Double Spin with a Twist: Synthesis and Characterization of a Neutral Mixed-Valence Organic Stable Diradical

A convenient design strategy opens access to neutral open-shell mixed-valence species via the redox transformation of charged stable precursors, i.e., the spiro-fused borate anions. We have implemented this strategy for the synthesis of the first neutral mixed-valence diradical: two neutral mixed-valence radical fragments were assembled via a twisted biphenyl bridge. The diradical is a crystalline solid obtained in almost quantitative yield by using a facile synthetic procedure. It is stable at room temperature in the triplet ground state with a very small singlet/triplet gap. This metal-free diradical can reversibly form five redox states. The diradical exhibits an intense IVCT band in the NIR region and can be assigned as a Class 2 Robin-Day MV (mixed valence) system with weakly interacting redox centers. Computations suggest that this diradical finds itself in a unique tug-of-war between two electron delocalization patterns, Kekulé and non-Kekulé, which gives rise to two geometric isomers that are close in energy but drastically different in spin distribution and polarity. Such bistable spin-systems should be intrinsically switchable and promising for the design of functional spin devices. The scope and limitations of the new redox-strategy for the neutral MV radicals were also tested on other types of spiro-fused borates, revealing structural factors responsible for the evolution from transient to persistent and then to stable radicals.

Mixed-ligand, radical, gold bis(dithiolene) complexes: from single-component conductors to controllable NIR-II absorbers

Neutral radical bis(dithiolene) gold complexes [Au(dt)2]˙ are known to exhibit a strong absorption in the 1400-2000 nm NIR absorption range. Here, we demonstrate that the NIR signature of mixed-ligand bis(dithiolene) gold complexes [Au(dtA)(dtD)]˙ associating two different dithiolene, dtA and dtD, is found at higher energy, out of the range of the homoleptic analogs [Au(dtA)2]˙ and [Au(dtD)2]˙, in the looked-after NIR-II 1000-1400 nm absorption range. An efficient synthetic approach towards precursor mixed-ligand monoanionic gold bis(dithiolene) complexes [Au(dtA)(dtD)]-1 is reported. Using this strategy, no symmetrical complexes are formed and, upon electrocrystallization, no scrambling was observed in solution, allowing for the isolation of radical gold bis(dithiolene) complex such as [Au(bdt)(Et-thiazdt)]˙ (bdt: benzene-1,2-dithiolate; Et-thiazdt: N-ethyl-thiazoline-2-thione-3,4-dithiolate), which behaves as a single-component conductor. It is shown from theoretical calculations that the spin polarization induced by electron repulsions leads to a strong localization of the spin-orbitals, and provides a sound basis to understand, (i) the different ligand-based oxidation potentials, (ii) the NIR optical absorption at notably higher energies and (iii) the larger potential difference of the two redox processes than in the parent symmetric complexes. The solid-state properties of the radical complex [Au(bdt)(Et-thiazdt)]˙ are the consequence of a strongly 1D electronic structure with weakly dimerized chains and electronic localization favoring a semiconducting behavior, stable under pressures up to 18.2 GPa. Altogether, the versatility of the preparation method of [Au(dtA)(dtD)]-1 salts opens the route for a wide library of different mixed-ligand radical complexes [Au(dtA)(dtD)]˙ with simultaneously an adaptable absorption in the NIR-II range and the rich structural chemistry of single-component conductors.

