Synthesis and characterization of transition metal systems containing phosphino-oligothiophene ligands for nonlinear optical materials

Article abstract of DOI:10.1021/om060224b

Polythiophenes exhibit interesting third-order nonlinear optical properties but poor solubilities. Incorporating polythiophenes into transition metal complexes should yield readily soluble materials, and the transition metal substituent may alter third-order nonlinear optical properties of the polythiophene. In this paper, we report the synthesis and characterization of 2-diphenylphosphinothianaphthene, A, and its trans-PdCl₂L ₂, B, and cis-PtCl₂L₂, C, analogues. Additionally 2-diphenylphoshino-5:2′,5′:2″-terthiophene, D, its trans-PdCl₂L₂ complex, E, both first prepared by Wolf,²⁵ the previously unprepared cis-PtCl₂L₂ analogue, F, and the oxide of A, G, were prepared and investigated for nonlinear optical (NLO) behavior. No appreciable third-order NLO behavior was observed for A, D, or G. However both E and F exhibited nonlinear refraction, as shown by Z-scan measurements, and nonlinear absorption. The nonlinear absorption was significantly stronger for E, indicating that the metal center has a significant affect on nonlinear absorption. This is the first reported observation of this type of nonlinear behavior in metal-phosphinothiophene systems.

Full text of DOI:10.1021/om060224b

Organometallics 2006, 25, 5045-5050
5045
Synthesis and Characterization of Transition Metal Systems
Containing Phosphino-Oligothiophene Ligands for Nonlinear Optical
Materials
R. Dustan Myrex and Gary M. Gray*
Department of Chemistry, UniVersity of Alabama-Birmingham, 201 Chemistry Building, 901 14th Street
South, Birmingham, Alabama 35294-1240
Amelia G. VanEngen Spivey† and Christopher M. Lawson
Department of Physics, UniVersity of Alabama-Birmingham, CH 310, 1530 Third AVenue South,
Birmingham, Alabama 35294-1170
ReceiVed March 10, 2006
Polythiophenes exhibit interesting third-order nonlinear optical properties but poor solubilities.
Incorporating polythiophenes into transition metal complexes should yield readily soluble materials, and
the transition metal substituent may alter third-order nonlinear optical properties of the polythiophene. In
this paper, we report the synthesis and characterization of 2-diphenylphosphinothianaphthene, A, and its
trans-PdCl2L2, B, and cis-PtCl2L2, C, analogues. Additionally 2-diphenylphoshino-5:2′,5′:2′′-terthiophene,
D, its trans-PdCl2L2 complex, E, both first prepared by Wolf,25 the previously unprepared cis-PtCl2L2
analogue, F, and the oxide of A, G, were prepared and investigated for nonlinear optical (NLO) behavior.
No appreciable third-order NLO behavior was observed for A, D, or G. However both E and F exhibited
nonlinear refraction, as shown by Z-scan measurements, and nonlinear absorption. The nonlinear absorption
was significantly stronger for E, indicating that the metal center has a significant affect on nonlinear
absorption. This is the first reported observation of this type of nonlinear behavior in metal-
phosphinothiophene systems.
to the transition metals.3-6,26-28 We have recently used this
approach to prepare both oligomeric complexes with phosphine-
substituted bithiophenes and monomeric complexes with similar
ligands that were models for the corners and edges in the
oligomers.13 The complexes exhibited high solubilities, but the
bithiophene groups contained too few thiophenes to exhibit
interesting optoelectronic properties.
To determine if complexes of phosphine-substitued oligoth-
iophenes with greater degrees of conjugation will exhibit
interesting optoelectronic properties, we have carried out a study
of the third-order nonlinear optical properties of two such
ligands, diphenylphosphinothianaphthene, A, and 2-diphe-
nylphoshino-5:2′,5′:2′′-terthiophene, D, their trans-PdCl2L2 and
cis-PtCl2L2 complexes, and 2-diphenyloxophosphino-5:2′,5′:2′′-
terthiophene using Z-scan and nonlinear absorption intensity-
dependent transmission measurements. The results of these
studies are reported in this paper. Syntheses and complete
multinuclear NMR spectroscopic characterizations for ligands
and complexes that have not been previously reported have also
been carried out and are summarized in this paper.
Introduction
There has been strong interest in conjugated polymers over
the last two decades because of their optoelectronic properties.
