Organometallic complexes for non-linear optics. 51. second- and third-order non-linear optical properties of alkynylgold complexes

Article abstract of DOI:10.1071/CH12054

The alkynes HC≡CC₆H₂-2,6-Et₂-4- C≡CC₆H₄-4-NO₂ (4) and HC≡CC ₆H₄-4-C≡CC₆H₂-2,6-Et ₂-4-C≡CC₆H₄-4-NO₂ (6

Full text of DOI:10.1071/CH12054

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2012, 65, 834–841
Full Paper
http://dx.doi.org/10.1071/CH12054
Organometallic Complexes for Non-Linear Optics. 51.
Second- and Third-Order Non-Linear Optical Properties
of Alkynylgold Complexes*
A
A
B C
,
A
Adam Barlow, Bandar Babgi, Marek Samoc,
T. Christopher Corkery,
D
D
D
Stijn van Cleuvenbergen, Inge Asselberghs, Koen Clays,
Marie P. Cifuentes, and Mark G. Humphrey
A
A E
,
A
Research School of Chemistry, Australian National University, Canberra,
ACT 0200, Australia.
B
Laser Physics Centre, Research School of Physics and Engineering, Australian National
University, Canberra, ACT 0200, Australia.
C
Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology,
Wroclaw 50-370, Poland.
D
Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200D,
3001 Leuven, Belgium.
E
Corresponding author. Email: Mark.Humphrey@anu.edu.au
The alkynes HCꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2 (4) and HCꢀCC6H4-4-CꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2
(6) and gold alkynyl complexes Au{CꢀCC6H2-2,5-(OEt)2-4-CꢀCC6H4-4-NO2}(PPh3) (7), Au(CꢀCC6H2-2,6-Et2-4-
CꢀCC6H4-4-NO2)(PPh3) (8), and Au(CꢀCC6H4-4-CꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2)(PPh3) (9) have been syn-
thesized. The linear optical properties and quadratic optical non-linearities of 7–9 have been measured, the latter by hyper-
Rayleigh scattering at 1064 nm, and compared with data for the previously reported complexes Au(CꢀCC6H4-4-NO2)
(PPh3) (10) and Au(CꢀCC6H4-4-CꢀCC6H4-4-NO2)(PPh3) (11). The optical absorption maximum red-shifts and the first
hyperpolarizabilities increase on p-system lengthening and on introduction of electron-releasing substituents on the p-
bridge ring adjacent to the metal centre. The cubic non-linear optical properties of 1,4-{(PCy3)Au(CꢀC)}2C6H4 (12) and
{(PCy3)Au(CꢀC-4-C6H4CꢀC)}6C6 (13) have been assessed by wide spectroscopic range femtosecond Z-scan studies; the
maximal values of the imaginary component and the effective two-photon absorption cross-section increase markedly on
proceeding from linear complex 12 to 6-fold-symmetric complex 13, an increase that is maintained when data are scaled by
relative molecular weight.
Manuscript received: 30 January 2012.
Manuscript accepted: 20 February 2012.
Published online: 16 April 2012.
Introduction
assessing quadratic non-linearity (1064 nm, corresponding to
the fundamental of a Nd : YAG laser) occurs at 532 nm, which is
often where the more efficient metal alkynyl complexes possess
material resonances (or at least where the ‘tail’ of the corre-
sponding absorption bands occurs). Resonance enhancement and
self-absorption at the second-harmonic tend to obscure the out-
comes of NLO measurements, rendering extraction of structure–
property NLO relationships problematic. While attempts have
been made to circumvent the resonance enhancement problem
by application of the so-called two-level correction, its applica-
bility to organometallic complexes is likely to be limited.
Gold alkynyl complexes frequently possess the advantage
that their linear absorption maxima occur in the UV region – they
are essentially optically transparent at 532 nm. We have previ-
ously used this fact to assess the effect on optical non-linearity of
The optical non-linearities of metal alkynyl and vinylidene
complexes have attracted considerable attention[1] since the first
reports in the late 1980s and 1990s.[2] One recurring theme in
photonic materials design has been the pursuit of molecular
structure–non-linear optical (NLO) property relationships to
define the key structural elements leading to significant non-
linearity, the idea being that, armed with such relationships, one
can design and construct highly efficient NLO materials. An
early observation was that increasing the metal valence electron
count resulted in an increase in non-linearity.[3] As a result,
studies of the NLO properties of 14 valence electron (phosphine)
gold alkynyl complexes[4] have been somewhat neglected in
favour of 18 valence electron metal alkynyl complexes. How-
ever, the second-harmonic of the most popular wavelength for
*Dedicated to Professor Allan White, premier chemist, prolific crystallographer, and polymath, with thanks for always stepping into the breach of local
diffractometer downtime, and for the crucial structural underpinnings to our first forays in this field.
Journal compilation Ó CSIRO 2012
www.publish.csiro.au/journals/ajc

Organometallic Complexes for Non-Linear Optics
835
specific bridge modifications in gold alkynyl complexes of
a donor–bridge–acceptor composition[5]; while absolute non-
linearities are lower than those found for the analogous 18
valence electron metal alkynyl complexes, the gold alkynyl
complexes possess the advantage in facilitating the development
of structure–property relationships. We have now extended these
gold alkynyl complex NLO studies to probe the influence on
quadratic non-linearity of varying the electron richness at the
bridge by introduction of alkyl and alkoxy groups. We also report
the first broad spectroscopic range femtosecond Z-scan study of
the cubic non-linearities of gold alkynyl complexes.
