Half-Wave Potential Splittings ΔE1/2 as a Measure of Electronic Coupling in Mixed-Valent Systems: Triumphs and Defeats

Palladium bis-pincer complexes with controlled rigidity and inter-metal distance

We report a series of redox-active bis(pincer) Pd(ii) complexes in which the redox active units are based on either a diarylamido or a carbazolide framework.

Carborane bridged ferrocenyl conjugated molecules: synthesis, structure, electrochemistry and photophysical properties

mono- and bis-carborane bridged ferrocenyl conjugated molecules 8–11 have been synthesized and systematically analyzed.

Controlling Through-Space and Through-Bond Exchange Pathways in Bis-Cobaltocenes for Molecular Spintronics

Pinching molecules via chemical strain suggests intuitive consequences, such as compression at the pinched site and clothespin‐like opening of other parts of the structure. If this opening affects two spin centers, it should result in reduced communication between them. We show that for naphthalene‐bridged biscobaltocenes with competing through‐space and through‐bond pathways, the consequences of pinching are far less intuitive: despite the known dominance of through‐space interactions, the bridge plays a much larger role for exchange spin coupling than previously assumed. Based on a combination of chemical synthesis, structural, magnetic, and redox characterization, and a newly developed theoretical pathway analysis, we can suggest a comprehensive explanation for this non‐intuitive behavior. These results are of interest for molecular spintronics, as naphthalene‐linked cobaltocenes can form wires on surfaces for potential spin‐only information transfer.

Bridge-Length-Dependent Intramolecular Charge Transfer in Bis(dianisylamino)-Terminated Oligo(p-phenylene)s

Radical cations of bis(dianisylamino)-terminated oligo(p-phenylene)s (OPPs) with up to five phenyl moieties were characterized by means of UV/Vis-NIR and variable-temperature ESR spectroscopy to investigate the bridge-length-dependence on intramolecular charge/spin self-exchange between two nitrogen redox-active centers. Additionally, a comparative study between bis(dianisylamine)-based mixed-valence (MV) radical cations connected by p-terphenylene and hexa-peri-hexabenzocoronene (HBC) π-bridging units also provided information on the influence of extended π-conjugation over the OPP-bridge due to the planarization between adjacent phenylene units on the strength of electronic coupling. The present study on a homologous series of organic MV systems clarifies the attenuation factor through the OPP-bridge and the linear relationship between the electrochemical potential splitting and the electronic coupling in the region of intermediate-to-weak electronic coupling regime.

Small anion-assisted electrochemical potential splitting in a new series of bistriarylamine derivatives: organic mixed valency across a urea bridge and zwitterionization

We report the synthesis of a new bistriarylamine series having a urea bridge and investigate its mixed-valence (MV) states by electrochemical and spectroelectrochemical methods. We found that the supporting electrolytes had unusual effects on potential splitting during electrochemical behavior, in which a smaller counteranion thermodynamically stabilized a MV cation more substantially than did a bulky one. The effects contrary to those reported in conventional MV systems were explained by zwitterionization through hydrogen bonding between the urea bridge and the counteranions, increasing the electronic interactions between two triarylamino units. Furthermore, we clarified the intervalence charge transfer characteristics of the zwitterionic MV state.

Anion and solvent dependency of the electronic coupling strength in mixed valent class II systems

The influence of the coordination and ion pairing properties of electrolyte anions on electronic coupling in cationic class II mixed valent species was studied. In order to cover a range of electronic coupling strengths within the class II regime, weakly coupled 2,5-diferrocenyl-3,4-thiadiazol, moderately coupled 2,5-diferrocenyl thiophene and strongly coupled N-(4-dimethylaminophenyl)-2,5-diferrocenyl-1H-pyrrole were chosen as analytes. The electrochemical properties of these compounds were determined by cyclic and square wave voltammetry using electrolytes with varying ion pairing capabilities, such as [NBu₄][Cl], [NBu₄][PF₆] and [NBu₄][BArF] ([NBu₄][B(C₆F₅)₄]), as well as solvents with increasing dielectric constants (dichloromethane (ε_r = 8.93), acetone (ε_r = 20.56), acetonitrile (ε_r = 35.94) and propylene carbonate (ε_r = 64.92)). It is shown that the choice of electrolyte has a considerable impact on the electrostatic and the electron transfer features of the mixed valent compounds when solvents of low polarity and low relative permittivity such as dichloromethane are used. For the use of more polar solvents such as propylene carbonate the electrochemical and spectroscopic properties are almost electrolyte independent. The solvatochromic and ion-related changes in the spectroscopic properties are most pronounced for weakly coupled systems and decrease with an increase in the electron transfer coupling strength.

