Metal-Containg and Metallosupramilecular Polymers and Materials

Chapter 19

Syntheses and Nonlinear Optical Properties of Alkynylruthenium Dendrimers 1

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M a r k G . Humphrey , Clem E. Powell , Marie P. Cifuentes , Joseph P. Morrall , and Marek Samoc Downloaded by COLUMBIA UNIV on August 10, 2012 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch019

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Department of Chemistry and Research School of Physical Sciences and Engineering, Australian National University, Canberra A C T 0200, Australia

Low-generation alkynylruthenium dendrimers with arylethynyl branching and spacer units have been synthesized. A "steric control" methodology to rapidly afford the necessary organometallic dendrons has been developed. The dendrimer complexes possess interesting nonlinear optical properties, including very large two-photon absorption cross-sections, the wavelength dependence of which has been examined in one case. They also undergo reversible oxidation in solution, which results in both linear and nonlinear electrochromism.

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© 2006 American Chemical Society

In Metal-Containing and Metallosupramolecular Polymers and Materials; Schubert, U., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Background The nonlinear optical (NLO) properties of materials have come under increasing scrutiny. This is because interactions of light with materials possessing N L O properties can result in new electromagnetic field components (e.g. with differing phase, frequency, amplitude, polarization, path, etc). N L O materials have potential applications in optical signal processing, switching and frequency generation and may also contribute to optical data storage, optical communication, and image processing. Inorganic salts were the initial focus of attention as second-order N L O materials, after the invention of the laser made it possible to observe these effects, but the potential of organic molecules was soon realized and this became a major area of study. More recently, organometallic complexes have attracted attention, as they possess the advantages of organic molecules (a purely electronic nonlinearity with a time domain of fs, ready processability into films, and ready access to systematically-modified compounds which permits structure-property studies and optimization of responses), together with greater structural diversity and the accessibility of more than one oxidation state for the metal.

Downloaded by COLUMBIA UNIV on August 10, 2012 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch019

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The N L O effects from organometallic complexes can be explained by considering the effect of a local electric field Ej acting on a molecule. This can distort the molecular electron density distribution p(r), the distortion being described in terms of changes in the dipole moment μ. Changes in the dipole moment induced by a weak field are linear with the magnitude of the field, but when E / is comparable in strength to the internal electric fields within the molecule, the distortion and the induced dipole moment should be treated as nonlinear functions of the field strength: oc

o c

μ = μο + aE

l0C

+PE

/ f l C

E

/ e c

+yE

/ o c

E

/ o c

E

/ o c

+ ...

(1)

The tensors α, β and γ defined by the above equation are the linear polarizability, the second-order or quadratic polarizability (the first hyperpolarizability) and the third-order or cubic polarizability (the second hyperpolarizability), respectively. Both μ and E / are vectors, α is a secondrank tensor, β is a third-rank tensor, and γ is a fourth-rank tensor. Many tensor components of α, β, and γ are equivalent by symmetry rules or equal to zero. Polarizabilities are invariant with respect to all point group symmetry operations, so all the components of β vanish in centrosymmetric point groups. o c

The electric field of a light wave can be expressed as: E(t) = E cos(œt) 0


260 Equation (1) can therefore be written as: 2

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μ ( 0 = μο + aE cos(û>t) + P E c o s ( œ t ) + yE cos3(©t) + ... 0