The extended π-conjugated networks and large concentration
of easily polarizable electrons possessed by such polymers can
lead to nonresonant third-order nonlinear optical properties in
these materials. Polythiophene systems are one type of such
polymers and have already seen use as antistatic coatings, hole-
injection materials for polymeric light-emitting diodes, and field-
effect transistors.1,2 One major limitation of the use of poly-
thiophenes as optoelectronic materials is that polythiophenes
with five or more repeat units exhibit very low solubilities that
make processing difficult. One popular method for improving
the solubilities of these materials is to substitute thiophene rings
at the â-position with alkyl groups, with butyl to octyl being
common choices.
Incorporation of the polythiophene groups into transition
metal complexes is an alternate and less studied approach for
improving the solubilities of polythiophenes. This approach has
the added advantage that substitution of the polythiophene by
a metal complex may modify the optoelectronic properties of
polythiophene. We and others have demonstrated that poly-
thiophene groups may be incorporated into transition metal
complexes via functionalization of the polythiophenes with
phosphines followed by coordination of the phosphine groups
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Batchelor, R. J.; Patrick, B. O.; Ishii, M. Inorg. Chem. 2003, 42 (8), 2704-
2713.
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620-627.
† Present address: Department of Physics, University of Puget Sound,
1500 N. Warner St. #1031, Tacoma, WA 98416-1031.
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V. R.; Fenf, Y. Katz, H. E.; Rogers, J. App. Phys. Lett. 1998, 73 (2),
142-144.
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Am. B 2005, 22, 852.
10.1021/om060224b CCC: $33.50 © 2006 American Chemical Society
Publication on Web 09/12/2006

5046 Organometallics, Vol. 25, No. 21, 2006
Myrex et al.
nylphosphine, and the solvent was removed on a rotary evaporator.
The residue was extracted with degassed ethyl ether to separate
the product from the LiCl byproduct, and the extracts were
combined and evaporated to dryness to yield 1.29 g (74.7%) of
Experimental Section
Materials. THF was first dried by allowing it to stand over
MgSO4 for at least 12 h, followed by distillation from CaH2, and
finally by distillation from Na/benzophenone onto molecular sieves.
The dry THF was used within a few hours of collection. Other
solvents were reagent-grade and were degassed using high-purity
(99.998%) nitrogen before use. Terthiophene, palladium chloride,
chlorodiphenylphosphine, and 1.6 M n-butyllithium were of suf-
ficient purity from suppliers for immediate use. Literature methods
were used for the dichloro(1,4-cyclooctadiene)platinum(II) chloride
precursor.7
NMR Spectroscopic Characterization. Multinuclear 31P{1H}
and 1H NMR spectra were run on a Bruker ARX-300 NMR
spectrometer. Chloroform-d solutions of the ligands and complexes
were prepared under nitrogen. The 31P{1H} spectra were referenced
to external 85% phosphoric acid in a coaxial tube that also contained
chloroform-d, while the 1H spectra were referenced to internal
tetramethylsilane.
2-Diphenylphosphino-5:2′,5′:2′′-terthiophene, C24H17PS3 (D).
A modification of Wolf’s method25 was used to prepare 2-diphe-
nylphosphino-5:2′,5′:2′′-terthiophene in much higher yield. A
mixture of 2.5 mL (4.0 mmol) of a solution of 1.6 M n-butyllithium
in hexanes and 20 mL of THF was added to a solution of 0.99 g
(4.0 mmol) of 2:2′,5′:2′′-terthiophene in 100 mL of THF. The
reaction mixture was cooled in an acetone/dry ice bath during the
reaction, and the addition took place over 30 min. When the addition
was completed, the reaction mixture was allowed to warm at room
temperature and stirred for 30 min. Next, 0.718 mL (4.0 mmol) of
neat chlorodiphenylphosphine was added to the reaction mixture
from a gastight syringe, and the resultant solution was stirred an
additional 15 min. A few drops of degassed, deionized water were
then added to quench any residual butyllithium or chlorodiphe-
1
crude D. H NMR (chloroform-d): δ 6.87-6.96 (3H, m), 7.07-
7.09 (2H, m), 7.12-7.15 (2H, m), 7.27-7.37 (10H, m). 31P{1H}
NMR (chloroform-d): δ -18.30 (s).
2-Diphenylphosphinothianaphthene, C20H15PS (A). Following
the procedure for D, 1.000 g (7.45 mmol) of thianaphthene, 4.66
mL (7.45 mmol) of n-butyllithium, and 1.34 mL (7.45 mmol) of
chlorodiphenylphosphine yielded 1.53 g (64.6%) of crude A as a
white solid. 1H NMR (chloroform-d): δ 7.25-7.47 (12H, m), 7.51-
7.53 (1H, m) 7.74-7.83 (2H, m). 31P{1H} NMR (chloroform-d):
δ -16.56 (s).
