A comparative study of third-order nonlinear optical properties of silver phenylacetylide and related compounds via ultrafast optical kerr effect measurements

Article abstract of DOI:10.1021/ic010408c

A comparative study of the third-order nonlinear optical properties, via the newly developed heterodyned optical Kerr effect (OHD-OKE) measurements, of silver phenylacetylide and related compounds is reported. [AgC≡CC₆H₅]_n (1) was found to exhibit efficient third-order nonlinear optical susceptibility X⁽³⁾ of 2.4 × 10^-14 esu, and second hyperpolarizability γ of 9.07 × 10^-32 esu. These results are compared with those of two related silver phenylacetylide compounds, namely, a double salt, (silver phenylacetylide)·(silver tert-butylthiolate) [AgC≡CC₆H₅·AgS (t-C₄H₉)]_n complex (2), and a cluster, triphenylphosphine silver phenylacetylide tetramer, [(C₆H₅)₃PAgC≡ CC₆H₅]₄ (3), as well as that of the related organic polymer polyphenylacetylene (4). These four compounds represent different types of phenylacetylide derivatives: 1 is an organometallic polymer, 2 a polymeric double salt, 3 a discrete metal cluster, and 4 an organic polymer. It was found that the third-order optical nonlinear response was enhanced by the incorporation of silver d electrons into the delocalized conjugated organic π system, and its magnitude is highly dependent upon the extent of the π delocalization. Specifically, the relative magnitudes of X⁽³⁾ and γ follow the order silver phenylacetylide polymer (1) > (silver phenylacetylide)·(silver tert-butylthiolate) double salt (2) > polyphenylacetylene polymer (4) > tetrameric (triphenylphosphine silver phenylacetylide)₄ cluster (3). The observed trend may be attributed to the decreasing length of π conjugation. It is interesting to note that the incorporation of Ag(I) into the polymeric framework of polyphenylacetylene enhances the X⁽³⁾ by 25-fold for the same degree of polymerization (n = 7). The signs of X⁽³⁾ and γ, which are related to the response mechanisms, were found to be solvent dependent.

Full text of DOI:10.1021/ic010408c

6794
Inorg. Chem. 2001, 40, 6794-6801
A Comparative Study of Third-Order Nonlinear Optical Properties of Silver
Phenylacetylide and Related Compounds via Ultrafast Optical Kerr Effect Measurements
Boon K. Teo* and Yi Hui Xu
Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607
Bing Yuan Zhong, Yuan Kang He, and Hui Ying Chen
Department of Chemistry, Peking University, Beijing 100871, China
Wei Qian, Yu Jun Deng, and Ying Hua Zou
Department of Physics, Peking University, Beijing 100871, China
ReceiVed April 17, 2001
A comparative study of the third-order nonlinear optical properties, via the newly developed heterodyned op-
tical Kerr effect (OHD-OKE) measurements, of silver phenylacetylide and related compounds is reported.
[AgCtCC6H5]n (1) was found to exhibit efficient third-order nonlinear optical susceptibility ø(3) of 2.4 × 10-14
esu, and second hyperpolarizability γ of 9.07 × 10-32 esu. These results are compared with those of two re-
lated silver phenylacetylide compounds, namely, a double salt, (silver phenylacetylide)‚(silver tert-butylthiolate)
[AgCtCC6H5‚AgS(t-C4H9)]n complex (2), and a cluster, triphenylphosphine silver phenylacetylide tetramer,
[(C6H5)3PAgCtCC6H5]4 (3), as well as that of the related organic polymer polyphenylacetylene (4). These four
compounds represent different types of phenylacetylide derivatives: 1 is an organometallic polymer, 2 a polymeric
double salt, 3 a discrete metal cluster, and 4 an organic polymer. It was found that the third-order optical nonlinear
response was enhanced by the incorporation of silver d electrons into the delocalized conjugated organic π system,
and its magnitude is highly dependent upon the extent of the π delocalization. Specifically, the relative magnitudes
of ø(3) and γ follow the order silver phenylacetylide polymer (1) > (silver phenylacetylide)‚(silver tert-butylthiolate)
double salt (2) > polyphenylacetylene polymer (4) > tetrameric (triphenylphosphine silver phenylacetylide)4
cluster (3). The observed trend may be attributed to the decreasing length of π conjugation. It is interesting to
note that the incorporation of Ag(I) into the polymeric framework of polyphenylacetylene enhances the ø(3) by
25-fold for the same degree of polymerization (n ) 7). The signs of ø(3) and γ, which are related to the response
mechanisms, were found to be solvent dependent.
Introduction
polymers and copolymers have been widely studied, the cor-
responding metal phenylacetylides, particularly silver pheny-
lacetylides, have not attracted much attention. The latter is be-
lieved to be polymeric with delocalized conjugated σ and π
bonding between silver and the triple bonds. Indeed, recent re-
lated studies have shown that phenylacetylene and metal phen-
ylacetylides play important roles in the formation of unusual
clusters with interesting electronic or optically active properties
through bridging or scaffolding.5-12
Recently, special attention has been focused on conjugated
organic polymers for their potential in the design and synthesis
of advanced materials.1,2 The π-electron delocalization in
conjugated systems contributes to the ultrafast response capabil-
ity and large third-order nonlinearity. Conjugated polymers with
efficient and fast optical response may find applications as fast
optical switches and modulators. In the past decade, investigation
of functionalized transition metals coordinated with organic
ligand systems has intensified due to the expectation that
incorporation of d electrons in the conjugated organic system
will greatly enhance the hyperpolarizability and the nonlinear
susceptibility.3,4 In this regard, though phenylacetylene and its
(5) Yam, V. W. W.; Lee, W. K.; Cheung, K. K. J. Chem. Soc., Dalton
Trans. 1996, 2335-2339.
(6) Wang, C. F.; Peng, S. M.; Chan, C. K.; Che, C. M. Polyhedron 1996,
15, 1853-1858.
(7) Page, H.; Blau, W.; Davey, A. P.; Lou, X.; Cardin, D. J. Synth. Met.
1994, 63 (3), 179-182.