3-sector VG Autospec mass spectrometer. Electrospray ioni-
zation (ESI) mass spectra were recorded using a Micromass/
Water’s LC-ZMD single quadrupole liquid chromatograph–
mass spectrometer and high resolution ESI mass spectra were
carried out utilizing a Bruker Apex 4.7T FTICR-MS instrument.
All mass spectrometry peaks are reported as m/z (relative
intensity, assignment).
Synthesis of 2,6-Diethyl-1,4-diodobenzene (1)
Schlenk techniques were not used for this preparation. 2,6-
Diethyl-4-iodoaniline (10 g, 36.5 mmol) was dissolved in a
H2SO4/H2O mixture (1 : 1, 200 mL) and cooled in an ice bath.
NaNO2 (2.6 g, 37.6 mmol) in water (5 mL) was added dropwise
maintaining the temperature below 58C. The mixture was then
allowed to stir for 30 min before the dropwise addition of a
solution of KI (6.5 g, 39.2 mmol) in H2O (10 mL). The resultant
mixture was stirred overnight at room temperature before
extraction with CH2Cl2. The organic layer was washed with H2O
followed by aqueous Na2S2O3 solution, dried, and the solvent
removed under vacuum. The residue was passed through a short
pad of silica, eluting with petrol, and the solvent was removed
from the eluate, to yield 2,6-diethyl-1,4-diiodobenzene 1 as a red
liquid (8.8g, 62 %). m/z (EI) 385 (100 %, Mþ), 370 (43, [M ꢁ
Me]þ). m/z (HR EI) 385.9031; C10H12I2 requires 385.9029. dH
(400 MHz, CDCl3) 1.20 (t, JHH 7.6, 6H, Me), 2.73 (q, JHH 7.6,
4H, CH2), 7.28 (s, 2H, H11). dC (101.5 MHz, CDCl3) 14.4 (CH3),
35.0 (CH2), 94.3 (C12), 106.8 (C9), 134.6 (C11), 149.3 (C10).
Experimental
General Remarks
All reactions were performed under a nitrogen atmosphere with
the use of Schlenk techniques unless otherwise stated. Dichloro-
methane was dried over calcium hydride before distillation; all
other solvents were used as received. ‘Petrol’ was a petroleum
fraction of boiling range 60–808C. Chromatography was per-
formed on ungraded basic alumina or Merck silica gel 60 (70-230
ASTM). The following were prepared by literature procedures:
AuCl(PPh3),[6] HCꢀCC6H2-2,5-(OEt)2-4-CꢀCC6H4-4-NO2,[7]
2,6-diethyl-4-iodoaniline,[8] 4-HCꢀCC6H4NO2,[9] 1,4-{(PCy3)
Au(CꢀC)}2C6H4,[10] and {(PCy3)Au(CꢀC-4-C6H4CꢀC)}6C6.[11]
Methods and Instrumentation
Microanalyses were carried out by the Microanalysis Service
Unit at the Australian National University. UV-Vis spectra of
solutions in 1 cm quartz cells were recorded using a Cary 5
spectrophotometer; bands are reported in the form frequency
(cmꢁ1) (extinction coefficient, 104 Mꢁ1 cmꢁ1). Infrared spectra
were recorded as dichloromethane solutions using a Perkin–
Elmer System 2000 FT-IR; peaks are reported in cmꢁ1. 1H (300
or 400 MHz), 13C (75 or 101.5 MHz), and 31P NMR (121 MHz
or 161 MHz) spectra were recorded using an Inova-300 NMR,
a Varian Mercury-300 FTNMRora Varian400MRspectrometer.
The spectra are referenced to residual chloroform (7.26 ppm),
CDCl3 (77.0 ppm), or external H3PO4 (0.0 ppm), respectively;
atom labelling follows the numbering scheme in Chart 1.
Electron impact (EI) mass spectra were recorded using a VG
Quattro II triple quadrupole mass spectrometer and high reso-
lution (HR) EI mass spectra were recorded using a Fisons
Synthesis of IC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2 (2)
2,6-Diethyl-1,4-diiodobenzene (1) (0.10 g, 0.26 mmol) and
4-ethynyl-1-nitrobenzene (0.040g, 0.27 mmol) were added to a
mixture of triethylamine (10 mL) and CH2Cl2 (5 mL). Catalytic
quantities of cis-[PdCl2(PPh3)2] (5.9mg, 0.008 mmol) and CuI
(1.2mg, 0.008 mmol) were added and the mixture was stirred for
18 h at room temperature. The solvent was removed under vac-
uum and the residue was passed through a short pad of silica,
eluting with petrol. The eluate solvent was reduced in volume to
give the product 2 as a yellow solid (0.080g, 76%). m/z (EI) 405
(100%, Mþ), 390 (23, [Mꢁ Me]þ). m/z (HR EI) 405.0228;
C18H16INO2 requires 405.0226. (Found: C 53.38, H 4.10, N 3.44.
