Tunneling versus Hopping in Mixed-Valence Oligo-p-phenylenevinylene Polychlorinated Bis(triphenylmethyl) Radical Anions

Pillar[5]arene-based Neutral Radicals with Doublet Red Emissions and Stable Chiroptical Properties

Stable organic radicals with unique luminescence show great importance in photoelectromagnetic materials. We herein report two unusual radical-based systems (P5N-TTM and P5B-TTM) using the concerted effects of planar chiral pillar[5]arenes and tris(2,4,6-trichlorophenyl)methyl (TTM) radicals. The steric effect and electronic doublet-spin character of these radicals allowed the optical resolution and the first red emissions (∼650 nm) for pillar[5]arene derivatives. Notably, cross-coupling with macrocyclic pillar[5]arene, in turn, considerably enhanced the configurational stability of TTM radicals.

Multifunctional Switch Based on Spin-Labeled Gold Nanoparticles

The fabrication of multifunctional switches is a fundamental step in the development of nanometer-scale molecular spintronic devices. The anchoring of active organic radicals on gold nanoparticles (AuNPs) surface is little studied and the realization of AuNPs-based switches remains extremely challenging. We report the first demonstration of a surface molecular switch based on AuNPs decorated with persistent perchlorotriphenylmethyl (PTM) radicals. The redox properties of PTM are exploited to fabricate electrochemical switches with optical and magnetic responses, showing high stability and reversibility. Electronic interaction between the radicals and the gold surface is investigated by UV-vis, showing a very broad absorption band in the near-infrared (NIR) region, which becomes more intense when PTMs are reduced to anionic phase. By using multiple experimental techniques, we demonstrate that this interaction is likely favored by the preferentially flat orientation of PTM ligands on the metallic NP surface, as confirmed by first-principles simulations.

Allocation of Ambipolar Charges on an Organic Diradical with a Vinylene-Phenylenediyne Bridge

Two redox and magnetically active perchlorotriphenylmethyl (^•PTM) radical units have been connected as end-capping groups to a bis(phenylene)diyne chain through vinylene linkers. Negative and positive charged species have been generated, and the influence of the bridge on their stabilization is discussed. Partial reduction of the electron-withdrawing ^•PTM radicals results in a class-II mixed-valence system with the negative charge located on the terminal PTM units, proving the efficiency of the conjugated chain for the electron transport between the two terminal sites. Counterintuitively, the oxidation process does not occur along the electron-rich bridge but on the vinylene units. The ^•PTM radicals play a key role in the stabilization of the cationic species, promoting the generation of quinoidal ring segments.

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.

Electrocatalytic oxidative Z/E isomerization of a stilbene favoured by the presence of an electroactive persistent radical

A push-pull-functionalized stilbene has been prepared, with an electroactive perchlorotriphenylmethyl (PTM˙) radical and dimethylamine units as electron-withdrawing and -donating moieties, respectively, showing an electrocatalytic redox-induced Z→E isomerization where the open-shell nature of PTM˙ plays a key role in the isomerization ocurrance.

Superexchange in the fast lane - intramolecular electron transfer in a molecular triad occurs by conformationally gated superexchange

Photoinduced electron transfer via hopping is generally considered to have a stronger temperature dependence than electron transfer via superexchange. However, in this work, an opposite trend of the temperature dependence is observed. This unexpected result is rationalized by considering the specific geometrical and electronic structure of the Ru-bis(terpyridine) photosensitizer.

