Aromatic Heterocyclic Rings as Active Components in the Design of

Chapter 16

Aromatic Heterocyclic Rings as Active Components in the Design of Second-Order Nonlinear Optical Chromophores 1

2

2

3

Bruce A. Reinhardt , Ram Kannan , Jay C. Bhatt , Jarek Zieba , and Paras N. Prasad Downloaded by COLUMBIA UNIV on April 22, 2013 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch016

3

1

Polymer Branch, Materials Directorate, U.S. Air Force Wright Laboratory, WL/MLBP, 2941 P St. Ste 1 Wright-Patterson Air Force Base, OH 45433-7750 Systran Corporation, 4126 Linden Avenue, Dayton, OH 45432 Photonics Research Laboratory, State University of New York, Buffalo, NY 14214 2

A series of second-order NLO model chromophores has been synthesized which contain only aromatic heterocyclic rings as the donors and acceptors. Experimentally determined values by EFISHG show that reasonably large values of the first hyperpolarizability β can be obtained with a much larger transparency window than normally demonstrated by convential chromophores with comparable βvalues. The dipole moments of these molecules can be controlled independently from β. These types of chromophore molecular structures can be incorporated into acetylene-terminated thermoset monomers which when incorporated into thin films of high Tg thermoplastics can be cured to produce composite films with stable second-order activity at 100°C.

The search for practical second-order nonlinear optical polymeric materials for use in frequency conversion, and integrated optics applications has been at the forefront of organic polymer research in recent years. The advantages of using organic polymers rather than organic or inorganic single crystalline materials include evironmental and optical stability, ease of fabrication and potential low cost. The figures of merit which a polymer system must meet to be device useable for second-order NLO applications have been summarized (1). Although some promising systems have been reported, all are based on polymer systems which incorporate NLO active elements containing standard electron donating and withdrawing groups such as dialkylamino and nitro or variations of other aliphatic polarizing functionality. Initially these types of polymers, many of which were based on acrylate type backbones, seemed to have the thermal stability necessary to meet the proposed device requirements. It is only recently that it has been realized that for an organic polymer to be truly device useable and ultimately commercially competitive especially for "on chip" applications it must be able to maintain a resonable second-order response (> 30 pm/V) during routine microelectronic circuit fabrication procedures (7,2 ). Such fabrication procedures include exposure to the high temperatures of solder baths which can be as high as 320°C for 20 0097-6156/95A)601-0205$12.00A) © 1995 American Chemical Society In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by COLUMBIA UNIV on April 22, 2013 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch016

206

POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

minutes. Long term thermal stability requirements are also necessary with military applications demanding that a material maintain 95% of it's original EO coefficient after 10 years at 125° C. In conjunction with the necessary increased thermal stability the optical quality of the polymerfilmmust also be preserved with losses below 1 dB/cm. Processing ease is also required for a polymer material to compete favorably in the commercial sector. The required processibility includes spin coatability and the necessary physical properties to either be self-assembled or electrically field poled into the noncentric bulk materials forms necessary for second-order activity. With the need for higher performance, the earlier high optical quality acrylate based polymers are no longer viable candidates for use in potential commercial "on chip" devices unless changes in the standard commercial processing procedures are instituted. Since changes in the current processing techniques would be extremely costly, the more logical approach involves the design of polymer second-order materials which have much greater thermal stability. New Approaches to Improved Thermal Stability Over the past two years there have been some promising approaches to synthesize new EO polymer systems which have increased thermal stability. Most of these are based on guest-host or chromophore side chain polymer systems involving thermoplastic polymer matricies with higher glass transition temperatures (Tg). Chromophore design involved the synthesis of new structures which have standard electron donor and acceptors connected by heterocyclic n electronic bridging systems with improved thermal stability (3,4). Additional studies have centered on the modification of standard donor groups to improve their thermal stability (5). It remains to be seen how successful any of the approaches will eventually be but certain key problems currently exist with all of them which must be solved if any of the alternate approaches to improved thermal stability will ultimately lead to practical EO devices. It must always be remembered that the underlying factor for any system to be commercially successful involves cost/performance considerations. Whatever the system turns out to be it must be of reasonable cost to produce, perform better than the state of the art materials, have minimal environmental impact, and answer the requirements of disposal and/or recyclability. Thermal stability of a second-order NLO material can be subdivided into 2 types: (I) The intrinsic thermal stability (ITS) of the NLO activity moeity which is dependent on the temperature above which thermal degradation of the active portion of the molecules reduces the NLO activity. (II) The alignment thermal stability (ATS) of the bulk polymer system which is dependent on the glass transition temperature of the aligned NLO active material. At temperatures approaching Tg thermal relaxation of nonthermodynamically stable states will be greatly accelerated. Both types of thermal stability must be considered when designing new molecules for use in practical devices. Our approach to the design of new molecules which will address both types of thermal stability issues centers around the synthesis of new NLO second-order active chromophores which have a higher degree of aromaticity and which can be incorporated into high temperature thermoset resins which are highly compatible with both NLO active or inactive high molecular weight thermoplastics. Theory and Molecular Design. It is generally recognized that organic ring compounds that possess (4n +2) n electrons (where n = 0, 1, 2, 3, etc.) have increased chemical and thermal stability over organic ring structures which do not meet this electronic criteria. Aromatic In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


