Intercage Electron Transfer Driven by Electric Field in Robin–Day‐Type Molecules

A new class of isomers, namely, intercage electron‐transfer isomers, is reported for fluorinated double‐cage molecular anion e^?@C₂₀F₁₈(NH)₂C₂₀F₁₈ with C₂₀F₁₈ cages: 1 with the excess electron inside the left cage, 2 with the excess electron inside both cages, and 3 with the excess electron inside the right cage. Interestingly, the C₂₀F₁₈ cages may be considered as two redox sites existing in a rare nonmetal mixed‐valent (0 and ?1) molecular anion. The three isomers with two redox sites may be the founding members of a new class of mixed‐valent compounds, namely, nonmetal Robin–Day Class II with localized redox centers for 1 and 3 , and Class III with delocalized redox centers for 2 . Two intercage electron‐transfers pathways involving transfer of one or half an excess electron from one cage to the other are found: 1) Manipulating the external electric field (?0.001 a.u. for 1 → 3 and ?0.0005 a.u. for 1 → 2 ) and 2) Exciting the transition from ground to first excited state and subsequent radiationless transition from the excited state to another ground state for 1 and 3 . For the exhibited microscopic electron‐transfer process 1 → 3 , 2 may be the transition state, and the electron‐transfer barrier of 6.021 kcal mol^?1 is close to the electric field work of 8.04 kcal mol^?1.     

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.     

Electronic coupling in a highly preorganized bimetallic complex comprising pyrazolate-bridged CpMn(CO)2 moieties

By means of a multistep synthetic procedure a dimanganese complex has been prepared, in which a N,N'-bridging pyrazolate ligand spans two CpMn(CO)(2) subunits in a highly preorganized chelate arrangement. The Xray crystallographic analyses of the Mn(I)Mn(I) complex K(+)1(-) and of its non-chelate precursor complex elucidate details of the molecular structure, in particular an unusual pyrazolate binding mode in the solid state and intertwining of the CO ligands in the crowded bimetallic array 1(-). The Mn(I)Mn(I) compound (1(-)), the mixed-valent Mn(I)Mn(II) (1), and the oxidized Mn(II)Mn(II) form (1(+)) have been characterized by various analytical and spectrosopic methods, such as electrochemistry, variable-temperature EPR spectroscopy, IR spectroelectrochemistry, and UV/Vis/NIR spectroelectrochemistry as well as by DFT and TD-DFT calculations. Strong electronic coupling in the mixed-valent complex is observed, but time- (and temperature-) dependent valence detrapping occurs, thus placing 1 in class II according to the Robin and Day assignment, close to the class II/III transition. From variable-temperature EPR spectroscopy a rough estimate of the activation energy and rate for thermal electron transfer can be deduced, with E(th) ( not equal )=13.6 kJ mol(-1) and k(th)=2.6 x 10(10) s(-1) at 298 K. Unexpectedly, no intervalence CT transition for 1 is detected in solution, but one appears in the optical spectrum of solid 1. The conclusions drawn from experiments are fully supported by DFT calculations that were carried out for all three forms of the dimanganese complex. A broken symmetry treatment for mixed-valent 1 reveals almost perfect localization of both spin and charge on one Mn center. According to TD-DFT the first excited states of 1 give rise to the IT processes in the NIR-energy region, as observed in the solid-state spectrum. The HOMOs are located at the Mn ions and are favorably arranged for pi interactions with the bridging pyrazolate.     

Mixed‐Valence Ruthenium Complexes Rotating through a Conformational Robin–Day Continuum