Electron Transfer across o-Phenylene Wires

Photoinduced electron transfer across rigid rod-like oligo- p-phenylenes has been thoroughly investigated in the past, but their o-connected counterparts are yet entirely unexplored in this regard. We report on three molecular dyads comprised of a triarylamine donor and a Ru(bpy)₃²⁺ (bpy =2,2'-bipyridine) acceptor connected covalently by 2 to 6 o-phenylene units. Pulsed excitation of the Ru(II) sensitizer at 532 nm leads to the rapid formation of oxidized triarylamine and reduced ruthenium complex via intramolecular electron transfer. The subsequent thermal reverse charge-shift reaction to reinstate the electronic ground-state occurs on a time scale of 120-220 ns in deaerated CH₃CN at 25 °C. The conformational flexibility of the o-phenylene bridges causes multiexponential transient absorption kinetics for the photoinduced forward process, but the thermal reverse reaction produces single-exponential transient absorption decays. The key finding is that the flexible o-phenylene bridges permit rapid formation of photoproducts storing ca. 1.7 eV of energy with lifetimes on the order of hundreds of nanoseconds, similar to what is possible with rigid rod-like donor-acceptor compounds. Thus, the conformational flexibility of the o-phenylenes represents no disadvantage with regard to the photoproduct lifetimes, and this is relevant in the greater context of light-to-chemical energy conversion.

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.

Tuning Spin-Spin Interactions in Radical Dendrimers

Two generations of polyphosphorhydrazone (PPH) dendrimers were synthesized and fully functionalized with TEMPO radicals via acrylamido or imino group linkers to evaluate the impact of the linker substitution on the radical-radical interactions. A drastic change in the way that the radicals interacted among them was observed by EPR and CV studies: while radicals in Gn -imino-TEMPO dendrimers presented a strong spin-spin interaction, in the Gn -acrylamido-TEMPO ones they acted mainly as independent radicals. This shows that these interactions could be tuned by the solely substitution of the radical linker, opening the perspective of controlling and modulating the extension of these interactions depending on each application. The chemical properties of the linker strongly influence the spin-spin exchange between pendant radicals.

High Electrical Conductivity of Single Metal-Organic Chains

Molecular wires are essential components for future nanoscale electronics. However, the preparation of individual long conductive molecules is still a challenge. MMX metal-organic polymers are quasi-1D sequences of single halide atoms (X) bridging subunits with two metal ions (MM) connected by organic ligands. They are excellent electrical conductors as bulk macroscopic crystals and as nanoribbons. However, according to theoretical calculations, the electrical conductance found in the experiments should be even higher. Here, a novel and simple drop-casting procedure to isolate bundles of few to single MMX chains is demonstrated. Furthermore, an exponential dependence of the electrical resistance of one or two MMX chains as a function of their length that does not agree with predictions based on their theoretical band structure is reported. This dependence is attributed to strong Anderson localization originated by structural defects. Theoretical modeling confirms that the current is limited by structural defects, mainly vacancies of iodine atoms, through which the current is constrained to flow. Nevertheless, measurable electrical transport along distances beyond 250 nm surpasses that of all other molecular wires reported so far. This work places in perspective the role of defects in 1D wires and their importance for molecular electronics.

Synthesis of Nanometer Sized Bis- and Tris-trityl Model Compounds with Different Extent of Spin-Spin Coupling

Tris(2,3,5,6-tetrathiaaryl)methyl radicals, so-called trityl radicals, are emerging as spin labels for distance measurements in biological systems based on Electron Paramagnetic Resonance (EPR). Here, the synthesis and characterization of rigid model systems carrying either two or three trityl moieties is reported. The monofunctionalized trityl radicals are connected to the molecular bridging scaffold via an esterification reaction employing the Mukaiyama reagent 2-chloro-methylpyridinium iodide. The bis- and tris-trityl compounds exhibit different inter-spin distances, strength of electron-electron exchange and dipolar coupling and can give rise to multi-spin effects. They are to serve as benchmark systems in comparing EPR distance measurement methods.

Understanding the charge transport properties of redox active metal-organic conjugated wires

For Rh₂-organic molecular wires, we found that weaker coupling systems built using longer bridging ligands exhibit better electrical conductance.

