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

The beta-diphenylethyne-linked porphyrin dimers ZnFbU-beta (nonlinking meso-mesityl substituents) and F(30)ZnFbU-beta (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 beta-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 (beta-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-beta and (24ps)(-1) for F(30)ZnFbU-beta. (2) The rate of hole/electron hopping in the monooxidized bis-Zn complex [F30Zn2U-beta](+) is in the fast-exchange limit and is at least comparable to that for [Zn2U-beta](+) These findings indicate that the presence of pentafluorophenyl groups causes enhancement of electronic communication in the beta-linked dimers but attenuation in the meso-linked dimers. These opposite effects in the beta- versus meso-linked dimers are explained by the fact that both pentafluorophenyl-substituted dimers have a(1u) HOMOs, which exhibit significant beta-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 beta-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.