Sergej Naumov

Publications of Sergej Naumov

• Single-particle and ensemble diffusivities--test of ergodicity.

  Authors: Florian Feil, Sergej Naumov, Jens Michaelis, Rustem Valiullin, Dirk Enke, Jörg Kärger, Christoph Bräuchle

  Angewandte Chemie (International ed. in English). 01/2012; 51(5):1152-5.

• Single-Particle and Ensemble Diffusivities-Test of Ergodicity.

  Authors: Florian Feil, Sergej Naumov, Jens Michaelis, Rustem Valiullin, Dirk Enke, Jörg Kärger, Christoph Bräuchle

  Angewandte Chemie (International ed. in English). 12/2011;

  Coincidence between the mean squared displacements determined for the same molecule over sequential time intervals and for all molecules over the same time interval follows as a consequence of theCoincidence between the mean squared displacements determined for the same molecule over sequential time intervals and for all molecules over the same time interval follows as a consequence of the ergodic theorem. In their Communication (DOI: 10.1002/anie.201105388), C. Bräuchle et al. confirm experimentally this fundamental principle of thermodynamics by diffusion studies using fluorescence microscopy and NMR ensemble measurements with pulsed field gradients.

• How hydrogen bonds influence the mobility of imidazolium-based ionic liquids. A combined theoretical and experimental study of 1-n-butyl-3-methylimidazolium bromide.

  Authors: Miriam Kohagen, Martin Brehm, Yves Lingscheid, Ralf Giernoth, Joshua Sangoro, Friedrich Kremer, Sergej Naumov, Ciprian Iacob, Jörg Kärger, Rustem Valiullin, Barbara Kirchner

  The journal of physical chemistry. B. 11/2011; 115(51):15280-8.

  The virtual laboratory allows for computer experiments that are not accessible via real experiments. In this work, three previously obtained charge sets were employed to study the influence ofThe virtual laboratory allows for computer experiments that are not accessible via real experiments. In this work, three previously obtained charge sets were employed to study the influence of hydrogen bonding on imidazolium-based ionic liquids in molecular dynamics simulations. One set provides diffusion coefficients in agreement with the experiment and is therefore a good model for real-world systems. Comparison with the other sets indicates hydrogen bonding to influence structure and dynamics differently. Furthermore, in one case the total charge was increased and in another decreased by 0.1 e. Both the most acidic proton as well as the corresponding carbon atom were artificially set to zero, sequentially and simultaneously. In the final setup a negative charge was placed on the proton in order to introduce a barrier for the anion to contact the cation via this most acidic hydrogen atom. The following observations were made: changing the hydrogen bonding ability strongly influences the structure while the dynamic properties, such as diffusion and viscosity, are only weakly changed. However, the introduction of larger alterations (stronger hydrogen bonding and antihydrogen bonding) also strongly influences the diffusion coefficients. The dynamics of the hydrogen bond, ion pairing, and the ion cage are all affected by the level of hydrogen bonding. A change in total charges predominantly influences transport properties rather than structure. For ion cage dynamics with respect to transport porperties, we find a good correlation and a weak or no correlation for the ion pair or the hydrogen bond dynamics, respectively. Nevertheless, the hydrogen bond does influence ion cage dynamics. Therefore, we confirm that ionic liquids rather consist of loosely interacting counterions than of discrete ion pairs. Hydrogen bonding affects the properties only in a secondary or indirect manner.

• Nanoporous glass as a model system for a consistency check of the different techniques of diffusion measurement.

  Authors: Christian Chmelik, Dirk Enke, Petrik Galvosas, Oliver Gobin, Andreas Jentys, Hervé Jobic, Jörg Kärger, Cordula B Krause, Jens Kullmann, Johannes Lercher, Sergej Naumov, Douglas M Ruthven, Tobias Titze

  Chemphyschem : a European journal of chemical physics and physical chemistry. 03/2011; 12(6):1130-4.

