Anion-induced Electron Transfer Research Articles

Photochromism and electrochromism in pyridinium-annelated perylene diimide salts via charge-assisted anion-π interactions

Organic chromic materials with photochromic and electrochromic properties are promising materials for photoelectric applications. Despite some reports indicating the presence of these properties in aromatic imide derivatives, the advancement of their enduring discoloration performance remains an unresolved challenge. Herein, we incorporated of charge-assisted anion-π interactions into aromatic imide derivatives as a strategy for improving the stability of generated free-radical, and a series of pyridinium-annelated perylene diimides salts (PAPDI-X, X = I−, Br−, Cl−, NO3−, PF6−) were synthesized. With variations in the electron-donating capabilities (Lewis basicity) of the anions, the nature of anion-PDI interactions undergoes a transition from non-chromogenic anion-π interaction to chromogenic anion-induced electron transfer (ET) phenomenon, and these interactions have been fully characterized by UV/Vis, NMR, and EPR spectroscopies. Among the evaluated anions, Br- anion with moderate Lewis basicity participates in ET with PAPDI by light or voltage stimuli, and generate stable PAPDI-Br•− radical anion and PAPDI-Br2- dianion in aprotic solvent or in solid state, thus making PAPDI-Br exhibit distinctive photo/electrochromic dual responsive behavior based on radical formation. This meaningful design provides new concepts for the design of organic chromic materials and shows great prospects in optoelectrical materials.

Here’s looking at the reduction of noninnocent copper corroles via anion induced electron transfer

Here’s looking at the reduction of noninnocent copper corroles via anion induced electron transfer

Electrosynthesis of π-Extended Porphyrins Via Reductive Decyanation

The development of new electrochemically driven synthetic routes to obtain π-extended porphyrins, or new synthetic porphyrins in general, is highly desirable since electrosynthesis is often able to afford a clean conversion of reactants to products without a great deal of contamination. At the same time, electrosynthesis, when combined with classical electrochemical measurements, can often provide unique insights into the mechanism and intermediates involved in formation of the desired product. This is explored in the current work where a novel electrosynthetic method for generation of π-extended porphyrins and the corresponding reductive decyanation mechanism is described. The heterogenous anion induced electrosynthetic method is also compared to a homogenous chemical synthesis method to obtain the same products, using anion-induced electron transfer (AIET). The starting porphyrins are β,meso-fused malononitrile derivatives are represented as MTPP(MN)2 and the π-extended, di-fused products as MTPP(VCN)2, where TPP = the dianion of tetraphenylporphyrin, MN = malononitrile, VCN = vinyl cyanide and M = H2, NiII, CuII and ZnII. Structures of these porphyrins are shown in Chart 1. The easily synthesized π-extended porphyrins with two fused cyanobenzene (benzonitrile) rings are characterized by three facile and reversible one-electron ring-centered reductions in THF containing 0.1 M TBAP. There is also a fourth irreversible reduction at the edge of the solvent. Figure 1

Facile Heterogeneous and Homogeneous Anion Induced Electrosynthesis: An Efficient Method for Obtaining π-Extended Porphyrins.

Two closely related electrosynthetic approaches are applied for the preparation of novel π-extended tetraphenylporphyrins from malononitrile-appended meso-β di-fused porphyrins, represented as MTPP(MN)2, where TPP = the dianion of tetraphenylporphyrin and MN = malononitrile. The first method involves application of a controlled reducing potential at a platinum electrode in CH2Cl2, while the second proceeds via cyanide anion induced electron transfer. Both methods produced the same decyanated, π-extended di-fused porphyrins represented as MTPP(VCN)2 where VCN = vinyl cyanide and M = H2, NiII, CuII, or ZnII in almost quantitative yields. The final isolated and purified porphyrin products are characterized by a split Soret band ranging from 411-497 nm and two broad intense Q bands. The new π-extended porphyrins are easier to reduce than the parent MTPP or MTPP(MN)2 compounds by 760-800 mV and 180-190 mV, respectively, and possess an electrochemical HOMO-LUMO gap ranging from 1.48 to 1.66 V. They are also characterized by two reversible one-electron ring-centered reductions in CH2Cl2 and three reversible one-electron ring-centered reductions in THF. A fourth irreversible reduction is seen in THF at more negative potentials and is assigned to one or two of the fused cyanobenzene rings of the macrocycle.