A Hole Delocalization Strategy: Photoinduced Mixed-Valence MLCT States Featuring Extended Lifetimes

Bimetallic trans-[Ru^II(tpm)(bpy)(μNC)Ru^II(L)₄(CN)]²⁺, where bpy is 2,2'-bipyridine, tpm is tris(1-pyrazolyl)methane and L = 4-methoxypyridine (MeOpy) or pyridine (py), was examined using ultrafast vis-NIR transient absorption spectroscopy. Of great relevance are the longest-lived excited states in the form of strongly coupled photoinduced mixed-valence systems, which exhibit intense photoinduced absorptions in the NIR and are freely tunable by the judicious choice of the coordination spheres of the metallic ions. Using the latter strategy, we succeeded in tailoring the excited state lifetimes of bimetallic complexes and, in turn, achieving significantly longer values relative to related monometallic complexes. Notable is the success in extending the lifetimes, when considering the higher density of vibrational states, as they are expected to facilitate nonradiative ground-state recovery.

A Spectroscopic and Computationally Minimal Approach to the Analysis of Charge-Transfer Processes in Conformationally Fluxional Mixed-Valence and Heterobimetallic Complexes

Class II mixed-valence bimetallic complexes {[Cp'(PP)M]C≡C-C≡N[M'(PP)'Cp']}²⁺ (M, M'=Ru, Fe; PP=dppe, (PPh₃ )₂ ; Cp'=Cp*, Cp) exist as conformational ensembles in fluid solution, with a population of structures ranging from cis- to trans-like geometries. Each conformer gives rise to its own series of low-energy intervalence charge-transfer (IVCT) and local d-d transitions, which overlap in the NIR region, giving complex band envelopes in the NIR absorption spectrum, which prevent any meaningful attempt at analysis of the band shape. However, DFT and time-dependent (TD)DFT calculations with dispersion-corrected global-hybrid (BLYP35-D3) or local hybrid (lh-SsirPW92-D3) functionals on a small number of optimised structures chosen to sample the ground state potential energy hypersurfaces of each of these complexes has proven sufficient to explain the major features of the electronic spectra. Although modest in terms of computational expense, this approach provides a more accurate description of the underlying molecular electronic structure than would be possible through analysis of the IVCT band by using the static point-charge model of Marcus-Hush theory and derivatives, or TDDFT calculations from a single (global) minimum energy geometry.

Red-shifted delayed fluorescence at the expense of photoluminescence quantum efficiency - an intramolecular charge-transfer molecule based on a benzodithiophene-4,8-dione acceptor

Employing the thiophene based quinone, benzo[1,2-b:4,5-b']dithiophene-4,8-dione, as the electron-accepting moiety alongside N-phenylcarbazole donors to produce a donor-π-acceptor-π-donor (D-π-A-π-D) molecule has yielded a new red emitter displaying delayed fluorescence. This new molecule shows strongly (over 100 nm) red-shifted emission when compared to an anthraquinone based analogue. Cyclic voltammetry complemented by computational insights prove that this red-shift is due to the significantly stronger electron-accepting ability of the thiophene quinone compared to anthraquinone. Photophysical and computational studies of this molecule have revealed that while the presence of the thiophene containing acceptor facilitates rapid intersystem crossing which is comparable to anthraquinone analogues, the reverse intersystem crossing rate is slow and non-radiative decay is rapid which we can attribute to low-lying locally excited states. This limits the total photoluminescence quantum efficiency to less than 10% in both solution and the solid state. These results provide a useful example of how very minor structural variations can have a defining impact on the photophysical properties of new molecular materials.