Luminescent Radicals

Organic radicals are attracting increasing interest as a new class of molecular emitters. They demonstrate electronic excitation and relaxation dynamics based on their doublet or higher multiplet spin states, which are different from those based on singlet-triplet manifolds of conventional closed-shell molecules. Recent studies have disclosed luminescence properties and excited state dynamics unique to radicals, such as highly efficient electron-photon conversion in OLEDs, NIR emission, magnetoluminescence, an absence of heavy atom effect, and spin-dependent and spin-selective dynamics. These are difficult or sometimes impossible to achieve with closed-shell luminophores. This review focuses on luminescent organic radicals as an emerging photofunctional molecular system, and introduces the material developments, fundamental properties including luminescence, and photofunctions. Materials covered in this review range from monoradicals, radical oligomers, and radical polymers to metal complexes with radical ligands demonstrating radical-involved emission. In addition to stable radicals, transiently formed radicals generated in situ by external stimuli are introduced. This review shows that luminescent organic radicals have great potential to expand the chemical and spin spaces of luminescent molecular materials and thus broaden their applicability to photofunctional systems.

Emergent Spin Frustration in Neutral Mixed-Valence 2D Conjugated Polymers: A Potential Quantum Materials Platform

Two-dimensional conjugated polymers (2DCPs)─organic 2D materials composed of arrays of carbon sp² centers connected by π-conjugated linkers─are attracting increasing attention due to their potential applications in device technologies. This interest stems from the ability of 2DCPs to host a range of correlated electronic and magnetic states (e.g., Mott insulators). Substitution of all carbon sp² centers in 2DCPs by nitrogen or boron results in diamagnetic insulating states. Partial substitution of C sp² centers by B or N atoms has not yet been considered for extended 2DCPs but has been extensively studied in the analogous neutral mixed-valence molecular systems. Here, we employ accurate first-principles calculations to predict the electronic and magnetic properties of a new class of hexagonally connected neutral mixed-valence 2DCPs in which every other C sp² nodal center is substituted by either a N or B atom. We show that these neutral mixed-valence 2DCPs significantly energetically favor a state with emergent superexchange-mediated antiferromagnetic (AFM) interactions between C-based spin-¹/₂ centers on a triangular sublattice. These AFM interactions are surprisingly strong and comparable to those in the parent compounds of cuprate superconductors. The rigid and covalently linked symmetric triangular AFM lattice in these materials thus provides a highly promising and robust basis for 2D spin frustration. As such, extended mixed-valence 2DCPs are a highly attractive platform for the future bottom-up realization of a new class of all-organic quantum materials, which could host exotic correlated electronic states (e.g., unusual magnetic ordering, quantum spin liquids).

Exploring the potential as molecular quantum-dot cellular automata of a mixed-valence Ru2 complex deposited on a Au(111) surface

A Ru2+ complex deposited on a Au(111) surface in the presence of a counterion presents excess charge localized on one side of the molecule. The switching can be promoted by an applied electric field, E, stronger than the critical field strength Ec.

Correspondence on "How Aromatic Are Molecular Nanorings? The Case of a Six-Porphyrin Nanoring"

A recent Research Article in this journal by Matito and co-workers claimed that none of the oxidation states of a butadiyne-linked six-porphyrin nanoring exhibit global aromaticity or antiaromaticity. Here we show that this conclusion is incorrect. Experimental data from NMR spectroscopy for a whole family of nanorings provide strong evidence for global ring currents. The NMR data reveal these ring currents directly, without needing analysis by density functional theory (DFT). Furthermore, DFT calculations reproduce the experimental results when a suitable functional is used.

Electron-Rich Diruthenium Complexes with π-Extended Alkenyl Ligands and Their F4 TCNQ Charge-Transfer Salts