Layer-by-layer assembly of the dirhodium complex [Rh₂(O₂CCH₃)₄] (Rh₂) with linear N,N′-bidentate ligands pyrazine (L_S) or 1,2-bis(4-pyridyl)ethene (L_L) on a gold substrate has developed two series of redox active molecular wires, (Rh₂L_S)_n@Au and (Rh₂L_L)_n@Au (n = 1–6). By controlling the number of assembling cycles, the molecular wires in the two series vary systematically in length, as characterized by UV-vis spectroscopy, cyclic voltammetry and atomic force microscopy. The current–voltage characteristics recorded by conductive probe atomic force microscopy indicate a mechanistic transition for charge transport from voltage-driven to electrical field-driven in wires with n = 4, irrespective of the nature and length of the wires. Whilst weak length dependence of electrical resistance is observed for both series, (Rh₂L_L)_n@Au wires exhibit smaller distance attenuation factors (β) in both the tunneling (β = 0.044 Å^–1) and hopping (β = 0.003 Å^–1) regimes, although in (Rh₂L_S)_n@Au the electronic coupling between the adjacent Rh₂ centers is stronger. DFT calculations reveal that these wires have a π-conjugated molecular backbone established through π(Rh₂)–π(L) orbital interactions, and (Rh₂L_L)_n@Au has a smaller energy gap between the filled π*(Rh₂) and the empty π*(L) orbitals. Thus, for (Rh₂L_L)_n@Au, electron hopping across the bridge is facilitated by the decreased metal to ligand charge transfer gap, while in (Rh₂L_S)_n@Au the hopping pathway is disfavored likely due to the increased Coulomb repulsion. On this basis, we propose that the super-exchange tunneling and the underlying incoherent hopping are the dominant charge transport mechanisms for shorter (n ≤ 4) and longer (n > 4) wires, respectively, and the Rh₂L subunits in mixed-valence states alternately arranged along the wire serve as the hopping sites.

Study of the E–Z stilbene isomerisation in perchlorotriphenyl-methane (PTM) derivatives

No reversibility in the E → Z isomerization of two vinylidene derivatives due to steric effects on the Z isomer is demonstrated.

Three Redox States of a Diradical Acceptor-Donor-Acceptor Triad: Gating the Magnetic Coupling and the Electron Delocalization

The diradical acceptor-donor-acceptor triad 1(••), based on two polychlorotriphenylmethyl (PTM) radicals connected through a tetrathiafulvalene(TTF)-vinylene bridge, has been synthesized. The generation of the mixed-valence radical anion, 1(•-), and triradical cation species, 1(•••+), obtained upon electrochemical reduction and oxidation, respectively, was monitored by optical and ESR spectroscopy. Interestingly, the modification of electron delocalization and magnetic coupling was observed when the charged species were generated and the changes have been rationalized by theoretical calculations.

Dynamics of intramolecular spin exchange interaction of a nitronyl nitroxide diradical in solution and on surfaces

In this paper we report the study of the dynamics of a thermally modulated intramolecular spin exchange interaction of a novel diradical nitronyl nitroxide-substituted disulfide in solution and when it is grafted on a gold surface. The structure of this diradical was designed to have flexible chains leading to intramolecular collisions and hence spin exchange interaction, and with an appropriate binding group to be grafted on the gold surface to study its behavior on the surface. In solution, this diradical shows a strong spin exchange interaction between both radicals which is modulated by temperature, but also gold nanoparticles (AuNPs) functionalized with this diradical permit investigation of such a phenomenon in surface-grafted radicals. The spin-labelled AuNP synthesis was optimized to obtain high coverage of spin labels to lead to high spin exchange interaction. The obtained AuNPs were studied by Electron Paramagnetic Resonance (EPR), UV-Vis, and IR spectroscopies, HR-TEM microscopy, Cyclic Voltammetry (CV), Energy Dispersive X-ray analysis (EDX) and Thermogravimetric Analysis (TGA). This inorganic-organic hybrid material also showed dipolar interactions between its radicals which were confirmed by the appearance in the EPR spectra of an |Δms| = 2 transition at half-field. This signal gives direct evidence of the presence of a high-spin state and permitted us to study the nature of the magnetic coupling between the spins which was found to be antiferromagnetic. Self-Assembled Monolayers (SAMs) of these radicals on the Au (111) substrate were also prepared and studied by contact angle, X-Ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), Cyclic Voltammetry and EPR. The magnetic as well as the electrochemical properties of the hybrid surfaces were studied and compared with the properties of this diradical in solution. Analogies between the properties of AuNPs with high coverage of radicals and those of SAM were observed.

