Ion-pair Charge-transfer Band Research Articles

Electrochemical and spectral properties of hexacoordinate polypyridyl silicon complexes

Various salts of tetracationic tris(2,2′-bipyridine)silicon(IV) and bis(2,2′;6′,2″-terpyridine)silicon(IV) were synthesized by reaction of SiI 4 with the appropriate ligand followed by metathesis reactions. The resulting salts exhibit multiple, chemically reversible one-electron reduction waves that are significantly shifted to less negative reduction potentials than the corresponding ruthenium analogs. The salts also exhibit anion dependent ion pair charge transfer bands throughout the visible spectrum. Weak, low energy emission is observed from Si(bipy) 3Br 4 and Si(terpy) 2Br 4 in the solid state at 77 K when the sample is excited in the ion pair charge transfer band.

Fluorescence properties of ion pair charge transfer complex with bithiophene group

The fluorescence properties of ion pair charge transfer (IPCT) complex with bithiophene group were studied by steady-state and time-resolved measurements in various solvents. The photophysical properties of 5,5′-di(4-pyridyl)-2,2′-bithiophene (DPBT) were identical to the unsubstituted bithiophene except for the peak wavelength. The absorption of polymeric 5,5′-di(4-pyridiniumyl)-2,2′-bithiophene (PBT 2+) salts showed both the locally excited state (LE) and intramolecular charge transfer (ICT) bands in all solvents studied. In less-polar solvents such as cyclohexane and toluene, absorption spectra showed an IPCT band in addition to the LE and ICT bands. The fluorescence spectra of polymeric PBT 2+ salts showed dual emission which originated from the LE and ICT states in polar solvents. The ICT fluorescence lifetime strongly depends on the solvent polarity. The fluorescence from the PBT 2+ salts was effectively quenched by their counter ion in less-polar solvent. These results indicate that the counter halide anion affects on the excited-state relaxation process.

Ion-pair charge transfer complexes with intense near IR absorption: Syntheses, crystal structures, electronic spectra and DFT calculations

Five ion-pair complexes, consisting of R-benzylidene-1-aminopyridinium derivatives and [Ni(mnt) 2] 2− (R = p-nitro ( 1), p-methyl ( 2), p-bromo ( 3), p-chloro ( 4) and m-nitro ( 5); mnt 2− = maleonitriledithiolate), were synthesized and structurally characterized. As for 1, it is interesting to observe a large deviation from square-planar coordination geometry for the Ni atom, while no deviation is observed in the other four complexes. In the solid state, UV–Vis–NIR spectra of 2– 5 show similar properties with intense absorption in the 200–750 nm and moderate near IR absorption in the 750–1000 nm region, whereas 1 exhibits an intense absorption from UV/Visible to near-IR region (200–1100 nm). This unique spectral feature of 1 is attributed to its distinctive structural differences from 2 to 5, namely the strong intermolecular packing interactions between anions and cations, as well as a significant deviation from the planarity of the anion. Based on DFT and TDDFT calculations, near-IR absorbance bands in 1– 5 were assigned to combined transitions of d–d, MLCT and π–π ∗ in the [Ni(mnt) 2] 2− anion as well as the ion-pair charge transfer (IPCT) from the anionic HOMO to the cationic LUMO. The IPCT band position in acetonitrile is independent of the substituent group feature in benzene ring of cations for 1– 5, which could be interpreted that the substituent group in benzene ring only has a minor contribution to the cationic LUMO.