(1) Salaneck, W. R., Stafstrom, S., Bredas, J. L., Eds. Conjugated Polymer
Surfaces and Interfaces, Electronic and Chemical Structure of
Interfaces for Light Emitting DeVices; Cambridge University Press:
Cambridge, 1996; p 154.
(2) Salaneck, W. R., Landstrom, I., Ranby, B., Eds. Conjugated Polymers
and Related MaterialssThe Interconnection of Chemical and Elec-
tronic Structure; Proceedings, 81st Nobel Symposium, (June 18, 1991);
Oxford University Press: Oxford, 1993; p 502.
(8) Yam, V. W. W.; Lo, K. K. W.; Fung, W. K. M.; Wang, C. R. Coord.
Chem. ReV. 1998, 171, 17-41.
(9) Chan, W. H.; Zhang, Z. Z.; Mak, T. C. W.; Che, C. M. J. Organomet.
Chem. 1998, 556, 169-172.
(10) (a) Deng, Y.; Xu, Y. H.; Lin, L.; Qian, W.; Xia, Z. J.; Teo, B. K.;
Zou, Y. H. J. Mater. Sci. Lett. 2000, 19, 549-551. (b) Xu, Y. H.; He,
X. R.; Hu, C. F.; Teo, B. K.; Chen. H. Y. Rapid Commun. Mass
Spectrom. 2000, 14, 298-303.
(3) Long, N. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 21-38.
(4) Bruce, D. W., O’Hare, D., Eds. Inorganic Materials, 2nd ed.; John
Wiley & Sons: Chichester, 1996; pp 122-169.
(11) Hissler, M.; Connick, W. B.; Geiger, D. K.; Eisenberg, R. Inorg. Chem.
2000, 39 (3), 447-457.
10.1021/ic010408c CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/15/2001

Third-Order Nonlinear Optical Properties of [AgCtCC6H5]n
Inorganic Chemistry, Vol. 40, No. 26, 2001 6795
2. Preparation of [AgS(t-Bu)]n. To a solution of 22.05 g (0.130
mol) of silver nitrate in 300 mL of acetonitrile, under nitrogen, was
added a solution of 49.6 mL (0.44 mol) of tert-butylthiol. The resulting
white precipitate was collected, washed thoroughly with 3 × 100 mL
methyl alcohol and 250 mL of acetonitrile, and dried under vacuum.
Yield: 22.99 g (90%).
We report herein a comparative study of the third-order
nonlinear optical properties of silver phenylacetylide and three
related compounds via the heterodyned optical Kerr effect
(OHD-OKE) measurements. [AgCtCC6H5]n (1) was found to
exhibit efficient third-order nonlinear optical susceptibility ø(3)
of 2.4 × 10-14 esu, and second hyperpolarizability γ of 9.07 ×
10-32 esu. These results are compared with those of two related
silver phenylacetylide compounds, namely, a double salt, (silver
phenylacetylide)‚(silver tert-butylthiolate) complex, [AgCt
CC6H5‚AgS(t-C4H9)]n (2), and a tetrameric cluster, triphen-
ylphosphine silver phenylacetylide, [(C6H5)3PAgCtCC6H5]4 (3),
as well as that of a closely related organic polymer, polyphen-
ylacetylene (4). These four compounds were chosen to represent
different types of phenylacetylide derivatives: 1 is an organo-
metallic polymer, 2 a polymeric double salt, 3 a discrete metal
cluster, and 4 an organic polymer. It was found that the third-
order optical nonlinearity was enhanced by the incorporation
of silver d electrons in the delocalized conjugated π system and
its magnitude is highly dependent upon the extent of the π
delocalization. Specifically, the relative magnitudes of ø(3) and
γ follow the order silver phenylacetylide polymer (1) > (silver
phenylacetylide)‚(silver tert-butylthiolate) double salt (2) >
polyphenylacetylene polymer (4) > (silver phenylacetylide)4-
(triphenylphosphine)4 cluster (3). This latter trend may be
attributed to the decreasing length of π conjugation. The signs
of ø(3) and γ, which are related to the response mechanisms,
were found to be solvent dependent.
Elemental anal. (%) Found (calcd): C, 24.62 (24.38); H, 4.69
1
(4.60); Ag, 54.70 (54.70). H NMR (CDCl3): 1.585 (s, -C(CH3)3).
3. Preparation of Silver Double Salt [(AgCtPh)(AgS(t-Bu))]n (2).
Silver tert-butyl thiolate (0.11 g, 0.558 mmol) was mixed with a 40
mL solution of 0.12 g (0.558 mmol) of silver phenylacetylide in 1:1
dimethyl sulfoxide (DMSO)/chloroform (CHCl3). The reaction mixture
turned silvery-pinkish-beige and was stirred vigorously under nitrogen
for 24 h. The precipitate was collected by filtration, washed with ethyl
alcohol, and dried under vacuum. Yield: 0.159 g (70.18%).
Elemental anal. (%) Found (calcd): C, 34.45 (35.49); H, 3.26
(3.47); Ag, 53.1 (53.13). 1H NMR (DMSO/CDCl3): 1.534 (s, 9H,
-C(CH3)3), 7.287-7.377 (m, 5H, -C6H5).
4. Preparation of Tetrameric Cluster [(Ph3 P)Ag(CtCPh)]4 (3).
Silver phenylacetylide (1.58 g, 7.56 mmol) was suspended in 200 mL
of dichloromethane in a 250 mL Erlenmeyer flask with stirring,
followed by the addition of 6.016 g (22.94 mmol) of triphenylphosphine.
Almost immediately the solid ingredients began to dissolve and the
solution became clear and turned pale yellow. The reaction mixture
was then stirred in the dark for 4 h. Slow evaporation in the dark
afforded colorless crystals, which were collected and recrystallized from
dichloromethane. The final product obtained was colorless glistening
rectangular columnar crystals with a sharp melting point of 176-178
°C. Yield: 2.86 g (80.3%).
Elemental anal. (%) Found (calcd): C, 66.39 (66.26); H, 4.63 (4.28).