Electron Transfer Rate Maxima at Large Donor-Acceptor Distances

Because of their low mass, electrons can transfer rapidly over long (>15 Å) distances, but usually reaction rates decrease with increasing donor-acceptor distance. We report here on electron transfer rate maxima at donor-acceptor separations of 30.6 Å, observed for thermal electron transfer between an anthraquinone radical anion and a triarylamine radical cation in three homologous series of rigid-rod-like donor-photosensitizer-acceptor triads with p-xylene bridges. Our experimental observations can be explained by a weak distance dependence of electronic donor-acceptor coupling combined with a strong increase of the (outer-sphere) reorganization energy with increasing distance, as predicted by electron transfer theory more than 30 years ago. The observed effect has important consequences for light-to-chemical energy conversion.

Magnetic and Electrochemical Properties of a TEMPO-Substituted Disulfide Diradical in Solution, in the Crystal, and on a Surface

A study of the magnetic and electrochemical properties of a TEMPO-substituted disulfide diradical in three different environments was carried out: in solution, in the crystal, and as a self-assembled monolayer (SAM) on an Au(111) substrate, and the relationship between them was explored. In solution, this flexible diradical shows a strong spin-exchange interaction between the two nitroxide functions that depends on the temperature and solvent. Structural, dynamic, and thermodynamic information has been extracted from the EPR spectra of this dinitroxide. The magnetic interactions in the crystal include intra- and intermolecular contributions, which have been studied separately and shown to be antiferromagnetic in both cases. Finally, we demonstrate that both the magnetic and electrochemical properties are preserved upon chemisorption of the diradical on a gold surface. The resulting SAM displayed anisotropic magnetic properties, and angle-resolved EPR spectra of the monocrystal allowed a rough determination of the orientation of the molecules in the SAM.

Unusual distance dependences of electron transfer rates

There are regimes in which electron transfer rates increase with increasing donor–acceptor distance, leading to rate maxima at large donor–acceptor separations.

Optimized polarization build-up times in dissolution DNP-NMR using a benzyl amino derivative of BDPA

A BAm-BDPA radical has proved to be a quick and efficient polarizing agent, showing optimum ¹³C NMR signal enhancements per polarization time unit.

Synthesis, X-Ray Structure, Magnetic Properties, and a Study of Intra/Intermolecular Radical-Radical Interactions of a Triradical TEMPO Compound

A novel triradical compound with a P=S core and three branches functionalized with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals is synthesized and characterized by IR, (1) H NMR, (31) P NMR, and EPR spectroscopy and MALDI-TOF mass spectrometry, and its chemical structure is confirmed by X-ray diffraction analysis. The triradical shows neither spin exchange interactions between its radical units nor detectable dipolar interactions. This is consistent with the separation between the radical units found in its X-ray diffraction structure, and discounts the existence of intramolecular interactions. This conclusion is confirmed by an EPR concentration study. The concentration at which intermolecular interactions start to appear is determined (5×10(-3)  m) and this concentration should be taken into account as a higher concentration limit when studies on intramolecular radical-radical interactions in polyradicals with similar structure are required. SQUID magnetometry analysis of the compound shows antiferromagnetic interactions between the spin carriers of different molecules; that is, antiferromagnetic intermolecular interactions.