High-Performance Electrofluorochromic Switching Devices Using a Novel Arylamine-Fluorene Redox-Active Fluorophore

Fluorescent light modulation by small electric potentials has gained huge interest in the past few years. This phenomenon, called electrofluorochromism, is of the utmost importance for applications in optoelectronic devices. Huge efforts are being addressed to developing electrofluorochromic systems with improved performances. One of the most critical issue is their low cyclability, which hampers their widespread use. It mostly depends on the intrinsic reversibility of the electroactive/fluorophore molecular system and on device architecture. Here we show a novel fluorene-based mixed-valence electrofluorochromic system that allows direct electrofluorochromic switching and exhibits incomparable electrochemical reversibility and device cyclability of more than 10 000 cycles.

Intramolecular Charge Transfer in Kekulé- and Non-Kekulé-Bridged Bis(triarylamine) Radical Cations: Missing Key Compounds in Organic Mixed-Valence Systems

From the viewpoint of para-meta topological bridging effect on the electronic coupling in organic mixed-valence (MV) systems, the optically induced and thermally assisted intramolecular charge/spin transfer (ICT/IST) processes have been investigated for three bis(triarylamine) (BTA) radical cations as missing key compounds in very basic BTA MV systems. In contrast to the case of p- and m-dinitrobenzene radical anions, the difference in the strength of electronic coupling (V) was not so large for the present BTA MV radical cations, although they still fall within the paradigm of strong V for para-linkage and weak V for meta-linkage. Unexpectedly, it has been found that meta-phenylenediamine radical cation has an electronic coupling comparable to those in the para-conjugated BTA-based MV species, and the ICT/IST rate exceeds the ESR time-scale. This finding is very encouraging considering that sufficient electronic communication can be ensured even when the redox-active centers are linked directly by the meta-phenylene bridge, thus broadening the selection of π-bridging units for molecule-based optoelectronics.

Inversion of donor–acceptor roles in photoinduced intervalence charge transfers

Upon MLCT photoexcitation, {(tpy)Ru} becomes the electron acceptor in the mixed valence {(tpy˙⁻)Ru^III−δ-NC-M^II+δ} moiety, reversing its role as the electron donor in the ground-state mixed valence analogue.

Diruthenium(ii)-capped oligothienylethynyl bridged highly soluble organometallic wires exhibiting long-range electronic coupling

Organometallic molecular wires with π-conjugation along their molecular backbones are of considerable interest for application in molecular-scale electronics. In this regard, thienylethynyl-based π-conjugated oligomers of three, five and seven thienylethynyl units with -C[triple bond, length as m-dash]C-H termini have been successfully synthesized through stepwise Pd(0)/Cu(i)-catalyzed Sonogashira coupling. The corresponding highly soluble diruthenium(ii) diacetylide complexes (O1-Ru2, O3-Ru2, O5-Ru2 and O7-Ru2, respectively) have been prepared by the reaction of cis-Ru(dppe)2Cl2 and NaPF6 in DCM with the corresponding rigid rod-like thienylethynyl oligomers with one, three, five and seven thienylethynyl π-conjugated segments containing alkynyl termini (O1, O3, O5 and O7). These Ru(ii)-Cl capped diacetylide complexes have been further functionalized by incorporating a phenylacetynyl moiety to afford [Ru(ii)-C[triple bond, length as m-dash]C-Ph]-capped diacetylide organometallic wires (O1-Ru2-Ph, O3-Ru2-Ph, O5-Ru2-Ph and O7-Ru2-Ph). The photophysical properties of the highly soluble thienylethynyl-based oligomers and Ru(ii)-organometallic wires have been explored to understand their electronic properties. Electrochemical studies of the binuclear ruthenium(ii)-alkynyl complexes showed highly interesting results, revealing long-range electrochemical communication between the two remote Ru(ii) termini connected even with five and seven thienylethynyl units. DFT computational studies further support the long range electrochemical communication between the redox active metal termini through heavy participation of the thienylethynyl bridge in the corresponding mono-oxidized mixed valence species of the organometallic wire-like complexes.

Mixed-Valent Molecular Triple Deckers

Two phenothiazine (PTZ) moieties were connected via naphthalene spacers to a central arene to result in stacked PTZ-arene-PTZ structure elements. Benzene and tetramethoxybenzene units served as central arenes mediating electronic communication between the two PTZ units. Based on cyclic voltammetry, UV/Vis-NIR absorption, EPR spectroscopy, and computational studies, the one-electron oxidized forms of the resulting compounds behave as class II organic mixed-valence species in which the unpaired electron is partially delocalized over both PTZ units. The barrier for intramolecular electron transfer depends on the nature of the central arene sandwiched between the two PTZ moieties. These are the first examples of rigid organic mixed-valent triple-decker compounds with possible electron-transfer pathways directly across a stacked structure, and they illustrate the potential of oligo-naphthalene building blocks for long-range electron transfer and a future molecular electronics technology.