A universal approach for optimizing charge extraction in electron transporting layer-free organic solar cells via Lewis base doping

Applying anion-induced electron transfer doping with a series of TXABr salts to non-fullerene organic solar cells enables to tune the doping efficiency and photovoltaic device performance.

Anion-Induced Electron Transfer.

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 πA2- 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°ET < 0). Depending on the relative HOMO and LUMO energies of participating anions and π-acids, respectively, the anion-induced ET (AIET) events take placeeither 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°ET) 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 Ka 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 Ka (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 thermodynamic rules. The paradigm-shifting discovery of AIET not only exposed a common misconception about the electron donating ability of F- but also enabled naked-eye detection of toxic anions, electrode-free silver plating, luminescent silver nanoparticle synthesis, light-harvesting, and conductivity enhancement of conjugated polymers, with more innovative applications still to come.

Doping Versatile n-Type Organic Semiconductors via Room Temperature Solution-Processable Anionic Dopants.

In this study, we describe a facile solution-processing method to effectively dope versatile n-type organic semiconductors, including fullerene, n-type small molecules, and graphene by commercially available ammonium and phosphonium salts via in situ anion-induced electron transfer. In addition to the Lewis basicity of anions, we unveiled that the ionic binding strength between the cation and anion of the salts is also crucial in modulating the electron transfer strength of the dopants to affect the resulting doping efficiency. Furthermore, combined with the rational design of n-type molecules, an n-doped organic semiconductor is demonstrated to be thermally and environmentally stable. This finding provides a simple and generally applicable method to make highly efficient n-doped conductors which complements the well-established p-doped organics such as PEDOT:PSS for organic electronic applications.

Enhancing stability and efficiency of perovskite solar cells with crosslinkable silane-functionalized and doped fullerene.

The instability of hybrid perovskite materials due to water and moisture arises as one major challenge to be addressed before any practical application of the demonstrated high efficiency perovskite solar cells. Here we report a facile strategy that can simultaneously enhance the stability and efficiency of p–i–n planar heterojunction-structure perovskite devices. Crosslinkable silane molecules with hydrophobic functional groups are bonded onto fullerene to make the fullerene layer highly water-resistant. Methylammonium iodide is introduced in the fullerene layer for n-doping via anion-induced electron transfer, resulting in dramatically increased conductivity over 100-fold. With crosslinkable silane-functionalized and doped fullerene electron transport layer, the perovskite devices deliver an efficiency of 19.5% with a high fill factor of 80.6%. A crosslinked silane-modified fullerene layer also enhances the water and moisture stability of the non-sealed perovskite devices by retaining nearly 90% of their original efficiencies after 30 days' exposure in an ambient environment.

Photoinduced electron transfer in a supramolecular triad produced by porphyrin anion-induced electron transfer from tetrathiafulvalene calix[4]pyrrole to Li(+)@C60.

Binding of a porphyrin carboxylate anion () to tetrathiafulvalene calix[4]pyrrole (TTF-C4P) results in electron transfer from TTF-C4P to Li(+)@C60 to produce the charge-separated state (1/TTF-C4P˙(+)/Li(+)@C60˙(-)) in benzonitrile. Upon photoexcitation of , photoinduced electron transfer from the triplet excited state of to TTF-C4P˙(+) occurs to produce the higher energy charge-separated state (˙(+)/TTF-C4P/Li(+)@C60˙(-)), which decays to the ground state with a lifetime of 4.8 μs.

A new insight into fluoride anion in electron transfer reactions

Recently, scientists have developed fluoride anion recognition synthetic macromolecules for potential applications in environmental monitoring. However, the actual role of fluoride anion remains highly debated in the reaction of anion-induced electron transfer (ET) to π-electron-deficient naphthalenediimides. The present study demonstrates that the ET reaction is induced by the complexation of F− with the cation radical formed with the assistance of polar aprotic solvent. The results present an alternative mechanism of the actual role of F− in ET reactions in polar aprotic solvents that could provide novel implications for anion-mediated ET reactions.