Fluoride binding to an organoboron wire controls photoinduced electron transfer

We demonstrate that the rates for long-range electron transfer can be controlled actively by tight anion binding to a rigid rod-like molecular bridge. Electron transfer from a triarylamine donor to a photoexcited Ru(bpy)32+ acceptor (bpy = 2,2'-bipyridine) across a 2,5-diboryl-1,4-phenylene bridge occurs within less than 10 ns in CH2Cl2 at 22 °C. Fluoride anions bind with high affinity to the organoboron bridge due to strong Lewis base/Lewis acid interactions, and this alters the electronic structure of the bridge drastically. Consequently, a large tunneling barrier is imposed on photoinduced electron transfer from the triarylamine to the Ru(bpy)32+ complex and hence this process occurs more than two orders of magnitude more slowly, despite the fact that its driving force is essentially unaffected by fluoride addition. Electron transfer rates in proteins could potentially be regulated via a similar fundamental principle, because interactions between charged amino acid side chains and counter-ions can modulate electronic couplings between distant redox partners. In artificial donor-bridge-acceptor compounds, external stimuli have been employed frequently to control electron transfer rates, but the approach of exploiting strong Lewis acid/Lewis base interactions to regulate the tunneling barrier height imposed by a rigid rod-like molecular bridge is conceptually novel and broadly applicable, because it is largely independent of the donor and the acceptor, and because the effect is not based on a change of the driving-force for electron transfer. The principle demonstrated here can potentially be used to switch between conducting and insulating states of molecular wires between electrodes.

Planarization, fusion, and strain of carbon-bridged phenylenevinylene oligomers enhance π-electron and charge conjugation: a dissectional vibrational Raman study

We have used Raman spectroscopy to study the molecular and electronic structures of the radical cations and dications of carbon-bridged oligo(para-phenylenevinylene)s (COPVn, n = 1-6) possessing consecutive fused pentagons and hexagons, up to 19, along with COPV derivatives having electron-donating and -withdrawing groups. This study was made possible by the outstanding stability of the charged states of COPVs. We could untangle the effects of π-conjugation in the planar structure on the Raman frequency by distinguishing it from other structural effects, such as strain in the vinylene groups shared by the two pentagons. The analyses showed that the radical cations have benzo-quinoidal structures confined in the center of the molecule, as well as benzo-aromatic rings at the terminal sites. In contrast, dications of COPVn longer than n = 3 exhibit a biradicaloid character because of the recovery of aromaticity in the central rings and quinoidal rings at the terminal positions. These biradicaloids favor a singlet nature in their ground electronic states because of the double spin polarization. The introduction of electron-donating and -withdrawing groups on the termini of a COPV core affords, upon oxidation or reduction, a fully delocalized class III mixed valence system because of the high degree of conjugation of the COPV platform, which favors extensive charge delocalization.

A New Type of Electron Relay Station in Proteins: Three-Piece S:Π∴S↔S∴Π:S Resonance Structure

A type of relay station for electron transfer in proteins, three-piece five-electron bonding, is introduced in this paper, which is also first proposed here. The ab initio calculations predict the formation of S:Π∴S↔S∴Π:S resonance binding with an aromatic ring located in the middle of two sulfur-containing groups, which may participate in electron-hole transport in proteins. These special structures can lower the local ionization energies to capture electron holes efficiently and may be easily formed and broken because of their proper binding energies. In addition, the UV-vis spectra provide evidence of the formations of the three-piece five-electron binding. The cooperation of three adjacent pieces may be advantage to promote electron transfer a longer distance.

Structural control over spin localization in triarylmethyls

Using density functional theory calculations we demonstrate the existence of a general relation between structure and spin localisation in an important class of organic radicals, and point towards its potential use in future applications.

On the relation of energy and electron transfer in multidimensional chromophores based on polychlorinated triphenylmethyl radicals and triarylamines

Chromophores with many donors and acceptors show electron transfer which is identical to energy transfer.