5. Polymerization of Phenylacetylene (4). Phenylacetylene was
polymerized by means of molecular sieve NHSY-3 (SiO2‚Al2O3‚Na2O,
treated with oxalic acid and baked at 300 °C before use). Pulverized
NHSY-3 (0.7 g) was placed in a dried polymerization tube. The tube
was purged at least four times with pure nitrogen before the pres-
sure in the tube was kept at 0.7 mm Hg. Phenylacetylene (0.8 mL)
was then injected into the tube. Polymerization of phenylacetylene
occurred immediately and exothermally. The darkened molecular sieve
was removed, washed with absolute alcohol, dried, and placed in a
plastic container. Dropwise hydrofluoric acid was added into the con-
tainer to destroy and dissolve the molecular sieve. The resulting light
yellow liquid was neutralized with sodium hydroxide to pH 7. The
solution mixture was allowed to settle and then extracted with benzene.
A viscous liquid thus obtained was redissolved in a minimal amount
of benzene, followed by the addition of an excess amount of methanol
to precipitate out the product. The reaction mixture was then allowed
to stand overnight. The powdered polyphenylacetylene product was
obtained by centrifugation and dried under vacuum. Molecular weight
determination was performed with vapor pressure osmometry (KNAU-
ER D-1000) at 90 °C, with dimethylformamide (DMF) as solvent. Poly-
phenylacetylene with an average degree of polymerization 7 was chosen
in this study in order to compare with the silver phenylacetylide 1.
Experimental Section
Reagents. Phenylacetylene (ACROS, 98%) was purified by distil-
lation under nitrogen just prior to use. 2-Methyl-2-propanethiol (tert-
butyl mercaptan) (ACROS, 99%) was used as purchased.
Instrumentation. Micro FT-IR absorption spectra were recorded
with a Nicolet Magna 750 instrument. UV-vis spectra were taken with
a Shimadzu 2100 instrument. Elemental analysis was performed with
a CARLO ERBA model-1102 instrument. Silver content was deter-
mined with inductively coupled plasma atomic emission spectrometry
1
(IRIS AP). H nuclear magnetic resonance (1H NMR) spectra were
taken with ARX-400. Differential scanning calorimetry (DSC) analysis
was done with TA DSC-2010 with a temperature rising rate of 10
°C/min. X-ray photoelectron spectroscopy (XPS) measurements were
done with Kratos XSAM 800 (U.K.).
Preparation and Characterization. 1. Preparation of [AgCt
CPh]n (1). (a) Two-Step Synthesis. In a 200 mL round-bottomed flask,
0.834 g (4.91 mmol) of silver nitrate was dissolved in 125 mL of
acetonitrile to form a colorless solution. NH3 gas was bubbled through
the solution, and white precipitates formed immediately. After about 1
h the NH3 gas was stopped and 0.65 mL (5.93 mmol) of phenylacetylene
was added. The color of the suspension turned to light brown. After
stirring for 30 h, the precipitate was collected by filtration, washed
with three portions of 50 mL of methyl alcohol, and dried under
vacuum. Yield: 0.943 g (91.8%).
Results and Discussion
Syntheses and Characterization. Four distinct types of
phenylacetylene derivatives and/or related compounds have been
prepared and studied in this paper. The first is the polymeric
silver phenylacetylide (1). Generally, 1 can be prepared from
the reaction of [Ag(NH3)2]+ with phenylacetylene as prescribed
in preparation 1(a) in the Experimental Section and in the
literature.13-16 We report here a one-step synthesis of 1 via a
homogeneous reaction of silver nitrate with phenylacetylene,
Elemental anal. (%) Found (calcd): C, 45.67 (45.98); H, 2.27
(2.41); Ag, 51.0 (51.61).
(b) One-Step Synthesis. Silver nitrate (1.0439 g, 6.15 mmol) was
disssolved in 70 mL of acetonitrile in a 100 mL Erlenmeyer flask
followed by successive additions of 2.03 mL (18.45 mmol) of
phenylacetylene and 2.56 mL (18.45 mmol) of triethylamine with
vigorous stirring. The reaction mixture was stirred for 48 h. The white
precipitate formed was collected by filtration, washed thoroughly with
3 × 50 mL of acetonitrile and 3 × 50 mL of methyl alcohol
successively, and dried under vacuum. Yield: 1.118 g (87.07%).
Elemental anal. (%) Found (calcd): C, 45.41 (45.90); H, 2.47
(13) Brasse, C.; Raithby, P. R.; Russell, C. A.; Wright, D. S. Organome-
tallics 1996, 15, 639-644.
(14) Osakada, K.; Takizawa, T.; Yamamoto, T. Organometallics 1995, 14,
3531-3538.
1
(2.41); Ag, 51.0 (51.61). H NMR (323 K, DMSO/CDCl3): 6.821-
7.455 (m, -C6H5).
(15) Yam, V. W.-W. J. Photochem. Photobiol. A 1997, 106 (1-3), 75-
84.
(16) Yam, V. W. W.; Lee, W. K.; Cheung, K. K. J. Chem. Soc., Dalton
Trans. 1996, 11, 2335-2339.
(12) Xu, H. Y.; Tang, B. Z. J. Macromol. Sci. 1999, 36, 1197-1207.

6796 Inorganic Chemistry, Vol. 40, No. 26, 2001
Teo et al.
Figure 2. DSC curves of (a) [AgCtCPh]n, (b) [AgStBu]n, (c) [(AgCt
CPh)(AgStBu)]n, and (d) [(AgCtCPh)(AgStBu)]n from a 1:2 [AgCt
CPh]n/[AgStBu]n feeding mole ratio.
of intense mass peaks centered at m/z 315, 523, 731, 939, 1147,
etc. Within each group, the peak intensities follow the isotopic
distribution expected from the number of silver atoms. (Note
that silver has a natural abundance of 51.82% of mass 107 and
48.18% of mass 109.) The mass differences between adjacent
groups of peaks are all identical, and equal to the mass number
of 218, which is equivalent to a single silver phenylacetylide
unit (AgCtCC6H5), or the monomer. This latter observation
implies that the monomeric unit remains intact during ionization
and acceleration. Moreover, since the m/z value of each of the
molecular ions was exactly 107 Da more than that of the
corresponding value of [nM] (n ) integer), the general
composition of the molecular ions can be expressed as [nM +
Ag]+, where n ) 1, 2, 3, 4, ..., referred to the degree of
oligomerization. As mentioned earlier, each [nM + Ag]+ ion
exhibits the expected isotopic distribution of the (n+1)Ag atoms,
implying that the formation of the observed molecular ions is
through the silver cationization.