  The remarkable differences in the guest diffusivities in nanoporous materials commonly found with the application of different measuring techniques are usually ascribed to the existence of aThe remarkable differences in the guest diffusivities in nanoporous materials commonly found with the application of different measuring techniques are usually ascribed to the existence of a hierarchy of transport resistances in addition to the diffusional resistance of the pore system and their differing influence due to the differing diffusion path lengths covered by the different measuring techniques. We report diffusion measurements with nanoporous glasses where the existence of such resistances could be avoided. Molecular propagation over diffusion path lengths from hundreds of nanometers up to millimeters was thus found to be controlled by a uniform mechanism, appearing in coinciding results of microscopic and macroscopic diffusion measurement.

• Shedding light into the detailed excited-state relaxation pathways and reaction mechanisms of thionaphthol isomers.

  Authors: Yasser M Riyad, Sergej Naumov, Ralf Hermann, Bernd Abel

  The journal of physical chemistry. A. 02/2011; 115(5):718-25.

  Nanosecond laser flash photolysis employing transient detection of emission and absorption in combination with pulse radiolysis and quantum theory has been employed to shed light into the kinetics,Nanosecond laser flash photolysis employing transient detection of emission and absorption in combination with pulse radiolysis and quantum theory has been employed to shed light into the kinetics, quantum yields, and mechanisms of the deactivation of the first excited singlet state of 1- and 2-thionaphthols (NpSH(S(1))). In contrast to thiophenols (ArSH(S(1))), the results revealed that the decay of the first excited singlet state of 1- and 2-thionaphthols (NpSH(S(1))) is governed by radiationless internal conversion (Φ(IC) = 0.29-0.46; 0.016-0.190) and intersystem crossing (Φ(ISC) = 0.14-0.15; 0.4-0.6), respectively, with pronounced S-H photodissociation (Φ(D) = 0.40-0.55; 0.35-0.40). Fluorescence as a deactivation channel plays a minor role (Φ(F) = 0.001-0.010; 0.010-0.034). Quantum chemical calculations helped in understanding the formation of naphthylthiyl radicals and rationalizing the differences in the efficiency of intersystem crossing of the 1- and 2-thionaphthol systems.

• Guest diffusion in interpenetrating networks of micro- and mesopores.

  Authors: Filipe Furtado, Petrik Galvosas, Maraisa Gonçalves, Frank-Dieter Kopinke, Sergej Naumov, Francisco Rodríguez-Reinoso, Ulf Roland, Rustem Valiullin, Jörg Kärger

  Journal of the American Chemical Society. 02/2011; 133(8):2437-43.

  Pulsed field gradient NMR is applied for monitoring the diffusion properties of guest molecules in hierarchical pore systems after pressure variation in the external atmosphere. Following previousPulsed field gradient NMR is applied for monitoring the diffusion properties of guest molecules in hierarchical pore systems after pressure variation in the external atmosphere. Following previous studies with purely mesoporous solids, also in the material containing both micro- and mesopores (activated carbon MA2), the diffusivity of the guest molecules (cyclohexane) is found to be most decisively determined by the sample "history": at a given external pressure, diffusivities are always found to be larger if they are measured after pressure decrease (i.e., on the "desorption" branch) rather than after pressure increase (adsorption branch). Simple model consideration reproduces the order of magnitude of the measured diffusivities as well as the tendencies in their relation to each other and their concentration dependence.

• Charge transport and diffusion of ionic liquids in nanoporous silica membranes.

  Authors: Ciprian Iacob, Joshua Rume Sangoro, Periklis Papadopoulos, Tilman Schubert, Sergej Naumov, Rustem Valiullin, Jörg Kärger, Friedrich Kremer

  Physical chemistry chemical physics : PCCP. 11/2010; 12(41):13798-803.

