A binary ionic liquid (ILs)-based electrolyte with advanced physicochemical properties has been successfully prepared.The ionic conductivity (σ) values of ILs [C4mim][PF6] and [C4mim][Tf2N] were measured using electrochemical impedance spectroscopy are 2.12 and 4.75 mS/cm at 30 C, respectively.The results of electrode kinetics measurement using cyclic voltammetry revealed a value of the diffusion coefficient (D) of ferrocene in [C4mim][PF6] of 3.3 x 10 -7 cm 2 /s and [C4mim][Tf2N] 1.60 x 10 -6 cm 2 /s.The ferrocene complex obtained has an electron transfer rate constant (ks) of 7.9 x 10 -4 cm/s in [C4mim][PF6] and 1.82 x 10 -3 cm/s in [C4mim][Tf2N].The addition of [C4mim][Tf2N] to [C4mim][PF6] increased ionic conductivity, diffusion coefficient, and electron transfer rate constant overall, but only to the range of pure ILs.Furthermore, changes in conductivity, diffusion coefficient, and electron transfer rate constant value are proportional to temperature changes, according to the Arrhenius equation.Based on the diffusion activation energy (EaD) and the slight decrease in conductivity with temperature, [C4mim][PF6]0.50-[C4mim][Tf2N]0.50 is a superior electrolyte candidate at low-temperature operation.

The rotational and translational diffusion of negatively charged and uncharged spin probes in five imidazolium-based room-temperature ionic liquids (RTILs), 1-ethyl-3-methylimidazolium tetrafluoroborate, emimBF4, 1-butyl-3-methylimidazolium tetrafluoroborate, bmimBF4, 1-octyl-3-methylimidazolium tetrafluoroborate, omimBF4, 1-octyl-3-methylimidazolium hexafluorophosphate, omimPF6, and 1-octyl-3-methylimidazolium chloride, omimCl, has been studied by means of electron paramagnetic resonance spectroscopy. Detailed analyses of the spin-Hamiltonian parameters and spin exchange interactions have been carried out. The temperature dependences of the line broadening induced by the electronic dipole-dipole interaction and the electron spin exchange coupling are determined. The translational mobility of spin probes is semiquantitatively characterized and successfully explained in the framework of a hypothesis based on the assumption of polar and unpolar domains within the RTILs.

Temperature dependences of electron paramagnetic resonance (EPR) spectra of an imidazoline nitroxide biradical spin probe in a series of room-temperature ionic liquids in the temperature range 124-390 K have been quantitatively simulated. The unusual asymmetric EPR spectrum shape previously observed in these systems [Kokorin et al., Appl. Magn. Res. 48 (2016) 287] is shown to originate from anisotropic rotational diffusion of the probe molecule. All experimental spectra were quantitatively reproduced in simulation using a unified set of geometrical and magnetic parameters of the spin probe, which were found to be fully consistent with the biradical geometry obtained from density functional theory calculations. Temperature dependences of rotation diffusion coefficient of the probe characterize the molecular mobility of the ionic liquid, whereas the temperature dependences of the spin-exchange integral J and of the isotropic hyperfine interaction constant, aN, are shown to reflect the intramolecular conformation motions of the biradical probe.

Three water-soluble tris-heteroleptic ruthenium(II) polypyridyl complexes [Ru(bpy)(phen)(bpg)](2+) (1), [Ru (bpy)(dppz)(bpg)]2+ (2), and [Ru(phen)(dppz)(bpg)]2+ (3) (where bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, dppz = dipyrido[3,2-a:2',3'-c] phenazine, bpg = 4b,5,7,7a-tetrahydro-4b,7a-epiminomethanoimino-6H-imidazo[4,5 f] [1,10] phenanthroline-6,13-dione) have been synthesized and characterized. Molecular structures of complexes 1 and 3 are confirmed by single crystal X-ray structure determination. Interaction of complexes 1-3 with DNA is explored by various spectroscopic techniques. The complexes 1-3 show solvent dependent photophysical properties. Complexes 2 and 3 show extensive "molecular light switch" effect for DNA. The complexes 1-3 are low toxic towards HeLa (human cervical cancer) and HL-60 (human promyelocytic leukemia) cell lines. Further, the cellular uptake of complexes 2 and 3 by cells shows that complexes mainly localised on the nucleus of the cells.

