Length-dependent convergence and saturation behavior of electrochemical, linear optical, quadratic nonlinear optical, and cubic nonlinear optical properties of dipolar alkynylruthenium complexes with oligo(phenyleneethynylene) bridges

Article abstract of DOI:10.1021/ja902793z

The syntheses of trans-[Ru{4,4′-C≡CC₆H ₂[2,5-(OEt)₂]C≡CC₆H₄NO ₂}Cl(dppm)₂] (19), trans-[Ru{4,4′,4″- C≡CC₆H₄C≡CC₆H₂[2,5-(OEt) ₂

Full text of DOI:10.1021/ja902793z

Published on Web 07/06/2009
Length-Dependent Convergence and Saturation Behavior of
Electrochemical, Linear Optical, Quadratic Nonlinear Optical,
and Cubic Nonlinear Optical Properties of Dipolar
Alkynylruthenium Complexes with
Oligo(phenyleneethynylene) Bridges
Bandar Babgi,† Luca Rigamonti,†,‡ Marie P. Cifuentes,† T. Christopher Corkery,†
Michael D. Randles,† Torsten Schwich,† Simon Petrie,† Robert Stranger,†
Ayele Teshome,§ Inge Asselberghs,§ Koen Clays,§ Marek Samoc,⊥,4 and
Mark G. Humphrey*,†
Research School of Chemistry, Australian National UniVersity, Canberra, ACT 0200, Australia,
Dipartimento di Chimica Inorganica, Metallorganica e Analitica ‘Lamberto Malatesta’,
UniVersita` degli Studi di Milano, Via Venezian 21, 20133 Milano, Italy, Department of
Chemistry, UniVersity of LeuVen, Celestijnenlaan 200D, B-3001 LeuVen, Belgium, Laser Physics
Centre, Research School of Physics and Engineering, Australian National UniVersity, Canberra,
ACT 0200, Australia, and Institute of Physical and Theoretical Chemistry, Wroclaw UniVersity
of Technology, 50-370 Wroclaw, Poland
Received April 7, 2009; E-mail: Mark.Humphrey@anu.edu.au
Abstract: The syntheses of trans-[Ru{4,4′-CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (19), trans-
[Ru{4,4′,4′′-CtCC6H4CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (20), trans-[Ru{4,4′,4′′,4′′′-CtCC6H4-
CtCC6H2[2,5-(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppe)2] (21), trans-[Ru{4,4′,4′′,4′′′-CtCC6H4-
CtCC6H2[2,5-(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (22), trans-[Ru{4,4′,4′′,4′′′-CtCC6H4Ct
CC6H4CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (23), and trans-[Ru{4,4′,4′′,4′′′,4′′′′-CtCC6H4-
CtCC6H4CtCC6H2[2,5-(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (24) are reported, together
with those of precursor alkynes, complexes with the donor-π-bridge-acceptor formulation that affords
efficient quadratic and cubic NLO compounds; the identity of 19 was confirmed by a structural study. The
electrochemical properties of 19-24 and related complexes with shorter π-bridge ligands were assessed
by cyclic voltammetry, and the linear optical, quadratic nonlinear optical, and cubic nonlinear optical properties
were assayed by UV-vis-NIR spectroscopy, hyper-Rayleigh scattering studies at 1064 and 1300 nm,
and broad spectral range femtosecond Z-scan studies, respectively. The RuII/III oxidation potentials and
wavelengths of the optical absorption maxima decrease on π-bridge lengthening, until the tri(phenylene-
ethynylene) complex is reached, further chain lengthening leaving these parameters invariant; theoretical
studies employing time-dependent density functional theory have shed light on this behavior. The quadratic
nonlinearity ꢀ1064 and two-photon absorption cross-section reach maximal values at this same π-bridge
length, a similar saturation behavior that may reflect a common importance of ruthenium-to-alkynyl ligand
charge transfer in electronic and optical behavior in these molecules.
donor or acceptor strength or by bridge modification,7 while
Introduction
compounds with extended π-systems were shown to possess
Considerable interest has been shown in the nonlinear optical
(NLO) properties of organometallic complexes,1-6 with the
majority of studies involving propagation of structure-property
relationships developed in organic systems into the organome-
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‡ Universita` degli Studi di Milano.
§ University of Leuven.
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J. AM. CHEM. SOC. 2009, 131, 10293–10307 10293

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10294 J. AM. CHEM. SOC. VOL. 131, NO. 29, 2009

NLO Properties of Dipolar Alkynylruthenium Complexes
A R T I C L E S
one or two phenyl rings coupled together in various ways; our
only study examining π-bridge lengthening, in proceeding from
trans-[Ru(4-CtCC6H4NO2)Cl(dppm)2] to trans-[Ru(4,4′-Ct
CC6H4CtCC6H4NO2)Cl(dppm)2] and then trans-[Ru(4,4′,4′′-
CtCC6H4CtCC6H4CtCC6H4NO2)Cl(dppm)2], resulted in a
nonlinear increase in quadratic nonlinearity as measured by
hyper-Rayleigh scattering (HRS) at 1064 nm [ꢀ1064: 767 to 833
to 1379; ꢀ0: 129 to 161 to 365 (10-30 esu)].68 [We have very
recently prepared and assessed further examples with π-bridges
containing three phenyl rings linked by a variety of yne and
E-ene groups and again noted an increase in quadratic
nonlinearity.1,92] Unfortunately, attempts to extend the oli-
go(phenyleneethynylene) series to the tetra(phenyleneethy-
nylene)-containing example resulted in an alkynyl complex with
minimal solubility, which hampered reaction and complex
characterization and rendered solution NLO studies impossible.
We report herein several new oligo(phenyleneethynylene)-
containing alkynes incorporating solubilizing substituents, the
corresponding ruthenium alkynyl complex derivatives, assess-
ment of the impact of π-bridge lengthening on electrochemical
and linear optical properties, theoretical studies using time-
dependent density functional theory (TD-DFT) directed at
rationalizing our experimental observations, quadratic nonlinear
optical properties from hyper-Rayleigh scattering studies at two
wavelengths, and wide-spectral-range wavelength-dependence
studies of the cubic nonlinear optical properties of selected
complexes from Z-scan studies; these studies demonstrate that
electrochemical and optical properties in this system converge
at the tri(phenyleneethynylene) π-bridge length.
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Experimental Section
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Materials. All reactions were performed under a nitrogen
atmosphere with the use of Schlenk techniques unless otherwise
stated. Tetrahydrofuran (THF) was dried by distilling over sodium/
benzophenone; all other solvents were used as received. Petrol is
a fraction of boiling range 60-80 ꢁC. Chromatography was
performed on silica gel or ungraded basic alumina. Ethynyltrim-
ethylsilane (Wacker), tert-butyllithium (concentration determined
prior to reaction by titration against diphenylacetic acid), 1,2-
diiodoethane, 4-bromo-1-iodobenzene, bromine, iodine, sodium
hexafluorophosphate, tetra-n-butylammonium fluoride, copper(I)
iodide, sodium sulfite, magnesium sulfate, sodium acetate, sodium
bicarbonate, potassium iodate, PdCl2(PPh3)2, and Pd(PPh3)4 (Ald-
rich) were used as received. The following were prepared by
literature procedures: 1,4-diethoxybenzene,93 1,4-dibromo-2,5-
diethoxybenzene,94 4-Me3SiCtCC6H4I, 4-HCtCC6H4NO2,95
4-HCtCC6H4CtCSiPri3,96 4,4′-IC6H4CtCC6H4CtCSiPri3,97 cis-
[RuCl2(dppe)2], cis-[RuCl2(dppm)2],98 trans-[Ru(4-CtCC6H4-
NO2)Cl(dppe)2] (25),99 trans-[Ru(4,4′-CtCC6H4C≡CC6H4NO2)-
Cl(dppe)2] (26), trans-[Ru(4,4′,4′′-CtCC6H4CtCC6H4CtCC6H4-
NO2)Cl(dppe)2] (27),1 trans-[Ru(4-CtCC6H4NO2)Cl(dppm)2]
(28),100 trans-[Ru(4,4′-CtCC6H4C≡CC6H4NO2)Cl(dppm)2] (29),
trans-[Ru(4,4′,4′′-CtCC6H4CtCC6H4CtCC6H4NO2)Cl(dppm)2]
(30).68 The syntheses of 1-18 are given in the Supporting
Information.