The conformational energy landscape and the associated electronic structure and spectroscopic properties (UV/Vis/near‐infrared (NIR) and IR) of three formally d⁵/d⁶ mixed‐valence diruthenium complex cations, [{Ru(dppe)Cp*}₂(μ‐C≡CC₆H₄C≡C)]⁺, [ 1 ]⁺, [trans‐{RuCl(dppe)₂}₂(μ‐C≡CC₆H₄C≡C)]⁺, [ 2 ]⁺, and the Creutz–Taube ion, [{Ru(NH₃)₅}₂(μ‐pz)]⁵⁺, [ 3 ]⁵⁺ (Cp=cyclopentadienyl; dppe=1,2‐bis(diphenylphosphino)ethane; pz=pyrazine), have been studied using a nonstandard hybrid density functional BLYP35 with 35 % exact exchange and continuum solvent models. For the closely related monocations [ 1 ]⁺ and [ 2 ]⁺, the calculations indicated that the lowest‐energy conformers exhibited delocalized electronic structures (or class III mixed‐valence character). However, these minima alone explained neither the presence of shoulder(s) in the NIR absorption envelope nor the presence of features in the observed vibrational spectra characteristic of both delocalized and valence‐trapped electronic structures. A series of computational models have been used to demonstrate that the mutual conformation of the metal fragments—and even more importantly the orientation of the bridging ligand relative to those metal centers—influences the electronic coupling sufficiently to afford valence‐trapped conformations, which are of sufficiently low energy to be thermally populated. Areas in the conformational phase space with variable degrees of symmetry breaking of structures and spin‐density distributions are shown to be responsible for the characteristic spectroscopic features of these two complexes. The Creutz–Taube ion [ 3 ]⁵⁺ also exhibits low‐lying valence‐trapped conformational areas, but the electronic transitions that characterize these conformations with valence‐localized electronic structures have low intensities and do not influence the observed spectroscopic characteristics to any notable extent.     

Transition from a Metal‐Localized Mixed‐Valence Compound to a Fully Delocalized and Bridge‐Biased Electrophore in a Ruthenium–Amine–Ruthenium Tricenter System

A series of cyclometalated diruthenium complexes with a redox‐active amine bridge were synthesized. Depending on the terminal ligands of the ruthenium components and the substituent on the amine unit, the one‐electron‐oxidized state can be either in the form of a weakly or strongly coupled mixed‐valence diruthenium complex, a fully charge‐delocalized three‐center system, or a bridge‐biased electrophore. This transition among different electronic forms was supported by electrochemistry, near‐infrared absorption, electron paramagnetic resonance, and density functional theory analysis.     

Functional Near‐Infrared Fluorescent Probes

Near‐infrared (NIR) fluorescent probes have attracted much attention, but despite the availability of various NIR fluorophores, only a few functional NIR probes, that is, probes whose absorption and/or fluorescence spectra change upon specific reaction with biomolecules, have been developed. However, functional probes operating in the NIR range that can be targeted to protons, metal ions, nitric oxide, β‐galactosidase, and cellular stress markers are expected to be effective for fluorescence imaging in vivo. This Focus Review concentrates on these functional NIR probes themselves, not their applications.     

Electronic Coupling between Two Amine Redox Sites through the 5,5′‐Positions of Metal‐Chelating 2,2′‐Bipyridines

Electron delocalization of new mixed‐valent (MV) systems with the aid of lateral metal chelation is reported. 2,2′‐Bipyridine (bpy) derivatives with one or two appended di‐p‐anisylamino groups on the 5,5′‐positions and a coordinated [Ru(bpy)₂] (bpy=2,2′‐bipyridine), [Re(CO)₃Cl], or [Ir(ppy)₂] (ppy=2‐phenylpyridine) component were prepared. The single‐crystal molecular structure of the bis‐amine ligand without metal chelation is presented. The electronic properties of these complexes were studied and compared by electrochemical and spectroscopic techniques and DFT/TDDFT calculations. Compounds with two di‐p‐anisylamino groups were oxidized by a chemical or electrochemical method and monitored by near‐infrared (NIR) absorption spectral changes. Marcus–Hush analysis of the resulting intervalence charge‐transfer transitions indicated that electron coupling of these mixed‐valent systems is enhanced by metal chelation and that the iridium complex has the largest coupling. TDDFT calculations were employed to interpret the NIR transitions of these MV systems.     

