Charge-Gating Dibenzothiophene‑S,S‑dioxide Bridges in Electron Donor–Bridge–Acceptor Conjugates

The synthesis of a series of new electron donor–bridge–acceptor (D–B–A) conjugates (18–20) comprising electron-donating zinc­(II) porphyrins (ZnPs) and electron-accepting fullerenes (C₆₀s) connected through various co-oligomer bridges containing both dibenzothiophene-S,S-dioxide and fluorene units is reported. Detailed investigations using cyclic voltammetry, absorption, fluorescence, and femto/nanosecond transient absorption spectroscopy in combination with quantum chemical calculations have enabled us to develop a detailed mechanistic view of the charge-transfer processes that follow photoexcitation of ZnP, the bridge, or C₆₀. Variations in the dynamics of charge separation, charge recombination, and charge-transfer gating are primarily consequences of the electronic properties of the co-oligomer bridges, including their electron affinity and the energy levels of the excited states. In particular, placing one dibenzothiophene-S,S-dioxide building block at the center of the molecular bridge flanked by two fluorene building blocks, as in 20, favors hole rather than electron transfer between the remote electron donors and acceptors, as demonstrated by exciting C₆₀ rather than ZnP. In 18 and 19, in which one dibenzothiophene-S,S-dioxide and one fluorene building block constitute the molecular bridge, photoexcitation of either ZnP or C₆₀ results in both hole and electron transfer. Dibenzothiophene-S,S-dioxide is thus shown to be an excellent building block for probing how subtle structural and electronic variations in the bridge affect unidirectional charge transport in D–B–A conjugates. The experimental results are supported by computational calculations.