Materials for Nonlinear Optics - ACS Publications - American

Chapter 40

Transition Metal Acetylides for Nonlinear Optical Properties 1

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Todd B. Marder , Gerry Lesley , Zheng Yuan , Helen B. Fyfe , Pauline Chow , Graham Stringer , Ian R. Jobe , Nicholas J. Taylor , Ian D. Williams , and Stewart K. Kurtz 1

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Downloaded by CORNELL UNIV on October 19, 2016 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch040

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Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Materials Research Laboratory, Pennsylvania State University, University Park, PA 16802

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Transition metal acetylide complexes represent a class of linear conjugated molecules which can exhibit both second- and third-order optical nonlinearities. We have prepared several classes of such complexes including both symmetrically (X = Y) and unsymmetrically (Χ ≠Y) substituted bis(acetylide) complexes trans-[Pt(PMe Ph) (C=C-X)(C=C-Y)]. Complexes where X is a strong π-donor and Y is a strong π-acceptor have the fundamental electronic and structural relationships required for good X materials. Binuclear and polymeric rhodium compounds containing bridging M-C=C-(C H - -) -C=C-M units (n = 0, 1, 2) have also been prepared. Second Harmonic Generation was observed in powder samples of all unsymmetric platinum bis(acetylides), with the largest powder efficiencies being on the order of urea. Single-crystal X-ray diffraction studies on several symmetric and unsymmetric compounds demonstrate both intra- and intermolecular parallel alignment of all acetylide units. The complex trans-[Pt(PMe Ph) (C=C-C H -4-OMe)(C=C-C H -4-NO )], for example, crystallizes with point group symmetry 1 (space group P1, Ζ = 1), which givesriseto parallel alignment of all molecular dipoles. 2

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The second- and third-order nonlinear optical properties of organic molecules and polymers are the subject of much current interest (1-5). The molecular properties which are critical include a delocalized π-system. For second-order effects, an accessible charge-transfer transition which gives rise to a large change in dipole moment, and a non-centrosymmetric molecular structure are required. In addition, for a bulk material, such as a single-crystal, to exhibit optimal second-order effects, a non-centrosymmetric arrangement of molecules is required in which there is a large degree of alignment of the molecular dipoles. Acetylenes R-C=C-R' and higher oligomers, such as diynes R - ( C = C ) 2 - R ' , triynes R-(C=C)3-R', and tetraynes R-(CHC)4-R', have the requisite conjugation required for both χ( ) and χ( ) effects. With suitable substitution patterns, R = π2