0

0


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Trigonometric relations such as cos (©t) = 1/2 + l/2cos(2©t) reveal that the nonlinear terms in the dipole moment expansion introduce contributions at different frequencies, the second-order (β) term introducing a time-independent (d.c.) contribution and a term oscillating at 2ω (second-harmonic generation), and providing a frequency mixing phenomenon i f the input field is a sum of two components with different frequencies. A constant (d.c.) field may influence an oscillating field i f the two are combined in a medium containing second-order nonlinear molecules (the electro-optic effect). The cubic term in Equation (1) leads to several N L O effects, one being oscillation of the induced dipoles at 3ω (third-harmonic generation). The magnitude of these effects depends on the magnitude of the N L O coefficients β and γ, so a major focus of studies has been to prepare materials with large N L O coefficients that are stable to processing and subsequent device operating conditions. Second-order effects have been the subject of an enormous number of studies, and consequently structure-NLO activity relationships have been developed that enable one to design compounds with optimized quadratic N L O response. While less is known of molecular structure-NLO activity relationships for third-order properties than is known for second-order properties, it has been established with organic compounds that increase in π-delocalization possibilities (e.g. progressing from small molecules to π-conjugated polymers), the introduction of strong donor and acceptor functional groups, controlling chain orientation, packing density, and conformation, and increasing dimensionality can all result in increased cubic nonlinearity. However, with the exception of several studies of organometallic polymers, few large organometallic molecules have been examined, and so structure-cubic N L O activity studies are sparse. Dendrimers are monodisperse hyperbranched molecules that have attracted significant interest recently as novel materials with possible uses in medical diagnostics, molecular recognition, catalysis, and photoactive device engineering. Although organic dendrimers dominate the field, organometallic dendrimers have been the focus of considerable interest because the metal may imbue the dendritic material with specific optical, electronic, magnetic, catalytic, and other properties. The great majority of organometallic dendrimers are peripherally-metalated organic dendrimers. In contrast, considerably fewer coremetalated and other-shell-metalated dendrimers have been reported.


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Organometallic dendrimers with transition metals in every generation are comparatively rare, and rigid, π-delocalizable examples even more so. Examples incorporating 16-electron group 10 metals within an arylalkynyl structure have been recently reported by several groups. Dendrimers of this type are also of interest because of possible applications in nonlinear optics: N L O materials with a dendritic construction may have enhanced nonlinearities coupled to favorable transparency and processing characteristics, because the 1,3,5-trisubstituted benzene branching points in arylalkynyl dendrimers may permit extensive πdelocalization without appreciable red-shift of the important linear optical absorption band(s).


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Objective The discussion above suggests that metal-containing π-delocalizable dendrimers may have enhanced nonlinearities. This study involved development of synthetic procedures to electron-rich ( 18-valence-electron) bis(diphosphine)ruthenium-containing arylalkynyl dendrimers and investigation of their cubic N L O properties; the latter has revealed very large two-photon absorption cross-sections, the wavelength dependence of which has been examined. As these compounds have fully reversible redox processes, the possibility of "switching" the N L O properties of these complexes using electrochemical stimuli has also been probed.

Experimental Experimental procedures to and spectroscopic characterization of the complexes l,3,5-C H {4-CsCC H4CsC-/ra«5-[RuX(dppe) ]}3 (X = O C P h (1), 4 - O C C H N 0 (2), 4 - O C C H N E t (3)), 1 , 3 , 5 - C H ( 4 - C S C C H C S C 6

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/raw-tRu(dppe)2]CsC-3,5-C H3{4-CsCC H CsC-/ran5-[RuX(dppe)2]}2)3 ( X = O C P h (4), 4 - O C C H N 0 (5)), 1,3,5-C H (4-CSCC H CSC-3,5C H { C s C - / ^ 5 - [ R u X ( d p p e ) ] } ) ( X - O C P h (6), 4 - O C C H N 0 (7)) (4,5) and l,3,5-C H {4-CsCC H CsC-/ra/w-[RuCl(dppe) ]} (8) have been reported in detail elsewhere. 6

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UV/vis spectra were recorded as THF solutions in 1 cm cells using a Cary 5 spectrophotometer. The cyclic voltammetric measurement was recorded using a MacLab 400 interface and MacLab potentiostat from ADInstruments (using Pt disc working, Pt auxiliary and Ag-AgCl reference mini-electrodes from Cypress Systems). The scan rate was 100 mV s-*. The electrochemical solution contained