  Charge transport in 1-hexyl-3-methylimidazolium hexafluorophosphate ionic liquid in oxidized nanoporous silicon membranes is investigated in a wide frequency and temperature range by a combination ofCharge transport in 1-hexyl-3-methylimidazolium hexafluorophosphate ionic liquid in oxidized nanoporous silicon membranes is investigated in a wide frequency and temperature range by a combination of Broadband Dielectric Spectroscopy (BDS) and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). By applying the Einstein-Smoluchowski relations to the dielectric spectra, diffusion coefficient is obtained in quantitative agreement with independent PFG NMR measurements. More than 10-fold systematic decrease in the effective diffusion coefficient from the bulk value is observed in hydrophilic silica nanopores. A model assuming a reduced mobility at the pore-matrix interface is shown to provide a quantitative explanation for the remarkable decrease of effective transport quantities (such as diffusion coefficient, dc conductivity and consequently, the dielectric loss) of the ionic liquid in non-silanized membranes. This approach is supported by the observation that silanization of porous silica membranes results in a significant increase of the effective diffusion coefficient, which approaches the value for the bulk liquid.

• Free electron transfer--relations between molecule dynamics and reaction kinetics.

  Authors: Ortwin Brede, Sergej Naumov

  Chemical Society reviews. 08/2010; 39(8):3057-71.

  Electron transfer in non-polar media (alkanes, alkyl chlorides) exhibits some essential peculiarities. For instance the reaction of hetero-substituted aromatics with parent solvent radical cationsElectron transfer in non-polar media (alkanes, alkyl chlorides) exhibits some essential peculiarities. For instance the reaction of hetero-substituted aromatics with parent solvent radical cations results in the parallel formation of metastable donor radical cations and fragmentation products, in comparable amounts. The fragmentation products originate from a dissociative donor radical cation which decays extremely rapidly, i.e., in a few femtoseconds. This phenomenon is explained in terms of intramolecular dynamic motions which cause changes of the electron density (pi- and n-orbitals) in dependence on the deformation angle between the substituents and the aromatic ring. Hence femtosecond dynamics is reflected in the nanosecond time range and can be observed with real-time spectroscopy. Therefore, the process is named free electron transfer (FET) which corresponds to an unhindered electron jump occurring in the first approach of the reactants. This dynamic controlled process is compared with the classical electron transfer theories which are based on equilibrium kinetics. From the FET mechanism some new aspects for chemical reaction kinetics can be derived (critical review, 69 references).

• Free Electron Transfer with Bifunctional Donors: p-Aminotritylsilanes.

  Authors: Nikolaos Karakostas, Sergej Naumov, Ortwin Brede

  The journal of physical chemistry. A. 10/2009;

  This work provides an in-depth look at the bimolecular free-electron transfer (FET) from bisubstituted (amine and -CR(2)SiMe(3) groups) aromatic molecules to the solvent radical cations of n-BuCl.This work provides an in-depth look at the bimolecular free-electron transfer (FET) from bisubstituted (amine and -CR(2)SiMe(3) groups) aromatic molecules to the solvent radical cations of n-BuCl. Because of the low rotational barriers, the substrates obtain all possible arrangements in solution. The electron jump is an unhindered process that does not require a defined encounter complex. The resulting radical cations show great conformer diversity because they directly inherit the geometry of their mobile precursors. One part of the radical cations is unstable and dissociates instantly, but the other one is metastable (microsecond lifetime). The two substituents reduce the barrier of internal rotation, resulting in stabilization of the otherwise good leaving group -SiMe(3). The amine group governs the reactivity of the system because it receives most of the electron density of the fluctuating highest MOs: primary and secondary amine groups lead to both instant and delayed formation of aminyl radicals; tertiary amines cause the rapid loss of an alpha-H(+) to yield alpha-aminoalkyl radicals.

• Understanding adsorption and desorption processes in mesoporous materials with independent disordered channels.

  Authors: Sergej Naumov, Rustem Valiullin, Jörg Kärger, Peter A Monson

  Physical review. E, Statistical, nonlinear, and soft matter physics. 09/2009; 80(3 Pt 1):031607.

