A two-state, four-orbital, independent electron analysis of the first optical molecular hyperpolarizability, beta, leads to the prediction that |beta| maximizes at a combination of donor and acceptor strengths for a given conjugated bridge. Molecular design strategies that focus on the energetic manipulations of the bridge states are proposed for the optimization of beta. The limitations of molecular classes based on common bridge structures are highlighted and more promising candidates are described. Experimental results supporting the validity of this approach are presented.

Conjugated organic compounds with 3-phenyl-5-isoxazolone or N,N'-diethylthiobarbituric acid acceptors have large first molecular hyperpolarizabilities (beta) in comparison with compounds with 4-nitrophenyl acceptors. For example, julolidinyl-(CH=CH)(3)-CH=N,N'- diethylthiobarbituric acid, which has 12 atoms between the donor and acceptor, has a beta(0) of 911 x 10(-30) electrostatic units, whereas (CH(3))(2)NC(6)H(4),-(CH=CH)(4)-C(6)H(4)NO(2), with 16 atoms between its donor and acceptor, has a beta(0) of 133 x 10(-30) electrostatic units. The design strategies demonstrated here have resulted in chromophores that when incorporated into poled-polymer electrooptic modulators exhibited significant enhancements in electrooptic coefficients relative to polymers containing the commonly used dye Disperse Red-1. Poled polymer devices based on these or related chromophores may ultimately lead to high-speed electrooptic switching elements with low drive-power requirements, suitable for telecommunications applications.

Synthesis of l,3-diethyl-5-(5-(N,N'-diethylamino)penta-2,4-dienylidene) barbituric acid, 3.The synthesis of the starting material l,3-diethyl-5-(5-acetanilidopenta-2,4-dienylidene) barbituric acid, A, has been reported previously.9 1.3g (3.41 mmol) of A was dissolved in 25ml of methylene chloride.While gently warming the solution, 0.9ml (8.72 mmol) o f diethylamine was added with swirling, causing an immediate color change to magenta. Excess diethylamine and methylene chloride were removed under reduced pressure, resulting in an inky oil with a metallic greenish sheen. The oil was redissolved in 10ml of fresh methylene chloride. Diethyl ether (100 ml) was added dropwise, resulting in crystal precipitation. The crystals were filtered, washed with ether and dried in air. Yield 0.950 g (87%).Characterizing data for 3: lH NMR. 5 8.00 (d, J = 13.3 Hz, 1H), 7.72 (dd, J = 13.3, 13.2 Hz, 1H), 7.26 (dd, J = 13.2, 12.2 Hz, 1H), 7.05 (d, J = 12.3 Hz, 1H), 5.68 (dd, J = 12.3, 12.2 Hz, 1H), 4.00 (q, J = 7.0 Hz, 4H), 3.37 (m, 4H), 1.27 (m, 6H), 1.23 (t, J = 7.0 Hz, 3H), 1.22 (t, J = 7.0 Hz, 3H). Elemental Analysis: Calculated for C 17H 2 5 N 3 O3 : C, 63.93; H, 7.89; N, 13.16; Found: C, 63.92; H, 7.91 The synthesis of the starting material l,3-diethyl-5-(5-acetanilidopenta-2,4-dienylidene)-2-thiobarbituric acid, C, has been reported previously.^ 1.3g (3.27 mmol) of C was dissolved in 25ml of methylene H, 7.51; N, 12.53; S, 9.56. Found: C, 60.77; H, 7.56; N, 12.44; S, 9.63 The synthesis of the starting material 1,3-diethyl-5-anilinylidene barbituric acid, D, has been reported were added to 50ml toluene while gently heating the solution. Then, 2.5ml of triethylamine was added and the reaction mixture refluxed for 2.5 hours at 110°C. Toluene was removed under vacuum and water was added to the remaining residue. An orange precipitate formed upon addition of the water.This precipitate was dissolved in toluene and dichloromethane and separated from the water layer. The solution was dried over Magnesium Sulfate, filtered and the remaining solvent removed under vacuum.The residue was recrystallized from toluene. Yield 1.98g (34.2% ). A ny additional Data:Weights w are calculated as l/cr2(F2); variances (cr2(F2)) were derived from counting statistics plus an additional term, (0.014I)2; variances of the merged data were obtained by propagation of error plus another additional term, (0.014I)2. Definitions:where n -number of data p = number of parameters refined.

A series of mixed-ligand transition-metal diimine dithiolate complexes have been prepared, and their nonlinear optical properties have been investigated. Molecular hyperpolarizabilities have been obtained from electric field induced second harmonic generation experiments at 1.9 µm and range in magnitude from 0 to -39 × 10 -30 esu. Variation in molecular design of the complexes, through changes in the metal ion and diimine and dithiolate ligands, can be used to affect changes in the charge-transfer-to-diimine absorption band frequency and oscillator strength. Correlation between the optical charge-transfer transition and the nonlinear optical properties appears to agree with the two-state model.