262 0.1 M (NWBU4)PFÔ and ca. Ι Ο

- 3

M complex in C H 2 C I 2 . The solution was purged

and maintained under an atmosphere o f argon, and referenced to an internal ferrocene/ferrocenium couple (E° at 0 . 5 6 V ) . Spectroelectrochemical data were recorded on a Cary 5 spectrophotometer ( 4 5 0 0 0 - 4 0 0 0 cm-1) in C H C ! . The solution spectra of the oxidized species at 2 5 3 Κ were obtained by electrogeneration (Thompson 4 0 I E potentiostat) at a Pt gauze working electrode within a cryostatted optically transparent thin-layer electrochemical (OTTLE) cell, path-length 0 . 5 mm, mounted within the spectrophotometer. The electrogeneration potential was ca. 3 0 0 mV beyond E1/2, to ensure complete electrolysis. The efficiency and reversibility was tested by applying a sufficiently negative potential to regenerate the starting compound; stable isosbestic points were observed in the spectral progression. Downloaded by COLUMBIA UNIV on August 10, 2012 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch019

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Z-scan measurements at 8 0 0 nm ( 1 2 5 0 0 cm- ) were performed using 100 fs pulses from a system consisting o f a Coherent Mira Ti-sapphire laser pumped with a Coherent Verdi cw pump and a Ti-sapphire regenerative amplifier pumped with a frequency-doubled Q-switched pulsed N d : Y A G laser (Spectra Physics G C R ) at 3 0 H z and employing chirped pulse amplification. Tetrahydrofuran solutions were examined in a 0.1 cm path length cell. The closed-aperture and open-aperture Z-scans were recorded at a few concentrations of each compound and the real and imaginary part of the nonlinear phase shift determined by numerical fitting using equations given in reference. The real and imaginary part o f the hyperpolarizability o f the solute was then calculated by linear regression from the concentration dependencies. The nonlinearities and light intensities were calibrated using measurements of a 1 mm thick silica plate for which the nonlinear refractive index Λ ~ 3 χ 1 0 " c m W* was assumed. 6

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"Switching" the cubic nonlinearity at 8 0 0 nm was performed as follows. A n argon-saturated dichloromethane solution containing ca. 0.3 M ( N W B U ^ P F Ô supporting electrolyte was examined in an O T T L E cell (with Pt auxiliary-, Pt working- and Ag-AgCl reference electrodes), path length 0.5 mm, with the 8 0 0 nm laser beam passing through a focussing lens and directed along the axis passing through a 1.5 mm diameter hole in the Pt sheet working electrode. The electrochemical cell was mounted on a computer driven translation stage, as usual in Z-scan measurements. The wo parameter of the beam (the radius at the 1/e intensity point) was chosen to be in the range 3 5 - 4 5 μπι. The Rayleigh 6

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length z

R

=— -

f

where WQ is the Gaussian beam waist and λ is the

wavelength, was therefore taken to be z > 3 mm. A "thin sample" assumption was therefore considered to be justified. In effect, one can then treat the total effect o f the third-order nonlinearity of all the components o f the system, the solution (solvent and dissolved materials) and the glass walls of the cell, as being R


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an additive quantity. The beam "cropping' by the aperture was also negligible over the range of travel of the cell (z = -3 cm to +3 cm from the focal plane), the beam radius growing by roughly a factor of ten (i.e. to about 350-450 μηι) over the distance of ten Rayleigh lengths, but still providing for almost complete transmission through the 1.5 mm aperture. The beam transmitted through the electrochemical cell was split in two, one part being focussed on an "open aperture" detector, the other part being transmitted through a 1 mm aperture to provide the "closed aperture" signal. Z-scans were collected with the electrochemical cell. The electrogeneration potential was 0.8 V to ensure complete electrolysis; this required approx. 5 min. The Z-scan measurements were carried out during the electrolysis and were continued while the electrode potential was cycled from zero to 0.8 V and back to zero again. The real and imaginary parts of the nonlinear phase change were determined for the resting state and electrochemicaly modified molecules in the same way as for our standard Z-scan measurements. For absorbing solutions it was assumed that the absorption saturation process can be modelled by a linear dependence of the absorption coefficient on the light intensity. The nonlinearities and light intensities were again calibrated against silica. 7