Dichlorobis(2-diphenylphosphino-5:2′,5:2′-terthiophene)pal-
ladium(II) Chloride, PdCl2(C24H17PS3)2‚CH2Cl2 (E‚CH2Cl2). A
mixture of 0.113 g (0.639 mmol) of palladium chloride powder
and 0.553 g (1.278 mmol) of E in 75 mL of degassed acetonitrile
was stirred for 18 h at ambient temperature under nitrogen. Then,
the solid that had precipitated from solution was collected by
filtration and recrystallized from a dichloromethane/hexanes mixture
to yield 2.91 g (43.7%) of analytically pure E‚CH2Cl2 as a red,
1
crystalline solid. H NMR (chloroform-d): δ 7.00-7.24 (6H, m),
7.35-7.47 (6H, m), 7.65-7.75 (5H, m). 31P{1H} NMR (chloroform-
d): δ 13.19 (s). Anal. Calcd: C, 51.90; H, 3.10. Found: C, 51.60;
H, 3.23.
Dichlorobis(2-diphenylphosphinothianaphthene)palladium-
(II) Chloride, PdCl2(C20H15PS)2 (B). Using the procedure for E,
0.177 g (0.100 mmol) of palladium chloride and 0.637 g (0.200
mmol) of A yielded 0.701 g (86.1%) of analytically pure B as large
1
orange needles. H NMR (chloroform-d): δ 7.32-7.51 (8H, m),
7.73-7.83 (6H,m), 8.14 (1H, m). 31P{1H} NMR (chloroform-d):
δ 14.53 (s). Anal. Calcd: C, 59.02; H, 3.71. Found: C, 58.96; H,
3.71.
(10) Sun, W.; Byeon, C. C.; McKerns, M. M.; Lawson, C. M.; Gray, G.
M.; Wang, D. Appl. Phys. Lett. 1998, 73, 1167.
(11) Sheik-Bahae, M.; Said, A. A.; Wei, T.-H.; Hagan, D. J.; Van
Stryland, E. W. IEEE J. Quantum Electron. 1990, 26, 760.
(12) Zhao, W.; Palffy-Muhoray, P. Appl. Phys. Lett. 1993, 63, 1613.
(13) Myrex, R. D.; Colbert, C. S.; Gray, G. M.; Duffey, C. H.
Organometallics 2004, 23, 409.
Dichlorobis(2-diphenylphosphino-5:2′,5′:2′- terthiophene)-
platinum(II) Chloride‚CH2Cl2, PtCl2(C24H17PS3)2‚CH2Cl2 (F‚
CH2Cl2). To approximately 75 mL of degassed dichloromethane
were added 0.748 g (2.0 mmol) of dichloro(1,4-cyclooctadiene)-
platinum(II) chloride and 1.730 g (4.0 mmol) of D. This solution
was stirred at ambient temperature for 18 h, and then the solvent
was removed under vacuum. Soxhlet extraction with hexanes
removed the residual D and yielded 1.66 g (80.0%) of analytically
(14) (a) Redfield, D. A.; Cary, L. W.; Nelson, J. H. Inorg. Chem. 1975,
14, 50. (b) Verstuyft, A. W.; Redfield, D. A.; Cary, L. W.; Nelson, J. H.
Inorg. Chem. 1977, 16, 2776. (c) Varshney, A.; Gray, G. M. Inorg. Chem.
Acta 1988, 148, 215. (d) Smith, D. C. Jr.; Gray, G. M. Inorg. Chem. 1998,
37, 1792. (d) Smith, D. C., Jr.; Gray, G. M. J. Chem. Soc., Dalton Trans.
2000, 677.
1
pure F‚CH2Cl2 as a golden-yellow powder. H NMR(chloroform-
d): δ 6.90-6.97 (4H, m), 7.01 (1H, s), 7.05-7.07 (1H, m), 7.10-
7.12 (1H, m), 7.14-7.17 (2H, m), 7.22 (1H, m), 7.26-7.35 (3H,
m), 7.54-7.57 (3H, m). 31P{1H} NMR (chloroform-d): δ 5.33(s),
1J(31P,195Pt) 3698 Hz). Anal. Calcd: C, 49.00; H, 2.99. Found: C,
48.95; H, 3.08.
(15) Pregosin, P. S.; Kunz, R. W. In 31P and 13C NMR of Transition
Metal Phosphine Complexes; Diehl, P., Flack, F., Kosfeld, R., Eds.; NMR:
Basic Principles and Progress 1; Springer-Verlag: Berlin, 1979; Vol. 16.