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0097-6156/91/0455-0605$06.00/0 © 1991 American Chemical Society Marder et al.; Materials for Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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donor and R' = π-acceptor, low-lying charge transfer transitions and molecular asymmetry can be incorporated to enhance molecular hyperpolarizability, β. The remaining requirement for a crystalline material, i.e. bulk dipole alignment in a crystal lattice, is somewhat more difficult to control. Recently, several groups have demonstrated (6-13) large χ( ) values for donoracceptor substituted diphenyl acetylenes 4-D-C6H4-&C-C6H4-4-A and their higher oligomers 4-D-C6H4-(CsC) -C6H4-4-A. In one case (11), phase-matched second harmonic generation (SHG) was confirmed for the compound 4-MeO-C6H4-OCC5H4-4-NO2. Organotransition metal complexes, including square-planar compounds of the general form trans-[M(PR3)2(X)(C6H4-4-A)] ( M = Pd, Pt), have also been shown (3.14-24) to exhibit large second-order optical nonlinearities. SHG values as high as ca. 220x urea have now been reported for certain ferrocene derivatives (3.2224). In addition, a series of symmetrically substituted square-planar palladium and platinum acetylides, and their polyyne polymers, have been reported (25-28) to exhibit significant χ( ) values, and their incorporation into electro-optic devices has now been achieved (29). Transition metal acetylides combine the properties of acetylenes with those of the transition metals, offering flexibility in the tuning of structural and electronic properties of both the organic and inorganic constituents. Optimization of the molecular and bulk crystalline properties is envisaged to lead to a new class of useful nonlinear optical materials. The metal acetylides are inherently linear molecules due to the ca. 180° bond angles in an L M - ( C = C ) - R unit. Linearity can be extended by use of transbis(acetylides), R-iC=Qn-MLn-iC=Qn-R', in which the ( O C ) - M - ( O C ) angle is also 180°. The transition metal acetylide σ-bonds are extremely strong, giving rise to the thermodynamic stability in complexes with < 8 d-electrons. Of importance in terms of optimizing optical nonlinearities is the degree of interaction between the metal d-block orbitals and the acetylene ρ-π and ρ-π* systems. Molecular orbital calculations (31.33) on several L M - O C - H complexes indicated weak π-donor and π-acceptor properties for the - O C - H group. However, calculations on diynes and on acetylide groups with strong π-acceptor substituents, -C=C-A (Marder, T.B.; unpublished results), suggest that such perturbations can significantly alter the π and π* orbital energies and atomic coefficients resulting in excellent π-acceptor properties. Recent electrochemical experiments (34). on a series of (chelate)Rh-C=C-A complexes, have confirmed this hypothesis. In contrast, substitution with strong πdonors should raise the energies of the filled acetylide π-orbitals allowing for the design of good π-donor ligands. The combination of a good π-donor acetylide and a good π-acceptor acetylide in trans-positions on a metal center would provide the push-pull relationship resulting in low-lying intramolecular charge transfer transitions, and lack of a molecular center of symmetry. Thus, the molecular hyperpolarizability, β, should be large, and if the crystal packing can be controlled, large χ( ) values should be achieved. During the course of our studies (30-32) of the synthesis and structures of rhodium acetylide and hydrido-acetylide complexes, we developed (32) a step-wise route to trans-bis(acetylides) of the general form mer-ira>ts-[Rh(PMe3)3(H)(C=CR) (C=CR*)]. Unfortunately, scrambling processes have thus far precluded the preparation of the unsymmetrically substituted complexes (R Φ R') in the absence of 2


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Marder et al.; Materials for Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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their symmetric (R = R') counterparts. However, single crystal X-ray diffraction studies (32) on three of the symmetric hydrido-bis(acetylides) demonstrated an interesting feature of the crystal packing of such complexes. We observed that, in each case, all acetylide moieties are aligned in a parallel fashion throughout the crystal lattice. The possibility that such a packing arrangement might be a general feature of bis(acetylides) led us to prepare a series of symmetrically substituted transbis(acetylide) complexes of platinum. In contrast to the rhodium systems, which are octahedral d -compounds, the platinum complexes are square-planar d -complexes. We have developed a successful synthetic route to the unsymmetrically substituted platinum bis(acetylides) which avoids the formation of significant quantities of the symmetric counterparts. Finally, we developed routes to both linear bimetallic and polymeric acetylide complexes of Rh, Pd, and Pt, which will allow tuning of the electronic nature of the linker groups. This paper reports the synthesis, representative single-crystal X-ray structural data, optical spectra, and preliminary measurements of powder SHG efficiencies of the new compounds.


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Preparation of Terminal Alkvnes and Diynes The terminal alkynes (35.36) 4 - R - C H - O C H (R = N 0 , l a ; C N , l b ; MeO, l c ; MeS, I d ; H2N, l e ; Me2N, If), 4-ethynylpyridine 2 Q2), and ethynylferrocene (FcC^CH) 3 (38). were prepared by modifications of literature routes. Compound Id, which was not previously reported, was prepared by coupling of 4-MeS-C6H4Br with Me3SiC=CH, by analogy with the other para substituted phenyl acetylenes. Terminal diynes, R-C=C-C=CH (R = Ph, 4a; Fc, 4b), were prepared by coupling of the appropriate alkyne, R-C=CH with ri.s-ClHC=CHCl, followed by treatment with base (22). Diacetylene (5) was prepared by treatment of C1CH2&CCH2C1 with base (40), and the compounds 4-HC=C-(C6H4) -C=CH (n = 1, 6a; η = 2, 6b) were prepared via coupling (41) of either Me3SiC=CH or HOC-C(OH)Me2 with the 1,4C6H4Br2 or 4,4'-BrC6H4C6H4Br, followed by deprotection. We also prepared and structurally characterized 9,10-bis(trimethylsilylethynyl) anthracene; however, clean deprotection to 9,10-bis(ethynylanthracene) has not yet been achieved. 6

