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.     

Recognizing Through‐Bond and Through‐Space Self‐Exchange Charge/Spin Transfer Pathways in Bis(triarylamine) Radical Cations with Similar Geometrical Arrangements

Radical cations of bis(triarylamine)s, 3 and 4 , in which the triarylamine redox centers are bridged by an ortho ‐phenylene and ortho ‐carborane cluster, respectively, have been prepared to elucidate the difference in intramolecular charge/spin‐transfer (ICT/IST) pathway owing to the two different bridging units affording similar geometrical arrangements between the redox centers. Electrochemistry, absorption spectroscopy, VT‐ESR spectroscopy, and DFT calculations reveal that 3 ^.+ and 4 ^.+ are classified into class II and class I mixed‐valence systems, respectively, and therefore, through‐bond and through‐space mechanisms are dominant for the ICT/IST phenomena in 3 ^.+ and 4 ^.+, respectively. Moreover, SQUID measurements for dicationic species provide the fact that virtually no spin‐exchange interaction is observed for spins in 4 ²⁺, while the antiferromagnetic interaction for spins in 3 ²⁺, in accordance with the existence of a conjugation pathway for the ortho ‐phenylene bridge.     

Conformation‐Determined Through‐Bond versus Through‐Space Electronic Communication in Mixed‐Valence Systems with a Cross‐Conjugated Urea Bridge

Bis‐triarylamine 2 and cyclometalated diruthenium 6 (PF₆)₂ with a linear trans,trans‐urea bridge have been prepared, together with the bis‐triarylamine 3 and cyclometalated diruthenium 8 (PF₆)₂ with a folded cis,cis‐N,N‐dimethylurea bridge. The linear or folded conformations of these molecules are supported by single‐crystal X‐ray structures of 2 , 3 , and other related compounds. These compounds display two consecutive anodic redox waves (N ^{. +/0} or Ru^III/II processes) with a potential separation of 110–170 mV. This suggests that an efficient electronic coupling is present between two redox termini through the cross‐conjugated urea bridge. The degree of electronic coupling has been investigated by using spectroelectrochemical measurements. Distinct intervalence charge‐transfer (IVCT) transitions have been observed for mixed‐valent (MV) compounds with a linear conformation. The IVCT transitions can also be identified for the folded MV compounds, albeit with a much weaker intensity. DFT results support that the electronic communication occurs by a through‐bond and through‐space pathway for the linear and folded compounds, respectively. The IVCT transitions of the MV compounds have been reproduced by TDDFT calculations. For the purpose of comparison, a bistriarylamine and a diruthenium complex with an imidazolidin‐2‐one bridge and a urea‐containing mono‐triarylamine and monoruthenium complex have been synthesized and studied.     

An Organic Mixed‐Valence Ligand for Multistate Redox‐Active Coordination Networks

The multistate redox‐active/multi‐interactive ligand 5,5′,8,8′‐tetra(4‐pyridyl)‐2,2′‐(1,4‐phenylene)bis‐1H‐perimidine (H₂TPP) was designed and synthesized. H₂TPP undergoes four one‐electron oxidation steps, and was used for the preparation of a multistate redox‐active coordination network in a solid–liquid interface reaction using molten Cd²⁺ salts. The multiple redox states of H₂TPP were confirmed spectroscopically by stepwise four‐electron oxidation. Spectroscopic analysis indicated that the mixed‐valence states of the ligand are class II on the UV/Vis/NIR timescale and borderline class II/class III on the ESR timescale.     

Generation and detection of the radical cation and the dication derived from 4,9‐diethyl[1,4]dihydrodithiino[5,6‐f]benzotrithiole and its 5‐oxide

4,9‐Diethyl[1,4]dihydrodithiino[5,6‐f]benzotrithiole (DTBT) gave a radical cation, DTBT(•+), and a dication, DTBT(2+), on treatment with a single‐electron oxidizing reagent. Both compounds showed an ESR signal, whereas the dication, generated by this procedure, was silent for ¹H NMR. Hydrolysis of DTBT(2+) gave DTBT 1‐oxide (DTBT 1‐O) and 2‐oxide (DTBT 2‐O) together with DTBT and a mixture of several dioxides. A singlet‐state dication, DTBT(2+)‐S, which was generated upon treatment of DTBT 5‐oxide (DTBT 5‐O) with concentrated D₂SO₄, was detected by ¹H and ¹³C NMR. After 20 h, the NMR signals disappeared while the solution was active for ESR. The results suggest that (i) a species generated from DTBT by oxidation with the single‐electron oxidizing reagent is a triplet‐state dication, DTBT(2+)‐T, and (ii) DTBT(2+)‐S, initially generated, gradually isomerizes to DTBT(2+)‐T in the solution, and DTBT(2+)‐T forms a partial spin pair. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:394–401, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20445     

