Anion-Induced Electron Transfer

Abstract

As counterintuitive as it might seem, in aprotic media, electron transfer (ET) from strong Lewis basic anions, particularly F, OH, and CN, to certain π-acids (πA) is not only spectroscopically evident from the formation of paramagnetic πA radical anions and πA dianions, but also thermodynamically justified because these anions' highest occupied molecular orbitals (HOMOs) lie above the π-acids' lowest unoccupied molecular orbitals (LUMOs) creating negative free energy changes (Δ G° < 0). Depending on the relative HOMO and LUMO energies of participating anions and π-acids, respectively, the anion-induced ET (AIET) events take place either in the ground state or upon photosensitization of the π-acids. The mild basic and charge-diffuse anions with lower HOMO levels fail to trigger ET, but they often form charge transfer (CT) and anion-π complexes. Owing to their high HOMO levels in aprotic environments, strong Lewis basic anions, such as F enjoy much greater ET driving force (Δ G°) than mild and non-basic anions, such as iodide. In protic solvents, however, the former become more solvated and stabilized and lose their electron donating ability more than the latter, creating an illusion that F is a poor electron donor due to the high electronegativity of fluorine. However, UV-vis, EPR, and NMR studies consistently show that in aprotic environments, F reduces essentially any π-acid with LUMO levels of -3.8 eV or less, revealing that contrary to a common perception, the electron donating ability of F anion is not dictated by the electronegativity of fluorine atom but is a true reflection of high Lewis basicity of the anion itself. Thus, the neutral fluorine atoms with zero formal charge and F anion have little in common when it comes to their electronic properties. The F anion can also legitimately act as a Brønsted base when the proton source has a p K lower than that of its conjugate acid HF (15), not the other way around, and ET from F to a poor electron acceptor is not thermodynamically feasible. While there is no shortage of indisputable evidence and clear-cut thermodynamic justifications for ET from F and other Lewis basic anions to various π-acids in aprotic solvents, because of the aforesaid misconception, it had been posited that F perhaps formed diamagnetic Meisenheimer complexes via nucleophilic attack, deprotonated an aprotic solvent DMSO against an insurmountably high p K (35) leading to a π-acid reduction, or formed [F/πA] complexes via a thermodynamically prohibited oxidation of π-acids. Unlike AIET, however, none of these hypotheses was thermodynamically viable nor supported by any experimental evidence. First, by defining the thermodynamic criteria of AIET pathways and all other alternate hypotheses and then evaluating the spectroscopic signals emanating from the interactions between different anions and π-acids and Lewis acids in the light of these criteria, this Account comes to a conclusion that AIET is the only viable mechanism that can rationalize the reduction of π-acids without violating any ther...

How Radical are ‘Radical’ Photocatalysts? A Closed-Shell Meisenheimer Complex is Identified as a Super-reducing Photoreagent

“…Treatment of rylene imides with fluoride sources in some polar aprotic solvents was previously reported to result in one electron reduction, in the dark. 32,33 The mechanism of this process has been debated in the literature, with initial reports suggesting direct fluoride oxidation 32,28 later revised by reports of fluoride-mediated solvent oxidation, 33 wherein a protoncoupled electron transfer (PCET) process involving solvent deprotonation and formation of bifluoride is concomitant with electron transfer from solvent to rylene imide. In the case of rylene bisimides, for which two-electron reductions are fully reversible, this process was demonstrated to terminate at one electron.…”

“…Fluoride-mediated solvent oxidation may result in dimethylacetamidyl (from DMAc) or dimsyl (from DMSO) anion addition to NMI. 32,33 Additionally, of possible relevance to several photochemical cycles invoking two-photon radical anion photocatalysis, a photon-mediated formation of a triethylamine (TEA) σ-complex may be envisioned wherein photooxidation of TEA followed by deprotonation produces a radical pair consisting of the NMI radical anion and the neutral TEA radical. A σ-complex formed from this radical pair is another possible photoactive Meisenheimer adduct.…”

Cationic Organophosphorus Chromophores: A Diamond in the Rough among Ionic Dyes

“…The resulting phenyl‐substituted salts 26 a – c with moderate ICT are intense blue emitters in solution ( λ em =446 nm R=H, 476 nm R=OMe, CH 2 Cl 2 ) with quantum yields ranging from 0.75 to 0.89 for PF 6 − or OTf − anions [48g, 53] . In the case of iodide, the intensity of fluorescence is systematically lower ( Φ em =0.36–0.80) that might be a result of anion‐π charge transfer interaction [65] . The bulkier groups at the phosphorus atom lead to a certain improvement of quantum efficiency by suppressing non‐radiative decay rate; for example, the methylated derivative 26 a shows twice larger k nr (3×10 7 s −1 ) than ethylated and phenylated congeners 26 b , c ( k nr =1.4×10 7 s −1 ) [48g] …”

Radical Anion Yield, Stability, and Electrical Conductivity of Naphthalene Diimide Copolymers n-Doped with Tertiary Amines

“…[36][37][38] In this case, the chemical functionality responsible for doping, which may be a tertiary amine or quaternary ammonium group, is covalently linked to the conjugated polymer backbone. [38][39][40][41] While the mechanism involves doping via electron transfer from the amine to the electron acceptor, 40,[42][43][44] in case of quaternary ammonium-containing side chains details of this mechanism may differ. 12,[38][39][40]43,45,46 Next to these mechanistic studies, such doping processes are often correlated with device characteristics such as electrical conductivity for a given system, 41 but structure-function relationships are more often reported for transistor applications.…”

“…[38][39][40][41] While the mechanism involves doping via electron transfer from the amine to the electron acceptor, 40,[42][43][44] in case of quaternary ammonium-containing side chains details of this mechanism may differ. 12,[38][39][40]43,45,46 Next to these mechanistic studies, such doping processes are often correlated with device characteristics such as electrical conductivity for a given system, 41 but structure-function relationships are more often reported for transistor applications. [47][48][49] Previously, we have used binary blends of small molecule NDI derivatives as a model system to study electron transfer from dimethylaminopropyl (DMAP) side chains attached to NDI to a second NDI derivative of varying energy level.…”