Günter Grampp

Time-resolved magnetic field effects distinguish loose ion pairs from exciplexes.

We describe the experimental investigation of time-resolved magnetic field effects in exciplex-forming organic donor–acceptor systems. In these systems, the photoexcited acceptor state is predominantly deactivated by bimolecular electron transfer reactions (yielding radical ion pairs) or by direct exciplex formation. The delayed fluorescence emitted by the exciplex is magnetosensitive if the reaction pathway involves loose radical ion pair states. This magnetic field effect results from the coherent interconversion between the electronic singlet and triplet radical ion pair states as described by the radical pair mechanism. By monitoring the changes in the exciplex luminescence intensity when applying external magnetic fields, details of the reaction mechanism can be elucidated. In this work we present results obtained with the fluorophore-quencher pair 9,10-dimethylanthracene/N,N-dimethylaniline (DMA) in solvents of systematically varied permittivity. A simple theoretical model is introduced that allows discriminating the initial state of quenching, viz., the loose ion pair and the exciplex, based on the time-resolved magnetic field effect. The approach is validated by applying it to the isotopologous fluorophore-quencher pairs pyrene/DMA and pyrene-d10/DMA. We detect that both the exciplex and the radical ion pair are formed during the initial quenching stage. Upon increasing the solvent polarity, the relative importance of the distant electron transfer quenching increases. However, even in comparably polar media, the exciplex pathway remains remarkably significant. We discuss our results in relation to recent findings on the involvement of exciplexes in photoinduced electron transfer reactions.

Cyclic Voltammetric Study of Heterogeneous Electron Transfer Rate Constants of Various Organic Compounds in Ionic liquids: Measurements at Room Temperature

Abstract Room temperature ionic liquids (RTILs) are of growing interest due to their outstanding solvent properties. The high conductivity and large electrochemical window of RTILs have enabled their use in electrochemistry without adding supporting electrolyte. Heterogeneous electron transfer rate constants (khet) and diffusion coefficients (D) of ferrocene, 2,6-dimethylbenzoquinone, bromanil, tetracyanoethylene, tetrathiofulvalene, methylviologen, and ethylviologen were determined in several RTILs such as [emim][BF4], [bmim][OTf], [bmim][BF4] and [bmim][PF6] using cyclic voltammetry. The results obtained for khet and D, range from 0.25–29.6 × 104 cm s-1 and 1.27–25.5 × 108 cm2 s-1 respectively. Both were significantly lower than those found in organic solvents like acetonitrile (MeCN), dimethylformamide (DMF), etc. It was found that khet and D were two to three orders of magnitude lower in more viscous RTILs. Diffusion coefficients were inversely proportional to the viscosity of the RTILs for all substances under investigation. Marcus theory was applied to compare the khet. The main problem arising is to understand the role of solvent reorganization energy (λo). Whereas Marcus theory describes λo in two parts of polarization, a fast electronic and a slower orientational contribution both expressed by the Pekar factor γ = (1/n2 - 1/ɛs), the solvent is treated as a continuum having a dielectric constant (ɛs) and a refractive index (n). Such a concept seems to be not applicable to ionic liquids.

Intramolecular electron exchange in the 2,7-dinitronaphthalene radical anion : electron paramagnetic resonance and kinetic data

The rates of intramolecular electron exchange in the radical anion of 2,7-dinitronaphthalene in protic and aprotic solvents have been determined from alternating line-broadening effects in EPR spectra. Rate constants in alcohols range from 9.0 × 105 s–1(methanol, 297 K) to 1.7 × 108 s–1(propan-2-ol, 361 K), while in aprotic solvents they range from 1.2 × 108 s–1(acetonitrile, 232 K) to 3.7 × 109 s–1(dimethylformamide, 342 K). The reactions were found to be adiabatic and uniform.Applying Marcus theory and the Rips–Jortner approach to the solvent dependence of the rates, the transferred charge ze0 was determined. It was found that z decreases with increasing temperature, changing from 1.0 at 260 K to 0.98 at 320 K in protic solvents and from 0.58 at 280 K to 0.47 at 340 K in aprotic solvents. This result is explained on the basis of the effect of temperature on the asymmetric solvation of the anions.The outer-sphere reorganization energy, λ0, was obtained by applying the ellipsoidal cavity model and was found to be strongly dependent on the solvent polarity, changing at 293 K from 57.7 kJ mol–1 in hexamethylphosphoric triamide to 235.8 kJ mol–1 in methanol. Activation energies were found to be higher in protic than in aprotic solvents.Results for the 2,7-dinitronaphthalene radical anion are compared with previous data reported for the 1,3-dinitrobenzene radical anion. Rate constants are smaller in the former while outer-sphere reorganization energies are larger. Activation parameters were found of the same order of magnitude for both radical anions.