Tuning the Electronic Coupling in Cyclometalated Diruthenium Complexes through Substituent Effects: A Correlation between the Experimental and Calculated Results

A common bridging ligand, 3,3′,5,5′‐tetrakis(N‐methylbenzimidazol‐2‐yl)biphenyl, and four terpyridine terminal ligands with various substituents (amine, tolyl, nitro, and ester groups) have been used to synthesize ten cyclometalated diruthenium complexes 1 ²⁺– 10 ²⁺. Among them, compounds 1 ²⁺– 6 ²⁺ are redox nonsymmetric, and others are symmetric. These complexes show two Ru^III/II processes and an intervalence charge transfer (IVCT) transition in the one‐electron oxidized state. The potential separation (ΔE) of 1 ²⁺– 10 ²⁺ has been correlated to the energy difference ΔG⁰, the energy of the IVCT band E_op, and the ground‐state delocalization coefficient α². Time‐dependent (TD)DFT calculations suggest that the absorptions in the visible region of 1 ²⁺– 6 ²⁺ are mainly associated with the metal‐to‐ligand charge‐transfer transitions from both ruthenium ions and to both terminal ligands and the bridging ligand. However, the energies of these transitions vary significantly. DFT calculations have been performed on 1 ²⁺– 6 ²⁺ and 1 ³⁺– 6 ³⁺ to give information on the electronic structures and spin populations of the mixed‐valent compounds. The TDDFT‐predicted IVCT excitations reproduce well the experimental trends in transition energies. In addition, three monoruthenium complexes have been synthesized for a comparison study.     

Coexistence of Two Thermally Induced Intramolecular Electron Transfer Processes in a Series of Metal Complexes [M(Cat‐N‐BQ)(Cat‐N‐SQ)]/[M(Cat‐N‐BQ)2] (M=Co,Fe, and Ni) bearing Non‐Innocent Catechol‐Based Ligands: A Combined Experimental and Theoretical Study

The different thermally induced intermolecular electron transfer (IET) processes that can take place in the series of complexes [M(Cat‐N‐BQ)(Cat‐N‐SQ)]/[M(Cat‐N‐BQ)₂], for which M=Co ( 2 ), Fe ( 3 ) and Ni( 4 ), and Cat‐N‐BQ and Cat‐N‐SQ denote the mononegative (Cat‐N‐BQ^?) or dinegative (Cat‐N‐SQ^2?) radical forms of the tridentate Schiff‐base ligand 3,5‐di‐tert‐butyl‐1,2‐quinone‐1‐(2‐hydroxy‐3,5‐di‐tert‐butylphenyl)imine, have been studied by variable‐temperature UV/Vis and NMR spectroscopies. Depending on the metal ion, rather different behaviors are observed. Complex 2 has been found to be one of the few examples so far reported to exhibit the coexistence of two thermally induced electron transfer processes, ligand‐to‐metal (IET^LM) and ligand‐to‐ligand (IET^LL). IET^LL was only found to take place in complex 3 , and no IET was observed for complex 4 . Such experimental studies have been combined with ab initio wavefunction‐based CASSCF/CASPT2 calculations. Such a strategy allows one to solicit selectively the speculated orbitals and to access the ground states and excited‐spin states, as well as charge‐transfer states giving additional information on the different IET processes.     

Class III Delocalization in a Cyanide‐Bridged Trimetallic Mixed‐Valence Complex

The NIR and IR spectroscopic properties of the cyanide‐bridged complex, trans‐[Ru(dmap)₄{(μ‐CN)Ru(py)₄Cl}₂]³⁺ (py=pyridine, dmap=4‐dimethylaminopyridine) provide strong evidence that this trimetallic ion behaves as a Class III mixed‐valence species, the first example reported of a cyanide‐bridged system. This has been accomplished by tuning the energy of the fragments in the trimetallic complex to compensate for the intrinsic asymmetry of the cyanide bridge. Moreover, (TD)DFT calculations accurately predict the spectra of the trans‐[Ru(dmap)₄{(μ‐CN)Ru(py)₄Cl}₂]³⁺ ion and confirms its delocalized nature.     