Anal. Calc. for C18H16INO2: C 53.35, H 3.98, N 3.46%.) dH
(400MHz, CDCl3) 1.24 (t, JHH 7.6, 6H, Me), 2.81 (q, JHH 7.6, 4H,
O
C12 C13
C4 C5
C1 C2 C3
C8 C7
Ph2P Au
C6 C9 C10 C11
C14 NO2
Ci
O
Co
Cm
Cp
C16 C17
C12 C13
C14 C15 C18 NO2
C4 C5
C10 C11
C9
Ph2P Au C1 C2 C3
C6 C7 C8
Ci
Co
Cp Cm
Chart 1. Atom labelling for NMR assignments.

836
A. Barlow et al.
CH2), 7.23 (s, 2H, H11), 7.66 (AA0BB0, 2H, H16), 8.19 (AA0BB0,
2H, H17). dC (101.5 MHz, CDCl3) 14.4 (CH3), 35.4 (CH2), 87.9
(C14), 94.2 (C13), 108.9 (C9), 121.8 (C12), 123.6 (C17), 128.7
(C16), 130.1 (C15), 132.2 (C11), 146.9 (C18), 147.7 (C10).
14.6 (CH3), 27.9 (CH2), 88.1 (C14), 88.6 (C8), 95.1 (C13),
96.5 (C2), 98.7 (C7), 104.6 (C1), 121.7 (C9), 121.8 (C12),
123.2 (C6), 123.5 (C3), 123.8 (C17), 128.7 (C16), 130.3 (C15),
131.2 (C4), 132.1 (C5), 132.3 (C11), 146.8 (C10), 147.0 (C18).
Synthesis of Me3SiCꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2 (3)
Synthesis of HCꢀCC6H4-4-CꢀCC6H2-2,6-Et2-4-
CꢀCC6H4-4-NO2 (6)
Catalytic quantities of cis-[PdCl2(PPh3)2] (5.9 mg, 0.008 mmol)
and CuI (1.2 mg, 0.008 mmol) were added to a solution of
IC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2 (2) (0.8 g, 1.9 mmol) in
deoxygenated triethylamine (15 mL). Ethynyltrimethylsilane
(0.3 mL, 2.1 mmol) was then added and the solution was stirred
at room temperature for 18 h. The solvent was removed and the
residue passed through a short pad of silica, eluting with petrol.
The eluate solvent was reduced in volume, to give the product 3
as a yellow solid (0.60 g, 81 %). m/z (EI) 375 (100 %, Mþ), 360
(79, [M ꢁ Me]þ). m/z (HR EI) 375.1655; C23H25NO2Si requires
375.1650. (Found: C 73.25, H 6.97, N 3.99. Anal. Calc. for
C23H25NO2Si: C 73.56, H 6.71, N 3.73 %.) dH (400 MHz,
CD2Cl2) 0.27 (s, 9H, SiMe3), 1.26 (t, JHH 9.6, 6H, Me), 2.83 (q,
JHH 9.6, 4H, CH2), 7.29 (s, 2H, H11), 7.69 (AA0BB0, 2H, H16),
8.23 (AA0BB0, 2H, H17). dC (101.5 MHz, CDCl3) 0.4 (SiMe3),
14.8 (CH3), 28.3 (CH2), 88.9 (C14), 95.6 (C13), 101.5 (C8), 104.8
(C7), 121.0 (C12), 123.4 (C9), 124.1 (C17), 129.1 (C16), 130.7
(C15), 132.7 (C11), 147.4 (C18), 147.6 (C10).
Me3SiCꢀCC6H4-4-CꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2 (5)
(0.25 g, 0.53 mmol) was dissolved in distilled CH2Cl2 (20 mL)
and NBun4F (1 M solution in THF, 0.2 mL) was added. The
mixture was stirred for 2 h. The solvent was then removed and
the residue was passed through a short pad of silica, eluting with
2 : 1 petrol/CH2Cl2. The eluate solvent was reduced in volume,
affording the product 6 as a bright yellow solid (0.12 g, 82 %).
m/z (EI) 403 (100 %, Mþ), 373 (65, [M ꢁ 2Me]þ). m/z (HR EI)
403.1573; C28H21NO2 requires 403.1572. (Found: C 83.31,
H 5.02, N 3.25. Anal. Calc. for C28H21NO2: C 83.35, H 5.25,
N 3.47 %.) dH (300 MHz, CDCl3) 1.32 (t, JHH 7.8, 6H, Me), 2.90
(q, JHH 7.8, 4H, CH2), 3.20 (s, 1H, HCꢀ), 7.30 (s, 2H, H11), 7.48
(s, 4H, H4 þ H5), 7.67 (AA0BB0, 2H, H16), 8.23 (AA0BB0, 2H,
H17). dC (101.5 MHz, CDCl3) 14.5 (CH3), 27.9 (CH2), 79.0 (C1),
83.2 (C2), 88.1 (C14), 88.6 (C8), 95.0 (C13), 98.7 (C7), 121.7 (C9),
122.0 (C12), 122.6 (C6), 123.6 (C17), 123.8 (C3), 128.7 (C16),
130.1 (C15), 131.2 (C4), 132.1 (C5), 132.3 (C11), 146.8 (C10),
147.0 (C18).