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Asselberghs, I.; Clays, K.; Persoons, A.; Isoshima, T. J. Am. Chem.
Soc. 2006, 126, 10819.
Methods and Instrumentation. Microanalyses were carried out
at the Australian National University. UV-vis spectra of solutions
in 1 cm quartz cells were recorded as dichloromethane solutions
using a Cary 5 spectrophotometer; bands are reported in the form
frequency (cm-1) [extinction coefficient (104 M-1 cm-1)]. Infrared
spectra were recorded as solutions in dichloromethane or as KBr
disks using a Perkin-Elmer System 2000 FT-IR; data are reported
1
in cm-1. H (300 MHz), 13C (75 MHz), and 31P NMR (121 MHz)
spectra were recorded using a Varian Gemini-300 FT NMR
spectrometer and are referenced to residual chloroform (7.26 ppm),
CDCl3 (77.0 ppm), or external H3PO4 (0.0 ppm), respectively; atom
(93) Yu, B. Z.; Li, M. K.; Lu, M.; Li, H. L. Appl. Phys. A: Mater. Sci.
Process. 2003, 76, 593.
(94) Maruyama, S.; Kawanishi, Y. J. Mater. Chem. 2002, 2245.
(95) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 627.
(96) Lavastre, O.; Olivier, L.; Dixneuf, P. H.; Sibandhit, S. Tetrahedron
1996, 52, 5495.
(97) Dalton, G. T.; Cifuentes, M. P.; Watson, L. A.; Petrie, S.; Stranger,
R.; Samoc, M.; Humphrey, M. G. Inorg. Chem., in press. DOI:
10.1021/ic900469y.
(89) Powell, C. E.; Cifuentes, M. P.; Humphrey, M. G.; Willis, A. C.;
Morrall, J. P.; Samoc, M. Polyhedron 2007, 26, 284.
(90) Samoc, M.; Morrall, J. P.; Dalton, G. T.; Cifuentes, M. P.; Humphrey,
M. G. Angew. Chem., Int. Ed. 2007, 46, 731.
(91) Vicente, J.; Chicote, M. T.; Abrisqueta, M. D.; Ramˆırez de Arellano,
M. C.; Jones, P. G.; Humphrey, M. G.; Cifuentes, M. P.; Samoc,
M.; Luther-Davies, B. Organometallics 2000, 19, 2968.
(92) Cifuentes, M. P.; Humphrey, M. G.; Samoc, M.; Babgi, B.; Dalton,
G. T.; Rigamonti, L. Poly. Prepr., in press. Paper no 1288516.f
(98) Chaudret, B.; Commenges, G.; Poilblanc, R. J. Chem. Soc., Dalton
Trans. 1984, 1635.
(99) Touchard, D.; Haquette, P.; Guesmi, S.; Pichon, L. L.; Daridor, A.;
Toupet, L.; Dixneuf, P. H. Organometallics 1997, 16, 3640.
(100) McDonagh, A. M.; Cifuentes, M. P.; Whittall, I. R.; Humphrey,
M. G.; Samoc, M.; Luther-Davies, B.; Hockless, D. C. R. J.
Organomet. Chem. 1996, 526, 99.
9
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A R T I C L E S
Babgi et al.
labeling follows the numbering scheme in Chart S1. Electrospray
ionization mass spectra (ESI-MS) were recorded using a Micromass/
Water’s LC-ZMD single quadrupole liquid chromatograph MS,
high-resolution (HR) ESI mass spectra were carried out utilizing a
Bruker Apex 4.7T FTICR-MS instrument, and electron impact mass
spectra (EI MS) were recorded using a VG Quattro II triple
quadrupole MS; all mass spectrometry peaks are reported as m/z
(assignment, relative intensity). Cyclic voltammetry measurements
were recorded using a MacLab 400 interface and MacLab poten-
tiostat from ADInstruments. The supporting electrolyte was 0.1 M
at idealized positions, riding on the atoms to which they are bonded,
and their positions were frequently recalculated. Conventional
residuals R and Rw on |F| were 4.3% and 15.2%, respectively; a
Chebyshev weighting function was employed. The largest peaks
in the final difference electron density map are located near the Ru
atom. Molecular graphics were displayed using the PLATON
software.107
Crystal/Refinement Data: C70H60ClNO4P4Ru, M ) 1239.67,
monoclinic, P21/c, a ) 22.2245(2) Å, b ) 12.7995(1) Å, c )
23.5634(2) Å, ꢀ ) 116.4624(5)°, V ) 6000.62(9) Å3. Dc (Z ) 4)
) 1.327 g cm-3. µMo ) 0.46 mm-1; specimen: 0.40 × 0.25 × 0.24
mm; Tmin/max ) 0.857/0.916. 2θmax ) 55°; Ntotal ) 125 417 (CCD
diffractometer, monochromatic Mo KR radiation, λ ) 0.71073 Å;
T 200 K) merging to 13 727 unique (Rint ) 0.044), No ) 3517 (I
g 3σ(I)) refining to R ) 0.0425, Rw ) 0.152.
(NBun )PF6 in distilled, deoxygenated CH2Cl2. Solutions containing
4
ca. 1 × 10-3 M complex were maintained under argon. Measure-
ments were carried out at room temperature using Pt disk working,
Pt wire auxiliary, and Ag/AgCl reference electrodes, such that the
ferrocene/ferrocenium redox couple was located at 0.56 V (peak
separation ca. 0.09 V). Scan rates were typically 100 mV s-1
.
Theoretical Studies. Calculations were performed using the
Amsterdam Density Functional (ADF) package ADF2006.01,108
developed by Baerends et al.109,110 These calculations were
undertaken to characterize the lowest frequency allowed single-
photon transitions of a set of model compounds containing the
oligo(phenyleneethynylene) bridges of compounds 19 to 24. These
models were of the form [Ru](C2C6H4)iC2C6H4NO2, where [Ru] )
trans-RuCl(PH2CH2PH2)2 and i ) 1-4. In the discussion that
follows, the models are denoted as 19M (i ) 1), 20M (i ) 2),
23M (i ) 3), and 24M (i ) 4) to indicate the laboratory compounds
of which they are structural homologues. Note that, for reasons of
computational expediency, the solubilizing ethoxy substituents of
the laboratory compounds have been omitted in our model
calculations. The presence or absence of alkoxy substituents on
the phenylene rings has been found to exert only a minor influence
on the intensity and wavelength of major single-photon transitions
in a detailed computational study of related compounds.97 Omission
of the ethoxy substituents in this instance allowed the structural
symmetry of the models to be constrained to C2V throughout. In all
calculations and for all atoms, the Slater-type orbital basis sets used
were of triple- -plus-polarization quality (TZP). Electrons in orbitals
up to and including 1s {C, N, O}, 2p {P, Cl}, and 4d {Ru} were
treated in accordance with the frozen-core approximation. Geometry
optimizations employed the gradient algorithm of Versluis and
Ziegler.111 Functionals used in the optimization calculations were
the local density approximation (LDA) to the exchange potential,112
the correlation potential of Vosko, Wilk, and Nusair (VWN),113
and the nonlocal corrections of Perdew, Burke, and Enzerhof
(PBE).114 Following optimization of the model compounds, time-
dependent density functional theory (TD-DFT) calculations were
pursued using either PBE or the asymptotically correct functional
of van Leeuwen and Baerends (LB94).