16. REINHARDT ET AL.

Aromatic Heterocyclic Rings as Design Components

rings have long been the integral structural units in the design of high temperature organic polymers (6). It appears logical that the design of new second-order NLO chromophores should include structural units which are highly aromatic. A second-order NLO chromophore no matter what it's thermal stability requires an asymmetric charge distribution. If the structural building blocks are to be limited only to aromatic rings, charge asymmetry can only be accomplished by using aromatic heterocyclic rings. Here the presence of heteroatoms and the size of the rings can produce structural units which are either electronrichor electron poor when compared to benzene. Benzene contains six n electrons distributed equally over six atoms. The n electron densities at each of the atoms can be calculated using simple molecular orbital theory and in the case of the symmetrical benzene is found to have a value of 1.0. In five-membered heterocyclic rings containing one hetero atom, two of the 6K electrons which make up the aromatic sextet reside in a p orbital provided by the heteroatom. There are 6 n electrons somewhat unsymmetrically distributed over only 5 atoms and thus the HMO calculated n electron densities are greater than 1.0. Heterocyclic rings of this type are termed electron excessive (7) and exhibit higher reactivity to electrophilic aromatic substitution when compared to benzene. Six-membered rings containing one or more nitrogen atoms have their n electron density unsymmetrically distributed due to the higher electronegativity of the nitrogen atom. At carbon atoms which are ortho or para to the more electronegative nitrogen the 7C electron density is calculated by HMO theory to be less than 1.0. Theseringsare considered to be electron deficient (7 ) and behave as highly deactivated benzeneringstoward electrophillic aromatic substitution. Figure 1 illustrates the relative electron excessive (electron donating) or electron defficient nature of various aromatic heterocyclic rings. Calculated n electron densities are also included for comparison. Previous to our earlier reports (8-10) aromatic heterocyclicringshave been investigated as electronrichand electron poor centers in conductive polymers (77), and as donor (9 ), bridging (72 ) and acceptor (73 ) groups in second-order NLO molecules. In this report we describe our continuing studies which systematically attempt to evaluate the affect of electron excessive and electron deficient heterocyclic aromatic rings on the molecular first hyperpolarizability in a given series of structurally similar molecules. Model Chromophores. As an initial attempt to ascertain the feasibility of using aromatic heterocyclicringsas donors and acceptorsfivegeneral types of molecules were synthesized. The compounds synthesized within these 5 general types contained various combinations of electronrichheterocyclic (ERH) and electron deficient heterocyclic (EDH)rings.The ERH and the EDHringswere connected either directly together or with the use of a simple carbon carbon double bond. Although a simple double bond would not provide the most thermally stable bridge, it did provide ease of synthesis of a wide variety of compounds for proof of concept. Upon concept validation a polarizable aromatic bridge would be substituted for the double bond for improved thermal stability. Thefivegeneral types of compounds synthesized are depicted graphically in Figure 2. Synthesis. The syntheses of the model chromophores were carried out via the acetic anhydride or base catalyzed condensation of thiophene or bithiophene carboxaldehyde with the appropriate heterocyclic methyl compound. Yields of