  Using a lattice-gas model in mean-field theory, we discuss the problem of how adsorption and desorption of fluids in independent cylinderlike pores is influenced by variations in the pore diameterUsing a lattice-gas model in mean-field theory, we discuss the problem of how adsorption and desorption of fluids in independent cylinderlike pores is influenced by variations in the pore diameter along the length of the pore, surface roughness of the pore walls, and chemical heterogeneity. We also consider the impact of contact with the bulk phase via the pore opening and the possibility of interactions between neighboring pores via a liquid film on the external surface of the material. We find that a combination of pore size variation along the length of the pore and surface roughness yields sorption hysteresis similar to that found in systems with three-dimensional disordered pore networks such as porous glasses. Our results are especially relevant to adsorption and desorption in porous silicon materials with independent linear pores and apparently anomalous features of the behavior in these systems can be accounted for within the context of the present model.

• Understanding capillary condensation and hysteresis in porous silicon: network effects within independent pores.

  Authors: Sergej Naumov, Alexey Khokhlov, Rustem Valiullin, Jörg Kärger, Peter A Monson

  Physical review. E, Statistical, nonlinear, and soft matter physics. 01/2009; 78(6 Pt 1):060601.

  The ability to exert a significant degree of pore structure control in porous silicon materials has made them attractive materials for the experimental investigation of the relationship between poreThe ability to exert a significant degree of pore structure control in porous silicon materials has made them attractive materials for the experimental investigation of the relationship between pore structure, capillary condensation, and hysteresis phenomena. Using both experimental measurements and a lattice gas model in mean field theory, we have investigated the role of pore size inhomogeneities and surface roughness on capillary condensation of N2 at 77K in porous silicon with linear pores. Our results resolve some puzzling features of earlier experimental work. We find that this material has more in common with disordered materials such as Vycor glass than the idealized smooth-walled cylindrical pores discussed in the classical adsorption literature. We provide strong evidence that this behavior comes from the complexity of the processes within independent linear pores, arising from the pore size inhomogeneities along the pore axis, rather than from cooperative effects between different pores.

• Ionization of Amino-, Thio- and Hydroxy-naphtalenes via Free (Unhindered) Electron Transfer.

  Authors: Aliaksandr Baidak, Sergej Naumov, Ralf Hermann, Ortwin Brede

  The journal of physical chemistry. A. 11/2008;

  The electron transfer from various monosubstituted naphthyl derivatives (naphtols, NpOH; naphtylamines, NpNH 2; and thionaphtols, NpSH) to parent n-BuCl radical cations was studied by means of pulseThe electron transfer from various monosubstituted naphthyl derivatives (naphtols, NpOH; naphtylamines, NpNH 2; and thionaphtols, NpSH) to parent n-BuCl radical cations was studied by means of pulse radiolysis. The experiments reveal the synchronous and direct formation of two types of transients: the metastable solute radical cation (NpXH (*+), X = heteroatom) and the corresponding heteroatom-containing radical (NpX (*)) in comparable amounts. This is explained in terms of the free (unhindered) electron transfer in nonpolar solvents, which is a bimolecular process reflecting femtosecond time scale events of intramolecular dynamic motions accompanied by significant changes of the electron distribution within the donor molecule.

• Kinetic and Energetic Analysis of the Free Electron Transfer.

  Authors: Aliaksandr Baidak, Sergej Naumov, Ortwin Brede

  The journal of physical chemistry. A. 10/2008;

