Molecular Design for Enhanced Electric Field ... - ACS Publications

Chapter 17

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch017

Molecular Design for Enhanced Electric Field Orientation of Second-Order Nonlinear Optical Chromophores H. E. Katz, M. L. Schilling, W. R. Holland, and T. Fang AT&T Bell Laboratories, Princeton, NJ 08540

Three synthetic approaches to donor-acceptor-substituted conjugated molecules with enhanced orientability in electric fields, potentially applicable to the preparation of electro-optic polymers via electric field poling, are summarized. The three approaches are parallel attachment of chromophores to a common framework, embedding the chromophore in a zwitterion, and head-to-tail oligomerization of chromophores. The oligomerization method as well as the use of dyes as curing agents are briefly discussed in relation to the stability of electric field-induced polar order in polymer matrices. Two of the most important nonlinear optical (NLO) processess, electro-optic switching and second harmonic generation, are second order effects. As such, they occur in materials consisting of noncentrosymmetrically arranged molecular subunits whose polarizability contains a second order dependence on electric fields. Excluding the special cases of noncentrosymmetric but nonpolar crystals, which would be nearly impossible to design from first principles, the rational fabrication of an optimal material would result from the simultaneous maximization of the molecular second order coefficients (first hyperpolarizabilities, P) and the polar order parameters of the assembly of subunits. (1) The desire to increase p values above those of molecules used in the earliest materials has led to the exploration of organic compounds as the active components of second order N L O devices. Considerable effort has been expended in the synthesis and analysis of candidate molecules, which are largely donor-acceptor substituted conjugated n systems. (2) Examples of compound classes whose members display large nonresonant P include azo dyes, stilbenes, polyenes, merocyanines, stilbazolium salts, and quinoid

0097-6156/91AM55-0267$06.00/0 © 1991 American Chemical Society In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

compounds. Structure-property relationships governing the effects of substituents, molecular length, and the nature of the molecular skeleton on (J have been deduced for many of these types of compounds (3) and are fairly well established. >From these relationships, it might be possible to conjure molecules which might exhibit still larger (}. However, there are diminishing returns in the increase in P as one increases the length of the n system or the donor-acceptor strength of the substituents beyond certain limits (4-6), and increases in low energy absorbances limit the utility of some chromophores exhibiting very high P, especially when the coefficient is enhanced primarily by resonance. In any event, the means of maximizing the second order hyperpolarizability are well laid out. Methods for achieving the orientational order required for a second order N L O bulk material have also been intensively studied, but remain much more problematical. The most effective ways to impart this order consist of noncentrosymmetric crystallization, self-assembly at interfaces, and electric field poling of the active chromophores. Crystallization suffers from being difficult to model thermodynamically, although some intriguing results concerning rationally predicted polar crystallization have recently been reported. (7-9) Crystals are also troublesome to employ in waveguiding and integrated modes, although they are generally quite stable. The highest degrees of polar order are achieved in organized thin films, either deposited as Z-type Langmuir-Blodgett films (10,11) or chemisorbed from solution. (12,13) The main drawbacks to these systems are the tedium in building up enough mass to serve as a bulk device material and the fragility of the multilayer assemblies. More facile deposition techniques leading to more robust materials are currently being investigated. (14) Electric field poling has been widely pursued as a means of orientation, generally in thin polymer films. (15-17) One advantage is that the process may be modeled thermodynamically. (1) A corresponding disadvantage is that the thermodynamic model (correctly) predicts maximum order parameters (excess projection of the principal molecular moment in one direction versus an isotropic ensemble) of only 10-20% for most chromophores considered. (18) Additional problems center on the stringent conditions necessary for producing films that retain optical quality and dielectric strength during poling, and that fully retain orientational order after poling. On the other hand, polymer thin films can be deposited and oriented in situ relative to other components with which they may be integrated, and are well suited to waveguide applications. (19) For these latter reasons, much of our research has been directed towards new compounds which may be useful in second order N L O materials prepared by electric field poling. Very little effort has been devoted to the design and synthesis of compounds in which the susceptibility to electric field alignment has been enhanced without significantly perturbing the electronic states of the chromophoric moieties. Similarly, very few compounds have been prepared for the express purpose of improving orientational stability after poling through judicious functional group placement. The primary purpose of this presentation

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

17. KATZETAL.

Molecular Design for Enhanced Electric Field Orientation 269

is to describe the syntheses of several new compounds which are aimed at these issues of electric field-induced orientational order, and to demonstrate the potential for traditional organic synthesis to further the advancement of this field.