(16) Tolman, C. A. Chem. ReV. 1977, 77, 313.
(17) Stott, T. L.; Wolf, M. O. J. Phys. Chem. B 2004, 108, 18815.
(18) Calvete, M.; Yang, G. Y.; Hanack, M. Synth. Met. 2004, 141, 231.
(19) Si, J.; Yang, M.; Wang, Y.; Zhang, L.; Li, C.; Wang, D.; Dong, S.;
Sun, W. Appl. Phys. Lett. 1994, 64, 3083.
(20) Perry, J. W.; Khundkar, L. R.; Coulter, D. R.; Alvarez, D., Jr.;
Marder, S. R.; Wei, T. H.; Sence, M. J.; Van Stryland, E. W.; Hagan, D.
J. In Organic Molecules for Nonlinear Optics and Photonics; Messier, J.,
et al., Eds.; Excited-state absorption and optical limiting in solutions of
metallophthalocyanines; Kluwer Academic Publishers: Netherlands, 1991.
(21) Shirk, J. S.; Lindle, J. R.; Bartoli, F. J.; Boyle, M. E. J. Phys. Chem.
1992, 96, 5847.
(22) Unnikrishnan, K. P.; Thomas, J.; Nampoori, V. P. N.; Vallabhan,
C. P. G. Opt. Commun. 2002, 204, 385.
(23) Sutherland, R. L. Handbook of Nonlinear Optics; Marcel Dekker:
New York, 1996.
(24) Nikogosyan, D. N. Properties of Optical and Laser-Related Materi-
als: A Handbook; John Wiley and Sons: New York, 1997.
(25) Clot, O.; Wolf, M. O.; Patrick, B. O. J. Am. Chem. Soc. 2001, 123,
9963-9973.
(26) Zhai, T.; Lawson, C. M.; Gale, D. C.; Gray, G. M. Opt. Mat. 1995,
4, 455-460.
Dichlorobis(2-diphenylphosphinothianaphthene)platinum-
(II) Chloride‚CH2Cl2, PtCl2(C20H15PS)2‚CH2Cl2 (C‚CH2Cl2). A
solution of 0.187 g (0.500 mmol) of dichloro(1,4-cyclooctadiene)-
platinum(II) and 0.318 g (1.00 mmol) of A in 75 mL of degassed
dichloromethane was stirred at ambient temperature for 18 h, and
then the solvent was removed under vacuum. The resulting yellow
powder was washed with several portions of degassed hexanes and
then dried under high-vacuum to yield 0.320 g (70.9%) of
1
analytically pure C‚CH2Cl2 as a pale yellow powder. H NMR
(chloroform-d): δ 7.10-7.16 (3H, m), 7.24-7.30 (3H, m), 7.32-
7.36 (1H, m), 7.40-7.47 (1H, m), 7.49-7.58 (4H, m), 7.66-7.76
(3H, m). 31P{1H} NMR (chloroform-d): δ 7.62 (s and d, 1J(31P,195
-
Pt) 3697 Hz). Anal. Calcd: C, 49.86; H, 3.16. Found: C, 49.83;
H, 3.17.
2-Diphenyloxophosphino-5,2′:5′,2′′-terthiophene, C24H17OPS3
(G). A solution 0.250 g (0.578 mmol) of D and 1.000 g of urea-
stabilized hydrogen peroxide pellet (35% peroxide by weight), from
Acros Organics, in 75 mL of methanol was stirred at ambient
temperature for 21 h, and then the solvent was removed under
(27) Sun, W.; Byeon, C. C.; McKerns, M. M.; Lawson, C. M.; Dunn, J.
M.; Hariharasarma, M.; Gray, G. M. Opt. Mater. 1998, 11, 87-93.
(28) Gale, D. C.; Gray, G. M.; Lawson, C. M. SPIE Proc. 1997, 54-
60, 3146.

Phosphino-Oligothiophene Ligands
Organometallics, Vol. 25, No. 21, 2006 5047
Figure 1. Experimental configuration for nonlinear absorption
(NLA) measurements. W ) half-wave plate, GLP ) Glan-laser
polarizer, B ) beam splitter, L1 ) 100 cm focal length lens, L2 )
20 cm focal length lens, KE ) knife edge, D1, D2 ) detectors.
