Transport and Spectroscopy in Conjugated Molecules: Two Properties and a Single Rationale

Abstract

In the context of electron dynamics simulations, when the charge density of a molecule is subject to a perturbation in the form of a short electric field pulse, density fluctuations develop in time. In the absence of dissipation, these oscillations continue indefinitely, reflecting the resonances of the electronic system; as a matter of fact, from the Fourier transform of the time dependent dipole arising from them, the absorption spectrum of the molecule can be calculated. Since these oscillations are the result of the electrons moving through the molecular structrure, it seems plausible that they carry information on the transport properties of the system. This is the idea explored in the present article for the case of conjugated polymers. Specifically, we depart from a nonequilibrium state with the charge concentrated on the ends of the molecule, and estimate the currents flowing back and forth during the evolution of electron dynamics simulations. These show that the charge oscillates between the sides of the polymer with the predominance of a frequency that is coincident with one of the main bands in the absorption spectrum, which can be ascribed to a charge transfer transition. Thus, from the charge transfer band frequency appearing in the absorption spectrum, the molecular conductance of a conjugated molecule can be calculated. Also interestingly, we find that, while a perturbation excites all resonances of an electronic system, the form in which this perturbation is applied can be manipulated to determine the relative manifestation of the response. The electric field pulse excites all resonances according to the transition dipole moment and is then appropriate to produce the absorption spectrum. A charge separated initial state, however, specifically stimulates the charge transfer mode and is then suitable to calculate transport properties. This allows us to propose a simple approach to obtain molecular conductances and tunneling decay constants in agreement with results from much more demanding electronic structure techniques.