Dipole Additivity and Increased Polar Order The degree of orientation achievable by applying an electric field (E) to an ensemble of dipoles increases with the magnitude of the dipole moments ( \i ) involved. The increase is approximately linear when the product | i E is substantially below kT. (18) Since most poling processes for N L O occur in the linear regime, increases in the effective | i should lead to improved ordering of the dipolar chromophores. We have examined three synthetic strategies for enforcing the superimposition or additivity of dipole moments coincident with the principal moments of established NLO-active chromophores. The strategies are 1) projecting two chromophores in parallel directions from a rigid molecular backbone, 2) surrounding a weakly dipolar chromophore with separated, full charges that define a much larger | i , and 3) linking dipoles head-to-tail so that the ordering force acts on a cumulative effective | i . A l l three strategies have been demonstrated by actual syntheses, and in some cases, physical measurements as well. However, extensive materials science would still be required in order to implement these schemes in actual bulk systems. The 2,5-endo bonds of simple norbornanes are within 30° of being perfectly parallel, and the 1,4-trans bonds of substituted piperazines are even closer to being parallel, according to molecular models. Accordingly, we synthesized compound 1 as shown in Equation 1. (20) The reaction is highly stereoselective, with no exo substituents observed. A n x-ray structure of the analogue without the nitro groups is shown in Figure 1. Unfortunately, the limited deviations from parallelism at each ring-ring bond and steric distortions of the norbornane skeleton force the aminophenyl residues outward to almost 90° angles. The dipole moment of 1 in dioxane is 8.9 D, compared to 6.8 D for N,N-dimethyl-p-nitroaniline. This reflects the vector addition of the two main moments of 1 at approximate right angles, which is mathematically identical to having two unattached chromophores with no enforced additivity at all. Even so, there could be some advantage to an arrangement like 1. In a poled polymer, oriented 1 would have to sweep out a much larger volume in a disorientation process than would a monomelic chromophore, and thus might be more orientationally stable. Interaction of a polymeric or dipolar functional group with the basic sites in the cavity of 1 might further improve the magnitude or stability of orientation. A more rigidly parallel pair of bonds for the projection of chromophores are the 1,8 positions of anthracene and anthraquinone. The respective 1,8dichlorides undergo a limited substitution chemistry, which we extended as shown in Equation 2 to synthesize parallel-directed but weakly dipolar phthalimides. In principle, the use of donor-substituted phthalimide nucleophiles in the reaction of Equation 2 would give a fully additive pair of strong dipoles; however, this has not yet been accomplished.




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Molecular Design for Enhanced Electric Field Orientation 271

The use of zwitterions in poling would be desirable because the dipole moment of a long zwitterion is many times that of most polarized π systems. Indeed, the high molecular figures of merit for some donor- acceptor substituted quinoid compounds is based in part on large dipole moments that result from ground state zwitterionic character. (K.D. Singer, unpublished) Concomitant with zwitterionic character, however, is intractability in all but the most polar media. Thus, it is only with difficulty that partially zwitterionic chromophores may be dispersed in moderately polar polymers. We sought to superimpose a full zwitterion dipole moment on an established N L O chromophore, cyanovinylaniline, such that the known N L O properties of the chromophore would be maintained while ordering consistent with a much larger dipole would be achieved. The synthesis of such a molecule, 2, is illustrated in Scheme 1. Although the synthesis is multistep, most of the individual reactions are standard condensations or protecting group manipulations. The trifluoroacetyl protecting group was essential for the Vilsmeyer formylation to succeed, since more electron-rich amide groups reacted with the Vilsmeyer reagent. The two charged groups are rigidly separated, and define a dipole virtually parallel to the main moment of the chromophore. The 2-ethylhexyl substituent was selected for maximum lipophilicity and poor crystal packing without micelle formation. Despite this selection, the dipole-dipole intermolecular association apparently dominated the physical properties of pure, nonprotonated 2. It was not possible to dissolve 2 or a borane adduct of 2 in a sufficiently nonpolar solvent to measure a dipole moment, which is estimated from molecular models to be ca. 50 D (2 full opposite charges separated 1 nm). Compound 2 could not be dispersed in poly(methyl methacrylate) (PMMA), but films of 2 in poly(N-vinylpyrrolidone) were spun from aqueous N , N - dimethylformamide. Unfortunately, these films, while of good optical quality, were too conductive to pole, possibly because of partial protonation or low-level ionic contamination. Thus, the use of zwitterionic chromophores for second order N L O materials remains tantalizing; however, considerable material fine tuning will be required in order to disperse and pole such species in a continuous medium. The dipole moment augmentation strategy that has come closest to realization in a material is the head-to-tail linkage of semirigid dipolar chromophoric subunits in an oligomer. This strategy has precedent in work published by Williams and Willand. (21,22) While the preceeding work focussed on high molecular weight polymers with limited registry among subunits, our thrust has been toward smaller assemblies in which the geometrical relationships among subunits are well defined. Examples of compounds we have studied in this light are dimers 3 and 4 and oligomers 5 and 6. (23,24) Once again, we have taken advantage of the piperazine ring to combine electron donating ability and conformational definition in as small a subunit as possible. The amide group provides electron withdrawing character and a linkage with little angular variability. A crystal structure of the non-



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Figure 1. norbornane.

X-ray

crystal

structure

of

endo,endo-2,5-bis(4-phenylpiperazinyl)-

2

Scheme 1. Abbreviations: TFAA, trifluoroacetic anhydride; DMF, Ν,Νdimethylformamide; 2EHCOC1, 2-ethylhexanoyl chloride; LAH, lithium aluminum hydride; MeOTf, methyl trifluoromethanesulfonate.


Molecular Design for Enhanced Electric Field Orientation


17. KATZ ET AL.



274


nitrated analogue of 3, shown in Figure 2, clarified both the stereochemistry and relative conformations of the two chromophores in these compounds. Other related compounds were judged to be stereochemically analogous based on N M R spectra. The key steps in the syntheses were formation of the vinylic double bonds through Knoevenagel condensations. Dipole moments of 3, 4, and tetrameric 5 were 9.1, 13.0, and 16.6 D respectively, consistent with vector additivity of the chromophore moments at 110° angles as predicted by the crystal structure and molecular models. These data also indicate the preference of higher oligomers for extended conformations even though doubled-back conformations are available to these oligomers that are not accessible to dimers. The tetrameric material was also poled as a 10% mixture in P M M A at 120 °C in a field of 1 MV/cm; the electro-optic coefficient measured for this sample was 0.8 pm/V, and the birefringence was 0.006. These values are consistent with an extended conformation in that an ensemble of monomers poled at similar concentration should have had an electro-optic coefficient

Molecular Design for Enhanced Electric Field ... - ACS Publications

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