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

Abstract

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 (C 60 s) 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 C 60 . Variations in the dynamics of charge separation, charge recombination, and chargetransfer 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 C 60 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 C 60 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.

Designing Cascades of Electron Transfer Processes in Multicomponent Graphene Conjugates

“…This approach is similar to that we explored in fullerene‐based electron donor‐acceptor conjugates, which exhibit long‐lived charge‐separated states as the product of a cascade of several charge‐transfer events [32] . In particular, we focused on the synthesis of β‐modified porphyrins, which feature electron‐accepting C 60 [32–34] and/or electron‐donating ferrocenes at the β‐, meso ‐, or 4‐phenyl positions [35–41] . Our past work provided fundamental insights into the unambiguous identification of all the different species that evolve as products of any charge‐transfer reactions, namely charge separation, charge shift, charge recombination, etc.…”

Screening Nanographene‐Mediated Inter(Porphyrin) Communication to Optimize Inter(Porphyrin–Fullerene) Forces

“…[7] Linking porphyrins and fullerenes covalently not only secures sufficient electronic coupling between them to power, for example, electron transfer, but also to fine-tune it by choosing the right molecular bridge. [8,9] A major drawback of linking them is the need of functionalization that alters their physicochemical properties.…”

Bridge-Length- and Solvent-Dependent Charge Separation and Recombination Processes in Donor–Bridge–Acceptor Molecules

“…Photoinduced charge separation (CS) that produces ion pairs has been widely studied in chemical, biological, and physical systems for its crucial roles in photocatalysis, energy conversion, photosynthesis, and so on. − Understanding the mechanism of intramolecular CS is a key step for designing electronic materials with tailored properties and fully understanding how to improve the efficiency of organic light-conversion systems. ,− The relationship between the molecular structure and the lifetime of the CS state has also been an important subject because of the pivotal importance in light-to-chemical energy conversion and molecular photoelectronics. ,− Generally, CS occurs in donor (D)–acceptor (A) molecules that comprise an electron D and an electron A, named the D–A or D–bridge–A (D–B–A) systems. The well-known theoretical model of the kinetic effect of CS is described using the Marcus equation (eq ), which allows the CS rate constant to be calculated as a function of three variables that can be determined computationally or experimentally − where Δ G CS is the Gibbs free energy difference, also called the thermodynamic driving force, which can be used to judge whether electron transfer is energetically favorable in the excited state; V DA is the electronic coupling matrix between the initial and final states at the transition-state nuclear configuration; λ is the recombination energy that contains inner- (λ i ) and outer-sphere (λ o ) contributions, which reflects the energy required for nuclear recombination of D and A during the electron-transfer process, as well as the recombination of their surrounding environment (e.g., solvent molecules or counterions).…”