n-Dopants Based on Dimers of Benzimidazoline Radicals: Structures and Mechanism of Redox Reactions

Study of the electron-doping mechanism in single-walled carbon nanotubes using dimethylbenzimidazole

Single-walled carbon nanotubes (SWCNTs) exhibit p-type properties in air, necessitating electron doping using n-dopants (e.g., reducing agents) for the development of SWCNT-based electronic devices. Dimethylbenzimidazole (DMBI-H) derivatives serve as effective electron dopants, not only for SWCNTs, but also for various organic semiconducting materials. However, the doping reaction is still a subject of debate. In this study, the electron-doping reactions of ortho-methoxy-substituted DMBI-H for SWCNTs were analyzed in protic and aprotic solvents in the presence and absence of dioxygen (O2). The presence of O2 was found to cause the reduction of O2 on the SWCNT surface in the protic solvent, resulting in the production of DMBI cations and water through proton-coupled electron transfer (PCET) from the n-doped SWCNT and ethanol. This work elucidates the mechanism behind the air-stability of n-type SWCNTs.

Reductive dimerization of benzothiazolium salts

Three different types of reaction products were obtained from the reduction of 2-substituted 3-methylbenzothiazolium salts using Na : Hg (1 wt%). Depending on the 2-substituents, two types of dimeric compounds were obtained: the 2-cyclohexyl-, 2-phenyl-, and 2-(p-tolyl)-substituted species are reduced to the corresponding 2,2'-bibenzo[d]thiazoles, while their 2-((p-OMe)C6H4)- and 2-((p-NMe2)C6H4)-substituted derivatives afford cis-[1,4]benzothiazino[3,2-b][1,4]benzothiazines. Furthermore, in the presence of molecular O2, new disulfide derivatives were obtained from the bibenzo[d]thiazoles. The products were obtained in a moderate to good yield, and the structures were confirmed using single-crystal X-ray diffraction. The electrochemistry and further reactivity towards different oxidants of the dimeric compounds were studied; the 2,2'-bibenzo[d]thiazoles show oxidation potentials similar to that of ferrocene and are converted back to the corresponding benzothiazolium cations by mild oxidants such as TCNQ. In contrast, the benzothiazino-benzothiazines show no oxidations in the solvent window of THF.

Beyond n-dopants for organic semiconductors: use of bibenzo[d]imidazoles in UV-promoted dehalogenation reactions of organic halides

2,2'-Bis(4-dimethylaminophenyl)- and 2,2'-dicyclohexyl-1,1',3,3'-tetramethyl-2,2',3,3'-tetrahydro-2,2'-bibenzo[d]imidazole ((N-DMBI)2 and (Cyc-DMBI)2) are quite strong reductants with effective potentials of ca. -2 V vs ferrocenium/ferrocene, yet are relatively stable to air due to the coupling of redox and bond-breaking processes. Here, we examine their use in accomplishing electron transfer-induced bond-cleavage reactions, specifically dehalogenations. The dimers reduce halides that have reduction potentials less cathodic than ca. -2 V vs ferrocenium/ferrocene, especially under UV photoexcitation (using a 365 nm LED). In the case of benzyl halides, the products are bibenzyl derivatives, whereas aryl halides are reduced to the corresponding arenes. The potentials of the halides that can be reduced in this way, quantum-chemical calculations, and steady-state and transient absorption spectroscopy suggest that UV irradiation accelerates the reactions via cleavage of the dimers to the corresponding radical monomers.

Benzoimidazolium-derived dimeric and hydride n-dopants for organic electron-transport materials: impact of substitution on structures, electrochemistry, and reactivity

1,3-Dimethyl-2,3-dihydrobenzo[d]imidazoles, 1H, and 1,1',3,3'-tetramethyl-2,2',3,3'-tetrahydro-2,2'-bibenzo[d]imidazoles, 12, are of interest as n-dopants for organic electron-transport materials. Salts of 2-(4-(dimethylamino)phenyl)-4,7-dimethoxy-, 2-cyclohexyl-4,7-dimethoxy-, and 2-(5-(dimethylamino)thiophen-2-yl)benzo[d]imidazolium (1g-i+, respectively) have been synthesized and reduced with NaBH4 to 1gH, 1hH, and 1iH, and with Na:Hg to 1g2 and 1h2. Their electrochemistry and reactivity were compared to those derived from 2-(4-(dimethylamino)phenyl)- (1b+) and 2-cyclohexylbenzo[d]imidazolium (1e+) salts. E(1+/1•) values for 2-aryl species are less reducing than for 2-alkyl analogues, i.e., the radicals are stabilized more by aryl groups than the cations, while 4,7-dimethoxy substitution leads to more reducing E(1+/1•) values, as well as cathodic shifts in E(12•+/12) and E(1H•+/1H) values. Both the use of 3,4-dimethoxy and 2-aryl substituents accelerates the reaction of the 1H species with PC61BM. Because 2-aryl groups stabilize radicals, 1b2 and 1g2 exhibit weaker bonds than 1e2 and 1h2 and thus react with 6,13-bis(triisopropylsilylethynyl)pentacene (VII) via a "cleavage-first" pathway, while 1e2 and 1h2 react only via "electron-transfer-first". 1h2 exhibits the most cathodic E(12•+/12) value of the dimers considered here and, therefore, reacts more rapidly than any of the other dimers with VII via "electron-transfer-first". Crystal structures show rather long central C-C bonds for 1b2 (1.5899(11) and 1.6194(8) Å) and 1h2 (1.6299(13) Å).