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Preparation of Symmetric and Unsymmetric rrafl,y-Bis(acetylide) Complexes of Platinum The symmetric bis(acetylides), trans-[Pt(PMe2Ph)2(OC-R)2] (R = MeO, 7a; NO2, 7b; 4-py, 7c), can be prepared conveniently, and in high yield, by room temperature (ca. l-2hr) reaction of two equivalents of R - & C H with [Pt(PMe2Ph)2Cl2] in Et2NH solution in the presence of Cul, a route initially reported by Hagihara et al. (42). The bis(acetylides), iraAi5-[Pt(PMe2Ph)2(C=C-C=C-R)2] (R = Ph, 8a; Fc, 8b), were also prepared by this route. Preparation of the unsymmetric complexes was more difficult in that the two acetylides must be attached sequentially, and scrambling must be avoided. Refluxing a solution of [Pt(PMe2Ph)2Cl2] and l a or l b in CHCI3 in the presence of Et2NH for 3 days yielded the mono-acetylide species trans[Pt(PMe2Ph)2(Cl)C=C-C H -4-A)] (9a, A = N 0 , 88%; 9b, A = C N , 98%). This 6

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general procedure, reported by Furlani et al. (42), avoids undesired formation of significant quantities of the symmetric bis(acetylide) complexes. We find that with careful control of reaction time and reagent stoichiometry, the mono-acetylides (9a,b) react with the donor-alkynes lc-f, 2, and 3 in C H C I 3 in the presence of small amounts of E t 2 N H and C u l , yielding the novel unsymmetrically substituted complexes irani-[Pt(PMe2Ph) (C=C-C6H -4-A)(C^C-D)] (10,11). Using short reaction times (< 1 hr, room temperature) and relatively small amounts of E t 2 N H (ca. 100 equiv.) and Cul (ca. 0.1 equiv.), acetylide scrambling is avoided. 2

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PMe Ph I D-(SC-Pt-C=C-^(J>-A PMe Ph


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Preparation of Dinuclear and Polymeric Rhodium Complexes Linked by Acetylides Our previously reported synthetic strategies (30-32). based on oxidative addition of terminal alkyne C-H bonds to RhW centers, could be extended to include bifunctional alkynes H C = C - ( C 6 H - 4 - ) - C = C H (n = 0-2). Thus, reaction of 2 equiv. of [(PMe3) Rh]Cl with 5 or 6a,b in T H F suspension gave the dinuclear hydridoacetylides cz y-cw-[{Rh(PMe3)4(H)}2{μ-C C-(C6H -4-)n-C C-}] [Cl2] • (n = 0, 12a; η = 1,12b; η = 2,12c) in high yields as white precipitates. 4

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Reaction of [(PMe3) RhMe] with two equiv. of 6a,b yields the bis(acetylide)hydride complexes mer-irû« y-[Rh(PMe3)3(H){-C=C-(C6H -4-) -C=CH}2] (η = 1, 13a; η = 2, 13b) in which one terminal hydrogen still remains on each end for subsequent reactions. 4

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A similar reaction conducted with 1:1 stoichiometry yields the rigid-rod polymeric species mer-ira/i5-[Rh(PMe3)3(H){C=C-(C6H -4-) C=C}-]x (η = 1,14a; η = 2,14b) directly as insoluble powders. 4

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14a,b These new complexes and polymers are related to the square-planar palladium and platinum polyynes which have recently been shown (25-28) to exhibit interesting χ(3) behavior. We have not yet measured the χ ( ) properties of our rhodium complexes, or the molecular weights of the polymers. Current synthetic work is directed towards understanding the influence of the linker groups on electronic communication between the metal centers, and on designing new linkers with lowlying π* levels to improve conjugation.