Pump‐Pump‐Probe Spectroscopy of a Molecular Triad Monitoring Detrimental Processes for Photoinduced Charge Accumulation

Controlling light‐induced accumulation of electrons or holes is desirable in view of multi‐electron redox chemistry, for example for the formation of solar fuels or for photoredox catalysis in general. Excitation with multiple photons is usually required for electron or hole accumulation, and consequently pump‐pump‐probe spectroscopy becomes a valuable spectroscopic tool. In this work, we excited a triarylamine‐Ru(bpy)₃²⁺‐anthraquinone triad (bpy = 2,2′‐bipyridine) with two temporally delayed laser pulses of different color and monitored the resulting photoproducts. Absorption of the first photon by the Ru(bpy)₃²⁺ photosensitizer generated a triarylamine radical cation and an anthraquinone radical anion by intramolecular electron transfer. Subsequent selective excitation of either one of these two radical ion species then induced rapid reverse electron transfer to yield the triad in its initial (ground) state. This shows in direct manner that after absorption of a first photon and formation of the primary photoproducts, the absorption of a second photon can lead to unproductive electron transfer events that counteract further charge accumulation. In principle, this problem is avoidable by careful excitation wavelength selection in combination with good molecular design.     

Intramolecular Spin Alignment within Mono‐Oxidized and Photoexcited Anthracene‐Based π Radicals as Prototypical Photomagnetic Molecular Devices: Relationships Between Electrochemical,Photophysical, and Photochemical Control Pathways

AnOV is a π‐conjugated radical built from an anthracene (An) unit linked by a p‐phenylene to an oxoverdazyl (OV) moiety. The mono‐oxidized (cationic) form of AnOV was generated both electrochemically and photochemically (in the presence of an electron acceptor). The triplet nature (S=1) of the electronic ground state of AnOV ⁺ was demonstrated by combining spectroelectrochemistry, electron‐spin resonance (ESR) experiments, and ab initio molecular orbital (MO) calculations. The intramolecular spin alignment (ISA) within AnOV ⁺ results from the ferromagnetic coupling (J_electrochem>0) of the two unpaired electrons located on the oxidized electron donor (An⁺) and on the pendant OV radical. The spin‐density distribution pattern of AnOV ⁺ is akin to that of AnOV when photopromoted ( AnOV *) to its high‐spin (HS) lowest excited quartet (S=3/2) state. This high‐spin state results from the ferromagnetic coupling (J_photophys>0) of the triplet locally excited state of An (³An*) with the doublet ground state of OV. As a shared salient feature, AnOV ⁺ and AnOV * (HS) show a spin delocalization within the domain of activated An in either An⁺ or ³An* (nexus states) forms. The present study essentially contributes to establish and clarify relationships between electrochemical, photophysical, and photochemical pathways to achieve ISA processes within AnOV . In particular, we discuss the impact of the spin polarization of the unpaired electron of OV on electronic features of the An electron‐donating subunit. Close analysis of this polarizing interplay allows one to derive a novel functional paradigm to manipulate electron spins at the intramolecular level with light and under an external magnetic field. Indeed, two original functional elements are identified: light‐triggered donors of spin‐polarized electrons and spin‐selective electron acceptors, which are of potential interest for molecular spintronics.     

Observation of a Metastable P‐Heterocyclic Radical by Muonium Addition to a 1,3‐Diphosphacyclobutane‐2,4‐diyl

A 1,3‐diphosphacyclobutane‐2,4‐diyl contains a unique unsaturated cyclic unit, and the presence of radical‐type centers have been expected as a source of functionality. This study demonstrates that the P‐heterocyclic singlet biradical captures muonium (Mu=[μ⁺e^?]), the light isotope of a hydrogen radical, to generate an observable P‐heterocyclic paramagnetic species. Investigation of a powder sample of 2,4‐bis(2,4,6‐tri‐t‐butylphenyl)‐1‐t‐butyl‐3‐benzyl‐1,3‐diphosphacyclobutane‐2,4‐diyl using muon (avoided) level‐crossing resonance (μLCR) spectroscopy revealed that muonium adds to the cyclic P₂C₂ unit. The muon hyperfine coupling constant (A_μ) indicated that the phosphorus atom bearing the t‐butyl group trapped muonium to provide a metastable P‐heterocyclic radical involving the ylidic MuP(<)=C moiety. The observed regioselective muonium addition correlates the canonical formula of 1,3‐diphosphacyclobutane‐2,4‐diyl.     