Synthesis of trans-[Ru{4,4′-CtCC6H2[2,5-(OEt)2]CtCC6H4-
NO2}Cl(dppm)2] (19). cis-[RuCl2(dppm)2] (132 mg, 0.14 mmol) and
NaPF6 (31.3 mg, 0.17 mmol) were added to a solution of 5 (47.1
mg, 0.14 mmol) in CH2Cl2 (30 mL). The yellow mixture was stirred
at room temperature overnight. NEt3 (1 mL) was added and the
red mixture stirred at room temperature for 6 h. The reaction mixture
was purified by passing through a short pad of alumina, eluting
with CH2Cl2/petrol/NEt3 (10:10:1). Reduction in volume of the
solvent on a rotary evaporator afforded 19 as a dark red powder
(120 mg, 69%). ESI-MS: 1240 ([M]+, 100), 1205 ([M - Cl]+, 10).
Anal. Calcd for C70H60ClNO4P4Ru·0.25CH2Cl2: C, 66.92; H, 4.84;
N, 1.11. Found: C, 66.95; H, 5.11; N, 1.25. UV-vis: 19 600 [1.8],
1
26 500 [2.2], 30 400 [1.9]. IR (CH2Cl2): 2200, 2064 (CtC). H
NMR: δ 1.25, 1.41 (2 t, 6H, JHH ) 7 Hz, 2CH3), 3.67, 3.82 (2 q,
4H, JHH ) 7 Hz, 2CH2), 4.80, 5.32 (2 m, 4H, 2PCH2), 5.19, 6.77
(2 s, 2H, H16, H19), 5.28 (s, 0.5H, CH2Cl2), 6.99-7.60 (m, 44H,
H24, Ph), 8.20 (d, JHH ) 8 Hz, 2H, H25). 13C NMR: δ 15.1, 15.2
(CH3), 47.6 (PCH2), 63.7, 64.4 (CH2), 91.6, 94.2 (C21, C22), 104.2,
110.4 (C15, C18), 114.1, 117.4 (C16, C19), 123.1 (C23), 123.6 (C25),
127.5 (Ph), 129.0 (d, JCP ) 36 Hz, Ph), 131.6 (C24), 133.5 (d, JCP
) 38 Hz, Ph), 146.2 (C26), 152.4, 153.7 (C17, C20), C13, C14 not
observed. 31P NMR: δ -4.7. The syntheses of 20-24 are similar
and are given in the Supporting Information.
X-ray Structural Study of trans-[Ru{4,4′-CtCC6H2[2,5-(OEt)2]-
CtCC6H4NO2}Cl(dppm)2] (19). A crystal suitable for the structural
study was grown by slow diffusion of methanol into a dichlo-
romethane solution of 19 at -20 °C. A unique diffractometer data
set was collected on a Nonius Kappa CCD diffractometer using
the ω scan technique (graphite-monochromated Mo KR radiation,
0.71073 Å). The unit cell parameters were obtained by least-squares
refinement101 of 13 727 reflections with 2.6° e θ e 27.5° at 200
K. An analytical absorption correction was applied, using numerical
methods,102 implemented from MAXUS,103 and equivalent reflec-
tions were merged. The structure was solved by direct methods,
expanded using Fourier techniques,104 and refined using the
CRYSTALS software package.105,106 One of the phenyl rings
attached to a dppm moiety was found to be disordered, and so each
atom was split. The total site occupancies were set to be one for
each set of split atoms. All other non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included in the refinement
HRS Measurements. An injection-seeded Nd:YAG laser (Q-
switched Nd:YAG Quanta Ray GCR, 1064 nm, 8 ns pulses, 10
Hz) was focused 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
collected by an efficient condenser system and detected by a
(107) Spek, A. L. PLATON-Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 2008.
(101) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter C. W.,
Jr., Sweet, R. M., Eds., Academic Press: New York, 1997; Vol. 276,
p 307.
(102) Coppens, P. In Crystallographic Computing; Ahmed, F. R., Hall,
S. R., Huber, C. P., Eds.; Munksgaard: Copenhagen, 1970; p 255.
(103) Mackay, S.; Gilmore, C. J.; Edwards, C.; Tremayne, M.; Stewart,
N.; Shankland, K., maXus Computer Program for the Solution and
Refinement of Crystal Structures; Nonius: The Netherlands, Mac-
Science, Japan, The University of Glasgow, 2000.
(104) Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 425.
(105) Watkin, D. J.; Prout, C. K. J. Appl. Crystallogr. 2003, 36, 1487.
(106) Watkin, D. J.; Prout, C. K.; Pearce, L. J. CAMERON; Chemical
Crystallography Laboratory, Oxford University: Oxford.
(108) Baerends, E. J.; et al. Amsterdam Density Functional V.2006.01;
S.C.M., Theoretical Chemistry, Vrije Universiteit: Amsterdam, The
Netherlands, http://www.scm.com, 2006.
(109) Fonseca Guerra, C. F.; Snijders, J. G.; te Velde, G.; Baerends, E. J.
Theor. Chem. Acc. 1998, 99, 391.
(110) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.; van
Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem.
2001, 22, 931.
(111) Versluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322.
(112) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
(113) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(114) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,
3865.
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NLO Properties of Dipolar Alkynylruthenium Complexes
A R T I C L E S
photomultiplier. 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 p-
nitroaniline (ꢀ ) 21.4 × 10-30 esu115) as a reference. Solutions
were sufficiently dilute that absorption of scattered light was
negligible.
For studies at 1.300 µm, a Tsunami-pumped OPAL (model
Spectra-Physics) was used. With a high repetition rate of the laser,
high-frequency demodulation of fluorescence contributions can be
effected, a full description being given in ref 116. All measurements
were performed in THF using Disperse Red 1 (DR1, ꢀ ) 54 ×
10-30 esu in chloroform) as a reference. Experiments utilized low
chromophore concentrations, the linearity of the HRS signal as a
function of the chromophore concentration confirming that no
significant self-absorption of the SHG signal occurred.
Z-Scan Studies. The compounds were dissolved in dichlo-
romethane at concentrations in the range 0.5-1.2% w/w, and the
solutions were placed in 1 mm path length Starna glass cells,
stoppered, and sealed with Teflon tape. Due to experimental
limitations, only one concentration of each compound was exam-
ined. 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 decomposition. The experiments were carried out
at a number of wavelengths in the range 530-1500 nm. To obtain
the relevant wavelengths, the optical parametric amplifier was tuned
appropriately 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, color glass filters,
and spatial filtering.
the light intensity. From the comparison of n2 values for silica
compiled by Milam118 we employed
n2,silica(ν¯) ) 2.82 × 10-16 - 3 × 10-21ν¯ +
2 × 10-25ν¯2(cm2/W)
for the spectral dependence of n2 for silica, where νj ) 1/λ is the
wavenumber expressed in cm-1. This equation is a good ap-
proximation for values of n2,silica in the range of λ ) 500-1500
nm. Note that most of our previous papers assumed n2,silica ) 3 ×
10-16 cm2/W independent of the wavelength, which is also a
reasonably good approximation.