Reduction of 2-H-substituted pyrrolinium cations: the carbon-carbon single bond in air stable 2,2'-bipyrrolidines as a two-electron-source

Reduction of 2-H-substituted pyrrolinium cations via initially formed secondary radicals results in either dimerisation or H-abstracted products, while the outcome depends on the N-substituents. The resultant central carbon-carbon single bond in the dimerised 2,2'-bipyrrolidine derivatives can be oxidised chemically and electrochemically. The notably air and moisture-stable dimers were subsequently utilised as a source of two electrons in various chemical transformations.

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF₆ ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC₈ results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.

Organometallic and Organic Dimers: Moderately Air-Stable, Yet Highly Reducing, n-Dopants

ConspectusElectrical doping using redox-active molecules can increase the conductivity of organic semiconductors and lower charge-carrier injection and extraction barriers; it has application in devices such as organic and perovskite light-emitting diodes, organic and perovskite photovoltaic cells, field-effect transistors, and thermoelectric devices. Simple one-electron reductants that can act as n-dopants for a wide range of useful semiconductors must necessarily have low ionization energies and are, thus, highly sensitive toward ambient conditions, leading to challenges in their storage and handling. A number of approaches to this challenge have been developed, in which the highly reducing species is generated from a precursor or in which electron transfer is coupled in some way to a chemical reaction. Many of these approaches are relatively limited in applicability because of processing constraints, limited dopant strength, or the formation of side products.This Account discusses our work to develop relatively stable, yet highly reducing, n-dopants based on the dimers formed by some 19-electron organometallic complexes and by some organic radicals. These dimers are sufficiently inert that they can be briefly handled as solids in air but react with acceptors to release two electrons and to form two equivalents of stable monomeric cations, without formation of unwanted side products. We first discuss syntheses of such dimers, both previously reported and our own. We next turn to discuss their thermodynamic redox potentials, which depend on both the oxidation potential of the highly reducing odd-electron monomers and on the free energies of dissociation of the dimers; because trends in both these quantities depend on the monomer stability, they often more-or-less cancel, resulting in effective redox potentials for a number of the organometallic dimers that are approximately -2.0 V vs ferrocenium/ferrocene. However, variations in the dimer oxidation potential and the dissociation energies determine the mechanism through which a dimer reacts with a given acceptor in solution: in all cases dimer-to-acceptor electron transfer is followed by dimer cation cleavage and a subsequent second electron transfer from the neutral monomer to the acceptor, but examples with weak central bonds can also react through endergonic cleavage of the neutral dimer, followed by electron-transfer reactions between the resulting monomers and the acceptor. We, then, discuss the use of these dimers to dope a wide range of semiconductors through both vacuum and solution processing. In particular, we highlight the role of photoactivation in extending the reach of one of these dopants, enabling successful doping of a low-electron-affinity electron-transport material in an organic light-emitting diode. Finally, we suggest future directions for research using dimeric dopants.

Doping Approaches for Organic Semiconductors

Electronic doping in organic materials has remained an elusive concept for several decades. It drew considerable attention in the early days in the quest for organic materials with high electrical conductivity, paving the way for the pioneering work on pristine organic semiconductors (OSCs) and their eventual use in a plethora of applications. Despite this early trend, however, recent strides in the field of organic electronics have been made hand in hand with the development and use of dopants to the point that are now ubiquitous. Here, we give an overview of all important advances in the area of doping of organic semiconductors and their applications. We first review the relevant literature with particular focus on the physical processes involved, discussing established mechanisms but also newly proposed theories. We then continue with a comprehensive summary of the most widely studied dopants to date, placing particular emphasis on the chemical strategies toward the synthesis of molecules with improved functionality. The processing routes toward doped organic films and the important doping-processing-nanostructure relationships, are also discussed. We conclude the review by highlighting how doping can enhance the operating characteristics of various organic devices.