Figure 1. Micro FT-IR spectra of silver phenylacetylide from (a) two-
step process and (b) one-step process.
Due to the higher sensitivity of the linear mode (see Figure
1a of ref 10b), there are detectable molecular ion peaks at up
to m/z 4000, corresponding to a degree of polymerization as
high as 20 (though the intensity drops off significantly beyond
n ) 7), whereas in the reflected spectrum (Figure 1b of ref 10b)
no detectable mass peaks can be observed beyond m/z 1600
(which corresponds to n ) 7).
in the presence of triethylamine, resulting in higher purity and
better yield. As shown in Figure 1, spectra 1a and 1b of silver
phenylacetylide, prepared by the two methods, respectively, are
virtually identical. Other than the typical benzene C-H absorp-
tion at around 3053, 689, and 753 cm-1 and the CdC stretching
absorption at 1440 and 1483 cm-1, respectively, there is a set
of four weak combination bands at 1754-1947 cm-1, typical
of a monosubstituted benzene ring. The acetylene CtC stretch-
ing absorption at 2055 cm-1 is consistent with the formation of
silver phenylacetylide, [AgCtCPh]n. Furthermore, XPS (un-
published results) showed that there is only one kind of silver
atom in 1 (Ag 3d3/2, 374.8 eV; 3d5/2, 368.8 eV) and, as expected,
there are two types of carbons (C 1s, 284.7 and 283.3 eV (sh)
in approximately a 3:1 ratio). These spectroscopic results are
not inconsistent with the following generally accepted structure
of 1 (though other more complicated structures cannot be ruled
out).
The second type of compound studied here is the silver double
salt [AgCtCPh‚AgS(t-Bu)]n complex (2). Experimental evi-
dence strongly suggests that the 1:1 adduct (AgCtCPh)‚(AgS-
(t-Bu)), formed by silver phenylacetylide and silver tert-
butylthiolate, is a“double salt” composite compound. To the best
of our knowledge, 2 is the first example of a silver double salt
containing both acetylide and thiolate anions. As such, it opens
the door to other multiple salts of silver involving these two
functional groups. In fact, these two functional groups are highly
compatible; and 2 may be considered as a polymeric adduct as
indicated by the differential scanning calorimetry (DSC) data
depicted in Figure 2 for silver phenylacetylide (curve a), silver
tert-butylthiolate (curve b), and the AgCtCPh‚AgS(t-Bu)
double salt (curve c). It can be seen that these are distinct
compounds, with melting points at 138.5 and 155.3 °C for silver
phenylacetylide (1) and 252.7 and 276.1 °C for silver tert-
butylthiolate (5). The melting behavior of the [AgCtCPh‚AgS-
(t-Bu)]n double salt is totally different from that of its two
constituents 1 and 5. In curve c, there are two melting points at
164.0 and 180.9 °C and a new exothermal peak with a maximum
at 210.6 °C, implying further ordering or polymerization. Thus
we can rule out the possibility of a physical mixture of [AgCt
CPh]n and [AgS(t-Bu)]n. Further evidence of this statement was
found in our observation that if the feeding mole ratio of AgCt
CPh to AgS(t-Bu) was varied, the only product obtained
remained the 1:1 adduct, [AgCtCPh‚AgS(t-Bu)]n, with virtually
the same elemental analysis data and DSC thermal transition
The MALDI-TOF mass spectra of silver phenylacetylide had
been reported by us (see Figure 1 of ref 10b). There are groups

Third-Order Nonlinear Optical Properties of [AgCtCC6H5]n
Inorganic Chemistry, Vol. 40, No. 26, 2001 6797
absorption maximum of silver phenylacetylide (1) appears at
260 nm and extends somewhat into the visible region with fine
structures of two shoulders at 272 and 280 nm (curve b). The
observed red shift of the main absorption peak and the
occurrence of the fine structures (shoulders) are indicative of
an extended conjugated π system of phenylacetylides interacting
with the silver d electrons. Specifically, the major absorption
band can be assigned to the metal-to-ligand charge transfer
(MLCT) involving the excitation of electrons from the filled
metal d orbitals (Ag(I) d10 configuration) to the virtual π*
orbitals of the CtC bonds. This MLCT will be referred to as
(Ag) d10 f π* (CtC) or, simply, d10 f π* transitions in later
discussions.
It is interesting to note that, in a 1:1:1 DMSO/CHCl3/
CH2Cl2 mixed solvent system, the absorption curve of silver
phenyacetylide (1) is also a broadened peak with a maximum
absorption at 270.6 nm and extends into 350 nm, while that of
the tetrameric triphenylphosphine silver phenylacetylide cluster
(4) is a narrow peak with an absorption maximum at 257.0 nm
with no absorption beyond 300 nm. For the (silver phenylacety-
lide)‚(silver tert-butylthiolate) double salt (2), the absorption
peak is also quite broad with a maximum at 270.7 nm and
extends to 350 nm. This behavior is similar to that of silver
phenylacetylide, indicating that there exists charge transfer in
these two compounds. Finally, though the absorption of
polyphenylacetylene (4) is much weaker than that of the
phenylacetylide-containing compounds, it has a broad absorption
peak with a maximum at 260.8 nm and extends into 400 nm
(curve c in Figure 3), also indicating an extended π conjugated
system, but involving CdC bonds instead of CtC bonds.
