Interplay of orbital tuning and linker location in controlling electronic communication in porphyrin arrays

The β-diphenylethyne-linked porphyrin dimers ZnFbU-β (nonlinking meso- mesityl substituents) and F₃₀ZnFbU-β (nonlinking meso-pentafluorophenyl substituents) and their bis-Zn analogues have been examined by static spectroscopic (absorption, fluorescence, electron paramagnetic resonance), time-resolved spectroscopic (absorption, fluorescence), and electrochemical (cyclic and square-wave voltammetry, coulometry) methods. The β-linked dimers were examined to test the hypothesis that the nature of the porphyrin HOMO (a(1u) versus a(2u)) in concert with the position of the linker (β- pyrrole or meso carbon) mediates electronic communication (excited-state energy transfer, ground-state hole-hopping). The major findings are as follows: (7) The rate of energy transfer is (56 ps)^-1 for ZnFbU-β and (24 ps)^-1 for F₃₀ZnFbU-β. (2) The rate of hole/electron hopping in the monooxidized bis-Zn complex [F₃₀Zn₂U-β]+ is in the fast-exchange limit and is at least comparable to that for [Zn₂U-β]+. These findings indicate that the presence of pentafluorophenyl groups causes enhancement of electronic communication in the β-linked dimers but attenuation in the meso- linked dimers. These opposite effects in the β- versus meso-linked dimers are explained by the fact that both pentafluorophenyl-substituted dimers have a(1u) HOMOs, which exhibit significant β-pyrrole electron density, whereas both mesityl-substituted dimers have a(2u) HOMOs, which exhibit large meso- carbon density. Thus, the combination of an a(1u) HOMO with a β-linker or an a(2u) HOMO with a meso linker results in optimal electronic communication. Collectively, these results demonstrate that the nature of the frontier orbitals and position of connection of a covalent linker (in addition to distance, orientation, and energetics) must be considered in the design architecture of molecular photonic devices.