We employed intensities on the order of 100 GW/cm2. Since
our measurements were always uniformly calibrated to the non-
linearity of silica glass, this avoided the need for detailed informa-
tion of beam geometry and pulse shape.
The real and imaginary parts of the second hyperpolarizability,
γ, of the solutes were computed assuming additivity of the nonlinear
contributions of the solvent and the solute and the applicability of
the Lorentz local field approximation.119 The values of the two-
photon absorption (TPA) cross-section σ2 were computed from the
absorptive part of the nonlinearity 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.
Results
Synthesis and Characterization of Alkynes. The acetylenes
required for the alkynyl complex syntheses were prepared by
extensions of established organic synthetic procedures (Schemes
1-6). Monobromination of 1,4-di(ethoxy)benzene using bro-
mine and sodium acetate in acetic acid afforded 1-bromo-2,5-
diethoxybenzene (1), in a procedure previously applied to afford
the 1-bromo-2,5-di(n-propoxy)benzene analogue (Scheme 1).120
Subsequent iodination with iodine/potassium iodate in an acetic
acid/carbon tetrachloride mixture afforded 4-bromo-2,5-di-
ethoxy-1-iodobenzene (2) in good yield. This facile synthesis
of 2 is preferable to the trans-halogenation of 1,4-dibromo-2,5-
di(ethoxy)benzene employing tert-butyllithium and 1,2-diiodo-
ethane.
Both the bromo and iodo functionalities in 2 are reactive.
4-Ethynyl-1-nitrobenzene and 4-ethynyl-1-(triisopropylsilyl)-
ethynylbenzene reacted with 2 under Sonogashira conditions
at room temperature to give 2,5-(EtO)2-4-BrC6H2CtC-4-
C6H4NO2 (3) and 4-Pri3SiCtCC6H4CtCC6H2-2,5-(OEt)2-4-Br
(6), respectively, in excellent yields (Scheme 2). The former
reacted with ethynyltrimethylsilane under Sonogashira condi-
tions to afford 4-Me3SiCtC-2,5-(EtO)2C6H2-1-CtCC6H4-4-
NO2 (4) in fair yield. The diyne 4 was desilylated to afford
4-HCtC-2,5-(EtO)2C6H2-1-CtCC6H4-4-NO2 (5).
Syntheses of tri(phenyleneethynylene)-containing compounds
are summarized in Scheme 3. 5 underwent sequential Sono-
gashira coupling with 4-bromo-1-iodobenzene and then ethy-
nyltrimethylsilane to afford 4-O2NC6H4CtCC6H2-2,5-(OEt)2-
4-CtCC6H4-4-CtCSiMe3 (7), which was desilylated by tetra-
n-butylammonium fluoride, giving 4-O2NC6H4CtCC6H2-2,5-
(OEt)2-4-CtCC6H4-4-CtCH(8).AsimilarsequentialSonogashira
coupling with 4-Br-2,5-(EtO)2C6H2I and then ethynyltrimeth-
ylsilane gave 4-O2NC6H4CtCC6H2-2,5-(OEt)2-4-CtCC6H2-2,5-
(OEt)2-4-CtCSiMe3 (10), which was similarly desilylated to
The laser beam was attenuated to energies in the µJ/pulse 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 focused so as to provide a
spot size in the range w0 ≈ 40-80 µm, ensuring that the Rayleigh
2
range zR ) πw0 /λ was always larger than the total thickness of
the sample (≈3 mm, which includes two glass walls and the solution
inside the cell). The cell travel was z ) -40 to 40 mm, and the
data were recorded using three Si or InGaAs photodiodes monitor-
ing 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.117 to calculate the theoretical curves. In this way,
the values of the real and imaginary part of the nonlinear phase
shift ∆Φ0 (corresponding to the refractive and absorptive nonlin-
earity, 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
∆Φ0 for such scans to be about 0.5-1 rad. The nonlinear refractive
index of silica, n2, is well known across a wide wavelength range,
so the real part of the nonlinear phase shift can be used to calculate
(115) Sta¨helin, M.; Burland, D. M.; Rice, J. E. Chem. Phys. Lett. 1992,
191, 245.
(118) Milam, D. Appl. Opt. 1998, 37, 546.
(116) Olbrechts, G.; Wostyn, K.; Clays, K.; Persoons, A. Opt. Lett. 1999,
403.
(119) Samoc, M.; Samoc, A.; Luther-Davies, B.; Humphrey, M. G.; Wong,
M.-S. Opt. Mater. 2002, 21, 485.
(117) Sheik-Bahae, M.; Said, A. A.; Wei, T.; Hagan, D. J.; van Stryland,
E. W. IEEE J. Quantum Electr. 1990, 26, 760.
(120) Meier, H.; Ickenroth, D.; Stalmach, U.; Koynov, K.; Bahtiar, A.;
Bubeck, C. Eur. J. Org. Chem. 2001, 4431.
9
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A R T I C L E S
Babgi et al.
Scheme 1. Syntheses of 1 and 2
Scheme 2. Syntheses of 3-6
Scheme 3. Syntheses of 7-11
4-O2NC6H4CtCC6H2-2,5-(OEt)2-4-CtCC6H2-2,5-(OEt)2-4-
CtCH (11).
4-HCtCC6H4CtCC6H2-2,5-(OEt)2-4-CtCC6H2-2,5-(OEt)2-4-
CtCC6H4-4-NO2 (14) in quantitative yield. 14 is also accessible
from a sequential Sonogashira coupling of 11 with 4-bromo-
1-iodobenzene and then ethynyltrimethylsilane, affording 4-Me3-
SiCtCC6H4CtCC6H2-2,5-(OEt)2-4-CtCC6H2-2,5-(OEt)2-4-
CtCC6H4-4-NO2 (13) and a subsequent desilylation. Syntheses
of tetra(phenyleneethynylene)-containing compounds in which
one ring bears solubilizing groups are summarized in Scheme
Syntheses of tetra(phenyleneethynylene)-containing com-
pounds in which two rings bear solubilizing groups are
summarized in Scheme 4. 4-Pri3SiCtCC6H4CtCC6H2-2,5-
(OEt)2-4-CtCC6H2-2,5-(OEt)2-4-CtCC6H4-4-NO2 (12) is avail-
able from reaction of 5 with 6 under Sonogashira conditions;
12 was desilylated by tetra-n-butylammonium fluoride to give
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NLO Properties of Dipolar Alkynylruthenium Complexes
A R T I C L E S
Scheme 4. Syntheses of 12-14
Scheme 5. Syntheses of 15-17
Scheme 6. Synthesis of 18
1
5. Sequential Sonogashira coupling of 8 with 4-bromo-1-
iodobenzene and then ethynyltrimethylsilane gave 4-Me3SiCt
CC6H4-1-CtCC6H4-4-CtCC6H2-2,5-(OEt)2-4-CtCC6H4-4-
NO2 (15) in a good yield, subsequent desilylation affording
4-HCtCC6H4CtCC6H4-4-CtCC6H2-2,5-(OEt)2-4-CtCC6H4-
4-NO2 (17). Alternatively, 5 was coupled to 4-IC6H4CtCC6H4-4-
CtCSiPri3 to afford 4-Pri3SiCtCC6H4-1-CtCC6H4-4-CtCC6H2-
2,5-(OEt)2-4-CtCC6H4-4-NO2 (16), which was desilylated to
give 17 in a better yield. The penta(phenyleneethynylene)
4-Pri3SiCtCC6H4-1-CtCC6H4-4-CtCC6H2-2,5-(OEt)2-4-
CtCC6H2-2,5-(OEt)2-4-CtCC6H4-4-NO2 (18) was obtained via
Sonogashira coupling of 11 with 4-IC6H4CtCC6H4-4-
CtCSiPri3 in a fair yield (Scheme 6).
