Chapter 11
Organic and Organometallic Compounds Second-Order Molecular and Macroscopic Optical Nonlinearities 1
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Seth R. Marder , Bruce G. Tiemann , Joseph W. Perry , Lap-Tak Cheng , Wilson Tam , William P. Schaefer , and Richard E. Marsh Downloaded by STANFORD UNIV GREEN LIBR on May 18, 2013 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch011
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Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 Central Research and Development Department, E. I. du Pont de Nemours and Company, Wilmington, DE 19880-0356 Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125 2
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Organic and organometallic stilbazolium cations can be crystallized with various counterions; some of the resulting salts exhibit large SHG powder efficiencies. Approximately linear stilbazolium cations have a greater tendency to crystallize in noncentrosymmetric space groups than do cations with substantial geometric asymmetry. The nonresonant quadratic molecular hyperpolarizabilities of several ferrocene and ruthenocene derivatives were studied by D C electric -field-induced second-harmonic generation (EFISH) experiments using fundamental radiation at 1.91 μm. Hyperpolarizabilities approaching that of 4-dimethylamino 4'-nitro-stilbene (DANS) were observed indicating that the ferrocene moiety can act as an effective donor. EFISH measurements indicate that indoaniline dyes with very polarizable πsystems have large molecular hyperpolarizabilities (β), in one case approaching three times that of DANS. Organic materials with second-order nonlinear optical (NLO) properties have been the subject of intense investigation owing to their potential use in a variety of technologies including telecommunications, optical information processing and storage.CL3) Large second-order molecular hyperpolarizabilities (P) are associated with chromophores comprised of electron donors and acceptors linked by a conjugated K svstem.(4-6) The nonlinear chromophore must reside in a noncentrosymmetric environment if P is to lead to an observable bulk effect such as second harmonic generation (SHG) or the linear electrooptic effect (LEO). In this paper we focus on factors affecting each of the above design criteria for second-order N L O materials. We will first show that in many instances variation of the counterion in ionic structures leads to materials with large bulk second-order susceptibilities (%3, C H 3 C 6 H 4 S O 3 , 1 , Br) all gave negligible SHG efficiencies. The exception was (3)-CH30C6H4-CH=CH-(2)-C5H4N(CH3)+ CF3SO3", which has an efficiency of 25 times urea. The nonlinear optical properties of this compound are worthy of consideration since, contrary to simple resonance considerations for the design of N L O chromophores, the donor, the methoxy group, and the acceptor, the cationic alkylated nitrogen atom, are cross conjugated. This gives rise to enhanced transparency in the visible in comparison to the isomer 4'methoxy-2-Af-methyl stilbazolium triflate, in which the donor and the acceptor are conjugated. In methanol solution, 3'-methoxy-2-N-methyl stilbazolium triflate has a ^max at 344 nm and a cutoff at 455 nm. In comparison, 4'-methoxy-2-Af-methyl stilbazolium triflate has a charge transfer band at 368 nm. In the solid state the cutoff for 3 -methoxy-2-Af-methyl stilbazolium triflate is at ~ 425 nm (for a ~100|im thick crystal). Although the molecular hyperpolarizability (P) of 3'-methoxy-2-Af-methyl stilbazolium triflate is undoubtedly smaller than 4'-methoxy-2-N-methyl stilbazolium triflate, its SHG efficiency suggests that it is not necessary to have very strong donors and acceptors or for the donor and the acceptor to be strongly coupled in order to achieve significant macroscopic nonlinearities.(Cheng, L . T.; Tarn, W.; Meredith, G . R.; Rikken, G ; Marder, S. R. J. A m . Chem . S o c . submitted for publication.) Crystal structures of several salts were determined in order to better understand how the chromophores align in the crystal lattice. Although it is difficult to generalize packing trends, in the nine crystal structures we have determined, a recurring structural motif is alternating parallel rows of cations and rows of anions. (Schaefer, W. P.; Marsh, R. E.; Marder, S. R., in preparation.) The compounds shown in Fig la-e follow this motif. In general, neutral dipolar molecules with geometrical asymmetry show a greater tendency to crystallize in noncentrosymmetric space groups than do more linear symmetric analogs. Thus, whereas crystals of 4nitroaniline are centrosymmetric, 2-methyl-4-nitroaniline crystallizes in the noncentrosymmetric space group Cc. Similarly, although crystals of 4-methoxy-4'nitrostilbene are most likely centrosymmetric (as surmised by no SHG activity), 3methyl-4-methoxy-4'-nitrostilbene (20) and 2-methoxy-4'-nitro-stilbene (Grubbs, R. B.; Marder, S. R.; Perry, J. W.; Schaefer, W. P. Chem.Mater.. Accepted for publication.) both crystallize in noncentrosymmetric space groups and give rise to large SHG efficiencies. The opposite trend is observed with the 2-N-methyl and the 4-N-methyl stilbazolium salts we have examined. Over half of the 4-N-methyl stilbazolium salts we have examined exhibit powder SHG efficiencies greater than urea, whereas only two of the 2-Af-methyl stilbazolium salts we have studied had powder efficiencies substantially greater than urea. Thus, whereas molecular asymmetry may tend to favor crystallographic noncentrosymmetry in neutral molecules, it appears from our limited sampling that the opposite is true for ionic chromophores.