Magnetic field effects on the pyrene—dicyanobenzene system: determination of electron self-exchange rates by MARY spectroscopy

An experimental determination has been made of electron self-exchange rates between the radical anions of 1,2-, 1,3- and 1,4-dicyanobenzene (DCB) and the respective neutral molecules applying steady-state field-modulated MARY (magnetic field effect on reaction yield) spectroscopy. For the first time this has been achieved successfully for compounds whose self-exchange rate constants can be obtained independently by alternative methods such as EPR linebroadening. In this study, pyrene was used as an electron donor to generate the spin-correlated radical ion pair (pyrene·+ DCB·−) essential for MARY spectroscopy. The radical ion pair is in equilibrium with an exciplex whose magnetic field affected fluorescence was recorded as a function of the magnetic field to yield the MARY spectrum. Due to lifetime uncertainty energy broadening of spin levels caused by electron self-exchange, the characteristic B 1/2 value increases with the concentration of DCB in the sample. The rate constant of self-exchange was obtained from the slope of the linear part in the plot of B1/2 versus DCB concentration. The values range between 6 x 108 M−1 s−1 and 1.4 x 1010 M−1 s−1, depending on the DCB isomer and solvent. Comparison with literature data from EPR linebroadening measurements shows good agreement.

Rotational and Translational Diffusion of Spin Probes in Room-Temperature Ionic Liquids

We have studied the rotational and translational diffusion of the spin probe 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPOL) in five imidazolium-based room-temperature ionic liquids (RTILs) and glycerol by means of X-band electron paramagnetic resonance (EPR) spectroscopy. Rotational correlation times and rate constants of intermolecular spin exchange have been determined by analysis of the EPR line shape at various temperatures and spin probe concentrations. The model of isotropic rotational diffusion cannot account for all spectral features of TEMPOL in all RTILs. In highly viscous RTILs, the rotational mobility of TEMPOL differs for different molecular axes. The translational diffusion coefficients have been calculated from spin exchange rate constants. To this end, line shape contributions stemming from Heisenberg exchange and from the electron-electron dipolar interaction have been separated based on their distinct temperature dependences. While the Debye-Stokes-Einstein law is found to apply for the rotational correlation times in all solvents studied, the dependence of the translational diffusion coefficients on the Stokes parameter T/η is nonlinear; i.e., deviations from the Stokes-Einstein law are observed. The effective activation energies of rotational diffusion are significantly larger than the corresponding values for translational motion. Effects of the identity of the RTIL cations and anions on the activation energies are discussed.

Fluorescence quenching of aromatic hydrocarbons by nitroxide radicals: a mechanismatic study

The fluorescence quenching of phenanthrene (Phen), 9-cyanophenanthrene (CPhen), 9-cyanoanthracene (CA), perylene (Per), 9,10-dicyanoanthracene (DCA), and 9,10-diphenylanthracene (DPA) using stable nitroxide radicals as quenchers has been studied by steady state and flash photolysis measurements. Both linearity and deviation from linearity in the Stern-Volmer plots have been observed. The active sphere model was used to discuss the upward curvature of the Stern-Volmer plots in case of Per, DCA, and DPA. The bimolecular quenching rate constant (kq) of Phen, CPhen and CA was found to be diffusion controlled while in other cases it is lower than the diffusion limit. On the basis of flash photolysis measurements as well as the overlap between the emission spectra of hydrocarbons and the absorption spectra of radicals, a resonance energy transfer mechanism is taken place in case of Per, DPA, DCA, and CA. For Phen and CPhen where the energy gap between the first excited singlet and the nearest lower triplet state is small, an induced intersystem crossing was suggested. Finally, the quenching process was discussed in terms of the free energy dependence (ΔG) of the electron transfer from nitroxide radicals to the excited hydrocarbons.

