Interplay of Orbital Tuning and Linker Location in Controlling Electronic Communication in Porphyrin Arrays

Abstract

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 (a1u versus a2u) 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: (1) 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 a1u HOMOs, which exhibit significant β-pyrrole electron density, whereas both mesityl-substituted dimers have a2u HOMOs, which exhibit large meso-carbon density. Thus, the combination of an a1u HOMO with a β-linker or an a2u 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.