In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

208


increasing electron deficiency or accepting ability

increasing electron donating ability

0.90 rl .04


1.03

I

'

0.91 1.05

09 0.93

^1.07 1.10

0.83

S 0.94

.000

^ N

0 . S 4 ©

Figure 1. Relative electron donating and accepting ability for aromatic heterocyclic rings.

Generalized Structure

Type

Specific Examples

la

lb

Ic

Ila

lib

Figure 2. Generalized structures for model chromophores


16. REINHARDT ET AL.

Aromatic Heterocyclic Rings as Design Components

purified products ranged from 30% to quantitative. The 3 synthetic reaction pathways and the structures of the compounds synthesized are depicted in Schemes A-C and Figures 3-5. Model Chromophore Nonlinear Optical Characterization. The model chromophores were characterized using electric field induced second harmonic generation (EFISH) at 1.063 pm. The effective y determined by EFISH measurements is the sum of the scaler orientationally averaged electronic part of the second hyperpolarizability and p,p/5kT, the contribution to the total y from aligned dipoles (Equation 1). In many cases the electronic part of y was y

=


eff

y

+ elec

(!) 5kT

considered to be negligible when compared to the contribution from the aligned dipoles. When the size of y i c was in question it was determined experimentally by degenerate four wave mixing (DFWM). The following approximations were made: For a p-nitroaniline (PNA) standard; Y(-2G); CO,CO) = y(-(0\ co,to,-co) = 4.8E35 esu. For Compound 2 ; y(-2a); a),©) = y(-G); (Q,G),-G)) = 5.0E-35 esu, and for Compound 12. y(-2to: ©,©) = y(-(o; ©,(0,-0)) = 3.16E-34 esu. For all other compounds 7(-2co; co,co) = 0. The dipole moments p were measured independently in xylene solution for all compounds investigated. e e

Model Chromophore Results and Discussion. Earlier studies (10 ) indicated that when single EDH rings and ERH rings are connected directly together without a bridge of polarizable K electrons(Type la and Ha chromophores) the values of the first hyperpolarizability P are relatively small. When a highly polarizable bridging carbon-carbon double bond is added (Type lb, Ic, and lib chromophores) the values of p increases substantially depending upon the ring system and the position of the molecular dipole with respect to the direction of conjugation. From the data in Figures 3, 4 and 5 some insights into the structure/NLO property relationships for these types of chromophores can be obtained. In the case of compound 1 the value calculated for p from EFISH measurements is much lower than that obtained for compound 2 due to the change in magnitude and direction of the net dipole of the molecule brought about by the change in point of attachment to the bridging double bond. Chromophore 2 has a p value which is slightly larger than that measured for p-nitroaniline (PNA) measured under identical conditions. Compound 2 crystallizes in noncentric almost colorless needles and has a much larger optical transparency window when compared to PNA. If one adds a second nitrogen atom ortho to the position of attachment to the double bond to form the six-membered pyrimidine ring (compound 6) the P increases only slightly. The addition of a fused ring on the side of the pyridine ring in compound 2 produces the quinoline ring system (compound 4). This again increases P only slightly but the addition of a second nitrogen atom and a change in linkage position to extend the conjugation into the fused benzene ring to form the quinoxaline ring system compound U increases p by more than 2 fold when compared to compound 2. The experimental results for Type Ic chromophores in which thiophene is replaced by bithiophene compounds


210


Scheme A

A ^ H O Het-CH

^-S

3

yjr-Het

esu

\nax nm gMW

Aromatic Heterocyclic Rings as Active Components in the Design of

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