Features of optoelectronic processes in a molecular junction based on a fluorophore with optically active Frontier π-orbitals: electrofluorochromism in a ZnPc-based junction

Abstract

A model of the optoelectronic process in a molecular junction has been developed, in which electron transfer occurs through transmission channels associated with the filling of the π and π* orbitals of the fluorophore with transferred electrons. The contribution of each channel to the formation of current and electroluminescence (EL) is determined by the probability of the realization of those electronic states of the molecule that, at a given bias voltage, are involved in electron transfer. It is shown that in the vicinity of critical bias voltage, stepwise changes in current and EL occur, and the height of each step is controlled by kinetic processes associated with both electron transfer and intramolecular transitions. Using the obtained analytical expressions for the relative intensities of the emission lines X⁰ and X⁺ and comparing theoretical results with experimental data on STM-induced EL in a ZnPc-based junction, we showed that the method for analyzing the behavior of current and EL near critical voltages can serve as an effective tool for understanding the physical mechanisms responsible for optoelectronic processes at the single-molecule level. The method also made it possible to obtain real values of the energy of the Frontier orbitals of the ZnPc molecule embedded between the electrodes, as well as the energies of those electronic states of the neutral and charged molecules that participate in the optoelectronic process, including electrofluorochromism.