Solution‐Processible Highly Conducting Fullerenes

n-Doping of solution-processible organic semiconductors: highly conductive fullerenes are demonstrated through solution-processed fulleropyrrolidinium iodide (FPI) and FPI-doped PCBM to reach a high conductivity (3.2 S/m). The n-doping proceeds via anion-induced electron transfer between the iodide on FPI and the fullerene in the solid state.

Tunable electronic interactions between anions and perylenediimide

Over the past decade anion-π interaction has emerged as a new paradigm of supramolecular chemistry of anions. Taking advantage of the electronic nature of anion-π interaction, we have expanded its boundaries to charge-transfer (CT) and formal electron transfer (ET) events by adjusting the electron-donating and accepting abilities of anions and π-acids, respectively. To establish that ET, CT, and anion-π interactions could take place between different anions and π-acids as long as their electronic and structural properties are conducive, herein, we introduce 3,4,9,10-perylenediimide (PDI-1) that selectively undergoes thermal ET from strong Lewis basic hydroxide and fluoride anions, but remains electronically and optically silent to poor Lewis basic anions, as ET and CT events are turned OFF. These interactions have been fully characterized by UV/Vis, NMR, and EPR spectroscopies. These results demonstrate the generality of anion-induced ET events in aprotic solvents and further refute a notion that strong Lewis basic hydroxide and fluoride ions can only trigger nucleophilic attack to form covalent bonds instead of acting as sacrificial electron donors to π-acids under appropriate conditions.

Electronically Regulated Thermally and Light-Gated Electron Transfer from Anions to Naphthalenediimides

Anion-induced electron transfer (ET) to π-electron-deficient naphthalenediimides (NDIs) can be channeled through two distinct pathways by adjusting the Lewis basicity of the anion and the π-acidity of the NDI: (1) When the anion and NDI are a strong electron donor and acceptor, respectively, positioning the HOMO of the anion above the LUMO of the NDI, a thermal anion → NDI ET pathway is turned ON. (2) When the HOMO of a weakly Lewis basic anion falls below the LUMO of an NDI but still lies above its HOMO, the thermal ET is turned OFF, but light can activate an unprecedented anion → (1)*NDI photoinduced ET pathway from the anion's HOMO to the photogenerated (1)*NDI's SOMO-1. Both pathways generate NDI(•-) radical anions.

Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers.

Inorganic anions strongly influence the electron transfer rate from the ascorbate to the ferrocene-terminated self-assembled monolayer (SAM) composed of 9-mercaptononyl-5'-ferrocenylpentanoate (Fc(CH2)4COO(CH2)9SH, MNFcP). At the 1 M concentration level of the supporting anion (sodium salt electrolyte), a more than 10-fold increase in the electrocatalytic oxidation rate constant of the ascorbate is observed in the following sequence: PF6-, ClO4-, BF4-, NO3-, Cl-, SO4(2-), NH2SO3- (sulfamate), and F-. The sequence corresponds to the direction of increasing hydration energy of the corresponding anion, suggesting that highly hydrated ions promote electrocatalytic electron transfer to the ferrocene-terminated SAMs, while poorly hydrated ions inhibit it. Fourier transform surface-enhanced Raman spectroscopy (FT-SERS), in combination with cyclic voltammetry, indicates the formation of surface ion pairs between the ferricinium cation (Fc+) and low hydration energy anions, while, on the contrary, no ion pairs were observed in the electrolytes dominated by the high hydration energy anions. Though it is evident that the ion-pairing ability of hydrophobic anions is directly responsible for the electrocatalytic electron transfer inhibition, an estimate of the free, ion-unpaired Fc+ surface concentration shows that it cannot be directly related to the electron transfer rate. This suggests that the principal reason of the anion-induced electron transfer rate modulation might be related to the molecular level changes of the physical and chemical properties as well as the structure of the self-assembled monolayer.