Wavelength dependence of the cubic nonlinearity was assessed by Z-scan measurements performed using two amplified femtosecond laser systems. The first system was based on a Coherent Mira-900D Ti-sapphire oscillator and included a chirped pulse Ti-sapphire amplifier operating at a repetition rate of 30 Hz. This system was used at wavelength 800 nm and provided ca 150 fs F W H M pulses. The second system was a Clark-MXR CPA-2001 regenerative amplifier pumping a Light Conversion TOPAS optical parametric amplifier. This system was operated at a repetition rate of 250 H z (reduced from the usual default rate of 1 k H z to minimize potential problems with thermal effects and sample photodecomposition). The output of the optical parametric amplifier was tuned in the range 650 nm to 1300 nm using the second harmonic of the signal, the second harmonic of the idler, or the signal, respectively, in three wavelength ranges for tuning the system. The pulse duration was ca 150 fs. The Z-scan set ups used lenses with the focal lengths suitable for creating focal spots with the 1/e radius, M> , being in the range 40 - 65 μηι. This resulted in the Rayleigh lengths being greater than 3 mm throughout the wavelength range employed; the measurements of solutions in 1 mm thick glass cells with ca. 1 mm thick glass walls could therefore be always treated in the thin sample approximation. Due to the deviations from Gaussian character of the beam from the ΟΡΑ, it was necessary to perform some spatial filtering of the beam. This resulted in the beam approximating the truncated Airy pattern case discussed by Rhee et aL* A l l measurements were calibrated against Z-scans taken on the pure solvent and on silica and glass plates of thicknesses in the range 1 - 2.5 mm. It was assumed that 2

0


264 the dispersion of the nonlinear refractive index of silica can be neglected in the range of wavelengths investigated, and so the value w = 3 χ 1 0 · c m W ' was adopted throughout the range. The light intensities used in the three different wavelength ranges differed somewhat, but as a rule the intensities were adjusted to obtain nonlinear phase shifts for the measured samples in the range 0.5 - 1.0 rd, which corresponded to peak intensities of the order of 100 G W cm- . 1 6

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Results & Discussion Our initial attempt to prepare a zero-generation dendrimer utilized 1,3,5triethynylbenzene, but while chloro(triphenylphosphine)gold(I) reacted cleanly to afford [ 1,3,5-{(Ph P)Au} C H ], excess ris-[RuCl (dppm) ] (dppm = bis(diphenylphosphino)methane) afforded the bis-adduct [\,3-trans[RuCI(dppm) ] -5-HC CôH3] only, structural studies confirming that steric constraints had restricted the extent of reaction. We therefore inserted arylethynyl "spacer" units between the 1,3,5-triethynylbenzene core and the ligated metal units and were successful in obtaining the desired dendrimers, the convergent procedure that we employed being shown in Figure 1. 3

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The lengthy syntheses in Figure 1 are not ideal when the intent is to promulgate physical property investigations, so we developed a more rapid methodology for dendron synthesis. Although an undesired product from an attempt to trimetalate triethynylbenzene, [l,3-/raw.s-[RuCl(dppm)2]2-5HC2C6H3] can also be considered as an archetypal organometallic dendron, with the A B 2 1,3,5-trisubstituted benzene composition required for alkynylruthenium dendrimer construction. Replacing the bidentate phosphine dppm with l,2-bis(diphenylphosphino)ethane (dppe) has permitted the reaction sequence shown in Figure 2, and thereby rapid access to nanometer-sized πdelocalized peripherally-metalated complexes. 4

Cubic N L O studies at 800 nm Linear optical and cubic N L O data for 1-5 are collected in Table 1. Increasing the size of the complexes in proceeding from 1 and 2 to 4 and 5, respectively, does not reduce optical transparency significantly; the small blue shift observed in proceeding from 1 to 4 may indicate a lack of co-planarity through the dendritic structure of 4. The significant y values for all complexes are indicative of two-photon absorption, which becomes important as X approaches the wavelength corresponding to 2ω (ω = frequency of incident radiation). The γ values and two-photon absorption cross-sections σ of these dendritic complexes are amongst the largest for organometallic complexes thus far(/0,//). imag

m a x

2



Figure 1. Synthesis of alkynylruthenium dendrimers by a convergent procedure.



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Figure 2. Synthesis of alkynylruthenium dendrimers by a convergent procedure, and employing "steric control" for dendron synthesis.


267 Table 1. Linear Optical and Cubic Nonlinear Optical Data Cmpd

h™

α

ε

h

Yreal

c

Metal-Containg and Metallosupramilecular Polymers and Materials

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