Photoinduced electron transfer across molecular bridges: electron- and hole-transfer superexchange pathways

Abstract

Photoinduced electron transfer plays key roles in many areas of chemistry. Superexchange is an effective model to rationalize photoinduced electron transfer, particularly when molecular bridges between donor and acceptor subunits are present. In this tutorial review we discuss, within a superexchange framework, the complex role played by the bridge, with an emphasis on differences between thermal and photoinduced electron transfer, oxidative and reductive photoinduced processes, charge separation and charge recombination. Modular bridges are also considered, with specific attention to the distance dependence of donor-acceptor electronic coupling and electron transfer rate constants. The possibility of transition, depending on the bridge energetics, from coherent donor-acceptor electron transfer to incoherent charge injection and hopping through the bridge is also discussed. Finally, conceptual analogies between bridge effects in photoinduced electron transfer and optical intervalence transfer are outlined. Selected experimental examples, instrumental to illustration of the principles, are discussed.

Efficient Charge Separation in Li+@C60 Supramolecular Complexes with Electron Donors

“…[52] However, the addition of tetra-n-hexylammonium chloride (THACl) as an anion source to a solution of TTF-C4P and Li + @C 60 in PhCN induced electron transfer from TTF-C4P to Li + @C 60 by binding of Cl À to the calix [4]pyrrole moiety to produce the radical ion pair complex Li…”

“…[53] Such enhanced electron-acceptor ability of Li + @C 60 has made it possible to achieve more efficient thermal and photoinduced electron-transfer reduction of Li + @C 60 than that of C 60 , as discussed below. Li + @C 60 has been reported to form a supramolecular complex with tetrathiafulvalene calix [4]pyrrole (TTF-C4P). [52,54,55] TTF-C4P has electron-rich TTF moieties and its structure is changed by the addition of an anion from the 1,3-alternate conformation into the cone conformation, which can include pristine C 60 .…”

“…Sulfonated meso-tetraphenylporphyrins ((Bu 4 N + ) 4 MTPPS 4À : M= Zn, H 2 ) form 1:1 supramolecules with Li + @C 60 in PhCN; these are connected through ionic and p-p interactions. [66] The formation constants (K) of MTPPS 4À /Li + @C 60 were determined to be 1.…”

Long‐Lived, Emissive Excited States in Direct and Amide‐Linked Thienyl‐Substituted RuII Complexes

“…[4,[13][14][15] One general strategy to extend the lifetime of these CS states is to lengthen the bridge or linker between the ruthenium core and the peripheral electron-donating/accepting moieties thereby decreasing the rate of back electron transfer. [16][17][18][19][20] Ruthenium based dyads and triads have been investigated with both saturated and π-conjugated linkers between the core and the peripheral substituents, in an effort to mediate electron transfer. [4,[21][22][23] As expected, it has been shown that both the length and the nature of the linker greatly influence the properties of molecular wire-like electron and energy transfer behavior within these assemblies.…”

BioLeT: A new design strategy for functional bioluminogenic probes

“…In the field of FLI, a versatile fluorescence off/on approach, named photoinduced electron transfer (PET), is widely used for the fluorogenic probe design [13]. In general, PET refers to an excited state electron transfer process by which excited electron is transferred from the donor to acceptor [14]. Recently, many PETbased fluorescent probes were designed and utilized to detect the quantity of biomacromolecules, such as GPCRs, ion channels, DNAs, and RNAs [15].…”