Ion Pairs between Maleonitriledithiolato Complex Dianions of Cobalt and Nickel and Macrocyclic Ligand Complex Dications of Nickel(II) — Control of Intrapair Interaction through Ligand Modification

Abstract The ion pairs {[Ni(cyclam)]2+[M(mnt)2]2−} (M = Co, Ni: 1a,b), {[Ni(tmc)]2+[M(mnt)2]2−} (M = Co, Ni: 2a,b) and the solvent complexes {[Ni(cyclam)(dmso)2]2+[M(mnt)2]2−} (M = Co, Ni, mnt2− = maleonitriledithiolato = (Z)1,2-dicyano-1,2-ethylenedithiolato, cyclam = 1,4,8,11-tetraazacyclotetradecane, tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) were isolated and structurally characterised. The nature of the macrocyclic ligand and the presence of coordinating solvents determine whether the dication is present as a tetra-, penta- or hexacoordinated complex. In 1a the tetracoordinated dications and dianions are arranged in a mixed stack structure; the absences of any short contacts and of a mutual ion pair charge transfer band indicate the absence of charge transfer interaction. The highly puckered cyclam ligand will prevent a close approach of the planar bis(maleonitriledithiolato)cobaltate(II). When cyclam is replaced by its tetramethyl derivative tmc, a strong coordinative interaction is introduced as indicated by the structure of 2a. One dianion coordinates to two dications through two trans-cyano groups. The resulting trinuclear dications {[Ni(tmc)][Co(mnt)2][Ni(tmc)]}2+ form segregated columns separated by the non-interacting, remaining dianion. Crystallisation in the presence of a coordinating solvent like dimethyl sulfoxide (dmso) leads to the ion pairs {[Ni(cyclam)(dmso)2]2+[M(mnt)2]2−}containing hexacoordinated dications. Magnetic susceptibility measurements at room temperature are in accord with the observed structures. While 1b is diamagnetic, 1a, 2a, 2b, and the two solvent complexes exhibit values characteristic of the isolated ions, in agreement with the absence of any electronic interaction between dication and dianion within the ion pair. Temperature- dependent measurements down to 1.7 K indicate that a weak antiferromagnetic interaction is present in 2a.

Chiral Metal-Dithiolene/Viologen Ion Pairs: Synthesis and Electrical Conductivity

Enantiomerically pure dithiolene complexes NBu4[Ni[(R,R)-diotte)2] and NBu4[Ni((S,S)-diotte]2] (diotte2- = a 1,3-dioxolane-tetrathiaethylene), were prepared from the corresponding enantiomers of a diotte2- precursor. The structure of the precursor was solved by single-crystal X-ray analysis; desulfurization afforded a novel tetrathiafulvalene derivative. Combination of the complex monoanion with the enantiomers of the viologen derivative bis(2-methyl-3-hydroxypropyl)-4,4'-dipyridinium (HiBV2+) afforded enantiomeric and diastereomeric ion-pair complexes of the type HiBV[Ni(diotte)2]2. For comparison, the analogous compounds A[Ni(diotte)2]2, (A2+ = methyl (MV2+), octyl (OV2-), stearyl (StV2+) viologen or two 2,2'-bipyridinium acceptors), HiBV-[Ni(diotte)L] [L = mnt2- (maleonitrile-1,2-dithiolate), dmit2- (2-thioxo-1,3-dithiol-4,5-dithiolate)], MV[Ni(dmit)2)]2, [Ni(diotte)2], and [Ni(diotte)(dmit)] were synthesized. An X-ray powder diffraction structural analysis of MV-[Ni(dmit)2)]2 revealed the presence of mixed stacks that contain the sequence anion-anion-cation. While no short contacts are observable within a stack, these are observed between the stacks for the dication-anion interaction by short S...H distances in the range of 2.77 to 2.86 A, and for the anion-anion interaction short S...S distances of 3.55 to 3.65 A. In agreement with the absence of intrastack interactions, no ion-pair charge-transfer band can be detected in this and the other complexes. ESR and UV/Vis data suggest that in [Ni(diotte)2]- electron delocalization is less pronounced than in the corresponding mnt2- and dmit2- complexes. The specific electrical conductivity (sigma) of pressed powder pellets ranges from 10(-2) to 10(-12) ohm(-1) cm(-1) and in all cases increases with increasing temperature (293 - 393 K) according to an Arrhenius law. Corresponding activation energies vary from 0.14 to 0.93 eV and increase linearly with log a for structurally similar ion pairs. Charge generation is postulated to occur by disproportionation of the monoanion as suggested by the almost linear increase of log(sigma) with decreasing disproportionation energy. The conductivity of diastereomers of ions with two unlike configurations like [(S,S)-HiBV]-[Ni[(R,R)-diotte]2]2 (1.1 x 10(-1) ohm(-1) cm(-1)) is one to two orders of magnitude higher as compared to the diastereomers with two like-configured ions.