Donor-acceptor electron transport mediated by solitons

We study the long-range electron and energy transfer mediated by solitons in a quasi-one-dimensional molecular chain (conjugated polymer, alpha-helical macromolecule, etc.) weakly bound to a donor and an acceptor. We show that for certain sets of parameter values in such systems an electron, initially located at the donor molecule, can tunnel to the molecular chain, where it becomes self-trapped in a soliton state, and propagates to the opposite end of the chain practically without energy dissipation. Upon reaching the end, the electron can either bounce back and move in the opposite direction or, for suitable parameter values of the system, tunnel to the acceptor. We estimate the energy efficiency of the donor-acceptor electron transport depending on the parameter values. Our calculations show that the soliton mechanism works for the parameter values of polypeptide macromolecules and conjugated polymers. We also investigate the donor-acceptor electron transport in thermalized molecular chains.

Diradicals acting through diamagnetic phenylene vinylene bridges: Raman spectroscopy as a probe to characterize spin delocalization

We present a complete Raman spectroscopic study in two structurally well-defined diradical species of different lengths incorporating oligo p-phenylene vinylene bridges between two polychlorinated triphenylmethyl radical units, a disposition that allows sizeable conjugation between the two radicals through and with the bridge. The spectroscopic data are interpreted and supported by quantum chemical calculations. We focus the attention on the Raman frequency changes, interpretable in terms of: (i) bridge length (conjugation length); (ii) bridge conformational structure; and (iii) electronic coupling between the terminal radical units with the bridge and through the bridge, which could delineate through-bond spin polarization, or spin delocalization. These items are addressed by using the "oligomer approach" in conjunction with pressure and temperature dependent Raman spectroscopic data. In summary, we have attempted to translate the well-known strategy to study the electron (charge) structure of π-conjugated molecules by Raman spectroscopy to the case of electron (spin) interactions via the spin delocalization mechanism.

Single-Molecule Conductance in a Series of Extended Viologen Molecules

Single-molecule conductance in a series of extended viologen molecules was measured at room temperature using a gold-molecule-gold scanning tunneling microscopy break junction arrangement. Conductance values for individual molecules change from 4.8 ± 1.2 nS for the shortest compound to 2.9 ± 1.0 nS for the compound with six repeating units and length of 11 nm. The latter value is almost 3 orders of magnitude higher than that reported for all-carbon-based aromatic molecular wires of comparable length. On the basis of the length of the molecules, an attenuation factor of only 0.06 ± 0.004 nm(-1) (0.006 ± 0.0004 Å(-1)) was obtained. To the best of our knowledge, this is the smallest value reported for the conductance attenuation in a series of molecular wires.

The perchlorotriphenylmethyl (PTM) radical

In spite of the considerable understanding and development of perchlorotriphenylmethyl (PTM) radical derivatives, the preparation of crystals of the pure unsubstituted PTM radical, C19Cl15, suitable for single-crystal X-ray diffraction has remained a challenge since its discovery, and only two studies dealing with the crystal structure of the unsubstituted PTM radical have been published. In one study, the radical forms clathrates with aromatic solvents [Veciana, Carilla, Miravitlles & Molins (1987).J. Chem. Soc. Chem. Commun.pp. 812–814], and in the other the structure was determinedab initiofrom powder X-ray diffraction data [Rius, Miravitlles, Molins, Crespo & Veciana (1990).Mol. Cryst. Liq. Cryst.187, 155–163]. We report here the preparation of PTM crystals for single-crystal X-ray diffraction and their resolution. The structure, which shows monoclinic symmetry (C2/c), revealed a nonsymmetric molecular propeller conformation (D3symmetry) caused by the steric strain between theortho-Cl atoms, which protect the central C atom (sp2-hybridization and major spin density) and give high chemical and thermal persistence to the PTM. The supramolecular structure of PTM shows short Cl...Cl intermolecular interactions and can be described in terms of layers formed by rows of molecules positioned in a head-to-tail manner along thecaxis.