  In this paper, the bimolecular free (unhindered) electron transfer (FET) from various trityl-containing compounds to the solvent radical cations of n-BuCl is described. In good agreement with theIn this paper, the bimolecular free (unhindered) electron transfer (FET) from various trityl-containing compounds to the solvent radical cations of n-BuCl is described. In good agreement with the previously studied cases, the FET involving trityl-derived compounds results in the formation of two different types of the radical cation, which undergo the subsequent fragmentation via two alternative reaction channels. This unusual effect is caused by the intramolecular rotational motion in the ground-state molecules around the arrow-marked bond Ar-right curved arrow -X-CPh 3 (Ar = aromatic moiety; X = S, O, NH, CH 2), since such oscillations are directly connected with the electron distribution within the molecule. An unhindered electron jump from the donor trityl compound to the solvent radical cation, taking place in the subfemtosecond time range, generates the solute radical cation with the inherited geometry and the electron distribution of its precursor. Among the whole variety of produced radical cations, two extreme conformer states can be distinguished, namely, a planar and a twisted state. The planar type represents the structures with minimum energy, whereas the twisted type is destabilized by the increased value of the rotational barrier in the ionized state. The difference in the energetic profiles between planar and twisted radical cations plays a crucial role in their subsequent fragmentation. The planar radical cation follows the thermodynamically favored pathway generating ArX (*) and Ph 3C (+). A distinct part of the twisted radical cation dissociates faster than it relaxes into the more preferable planar conformation and, therefore, produces a thermodynamically unfavorable couple of products: ArX (+) and Ph 3C (*). This fragmentation channel is exclusively caused by FET. The undertaken quantum chemical calculations enable the judgment of the energetics of the different dissociation channels of the radical cations of the trityl derivatives.

• Dynamical aspects of the adsorption hysteresis phenomenon.

  Authors: Rustem Valiullin, Sergej Naumov, Petrik Galvosas, Jörg Kärger, Peter A Monson

  Magnetic resonance imaging. 06/2007; 25(4):481-4.

  Equilibrium and nonequilibrium transport properties of adsorbates in mesoporous Vycor porous glass have been experimentally studied using nuclear magnetic resonance techniques. With the knownEquilibrium and nonequilibrium transport properties of adsorbates in mesoporous Vycor porous glass have been experimentally studied using nuclear magnetic resonance techniques. With the known geometrical characteristics of porous glass and with measured self-diffusivities, transient sorption curves have been quantitatively compared to those predicted within a Fick's law model. This model correctly describes data outside a hysteresis region. In contrast, in the hysteresis region, a two-step mechanism of density relaxation is required to explain the behavior. These two mechanisms are identified as diffusion at early stages and activated density redistribution at later stages of adsorption. The latter mechanism, being intrinsically slow in nature, is anticipated to prevent the system from reaching equilibrium.

• Ionization of aromatic sulfides in nonpolar media: free vs reaction-controlled electron transfer.

  Authors: Nikolaos Karakostas, Sergej Naumov, Ortwin Brede

  The journal of physical chemistry. A. 02/2007; 111(1):71-8.

  The primary products of the bimolecular free electron transfer (FET) from aromatic sulfides (PhSCH2Ph, PhSCHPh2, PhSCPh3) to n-butyl chloride radical cations are two radical cation conformers: aThe primary products of the bimolecular free electron transfer (FET) from aromatic sulfides (PhSCH2Ph, PhSCHPh2, PhSCPh3) to n-butyl chloride radical cations are two radical cation conformers: a dissociative and a metastable one. In analogy with formerly studied donor systems, this result seems to reflect femtosecond oscillations in the ground state of the sulfides such as torsion motions around the Ar-S bond. This motion is accompanied by a marked electron fluctuation within the HOMO (or the n) orbitals. The FET products observed in the nanosecond time scale such as the metastable sulfide radical cations (Ar-S-CR3*+), the dissociation products R3C+; and R3C*, and their (experimentally) nondetectable counterparts Ar-S* as well as Ar-S+ can be understood with the simplified assumption of two extreme conformations, namely a planar and a twisted donor molecule. Using mediator radical cations (benzene, butylbenzene, biphenyl), the stepwise reduction of the free energy of the electron transfer from -DeltaH = 2.5 to <or=0.5 eV changes the dissociation pattern of the sulfide radical cations toward a uniform product such as the metastable sulfide radical cation. From that, we tentatively interpret the mechanism of the studied electron transfer as diffusion-controlled (collision-determined FET) at higher -DeltaH values and as reaction-controlled at -DeltaH below 0.5 eV.