Ultrafast Optical Kerr Measurements. In this study, third-
order optical nonlinear susceptibilities of silver phenylacetylide
(1) and three related compounds (2-4) were investigated via
femtosecond optically heterodyned optical Kerr effect (OHD-
OKE)19-21 measurements. In general, third-order optical non-
linear susceptibility ø(3) is a complex quantity. However, for
the majority of photonics applications, such as all-optical
switching, signal processing, and data storage, the real compo-
nents of ø(3) are more relevant. Unfortunately, in conventional
optical Kerr effect (OKE) experiments, the detected signal is
proportional to the quadratics of the third-order optical nonlinear
susceptibility ø(3). Hence, the real components of ø(3) cannot be
determined directly from the experiment. Recently, by introduc-
ing a local oscillator field with or without 90° optical phase
bias with respect to that of optical Kerr signal, a new
experimental technique called optically heterodyned optical Kerr
effect (OHD-OKE) has been developed. This new technique
allows the determination of the magnitudes and signs of both
the real and the imaginary components of ø(3) with greatly
improved signal-to-noise ratios.
Figure 3. UV-vis absorption spectra of (a) phenylacetylene in 1:1
DMSO/CHCl3; (b) silver phenylacetylide in 1:1 DMSO/CHCl3; and
(c) polyphenylacetylene in 1:1:1 DMSO/CHCl3/CH2Cl2.
behavior. One such example is shown in curve d of Figure 2
for a 1:2 feeding molar ratio.
The third type of compound studied here is the tetrameric
metal cluster [Ph3PAgCtCPh]4 (3), which has been synthesized
and characterized by one of us.17 The structure of 3, as
determined by single-crystal X-ray crystallography, can be
described as two linear anionic [Ag(CtCPh)2]- units bridged
by two [(Ph3P)2Ag]+ cationic moieties. The silver atom in each
of the two anionic [Ag(CtCPh)2]- units is σ bonded to the
two acetylides in a linear fashion. The silver coordination of
the cationic unit may be considered as distorted tetrahedral with
two phosphines and two dative π bonds from two CtC bonds.
The four silver atoms form a distorted square with Ag-Ag
bonds of 2.9-3.1 Å. It should be mentioned that the overall
structure of 3 is similar but not identical to that of (Ph3P)2Ag2-
Au2(CtCPh)4.18 In the latter cluster, two linear anionic
[Au(CtCPh)2]- units are bridged by two [(Ph3P)Ag]+ cationic
moieties whose coordination may be considered as a distorted
trigonal arrangement of one phosphine ligand and two dative π
bonds.
The experimental arrangement for OHD-OKE, shown in
Figure 4, is basically the same as that of OKE, which has been
described elsewhere.22 The laser source is an ultrafast dye laser
synchronously pumped by a CW mode-locked Nd:YAG laser
operating at 76 MHz. The average power of the output is about
160 mW with wavelength of 647.0 nm. The full width at half-
maximum of pulses is about 165 fs, which is deduced from a
The π conjugated nature of silver phenylacetylide may be
illustrated by its UV-vis absorption spectrum in comparison
with that of the phenylacetylene, as shown in Figure 3. The
absorption maximum of phenylacetylene occurs at 247 nm with
no absorption peaks beyond 260 nm (curve a), while the
(19) Orczyk, M. E.; Samoc, M.; Swiatkiewicz, J.; Prasad, P. N. J. Chem.
Phys. 1993, 98, 2524-2533.
(20) Orczyk, M. E.; Swiatkiewicz, J.; Huang, G.; Prasad, P. N. J. Phys.
Chem. 1994, 98, 7307-7312.
(21) Lin, L.; Qian, W.; Wang. C. F.; Zou, Y. H.; Wang, Q.; Chen, H. Y.
SPIE 1999, 3796, 131-142.
(17) Teo, B. K.; Dang, H.; Zhang, H. To be published.
(18) Abu-Salah, O.; Knobler, C. B. J. Organomet. Chem. 1986, 302, C10-
(22) Yuan, P.; Xia, Z. J.; Zou, Y. H.; Qiu, L.; Shen, J. F.; Xu, J. H. Chem.
Phys. Lett. 1994, 224, 101-105.
12.

6798 Inorganic Chemistry, Vol. 40, No. 26, 2001
Teo et al.
(3)
Z2 ) (-4π2ω2d/κ1c3)R2n-1/2(øe(3ff))realI2I1
Here I1 and I2 are the pump and probe beam intensities,
respectively; d is the thickness of the sample cell; κ1 is the value
of the probe beam wave vector; R2 represents a correction factor
for the attenuation of beams in the sample due to linear
absorption; n is the index of refraction of the sample; and (
ø3 )real is the real component of the effective third-order
eff
nonlinear optical susceptibility of the sample. From eq 3, we
know that if we select a standard sample as reference we can
derive the following formula:
1/2
ns drefZs
(3)
eff,s real
(3)
(ø
)
)
(ø
)
eff,ref real
(4)
( )
nref dsZref
Figure 4. Femtosecond optical Kerr effect experimental arrangement.
FDL: femtosecond laser. W: λ/4-plate. S: sample. P1: polarizer. P2:
polarization analyzer. C: chopper. ODL: optical delay line. SM: step
motor. L: lens. OD1, OD2: photoelectronic diodes. LA: lock-in
amplifier.
Here, the subscripts s, ref, and real denote the sample, the
reference, and the real component of third-order nonlinear
susceptibility, respectively. In our experiment, dref ) ds ) 80
(3)
µm, nref and (ø
)
are known, and Zs and Zref can be
real
eff,ref
pump-probe autocorrelation curve measured by second-
harmonic generation in KDP at the sample position. The probe
and pump pulses are split from the dye laser output (Iprobe:Ipump
) 1:7). The probe beam is passed through a polarizer, P1, whose
polarization direction is directed at 45° with respect to that of
the pump beam. The pump beam is chopped at 2770 Hz, and
the probe beam goes through a motorized optical delay line
controlled by a computer. The two beams are focused on the
same spot of the sample with a spot size of about 10 µm by a
lens with a 5 cm focal length. A polarization analyzer, P2, is
placed behind the sample at the cross polarization with respect
to the input polarizer P1 in order to filter out the residual probe
beam. The signal is finally detected by a photodiode connected
with a lock-in amplifier and stored in the computer equipped
with an analog-to-digital card. As mentioned above, in order to
measure the real components of ø(3) we need to provide a local
oscillator field with a 90° optical phase bias with respect to
that of optical Kerr signal. To convert from the OKE to the
OHD-OKE experimental setup, we insert a quarter-wave plate
W between the input polarizer P1 and the focusing lens L. The
optic axis of the quarter-wave plate is also directed at 45° with
respect to the polarization direction of the pump beam. Through
slight rotation of the input polarizer P1, a local oscillator field
with a 90° optical phase bias with respect to that of optical Kerr
signal can be generated. We define the angle Φ between the
polarization direction of polarizer P1 and the optic axis of
quarter-wave plate W as the “heterodyning angle.”
