Electronic transport in radial π-conjugated macrocyclic molecules: a density functional theory study

Radially conjugated macrocyclic molecules offer a unique approach to tuning frontier orbital energies by manipulating ring geometry and donor-acceptor (D-A) interactions. Here, we present a systematic DFT study of the electronic structure and metal-molecule energy-level alignment for a series of macrocyclic molecules and their linear counterparts, which include thiophene, diketopyrrolopyrrole (DPP), benzodithiophene (BDT), dithienobenzodithiophene (DTBDT), and benzothiazole (BT) units. Our calculations indicate that macrocyclization induces a system-specific change in the HOMO-LUMO energy gap, with the direction and magnitude depending on the balance between ring strain and D-A coupling strength for each molecule. Among the studied systems, [DTBDT-DPP]₃ has the smallest HOMO-LUMO gap, decreasing from 1.13 eV in the isolated macrocycle to 1.44 eV in the Au18 junction model due to electrode-induced orbital hybridization. Its HOMO at -5.31 eV aligns most closely with the Au18 Fermi level, resulting in a hole injection barrier. (Φ _h) of approximately 0.01-0.21 eV, making it the most promising candidate for hole injection in this series. These results establish quantitative structure-property relationships across five D-A macrocyclic architectures and offer a computational foundation for the rational design of macrocycle-based organic semiconductors.