Synthesis of HCꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2 (4)
Synthesis of Au{CꢀCC6H2-2,5-(OEt)2-4-
CꢀCC6H4-4-NO2}(PPh3) (7)
Me3SiCꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2 (3) (22 mg, 0.07
mmol) was dissolved in a deoxygenated mixture of CH2Cl2 and
MeOH (1 : 1, 50 mL). K2CO3 (20 mg, 0.14 mmol) was added
and the resultant mixture was stirred for 2 h. The solvent was
then removed under vacuum and the residue was passed through
a short pad of silica, eluting with 1 : 1 petrol/CH2Cl2. The eluate
solvent was reduced in volume, to afford the product 4 as
a bright yellow solid (20 mg, 93 %). m/z (EI) 303 (100 %, Mþ).
m/z (HR EI) 303.1260; C20H17NO2 requires 303.1259. (Found:
C 78.71, H 5.81, N 4.44. Anal. Calc. for C20H17NO2: C 79.19,
H 5.65, N 4.62 %.) dH (400MHz, CDCl3) 1.25 (t, JHH 8, 6H, Me),
2.82 (q, JHH 8, 4H, CH2), 3.55 (s, 1H, HCꢀ), 7.24 (s, 2H, H11),
7.64 (AA0BB0, 2H, H16), 8.20 (AA0BB0, 2H, H17). dC
(101.5 MHz, CDCl3) 14.5 (CH3), 27.7 (CH2), 79.9 (C7), 86.6
(C8), 88.4 (C14), 94.8 (C13), 121.8 (C12), 121.9 (C9), 123.6 (C17),
128.5 (C16), 130.1 (C15), 132.2 (C11), 146.9 (C18), 147.4 (C10).
A solution of NaOMe in MeOH (3.5 mL, 0.5 M) was added
to a solution of HCꢀCC6H2-2,5-(OEt)2-4-CꢀCC6H4-4-NO2
(61.3 mg, 0.183 mmol) and AuCl(PPh3) (84.4 mg, 0.171 mmol)
in CH2Cl2 (10 mL). The mixture was stirred at room temperature
for 20 h and then reduced in volume. The precipitate was col-
lected, washed with MeOH (20 mL) and then petrol (20 mL), to
afford the product as an orange powder (102.3 mg, 76 %). m/z
(HR EI) 793.1660; C38H31AuNO4P requires 793.1656. (Found:
C 57.56, H 4.23, N 1.88. Anal. Calc. for C38H31AuNO4P:
C 57.51, H 3.94, N 1.76 %.) lmax/cmꢁ1 (e/104 Mꢁ1 cmꢁ1) 24750
(4.1), 32360 (4.3). nmax (CH2Cl2)/cmꢁ1 2102 n(CꢀC). dH
(300 MHz, CDCl3) 1.47 (m, 6H, CH3), 4.10 (m, 4H, OCH2), 6.95
(s, 1H, H8), 7.08 (s, 1H, H5), 7.46–7.59 (m, 15H, PPh3), 7.64
(AA0BB0, H12), 8.20 (AA0BB0, H13). dC (101.5 MHz, CDCl3)
14.9 (CH3), 64.8, 65.02 (OCH2), 92.2 (C10), 110.4, 117.4 (C3,
C6), 116.7, 118.3 (C5, C8), 123.6 (C13), 129.2 (d, JCP 18, Cm),
131.5 (Cp), 134.3 (d, JCP 22, Co) (PPh3), 132.0 (C12), 146.7
(C14), 153.6 (C4, C7) C1, C2, Ci, C9 not observed. dP 43.1.
Synthesis of Me3SiCꢀCC6H4-4-CꢀCC6H2-2,6-Et2-4-
CꢀCC6H4-4-NO2 (5)
HCꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2 (4) (0.41 g, 1.35 mmol)
was dissolved in
a deoxygenated mixture of CH2Cl2
Synthesis of Au(CꢀCC6H2-2,6-Et2-4-
CꢀCC6H4-4-NO2)(PPh3) (8)
(15 mL) and triethylamine (30 mL) containing 1-iodo-4-
trimethylsilylethynylbenzene (0.39 g, 1.36 mmol). A catalytic
amount of cis-[PdCl2(PPh3)2] (16 mg, 0.02 mmol) and CuI
(4.1 mg, 0.019 mmol) was added and the resultant mixture was
stirred at room temperature for 18 h. The solvent was then re-
moved under reduced pressure and the residue passed through a
short pad of silica, eluting with petrol. The eluate solvent was
reduced in volume, to afford the product 5 as a yellow solid
(0.49g, 96%). m/z (EI) 475 (45 %, Mþ), 445 (100, [Mꢁ 2Me]þ).
