Optical properties of fully conjugated cyclo[n]thiophenes - An experimental and theoretical approach.

Optical properties of two series of fully conjugated cyclo[n]thiophenes were analyzed experimentally and theoretically. The absorption spectra reveal a shift to higher wavelengths with increasing size of the cycles, which can be successfully described by an excitonic approach based on a Frenkel exciton Hamiltonian. Furthermore, intriguing new bands in the absorption and fluorescence spectra of the smaller macrocycles disclose the dominance of their ring strain.

Diffusion process with two reflecting barriers in a time-dependent potential.

We consider a Brownian particle which is driven by a harmonically oscillating force, the motion of which is restricted by two reflecting boundaries. We solve the Fokker-Planck equation using the finite-element method and focus on the dynamics of the mean position of the particle in the time-asymptotic regime. As a function of the strength of the external force, the response of the system, i.e., the amplitude of the mean position and the dynamical shift, in the stationary limit shows a resonancelike behavior as a function of the diffusion coefficient for certain parameter regimes. We explain these numerical results heuristically and give some qualitative estimates.

Probability distribution of work done on a two-level system during a nonequilibrium isothermal process.

We present an exact calculation of the probability density for the work done by an external agent on a two-level system. Due to the external drive, both the transition rates between the two states and their energies depend on time. Within this setting we calculate the probability of every possible sample path of the system evolution and also the work done along any such path. The general procedure yields an evolution equation for the characteristic function of the work. Assuming that the energies change with constant rates, the properties of the work distribution are controlled by a single parameter representing the ratio of the time scales of the driving protocol, and of the internal dynamics, respectively. We calculate the mean work and characterize those sample paths which are not in agreement with the second law. In the slow driving limit, the probability density for the work collapses to a delta function localized at the reversible work. In the strongly nonequilibrium regime, the most probable work is smaller and the mean work is bigger than the reversible work.

The formation of planar ribbonlike aggregates from stiff polyanions in the presence of anisotropic cations.

A dilute salt-free solution of rodlike polyanions in the presence of anisotropic (chain) cations consisting of neutral tails and charged heads is studied. Using Monte Carlo simulation within the framework of the primitive model, different Coulomb coupling regimes were considered. While aggregation in the strong coupling limit is expected, we report new morphology, namely, the formation of ribbonlike nanostructures. At strong electrostatic interaction, the system is found to undergo the self-organization resulting in the formation of planar aggregates that look like a "ladder" of polyanions sandwiched between cationic chains. We investigate the stability of different morphologies and find that these aggregates are thermodynamically stable. Focus has been made on how the chemical structure of anisotropic cations affects the morphology of the aggregates.

Mori-Zwanzig memory analysis in single-molecule spectroscopy.

We study the dynamical behavior of the Rhodospirillum molischianum LH2 complex based on intensity time series obtained from single-molecule spectroscopy experiments. This is achieved by reconstructing the memory function describing the time-dependent fluctuations of the excited states. We conclude that the apparent stochastic evolution of the dynamics is controlled by at least two different non-Markovian main processes.

Effect of low-molecular-weight salt on the nematic ordering in solutions of rodlike polyelectrolytes.

Nematic ordering is studied in solutions of rodlike macroions in the presence of monovalent salt ions. The correlation free energy of electrostatic interactions is calculated within the framework of the Debye-Hückel approach. The phase diagram of the system is obtained. We have shown that the addition of salt destroys the weakly ordered nematic phase which is stable at small polymer concentrations. On the other hand, the salt widens the region of the phase separation between the isotropic (or weakly ordered nematic) and the highly ordered nematic phases at high polymer concentrations. An explanation of these effects is proposed.

Analysis of stochastic resonances.

We investigate the one-dimensional diffusion of a particle in a piecewise linear potential superimposed with a step of a harmonically modulated height. Employing the matching conditions, we solve the corresponding Fokker-Planck equation and we analyze nonlinear features of the particle's mean position as a function of time. We present detailed results in two physically relevant cases. First, we take the unperturbed potential as a symmetrical up-oriented tip, which is placed between two reflecting boundaries and we add the jump at the tip coordinate. The setting yields resonance-like behavior of the stationary-response amplitude. Second, if the discontinuity at origin is combined with the constant force in the symmetrical region between the boundaries, the stationary response displays a time-independent shift against the potential slope. The driving-induced force exhibits a resonance-like behavior both with respect to the diffusion constant and the slope of the unperturbed potential.

Complex formation in systems of oppositely charged polyelectrolytes: a molecular dynamics simulation study.

Results of molecular dynamics simulations for systems with two flexible, oppositely charged polymer chains are presented. The lengths N and interaction strength lambda of the chains are varied. We find that the chains remain separated for small values of lambda. For large interaction strengths, i.e., large Bjerrum lengths, we find glasslike structures and order on the length scale of a few monomer diameters. Between these two limits of the interaction strengths, the chains of various lengths collapse into compact complexes that exhibit self-similar structures. The scaling behavior of the radius of gyration is discussed as a function of chain length and interaction strength. In addition, the local structure of the collapsed systems is analyzed and the dependence of the density of the aggregate on the interaction strength is discussed.