The molecular structures of three compounds, LD 425 (C(13)H(14)N(2)O(3)) (1), LD 489 (C(15)H(15)F(3)N(2)O(2)) (2) and LD 473 (C(17)H(19)F(3)N(2)O) (3), are determined by single-crystal X-ray diffraction (XRD) at 180 K. Azacoumarins (1) and (2) possess para-quinoidal bond-length patterns in their benzene rings due to intramolecular charge transfer (ICT) from these rings to the adjoining rings. In contrast, substitution of O with N within the coumarin heterocycle, to form a 2-quinolone, results in the suppression of this ICT effect. Instead, charge transfer within the heterocycle is shown to become more pronounced. Resonance theory is employed to discuss these bond pattern differences and characteristic spectral blue shifts in relation to their coumarin analogues. The application of this theory offers an intuitive understanding of the structure-property relationships in azacoumarins and 2-quinolones which is further supported by quantum chemical calculations. Such an understanding is important for recognizing ICT mechanisms in these compounds which can then be used to facilitate the molecular design of new laser dyes with the desired spectral shifts.

Advanced organic nonlinear optical (NLO) materials have attracted increasing attention due to their multitude of applications in modern telecommunication devices. Arguably the most important advantage of organic NLO materials, relative to traditionally used inorganic NLO materials, is their short optical response time. Geminal amido esters with their donor-π-acceptor (D-π-A) architecture exhibit high levels of electron delocalization and substantial intramolecular charge transfer, which should endow these materials with short optical response times and large molecular (hyper)­polarizabilities. In order to test this hypothesis, the linear and second-order nonlinear optical properties of five geminal amido esters, (E)-ethyl 3-(X-phenylamino)-2-(Y-phenylcarbamoyl)­acrylate (1, X = 4-H, Y = 4-H; 2, X = 4-CH3, Y = 4-CH3; 3, X = 4-NO2, Y = 2,5–OCH3; 4, X = 2-Cl, Y = 2-Cl; 5, X = 4-Cl, Y = 4-Cl) were synthesized and characterized, whereby NLO structure–function relationships were established including intramolecular charge transfer characteristics, crystal field effects, and molecular first hyperpolarizabilities (β). Given the typically large errors (10–30%) associated with the determination of β coefficients, three independent methods were used: (i) density functional theory, (ii) hyper-Rayleigh scattering, and (iii) high-resolution X-ray diffraction data analysis based on multipolar modeling of electron densities at each atom. These three methods delivered consistent values of β, and based on these results, 3 should hold the most promise for NLO applications. The correlation between the molecular structure of these geminal amido esters and their linear and nonlinear optical properties thus provide molecular design guidelines for organic NLO materials; this leads to the ultimate goal of generating bespoke organic molecules to suit a given NLO device application.