by a combination of IR, H, 13C, and 31P NMR spectroscopies
and ESI mass spectrometry. The IR spectra contain characteristic
ν(CtC) bands at 2063-2074 cm-1 for the metal-bound alkynyl
group, with the higher values corresponding to complexes with
the longer π-bridges (22-24). The 31P NMR spectrum of the
dppe-containing complex 21 contains one singlet resonance at
49.8 ppm, confirming the trans-disposed diphosphine ligands.
The spectra of the dppm-containing complexes similarly contain
a singlet; the 31P resonance for the complex with ethoxy
substituents on the ring adjacent to the metal center (19: -4.7
ppm) is somewhat downfield of those of other dppm-containing
complexes (-5.5 ppm). Mass spectra contain molecular ions
and/or fragment ions corresponding to loss of a chloro ligand.
The identity of 19 was confirmed by a single-crystal X-ray
diffraction study. Figure 1 contains an ORTEP plot showing
the molecular geometry, together with selected bond lengths
Synthesis and Characterization of Alkynyl Complexes. The
new alkynyl complexes 19-24 were prepared using established
methodologies (Schemes 7 and 8)77,99,121-131 and characterized
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A R T I C L E S
Babgi et al.
Scheme 7. Syntheses of 19-23
Scheme 8. Synthesis of 24
and angles. Bond lengths and angles are similar to those of the
structurally characterized analogues trans-[Ru(CtC-4-
C6H4CtCC6H4-4-X)Cl(dppm)2] (X ) H,68 NdNC6H4-4-
NO2124), the only notable exception being Cl1-Ru1-Cl
170.33(9)° (19), cf. 175.9(2)° (X ) H), 175.1(17)° (X )
NdNC6H4-4-NO2), a distortion in 19 likely to derive from the
need to minimize unfavorable steric interactions between the
C4-ethoxy group and the P-phenyl rings. The dihedral angle
between the C3/C8 and C11/C16 phenyl planes in 19 (5.7(2)°)
is much smaller than those in precedent structural studies (26.75°
(X ) H), 21.9° (X ) NdNC6H4-4-NO2), 33.9° (trans-
[Ru(CtC-4-C6H4CtC-4-C6H4CtCPh)2(dmpe)2]132)). Because
coplanar phenyl rings in extended π-systems enhance π-delo-
calization, this structural difference is likely to favorably
influence nonlinearity in solid-state materials; however, the
centrosymmetric packing in the crystal structure of 19 will result
in no bulk second-order susceptibility.
Electrochemical and Linear Optical Studies. Electrochemical
properties of ruthenium alkynyl complexes are of significant
interest (see ref 92 and references therein). The results of cyclic
voltammetric studies of the new ruthenium alkynyl complexes
19-24 are collected in Table 1, together with data from related
complexes 25-30.
Figure 1. Molecular geometry and atomic labeling scheme for trans-[Ru{4,4′-CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (19). Selected bond lengths
(Å) and angles (deg): Ru1-P1 2.3291(8), Ru1-P2 2.3362(8), Ru1-P3 2.3613(8), Ru1-P4 2.3504(8), Ru1-Cl1 2.5176(7), Ru1-C1 1.988(3), C1-C2
1.217(5), C2-C3 1.421(4); P1-Ru1-P2 71.76(3), P3-Ru1-P4 70.87(3), Cl1-Ru1-C1 170.33(9), Ru1-C1-C2 177.2(3), C1-C2-C3 175.8(4).
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NLO Properties of Dipolar Alkynylruthenium Complexes
A R T I C L E S
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A R T I C L E S
Babgi et al.
Figure 2. Optical spectra of trans-[Ru(4,4′-CtCC6H4CtCC6H4NO2)-
Cl(dppm)2] (29) and trans-[Ru{4,4′-CtCC6H2[2,5-(OEt)2]CtCC6H4-
NO2}Cl(dppm)2] (19).
Figure 5. Optical spectra of trans-[Ru{4,4′,4′′,4′′′-CtCC6H4CtCC6H4Ct
CC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2](23),trans-[Ru{4,4′,4′′,4′′′,4′′′′-
CtCC6 H4 CtCC6 H4 CtCC6 H2 [2, 5-(OEt)2 ]CtCC6 H2 [2, 5-
(OEt)2]CtCC6H4NO2}Cl(dppm)2] (24), trans-[Ru(4-CtCC6H4NO2)-
Cl(dppm)2] (28), trans-[Ru(4,4′-CtCC6H4CtCC6H4NO2)Cl(dppm)2] (29),
and trans-[Ru(4,4′,4′′-CtCC6H4CtCC6H4CtCC6H4NO2)Cl(dppm)2] (30).
be deconvoluted by examination of the more extensive data
available for the dppm-containing complexes. Thus, introduction
of solubilizing substituents at the π-bridge phenyl ring adjacent
to the metal (proceeding from 29 to 19) results in a significant
increase in the ease of oxidation (∆E0 0.07 V), whereas
ox
solubilizing group introduction more remote from the metal
(proceeding from 30 to 20) and further increase in solubilizing
groups (proceeding from 23 to 22) results in no change in
oxidation potential; these results, which suggest electron density
at the metal is particularly sensitive to adjacent ring modification,
are consistent with those seen upon nitro group incorporation
into such complexes (across the series 19-30, oxidation
potential is significantly higher for complexes in which the nitro
group is attached to the ring adjacent to the metal, namely, 25
and 28). These results suggest that the effect of π-bridge
lengthening on ease of oxidation should only be assessed with
complexes incorporating the same number of solubilizing
groups. With this in mind, the present data suggest π-bridge
lengthening (proceeding from 28 to 29 and then 30, from 20 to
23, and from 22 to 24) progressively increases the ease of
oxidation until the tri(phenyleneethynylene) is reached, after
which the oxidation potential is invariant.
Figure 3. Optical spectra of trans-[Ru(4,4′,4′′-CtCC6H4CtCC6H4Ct
CC6H4NO2)Cl(dppm)2] (30) and trans-[Ru{4,4′,4′′-CtCC6H4CtCC6H2[2,5-
(OEt)2]CtCC6H4NO2}Cl(dppm)2] (20).
Absorption maxima and intensities from electronic spectra
are collected in Table 2, and the evolution in optical spectra on
systematic structural modification is illustrated in Figures 2-5.
