Syntheses, Characterization and Acidochromic Properties of Donor‐Acceptor‐Donor Compounds Based on 2‐Amino‐Pyrimidine

Organic materials with a donor-acceptor-donor (D−A−D) system of π-conjugation exhibit strong intermolecular interactions. This study reports the acid-induced color-changing properties of two D−A−D type π-conjugated compounds containing 2-aminopyrimidine. When the 3,4-ethylenedioxythiophene (EDOT) moiety is introduced into the D−A−D compound as an electron donor and analyzed through UV-visible spectroscopy, fluorescence, and thermal characterization, the compound is found to have a narrow bandgap, good thermal stability, and detection ability. The detection limit for methanesulfonic acid is 1.24×10−9 mol/L. Density functional theory (DFT) indicates that both the protonated and unprotonated D−A−D compounds achieve a coplanar geometry. In the acid-induced color change, the protonated AETP, EDOT′s C1 atom, and the C2 atom of the pyrimidine ring rotate into a head-to-head structure, providing sufficient space for proton and pyrimidine N closure. Additionally, The HOMO and LUMO energy levels of ATP and AETP exhibit a notable difference with the former at 2.127 eV and the latter at 2.039 eV. The protonation of these compounds led to a decrease in molecular orbital energy by approximately 50 %, indicating an enhancement in stability relative to their unprotonated counterparts. This observation has significant implications for the design of molecular structures that are responsive to changes in acid concentration, making it valuable for future studies in the field of acid color-changing compounds.