Azobipyridinium Dications and Radical Monocations as Acceptors

Combination of the planar redox-active ions [ML2]2– (L = mnt2– = maleonitrile-1,2- dithiolate; M = Ni (1), Pd (2), Pt (3); dmit2– = 2-thioxo-1,3-dithiol-4,5-dithiolate: M = Ni (4), Pd (5) and trans-4,4′-azobis(1-methyl-pyridinium), (a2+) affords 1:1 ion pairs exhibiting partial and complete electron transfer as evidenced by UV-Vis and EPR spectra. Replacement of planar a2+ by a non-planar dipyridinium ketone b2+ leads to the complexes 1b and 4b. 1a, 2a, and 3a are predominantly composed of dications and dianions while 4a, 5a, and 4b are rare examples of ion pairs consisting of two radical ions. Single crystal X-ray structural analyses was performed on 4a, a(PF6)2, and a(MeSO4)2 while the structure of 1a was resolved by powder X-ray diffractometry. The latter consists of mixed stacks of planar dianions and dications forming a slipped arrangement with the centers of the two ions displaced relative to each other by 250 pm. The short interplanar distances of 340 pm are in agreement with the presence of a weak charge-transfer interaction as indicated by the corresponding absorption band at about 1400 nm. A mean reorganization energy of 0.85 ±0.04 eV is calculated from the Hush equation for complexes 1a, 2a, and 3a. No ion pair charge-transfer band is observable for 4a, 5a, and 4b. Surprisingly, in the solid state structure of 4a the [Ni(dmit)2]− monoanions do not form segregated columns as found in the many ion pairs with redox inert counterions, but prefer a mixed stack arrangement as observed also for 1a.The specific electrical conductivity of pressed powder pellets of complexes exhibiting a charge-transfer band is in the range of 2 × 10−7 to 1 × 10−5 Ω−1cm−1, increasing with increasing driving force of electron transfer in accordance with previous results. Different to that, the conductivity of the other complexes does not depend on driving force and is much higher (2 × 10−4 to 4 × 10−4 Ω−1cm−1).

Ultrafast optical switching by photoinduced electrochromism in cast films of polymeric 4,4′-bipyridinium salts with di-iodides

Femtosecond dynamics of color changes due to photoinduced electron transfer and reverse reactions in polymeric 4,4′-bipyridinium di-iodides salts were studied with a fs laser pump–probe technique. The characteristic color change was shown to take place in less than 1 ps by fs-laser excitation of an ion-pair charge-transfer band, which may be applied to ultrafast THz all-optical switching devices using visible light.

Ion-pairing and charge-transfer interactions between octacyano-molybdates(IV) and -tungstates(IV) and diquaternary bipyridines

New salts of formula [pq]2[M(CN)8] [M = Mo or W, pq = paraquat (1,1′-dimethyl-4,4′-bipyridinediium)] have been synthesized. The systems {A2+, M(CN)84–} where A2+ = pq or dq (diquat, 6,7-dihydrodipyrido[1,2-a∶2′,1′-c]pyrazinediium), which show optical electron transfer from anion to cation, were studied. The first association constants in aqueous solution were determined spectroscopically and the properties of the ion-pair charge-transfer bands were used for calculation of the thermodynamic parameters of the electron transfer and construction of potential-energy curves on the basis of the Hush model.