Incorporating solubilizing groups on the ring adjacent to the
metal (proceeding from 29 to 19) results in a red-shift and
decrease in intensity of the absorption bands (Figure 2), while
introducing the solubilizing groups at a more remote ring
(proceeding from 30 to 20) results in a decrease in intensity of
the higher energy bands (Figure 3); a further increase in number
of solubilizing groups remote from the metal (proceeding from
23 to 22) results in a slight increase in intensity of bands, but
again little shift in the location of the absorption maxima (Figure
4). Minimizing solubilizing substituents while π-bridge length-
ening, in proceeding from 28, through 29, 30, and 23, to 24
(Figure 5), results in a distinct blue-shift from the mono(phe-
nyleneethynylene) 28 to the tri(phenyleneethynylene) 30, with
little further change in νmax, but an increase in absorptivity, in
proceeding to the penta(phenyleneethynylene) 24. Closer in-
spection of the spectra reveals that the next-higher-energy bands
red-shift and gain in intensity on chain-lengthening, to eventually
coalesce with the lowest energy band for 23 and 24.
Figure 4. Optical spectra of trans-[Ru{4,4′,4′′,4′′′-CtCC6H4CtCC6H2[2,5-
(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (22) and trans-
[ R u { 4 , 4 ′ , 4 ′ ′ , 4 ′ ′ ′ - C t C C 6 H 4 C t C C 6 H 4 C t C C 6 H 2 [ 2 , 5 -
(OEt)2]CtCC6H4NO2}Cl(dppm)2] (23).
As with other trans-bis(bidentate diphosphine)ruthenium
monoalkynyl complexes, the cyclic voltammograms of the new
complexes contain a reversible or quasi-reversible anodic wave
assigned to the RuII/III oxidation process, in the range 0.50-0.56
0/-1
V, and a less reversible cathodic wave assigned to the NO2
reduction process, in the range -0.86 to -1.16 V. These data,
together with those of the previously reported complexes 25-30,
permit comment on the effect of π-bridge lengthening and
solubilizing group incorporation upon metal-localized oxidation
potential. For the dppe-containing complexes, OPE increase
(proceeding from 25 to 26 to 27 and finally 21) leads to a
progressive decrease in RuII/III oxidation potential, but this is
not clear-cut because of the introduction of ethoxy groups at
21. The effect of these solubilizing substituents can in principle
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NLO Properties of Dipolar Alkynylruthenium Complexes
A R T I C L E S
Table 3. Computed Significantly Allowed Single-Photon Transitions (those with oscillator strengths exceeding f ) 0.3 au) for
[Ru](C2C6H4)iC2C6H4NO2 (i ) 1-4), Obtained through PBE/TZP and LB94/TZP//PBE/TZP Calculations
PBE/TZP
LB94/TZP
model complexa
symmb
nc
E/eVd
f/aue
occ.f
virt.f
wt %g
nc
E/eV d
f/aue
occ.f
virt.f
wt %g
19M
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
1
1.533
2.810
1.112
2.162
2.975
0.43
0.81
0.33
0.83
1.01
14B2
14B2
17B2
17B2
14B2
15B2
16B2
18B2
19B2
18B2
94
74
97
88
42
1
3
1
3
5
7
10
3
5
6
7
9
2
3
4
5
6
7
1.167
2.441
0.749
1.870
2.527
2.801
3.206
1.507
2.086
2.244
2.326
2.695
1.034
1.264
1.393
1.764
1.876
2.064
2.374
2.620
0.35
0.67
0.30
0.78
0.34
0.70
0.36
0.62
0.34
0.20
0.84
0.76
0.13
0.41
0.10
0.28
0.38
0.88
0.90
1.01
14B2
14B2
17B2
17B2
14B2
17B2
15B2
20B2
17B2
20B2
19B2
18B2
22B2
23B2
21B2
20B2
23B2
22B2
21B2
22B2
15B2
16B2
18B2
19B2
18B2
20B2
19B2
22B2
21B2
23B2
22B2
22B2
24B2
25B2
24B2
24B2
26B2
25B2
25B2
26B2
92
44
96
65
65
70
49
84
75
79
73
53
96
94
85
79
89
80
42
56
3
1
3
5
20M
23M
24M
3
6
5
7
8
3
2
4
6
5
7
8
10
1.755
2.568
2.495
2.657
2.947
1.514
1.490
1.942
2.282
2.112
2.346
2.620
2.797
0.69
0.46
0.84
0.48
0.52
0.32
0.49
0.38
0.74
0.48
0.76
0.98
0.15
20B2
17B2
20B2
19B2
18B2
22B2
23B2
21B2
20B2
23B2
22B2
21B2
22B2
22B2
21B2
23B2
22B2
22B2
24B2
25B2
24B2
24B2
26B2
25B2
25B2
26B2
93
47
75
56
57
95
95
82
64
84
66
44
19
9
12
a Notation used for calculation on model compounds is consistent with that indicated in the main text. b Symmetry classification of the identified
transition. c Energy ranking of the identified transition within the indicated symmetry classification. d Calculated transition energy in electronvolts, at the
indicated level of theory. e Calculated transition oscillator strength, in atomic units, at the indicated level of theory. f Principal pair of occupied and
virtual orbitals involved in the identified transition. g Percentage contribution of principal-orbital character to the calculated transition, at the indicated
level of theory.
Theoretical Studies. Our TD-DFT calculations on the four
model compounds [Ru](C2C6H4)iC2C6H4NO2 (i ) 1-4) deliv-
ered, for each model, the 50 lowest energy symmetry-allowed
single-photon transitions. Of this number, only a handful of
calculated transitions for each model (all such transitions being
of A1 symmetry for the C2V-constrained models) have expecta-
tion values f greater than about 0.3 atomic unit (see Table 3).
Orbital characteristics for models 19M, 20M, 23M, and 24M
show considerable common features, as is to be expected from
their structural similarity. The HOMO in each case (11+3iB2) is
dominated by acetylenic character concentrated on the C2 unit
immediately adjacent to Ru, while the LUMO (12+3iB2) is
characterized by CdN bonding of the terminal phenylene to
the nitro group. The next few lower lying B2-symmetry occupied
orbitals show acetylenic character at progressively greater
separation from Ru at increasing distance from the “frontier”,
while the next few higher lying B2-symmetry virtual orbitals
are cumulenic in character, associated with C2 units that are
closer to the Ru as the orbital energy increases above the
LUMO.
Perhaps because the spatial separation of domains for the
HOMO and LUMO becomes more exaggerated as chain length
1
i is increased, the LUMOrHOMO excitation A1 becomes
progressively less important with increasing i, with the calcu-
lated f value for this transition decreasing monotonically at both
the PBE/TZP and LB94/TZP levels of theory. Instead, there is
a marked tendency toward the strongest transitions being those
where there is a close spatial correspondence between the
occupied and virtual domains.
Analysis of the TD-DFT results is hampered by inconsisten-
cies between the intensities calculated at the PBE/TZP level of
theory and those obtained at the LB94/TZP level.133 For
example, for 24M (see Figure 6) the most intense transitions at
the PBE/TZP level of theory are 6A1 (24B2r20B2), f ) 0.74 au;
8
7A1 (22B2r25B2), f ) 0.76 au; and A1 (21B2r25B2), f ) 0.98
(121) Touchard, D.; Haquette, P.; Pirio, N.; Toupet, L.; Dixneuf, P. H.
Organometallics 1993, 12, 3132.
au. For the same compound according to LB94/TZP, the latter
7
9
two transitions (here A1 and A1) are again calculated to be
intense, with f values of 0.88 and 0.90, very close to the PBE/
TZP results. However, the transition involving 24B2r20B2
character, here 5A1, has a much lower intensity (f ) 0.28), while
the most intense transition according to LB94/TZP, A1
(22B2r26B2), f ) 1.01 au, has only a weak counterpart (f )
0.15) in the PBE/TZP calculations. While the two TD-DFT
methods employed yield somewhat inconsistent results for the
identities of the most intense single-photon transitions, it remains
notable that the most intense transitions consistently display
(122) Hodge, A. J.; Ingham, S. L.; Kakkar, A. K.; Khan, M. S.; Lewis, J.;
Long, N. J.; Parker, D. G.; Raithby, P. R. J. Organomet. Chem. 1995,
488, 205.