π‐Extended Dihydrophenazines with Three‐State NIR Electrochromism Involving Large Conformational Changes

π‐Extended dihydrophenazines were successfully prepared by oxidation of 2‐(N ‐arylamino)anthracenes. Their roof‐type conformations were revealed by X‐ray diffraction analysis, and the analysis of the optical properties indicated the presence of intramolecular charge‐transfer processes. Upon chemical oxidation, the electronic absorption dramatically changed in a two‐step fashion. The electron spin resonance (ESR) analysis revealed that, depending on the amount of oxidant added, either a paramagnetic radical cation or a diamagnetic dication was generated. The NMR analysis revealed a conformational change upon oxidation, which was supported by theoretical calculations. A three‐state electrochromic behavior was observed during the electrochemical oxidation and reduction cycles, showing sequential switching between visible and near‐infrared (NIR) absorption properties upon application of electrochemical stimuli.     

Photo‐ and Electronically Switchable Spin‐Crossover Iron(II) Metal–Organic Frameworks Based on a Tetrathiafulvalene Ligand

A major challenge is the development of multifunctional metal–organic frameworks (MOFs), wherein magnetic and electronic functionality can be controlled simultaneously. Herein, we rationally construct two 3D MOFs by introducing the redox active ligand tetra(4‐pyridyl)tetrathiafulvalene (TTF(py)₄) and spin‐crossover Fe^II centers. The materials exhibit redox activity, in addition to thermally and photo‐induced spin crossover (SCO). A crystal‐to‐crystal transformation induced by I₂ doping has also been observed and the resulting intercalated structure determined. The conductivity could be significantly enhanced (up to 3 orders of magnitude) by modulating the electronic state of the framework via oxidative doping; SCO behavior was also modified and the photo‐magnetic behavior was switched off. This work provides a new strategy to tune the spin state and conductivity of framework materials through guest‐induced redox‐state switching.     

A new oxidation state of aniline pentamer observed in water‐soluble electroactive oligoaniline‐chitosan polymer

A novel water‐soluble electroactive polymer, aniline pentamer crosslinked chitosan (Pentamer‐c‐Chi), was prepared by condensation polymerization of the terminal carboxyl groups in aniline pentamer with the amino side groups in chitosan in aqueous solution. The carboxyl groups were activated by N‐hydroxysuccinimide (NHS) and N,N′‐dicyclohexylcarbodiimide (DCC). The electrochemical behavior of anilinepentamer in this kind of crosslinked polymer was studied in acidic aqueous solution by means of cyclic voltammetry (CV), UV–vis, and electron spin resonance (ESR) spectroscopy. There were three reversible redox peaks in the CV of Pentamer‐c‐Chi. A new emeraldine oxidization state in the form of radical cations was proposed, which was associated with the new absorption band at 370 nm in the UV–vis spectra. The ESR of the aqueous solution of Pentamer‐c‐Chi showed a single Lorentzian shaped signal, which suggested the existence of radical cations. The new redox state was pH dependent and appeared only at pH < 3. The stability of radical cations could be attributed to the hydrogen bonds between radical cations, water, and chitosan. Morphological structure of the Pentamer‐c‐Chi can be adjusted by varying the content of aniline pentamer. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1124–1135, 2008     

Redox‐Switchable 20π‐, 19π‐, and 18π‐Electron 5,10,15,20‐Tetraaryl‐5,15‐diazaporphyrinoid Nickel(II) Complexes

The first examples of air‐stable 20π‐electron 5,10,15,20‐tetraaryl‐5,15‐diaza‐5,15‐dihydroporphyrins, their 18π‐electron dications, and the 19π‐electron radical cation were prepared through metal‐templated annulation of nickel(II) bis(5‐arylamino‐3‐chloro‐8‐mesityldipyrrin) complexes followed by oxidation. The neutral 20π‐electron derivatives are antiaromatic and the cationic 18π‐electron derivatives are aromatic in terms of the magnetic criterion of aromaticity. The meso N atoms in these diazaporphyrinoids give rise to characteristic redox and optical properties for the compounds that are not typical of isoelectronic 5,10,15,20‐tetraarylporphyrins.     