Outer Sphere Metal-to-Ligand Charge Transfer in Organometallic Ion Pairs

A series of organometallic salts which comprise a fac-(b)ReI(CO)3(py)+ cation (b = 4,4‘,5,5‘-tetramethyl-2,2‘-bipyridine (tmb), 2,2‘-bipyridine (bpy), or 4,4‘-dicarbomethoxy-2,2‘-bipyridine (dmeb); py = pyridine) paired with the Co(CO)4- anion have been prepared and subjected to photophysical study. In nonpolar solvents the salts feature a broad, low-intensity ion-pair charge transfer (IPCT) absorption band. The energy of the IPCT band decreases with the LUMO energy of the diimine ligand, suggesting that the orbital basis of the transition is d (Co) → π* (b). An X-ray crystal structure of [(bpy)ReI(CO)3(py)+][Co(CO)4-] (2a) reveals that the anion occupies a lattice position which is directly below (or above) the plane defined by the bpy ligand, which supports the d (Co) → π* (b) IPCT assignment. Luminescence studies of the salts indicate that the dπ (Re) → π* (b) metal-to-ligand charge transfer (MLCT) excited state is quenched by reductive electron transfer from Co(CO)4-. Nominally IPCT and MLCT excitatio...

Metal-organic ion pairs with dithiooxalate d 8 metal donors and viologens. Spectroscopic, structural and electronic characterization

Ion-pair charge-transfer complexes of the type {A 2+[M(dto) 2] 2−}, where A 2+ represents bipyridinium and phenanthrolinium compounds of different redox potential, M=Ni, Pd, Pt, and dto=1,2-dithiooxalate, were synthesized and characterized with respect to their spectroscopic, structural and electronic properties. A weak absorption band at the red-tail of the low-energy two-band system of the complex donor component can be attributed to the ion-pair charge-transfer (IPCT) transition type. As expected from electrochemical data, the IPCT band of these systems is markedly hypsochromically shifted compared with ion pairs containing dithiolene donors. The different nature of the highest occupied MO may offer an explanation for the distinct electro- and photochemical properties. Applying the Hush model reorganization energies are estimated in the range 107–125 kJ mol −1 for the thermal electron transfer from [Ni(dto) 2] 2− to the acceptor site. The amount of charge that is transferred from the donor to the acceptor ( α 2) is calculated as 10 −5–10 −6, lowered by 2–3 orders of magnitude compared with the metal dithiolenes. As shown by X-ray analysis of HEM[Ni(dto) 2] (HEM= N-2-hydroxyethyl- N′-methyl-4,4′-bipyridinium cation), the molecular structure consists of alternating donor–acceptor stacks with a typical plane-to-plane distance of 345 pm. In addition to supramolecular interactions both components are bonded by a hydrogen bridge between the OH group of the cation and the electron-rich oxygen of the dto ligand.

Ion Pair Charge-Transfer Complexes of Dithiolatocobaltates with Cationic Copper or Nickel Macrocycles: Crystal Structure and Magnetic Properties

Abstract The complexes {[M(tim)]2+[Co(mnt)2]2−}, (M = Ni (NiCo), Cu (CuCo), tim = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene, mnt2− = cis-1,2-dicyano-1,2-ethylenedithiolato) have been isolated. NiCo and CuCo have isomorphous crystal structures (P1-), the basic feature of which is the presence of mixed donor–acceptor chains induced by short intrachain distances of 3.15 and 3.03 Å, respectively. For both NiCo and CuCo, an ion pair charge-transfer band is observed at 707 and 843 nm; application of the Hush model affords reorganization energies for electron transfer from the dianion to the dication of 89 and 104 kJ mol−1, respectively. The magnetic properties of the powder samples have been studied in the temperature range of 2 K to 300 K. In NiCo the diamagnetic [Ni(tim)]2+ acceptor induces antiferromagnetic coupling of the spin a half system [Co(mnt)2]2− along the a-axis over a distance of 6.3 Å. From the exchange coupling constant of J = −37 cm−1 a Weiss temperature of 26 K is calculated. Contrary to that, the magnetic orbitals of [Cu(tim)]2+ and [Co(mnt)2]2− remain uncoupled in CuCo.