Figure 2. Experimental configuration for top-hat Z-scans. “Open”
and “closed” aperture Z-scans are measured simultaneously by
detectors D2 and D3, respectively, while detector D1 serves as a
reference. W ) half-wave plate, GLP ) Glan-laser polarizer, A1,
A2 ) apertures, L ) 20 cm focal length lens, BS1, BS2 ) beam
splitters, D1, D2, D3 ) detectors, S ) sample.
vacuum. The residual urea was extracted from the residue with
deionized water, and the insoluble G was dried under high vacuum
overnight. 1H NMR (chloroform-d): δ 7.01-7.22 (7H, m), 7.23-
7.79 (10H, m). 31P{1H} NMR (chloroform-d): δ 21.80 (s).
Optical Characterization of the Ligands and Complexes. The
linear and nonlinear optical properties of the complexes in solution
were characterized by UV-visible absorption, nonlinear absorp-
tion,8-10 and Z-scan measurements.11 Together, this series of
measurements allowed both the absorptive and refractive compo-
nents of the optical nonlinearities of the complexes and the linear
optical properties to be probed.
records changes in transmission caused by both absorptive and
refractive nonlinearities. By taking the ratio of the closed-aperture
to open-aperture signals, one can isolate the change in transmission
I
due solely to the refractive nonlinearity quantified by n2 .
Results and Discussion
Ligand Syntheses. The phosphine-substituted terthiophene
and thianaphthene ligands A and D were prepared as shown in
Scheme 1. The reactions were run in a dry ice/acetone slush
bath (-80 °C) to minimize isomeric scrambling of the lithiated
thiophene and hence maximize the yield of the desired ligand.
Generation of the lithiated species was rapid, and addition of
the chlorodiphenylphosphine was carried out within 30 minutes
of completion of the n-butyllithium addition. The crude ligands
were used in subsequent reactions because the 31P{1H} NMR
Nonlinear absorption (NLA) measurements probe the percent
transmission of a sample as a function of the incident light fluence
(energy per unit area) or intensity. Fluence-dependent (i.e., non-
linear) transmission through a sample can be caused by physical
processes such as two-photon absorption, excited-state absorption,
nonlinear scattering, or a combination of such processes.8 Figure 1
shows the experimental configuration used for NLA measurements.
The frequency-doubled Nd:YAG laser produces 7 ns pulses at 532
nm with a repetition rate of 10 Hz. After passing through a wave
plate and polarizer combination, which is used to control the pulse
energy, part of the 8 mm diameter beam (about 5%) is reflected
by beam splitter B toward reference detector D1. The remaining
portion of the beam is focused by lens L1 (f ) 1 m) into the sample.
The translation stage and knife edge, KE, are used to measure the
beam radius at the focus, which is typically 100-120 µm. After
passing through the sample, the transmitted light is re-collimated
by L2 and sent into detector D2. During a NLA scan, the input
energy is varied using the wave plate-polarizer combination, and
the reference energy and transmitted signal energy are detected
simultaneously. The incident energy, Ein, is calculated from the
reference energy by measuring the ratio of energy at D2 to the
energy at D1 without a sample, and the incident and transmitted
1
spectra of the A and D were singlets and the H NMR spectra
contained only the resonances for the phenyl and either the
terthiophenyl or thianaphthenyl protons in the expected ratios.
The procedure used to prepare D is a modification of that
reported by Wolf.25 This modification avoids chromatographic
purification and gives D in a significantly higher yield (74.7%
versus 29%).
Syntheses of the Complexes. The palladium(II) chloride
complexes were synthesized by combining stoichiometric
amounts of palladium chloride and the appropriate ligand in
acetonitrile as shown in Scheme 2. The mixtures were stirred
for 18 h at ambient temperature to yield the desired products
as insoluble precipitates. The synthesis of complex D by Wolf
and co-workers has previously been reported.25 The platinum-
(II) chloride complexes were prepared by the reaction of
stoichiometric amounts of dichloro(1,4-cyclooctadiene)platinum-
(II) and the appropriate ligand in dichloromethane as also shown
in Scheme 2.
Oxidation of Ligand D. The oxide of ligand D, G, was
prepared by the reaction of the ligands with a urea/hydrogen
peroxide complex in dichloromethane. The urea byproduct was
separated from G by washing with water to give an essentially
quantitative yield of crude G. The 1H NMR and 31P{1H} NMR
spectra of this product contained no unexpected resonances, and
it was used without further purification.
NMR Characterization of the Ligands and Complexes.