calculated from the experiment signals. Due to the very low
concentration (about 10-4 M) of the sample in the solution, the
index of refraction of the sample solution, ns, can be assumed
to be the same as that of the solvent. From eq 4, the value of
(3)
eff,s
(ø
)
real
can be determined.
For a dilute solution, the real part of the second hyperpolar-
izability γ of the solute can be deduced from
(3)
real
ø
γreal
)
(5)
NcL
where Nc denotes the number density of the sample molecules
and L is the local field correction factor, which may be
approximated by the Lorentz expression,
L ) (n2 + 2)/3
(6)
Figure 5 shows the OHD-OKE signals of silver phenylacetyl-
ide 1 and the related compounds 2 and 4. Cluster 3 is not shown
because it exhibits a negligible third-order optical nonlinear
response. Numerical results of the OHD-OKE measurements,
along with the absorption maxima in the electronic spectra
(Figure 3), are listed in Table 1. From Table 1, it can be seen
that the absolute magnitude of the ø(3) or γ values of these
compounds follows the order of silver phenylacetylide (1) >
(silver phenylacetylide)‚(silver tert-butylthiolate) (2) > polyphen-
ylacetylene (4) > (triphenylphosphine silver phenylacetylide)4
cluster (3). This decreasing order can be attributed to the extent
of conjugation, or, equivalently, to a reduction in effective length
of electron delocalization. The largest ø(3) or γ values were
observed in 1, presumably due to its high degree of conjugation
as a result of oligomerization (n > 7). The second largest ø(3)
or γ values were observed in 2, though reduced by a factor of
2. This reduction in the absolute values of ø(3) or γ values in
going from 1 to 2 may be taken as an indication that the extent
of conjugation is greatly reduced or impeded as a result of the
double salt formation. It is also interesting to note from Table
1 that 1 and 2 have the same signs for ø(3) or γ and follow the
same solvent-induced sign changes (vide infra). Furthermore,
the conjugated system composed of repeat units of [AgCtCPh],
as in 1 and 2, seems to be more effective than that comprising
alternating carbon-carbon double bonds as in polyphenylacety-
lene 4. Finally, since only two CtC units are involved in the
conjugation, mediated by a silver atom, in a linear fashion (i.e.,
In our experiment, we measure the time evolution of the
intensity of the heterodyned Kerr signal by varying the delay
time between the pump and the probe pulses at several
heterodyning angles. According to the basic principle of OHD-
OKE, the dependence of the intensity, I(Φ), of the heterodyned
Kerr signal, at zero delay time, on heterodyning angle, Φ, can
be fitted with the functional form
I(Φ) ) Z1 + Z2Φ
(1)
Thus, by measuring the intensities of the heterodyned Kerr
signals (at zero delay time) at two different angles Φ1 and Φ2,
we can determine the value of Z2, as follows:
Z2 ) (I(Φ1) - I(Φ2))/(Φ1 - Φ2)
(2)
Finally, the coefficient Z2 is proportional to the real component
of ø(3), as follows:

Third-Order Nonlinear Optical Properties of [AgCtCC6H5]n
Inorganic Chemistry, Vol. 40, No. 26, 2001 6799
Figure 5. OHD-OKE signals of silver phenylacetylide (s), (silver phenylacetylide)‚(silver tert-butylthiolate) complex (‚‚‚) and polyphenylacetylene
(- - -).
Table 1. The Absorption Maximum in UV-Vis Spectra,
Third-Order NLO Susceptibility (ø(3)), and Second
less frequently, in systems with an odd number of atoms in the
conjugated unit. In our experiments, instead, the signs of ø(3)
and γ change from positive to negative with a minor change in
the polarity in the solvent system. (The electronic spectra,
discussed above, are consistent with the notion that the solvent
system of 1:1:1 DMSO/CHCl3/CH2Cl2 is more polar than that
of 1:1 DMSO/CHCl3.) Hence we conclude that the overall sign
of the susceptibility is highly solvent dependent.
Hyperpolarizability (γ) of Silver Phenylacetylide and Its Derivatives
λmax
ø(3)
×
γ ×
compound
[AgCtCPh]n
(nm) solventa 10-14 esu 10-32 esu
260.0
270.6
a
b
a
b
b
b
2.40
-1.11
1.10
9.07c
-10.59c
7.44c
[AgCtCPh‚AgS(t-Bu)]n 264.0
270.7
257.0
260.8
-0.68
-6.49c
Finally, it can be seen from Figure 5 that the signal profiles
of all compounds studied here are approximately symmetric with
respect to the delay time, implying a primarily pulse width
limited response, and, since the time constants of all samples
are less than the pulse duration, it is concluded that the relaxation
time of all samples is shorter than the laser pulse width (165
fs), which means that the ultrafast optical response originates
from the electron movement and/or is caused by distortion of
the electron densities. The origin and/or mechanism of such
dependence is currently under investigation.
[Ph3PAgCtCPh]4
[-CHdC(Ph)]7
0.094
0.52
a Solvent systems: (a) 1:1 DMSO/CHCl3; (b) 1:1:1 DMSO/CHCl3/
CH2Cl2; (c) assume n ) 7.
[PhCtCAgCtCPh]) in the tetrameric cluster 3, it has the
smallest ø(3) or γ values.