The Equally Important Role of Adenine Derivatives to That of Pyrimidine Derivatives in Near‐0 eV Electron‐Induced DNA Lesions

The role of adenine (A) derivatives in DNA damage is scarcely studied due to the low electron affinity of base A. Experimental studies demonstrate that low‐energy electron (LEE) attachment to adenine derivatives complexed with amino acids induces barrier‐free proton transfer producing the neutral N₇‐hydrogenated adenine radicals rather than conventional anionic species. To explore possible DNA lesions at the A sites under physiological conditions, probable bond ruptures in two models—N₇‐hydrogenated 2′‐deoxyadenosine‐3′‐monophosphate (3′‐dA(N7H)MPH) and 2′‐deoxyadenosine‐5′‐monophosphate (5′‐dA(N7H)MPH), without and with LEE attachment—are studied by DFT. In the neutral cases, DNA backbone breakage and base release resulting from C_3′?O_3′ and N₉?C_1′ bond ruptures, respectively, by an intramolecular hydrogen‐transfer mechanism are impossible due to the ultrahigh activation energies. On LEE attachment, the respective C_3′?O_3′ and N₉?C_1′ bond ruptures in [3′‐dA(N7H)MPH]^? and [5′‐dA(N7H)MPH]^? anions via a pathway of intramolecular proton transfer (PT) from the C_2′ site of 2′‐deoxyribose to the C₈ atom of the base moiety become effective, and this indicates that substantial DNA backbone breaks and base release can occur at non‐3′‐end A sites and the 3′‐end A site of a single‐stranded DNA in the physiological environment, respectively. In particular, compared to the results of previous theoretical studies, not only are the electron affinities of 3′‐dA(N7H)MPH and 5′‐dA(N7H)MPH comparable to those of hydrogenated pyrimidine derivatives, but also the lowest energy requirements for the C_3′?O_3′ and N₉‐glycosidic bond ruptures in [3′‐dA(N7H)MPH]^? and [5′‐dA(N7H)MPH]^? anions, respectively, are comparable to those for the C_3′?O_3′ and N₁‐glycosidic bond cleavages in corresponding anionic hydrogenated pyrimidine derivatives. Thus, it can be concluded that the role of adenine derivatives in single‐stranded DNA damage is equally important to that of pyrimidine derivatives in an irradiated cellular environment.     

Tetrathiafulvalene‐Based Mixed‐Valence Acceptor–Donor–Acceptor Triads: A Joint Theoretical and Experimental Approach

This work presents a joint theoretical and experimental characterisation of the structural and electronic properties of two tetrathiafulvalene (TTF)‐based acceptor–donor–acceptor triads (BQ–TTF–BQ and BTCNQ–TTF—BTCNQ; BQ is naphthoquinone and BTCNQ is benzotetracyano‐p‐quinodimethane) in their neutral and reduced states. The study is performed with the use of electrochemical, electron paramagnetic resonance (EPR), and UV/Vis/NIR spectroelectrochemical techniques guided by quantum‐chemical calculations. Emphasis is placed on the mixed‐valence properties of both triads in their radical anion states. The electrochemical and EPR results reveal that both BQ–TTF–BQ and BTCNQ–TTF–BTCNQ triads in their radical anion states behave as class‐II mixed‐valence compounds with significant electronic communication between the acceptor moieties. Density functional theory calculations (BLYP35/cc‐pVTZ), taking into account the solvent effects, predict charge‐localised species (BQ ^{. ?}–TTF–BQ and BTCNQ ^{. ?}–TTF–BTCNQ) as the most stable structures for the radical anion states of both triads. A stronger localisation is found both experimentally and theoretically for the BTCNQ–TTF–BTCNQ anion, in accordance with the more electron‐withdrawing character of the BTCNQ acceptor. CASSCF/CASPT2 calculations suggest that the low‐energy, broad absorption bands observed experimentally for the BQ–TTF–BQ and BTCNQ–TTF–BTCNQ radical anions are associated with the intervalence charge transfer (IV‐CT) electronic transition and two nearby donor‐to‐acceptor CT excitations. The study highlights the molecular efficiency of the electron‐donor TTF unit as a molecular wire connecting two acceptor redox centres.     

