Time-evolution study of photoinduced charge-transfer in tertiary amine-fluorophore systems

Abstract

Photoinduced charge transfer in a fluorescent sensor of organophosphorus nerve agents is examined by solution of the time dependent Schrödinger equation, under the influence of an external electromagnetic field and employing potential energy curves obtained by density functional theory (DFT) and time dependent DFT (TDDFT) calculations for the ground and the excited states, respectively. The systems of interest here consist of a tertiary amine-pyrene molecule, with different numbers of CH2 spacer units between the t-amine and the pyrene. Quenching of pyrene emission has been observed in these systems, to different extent depending on the number of spacer units, which is due to photoinduced electron transfer from the t-amine to pyrene. Here, we present a new approach based on time-evolution for the explanation of the experimental observation of the quenching of pyrene emission. In this work the inversion coordinate at nitrogen is considered as a generalized coordinate for the charge-transfer process. Vibrational levels, dipole moments and dipole transition moments of the ground, excited, and charge transfer electronic states along the inversion coordinate are computed by DFT and TDDFT calculations. The time evolution of the excitation probability between the vibrational levels of the ground state and the first excited state and the charge transfer state has been calculated for different excitation frequencies. The results are in agreement with experimental observations regarding the photoinduced electron transfer and decreasing probability for charge transfer with increasing number of CH2 spacer units between the t-amine and the pyrene.