• Exploration of molecular dynamics during transient sorption of fluids in mesoporous materials.

  Authors: Rustem Valiullin, Sergej Naumov, Petrik Galvosas, Jörg Kärger, Hyung-June Woo, Fabien Porcheron, Peter A Monson

  Nature. 11/2006; 443(7114):965-8.

  In recent years, considerable progress has been made in the development of novel porous materials with controlled architectures and pore sizes in the mesoporous range. An important feature of theseIn recent years, considerable progress has been made in the development of novel porous materials with controlled architectures and pore sizes in the mesoporous range. An important feature of these materials is the phenomenon of adsorption hysteresis: for certain ranges of applied pressure, the amount of a molecular species adsorbed by the mesoporous host is higher on desorption than on adsorption, indicating a failure of the system to equilibrate. Although this phenomenon has been known for over a century, the underlying internal dynamics responsible for the hysteresis remain poorly understood. Here we present a combined experimental and theoretical study in which microscopic and macroscopic aspects of the relaxation dynamics associated with hysteresis are quantified by direct measurement and computer simulations of molecular models. Using nuclear magnetic resonance techniques and Vycor porous glass as a model mesoporous system, we have explored the relationship between molecular self-diffusion and global uptake dynamics. For states outside the hysteresis region, the relaxation process is found to be essentially diffusive in character; within the hysteresis region, the dynamics slow down dramatically and, at long times, are dominated by activated rearrangement of the adsorbate density within the host material.

• Femtodynamics reflected in nanoseconds: bimolecular free electron transfer in nonpolar media.

  Authors: Ortwin Brede, Sergej Naumov

  The journal of physical chemistry. A. 11/2006; 110(43):11906-18.

  The (free) electron transfer (FET) from electron donor molecules to parent solvent radical cations of alkanes and alkyl chlorides exhibits mechanistic peculiarities that are conditioned by the lowThe (free) electron transfer (FET) from electron donor molecules to parent solvent radical cations of alkanes and alkyl chlorides exhibits mechanistic peculiarities that are conditioned by the low polarity of these solvents. Because of the negligible solvation of ions in such systems and the almost complete lack of an activation barrier, the electron jump takes place at the very first encounter of the reactants and, as such, in extremely short times of <or=10(-15) s. Molecular oscillations (deformation, bending) occurring within the femtosecond time domain result directly in significant changes of the pi- or n-electron distribution in the HOMO ground state of the donor molecule, thereby generating a distribution of conformers. This is considered to be a rationale for a possible generation of different product radical cations in the free electron transfer, which exhibit different spin and charge distribution and, consequently different stability. Experimentally, the latter has been verified for aromatic donors, substituted with mobile, i.e., not rigidly fixed heteroatom-centered groups (various phenol type compounds, thiophenols, aromatic amines, benzyltrimethylsilanes etc.). The individually characteristic product distribution could be visualized and quantified by time-resolved spectroscopy in the nanosecond time domain. On the basis of a manifold of experimental data and supported by quantum-chemical calculations, the free electron transfer phenomenon is analyzed and discussed in detail in this summarizing report. The results presented here stand also for a, seemingly paradox, situation in which the products of a diffusion-controlled bimolecular reaction are governed by femtosecond events.

• Repair reactions of pyrimidine-derived radicals by aliphatic thiols.

  Authors: Aleksandra Wójcik, Sergej Naumov, Bronislaw Marciniak, Ortwin Brede

  The journal of physical chemistry. B. 07/2006; 110(25):12738-48.

