Importance of Through-Space Interaction of [2,2′]-Paracyclophane-oligo(p-phenylenevinylene) Molecular Wires for Photovoltaic Application and Effective Wire Design by Chemical Substitution

A theoretical study was performed to understand the effects of a [2,2′]-paracyclophane (pCp) moiety in zinc-porphyrin (ZnP) - C₆₀ connected pCp-oligo(p-phenylenevinylene) (pCp-oPPV) molecular wire for photovoltaic application. Quantum chemistry (QC) calculations showed that pCp changes a photoexcitation site from a wire part to a donor ZnP by comparison with a pure oPPV-based system. In addition, pCp was found to produce (1) stepwise block localized vacant frontier molecular orbitals (MOs) with their energy levels decreasing in the direction from cathode to anode and (2) a large difference in energy levels between occupied frontier MOs localized on ZnP and wire parts. The first and second features are expected to accelerate charge separation (CS) and suppress charge recombination (CR), respectively. QC calculations for wire models showed that the inclusion of pCp causes asymmetric features in orbital levels, that is, "stepwise" vacant and "degenerate" occupied MOs. It was found from our analysis that in vacant MOs, an effective orbital overlap between oPPV moieties through pCp results in energy splitting leading to stepwise vacant MOs. In contrast, in occupied MOs, impractical orbital overlap between oPPV moieties was found to result in degenerate occupied MOs. Such differences in orbital overlap also indicate asymmetric CS/CR properties in pCp-oPPV, that is, fast CS and slow CR, which was observed experimentally.