nanosecond scans) are much
longer than the corresponding pulse widths (21 ps and
4.1 ns, respectively), thermal lensing can be ignored in
fitting the closed-ap e rtur e Z scans.
4. Conclusions
The singlet and triplet excited-state refraction cross- sec-
tions at 532 nm of ten novel zinc phthalocyanine deriva-
tives with mono- or tetra-peripheral substituent(s) in
DMSO solution were obtained by using a five-level dy-
namic model to fit nanosecond and picosecond closed-
aperture Z-scan data. These results, in combination with
those for the singlet and triplet excited-state absorption
cross-sections previously obtained from open-aperture Z
scans, provide a complete picture of the excited-state op-
tical nonlinearities of the ten zinc phthalocyanine deriva-
tives. It has been demonstrated that both the number and
the position of the peripheral substituents dramatically
affect the triplet excited-state absorption and refraction
cross-sections. In addition, the wavelength-dependence of
the singlet excited-state refraction cross-section of a rep-
resentative complex was investigated over the range from
470 to 550 nm. No monotonic dependence of refraction
cross-section was observed.
5. Acknowledgements
W. Sun acknowledges the financial support from the NSF
NIRT program (DMI 0506531) and is grateful to Dr. Joy
Haley at the Air Force Research Laboratory for her help
in the singlet excited-state lifetime measurement. Sun is
also grateful to Professor Hongshan He at the South Da-
kota State University for his kind help in DFT calcula-
tions to obtain the geometry-optimized structures for
complexes 6 and 7. J. Huang acknowledges the financial
support from the National Natural Science Foundation of
China (Grant No. 20 20 1005).
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