Donor-substituted diphenylacetylene derivatives act as electron donors and acceptors

Despite the predominant electron donor character of p-phenylenediamine, our studies on extended p-phenylenediamine derivatives show that they can not only be chemically oxidized, giving well-known Wurster-type radical cations, but also be chemically reduced, giving radical anions. Making use of EPR/ENDOR spectroscopy and supported by DFT calculations, we were able to reveal the extent of π-electron delocalization in the paramagnetic species and to shed light onto the geometry and bond lengths. While for the radical anions spin was found to be mostly delocalized into the π-system, the radical cations can be described as essentially N-centered. Furthermore, we performed electrochemical characterizations using cyclic voltammetry to gain insight into the thermodynamics of the redox processes. The photophysical properties of the parent extended p-phenylenediamine were investigated by absorption, emission, and excitation spectroscopy. The fluorescence quantum yield and the excited-state lifetime of the neutral precursors in hexane and acetonitrile were determined to establish elementary differences originating from solvent effects.     

Study of structural and dynamic properties of liquid phenyltrimethoxysilane

Herein, we present a combined experimental and computational study of liquid phenyltrimethoxysilane. A femtosecond time-resolved optical Kerr effect experiment has been performed to study the rotational diffusion of the molecule. A new all-atoms molecular model of the compound, based on the OPLS force field, has been developed to reproduce the rotational diffusion time constant and other physical and dynamic properties available in the literature. The density obtained from the simulations is 1074 ± 4 kg m(-3), which is within 1% of the experimental value of 1062 kg m(-3). The viscosity from the simulations is 1.6 ± 0.1 mPa s while the experimental value is 2.1 mPa s. The average bulk dipole moment of 1.8 ± 0.5 Debye obtained from the simulation matches the experimental value of 1.77 Debye. The average relative dielectric constant from the simulations is 3.86 ± 0.04, which is within 13% of the experimental value (4.4). The rotational diffusion time of the dipole moment obtained from the simulations is 20.39 ± 0.06 ps, which is in excellent agreement with the experimental value of 20 ± 1 ps obtained from our measurements. The new model has also been used to calculate structural and dynamic properties of the molecule not yet determined experimentally.     

Magnetic field effects on exciplex-forming systems: the effect on the locally excited fluorophore and its dependence on free energy

This study addresses magnetic field effects in exciplex forming donor-acceptor systems. For moderately exergonic systems, the exciplex and the locally excited fluorophore emission are found to be magneto-sensitive. A previously introduced model attributing this finding to excited state reversibility is confirmed. Systems characterised by a free energy of charge separation up to approximately -0.35 eV are found to exhibit a magnetic field effect on the fluorophore. A simple three-state model of the exciplex is introduced, which uses the reaction distance and the asymmetric electron transfer reaction coordinate as pertinent variables. Comparing the experimental emission band shapes with those predicted by the model, a semi-quantitative picture of the formation of the magnetic field effect is developed based on energy hypersurfaces. The model can also be applied to estimate the indirect contribution of the exchange interaction, even if the perturbative approach fails. The energetic parameters that are essential for the formation of large magnetic field effects on the exciplex are discussed.     

Retracted: A comparative Raman study of graphite,single‐walled,and multi‐walled carbon nanotubes

Retraction: The following article from Journal of Raman Spectroscopy, ‘A comparative Raman study of graphite, single‐walled, and multi‐walled carbon nanotubes’, by Animesh K. Ojha and Arnulf Materny published online on 3 March 2009 in Wiley InterScience ( www.interscience.wiley.com ), has been retracted by agreement between the authors, the journal Editor in Chief, Wolfgang Kiefer, and the publisher John Wiley & Sons, Ltd. The retraction has been agreed due to lack of citation or acknowledgement of third party interests in certain data included in the article.     

Pathwise Numerical Approximations of SPDEs with Additive Noise under Non-global Lipschitz Coefficients

We consider the pathwise numerical approximation of nonlinear parabolic stochastic partial differential equations (SPDEs) driven by additive white noise under local assumptions on the coefficients only. We avoid the standard global Lipschitz assumption in the literature on the coefficients by first showing convergence under global Lipschitz coefficients but with a strong error criteria and then by applying a localization technique for one sample path on a bounded set.     

Transient grating studies of femtosecond processes in ultra-thin layers of PTCDA

Elementary processes like energy transfer, charge transport, and exciton diffusion in thin films occur on time scales of femtoseconds. Time-resolved photo-electron spectroscopy, a technique limited to ultra-high vacuum environment and the proper choice of a substrate, has been used to study ultrafast processes in sub-nanometer thin films so far. Herein we show that a transient (population) grating created by the interference of laser pulses can be used to study ultrafast processes in such films under ambient conditions. Our investigations of exciton dynamics in 1.4±0.2 nm and 0.4±0.2 nm thin films, formed by nanocrystals of 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) on glass and mica, show that the dynamics differ with the crystal size, possibly due to the confinement induced changes in the electronic structure. The technique is sensitive enough to investigate the dynamics in systems, where only 20 % of the surface is covered by nano-crystals. We expect such an optical technique that is sensitive enough to study dynamics in few to sub-nanometer thin layers under ambient conditions to become important in investigating ultrafast dynamics on surfaces, interfaces, functionalized materials, organic semiconductors, and quantum phenomena in ordered structures of reduced dimensions, such as quantum dots and graphene sheets.