Downloaded by STANFORD UNIV GREEN LIBR on May 18, 2013 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch011
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Second-Order N L O Properties of Metallocenes. Until recently.(21-26) the potential of organometallic compounds for quadratic nonlinear optics has been completely ignored. The observation that the ferrocene complex (Z)-{ l-ferrocenyl-2-(4nitrophenyl) ethylene} has an SHG efficiency 62 times urea demonstrates that organometallic compounds could exhibit large %( X(27) Given this observation, we synthesized the new compound (C5H5)Fe(C5H4)-CH=CH-(4)-C5H4N(CH3)+I" and measured its SHG powder efficiency by a modification of the Kurtz powder technique.(21) Powder S H G efficiencies were determined using 1907 nm 2
In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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190
MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES
fundamental radiation (SH at 953.5 nm) to avoid absorption of the SH by the dark colored salt. This salt has an SHG efficiency roughly 220 times urea, the largest efficiency known for an organometallic compound.QS) Furthermore, the magnitude of the powder SHG signal is sensitive to the nature of the counterion as shown in Table E. The crystal structure determination of the nitrate salt reveals the polar nature of the lattice (Fig. If). The results obtained from the Kurtz powder test, although tantalizing, provide little insight into molecular structure-property relationships since they are almost entirely determined by crystallographic, linear optics (i.e. birefringence), and dispersive factors. In addition, since molecular structure modification is often accompanied by crystallographic changes, powder testing cannot be used to systematically probe molecular structure-property relations. Solution-phase D C electric-field-induced second-harmonic (EFISH) generation (29) is a more appropriate method for hyperpolarizability studies. It allows extraction of a vectorial projection of the hyperpolarizability tensor (|3) along the molecular dipole (|x) direction. When experiments are carried out with radiation of sufficiently long wavelength, EFISH provides direct information on the intrinsic optical nonlinearity of a molecule. For organic compounds, structure-property trends concerning donor-acceptor strengths and the effectiveness of different conjugated backbones have been topics of many studies.(20. and Cheng, L . T.; Tarn, W.; Rikken, G . manuscript in preparation.) Our recent efforts have provided an extensive set of internally consistent results on many of the important molecular classes. (31) Organometallic compounds allow us to explore new variables. We can change the transition metal element, its oxidation state, the number of d electrons and examine the differences between diamagnetic and paramagnetic complexes and the effect of new bonding geometries and coordination patterns. Each of these factors creates new possibilities for the engineering of asymmetric polarizability. The considerations outlined above coupled with the large observed powder efficiencies of several ferrocene complexes (23.26.28) motivated us to undertake a study of the molecular hyperpolarizabilities of metallocene complexes. Given the aromatic character of the cyclopentadienyl (Cp) ring and the propensity of the metal center to undergo redox chemistry, one may speculate on the potential for effective charge-transfer when a metallocene is conjugated to an electron acceptor. However, since the metal is centrallyrc-bondedto two Cp rings and the ring aromaticity also results in a formal divalence on the metal center, the donating ability of the metallocene is potentially complicated. At the least, it will be dependent on the oxidation potential of the metal center and additional substituents on both fivemembered rings. To assess the effectiveness of using metallocene donors for nonlinear optics, we have characterized the hyperpolarizabilities of several ferrocene and ruthenocene derivatives and have examined various structural dependencies, summarized in Table III. Compounds i n . l and III.2 represent the cyclopentadienyl analogues of acceptor substituted benzenes. Compounds III.3 to III.7 carry structural resemblance to some nitrostilbenes whose nonlinear properties have been previously studied.QI) By comparing current results with those obtained for benzene and stilbene derivatives, the nonlinearities of the metallocene derivatives can be put into perspective. Several structural variations, including different metal centers, cis and trans isomers, extension of conjugation and symmetric electron donating substituents in the form of pentamethylcyclopentadienyl rings (Cp*) have been implemented.
In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
11.
MARDER ET AL.
191
Organic and Organometallic Compounds
Table II. Powder SHG efficiencies of (E)-(C H5)Fe(C5H4)-CH=CH-(4)-C H4N(CH3) X- salts with 1907 nm input (Urea = 1). +
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SHGeff.
B(C6H ) 5
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Br
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CI
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CF3SO3
BF
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PF
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NO3
CH3C6H4SO3
120
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Table III: Summary of linear and nonlinear optical data on metallocene derivatives X
Compound 111.1 111.2
M Fe Ru
Compound III.3 III.4 III.5 III.6 III.7 III.8
M Fe Fe Fe Ru Ru Fe
COCH3 H Me NO?
X H H Me H Me H
n
solvent p-Diox CH?C1 , ?
X