Tetraruthenium Metallamacrocycles with Potentially Coordinating Appended Functionalities

We present four new tetraruthenium macrocycles built from two 1,4-divinylphenylene diruthenium and two isophthalic acid building blocks with peripheral, potentially mono- or tridentate donor functions attached to the isophthalic linkers. These macrocycles are characterized by multinuclear NMR spectroscopy, mass spectrometry and, in the case of the thioacetyl-appended complex 4, by X-ray crystallography. Cyclic and square wave voltammetry establish that the macrocycles can be oxidized in four consecutive redox steps that come as two pairs of two closely spaced one-electron waves. Spectroscopic changes observed during IR and UV/Vis/NIR spectroelectrochemical experiments (NIR = near infrared) show that the isophthalate linkers insulate the electroactive divinylphenylene diruthenium moieties against each other. The macrocycles exhibit nevertheless pronounced polyelectrochromism with highly intense absorptions in the Vis (2+/4+ states) and the NIR (2+ states) with extinction coefficients of up to >100,000 M−1·cm−1. The strong absorptivity enhancement with respect to the individual divinylphenylene diruthenium building blocks is attributed to conformational restrictions imposed by the macrocycle backbone. Moreover, the di- and tetracations of these macrocycles are paramagnetic as revealed by EPR spectroscopy.

Macrocyclic Triruthenium Complexes Having Electronically Coupled Mixed-Valent States

5-Ethynyl-2-furancarboxylic acid and 3-ethynylbenzoic acid self-assemble with [HRu(CO)Cl(PiPr₃ )₂ ] to form macrocyclic C₃ -symmetric triangular triruthenium alkenyl complexes [{Ru(CO)(PiPr₃ )₂ (CH=CHArCOO)}₃ ] (Ar=C₆ H₄ : 1-B, Ar=C₄ H₂ O: 1-F), which were characterized by multinuclear NMR spectroscopy, high-resolution ESI mass spectrometry, and, in the case of 1-B, by X-ray crystallography. Electrochemical studies indicate that the macrocycles are oxidized in three consecutive one-electron steps. The mixed-valent states obtained by electrochemical or chemical oxidation show signs of valence delocalization, which makes these complexes rare examples of molecule-based conductive loops with through-bond charge delocalization.

Synthesis, electronic structure and redox properties of the diruthenium sandwich complexes [Cp*Ru(μ-C10H8)RuCp*]x (x = 0, 1+; Cp* = C5Me5; C10H8 = naphthalene)

Strong electronic coupling was observed between the metal centers in a naphthalene-bridged diruthenium complex.

Oxidatively stable ferrocenyl-π-bridge-titanocene D–π-A complexes: an electrochemical and spectroscopic investigation of the mixed-valent states

Coordination of Cu^I into ^RCp₂Ti(C₂Fc)₂ compounds results in well-separated Fe^III/II couples and mixed-valent states with distinct spectroscopic signatures.

Mixed valency in ligand-bridged diruthenium frameworks: divergences and perspectives

Emerging fundamental issues involving intramolecular electron transfer at the mixed valent diruthenium frameworks and its application prospects have been highlighted.

Formation of unexpected silicon- and disiloxane-bridged multiferrocenyl derivatives bearing Si-O-CH[double bond, length as m-dash]CH2 and Si-(CH2)2C(CH3)3 substituents via cleavage of tetrahydrofuran and trapping of its ring fragments

