Synthesis and Spectral-Luminescent Properties of 4,6-Diaryl-Substituted Tricyanopyridines: Influence of the Nature and Position of Substituents

A series of new 2-(4,6-diaryl-3-cyanopyridin-2(1H)-ylidene)malononitriles was synthesized by reacting para-substituted chalcones with malononitrile dimer. The influence of the nature of substituents in the aromatic fragments on the spectral-luminescent properties of the synthesized compounds in solution and in the solid state was studied using electronic spectroscopy combined with quantum mechanical calculations based on density functional theory (DFT). It was found that, from the perspective of creating donor-acceptor (D–π–A) chromophores, the most suitable configuration of functional groups is the placement of an electron-donating fragment at the C6-aryl group and an electron-withdrawing fragment at the C4-aryl group. This arrangement promotes high efficiency of intramolecular charge transfer during photoexcitation, which causes the most bathochromic shift of the absorption band of the compounds in solution. In turn, from the perspective of photoluminescent properties, tricyanopyridine derivatives bearing strong donor substituents on both aryl fragments appear most promising. This configuration allows delocalization of the HOMO onto them while the LUMO is localized on the cyano-substituted pyridine ring, which sharply increases the probability of radiative relaxation from the excited state.