Malonic Acid Derivatives on Duty as Electron‐Withdrawing Units in Push–Pull Molecules

Abstract

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.