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X-Rav Crystal Structure Determinations Single crystal X-ray studies on representative examples of the platinum bis(acetylides) were carried out to gain information on both the molecular geometry and nature of the packing of the acetylide moieties in the crystal. Selected crystal data for the various compounds are given in Table I. Full details of the structural analyses will be reported elsewhere. Table I. Crystal Data Collection and Refinement Parameters Compound Crystal System a (A) b(A) c(A) 3σ(Ι)) R R 3

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triclinic 8.063(1) 9.782(2) 11.240(1) 92.72(1) 102.97(1) 112.89(1) 768.8(2) PÎ 1 295 3.5 - 50.0

triclinic 8.762(1) 9.579(1) 11.110(2) 113.05(1) 95.73(1) 112.55(1) 757.4(2) PÎ

triclinic 5.985(1) 9.065(2) 14.105(3) 84.27(2) 88.12(2) 78.35(2) 745.7(3) PI

monoclinic 10.076(2) 8.530(1) 18.260(3) 94.70(2)

1 295 3.5 - 60.0

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monoclinic 20.961(5) 5.693(1) 28.551(8) 110.29(2) 3196(1) Cc or C/2c 4 170 3.5 - 60.0

2778 0.0235 0.0245

4202 0.0269 0.0295

12,978 0.0286 0.0292

2899 0.0353 0.0373

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1564.1(5) P2i/c 2 295 3.5 - 50.0

One important feature which has emerged is the proclivity of the acetylide units to align in a parallel manner. This has now been found to hold for the symmetric



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platinum bis(acetylides), [Pt(PMe2Ph)2(C=CC6H4X)2] (7a,b), which crystallize in the centrosymmetric triclinic space group P I , with one molecule per unit cell and the platinum atom residing on an inversion center. Although the two compounds are not isostructural, they have in common the fact that all the acetylides in the crystal are in perfect parallel alignment. This arises as a consequence of: a) the linear nature of each individual acetylide; b) the trans- configuration at each metal (compatible with their 1 site symmetry; and, c) the fact that the only other symmetry elements in the triclinic system are pure translations. The unsymmetrically substituted bis(acetylide) (10a) was also found to crystallize in the triclinic system, with Ζ = 1. Unlike its symmetric analogues, the lack of molecular symmetry precludes the platinum atom from sitting at a true inversion. Collection of all diffraction data including Friedel pairs, hkl and hkl, allowed successful refinement in space group P I , despite the severe pseudosymmetry. The lack of significant residual electron density in the region of the acetylide end groups OMe and NO2, and the observation (vide infra) of a second harmonic signal for the material, supported this choice of space group and indicated little or no end group disorder in the crystal. The packing diagram of 10a is shown in Figure 1 as representative of these triclinic materials. Another unsymmetric analogue, lOd (D = C6H4NMe2, A = NO2), was found to crystallize in the monoclinic system. The systematic absences were consistent with the space groups C2/c or Cc. Again, the centric choice would require molecular endto-end disorder in the crystal. Structure refinement in the non centric group Cc has so far proved unstable, possibly due to the similarity between N(CH3)2 and N O 2 substituents. A degree of disorder cannot be ruled out in spite of an observed SHG signal. Once again, the packing of all acetylides moieties in the crystal is found to be parallel (Figure 2). Extension of the linear chain system is possible by use of diacetylide (C4R) rather than acetylide (C2R) ligands. We have now carried out a single crystal structure determination of the symmetric bis(acetylide) complex rra«s-[Pt(PMe2Ph)2(C=C-OCPh)2l (8a). The structure is the first of any transition metal diacetylide, and the geometry is shown in Figure 3. Interestingly, the molecular packing for this compound does not result in the parallel alignment of all the acetylide moieties; a herringbone pattern is found (Figure 4). Second Harmonic Generation Measurements A l l samples were tested on a second harmonic analyzer based on a modified version of that designed by Dougherty and Kurtz (44). The instrument comprises a Nd-glass laser rod operating at 1.06 μπι. Fundamental and second harmonic signals are compared for time correlation to eliminate the possibility that the second harmonic signals are spurious. The system was calibrated using a quartz standard, ca. 10 mg graded 62 μπι quartz powder immersed in two drops of an index matching liquid (Cargille Inc., Cedar Grove, NJ) with η = 1.544. Approximate S H G powder efficiencies were obtained using the Kurtz powder technique (45). The compounds were ground or roughly crushed, depending on the crystallinity of the sample, to produce powders with particle sizes ranging approximately from 50-100 μπι. The indices of refraction were estimated (Becke line method) to be close to 1.50 for 10a. The same index matching fluid, η = 1.544, was used throughout.




