Charge transfer versus molecular conductance: molecular orbital symmetry turns quantum interference rules upside down† †Electronic supplementary information (ESI) available: Details on the synthesis and characterization of 1, 2mm, 2mp, 2pp, and 3, details on the experimental methodology, xyz-coordinates for all structures used in the calculations, details on the presence of destructive interference in S2mm, details on the orbital symmetry considerations. See DOI: 10.1039/c5sc01104c

Molecular orbital symmetry considerations can strongly affect the nature of quantum interference effects in charge transfer.. Destructive quantum interference has been shown to strongly reduce charge tunneling rates across molecular bridges. The current consensus is that destructive quantum interference occurs in cross-conjugated molecules, while linearly conjugated molecules exhibit constructive interference. Our experimental results on photoinduced charge transfer in donor-bridge-acceptor systems, however, show that hole transfer is ten times faster through a cross-conjugated biphenyl bridge than through a linearly conjugated biphenyl bridge. Electronic structure calculations reveal that the surprisingly low hole transfer rate across the linearly conjugated biphenyl bridge is caused by the presence of destructive instead of constructive interference. We find that the specific molecular orbital symmetry of the involved donor and acceptor states leads to interference conditions that are different from those valid in single molecule conduction experiments. Furthermore, the results indicate that by utilizing molecular orbital symmetry in a smart way new opportunities of engineering charge transfer emerge.