m/z (HR EI) 475.1968; C31H29NO2Si requires 475.1267. (Found:
C 77.98, H 6.02, N 2.79. Anal. Calc. for C31H29NO2Si: C 78.28,
H 6.15, N 2.94 %.) dH (400MHz, CDCl3) 0.26 (s, 9H, SiMe3),
1.34 (t, JHH 7.6, 6H, Me), 2.90 (q, JHH 7.6, 4H, CH2), 7.30 (s, 2H,
H11), 7.45 (s, 4H, H4 þ H5), 7.67 (AA0BB0, 2H, H16),
8.31 (AA0BB0, 2H, H17). dC (101.5 MHz, CDCl3) 0.4 (SiCH3),
Sodium (0.05 g, 2.17 mmol) was added to MeOH (25 mL). When
it had completely reacted, AuCl(PPh3) (0.16 g, 0.33 mmol) and
HCꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2 (4) (0.1 g, 0.33 mmol)
were added. The resultant mixture was stirred at room temper-
ature for 18 h, after which the solvent volume was reduced to
5 mL and the resulting yellow precipitate collected and washed
with n-pentane, to give Au(CꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-
NO2)(PPh3) as a yellow solid (0.20 g, 81 %). m/z (HR EI)
762.1835; C38H32AuNO2P requires 762.1836. (Found: C 59.55,
H 4.01, N 2.00. Anal. Calc. for C38H32AuNO2P: C 59.85, H 4.23,
N 1.84 %.) lmax/cmꢁ1 (e/104 Mꢁ1 cmꢁ1) 27300 (3.1), 32460
(3.1). nmax (CH2Cl2)/cmꢁ1 2105 n(CꢀC). dH (400 MHz, CDCl3)
1.31 (t, JHH 7.6, 6H, Me), 2.98 (q, JHH 7.6, 4H, CH2), 7.25 (s, 2H,

Organometallic Complexes for Non-Linear Optics
837
H11), 7.44–7.59 (m, 15H, PPh3), 7.64 (AA0BB0, 2H, H16), 8.24
(AA0BB0, 2H, H17). dC (101.5 MHz, CDCl3) 14.5 (CH3), 27.8
(CH2), 87.7 (C14), 96.1 (C13), 119.2 (C12), 123.6 (C17), 125.4
(C9), 128.0 (C16), 129.1 (d, JCP 11.7, Cm), 129.6 (Ci), 130.1 (C15),
130.7 (C8), 131.5 (Cp), 132.1 (C11), 134.3 (d, JCP 14.2, Co), 146.7
(C10), 147.0 (C18). C7 not observed. dP (161 MHz, CDCl3) 42.1.
decomposition. The experiments were carried out at several
wavelengths in the range 530–1500 nm. To obtain the relevant
wavelengths, the optical parametric amplifier was tuned appro-
priately and one of the following modes of output was selected:
idler-pump mixing, signal doubling, idler doubling, or the signal.
The unwanted components of the TOPAS output were discarded
through the use of polarizing optics, colour glass filters, and
spatial filtering.
Synthesis of Au(CꢀCC6H4-4-CꢀCC6H2-2,6-Et2-4-
CꢀCC6H4-4-NO2)(PPh3) (9)
The laser beam was attenuated to energies in the mJ pulseꢁ1
range and directed through a standard Z-scan setup equipped
with a beam splitter, allowing one to record open-aperture and
closed-aperture Z-scans simultaneously. The beam was focussed
so as to provide a spot size in the range w0 E 40–80 mm, ensuring
that the Rayleigh range zR ¼ pw20/l was always larger than the
total thickness of the sample (E3 mm which includes two glass
walls and the solution inside the cell). The cell travel was
z ¼ ꢁ40 to 40 mm and the data were recorded using three Si or
InGaAs photodiodes monitoring the input pulse energy, the
open-aperture signal, and the closed-aperture signal, respectively.
The outputs were fed into three channels of a boxcar averager,
which was GPIB-interfaced with a data collection computer.
The shapes of the closed-aperture scans, open-aperture scans,
and the curves obtained by dividing a closed-aperture curve by
an open-aperture curve were analyzed with the help of a custom
fitting program that used equations derived by Sheik-Bahae
et al.[14] to calculate the theoretical curves. In this way, the
values of the real and imaginary part of the non-linear phase shift
DF0 (corresponding to the refractive and absorptive non-linear-
ity, respectively) could be obtained. At each wavelength the
energy of the laser pulses was adjusted, based on closed-aperture
Z-scans obtained for 1 mm cells filled with the solvent alone
and/or scans for a 3 mm thick silica glass plate. Typically, we
required DF0 for such scans to be ,0.5–1 rad. The non-linear
refractive index of silica, n2, is known across a wide wavelength
range, so the real part of the non-linear phase shift can be used to
calculate the light intensity. From the comparison of n2 values
Sodium (0.06 g, 2.17 mmol) was added to MeOH (15 mL). After
reaction was complete, AuCl(PPh3) (0.32 g, 0.64 mmol) and
HCꢀCC6H4-4-CꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2 (6) (0.28 g,
0.69 mmol) were added. The resultant mixture was stirred at
room temperature for 18 h, after which the solvent volume was
reduced to 5 mL and the resulting yellow precipitate collected.
The product was further purified by trituration with n-pentane to
give Au(CꢀCC6H4-4-CꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2)
(PPh3) as a yellow solid (0.50 g, 90 %). m/z (HR EI) 862.2149;
C46H36AuNO2P require 862.2165. (Found: C 64.58, H 4.27,
N 1.29. Anal. Calc. for C46H36AuNO2P: C 64.12, H 4.09,
N1.63 %.)lmax/cmꢁ1 (e/104 Mꢁ1 cmꢁ1)27600(4.4),29500(3.5).
nmax (CH2Cl2)/cmꢁ1 2103 n(CꢀC). dH (400 MHz, CDCl3) 1.28
(t, JHH 7.2, 6H, Me), 2.86 (q, JHH 7.2, 4H, CH2), 7.22 (s, 2H,
H11), 7.38–7.55 (m, 19H, PPh3, H4 and H5), 7.63 (AA0BB0, 2H,
H16), 8.19 (AA0BB0, 2H, H17). dC (101.5 MHz, CDCl3) 14.5
(CH3), 27.9 (CH2), 87.7 (C14), 88.4 (C8), 95.2 (C13), 99.4 (C7),
121.3 (C9), 121.5 (C12), 123.1 (C6), 123.7 (C17), 125.1 (C3),
128.7 (C16), 129.1 (d, JCP 11, Cm), 129.4 (Ci), 129.9 (C2), 130.3
(C15), 131.0 (C4), 131.6 (Cp), 132.2 (C5), 132.3 (C11), 134.2
(d, JCP 13.4, Co), 146.6 (C10), 146.9 (C22). C1 not observed.
dP (161MHz, CDCl3) 42.9.