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Figure 3. Ortep diagram for 8a. Selected bond distances (Â) and angles (°) are: Pt(l)-P(l) 2.308(2), Pt(l)-C(l) 2.009(5), C(l)-C(2) 1.175(8), C(2)-C(3) 1.406(8), C(3)-C(4) 1.188(8), C(4)-C(5) 1.442(8), P(l)-Pt(l)-C(l) 88.7(2), Pt(l)-C(l)-C(2) 177.8(5), C(l)-C(2)-C(3) 177.1(6), C(2)-C(3)-C(4) 177.1(6), C(3)-C(4)-C(5) 174.9(6).

Figure 4. Packing diagram for 8a viewed down the a axis.


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In the case of the asymmetrically substituted platinum acetylides, SHG was detected in all samples (Table II). The symmetrically substituted bis(pyridylacetylide) complex 7c was found to be centrosymmetric. Signals were quantified by peak height analysis of several laser pulses. Table II. Second Harmonic Generation and UV-VIS Absorption Data in Platinum Acetylides


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17,250 0.005 368 19,500 0.01 326 30,300 0.33 342 19,200 0.40 378 30,200 0.50 338 Hg 26,250 11a 0.50 350 10a 38,800 1.00 386 SHG (1.064 μπι 0.532 μπι) relative to optimized, index-matched, 62 μπι quartz. Average of 3 measurements. For comparison, an unoptimized urea sample gave a signal of 1.5 χ quartz under these conditions. In C H 3 C N . Lowest energy absorption band.

The unsymmetric bis(acetylides) (10,11) are significantly more effective than the mono-acetylides 9a,b. The SHG efficiencies obtained by this method are probably not of sufficient accuracy to warrant detailed comparisons among samples of 10 and 11. It is interesting, however, that l l g , in which D = Ph, has a comparable powder SHG efficiency to l l a , b in which the donor groups contain MeO or MeS substituents in the para position. Preliminary X-ray powder diffraction studies indicate that l l g is not isostructural with l l a , b . Thus, an alternative packing arrangement may be responsible for the magnitude of the SHG signal for l l g . One important conclusion is that the entire class of compounds seems to crystallize in non centrosymmetric space groups. The completely parallel alignments observed for 10a,d are not optimum for SHG; however, this is an advantageous arrangement for second-order electro-optic effects (46). With the exception of lOf, which has a significant absorption tail into the region around 532 nm, the compounds are essentially transparent in this region. The lowest energy electronic transitions (Table II) are in the range of ca. 325-390 nm. Complete experimental and spectroscopic details for all new compounds will be reported elsewhere. Acknowled gments We thank the Natural Sciences and Engineering Research Council of Canada, the Ontario Centre for Materials Research, and the University of Waterloo for support, Johnson Matthey Ltd. for a loan of precious metal salts, the DuPont Company for a donation of materials and supplies, and Drs. S.R. Marder and A . Stiegman (JPLCaltech), W. Tam and L.T. Cheng (DuPont) for helpful discussions and preprints of several publications.


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