Mechanisms of excited-state energy-transfer gating in linear versus branched multiporphyrin arrays

We have investigated electrochemical switching of excited-state electronic energy migration in two optoelectronic gates with different architectures. Each gate consists of diarylethyne-linked subunits: a boron-dipyrrin (BDPY) input unit, a Zn-porphyrin transmission unit, a free-base-porphyrin (Fb-porphyrin) output unit, and a Mg-porphyrin redox-switched site connected either to the Fo porphyrin (linear gate) or to the Zn porphyrin (branched. T gate). Both the linear and branched architectures show Fb-porphyrin emission when the Mg porphyrin is neutral and nearly complete quenching when the ME porphyrin is oxidized to the Jr-cation radical. To determine the mechanism of gating, we undertook a systematic photophysical study of the gates and their dyad and triad components in neutral and oxidized forms, using static and time-resolved optical spectroscopy. Two types of photoinduced energy-transfer (and/or charge-transfer) processes are involved in gate operation: transfer between adjacent subunits and transfer between nonadjacent subunits. All of the individual energy-transfer steps that funnel input light energy to the fluorescent output element in the neutral systems are highly efficient, occurring primarily by a through-bond mechanism. Similarly efficient energy/transfer processes occur between the BDPY and the Zn and Fo porphyrins in the oxidized systems, but are followed by rapid and efficient energy/charge transfer to the redox-switched site and consequent nonradiative deactivation. Energy/charge transfer between nonadjacent porphyrins, which occurs principally by superexchange, is crucial to the operation of the T gate. Collectively, our studies elucidate the photophysics of gating and afford great flexibility and control in the design of more elaborate arrays for molecular photonics applications.