The synthesis of dinuclear ruthenium alkenyl complexes with {Ru(CO)(Pi Pr3 )2 (L)} entities (L=Cl- in complexes Ru2 -3 and Ru2 -7; L=acetylacetonate (acac- ) in complexes Ru2 -4 and Ru2 -8) and with π-conjugated 2,7-divinylphenanthrenediyl (Ru2 -3, Ru2 -4) or 5,8-divinylquinoxalinediyl (Ru2 -7, Ru2 -8) as bridging ligands are reported. The bridging ligands are laterally π-extended by anellating a pyrene (Ru2 -7, Ru2 -8) or a 6,7-benzoquinoxaline (Ru2 -3, Ru2 -4) π-perimeter. This was done with the hope that the open π-faces of the electron-rich complexes will foster association with planar electron acceptors via π-stacking. The dinuclear complexes were subjected to cyclic and square-wave voltammetry and were characterized in all accessible redox states by IR, UV/Vis/NIR and, where applicable, by EPR spectroscopy. These studies signified the one-electron oxidized forms of divinylphenylene-bridged complexes Ru2 -7, Ru2 -8 as intrinsically delocalized mixed-valent species, and those of complexes Ru2 -3 and Ru2 -4 with the longer divinylphenanthrenediyl linker as partially localized on the IR, yet delocalized on the EPR timescale. The more electron-rich acac- congeners formed non-conductive 1 : 1 charge-transfer (CT) salts on treatment with the F4 TCNQ electron acceptor. All spectroscopic techniques confirmed the presence of pairs of complex radical cations and F4 TCNQ.- radical anions in these CT salts, but produced no firm evidence for the relevance of π-stacking to their formation and properties.

Femtosecond X-ray Spectroscopy Directly Quantifies Transient Excited-State Mixed Valency

Quantifying charge delocalization associated with short-lived photoexcited states of molecular complexes in solution remains experimentally challenging, requiring local element specific femtosecond experimental probes of time-evolving electron transfer. In this study, we quantify the evolving valence hole charge distribution in the photoexcited charge transfer state of a prototypical mixed valence bimetallic iron-ruthenium complex, [(CN)₅Fe^IICNRu^III(NH₃)₅]^-, in water by combining femtosecond X-ray spectroscopy measurements with time-dependent density functional theory calculations of the excited-state dynamics. We estimate the valence hole charge that accumulated at the Fe atom to be 0.6 ± 0.2, resulting from excited-state metal-to-metal charge transfer, on an ∼60 fs time scale. Our combined experimental and computational approach provides a spectroscopic ruler for quantifying excited-state valency in solvated complexes.

A "Pretender" Croconate-Bridged Macrocyclic Tetraruthenium Complex: Sizable Redox Potential Splittings despite Electronically Insulated Divinylphenylene Diruthenium Entities

Careful optimization of the reaction conditions provided access to the particularly small tetraruthenium macrocycle ²Ru₂Ph-Croc, which is composed out of two redox-active divinylphenylene-bridged diruthenium entities {Ru}-1,4-CH=CH-C₆H₄-CH=CH-{Ru} (Ru₂Ph; {Ru} = Ru(CO)Cl(PⁱPr₃)₂) and two likewise redox-active and potentially non-innocent croconate linkers. According to single X-ray diffraction analysis, the central cavity of ²Ru₂Ph-Croc is shielded by the bulky PⁱPr₃ ligands, which come into close contact. Cyclic voltammetry revealed two pairs of split anodic waves in the weakly ion pairing CH₂Cl₂/NBu₄BAr^F₂₄ (BAr^F₂₄ = [B{C₆H₃(CF₃)₂-3,5}₄]⁻ electrolyte, while the third and fourth waves fall together in CH₂Cl₂/NBu₄PF₆. The various oxidized forms were electrogenerated and scrutinized by IR and UV/Vis/NIR spectroscopy. This allowed us to assign the individual oxidations to the metal-organic Ru₂Ph entities within ²Ru₂Ph-Croc, while the croconate ligands remain largely uninvolved. The lack of specific NIR bands that could be assigned to intervalence charge transfer (IVCT) in the mono- and trications indicates that these mixed-valent species are strictly charge-localized. ²Ru₂Ph-Croc is hence an exemplary case, where stepwise IR band shifts and quite sizable redox splittings between consecutive one-electron oxidations would, on first sight, point to electronic coupling, but are exclusively due to electrostatic and inductive effects. This makes ²Ru₂Ph-Croc a true “pretender”.