(123) Ge, Q.; Hor, T. Dalton Trans. 2008, 2929.
(124) Fondum, T. N.; Green, K. A.; Randles, M. D.; Cifuentes, M. P.;
Willis, A. C.; Teshome, A.; Asselberghs, I.; Clays, K.; Humphrey,
M. G. J. Organomet. Chem. 2008, 693, 1605.
(125) Gauthier, N.; Olivier, C.; Rigaut, S.; Touchard, D.; Roisnel, T.;
Humphrey, M. G.; Paul, F. Organometallics 2008, 27, 1063.
(126) Fillaut, J.-L.; Andries, J.; Marwaha, R. D.; Lanoe, P.-H.; Lohio, O.;
Toupet, L.; Williams, J. A. G. J. Organomet. Chem. 2008, 693, 228.
(127) Onitsuka, K.; Ohara, N.; Takei, F.; Takahashi, S. Dalton Trans. 2006,
3693.
(128) Lavastre, O.; Fiedler, J. Organometallics 2006, 25, 635.
(129) Wong, W.-Y.; Wong, C.-K.; Lu, G.-L. J. Organomet. Chem. 2003,
671, 27.
(133) The problems of selecting the most appropriate functional for
predicting absorption spectra using TD-DFT have been discussed
recently. See, for example: (a) Jacquemin, D.; Perpe`te, E. A.;
Scuseria, G. E.; Ciofini, I.; Adamo, C. J. Chem. Theory Comput.
2008, 4, 123. (b) Peach, M. J. G.; Benfield, P.; Helgaker, T.; Tozer,
D. J. J. Chem. Phys. 2008, 128, 044118. (c) Peach, M. J. G.; Le
Sueur, C. R.; Ruud, K.; Guillaume, M.; Tozer, D. J. Phys. Chem.
Chem. Phys. 2009, 11, 4465.
(130) Rigaut, S.; Perruchon, J.; Le Pichon, L.; Touchard, D.; Dixneuf, P. H.
J. Organomet. Chem. 2003, 670, 37.
(131) Fillaut, J.-L.; Perruchon, J. Inorg. Chem. Commun. 2002, 5, 1048.
(132) Wong, C.-Y.; Che, C.-M.; Chan, M. C. W.; Han, J.; Leung, K.-H.;
Phillips, D. L.; Wong, K.-Y.; Zhu, N. J. Am. Chem. Soc. 2005, 127,
13997.
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A R T I C L E S
Babgi et al.
Figure 6. Energy-level diagram (obtained at the PBE/TZP level of theory) and orbital diagram for calculated intense single-photon transitions of the model
compound 24M. Transition term values are shown for the PBE/TZP and LB94/TZP levels of theory, respectively; a term value in parentheses indicates that
the transition in question is not notably intense (f < 0.3 au) at the indicated level of theory. A common feature of these intense transitions is the generally
close spatial correspondence between the electronic domains of the relevant occupied (solid line) and virtual (dotted line) orbitals. All of the transitions
identified as intense have excitation energies, at the PBE/TZP level of theory, of 2.0 eV (∼16 000 cm-1) or higher. In contrast, the HOMO-LUMO gap is
much less, only 0.73 eV (∼6000 cm-1), but does not feature significant overlap between occupied and virtual domains and is calculated to contribute only
weakly (f ∼0.1 au) to the single-photon excitation spectrum of 24M.
strong domain-region overlap between the identified occupied
and virtual orbitals. An apparent consequence of the increasing
importance of this effect with increasing chain length is that
the frequencies of the most intense transitions do not decrease
as rapidly as does the HOMO/LUMO energy separation. Indeed,
as the crucial orbitals are displaced progressively more from
the frontier, it may well appear that the important transitions
become blue-shifted on chain lengthening.
A second mechanism also exists whereby the most prominent
transitions detected can appear to blue-shift on chain lengthen-
ing. While the lowest energy conformer for 24M, as for all of
its smaller congeners, features coplanarity of all of the coaxial
aryl rings, the barriers to intramolecular rotation along any of
the ethynyl spindles are not large, with the calculated rotational
transition states lying typically 10 kJ mol-1, or less, above the
all-coplanar conformer.134 Further, while the primary effect of
moderate on-axis aryl rotation (i.e., for dihedral angle values
of up to 30°) is generally to reduce the energy of each allowed
transition by a modest quantity (i.e., by 300 cm-1 or less), there
is a more prominent effect on the calculated oscillator strength
for each transition, with internal rotation dampening some
transitions and strengthening others. This phenomenon can lead
to apparent blue-shifting on chain lengthening (since increasing
chain length corresponds to an increase in the number of
opportunities for on-axis aryl rotation) when rotation enhances
the relative strength of a higher energy transition relative to its
lower energy neighbor. Since the spectral features observed in
the laboratory are typically broad (∼5000 cm-1 fwhm or
similar), a rotationally mediated change in the relative intensity
of two or more close-lying transitions may well manifest as
the blue-shifting of a single broad peak effectively enveloping
several transitions.
It is important to note that, while both of these phenomena
can result in blue-shifting on chain lengthening, neither effect
is exactly systematic: they therefore appear to be of little
predictive value. However, they provide a rationalization for
the somewhat counterintuitive observation that elongation of
the phenyleneethynylene “axis” can result in an increase in the
frequency of the observed spectral peak.
Quadratic Nonlinear Optical Studies. The quadratic nonlin-
earities of 19-24 have been determined at 1064 nm; these
results, together with previously reported measurements for the
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NLO Properties of Dipolar Alkynylruthenium Complexes
A R T I C L E S
Figure 7. Comparison of the dispersion of the complex hyperpolarizability of (a) trans-[Ru(4-CtCC6H4NO2)Cl(dppm)2] (28), (b) trans-[Ru(4,4′-
CtCC6H4CtCC6H4NO2)Cl(dppm)2] (29), (c) trans-[Ru{4,4′,4′′-CtCC6H4CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (20), (d) trans-[Ru{4,4′,4′′,4′′′-
CtCC6H4CtCC6H4CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (23), and (e) trans-[Ru{4,4′,4′′,4′′′,4′′′′-CtCC6H4CtCC6H4CtCC6H2[2,5-
(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (24), with the one-photon absorption spectra of the compounds (real component: solid squares;
imaginary component: open triangles). Note that the hyperpolarizability values were plotted against twice the measurement wavenumber. This permits
comparison of two-photon absorption (represented here as the imaginary part of the hyperpolarizability) with one-photon absorption to the same final state.
related complexes 25-30, are presented in Table 2. The major
focus of the present study is to assess the impact of π-bridge
lengthening upon optical properties. For the dppe-containing
complexes, π-bridge lengthening, in proceeding from 25 to 26,
27, and finally 21, results in nonlinearity peaking at the
tri(phenyleneethynylene)-containing complex 27, but there are
insufficient data to deconvolute the influence of the ethoxy
substituents introduced into 21 to ensure sufficient solubility.