Intramolecular electron spin exchange has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two long-chain flexible nitroxide biradicals existing in fluid solutions in three spectroscopy-different spatial conformations as a function of temperature, solvent viscosity and polarity. Certain thermodynamic parameters of the conformational transitions were calculated from the EPR spectra. The process of spin-exchange in these biradicals dissolved in five different alcohols was compared with that in the non-polar solvents (toluene) and aprotic (acetonitrile), as well as with two other biradicals studied earlier, and with thermodynamic characteristics of the solvents. A distinct correlation was found between macroscopic (solvent viscosity) characteristics of solvents and thermodynamic parameters of the intramolecular conformational transitions.

The heterogeneous rate constants (k(s)) for the oxidation of iron-and ruthenium-bipyridine complexes, [Fe(bpy)(3)](2+/3+) and [Ru(bpy)(3)](2+/3+) have been determined using Nicholson's method in imidazolium-based ionic liquids with five different types of cation/anion over a range of temperature from 298-318 K. The heterogeneous rate constants of these redox reactions range from 10(-4) to 10(-3) cm s(-1), depending on the dynamic viscosity and type of cation/anion of the ILs. Marcus-Hush theory is used to explain the activation energies, E-a. Similar activation energies are found for the Fe-bipyridine and Ru-bipyridine complexes indicating that inner-sphere reorganization energy has only a small influence. For the calculation of the solvent-dependent outer sphere reorganization energy a dipole-free expression is used, adopted to the different dielectric properties of ionic liquids compared to organic solvents. i-R-u drops have been compensated using the corresponding experimental uncompensated R-u-values obtained from temperature dependent Bode-plots. (C) 2019 The Electrochemical Society.

Donor-acceptor systems forming exciplexes are versatile models for the study of magnetic field effects (MFEs) on charge recombination reactions. The MFEs originate from singlet-triplet interconversion within transient radical ion pairs (RIPs), which exist in a dynamic equilibrium with the exciplexes. Here, we describe the synthesis and MFEs of the chain-linked N,N-dimethylaniline (DMA)/9-methylanthracene (MAnt) donor-acceptor system MAnt-(CH2) n -O-CH2-CH2-DMA for n = 6, 8, 10, and 16. The MFEs are found to increase with increasing chain length. Effects as large as 37.5% have been observed for the long-chain compound with n = 16. The solvent dependence of the MFEs at magnetic field intensity 75 mT is reported. For the range of solvent static dielectric constants εs = 6.0-36.0, the MFEs go through a maximum for intermediate polarities, for which the direct formation of RIPs prevails and their dissociation and reencounter are balanced. Field-resolved measurements (MARY spectra) are reported for solutions in butyronitrile. The MARY spectra reveal that for n = 8, 10, 16, the average exchange interaction is negligible during the coherent lifetime of the radical pair. However, singlet-triplet dephasing broadens the lineshape; the shorter the linker, the more pronounced this effect is. For n = 6, a dip in the fluorescence intensity reveals a nonzero average exchange coupling of the order of ±5 mT. We discuss the field-dependence in the framework of the semiclassical theory taking spin-selective recombination, singlet-triplet dephasing, and exchange coupling into account. Singlet recombination rates of the order of 0.1 ns-1 and various degrees of singlet-triplet dephasing govern the spin dynamics. In addition, because of a small free energy gap between the exciplex and the locally excited fluorophore quencher pair, a fully reversible interconversion between the RIP, exciplex, and locally excited fluorophore is revealed by spectrally resolved MFE measurements for the long-chain systems (n = 10, 16).

A short nitroxide biradical: O=S(OR6)2 (BS), where OR6 is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, has been studied by electron paramagnetic resonance spectroscopy at X-, Q-, and W-band in liquid and frozen toluene, ethanol and ionic liquid solutions. Variations of the intramolecular dynamics and conformational transitions in the biradical as a function of temperature in the range of 240–420 K, polarity and the ionic strength were characterized by changes in the isotropic 14N hyperfine splitting (hfs) constant a, values of the exchange integral |J|, and the empirical parameter γ3, the ratio between conformations with slow and fast transitions. Thermodynamic parameters of the conformational rearrangements are calculated. The obtained results were compared with the X-ray structural data and quantum chemical calculations of the geometries and intramolecular transitions of biradical BS. Possible mechanisms of the polarity and the ionic strength effect on the biradical behavior in solutions of different types are discussed.