Optically Induced Charge Transfer in Organic Mixed-Valence Systems: Wave Packet Dynamics and Femtosecond Transient Spectroscopy

We theoretically study the dynamics of charge transfer induced by femtosecond laser-pulse excitation. Models involving coupled electronic states of symmetrically bridged organic mixed-valence molecules are investigated, where the motion proceeds along two reaction coordinates. Linear absorption spectra of two species that differ in the energetical position of the bridge, relative to acceptor and donor states, are determined and compared to experimental results. From the wave packet dynamics it emerges that relaxation dominates the charge transfer. This behavior is reflected in transient absorption spectra, which are obtained from a directional decomposition of the time-dependent polarization. Due to the nature of the coupled dynamics the extraction of the relevant contributions needs an extension of well-known techniques for the decomposition.

Pre-Planarized Triphenylamine-Based Linear Mixed-Valence Charge-Transfer Systems

Three linear dimers with two redox-active planarized triphenylamines were synthesized and their structures verified by X-ray crystallography. Their radical cations, which exhibit electron self-exchange between the two redox centers, are of great interest. This process was thoroughly investigated by means of electron paramagnetic resonance spectroscopy, absorption spectroscopy, and (time-dependent) density functional theory calculations. A comparison of the key parameters of electron transfer with non-planarized nitrogen-centered building blocks emphasizes the impact of using redox centers with low internal reorganization energies. However, the distance-dependence attenuation factor of the super-exchange mechanisms remains similar.

Localized and Delocalized States of a Diamine Cation: Resolution of a Controversy

Recent Rydberg spectroscopy measurements of a diamine molecule, N,N'-dimethylpiperazine (DMP), indicate the existence of a localized electronic state as well as a delocalized electronic state. This implies that the cation, DMP⁺, can similarly have its positive charge either localized on one of the N atoms or delocalized over both. This interpretation of the experiments has, however, been questioned based on coupled cluster calculations. In this article, results of high-level multireference configuration interaction calculations are presented where a localized state of DMP⁺ is indeed found to be present with an energy barrier separating it from the delocalized state. The energy difference between the two states is in excellent agreement with the experimental estimate. The results presented here, therefore, support the original interpretation of the experiments and illustrate a rare shortcoming of CCSD(T), the "gold standard" of quantum chemistry. These results have implications for the development of density functionals, as most functionals fail to produce the localized state.

Electrochemical deposition of a semiconducting gold dithiolene complex with NIR absorption

Here, the synthesis of a new anionic gold dithiolene complex, NBu₄·[1-i], and that of its corresponding neutral gold complex 2 is reported. Complex 2 shows strong absorption into the IR, semiconductivity (σ_RT = 3.06 × 10^-7 S cm^-1) with an activation energy of 0.25 eV, and weakly temperature dependent paramagnetic susceptibility, indicating strong intermolecular interactions in the solid state. As a consequence of their strong intermolecular interactions, neutral gold dithiolene complexes are often highly insoluble, limiting their utility and processability. Electrochemical deposition is used to deposit conductive films of complex 2, which retain the NIR properties present in the bulk material, indicting that the strong intermolecular interactions are retained in the film.

Toward a First-Principles Evaluation of Transport Mechanisms in Molecular Wires

Understanding charge transport through molecular wires is important for nanoscale electronics and biochemistry. Our goal is to establish a simple first-principles protocol for predicting the charge transport mechanism in such wires, in particular the crossover from coherent tunneling for short wires to incoherent hopping for longer wires. This protocol is based on a combination of density functional theory with a polarizable continuum model introduced by Kaupp et al. for mixed-valence molecules, which we had previously found to work well for length-dependent charge delocalization in such systems. We combine this protocol with a new charge delocalization measure tailored for molecular wires, and we show that it can predict the tunneling-to-hopping transition length with a maximum error of one subunit in five sets of molecular wires studied experimentally in molecular junctions at room temperature. This suggests that the protocol is also well suited for estimating the extent of hopping sites as relevant, for example, for the intermediate tunneling-hopping regime in DNA.