The Quest for Photoswitches Activated by Near‐Infrared Light: A Theoretical Study of the Photochemistry of BF2‐Coordinated Azo Derivatives

Recently synthesized BF₂‐coordinated azo derivatives have been proposed as photoswitches that operate in the optical window (λ=600–1200 nm) for use in bioimaging applications. Herein, we have theoretically analyzed these compounds and modified some substituents to analyze which properties of the molecule govern its photochemistry. Our results compare rather well with the available experimental data, so our methodology, based on density functional theory (DFT) calculations for the ground electronic state and time‐dependent‐DFT for the first excited electronic state, is validated. Through systematic modification of different substituents of the parent system, we designed compounds that are predicted to operate fully within the optical window. We also analyzed several molecules for which the cis isomer is the more stable isomer, a quite unusual result for azobenzene derivatives that is a much coveted property for some applications of these photoactive molecules in pharmacology. Our results also provide insight into other properties relevant for photoswitches, such as the thermal stability of the less stable isomer and the magnitude of the gap between the wavelengths of the radiation that activates each isomerization process, which must be as large as possible to improve the yield of each photoisomerization. From a more general perspective, our results may provide a step towards the rational design of new photoswitches that fulfill a set of desired characteristics.     

Structural, magnetic coupling and oxidation state trends in models of the CaMn4 cluster in photosystem II

Density functional theory calculations are reported on a set of isomeric structures I, II and III that share the structural formula [CaMn4C9H10N2O16]q+.(H2O)3 (q= -1, 0, 1, 2, 3). Species I has a skeletal structure, which has been previously identified as a close match to the ligated CaMn4 cluster in Photosystem II, as characterized in the most recent 3.0 angstroms crystal structure. Structures II and III are rearrangements of I, which largely retain that model's bridging ligand framework, but feature metal atom positions broadly consistent with, respectively, the earlier 3.5 and 3.2 angstroms crystal structures for the Photosystem II water-oxidising complex (WOC). Our study explores the influence of the cluster charge state (and hence S state) on several important properties of the model structures; including the relative energies of the three models, their interconversion, trends in the individual Mn oxidation states, preferred hydration sites and favoured modes of magnetic coupling between the manganese atoms. We find that, for several of the explored cluster charge states, modest differences in the bridging-ligand geometry exert a powerful influence over the individual manganese oxidation states, but throughout these states the robustness of the tetrahedron formed by the Ca and three of the Mn atoms remains a significant feature and contrasts with the positional flexibility of the fourth Mn atom. Although structure I is lowest in energy for most S states, the energy differences between structures for a given S state are not large. Overall, structure II provides a better match for the EXAFS derived metal-metal distance parameters for the earlier S states (S0 to S2), but not for S3 in which a significant structural change is observed experimentally. In this S state structure III provides a closer fit. The implications of these results, for the possible action of the WOC, are discussed.     

Oligotriarylamines with a Pyrene Core: A Multicenter Strategy for Enhancing Radical Cation and Dication Stability and Tuning Spin Distribution

Monoamine 1 , diamines 2 – 4 , triamine 5 , and tetraamine 6 have been synthesized by substituting dianisylamino groups at the 1‐, 3‐, 6‐, and/or 8‐positions of pyrene. Diamines 2 – 4 differ in the positions of the amine substituents. No pyrene–pyrene interactions are evident in the single‐crystal packing of 3 , 4 , and 6 . With increasing numbers of amine substituents, the first oxidation potential decreases progressively from the mono‐ to the tetraamine. These compounds show intense charge‐transfer (CT) emission in CH₂Cl₂ at around 530 nm with quantum yields of 48–68 %. Upon stepwise oxidation by electrolysis or chemical oxidation, these compounds were transformed into radical cations 1 ^?+– 6 ^?+ and dications 2 ²⁺– 6 ²⁺, which feature strong visible and near‐infrared absorptions. Time‐dependent density functional theory studies suggested the presence of localized transitions from the pyrene radical cation and aminium radical cation, intervalence CT, and CT between the pyrene and amine moieties. Spectroscopic studies indicated that these radical cations and dications have good stability. Triamine 5 and tetraamine 6 formed efficient CT complexes with tetracyanoquinodimethane in solution. The results of EPR spectroscopy and density functional theory calculations suggested that the dications 2 ²⁺– 4 ²⁺ have a triplet ground state, whereas 5 ²⁺ and 6 ²⁺ have a singlet ground state. The dication of 1,3‐disubstituted diamine 4 exhibits a strong EPR signal.