The 31P{1H} NMR chemical shifts of ligands D and A are shown
in Table 1 and are consistent with those reported for 2-diphe-
nylphosphinothiophene (-19.43 ppm) and 5-diphenylphosphino-
2,2′-bithiophene (-18.57 ppm).13 The 31P{1H} NMR spectra
of the palladium complexes, B and E, both exhibited a singlet,
2
fluences are calculated from the energies using F ) E/(0.5πw0 ).
A refractive nonlinearity behaves as an intensity-dependent
I
contribution to the index of refraction, n ) n0 + n2 I, where n0 is
I
the index of refraction under low-intensity illumination and n2 is
the nonlinear refractive index. The Z-scan technique is used to
isolate and probe the refractive part of the optical nonlinearity.
Figure 2 shows the experimental configuration for the top-hat Z-scan
measurement.11,12 The frequency-doubled Nd:YAG laser produces
7 ns pulses at 532 nm with a repetition rate of 10 Hz. After passing
through the wave plate and polarizer combination that is used to
control the pulse energy, the 8 mm diameter laser beam is expanded
by a 3× telescope before hitting a 3 mm aperture, A1. The light
transmitted by aperture A1 is focused by the 20 cm lens to a spot
of radius approximately 50 µm in the 2 mm sample. After passing
through the sample, the beam is split into two portions for open-
aperture and closed-aperture Z-scan detection at detectors D2 and
D3, respectively. Detector D1 is used as a reference. The transmis-
sion of the closed aperture, A2, is typically between 0.02 and 0.05.
The open-aperture detector records changes in transmission due to
absorptive nonlinearities only, while the closed-aperture detector

5048 Organometallics, Vol. 25, No. 21, 2006
Myrex et al.
Scheme 1
Scheme 2
Table 1. 31P{1H} NMR Chemical Shifts and Coordination
Chemical Shifts for Compounds A-F
compound
δ (ppm)
∆δ (ppm)
C24H17PS3 (D)
C20H15PS (A)
trans-PdCl2D2 (E)
trans-PdCl2A2 (B)
cis-PtCl2D2 (F)
cis-PtCl2A2 (C)
-18.30
-16.56
13.19
14.53
5.34
31.49
31.09
23.64
24.18
Figure 3. Linear absorption spectra of the five compounds in
dichloromethane. The concentration is 1.0 × 10-5 mol/L for B, D,
E, F, and G. The inset shows the region centered around the
excitation wavelength of 532 nm.
7.62
indicating that only one of the geometrical isomers had formed.
The chemical shifts of the 31P{1H} NMR resonances of B and
E do not directly indicate whether the geometrical isomer was
cis or trans. However, the 31P NMR coordination chemical
shifts, ∆δ (δcoordinated - δfree), for both complexes are similar
to the 31P NMR coordination chemical shifts observed for other
trans-PdCl2(phosphine)2 complexes but approximately 10 ppm
smaller than those observed for cis-PdCl2(phosphine)2 com-
plexes.14 The 31P{1H} NMR spectra of the platinum complexes,
C and F, both exhibited a superimposed singlet and doublet,
indicating that only one of the geometrical isomers had formed.
The platinum complexes were assigned as cis-isomers based
on the 1J(195Pt,31P) of approximately 3700 Hz. This is consistent
with those observed for other cis-PtCl2(phosphine)2 complexes
and approximately 1000 Hz higher than those observed for
trans-PtCl2(phosphine)2 complexes.15
approximately 31 ppm, while those for the platinum complexes,
C and F, were both approximately 24 ppm. This suggests that
the changes in the SPS angles on going from the free to
coordinated ligand are very similar for ligands A and D and,
thus, that the differing substitution on the thienyl substituents
of ligands does not significantly affect the SPS of either the
free or coordinated ligands. This observation is consistent with
previous results that indicate that the number of thiophene units
in the oligothiophene substituent of Ph2P(C4H2S)nH ligands (n
) 1, 2, 3) has very little effect upon the chemical shift of the
free phosphine ligand.
