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 F 30 ZnFbU-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: (1) The rate of energy transfer is (56 ps) - 1 for ZnFbU-β and (24 ps) - 1 for F 30 ZnFbU-β. (2) The rate of hole/electron hopping in the monooxidized bis-Zn complex [F 30 Zn 2 U-β] + is in the fast-exchange limit and is at least comparable to that for [Zn 2 U-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.