The formation of a family of silicon- and siloxane-bridged multiferrocenyl derivatives carrying different functional groups attached to silicon, including Fc₂(CH₃)₃C(CH₂)₂SiCH[double bond, length as m-dash]CH₂ (5), Fc₂(CH₂[double bond, length as m-dash]CH-O)SiCH[double bond, length as m-dash]CH₂ (6), Fc₂(OH)SiCH[double bond, length as m-dash]CH₂ (7), Fc₂(CH₂[double bond, length as m-dash]CH-O)Si-O-Si(O-CH[double bond, length as m-dash]CH₂)Fc₂ (8) and Fc₂(CH₂[double bond, length as m-dash]CH-O)Si-O-SiFc₃ (9) is described. Silyl vinyl ether molecules 6, 8 and 9 and the heteroleptic vinylsilane 5 resulted from the competing metathesis reaction of lithioferrocene (FcLi), CH₂[double bond, length as m-dash]CH-OLi or (CH₃)₃C(CH₂)₂Li with the corresponding multifunctional chlorosilane, Cl₃SiCH[double bond, length as m-dash]CH₂ or Cl₃Si-O-SiCl₃. The last two organolithium species have been likely formed in situ by fragmentation of the tetrahydrofuran solvent. Diferrocenylvinyloxyvinylsilane 6 is noteworthy since it represents a rare example of a redox-active silyl mononomer in which two different C[double bond, length as m-dash]C polymerisable groups are directly connected to silicon. The molecular structures of the silicon-containing multiferrocenyl species 5, 6, 8 and 9 have been investigated by single-crystal X-ray diffraction studies, demonstrating the capture and storage processes of two ring fragments resulting from the cleavage of cyclic THF in redox-active and stable crystalline organometallic compounds. From electrochemical studies we found that by changing the anion of the supporting electrolyte from [PF₆]^- to [B(C₆F₅)₄]^-, the redox behaviour of tetrametallic disiloxane 8 can be switched from a poorly resolved multistep redox process to four consecutive well-separated one-electron oxidations, corresponding to the sequential oxidation of the four ferrocenyl moieties.

Pyrene Molecular Orbital Shuffle-Controlling Excited State and Redox Properties by Changing the Nature of the Frontier Orbitals

We show that by judicious choice of substituents at the 2- and 7-positions of pyrene, the frontier orbital order of pyrene can be modified, giving enhanced control over the nature and properties of the photoexcited states and the redox potentials. Specifically, we introduced a julolidine-like moiety and Bmes₂ (mes=2,4,6-Me₃ C₆ H₂ ) as very strong donor (D) and acceptor (A), respectively, giving 2,7-D-π-D- and unsymmetric 2,7-D-π-A-pyrene derivatives, in which the donor destabilizes the HOMO-1 and the acceptor stabilizes the LUMO+1 of the pyrene core. Consequently, for 2,7-substituted pyrene derivatives, unusual properties are obtained. For example, very large bathochromic shifts were observed for all of our compounds, and unprecedented green light emission occurs for the D/D system. In addition, very high radiative rate constants in solution and in the solid state were recorded for the D-π-D- and D-π-A-substituted compounds. All compounds show reversible one-electron oxidations, and Jul₂ Pyr exhibits a second oxidation, with the largest potential splitting (ΔE=440 mV) thus far reported for 2,7-substituted pyrenes. Spectroelectrochemical measurements confirm an unexpectedly strong coupling between the 2,7-substituents in our pyrene derivatives.

Rational Design of Molecular Hole-Transporting Materials for Perovskite Solar Cells: Direct versus Inverted Device Configurations

Due to a still limited understanding of the reasons making 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) the state-of-the-art hole-transporting material (HTM) for emerging photovoltaic applications, the molecular tailoring of organic components for perovskite solar cells (PSCs) lacks in solid design criteria. Charge delocalization in radical cationic states can undoubtedly be considered as one of the essential prerequisites for an HTM, but this aspect has been investigated to a relatively minor extent. In marked contrast with the 3-D structure of Spiro-OMeTAD, truxene-based HTMs Trux1 and Trux2 have been employed for the first time in PSCs fabricated with a direct (n-i-p) or inverted (p-i-n) architecture, exhibiting a peculiar behavior with respect to the referential HTM. Notwithstanding the efficient hole extraction from the perovskite layer exhibited by Trux1 and Trux2 in direct configuration devices, their photovoltaic performances were detrimentally affected by their poor hole transport. Conversely, an outstanding improvement of the photovoltaic performances in dopant-free inverted configuration devices compared to Spiro-OMeTAD was recorded, ascribable to the use of thinner HTM layers. The rationalization of the photovoltaic performances exhibited by different configuration devices discussed in this paper can provide new and unexpected prospects for engineering the interface between the active layer of perovskite-based solar cells and the hole transporters.