The highly polarizable pi-electron system of conjugated molecules forms the basis for their unique electronic and photophysical properties, which play an important role in numerous biological phenomena and make them important materials for technological applications. We present a theoretical investigation of the dynamics and relaxation of photoexcited states in conjugated polyfluorenes, which are promising materials for display applications. Our analysis shows that both fast (approximately 20 fs) and slow (approximately 1 ps) nuclear motions couple to the electronic degrees of freedom during the excited-state dynamics. Delocalized excitations dominate the absorption, whereas emission comes from localized (self-trapped) excitons. This localization is attributed to an inherent nonlinear coupling among vibronic degrees of freedom which leads to lattice and torsional distortions and results in specific signatures in spectroscopic observables. Computed vertical absorption and fluorescence frequencies as well as photoluminescence band shapes show good agreement with experiment. Finally, we demonstrate that dimerization such as spiro-linking does not affect the emission properties of molecules because the excitation becomes confined on a single chain of the composite molecule.

A simple method that accurately captures the dynamics of metal–molecule–metal junctions under the influence of time-dependent driving forces is presented. In the method, the metallic contacts are modeled explicitly as a discrete set of levels that are dynamically broadened via an artificial damping term in the equations of motion. The approximations that underlie the method are revealed via a derivation of the effective equations of motion within the framework of nonequilibrium Green’s functions (NEGF) theory. As shown, the method applies to junctions that can be described by an effective independent Fermion Hamiltonian, admits arbitrary time dependence in the molecular Hamiltonian, and is restricted to time-dependent voltages that are adiabatically slow. The method is trivial to implement computationally, has a well-defined range where the results are independent of artificial model parameters, and is numerically shown to quantitatively reproduce the time-dependent transport characteristics of a model molecular junction driven by laser fields as described by an exact NEGF method in the wide band limit. As such, it generalizes previous efforts to capture Landauer transport via effective Liouville equations of motion with damping terms and constitutes an intuitive and technically accessible method for modeling time-dependent transport phenomena in molecular junctions that are driven by electric fields or fluctuating environments.

The molecular Schrödinger equation is rewritten in terms of nonunitary equations of motion for the nuclei (or electrons) that depend parametrically on the configuration of an ensemble of generally defined electronic (or nuclear) trajectories. This scheme is exact and does not rely on the tracing out of degrees of freedom. Hence, the use of trajectory-based statistical techniques can be exploited to circumvent the calculation of the computationally demanding Born-Oppenheimer potential-energy surfaces and nonadiabatic coupling elements. The concept of the potential-energy surface is restored by establishing a formal connection with the exact factorization of the full wave function. This connection is used to gain insight from a simplified form of the exact propagation scheme. In order to describe the correlated motion of electrons and nuclei, many strategies have been proposed to transcend the picture where the nuclei evolve on top of a single Born-Oppenheimer potential-energy surface (BOPES) [1]. Using a time-independent basis-set expansion of the electron-nuclear wave function, full quantum studies provide a complete description of nonadiabatic dynamics [2]. The scaling of these methods (even for a time-dependent basis-set expansion [3]) is, however, limiting their use to describe a few degrees of freedom. The so-called direct dynamics techniques attempt to alleviate this problem by calculating the BOPESs on the fly [4]. Of particular interest here are those methods that use information from quantum chemistry or time-dependent density functional theory calculations in the form of forces. Ab initio surface hopping, Ehrenfest dynamics [5], or Gaussian wave packet methods (such as the multiple spawning method) [6] are all able to reproduce the dynamics of some systems of interest [7]. In most of these methods, however, the form of the nuclear wave function is restricted, as they use a local or classical trajectory-based representation of the nuclear wave packet. In addition to the difficulties of including external fields or calculating the nonadiabatic coupling elements (NACs), this introduces the problem of systematically accounting for quantum nuclear effects.In this Letter, we propose an exact propagation scheme aimed at the study of nonadiabatic dynamics in the presence of arbitrary external electromagnetic fields. The coupled electron-nuclear dynamics is separated without tracing out degrees of freedom, which lends itself to a rigorous starting point for systematically including nonadiabatic nuclear effects without relying on the computation of BOPESs and NACs. This work constitutes a multicomponent extension of the conditional formalism proposed in Refs. [8,9]. Further, the propagation scheme presented here generalizes the conditional formalism beyond its original hydrodynamic formulation [8]. This makes it suitable to be coupled with well established electronic structure methods.Throughout this Letter, we use atomic units, and electronic and nuclear coordinates are collectively denoted by r ¼ fr 1 ; …; r N ...

The thermodynamic driving force in the self-assembly of the secondary structure of a class of donor-acceptor oligorotaxanes is elucidated by means of molecular dynamics simulations of equilibrium isometric single-molecule force spectroscopy AFM experiments. The oligorotaxanes consist of cyclobis(paraquat-p-phenylene) rings threaded onto an oligomer of 1,5-dioxynaphthalenes linked by polyethers. The simulations are performed in a high dielectric medium using MM3 as the force field. The resulting force vs. extension isotherms show a mechanically unstable region in which the molecule unfolds and, for selected extensions, blinks in the force measurements between a high-force and a low-force regime. From the force vs. extension data the molecular potential of mean force is reconstructed using the weighted histogram analysis method and decomposed into energetic and entropic contributions. The simulations indicate that the folding of the oligorotaxanes is energetically favored but entropically penalized, with the energetic contributions overcoming the entropy penalty and effectively driving the self-assembly. In addition, an analogy between the single-molecule folding/unfolding events driven by the AFM tip and the thermodynamic theory of first-order phase transitions is discussed and general conditions, on the molecule and the cantilever, for the emergence of mechanical instabilities and blinks in the force measurements in equilibrium isometric pulling experiments are presented. In particular, it is shown that the mechanical stability properties observed during the extension are intimately related to the fluctuations in the force measurements.