Heterobimetallic complexes of IrM (M = FeII, CoII, and NiII) core and bridging 2-(diphenylphosphino)pyridine: electronic structure and electrochemical behavior

Three complexes based on an Ir-M (M = Fe^II, Co^II, and Ni^II) heterobimetallic core and 2-(diphenylphosphino)pyridine (Ph₂PPy) ligand were synthesized via the reaction of trans-[IrCl(CO)(Ph₂PPy)₂] and the corresponding metal chloride. Their structures were established by single-crystal X-ray diffraction as [Ir(CO)(μ-Cl)(μ-Ph₂PPy)₂FeCl₂]·2CH₂Cl₂ (2), [IrCl(CO)(μ-Ph₂PPy)₂CoCl₂]·2CH₂Cl₂ (3), and [Ir(CO)(μ-Cl)(μ-Ph₂PPy)₂NiCl₂]·2CH₂Cl₂ (4). Time-dependent DFT computations suggest a donor-acceptor interaction between a filled 5d_z² orbital on iridium and an empty orbital on the first-row metal atom, which is supported by UV-vis studies. Magnetic moment measurements show that the first-row metals are in their high-spin electronic configurations. Cyclic voltammetry data show that all the complexes undergo irreversible decomposition upon either reduction or oxidation. Reduction of 4 proceeds through an ECE mechanism. While these complexes are not stable to electrocatalysis conditions, the data presented here refine our understanding of the bonding synergies of the first-row and third-row metals.

Synthesis, Photophysical and Electronic Properties of New Red-to-NIR Emitting Donor-Acceptor Pyrene Derivatives

We synthesized new pyrene derivatives with strong bis(para-methoxyphenyl)amine donors at the 2,7-positions and n-azaacene acceptors at the K-region of pyrene. The compounds possess a strong intramolecular charge transfer, leading to unusual properties such as emission in the red to NIR region (700 nm), which has not been reported before for monomeric pyrenes. Detailed photophysical studies reveal very long intrinsic lifetimes of >100 ns for the new compounds, which is typical for 2,7-substituted pyrenes but not for K-region substituted pyrenes. The incorporation of strong donors and acceptors leads to very low reduction and oxidation potentials, and spectroelectrochemical studies show that the compounds are on the borderline between localized Robin-Day class-II and delocalized Robin-Day class-III species.

Luminescent Mono-, Di-, and Triradicals: Bridging Polychlorinated Triarylmethyl Radicals by Triarylamines and Triarylboranes

Up to three polychlorinated pyridyldiphenylmethyl radicals bridged by a triphenylamine carrying electron withdrawing (CN), neutral (Me), or donating (OMe) groups were synthesized and analogous radicals bridged by tris(2,6-dimethylphenyl)borane were prepared for comparison. All compounds were as stable as common closed-shell organic compounds and showed significant fluorescence upon excitation. Electronic, magnetic, absorption, and emission properties were examined in detail, and experimental results were interpreted using DFT calculations. Oxidation potentials, absorption and emission energies could be tuned depending on the electron density of the bridges. The triphenylamine bridges mediated intramolecular weak antiferromagnetic interactions between the radical spins, and the energy difference between the high spin and low spin states was determined by temperature dependent ESR spectroscopy and DFT calculations. The fluorescent properties of all radicals were examined in detail and revealed no difference for high and low spin states which facilitates application of these dyes in two-photon absorption spectroscopy and OLED devices.