A binary system of two ionic liquids, 1-ethyl-3-methylimidazolium and trioctylmethylammonium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2 ] and [OMA][NTf2 ], respectively), with varying molar fractions, is introduced. It allows the dynamic viscosity to remain constant over a range of almost 60 K; this means that any activated process can be studied independent of the temperature dependence of viscosity itself. This principle is proven upon reinvestigation of electron self-exchange kinetics of tetrathiafulvalene by continuous-wave ESR line-broadening experiments. From these results, it is also confirmed that this process is totally diffusion controlled.

Photooxidation kinetics of phenol, 1-naphthol, 2-naphthol, tyrosine (TyrOH) and N-acetyl-tyrosine (AcTyrOH), tryptophan (TrpH) by ruthenium(II) polypyridyl complexes: [Ru(bPY)(3)]Cl-2 (1), [Ru(phen)(3)]Cl-2 (2), [Ru(bpy)(phen)(bpg)]Cl-2 (3), and [Ru(dpg)(2)(bxbg)]Cl-2 (4) where bpy is 2,2'-bipyridine, phen - 1,10-phenanthroline, bpg - bipyridine-glycoluril, dpq - dipyrido[3,2-d:2',3'-f]quinoxaline, and bxbg - bis(oxylene)bipyridine-glycoluril are investigated. Rate constants have been measured by steady-state luminescence and phase-modulation fluorometry in aqueous solutions at different pH's. The rates for the oxidation of the phenols and phenolic aromatic amino acids spreads over a wide range from 4.2 x 10(5) to 6.8 x 109 M-1 s(-1), depending on pH and the nature of solutes. At pH > pK(a) of the quenchers, the presence of reactive species (PhO-) in the alkaline solutions is accounted for the rapid ET rates. In the pH range between 4 and 10 (pH < pK(a)), the ETPT mechanism becomes dominate and the rate constants are relatively low. It reveals that the important parameters that influence the quenching reaction rates, others than the driving forces AG are the steric and hydrophobic interactions arising from the structure of the compounds. This is clearly seen in the case of photoreaction between the Ru(phen)(3)(2+) complex and AcTyrOH. Phen ligands and acetyl group cause a steric effect, but strengthen the hydrophobic interactions and thus promote the quenching process. The pH-dependent equation of the observed rate constant for PhOH/AcTyrOH oxidation is expressed as a sum of rates for its protonated, neutral and deprotonated forms. (C) 2016 Elsevier B.V. All rights reserved.

To address the question whether donor substituents can be utilized to accelerate the hole transfer (HT) between redox sites attached in para- or in meta-positions to a central benzene bridge, we investigated three series of mixed valence compounds based on triarylamine redox centers that are connected to a benzene bridge via alkyne spacers at para- and meta-positions. The electron density at the bridge was tuned by substituents with different electron donating or accepting character. By analyzing optical spectra and by DFT computations we show that the HT properties are independent of bridge substituents for one of the meta-series, while donor substituents can strongly decrease the intrinsic barrier in the case of the para-series. In stark contrast, temperature-dependent ESR measurements demonstrate a dramatic increase of both the apparent barrier and the rate of HT for strong donor substituents in the para-cases. This is caused by an unprecedented substituent-dependent change of the HT mechanism from that described by transition state theory to a regime controlled by solvent dynamics. For solvents with slow longitudinal relaxation (PhNO2, oDCB), this adds an additional contribution to the intrinsic barrier via the dielectric relaxation process. Attaching the donor substituents to the bridge at positions where the molecular orbital coefficients are large accelerates the HT rate for meta-conjugated compounds just as for the para-series. This effect demonstrates that the para-meta paradigm no longer holds if appropriate substituents and substitution patterns are chosen, thereby considerably broadening the applicability of meta-topologies for optoelectronic applications.