Synthesis, structures, and reactivity of isomers of [RuCp*(1,4-(Me2N)2C6H4)]2

[RuCp*(1,3,5-R₃C₆H₃)]₂ {Cp* = η⁵-pentamethylcyclopentadienyl, R = Me, Et} have previously been found to be moderately air stable, yet highly reducing, with estimated D⁺/0.5D₂ (where D₂ and D⁺ represent the dimer and the corresponding monomeric cation, respectively) redox potentials of ca. -2.0 V vs. FeCp₂^+/0. These properties have led to their use as n-dopants for organic semiconductors. Use of arenes substituted with π-electron donors is anticipated to lead to even more strongly reducing dimers. [RuCp*(1-(Me₂N)-3,5-Me₂C₆H₃)]⁺PF₆^- and [RuCp*(1,4-(Me₂N)₂C₆H₄)]⁺PF₆^- have been synthesized and electrochemically and crystallographically characterized; both exhibit D⁺/D potentials slightly more cathodic than [RuCp*(1,3,5-R₃C₆H₃)]⁺. Reduction of [RuCp*(1,4-(Me₂N)₂C₆H₄)]⁺PF₆^- using silica-supported sodium-potassium alloy leads to a mixture of isomers of [RuCp*(1,4-(Me₂N)₂C₆H₄)]₂, two of which have been crystallographically characterized. One of these isomers has a similar molecular structure to [RuCp*(1,3,5-Et₃C₆H₃)]₂; the central C-C bond is exo,exo, i.e., on the opposite face of both six-membered rings from the metals. A D⁺/0.5D₂ potential of -2.4 V is estimated for this exo,exo dimer, more reducing than that of [RuCp*(1,3,5-R₃C₆H₃)]₂ (-2.0 V). This isomer reacts much more rapidly with both air and electron acceptors than [RuCp*(1,3,5-R₃C₆H₃)]₂ due to a much more cathodic D₂˙⁺/D₂ potential. The other isomer to be crystallographically characterized, along with a third isomer, are both dimerized in an exo,endo fashion, representing the first examples of such dimers. Density functional theory calculations and reactivity studies indicate that the central bonds of these two isomers are weaker than those of the exo,exo isomer, or of [RuCp*(1,3,5-R₃C₆H₃)]₂, leading to estimated D⁺/0.5D₂ potentials of -2.5 and -2.6 V vs. FeCp₂^+/0. At the same time the D₂˙⁺/D₂ potentials for the exo,endo dimers are anodically shifted relative to those of [RuCp*(1,3,5-R₃C₆H₃)]₂, resulting in much greater air stability than for the exo,exo isomer.

A Zirconium Photosensitizer with a Long-Lived Excited State: Mechanistic Insight into Photoinduced Single-Electron Transfer

Time-resolved emission spectroscopy for the luminescent zirconium complex Zr(^MePDP)₂ (^MePDP = 2,6-bis(5-methyl-3-phenyl-1 H-pyrrol-2-yl)pyridine) revealed a long-lived excited state with a lifetime τ = 325 ± 10 μs. Computational studies using time-dependent density functional theory were conducted to identify the nature of the luminescent excited state as a mixed triplet intraligand/ligand-to-metal charge-transfer state. Stern-Volmer experiments showed a strong dependence of the quenching rate on the redox potential of the quencher indicating photoinduced single-electron transfer (SET) as the quenching pathway. Mechanistic investigations of the photocatalytic homocoupling of benzyl bromide allowed the detection of organic radical intermediates during turnover and provided further evidence for SET mediated by Zr(^MePDP)₂. Isolation of the one-electron-reduced form of the photosensitizer, [Zr(^MePDP)₂]^-, enabled studies of its electronic structure by a combination of experimental and computational techniques and confirmed its role as a strong reductant. Additionally, the role of the benzimidazolium hydride derivatives as two-electron sacrificial reductants during photoredox catalysis was investigated. In combination, the results presented in this report establish a detailed mechanistic picture of a photoredox catalytic reaction promoted by an earth-abundant early transition metal photosensitizer.

Elementary steps in electrical doping of organic semiconductors

Fermi level control by doping is established since decades in inorganic semiconductors and has been successfully introduced in organic semiconductors. Despite its commercial success in the multi-billion OLED display business, molecular doping is little understood, with its elementary steps controversially discussed and mostly-empirical-materials design. Particularly puzzling is the efficient carrier release, despite a presumably large Coulomb barrier. Here we quantitatively investigate doping as a two-step process, involving single-electron transfer from donor to acceptor molecules and subsequent dissociation of the ground-state integer-charge transfer complex (ICTC). We show that carrier release by ICTC dissociation has an activation energy of only a few tens of meV, despite a Coulomb binding of several 100 meV. We resolve this discrepancy by taking energetic disorder into account. The overall doping process is explained by an extended semiconductor model in which occupation of ICTCs causes the classically known reserve regime at device-relevant doping concentrations.

Molecular doping is routinely used in organic semiconductor devices nowadays, but the physics at play remains unclarified. Tietze et al. describe it as a two-step process and show it costs little, energetically, to dissociate charge transfer complexes due to energetic disorder of organic semiconductors.

Doped Organic Transistors

Organic field-effect transistors hold the promise of enabling low-cost and flexible electronics. Following its success in organic optoelectronics, the organic doping technology is also used increasingly in organic field-effect transistors. Doping not only increases device performance, but it also provides a way to fine-control the transistor behavior, to develop new transistor concepts, and even improve the stability of organic transistors. This Review summarizes the latest progress made in the understanding of the doping technology and its application to organic transistors. It presents the most successful doping models and an overview of the wide variety of materials used as dopants. Further, the influence of doping on charge transport in the most relevant polycrystalline organic semiconductors is reviewed, and a concise overview on the influence of doping on transistor behavior and performance is given. In particular, recent progress in the understanding of contact doping and channel doping is summarized.