Coordination‐Induced Spin‐State Switching of an Aminyl‐Radical‐Bridged Nickel(II) Porphyrin Dimer between Doublet and Sextet States

A bis(Ni^II‐porphyrinyl)aminyl radical with meso‐C₆F₅ groups was prepared as a spin‐delocalized stable aminyl radical with a doublet spin state. Upon addition of pyridine, both Ni^II centers became hexacoordinated by accepting two axial pyridines, which triggered a spin‐state change of the Ni^II centers from diamagnetic (S=0) to paramagnetic (S=1). The resulting high‐spin Ni^II centers interact with the aminyl radical ferromagnetically to give rise to an overall sextet state (S=5/2). Importantly, this coordination‐induced spin‐state switching can be conducted in a reversible manner, in that washing of the high‐spin radical with aqueous hydrochloric acid regenerates the original doublet radical in good yield.     

EPR Study on the Complex Formed by Charge‐Transfer Process between Ground‐state Acceptor 2, 3‐Dicyano‐5, 6‐dichloro‐1, 4‐benzoquinone and Some Donors and on Cation Radical of Perylene (or Pyrene)

EPR study showed that the semi‐quinone radical anion of 2,3‐dicyano‐5,6‐dichloro‐1,4‐benzoquinone (DDQ) was formed in a charge transfer process between ground‐state DDQ as acceptor and each one of following ground state donors, i.e., 4‐methyl‐4′‐tridecyl‐2, 2′‐bipyridyl; 4‐methyl‐4′‐nonyl‐2, 2′‐bipyridyl; bis (2,2′‐bipyridyl) (4‐methyl‐4′‐heptadecyl‐2, 2′‐bipyridyl)ruthenium(2+) perchlorate and perylene. EPR study also showed that there are perylene cation radical and pyrene cation radical in the following experimental conditions: (a) in 98% sulfuric add. (b) 10^?3 mol/L perylene (or pyrene) was dissolved in trifluoroacetic acid‐nitrobenzene (1: 1 V/V).     

trans‐Bis(cyano‐κC)(1,4,8,11‐tetraazacyclotetradecane‐κ4N)manganese(III) perchlorate,a low‐spin manganese(III) complex

The crystal structure of the low‐spin (S = 1) Mn^III complex [Mn(CN)₂(C₁₀H₂₄N₄)]ClO₄, or trans‐[Mn(CN)₂(cyclam)](ClO₄) (cyclam is the tetradentate amine ligand 1,4,8,11‐tetra­aza­cyclo­tetra­decane), is reported. The structural parameters in the Mn(cyclam) moiety are found to be insensitive to both the spin and the oxidation state of the Mn ion. The difference between high‐ and low‐spin Mn^III complexes is that a pronounced tetragonal elongation of the coordination octahedron occurs in high‐spin complexes and a slight tetragonal compression is seen in low‐spin complexes, as in the title complex.     

Polycarboxylate‐Templated Coordination Polymers: Role of Templates for Superprotonic Conductivities of up to 10−1 S cm−1

Three coordination polymers (CPs) have been synthesized based on a [Co(bpy)(H₂O)₄]²⁺ chain (bpy=4,4′‐bipyridine) by a template approach. The frameworks are neutralized by different templated polycarboxylate anions (furan di‐carboxylate (fdc) in Co‐fdc, benzene tri‐carboxylate (btc) in Co‐tri and benzene tetra‐carboxylate (btec) in Co‐tetra). These templates with different degrees of protonation and ionic carrier concentration played significant role on crystal packing as well as formation of well‐directed H‐bonded networks which made these CPs perform well in proton conduction (PC). The PC value reaches to 1.49×10^?1 S cm^?1 under 80 °C and 98 % relative humidity (R.H.) for Co‐tri, which is the highest among CPs/MOFs/COFs and is an example of conductivity in the order of 10^?1 S cm^?1. Co‐tri and Co‐tetra are excellent proton conductors at mild temperature (40 °C) and 98 % R.H. (conductivities up to 2.92×10^?2 and 1.38×10^?2 S cm^?1, respectively).     

Synthesis and characterization of novel poly(ester‐carbonate)s by copolymerization of trans‐4‐hydroxy‐L‐proline with cyclic carbonate