Zwitterionic Mixed Valence: Internalizing Counteranions into a Biferrocenium Framework toward Molecular Expression of Half-Cells in Quantum Cellular Automata

Realization of molecular quantum cellular automata (QCA), a promising architecture for molecular computing through current-free processes, requires improved understanding and application of mixed-valence (MV) molecules. In this report, we present an electrostatic approach to creating MV subspecies through internalizing opposite charges in close proximity to MV ionic moieties. This approach is demonstrated by unsymmetrically attaching a charge-responsive boron substituent to a well-known organometallic MV complex, biferrocenium. Guest anions (CN^- and F^- ) bind to the Lewis acidic boron center, leading to unusual blue-shifts of the intervalence charge-transfer (IVCT) bands. To the best of our knowledge, this is the first reported example of a zwitterionic MV series in which the degree of positive charge delocalization can be varied by changing the bound anions, and serves to clarify the interplay between IVCT parameters. The key underlying factor is the variable zero-level energy difference in the MV states. This work provides new insight into imbuing MV molecules with external charge-responsiveness, a prerequisite of molecular QCA techniques.

Synthesis, photophysical and electronic properties of tetra-donor- or acceptor-substituted ortho-perylenes displaying four reversible oxidations or reductions

Via regioselective Ir-catalyzed C-H borylation and subsequent reactions (i.e., via Br4-Per or (BF3K)4-Per intermediates), we have introduced strong π-donors and acceptors at the 2,5,8,11-positions of perylene leading to unusual properties. Thus, incorporation of four donor diphenylamine (DPA) or four acceptor Bmes2 (mes = 2,4,6-Me3C6H2) moieties yields novel compounds which can be reversibly oxidized or reduced four times, respectively, an unprecedented behavior for monomeric perylene derivatives. Spectroelectrochemical measurements show NIR absorptions up to 3000 nm for the mono-cation radical of (DPA)4-Per and a strong electronic coupling over the perylene bridge was observed indicative of fully delocalized Robin-Day Class III behavior. Both (DPA)4-Per and (Bmes2)4-Per derivatives possess unusually long intrinsic singlet lifetimes (τ 0), e.g., 94 ns for the former one. The compounds are emissive in solution, thin films, and the solid state, with apparent Stokes shifts that are exceptionally large for perylene derivatives. Transient absorption measurements on (DPA)4-Per reveal an additional excited state, with a long lifetime of 500 μs, which sensitizes singlet oxygen effectively.

Charge-Separated Mixed Valency in an Unsymmetrical Acceptor-Donor-Donor Triad Based on Diarylboryl and Triarylamine Units

In this study, we report the generation of new mixed-valence (MV) subspecies with charge-separated (CS) characters from an unsymmetrical acceptor-donor-donor (A-D-D) triad. The triad was synthesized by attaching a dimesitylboryl group (A) to a D-D conjugate that consisted of triarylamine (NAr₃) units. The MV radical cation, obtained by chemical oxidation of the triad, exhibited a strong intervalence charge transfer (IVCT) absorption derived from the bis(NAr₃)^•+ moiety in the near-IR region. The charge-separated MV (CSMV) state, obtained by photoexcitation of the triad, caused a blue shift in IVCT energy in the femtosecond transient absorption spectra, reflecting a bias of positive charge distributions to the D end site. This resulted from increased electron density at the A site and restructuring of the central D site from NAr₃ to NAr₂ sites. Interestingly, any shift in the IVCT energy that was caused by the polarity of the solvent was minimal, reflecting the unique characteristics of the CSMV state. These findings represent the first detailed analysis of the CSMV state, including a comparison with conventional MV states. Therefore, this work provides new insights into counterion-free MV systems and their applications in molecular devices.

Optically Induced Electron Transfer in Mixed-Valence States: A Model Study on Electronic Transitions, Relaxation Dynamics, and Transient Absorption Spectroscopy

Quantum dynamical model calculations are performed on the optically induced electron transfer in a mixed-valence system interacting with different solvents. The simultaneously occurring processes of population transfer between electronic states and relaxation are studied in detail. Transient absorption traces, as recently recorded in our laboratory, are simulated, and the features of the spectra are related to the dynamics. The agreement with the experiment hints at the fact that the employed one-dimensional models catch the essentials of the photochemistry of the investigated systems and that they can be used for the interpretation of the transient absorption spectra. It is inferred that the ultrafast electron transfer processes take place on a sub-picosecond time scale and afterward relaxation occurs within several picoseconds.

