Symmetry breaking charge transfer and control of the transition dipole moment in excited octupolar molecules

The structure of the energy levels of excited symmetric donor-acceptor octupolar molecules suggests a completely symmetric state and a degenerate doublet. For most molecules, the doublet is the first excited state, which is called the normal level order, but there are molecules with the reverse level order. Symmetry breaking charge transfer (SBCT) and its effect on the transient dipole moment in these structures are studied. It has been established that for reverse level order, SBCT is possible only if the reorganization energy exceeds a certain threshold, whereas for the normal level order, there is no such threshold. The lowest completely symmetric excited state is shown to become bright after SBCT. The dependence of the fluorescence transition dipole moment on the SBCT extent is calculated. It was established that the direction and magnitude of the transition dipole moment change similarly to the change in the dipole moment for the reverse level order, whereas for the normal level order, the changes are opposite. The effect of solvent thermal fluctuations on the transition dipole moment is simulated and discussed. A way for controlling the direction of the transition dipole moment by an external electric field is suggested.