Ion-pair charge-transfer complexes of a dithiooxalate zinc donor component with viologens. Synthesis, structural and electronic characterization

Bipyridinium and phenanthrolinium acceptors of different reduction potentials form with zinc 1,2-dithiooxalates (dto) ion-pair charge-transfer complexes of the general formula {A 2+[Zn(dto) 2] 2−}. The contact ion pairs exhibit absorptions in the range 390–490 nm which can be attributed to the ion-pair charge-transfer (IPCT) type. On the base of spectroscopic, electrochemical and quantum-chemical investigations the relation between optical and thermal electron transfer within the ion pair applying the Hush theory is discussed. The mean reorganization energy of 12 complexes is 180 U mol −1 and exceeds the values found for the dithiolene systems. Due to the diminished donor ability of the dithiooxalate unit, a hypsochromic shift of the position of the IPCT band, compared with the metal dithiolenes, results. The extent of electron delocalization from [Zn(dto) 2] 2− to pQ 2+, as described by the parameter α 2, is calculated as 4.1 × 10 −6. X-ray analyses of BQ[Zn(dto) 2] and DP[Zn(dto) 2] reveal that the solid-state structure is largely determined by the geometry of the acceptor component.

Electron Transfer Modelling of Electrical Dark- and Photoconductivity of Redoxactive Ion Pairs

Abstract Ion pair charge-transfer (IPCT) complexes consisting of dianionic dithiolene metallates [ML2]2- M = Ni, Pd, Pt, Cu, Zn, and dicationic 4,4′- and 2,2′-bipyridinium derivatives (A2+) exhibit IPCT bands in the VIS-NIR region of the electronic absorption spectrum. When both components are planar, the solid state structure consists of mixed donor–acceptor stacks while that is not the case when one or both are non-planar. By proper selection of the two components the driving force of electron transfer (ΔG 12) from the dianion to the dication is varied from 0.7 to -0.1 eV. It is shown that the Hush relation between optical and thermal electron transfer is fulfilled for a number of thirty-two complexes. The reorganization energy is in the range of 60 kJ·mol−1 for the d8 complexes while it is almost two times larger for the d10 zinc compounds. The extent of charge delocalization is typical for outer-sphere complexes as indicated by the parameter α2 which is in the range of 10−4. Correspondingly, the interaction between the two redox states is rather weak as suggested by the values of 220-360 cm−1 calculated for the exchange matrix element. Free activation enthalpies ΔG∗ of electron transfer, as calculated within the Hush–Marcus model, amount from 0.15 to 0.73 eV. It is found that the electrical dark conductivity of these composite solids can be quantitatively predicted from ΔG∗ in the range from 10−10 to 10−3 ω−1 cm−1 in the case of complexes consisting of planar components (Class I) while in the presence of a nonplanar acceptor or copper as the central metal (Class II) no similar relation is observed. The electrical photoconductivity is wavelength dependent and exhibits a maximum in the region of the IPCT band. Laser excitation of the solids produces a transient photovoltage (Dember Voltage) with a half-life of about 20 milliseconds. The rcsults suggest that charge generation occurs by electron transfer and charge migration by a hopping mechanism. When a photoisomerizable olefinic acceptor is employed, the conductivity is ten times larger for the cis-isomer.

Ultrafast photon-mode recording by novel photochromic polymer via photoinduced electron transfer

The color change of a novel photochromic polymer film of 4,4′-bipyridinium-tetrakis [3,5-bis(trifluoromethyl)phenyl]borate was shown to occur in less than 20 ps by the ps-laser excitation of an ion-pair charge-transfer band due to photoinduced electron transfer, which may be applied to ultrafast photon-mode recording.