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.

Formation of the Charge-Localized Dimer Radical Cation of 2-Ethyl-9,10-dimethoxyanthracene in Solution Phase

Although dimer radical ions of aromatic molecules in the liquid-solution phase have been intensely studied, the understanding of charge-localized dimers, in which the extra charge is localized in a single monomer unit instead of being shared between two monomer units, is still elusive. In this study, the formation of a charge-localized dimer radical cation of 2-ethyl-9,10-dimethoxyanthracene (DMA), (DMA)₂ ^.+ is investigated by transient absorption (TA) and time-resolved resonance Raman (TR³ ) spectroscopic methods combined with a pulse radiolysis technique. Visible- and near-IR TA signals in highly concentrated DMA solutions supported the formation of non-covalent (DMA)₂ ^.+ by association of DMA and DMA^.+ . TR³ spectra obtained from 30 ns to 300 μs time delays showed that the major bands are quite similar to those of DMA except for small transient bands, even at 30 ns time delay, suggesting that the positive charge of non-covalent (DMA)₂ ^.+ is localized in a single monomer unit. From DFT calculations for (DMA)₂ ^.+ , our TR³ spectra showed the best agreement with the calculated Raman spectrum of charge-localized edge-to-face T-shaped (DMA)₂ ^.+ , termed DT^.+ , although the charge-delocalized asymmetric π-stacked face-to-face (DMA)₂ ^.+ , termed DF3^.+ , is the most stable structure of (DMA)₂ ^.+ according to the energetics from DFT calculations. The calculated potential energy curves for the association between DMA^.+ and DMA showed that DT^.+ is likely to be efficiently formed and contribute significantly to the TR³ spectra as a result of the permanent charge-induced Coulombic interactions and a dynamic equilibrium between charge localized and delocalized structures.

Game of Frontier Orbitals: A View on the Rational Design of Novel Charge-Transfer Materials

Since the first application of frontier molecular orbitals (FMOs) to rationalize stereospecificity of pericyclic reactions, FMOs have remained at the forefront of chemical theory. Yet, the practical application of FMOs in the rational design and synthesis of novel charge transfer materials remains under-appreciated. In this Perspective, we demonstrate that molecular orbital theory is a powerful and universal tool capable of rationalizing the observed redox/optoelectronic properties of various aromatic hydrocarbons in the context of their application as charge-transfer materials. Importantly, the inspection of FMOs can provide instantaneous insight into the interchromophoric electronic coupling and polaron delocalization in polychromophoric assemblies, and therefore is invaluable for the rational design and synthesis of novel materials with tailored properties.

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.

Existence of multi-radical and closed-shell semiconducting states in post-graphene organic Dirac materials

Post-graphene organic Dirac (PGOD) materials are ordered two-dimensional networks of triply bonded sp 2 carbon nodes spaced by π-conjugated linkers. PGOD materials are natural chemical extensions of graphene that promise to have an enhanced range of properties and applications. Experimentally realised molecules based on two PGOD nodes exhibit a bi-stable closed-shell/multi-radical character that can be understood through competing Lewis resonance forms. Here, following the same rationale, we predict that similar states should be accessible in PGOD materials, which we confirm using accurate density functional theory calculations. Although for graphene the semimetallic state is always dominant, for PGOD materials this state becomes marginally meta-stable relative to open-shell multi-radical and/or closed-shell states that are stabilised through symmetry breaking, in line with analogous molecular systems. These latter states are semiconducting, increasing the potential use of PGOD materials as highly tuneable platforms for future organic nano-electronics and spintronics.

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.