Photophysics of two prototypical molecular-wire building blocks: solvent-induced conformational dynamics?

The photophysics of two molecular wire building blocks of different lengths based on p-phenyleneethylene, namely, 1,4-bis[p-(N,N-dimethylamino)phenyl]-1,2-ethyne and 1,4-bis[p-(N,N-dihexylamino)phenylethynyl]benzene, are studied experimentally in a wide range of organic solvents. The band shape and position of the electronic absorption and fluorescence emission of both compounds are discussed in terms of the empirical Catalán linear solvent energy relationship and the analytical solvation model of Liptay. It is found that solute polarizability plays an important part in the description of the pronounced solvatochromism for these highly symmetric molecules. In addition, the dependence of the emission quantum yield and the excited-state lifetime on the solvent are measured. The experimental findings can only be partially rationalized by the common theoretical models. They indicate that not only torsion about the triple bonds but also solvent-solute reorganization must be taken into account.

Quantum yields of singlet and triplet recombination products of singlet radical ion pairs

The efficiencies of contact geminate recombination to either the ground or excited triplet state of neutral products are calculated for contact and remote starts of radical ion pairs initially created in the singlet state. Considering the spin-conversion in this pair as a stochastic process with given rate, the diffusional dependence of recombination and charge separation yields and corresponding efficiencies are specified. This is compared with the experimental data obtained for photo-excited perylene quenched by aromatic amines in dimethyl sulfoxide–glycerol mixtures, which allow for a wide variation of solvent viscosity with composition. The successful fitting of the theory to non-trivial viscosity dependences confirms that the spin-forbidden recombination is composed of two sequential stages. Considering that the radical-ion pair is created in the singlet state, the spin conversion should precede its recombination to the excited triplet product.

Nature of the lowest triplet states of 4′-substituted N-phenylphenothiazine derivatives

The nature of the lowest triplet state of the donor–acceptor N-phenylphenothiazine derivatives has been studied by means of phosphorescence and EPR spectroscopy in various low temperature glasses. The triplet excitation of N-phenylphenothiazine and N-(p-anisyl)-phenothiazine is localised within the phenothiazine subunit. On the contrary, the 77 K phosphorescence of the molecules containing an electron acceptor group (i.e. –CN, –COCH3 or –COC6H5 at the 4′ position) is similar to that for benzonitrile, acetophenone or benzophenone, respectively, although the energy levels of their T1 states are higher than that of phenothiazine. With increasing temperature, however, their phosphorescence becomes similar to that characteristic for phenothiazine. This finding has been explained in terms of the excited-state intramolecular energy transfer accompanied by the planarisation of the phenothiazine kernel. The results of crystallographic investigations support this hypothesis.

High pressure ESR studies of electron self-exchange reactions of organic radicals in solution.

Simple electron self-exchange reactions are often used to study the role of the reaction medium on a chemical process, commonly implying the use of various solvents with different physical properties. In principle, similar studies may be conducted using a single solvent, changing its physical properties by application of elevated pressures, but so far only little information is available on pressure dependent exchange reactions. In this work, we have used a recently constructed high pressure apparatus for use with electron spin resonance (ESR) spectroscopy to investigate simple electron self-exchange reactions involving 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and tetracyanoethylene (TCNE) and their respective radical anions as well as TMPPD and its radical cation in three different solvents. The self-exchange was observed by ESR line broadening experiments, yielding rate constants and volumes of activation. The experimental results were compared to theoretical calculations based on Marcus theory and taking into account solvent dynamic effects. The use of elevated pressures has enabled the study of solvent effects without commonly encountered problems like solubility issues or chemical reactions between solvent and solute which sometimes limit the range of useable solvents.