Hyper-Rayleigh Scattering (HRS) Measurements
This technique is described in detail elsewhere.[12] Briefly, an
injection seeded Nd : YAG laser (Q-switched Nd : YAG Quanta
Ray GCR, 1064 nm, 8 ns pulses, 10 Hz) was focussed into a
cylindrical cell (7 mL) containing the sample. The intensity of
the incident beam was varied by rotation of a half-wave plate
placed between crossed polarizers. Part of the laser pulse was
sampled by a photodiode to measure the vertically polarized
incident light intensity. The frequency doubled light was col-
lected by an efficient condenser system and detected by a pho-
tomultiplier. The harmonic scattering and linear scattering were
distinguished by appropriate filters; gated integrators were used
to obtain intensities of the incident and harmonic scattered light.
The absence of a luminescence contribution to the harmonic
signal was confirmed by using interference filters at different
wavelengths near 532 nm. All measurements were performed in
THF using 4-nitroaniline (b ¼ 21.4 ꢂ 10ꢁ30 esu[13]) as a refer-
ence. Solutions were sufficiently dilute that absorption of scat-
tered light was negligible.
for silica compiled by Milam,[15] we employed[16]
:
n2;silica ðnÞ ¼ 2:8203 ꢂ 10ꢁ16 ꢁ 3 ꢂ 10ꢁ21n
þ 2 ꢂ 10ꢁ25n2 ðcm2 Wꢁ1
Þ
for the spectroscopic dependence of n2 for silica, where n ¼ 1l is
the wavenumber expressed in cmꢁ1. This equation is a good
approximation for values of n2,silica in the range of l ¼ 500–
1500 nm. (Note that many of our previous papers assumed
n2,silica ¼ 3 ꢂ 10ꢁ16 cm2 Wꢁ1 independent of the wavelength,
which is also a reasonably good approximation.) We employed
intensities of the order of 50–100 GWcmꢁ2. Since our measure-
ments were always uniformly calibrated to the non-linearity of
silicaglass, thisavoidedthe needfor detailed information of beam
geometry and pulse shape.
The real and imaginary parts of the second hyperpolariz-
ability, g, of the solutes were computed assuming additivity of
the non-linear contributions of the solvent and the solute and the
applicability of the Lorentz local field approximation.[16] The
values of the two-photon absorption (TPA) cross-section s2
were computed from the absorptive part of the non-linearity
determined from the Z-scans. In all cases, errors of the relevant
quantities were estimated from the assessed accuracies of the
parameters for the fitting of Z-scans for the solutions and
the corresponding scans for the solvent.
Z-Scan Studies
The compounds were dissolved in dichloromethane at con-
centrations of 0.76 and 0.327 % w/w for 12 and 13, respectively,
and the solutions were placed in 1 mm path length Starna glass
cells, stoppered, and sealed with Teflon tape. The Z-scans
were carried out using a femtosecond laser system composed of
a Clark-MXR CPA-2001 regenerative amplifier acting as a
775 nm pump and a Light Conversion TOPAS optical parametric
amplifier. The system was operated at a repetition rate of 250 Hz
to minimize the influence of thermal effects and photochemical

838
A. Barlow et al.
SiMe3
NH2
l
l
(i) NaNO2/H2SO4
H2O
NO2
cat. PdII/CI
SiMe3
cat. PdII/CuI
NEt3
(ii) KI, H2O
I
I
NEt3, CH2Cl2
62 %
1
NO2
NO2
76 %
2
81 %
3
SiMe3
SiMe3
l
NBun4F
CH2Cl2
K2CO3
CH2Cl2/MeOH
cat. PdII/CI
NEt3
NO2
82 %
NO2
NO2
6
96 %
5
93 %
4
Scheme 1.
