The GaCl₃-Catalyzed Knoevenagel Condensation To Achieve Acceptor–Donor–Acceptor Small-Molecule Acceptors: A DFT Mechanistic Study

Herein, the reaction mechanism for the GaCl₃-catalyzed Knoevenagel condensation of 2-formylindacenodithieno[3,2-<i>b</i>]thiophene (ITIC-CHO) and active methylene compound 1,1-dicyanomethylene-3-indanone (IC) to synthesize ITIC in the presence of acetic anhydride was investigated using the density functional theory (DFT) method. The calculated results indicate that this reaction follows a bimolecular GaCl₃ catalytic mechanism. The free energy span for the monomolecular GaCl₃ catalytic mechanism is the highest (31.8 kcal/mol), followed by the trimolecular GaCl₃ catalytic mechanism (26.4 kcal/mol) and the bimolecular GaCl₃ catalytic mechanism (26.3 kcal/mol). The trimolecular GaCl₃ path and bimolecular GaCl₃ path are competitive, but the former path is limited by the concentration of GaCl₃. The inclusion of GaCl₃ could stabilize the transition states of C-H activation. Compared to the GaCl₃-catalyzed Knoevenagel condensation, that catalyzed by pyridine is not advantageous, owning a high energy span of 31.7 kcal/mol. These agree well with experimental results. This work could provide a novel theoretical understanding of the Knoevenagel condensation, which could inspire the development of a synthesis strategy for electron acceptor materials.