First charge resonance band observed by steady photolysis at room temperature in solution

The steady excitation of an ion-pair charge-transfer band of 1-octadecyl-4-(4-nitrostyryl)pyridium tetraphenylborate formed a fairly stable 1-octadecyl-4-(4-nitrostyryl)pyridinyl radical accompanying, for the first time, a charge resonance band owing to its interaction with a parent cation at room temperature in organic solutions.

Photoconductivity of metal dithiolate ion-pair complexes

Photoelectric measurements were performed on powder samples of ion-pair complexes of mono- and dianionic nickel and zinc dithiolenes with redox-active cations like 2,2′-and 4,4′-bipyridinium or simple tetraalkylammonium derivatives. From the dependence of the photoconductivity on temperature, applied electric field and wavelength of exciting light it is suggested that charge generation is dominated by optical ion-pair charge transfer (IPCT) from the anion to the cation. Accordingly, low energy photoconductivity maxima appear in the region of the IPCT bands and are shifted linearly to higher energy when, for a given donor like [ZnL 2] 2−, the reduction potential of the acceptor becomes more negative.

Charge effects on the photochemistry of the Co(EDTA) −·I − system in the presence of polyammonium macrocyclic receptors

The effects of polyammonium macrocycles on the spectroscopic and photochemical properties of the Co(EDTA) −·I − ion-pair have been investigated. The addition of a macrocycle to aqueous solutions containing Co(EDTA) − and I − causes an increase of the absorbance in the region of the ion-pair charge-transfer band, as well as an increase of the quantum yield for the intramolecular photooxidation reduction of the ion-pair. Both these effects are mainly, if not only, due to an increase of the association constant between Co(EDTA) − and I −, caused by the positive charge of the macrocycle bound to the complex. On the contrary no change was observed on the intrinsic photoreactivity of the excited ion-pair. This last result is discussed in comparison with the effects already observed on the ligand photodissociation of MC excited states of Co(III) cyanide complexes.

Novel photochromic polymer films containing ion-pair charge-transfer complexes of 4,4′-bipyridinium ions for optical recording

Elastic polymer films containing 4,4′-bipyridinium salts with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as part of the main chain showed persistent and reversible colour changes due to photoinduced electron transfer upon excitation of an ion-pair charge-transfer band (λex >365 nm)in vacuo with the lifetime of the coloured state markedly dependent on temperature.

Photochemistry of the Co(Sep) 3+I − system: evaluation of the quantum yield for the photochemical redox reaction of the ion pair

Spectrophotometric measurements in the region of the ion pair charge transfer band of the Co(Sepulchrate) 3+I − system were used to assess the role played by the adducts between the complex cation and iodide anion in the photochemical behaviour of the system. No clear evidence was obtained for the presence or absence of 1:2 or higher adducts and the association constant and molar absorptivity of the 1:1 adduct (ion pair) could not be determined with a satisfactory accuracy. Nevertheless, reliable values were evaluated for the fraction of the exciting light absorbed by the ion pair in the photochemical experiments, and these values were used to correct the previously reported quantum yields, which were based on the total amount of light absorbed by the solutions. The “true” quantum yields for the ion pair are discussed in relation to the proposed reaction mechanism.

Photodecarboxylation of citrate through ion pair photochemistry: the Co(Sep)3+·citraten− (n = 1,2,3) system

Abstract The Co(Sep) 3+ complex (Sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane) forms ion pairs with citrate in aqueous solution at pH 4.0, 5.6 and 8.5, where the forms CIT − , CIT 2− and CIT 3− (CIT = citrate) predominate respectively. Irradiation in the ion pair charge transfer band for the three pH values leads to the decarboxylation of citrate with the production of CO 2 and Co(Sep) 2+ . In the presence of colloidal platinum, CO 2 and H 2 are evolved. The quantum yields for formation of CO 2 , Co(Sep) 2+ and H 2 were obtained and are discussed in terms of their dependence on pH.