Rk. Lammi et al., Mechanisms of excited-state energy-transfer gating in linear versus branched multiporphyrin arrays, J PHYS CH B, 105(22), 2001, pp. 5341-5352
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-po
rphyrin) output unit, and a Mg-porphyrin redox-switched site connected eith
er to the Fo porphyrin (linear gate) or to the Zn porphyrin (branched. T ga
te). 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 t
heir dyad and triad components in neutral and oxidized forms, using static
and time-resolved optical spectroscopy. Two types of photoinduced energy-tr
ansfer (and/or charge-transfer) processes are involved in gate operation: t
ransfer between adjacent subunits and transfer between nonadjacent subunits
. All of the individual energy-transfer steps that funnel input light energ
y to the fluorescent output element in the neutral systems are highly effic
ient, occurring primarily by a through-bond mechanism. Similarly efficient
energy/transfer processes occur between the BDPY and the Zn and Fo porphyri
ns in the oxidized systems, but are followed by rapid and efficient energy/
charge transfer to the redox-switched site and consequent nonradiative deac
tivation. Energy/charge transfer between nonadjacent porphyrins, which occu
rs 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 mo
lecular photonics applications.