Bond overlap population analysis on through-space electron transfer in [3n]cyclophanes (n = 2 - 5)

Through-space electron transfer between the benzene rings of [3n]cyclophanes, n = 2 - 5, was investigated using density functional theory calculations, CAM-B3LYP/6-311++G**. An external point charge, q, was placed on the axis perpendicular to a benzene ring of the cyclophane molecule. The dependence of the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap and bond overlap population (BOP) of cyclophanes on the values of q and n are discussed. The HOMO-LUMO gap generally decreases as q decreases and n increases. It is found from the BOP analysis that electron transfer occurs mainly through LUMO + 2 for n = 2, LUMO for n = 3 and n = 4, and LUMO + 4 for n = 5. The result that the values of the BOP for n = 2 and 5 increase as q decreases means that the through-space electron transfer is enhanced by the negative charge on the neighboring molecule in the solid.