  Pyrimidinyl radicals of various structures (Pyr*) were generated in aqueous and alcohol-containing solutions by means of pulse radiolysis to determine the rate constants of their repair reactions byPyrimidinyl radicals of various structures (Pyr*) were generated in aqueous and alcohol-containing solutions by means of pulse radiolysis to determine the rate constants of their repair reactions by different thiols (RSH = cysteamine, 2-mercaptoethanol, cysteine, and penicillamine): Pyr* + RSH --> PyrH + RS*. C5-OH and C6-OH adduct radicals of the pyrimidines react with thiols with k9 = (1.2-10.0) x 10(6) dm3 mol(-1) s(-1). Similar repair rate constants were found for uracil- and thymine-derived N1-centered radicals, k31 = (1.5-6.1) x 10(6) dm3 mol(-1) s(-1). However, pyrimidine radical anions protonated at their C6 position and C6-uracilyl radicals, with carbonyl groups at their C5 position, react with thiols faster, with k24 = (0.5-7.6) x 10(7) dm3 mol(-1) s(-1) and k14 = (1.4-4.8) x 10(7) dm3 mol(-1) s(-1), respectively. Quantum chemical calculations, at the B3LYP/6-31G(d,p) and self-consistent reaction field polarizable continuum model level point to the combined effects of the energy gap between interacting molecular orbitals, charge distribution within different pyrimidine-derived radicals, and the coefficients of the atomic orbitals as the possible reasons for the differences in the rate constants of repair.

• Deactivation of the first excited singlet state of thiophenols.

  Authors: Yasser M Riyad, Sergej Naumov, Ralf Hermann, Ortwin Brede

  Physical chemistry chemical physics : PCCP. 04/2006; 8(14):1697-706.

  On the bases of picosecond and nanosecond laser flash photolysis with detection by emission and absorption spectroscopy, a quantitative description is given of all deactivation channels of the firstOn the bases of picosecond and nanosecond laser flash photolysis with detection by emission and absorption spectroscopy, a quantitative description is given of all deactivation channels of the first excited singlet state of thiophenols ArSH(S(1)) such as fluorescence, intersystem crossing (ISC), chemical dissociation into radicals, and radiation-less internal conversion (IC). For this purpose, the photolysis of thiophenol and its methyl-, methoxy-, and chloro-substituted derivatives was studied in solvents of increasing polarity: 1-chlorobutane, ethanol, and acetonitrile. The fluorescence lifetime of the thiophenols was found to range from some hundreds of picoseconds up to a few nanoseconds, correlating with fluorescence quantum yields between 0.001-0.040, at room temperature. Depending on the substitution pattern of the aromatic ring, the quantum yield of the S-H bond dissociation was found to be between 0.3-0.5, irrespective of the solvent polarity. In laser photolysis, no triplet formation of the investigated compounds could be observed neither by the direct way nor by subsequent sensitization with beta-carotene. As a difference to the total, the radiation-less internal conversion (Phi(IC)>or= 0.5) was found to be the dominating process.

• Free electron transfer from xanthenyl- and fluorenylsilanes (Me3 or Ph3) to parent solvent radical cations: effects of molecule dynamics.

  Authors: Nikolaos Karakostas, Sergej Naumov, Michael G Siskos, Antonios K Zarkadis, Ralf Hermann, Ortwin Brede

  The journal of physical chemistry. A. 01/2006; 109(51):11679-86.

  Parent radical cations of nonpolar solvents (alkanes and alkyl chlorides) ionize 9-(trimethylsilyl)xanthenes and 9-(trimethylsilyl)fluorenes in a diffusion-controlled electron transfer. The actualParent radical cations of nonpolar solvents (alkanes and alkyl chlorides) ionize 9-(trimethylsilyl)xanthenes and 9-(trimethylsilyl)fluorenes in a diffusion-controlled electron transfer. The actual electron jump as the deciding part of the process does not require a defined encounter complex, and therefore the reactants are not subjected to any geometry optimization. Considering the molecule dynamics of the donors, bending motions of the silyl group are concerted with fluctuations of the highest occupied molecular orbital electrons. Ionizing such a standing conformer mixture creates metastable (microsecond) as well as dissociative donor radical cations. A mobility restriction of the benzylic silane group in positions vertical to the phenyl plane stabilizes the radical cations and accounts for a declining amount of dissociative radical cations, which undergo C-Si bond fragmentation in the order benzylsilane > xanthenylsilane > fluorenylsilane.