The melt ring‐opening/condensation reaction of trans‐4‐hydroxy‐N‐benzyloxycarbonyl‐L‐proline (N‐CBz‐Hpr) with cyclic carbonate [trimethylene carbonate (tri‐MC) or tetramethylene carbonate (tetra‐MC)] at a wide range of molar fractions in the feed produced new degradable poly(ester‐carbonate)s. The influence of reaction conditions such as polymerization time and temperature on the yield and inherent viscosity of the copolymers was investigated. The polymerizations were carried out in bulk at 140 °C with 1.5 wt % stannous octoate as a catalyst for 30 h. The poly(ester‐carbonate)s obtained were characterized by Fourier transform infrared spectroscopy, ¹H NMR, differential scanning calorimetry, gel permeation chromatography, and Ubbelohde viscometry. The copolymers synthesized exhibited moderate molecular weights with rather narrow molecular weight distributions. The values of the glass‐transition temperature (T_g) of the copolymers depend on the molar fractions of cyclic carbonate. For the poly(N‐CBz‐Hpr‐co‐tri‐MC) system, with a decreased tri‐MC content from 93 to 16 mol %, the T_g increased from ?10 to 60 °C. Similarly, for the poly(N‐CBz‐Hpr‐co‐tetra‐MC) system, when the tetra‐MC content decreased from 80 to 8 mol %, the T_g increased from ?18 to 52 °C. The relationship between the poly(N‐CBz‐Hpr‐co‐tri‐MC) T_g and the compositions was in approximation with the Fox equation. In vitro degradation of these poly(N‐CBz‐Hpr‐co‐tri‐MC)s was evaluated from weight‐loss measurements. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1435–1443, 2003     

Generation and structure determination of 5,6‐bis(methylthio)‐4,7‐diethylbenzo[1,2,3]trithiole dication MBT(2+)

5,6‐Bis(methylthio)‐4,7‐diethylbenzo‐[1,2,3]‐trithiole [MBT] was oxidized with two equivalents of SbCl₅ to produce a dication, MBT(2+)ċ2SbCl, as a stable, dark‐brown solid. MBT(2+) was unexpectedly silent for ¹H‐NMR in CD₃CN, whereas it was active for ESR, suggesting that MBT(2+) is a triplet‐state dication MBT(2+)‐T. Meanwhile, treatment of 5‐ methylsulfinyl‐6‐methylthio‐4,7‐diethylbenzo[1,2,3]‐ trithiole [MBTMO] with D₂SO₄ produced MBT(2+), whose ¹H‐NMR gave no signals, whereas the solution is active for ESR. These results imply that MBT(2+) prepared from MBTMO is a triplet‐state dication, and a singlet‐state dication, MBT(2+)‐S, initially generated by acidification of MBTMO, isomerized to the triplet‐state dication, MBT(2+)‐T. Since MBT(2+)‐T is active for ESR at room temperature, two molecules of MBT(2+)‐T should form a spin pair in the solution with a sufficient distance between the two radical centers. The structures of MBT(2+)‐S and MBT(2+)‐T were optimized with the DFT method at the B3LYP6‐31G** level. The total energy difference between them was calculated to be 7.90 kcal/mol; MBT(2+)‐T was shown to be more stable than MBT(2+)‐S. A treatment of MBTMO with SbCl₅ gave a 1:1 complex. The structure of the complex was determined with X‐ray crystallography, which showed that the complex is the corresponding sulfonium salt, MBTMOċSbCl₅. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:111–222, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20078     

Crystalline‐State Photoreaction of 1‐Azido‐2‐nitrobenzene – Direct Observation of Heterocycle Formation by X‐Ray Crystallography

The crystalline‐state photoreaction of 1‐azido‐2‐nitrobenzene ( 1 ) was investigated by a combination of X‐ray crystallography, IR spectroscopy, electron‐spin resonance (ESR), and by means of theoretical calculations. Upon low‐temperature (80 K) photolysis of 1 , the formation of benzofuroxan ( 2 ) was directly observed by X‐ray single‐crystal analysis. ESR Measurements at 5 K suggested the presence of a triplet nitrene as an intermediate in the formation of the heterocycle. Temperature‐dependent IR spectroscopy also revealed that another intermediate, trans,trans‐1,2‐dinitrosobenzene, was produced at temperatures below 80 K.     

d‐electron count,ion‐pairing and diagonal twist angles in metallo‐bis(dithiolene) complexes

Electronic structure calculations for late transition metals coordinated by two dithiolene ligands are found to be consistent with existing structures and also predict the geometries of Ni(I) species for which no solid state structures have been reported. Of particular interest are the compounds [M(mnt)₂]^{n −} (M = Ni, Pd, and Pt with n = 1, 2, 3; M = Cu with n = 2). Calculations have been performed with and without ion‐paring with M(diglyme)⁺ (M = Li, Na, K) and R₄N⁺ (R = Me, Bu). The diagonal twist angle between two NiS₂ planes is found to depend on (i) the metal's d‐electron count, spanning from 0° (planar d⁷ and d⁸), to 42° (d⁹), to 90° (pseudo‐tetrahedral d¹⁰), and (ii) the identity of the ion‐paired cations. Calculated ion‐pairing energies are functions of the cation size and charge‐density, being larger for alkali‐metal coordinated diglyme and smaller for tetra‐alkyl ammonium cations. © 2016 Wiley Periodicals, Inc.