A short-chain imidazoline-type nitroxide biradical R 5 NO –CH=N–N=C(CH3)–R 5 N , B2, with nitroxide rings R 5 N and R 5 NO as 1-oxyl-2,2,5,5-tetramethyl-3-imidazoline and 1-oxyl-2,2,5,5-tetramethyl-3-N-oxide imidazoline, respectively, has been studied using X-band electron paramagnetic resonance (EPR) spectroscopy in CH3CN solutions at variable temperature T and pressure P. Changes of the solution viscosity on the intramolecular electron spin exchange in B2 is characterized by calculating the value of the exchange integral |J/a|, where a denotes the 14N hyperfine splitting (hfs) constant. It is revealed that the intramolecular dynamics in B2 do not follow the Debye–Stokes–Einstein law, while the Arrhenius dependence is fulfilled. Probable reasons of such behavior are discussed.

Three imidazoline-type nitroxide biradicals of the similar composition R 5 NO –CH=N–N=CH–R 5 N , B1, R 5 NO –CH=N–N=C(CH3)–R 5 N , B2, and R 5 N –C(CH3)=N–N=C(CH3)–R 5 N , B3, with R 5 N and R 5 NO denoting, respectively, the nitroxide rings 1-oxyl-2,2,5,5-tetramethyl-3-imidazoline and 1-oxyl-2,2,5,5-tetramethyl-3-N–oxide imidazoline, have been studied by X-band electron paramagnetic resonance (EPR) spectroscopy. Variations of the intramolecular electron spin exchange in these biradicals dissolved in ethanol and the room temperature ionic liquid bmimBF4 were characterized as a function of temperature by means of the analysis of the EPR lines shape. Thermodynamic parameters of the conformational rearrangements in ethanol were calculated. Analyzing the EPR spectra of these biradicals in bmimBF4, it was revealed that the two-conformational model does not describe their conformational transitions. Moreover, the observed EPR spectra are not central symmetric especially at low temperatures that cannot be described and explained in the framework of the current theory of the intramolecular spin exchange. Probable reasons of this “strange” behavior are discussed.

Although Mn(III) porphyrins were considered earlier to be very weakly emissive, the fluorescence displayed by Mn(III) complexes with the cationic TMPyP2+ ligand (H2TMPyP4+ = 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin) as well as with its other alkyl (such as hexyl and dodecyl) derivatives proved to be strong enough for a comparative study. Elongation of the alkyl substituent increased both the quantum yield and the lifetime of the emission for the Mn(III) porphyrins, while resulted in an opposite effect for the corresponding free bases in homogeneous solutions. The presence of cationic micelles, however, reversed this tendency regarding the emission lifetime of the complexes. These cationic metalloporphyrins were applied in a photocatalytic system involving triethanolamine (TEOA) as a sacrificial electron donor and methylviologen (MV2+) as an acceptor. In the first step of the catalytic process outer-sphere photoinduced reduction of the metal center took place via quenching of the triplet excited state of these metalloporphyrins by TEOA. The corresponding manganese(II) porphyrins formed in this way were also photoactive; they forwarded an electron to MV2+ upon irradiation, regenerating the starting complex. Elongation of the alkyl substituents increased the quantum yield of the formation of methylviologen radical (MV(sic)+) in this system, but considerably decreased the durability of the photocatalyst. Anionic micelles totally hindered the photoinduced generation of Mn(II) porphyrins, while cationic micellar environment appreciably increased the efficiency of the accumulation of MV(sic)+. (C) 2016 Elsevier B.V. All rights reserved.

Electron transfer reactions in ionic liquids are profoundly affected by solvent properties. The activation barriers cannot be generally accounted for by Marcus' theory.

New 1/9- aminoanthracene-triazine-3-aminobenzanthrone bichromophores for the study of excitation energy transfer were synthesized and characterised. Absorption and emission spectra and fluorescence quantum yields were measured. Equilibrium ground state conformations and singlet electronic excited states characteristics were calculated at semiempirical level. A decrease of qF in polar solvents may be explained by the CT state connected with electron transfer from the HOMO localized on the donor to the LUMO spread over the acceptor. A highly efficient energy transfer mediated by through-bond mechanism was observed for all compounds.

