An internal or external electric field F can drive the chemical structure, bond order alternation, and electronic structure of linear polymethine dyes from a neutral, bond-alternated, polyene-like structure, through a cyanine-like structure, and ultimately to a zwitterionic (charge-separated) bond-alternated structure. As the structure evolves under the influence of F, the linear polarizability alpha, the first hyperpolarizability beta, and the second hyperpolarizability gamma are seen to be derivatives, with respect to F, of their next lower order polarization (for alpha) or polarizability (for beta and gamma). These derivative relations provide a unified picture of the dependence of the polarizability and hyperpolarizabilities on the structure in linear polymethine dyes. In addition, they allow for predictions of structure-property relations of higher order hyperpolarizabilities.

The solvent dependence of the second hyperpolarizability, gamma, of a variety of unsaturated organic compounds has been measured by third harmonic generation at 1907 nanometers. It is seen that the measured gamma is a function of solvent polarity. These solvent-dependent hyperpolarizabilities are associated with changes in molecular geometry from a highly bond-length alternated, polyene-like structure for a formyl-substituted compound in non-polar solvents, to a cyanine-like structure, with little bond-length alternation, for a dicyanovinyl-substituted compound in polar solvents. By tuning bond-length alternation, gamma can be optimized in either a positive or negative sense for polymethine dyes of a given conjugation length.

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.

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