The signs of ø(3) and γ deserve some comments. As can be
seen from Table 1, both ø(3) and γ exhibit positive values when
1:1 DMSO/CHCl3 (solvent system a) was used as the solvent
system but change to negative values when 1:1:1 DMSO/CHCl3/
CH2Cl2 (solvent system b) was used. This sign change (from
positive to negative) as a result of the slight increase in polarity
of the solvent system (from a to b) is rather interesting. Since
the structure-property relationships of the third-order NLO
materials are not so well understood as those for the second-
order NLO materials, models for elucidating the response
mechanism for linear conjugated molecules have been suggested.
According to Marder and Dirk,23,24 at least three terms are
needed to describe the third-order nonlinear hyperpolarization,
i.e., a negative term related to the transition between the ground
state and the first excited state, a two-photon term, and a dipolar
term. The absolute value of γ depends on the competition of
contributions from these three terms. When the two-photon
contribution dominates, the susceptibility is expected to be
positive, while when the negative term dominates, the suscep-
tibility should be negative. Thus, positive values of susceptibili-
ties are commonly observed for centrosymmetric conjugated
structures, while negative susceptibilities are observed, though
Design Strategies for Third-Order Nonlinear Optical
Materials Based on Silver Acetylides. The title compounds
1-4 were deliberately chosen for their commonalities and their
differences. They were selected to represent four distinctive
types of chemical species: 1 being an organometallic polymer,
2 a polymeric double salt, 3 a discrete metal cluster, and 4 an
organic polymer. Despite their structural and chemical diversi-
ties, compounds 1-3 share the common characteristics of having
an extended π conjugation system composed of the highly
polarizable metal ion (Ag(I), d10 configuration) and unsaturated
organic ligand (phenylacetylide). These characteristics are
known to give rise to highly efficient nonlinear optical proper-
ties. The diverse structural types of compounds 1-4 allow us
to assess the relative importance of stereochemical and electronic
factors that may impact the third-order nonlinear optical
properties, namely, ø(3) and γ. On the basis of our results, the
following design criteria for metal acetylide derivatives as
nonlinear optical materials can be formulated.
First, since high polarizability is key to nonlinear optical
responses, it is advantageous to choose metals with a completely
filled d shell. Though our focus in this paper was on Ag(I),
other d10 transition metals such as Cu(I) and Au(I) as well as
Zn(II), Cd(II), and Hg(II) are also excellent candidates. It is
interesting to note that the incorporation of Ag(I) into the
(23) Dirk, C. W.; Cheng, L.-T.; Kuzyk, M. G. Int. J. Quantum Chem. 1992,
43, 27-36.
(24) Marder, S. R.; Torruellas, W. E.; Blanchard-Desce, M.; Ricci, V.;
Stegeman, G. I.; Gilmour, S.; Bredas, J. L.; Li, J.; Bublitz, G. U.;
Boxer, S. G. Science 1997, 276, 1233-1236.

6800 Inorganic Chemistry, Vol. 40, No. 26, 2001
Teo et al.
polymeric framework of polyphenylacetylene enhances the ø(3)
by 25-fold for the same degree of polymerization (n ) 7).
Similarly, although we selected phenylacetylene as the organic
ligand, other unsaturated organic molecules with highly polariz-
able π electrons can also be used. The combination of d10
transition metals and triple bonds gives rise to colorless materials
covering a wide spectral range (transparent in the near-ultraviolet
regime down to 270 nm, the entire visible spectrum, and the
near-infrared region up to 3.3 µm). This wide window of optical
transparency is highly desirable for laser applications.
Second, the low-lying π* orbitals of the CtC triple bond
give rise to strong metal-ligand charge transfer (MLCT)
transitions that contribute to the nonlinear optical behavior of
this class of compounds. Hence, it is advantageous to minimize
the excitation energies (∆E) of these MLCT bands and to
maximize their extinction coefficients. Fortunately, for these
organometallic compounds, the MLCT occur at about 260 nm
(near-UV region) and the extinction coefficients are very high
(on the order of 105).
Third, it is desirable to maximize the extent of π delocal-
ization. In the case of compound 1, the degree of oligomerization
n is estimated to be 7. Previous work on the nonlinear optical
properties of organic polymers suggests that ø(3) or γ is highly
dependent upon the degree of oligomerization n. For example,
the ultrafast time-resolved optical Kerr effect had been used to
determine the nonresonant third-order optical nonlinearity of
polybenzonitrile at a wavelenth of 467 nm.25 A power-law
dependence of ø(3) or γ on the average degree of polymerization
n in the form of γ ) anb, where b ) 2.4, was obtained. The
results indicate that ø(3) or γ increases rapidly (faster than
quadratic, or, as a power of 2.4) by 5-fold as the average degree
of polymerization n increases by 2-fold (from n ) 4.5 to n )
9.1), before tapering off at n greater than 10. Therefore, we
anticipate significant improvements of ø(3) and γ for this class
of materials involving silver phenylacetylides as the degree of
polymerization n increases. As n increases, the effective π
conjugation length increases and the excitation energies (∆E)
of the MLCT bands decrease, resulting in significant enhance-
ment of the third-order nonlinear optical responses.
Fourth, the ligand arrangement of the Ag coordination in
compounds 1 and 2 is presumably of an asymmetric nature,
with one σ and a number (m) of π interactions, which may be
designated as (σ + mπ). This arrangement is preferred over
the symmetrical arrangement observed in compound 3, since
the asymmetric arrangement of the ligands around the metal
atom produces asymmetric electronic distribution which is a
desirable attribute for nonlinear optical properties. In 3, each
of the two central silver atoms is σ bonded to two acetylides in
a linear fashion (designated as 2σ) whereas each of the two
peripheral silver atoms interacts with two acetylenes via π
bonding (designated as 2π). The 2σ-type symmetrical ligand
arrangement gives rise to very weak third-order nonlinear optical
responses.
Fifth, while metal-metal bonding may contribute to electron
delocalization in the cluster or polymer, it plays a minor role
here since Ag(I), with a filled d orbital manifold (d10), forms
weak Ag‚‚‚Ag interactions, if any, at approximately 3.0 Å.