The question whether chemical reactions and diffusion processes in ionic liquids are comparable with those taking place in classical organic liquids is a current issue in the literature. Pressure- and temperature-dependent investigations on simple electron self-exchange reactions between the two partners of a redox couple are good tools to get a better understanding of how the solvent influences such reactions. The electron self-exchange reaction between tetrathiafulvalene (TTF) and its radical cation has been investigated in two ionic liquids and two organic solvents using electron spin resonance (ESR) line broadening experiments at variable temperature and pressure. Rate constants are reported for the ionic liquids 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim(+)][Tf2N-]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim(+)][Tf2N-]) within a temperature range of 298K <= T <= 368K and a pressure range of 0.1 MPa <= p <= 100 MPa. The self-exchange reaction of the redox couple [TTF/TTF center dot+] has been found to be diffusion-controlled in the used ionic liquids over the entire temperature range. The observed rate constants in ionic liquids at higher pressures are larger than those predicted by common diffusion, and suggest that the electron transfer takes place within a solvent cage. Also, the self-exchange reaction of the [TTF/TTF center dot+] redox couple in classical solvents (dimethylphthalate (DMP) and acetonitrile) was investigated and compared to the results with those obtained in ionic liquids. The high viscosity of the ionic liquids makes it difficult to extract the electron transfer rate constants reliably, making interpretation within the framework of the Marcus Theory impossible.

Surface-modified titanium dioxides by highly dispersed NiO particles have an extended absorption in the visible light region and a reduced hole-electron pair recombination than unmodified TiO2. They have now been successfully applied as highly active heterogeneous photocatalysts in the visible light mediated direct cyclization of tertiary anilines with maleimides to give tetrahydroquinoline products in moderate to high yields at ambient temperature. In contrast with unmodified titanium dioxide catalysts that are conventionally used in a stoichiometric amount in combination with UVA light, only a catalytic amount (1 mol %) of the surface-modified TiO2 catalyst is needed along with visible light to efficiently catalyze the reaction. Compared with transition-metal complexes such as Ru(bpy)3Cl2 or Ir(ppy)2(dtbbpy)PF6, advantages of these surface-modified titanium dioxides as photocatalyst include high catalytic activity, low cost, ease of recovering, and being able to be used for at least nine times without significant decay of catalytic activity.

Intramolecular electron spin exchange as a function of temperature, solvent viscosity and polarity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy for two nitroxide biradicals containing mercury-organic groups in the bridge connecting two 1-oxyl-2,2,6,6-tetramethylpiperidine-3,4-ene-nitroxide rings, R. The temperature dependence of the isotropic hyperfine splitting (hfs) constant a N and the exchange integral value |J/a| of the biradicals were measured from EPR spectra and subsequently analyzed comparing to a N and a Hg hfs constants of ClHgR radical dissolved in the same solvents. In all cases, the interaction of solvent molecules (SM) with >N–O fragments of nitroxide rings led to a slight decrease in a values with increasing temperature. The |J/a| value varied slightly with temperature T changes. The changes of |J/a| are much less comparing to those with variation of the solvent polarity. The interaction between SM and Hg atoms inside the bridge is observed and discussed.

The triplet-triplet absorption spectra of three newly synthesized N-substituted 4,5,6,7-tetrachlorophthalimides (TCP) were measured experimentally and calculated with density functional theory. The heavy atom effect increases the intersystem crossing rate, and the transient triplet absorbance could be measured. Fluorescence emission was not observed. The transient absorption spectra show two peaks in the region of 385–410 nm and 770–830 nm having a red shift with increasing size of N substituent. The singlet and triplet states and their vertical transitions were investigated theoretically by NEVPT2/CASSCF(12,9) and CAM-B3LYP methods for a benchmark molecule N-Phenyl-maleimide, and by CAM-B3LYP for N-Phenyl-TCP. It is shown that the singlet excited S1 state and the pi-pi* triplet state have similar geometries, thus intersystem crossing is most likely for N-Phenyl-TCP. No crossing between the singlet ground state and the singlet excited states could be found on the linear synchronous transit paths of S0-S1-T1-S0 states using CAM-B3LYP method. The computed singlet ground state absorption and triple-triplet absorption spectra agree well with the experimental ones, in both spectra the first state is a pi-pi* state while the second state is the n-pi* state with an energetic difference of 0.1 eV.