We have a more extensive set of dppm-containing complexes
that permit further comment. Introduction of solubilizing groups
(proceeding from 29 to 19, or 30 to 20) results in a ca. 30%
reduction in ꢀ1064, while further increasing the ethoxy content
(proceeding from 23 to 22) results in no further change, within
the error margins. There is sufficient data to offer a comment
on the effect on ꢀ of π-bridge lengthening; proceeding from
28 to 29, and then 30 (no ethoxy substituents), 20 to 23 (two
ethoxy substituents), and 22 to 24 (four ethoxy substituents)
suggests that ꢀ peaks at the tri(phenyleneethynylene) stage, after
which a saturation effect is apparent.
Table 2 also contains a more limited set of data at 1300 nm
and two-level-corrected values for the two wavelengths. The
results for the former are in contrast to those at 1064 nm.
Introducing solubilizing substituents, in proceeding from 29 to
19, results in an increase in nonlinearity, while a further increase
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A R T I C L E S
Babgi et al.
Figure 8. Comparison of the two-photon absorption dispersion of (a) trans-[Ru(4-CtCC6H4NO2)Cl(dppm)2] (28), (b) trans-[Ru(4,4′-
CtCC6H4CtCC6H4NO2)Cl(dppm)2] (29), (c) trans-[Ru{4,4′,4′′-CtCC6H4CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (20), (d) trans-[Ru{4,4′,4′′,4′′′-
CtCC6H4CtCC6H4C≡CC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (23), and (e) trans-[Ru{4,4′,4′′,4′′′,4′′′′-CtCC6H4CtCC6H4CtCC6H2[2,5-
(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (24).
in ethoxy content (proceeding from 23 to 22) results in a
decrease in ꢀ value. The π-bridge lengthening does not result
in a saturation point being reached, ꢀ increasing on proceeding
from 28 to 29 (no ethoxy substituents) and 20 to 23 (two ethoxy
groups). Two-level-corrected data should be viewed with
extreme caution; while the trends are broadly similar to those
for the corresponding ꢀλ values, the shortcomings of the model
have been discussed extensively elsewhere (ref 100 and refs
therein).
photon absorption). A few data were collected with moderate
linear absorption, and these consequently have large error
factors.
With reference to the results depicted in Figure 7, there is
generally an approximate correspondence between the one-
photon absorption bands and the main two-photon absorption
band. The low-energy nonlinear absorption band shown in
Figure 7 in the range ca. 12000-18000 cm-1, and appearing
for laser wavelengths of ca. 1100-1500 nm, seems to have a
multiphoton character with possible strong contributions from
three-photon absorption.
Figure 8 shows the absorptive part of the cubic nonlinearity
of the five compounds replotted as the (effective) two-photon
cross-section versus wavelength of the incident laser beam, with
relevant data being collected in Table 2. The rapid increase of
the maximal value of the two-photon cross-section in proceeding
from 28, through 29, to 20 (at ∼900 nm for all the compounds)
Cubic Nonlinear Optical Studies. The cubic NLO properties
of selected complexes were assessed over a wide spectral range
(Figure 7), the specific complexes (20, 23, 24, 28, 29) being
chosen to ensure sufficient solubility for the Z-scan studies.
Figure 7 contains the hyperpolarizabiliy values plotted against
twice the measurement wavenumber, to facilitate comparison
with the linear optical spectrum included in the figure. The
studies were undertaken in essentially transparent regions of
the UV-vis-NIR spectra (from 6700 cm-1 to the onset of one-
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NLO Properties of Dipolar Alkynylruthenium Complexes
A R T I C L E S
is followed by stabilization of the value in proceeding to the
longer π-bridge compounds 23 and 24.
of OPEs end-functionalized by di(dodecyl)amino and nitro
groups has been examined, with the linear optical absorption
maximum blue-shifting on chain-length increase until the trimer
is reached, further π-system lengthening leaving λmax invari-
ant.139 Third-harmonic generation studies of these dialkylamino-
nitro-substituted OPEs were hampered by solubility problems.140
The present studies expand the scope of donor-acceptor-
functionalized OPEs to include examples incorporating a ligated
metal center. The blue-shift of λmax on chain-lengthening this
metal-containing system mirrors that seen in the examples above.
Our theoretical studies have identified two possible sources of
the observed blue-shift of λmax on chain-lengthening: decreasing
importance of the LUMO r HOMO transition and on-axis aryl
rotation-mediated change in the relative intensities of two or
more close-lying transitions.
Discussion
The present studies have afforded a series of OPEs end-
functionalized by ligated Ru and nitro groups. Increasing OPE
length is associated with decreasing solubility, which at the
tetra(phenyleneethynylene) length is sufficiently diminished as
to cause problems with chemical synthesis. Alkoxy substituents
have been employed to ensure adequate solubility both for
chemical synthesis and for NLO studies. Solubilizing groups
such as these are not necessarily electronically and optically
innocent, and so, recognizing that these groups are essential,
we have sought to define their influence. Introducing solubilizing
groups at the phenylene adjacent to the metal results in a
decrease in E0 and a red-shift in the low-energy absorption
ox
The major outcome of the present work has been to define
the effective saturation length of OPE examples end-function-
alized by ligated metal and nitro group (donor-bridge-acceptor
arylalkynylruthenium complexes), for which metal-centered
oxidation potential, linear optical absorption maximum, qua-
dratic optical nonlinearity assessed by HRS at 1064 nm, and
two-photon-absorption cross-section assessed by Z-scan all
plateau at the tri(phenyleneethynylene) complex. These studies
have therefore afforded the specific bridge length for the most
efficient optical materials with this metal-containing donor
group, as well as providing an interesting comparison of
saturation length with related, purely organic, compounds.
bands. We have not probed this modification theoretically, but
introduction of electron-donating substituents at the bridge
phenyl ring adjacent to the ruthenium is expected to increase
electron richness at the metal center, and thereby increase ease
of oxidation. This substitution will also destabilize the HOMO
and SHOMO, which are localized at the metal and adjacent
ring, resulting in a red-shift in the optical absorption maximum.
Introducing solubilizing groups at rings remote from the metal
center results in a much smaller red-shift in the optical
absorption maximum and does not affect the ease of oxidation,
consistent with the aforementioned HOMO/SHOMO localiza-
tion. Incorporation of these solubilizing groups results in a ca.
30% reduction in ꢀ1064 and ꢀ0. The calculations are consistent
with the dominant charge-transfer transition (which would be
expected to contribute significantly to the observed nonlinearity)
being metal-to-bridge in character and with a diminished
importance of this transition on solubilizing group incorporation
and thereby a decreased nonlinearity.
Acknowledgment. We thank the Australian Research Council
(M.G.H., M.P.C., M.S., R.S.), the Fund for Scientific Research-
Flanders (FWO-Vlaanderen; FWO G.0312.08) (K.C.), the Katho-
lieke Universiteit Leuven (GOA/2006/03) (K.C.), and Wroclaw
University of Technology (M.S.) for support of this work. M.G.H.
is an ARC Australian Professorial Fellow. B.B. thanks King
Abdulaziz University for sponsorship.
The π-bridge length-dependent evolution of optical properties
in organic oligomers has been of considerable interest.134,135
Poly(phenyleneethynylene)s have potential as photolumines-
cence or nonlinear optical materials or as fluorescent chemosen-
sors, and monodisperse examples of a wide range of lengths
have been prepared.136-138 Related to the present work, a series
Supporting Information Available: List of authors of ref 108,
the syntheses of compounds 1-18 and complexes 20-24, and
a cif file giving solution and refinement details and tables of
atomic coordinates, bond lengths, and bond angles for the X-ray
structural study of 19. This material is available free of charge
via the Internet at http://pubs.acs.org.
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