Metal-metal bonding may become important if a large en-
semble of metal atoms form a cluster with strong metal-metal
bonds.
engineering. For compound 1, substituents could be put on the
phenyl group or the acetylene itself. This may affect the mode
of bonding, the degree of oligomerization, the excitation energies
of the MLCT bands, etc., making this class of materials highly
tailorable. Compound 2 is a double salt involving silver acetylide
and silver mercaptide, (AgCCR)‚(AgSR′) where R ) Ph and
R′ ) t-Bu. Highly promising classes of nonlinear optical
materials can be generated by combining silver acetylides with
other silver salts to form double, triple, and quadruple salts with
a wide variety of structures. It is known that silver(I) forms
multiple salts of the following general formulas: double salts,
mAgX‚nAgY; triple salts, mAgX‚nAgY‚pAgZ; and quadruple
salts, mAgX‚nAgY‚pAg‚qAgW. Related to 2 are the double salt
Ag2C2‚8AgF,26 the triple salt Ag2C2‚AgF‚4AgSO3CF3‚RCN (R
) Me, Et),27 and the quadruple salt 2Ag2C2‚3AgCN‚15AgCO2-
CF3‚2AgBF4‚9H2O.28 For these highly interesting multiple silver
salts involving the acetylide dianion (CC)2-, “argentophilicity”29
promotes the formation of polyhedral cages of Ag(I) ions that
enclose the acetylides, which exhibit a wide range of coordina-
tion modes via σ, π, or mixed (σ + π) bonding with the metal
ions. For multiple silver salts involving the monosubstituted
acetylide monoanion (CCR)-, as in 2, partial cage formation
due to argentophilicity of Ag(I) ions will affect the composition
as well as the structure of the resulting polymer. More important
for our purpose, however, is the fact that each component of
the multiple silver salts can be separately engineered by
attaching various different substituents to the anionic ligands.
For example, a triple silver salt of the general formula (AgCCR)‚
(AgSR′)‚(AgSO3R′′) will have three tailorable substituents R,
R′, and R′′. Similarly, a quadruple silver salt of the general
formula (AgCCR)‚(AgSR′)‚(AgSO3R′′)‚(AgCO2R′′′) will have
four tailorable substituents R, R′, R′′, and R′′′.
It should be emphasized that the above-mentioned criteria/
factors are not totally independent or mutually exclusive. For
example, an asymmetric ligand bonding mode may, under
favorable conditions, produce a higher degree of oligomerization
and smaller MLCT excitation energies, thereby enhancing the
nonlinear optical properties.
In summary, for derivatives of compound 1, in order to
achieve high nonlinear optical activity, it is advantageous to
place substituents on the phenyl or the acetylene group that will
lead to a polymer with a lower MLCT excitation energy, an
asymmetric silver coordination, and a large degree of oligo-
merization. Similar arguments apply to double, triple, and
quadruple silver salts, but with the added dimensionalities of
molecular design and crystal engineering on each component
of the multiple silver salts. These materials have the advantage
of versatility and flexibility in design and fabrication. Further-
more, though not emphasized in this paper, these materials can
be cast into oriented thin films with highly anisotropic physical
properties. Finally, we believe that the stereoelectronic principles
and the design strategies developed here are generally applicable
to other organometallic polymers as well and, hopefully, will
lead to the discoveries of new materials with highly efficient
nonlinear optical properties.
Conclusion
It can be seen from the present work that silver phenylacetyl-
ide and its derivatives are promising third-order nonlinear optical
(26) Guo, G. C.; Zhou, G. D.; Wang, Q. G.; Mak, T. C. W. Angew. Chem.,
Int. Ed. 1998, 37, 630.
(27) Wang, Q. M.; Mak, T. C. W. J. Am. Chem. Soc. 2000, 122, 7608.
(28) Wang, Q. M.; Mak, T. C. W. J. Am. Chem. Soc. 2001, 123, 1501.
(29) (a) Pyykko, P. Chem. ReV. 1997, 97, 597. (b) Omary, M. A.; Webb,
T. R.; Assefa, Z.; Shankle, G. E.; Patterson, H. H. Inorg. Chem. 1998,
37, 1380.
Finally, this particular class of silver phenylacetylide com-
pounds, especially 1 and 2, is amenable to molecular and crystal
(25) Wang, C.; Zhao, X.; Chen, H.; Ai, X.; Xia, Z.; Zou, Y. Appl. Phys.
1997, B64, 4548.

Third-Order Nonlinear Optical Properties of [AgCtCC6H5]n
Inorganic Chemistry, Vol. 40, No. 26, 2001 6801
materials. The relative magnitudes of ø(3) and γ follow the order
silver phenylacetylide polymer (1) > (silver phenylacetylide)‚
(silver tert-butylthiolate) double salt (2) > polyphenylacetylene
polymer (4) > tetrameric (triphenylphosphine silver phenyl-
acetylide)4 cluster (3). It was found that the third-order optical
nonlinear response is very fast (in the femtosecond regime) and
that the ø(3) and γ values are correlated with (1) incorporation
of silver d electrons in the delocalized conjugated π system
network of the acetylides and (2) the extent of the π delocal-
ization. On the basis of these results, a set of design strategies
and stereochemical and electronic principles have been devel-
oped in order to improve the third-order nonlinear optical
responses. Specifically, the ø(3) and γ values of an organome-
tallic polymer can be significantly enhanced by utilizing a highly
polarizable metal atom (e.g., a d10 transition metal) with an
extended π conjugation organic system, coupled with a low
MLCT excitation energy, an asymmetric metal coordination,
and a large degree of oligomerization. Work is in progress to
employ these strategies to design a whole new class of nonlinear
optical materials based on the novel multiple (double, triple,
and quadruple) salts of silver described in the previous section.
The ease of incorporating desirable substituents on the acetylide
and other anions or ligands coordinated to the silver ions of
these multiple silver salts makes this interesting class of non-
linear optical materials highly tailorable.
Acknowledgment. We are grateful to the National Science
Foundation, USA (Materials Initiative Grant to B.K.T.) for
financial support of this research.
IC010408C