Linear and Nonlinear Optical Properties of the Complexes
in Solution. Figure 3 shows the linear absorption spectra of
dichloromethane solutions of five of the compounds at equal
concentrations. The dichloromethane solvent has negligible
linear absorption in this spectral region. All of the compounds
show an absorption band between 250 and 400 nm. These bands
undergo a bathochromic shift with both phosphine functional-
ization and metal coordination of the phosphine. With phosphine
functionalization, red shifts of 12 and 17 nm, respectively, were
observed in terthiophene and thianaphthene. Another red shift
occurs upon coordination to a metal. For D the shift is about 8
nm (368 f 376 nm) for palladium (E) and about 34 nm (368
f 402 nm) for platinum (B). This behavior is consistent with
results that have been observed by other groups.25 For example,
The 31P NMR coordination chemical shift depends on two
main factors: (1) a decrease in the electron density at the
phosphorus nuclei due to the donation of the lone pair of
phosphorus to the metal, and (2) a decrease in electron density
at the phosphorus due to expansion of the substituent-
phosphorus-substituent (SPS) angle upon coordination.16 Thus
comparing the 31P NMR coordination shifts of ligands A and
D provides a comparison of the manner in which these ligands
coordinate to metal centers. The 31P NMR coordination chemical
shifts for the palladium complexes, B and E, were both

Phosphino-Oligothiophene Ligands
Organometallics, Vol. 25, No. 21, 2006 5049
Figure 5. (a) Open-aperture Z-scan signal for 1.07 × 10-3 mol/L
solution of trans-PdCl2D2 (E) in CH2Cl2 measured using the top-
hat Z-scan configuration. Note the weak, but measurable, nonlinear
absorption. (b) Normalized ratio of closed- to open-aperture Z-scan
signals for the same sample. The peak-before-valley structure
I
indicates that n2 is negative.
Figure 4. Nonlinear absorption scans of transmission as a function
of incident fluence through 2 mm samples of the complexes in
solution. The concentration is 1.1 × 10-3 mol/L in CH2Cl2 for D,
E, F, and G. For B, the concentration is 2.5 × 10-3 mol/L in CH2-
Cl2. The decrease in transmission at high incident fluence is a
characteristic signature of reverse saturable absorption.
absorption (rather than two-photon absorption), since some
ground-state absorption is required for excited-state absorption
to take effect. Excited-state absorption has been studied in
porphyrins,18 expanded porphyrins,9,10,19 and phthalocyanines,20-22
but to our knowledge this is the first observation of nonlinear
absorption of this type in polythiophene complexes.
density functional theory has demonstrated that the oligoth-
iophene LUMO is stabilized via interaction with the phospho-
rus.17 This is thought to be a result of π-d back-bonding
between the π-orbitals of the thienyl moiety and an empty
d-orbital on the phosphorus. These shifts, although small,
demonstrate the tuning ability of thienylphosphine systems,
moving the absorption band deeper into the visible region.
Nonlinear absorption measurements were also performed on
the same five compounds. Figure 4 shows the optical transmis-
sion through a 2 mm sample solution of each complex as a
function of incident fluence. The nonlinear absorption properties
of the complexes are shown to vary widely. Whereas B and D
exhibit simple linear transmission, E, F, and G show clear
reverse saturable absorption, with the transmission decreasing
with increasing incident fluence. With 2.3 J/cm2 incident fluence,
the transmission through 2 mm of E (trans-PdCl2(D)2) drops
from its linear value of approximately 90% to approximately
55%. In contrast, the nonlinear absorption behavior of F and G
is much weaker. Because B did not exhibit nonlinear optical
behavior, the nonlinear optical properties of A and C were not
studied.
Figure 4 shows that the coordination of B to a metal (either
Pd (E) or Pt (F)) or oxidation of the phosphorus in B (G)
increases the reverse saturable absorption properties of the
complex. Conversely, the dichloropalladium(II) complex of A
does not exhibit reverse saturable absorption. A comparison of
Figure 4 (the NLA results) to Figure 3 (the linear absorption of
the complexes) shows that the complexes that exhibit reverse
saturable absorption are also those in which the absorption band
has shifted to the red, causing some small linear absorption at
532 nm (as shown in the inset in Figure 3). This suggests that
the nonlinear absorption in Figure 4 is caused by excited-state
The nonlinear absorption effect can also be observed in the
open-aperture Z-scan data, as seen in Figure 5a. However, the
NLA data in Figure 4 are more useful because measurements
are made over a wider range of fluences, which allow the
observation of the fluence at which reverse saturable absorption
begins to be observed.
The refractive part of the optical nonlinearity is also probed
using Z-scan measurements. Figure 5b shows the ratio of the
closed-aperture to open-aperture scans as a function of position
Z for a 1.07 × 10-3 mol/L solution of E in CH2Cl2. The peak-
I
valley structure of the Z-scan signal indicates that n2 < 0.
