Keywords:Charge transfer / Donor-acceptor systems / Nonlinear optics / Electrochemistry / PyrazineThirteen new, stable, push-pull systems featuring dimethylamino and pyrazine-2,3-dicarbonitrile moieties as the donor and acceptor, respectively, and systematically extended and varied π-linkers were prepared and investigated. Evaluation of the measured UV/Vis spectra, electrochemical data (cyclic voltammetry, rotating disc voltammetry, and polarography), X-ray data, and experimentally determined and calculated hyperpolarizability values enabled structure-

The photophysical and two-photon absorption (2PA) properties of two tri-podal molecules and of their quadrupolar and dipolar counterparts are reported for a series of solvents with varying polarity. The molecules possess a tri-phenylamine electron donating group and mono-cyano acceptors while olefinic and acetylenic p-linkers have been used. Branching led to an increase of the molar extinction coefficient and to a slight bathochromic shift of the absorption spectra while the fluorescence quantum yields decrease but they are maintained to relatively high values. Solvatochromic measurements in the tripodal molecules revealed an emitting state with a polar nature. The 2PA cross sections in general increase upon branching but the observed behaviour strongly depends on the type of solvent. The highest 2PA cross sections are obtained in solvents of medium polarity and values as high as 1420 GM are reported.

Potentiometric titration has been used to measure dissociation constants of 38 monosubstituted benzoic acids in water and in 6 organic solvents (methanol, ethanol, dimethylformamide, acetonitrile, sulfolane, acetone). The results have been analyzed and interpreted from the point of view of substituent and solvent effects on the dissociation of the title substrates. It has been proved that solvents affect not only the reaction constant but also the substituent constant of the Hammett equation.

Based on the 2‐(N‐piperidinyl)thiophene central donor, 32 model push–pull molecules with systematically varied malonic acid‐derived peripheral acceptors have been prepared. Further property tuning has been achieved by modifying the π‐linker and the structural arrangement (linear vs. quadrupolar D–π–A systems). Malonic acid derivatives such as cyanoacetic acid, malondinitrile, diethyl malonate, Meldrum′s acid, and N,N′‐dibutyl(thio)barbituric acid as well as 1,3‐diketo analogues dimedone and indan‐1,3‐dione were employed as acceptor moieties. Knoevenagel condensation with four thiophene aldehydes afforded the target chromophores in satisfactory yields. The electron‐withdrawing abilities of malonic acid acceptors were examined both by experiment including X‐ray analysis, differential scanning calorimetry, electrochemistry, and UV/Vis absorption spectroscopy, and by DFT calculations. Details of the structure–property relationships have been elucidated. According to the increasing electron‐withdrawing ability, the widely used malonic acid acceptor units can be ordered: diethyl malonate ≤ cyanoacetic acid < malondinitrile < Meldrum's acid < dimedone ≤ N,N′‐dibutylbarbituric acid < indan‐1,3‐dione ≤ N,N′‐dibutylthiobarbituric acid.