Results and Discussion
band in gold alkynyl complexes has been previously assigned as
metal-to-ligand charge transfer (MLCT)-like in nature, and is
accompanied by higher-energy bands assigned to s(Au’P) -
p*(PPh).[5] For this series of complexes, the MLCT band is
intense (e ¼ 25000–40000 Mꢁ1 cmꢁ1) and red-shifts on the
p-system lengthening from a phenylethynyl ligand to a longer
ligand (proceeding from 10 to 11) and increasing the electron
richness of the phenyl ring adjacent to the metal centre (pro-
ceeding from 11 to 8 and then 7); p-system lengthening together
with incorporation of the electron donating substituents else-
where in the alkynyl ligand (proceeding from 11 or 8 to 9) does
Extensions of established organic synthetic procedures were
employed to synthesize the new acetylenes. Diazotization of
2,6-diethyl-4-iodoaniline and subsequent iodination of the
intermediate diazonium salt afforded the iodo derivative 1 as a
pale orange oil in good yield (Scheme 1). Sonogashira coupling
of 1 with 4-ethynyl-1-nitrobenzene occurred at the 4-iodo
position and afforded 2 exclusively, the 1-iodo site remaining
untouched. A subsequent Sonogashira coupling with the more
reactive alkyne ethynyltrimethylsilane gave 3 in excellent
yield. The trimethylsilyl protecting group was removed using
mild base to afford 4. Subsequent Sonogashira coupling of 4
with 1-iodo-4-trimethylsilylethynylbenzene gave 5, which was
smoothly desilylated with fluoride to afford 6. The mass spectra
of compounds 1–6 all contain strong molecular ions, while the
1H NMR spectra of the terminal acetylenes 4 and 6 contain
characteristic CꢀCH resonances at 3.55 and 3.20 ppm,
respectively.
The syntheses of the gold alkynyl complexes are summarized
in Scheme 2, employing a similar procedure to that used for the
synthesis of the analogous 4-nitrophenylethynyl complex.[5]
Complexes 7–9 were characterized by a combination of IR,
1H NMR, 13C NMR, 31P NMR, and UV-vis spectroscopies, ESI
mass spectrometry, and satisfactory microanalyses. Character-
istic n(CꢀC) bands are seen in the IR spectra between 2103 and
2105 cmꢁ1, singlet 31P NMR resonances are found in the range
42.1–43.1 ppm, while the high-resolution ESI mass spectra
contain peaks assigned to molecular ions.
not result in a red-shift of lmax
.
We have determined the quadratic hyperpolarizabilities of
7–9 by HRS at 1064 nm; the data are given in Table 1, together
with the earlier results for 10 and 11 and the two-level corrected
non-linearities. The last-mentioned seeks to approximate the
zero-frequency b value by assuming that in such donor–bridge–
acceptor complexes there is one important charge-transfer
transition (see footnote to Table 1). The utility of such an
approximation is questionable – with metal alkynyl complexes,
there is likely to be more than one low-energy band that results in
significant charge displacement – but it does provide a useful
point of comparison and its use in the literature is widespread.
The shortcomings of the two-level model have been discussed in
more detail elsewhere.[17]
For this series of complexes, p-bridge lengthening (proceed-
ing from 10 to 11 and then to 9) results in increases in quadratic
non-linearity. Introduction of electron-donating substituents at
the ring adjacent to the metal centre (proceeding from 11 to 8
and then to 7) also results in increased molecular first hyperpo-
larizability. It is tempting to ascribe the significant increase in
proceeding from 8 to 7 to the stronger donor nature of alkoxy
The optical absorption maxima for 7–9 are summarized
in Table 1, together with those of the related previously
reported complexes Au(CꢀCC6H4-4-NO2)(PPh3) (10) and
Au(CꢀCC6H4-4-CꢀCC6H4-4-NO2)(PPh3) (11). The low-energy

Organometallic Complexes for Non-Linear Optics
839
OEt
OEt
NO2
OEt
Ph3PAu
NO2
NaOMe/MeOH/CH2CI2
OEt
76 % 7
NO2
AuCI(PPh3)
Ph3PAu
Ph3PAu
NO2
NaOMe/MeOH
8
81 %
NO2
NO2
NaOMe/MeOH
90 % 9
Scheme 2.
Table 1. Experimental linear optical and hyper-Rayleigh scattering-derived non-linear optical response parametersA
Complex
l
max [nm]
(e [104 Mꢁ1 cmꢁ1])
b1064
[10ꢁ30 esu]A
b0
[10ꢁ30 esu]B
Ref.
Au(CꢀCC6H4-4-NO2)(PPh3) (10)
338 (2.5)
362 (3.6)
404 (4.1)
366 (3.1)
362 (4.4)
22 ꢃ 3
59 ꢃ 9
12 ꢃ 2
28 ꢃ 4
[5]
[5]
Au(CꢀCC6H4-4-CꢀCC6H4-4-NO2)(PPh3) (11)
Au{CꢀCC6H2-2,5-(OEt)2-4-CꢀCC6H4-4-NO2}(PPh3) (7)
Au(CꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2)(PPh3) (8)
Au(CꢀCC6H4-4-CꢀCC6H2-2,6-Et2-4-CꢀCC6H4-4-NO2)(PPh3) (9)
414 ꢃ 65
180 ꢃ 25
130 ꢃ 50
150 ꢃ 23
83 ꢃ 11
65 ꢃ 25
This work
This work
This work
AConditions: measurements were carried out in THF; all complexes are optically transparent at 1064 nm.
BCorrected for resonance enhancement at 532 nm using the two-level model with b0 ¼ b[1 ꢁ (2lmax/1064)2][1 ꢁ (lmax/1064)2].
versus alkyl (and, in particular, while the latter has a stronger
favourable inductive effect, the former has a positive mesomeric
effect which will more strongly influence the p-system), but the
differing substitution sites of these substituents preclude further
comment. The influence of these electron donating solubilizing
substituents is highlighted in proceeding from 8 to 9; while an
increase in non-linearity may have been anticipated on this
p-system lengthening, it seems clear that these substituents must
be located at the ring with the appended (phosphine)gold-
ethynyl donor group for the most favourable combination.