Previous work from our group has shown that this is not solely
a thermal dη/dT effect but that some electronic contribution is
responsible for the observed nonlinearity.29 For the top-hat
Z-scan configuration used here, the magnitude of the nonlinear
refractive index can be calculated analytically using11,12
∆Tp-ν
2.7λ
2πI0Leff
I
|n2 | )
tanh-1
(1)
1.14
[
]
2.8(1 - S)
where ∆Tp-ν is the difference in normalized transmission
between the peak and valley of the Z-scan signal. The factor S
is the transmission of the closed aperture, λ is the wavelength,
and I0 is the intensity at the beam focus. The effective sample
length is given by Leff ) (1 - exp[-RsLs])/Rs, where LS is the
sample length and Rs is the linear absorption coefficient of the
sample. The intensity at the beam focus is given by I0
)
(29) Floyd, J. M.; Gray, G. M.; Spivey, A. G. V.; Lawson, C. M.;
Pritchett, T. M.; Ferry, M. J.; Hoffman, R. C.; Mott, A. G. Inorg. Chim.
Acta 2005, 358 (13), 3773-3785.

5050 Organometallics, Vol. 25, No. 21, 2006
Myrex et al.
Table 2. Linear Absorption Coefficient and Nonlinear
Refractive Index Values Measured at 532 nm with 7 ns
Pulses in CH2Cl2
spread out spatially in the far field after transmission through
the sample. By defocusing the beam in the far field, this
nonlinear refractive effect could conceivably enhance any
limiting of the transmitted fluence due to reverse saturable
absorption in the samples.
I
concentration
(mol L-1
R
n2
complex
)
(cm-1
)
(m2 W-1
)
I
trans-PdCl2A2 (B)
trans-PdCl2D2 (E)
C24H17PS3 (D)
cis-PtCl2D2 (F)
C24H17OPS3 (G)
2.5 × 10-3
1.1 × 10-3
1.1 × 10-3
1.1 × 10-3
1.1 × 10-3
0.07
0.30
0.07
0.15
0.34
|n2 | < 6 × 10-19
-1.8 × 10-18
Conclusion
I
|n2 | < 4 × 10-19
-1.5 × 10-18
I
|n2 | < 4 × 10-19
Both functionalization of terthiophene with a diphenylphos-
phino group to form D and the coordination of the diphe-
nylphosphino group of D to transition metals have significant
effects on the linear and third-order nonlinear optical properties
of the compounds. Bathochromic shifts in the λmax were as high
as 50 nm relative to the parent terthiophene molecule. Extending
the conjugation length of the thiophene segment and modifica-
tion of the substituents on phosphorus could allow for even
better tuning of the absorption window. Of particular interest
is that the palladium(II) and platinum(II) complexes of D are
the first transition metal complexes with phosphinothiophene
substituents that have been demonstrated to be active nonlinear
absorbers. Furthermore the third-order property responsible for
this behavior is reverse-saturable absorption rather than two-
photon absorption. Our preliminary results suggest that the
choosing of an appropriate metal center and increasing the
conjugation of the ligand should allow for the further optimiza-
tion of the NLO behavior in complexes of the type presented
herein.
2
x
πln2(n0E/w0 τ), where n0 is the index of refraction of the
solution, E is the pulse energy, and τ is the temporal pulse width.
The beam diameter is approximately w0 ) λf/d.12 In this work,
the index of refraction of the complexes in solution is assumed
to be the index of refraction of the solvent. (We use n0 for CH2-
Cl2 and n0 for CS2.23) Experimental accuracy is verified when
I
the n2 for CS2, measured under the same conditions, is in good
agreement with the literature value.24 Table 2 shows values of
n2I for solutions of five of the complexes, as well as the values
of Rs in each sample at 532 nm.
The results of the Z-scan measurements provide different and
complementary information to the NLA measurements in these
complexes. For example, although E, F, and G all exhibited
some nonlinear absorption in Figure 4, only E and F exhibit
measurable nonlinear refractive properties at these concentra-
tions. This suggests that although phosphine functionalization
changes the nonlinear absorptive properties of the terthiophene
ligand, it has a weaker effect on its nonlinear refractive
properties. When metal atoms are incorporated into the ligand,
the enhancement of the nonlinear absorption shown in Figure
4 depends strongly on which metal is added. However, the
Acknowledgment. The authors gratefully acknowledge
support from the Army Research Office under grant DAAD19-
03-1-0218, from the National Science Foundation under grant
EPS0447675, and from the University of Alabama-Birmingham
Faculty Development Grant to G.M.G. R.D.M. thanks the
Graduate School of the University of Alabama-Birmingham
for a Graduate Fellowship.
I
measured values of n2 for solutions of E and F are virtually
identical, suggesting that the nonlinear refractive properties are
much less dependent on the nature of the metal.
I
The fact that n2 is negative in these complexes indicates self-
defocusing in the solutions, meaning that a laser beam would
OM060224B