The increasing availability of tunable femtosecond light
sources has afforded the opportunity to assess NLO properties
across broad spectroscopic ranges. While several reports of
the cubic non-linearities of ruthenium alkynyl complexes
over a wide spectroscopic range have appeared,[7,18] there
have been no studies of gold alkynyl complexes thus far. We
have, therefore, examined two examples: the centrosymmetric
complex 1,4-{(PCy3)Au(CꢀC)}2C6H4 (12)[10] and the 6-fold
symmetric complex {(PCy3)Au(CꢀC-4-C6H4CꢀC)}6C6 (13).[11]
Fig. 1 contains the hyperpolarizability data for 12 and 13. The
short wavelength cut-off for these studies was chosen to ensure
that data were collected before the onset of one-photon absorp-
tion. The refractive components of the non-linearities peak at
approx. 700 and 900 nm (12) and 600 and 900 nm (13), but the
maximal values for the 6-fold symmetric species 13 are more
than an order of magnitude greater than those of the linear
complex 12. The significant errors associated with the real
components, which arise because of contributions to the mea-
sured response from the cell walls and solvent, render further
discussion unwarranted. The difference in performance of the
two complexes is even more stark when considering the imagi-
nary components, which are available with considerably more
precision because the cells and solvent do not contribute to the
experimental outcome: while 12 shows minimal activity (maxi-
mal value at ,600 nm), the maximal value for 13 is more than
two orders of magnitude greater.
Fig. 2 shows the absorptive non-linearities replotted as
the effective two-photon absorption cross-sections. Both com-
plexes reveal maxima at ,600 nm, mirroring the maxima for the
imaginary components of the non-linearities displayed in Fig. 1.
The increase in maximal value (12: 18 GM; 13: 9000 GM) is
enormous, and this significant increase is maintained when the
data are corrected for relative molecular weights (s2/M values
0.017 and 2.4, respectively) or the number of phenylethynyl
units in the bridge.
Conclusion
Quadratic optical non-linearities are still often assessed at
1064 nm because of the widespread availability of Nd : YAG

840
A. Barlow et al.
(b)
3.0Eꢀ32
2.0Eꢀ32
1.0Eꢀ32
0Eꢁ00
(a)
1.0Eꢀ33
5.0Eꢀ34
0Eꢁ00
Imag. part
Real part
Imag. part
Real part
ꢀ5.0Eꢀ34
ꢀ1.0Eꢀ33
ꢀ1.0Eꢀ32
500
700
900
1100
1300
1500
1700
500
700
900
1100
1300
1500
1700
Wavelength [nm]
Wavelength [nm]
Fig. 1. Comparison of the dispersion of the complex hyperpolarizability of (a) [1,4-{(PCy3)Au(CꢀC)}2C6H4] (12) and (b) [{(PCy3)Au(CꢀC-4-
C6H4CꢀC)}6C6] (13). Solid squares: real component; solid diamonds: imaginary component.
30
20
10
0
12000
10000
8000
6000
4000
2000
0
(a)
(b)
σ
σ
500
700
900
1100
1300
1500
1700
500
700
900
1100
1300
1500
1700
Wavelength [nm]
Wavelength [nm]
Fig. 2. Comparison of the two-photon absorption dispersion of (a) [1,4-{(PCy3)Au(CꢀC)}2C6H4] (12) and (b) [{(PCy3)Au(CꢀC-4-C6H4CꢀC)}6C6] (13).
s2: two-photon absorption (TPA) cross-section; GM: Goeppert-Mayer units ¼ 10ꢁ50 cm4 s moleculesꢁ1 photonꢁ1
.
lasers. While a major focus of organometallic NLO materials
research has been maximizing the values of the relevant
coefficients, the proximity of the absorption bands of many
complexes to 532 nm, the second-harmonic wavelength of a
Nd : YAG laser, has resulted in difficulty in establishing the
structure–property relationships that are needed to optimize
molecular design. Gold alkynyl complexes are particularly
useful in this regard because their optical absorption maxima are
far removed from 532 nm. The present studies have explored the
introduction of solubilizing substituents onto a p-bridge linking
a ligated gold centre to a nitro group, the classical organic
acceptor in compounds with a donor–bridge–acceptor composi-
tion, and have shown that proceeding from a non-functionalized
ring to one incorporating alkyl and then alkoxy substituents
results in a progressive increase in quadratic optical non-linearity,
but with a concomitant decrease in optical transparency.
The (tricyclohexylphosphine)gold unit is comparatively small
compared with other ligated metal centers in metal alkynyl
complexes, and this permits incorporation of six such units
around a central benzene ring. We have used the sterically less-
demanding nature of (Cy3P)Au to carry out an initial study of the
effect of proceeding from linear species to 6-fold symmetric
examples, and have noted a dramatic increase in the imaginary
component of the cubic non-linearity and the effective two-
photon absorption cross-section, even when scaled for molec-
ular size. The absolute value of the s2/M coefficient in 12
matches those achievable with ruthenium complexes,[16j] and it
does highlight the potential of highly symmetric species (it
should be mentioned, though, that the maximal value for the
gold complex is obtained at a short wavelength, and that an order
of magnitude smaller value is recorded near 800 nm, the
wavelength where the two-photon absorption peaks for many
ruthenium complexes). Further studies exploring highly sym-
metric species are currently underway.
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