A model study of the wavepacket dynamics around a Jahn–Teller conical intersection in a symmetric charge-transfer system Original Research Article

A model system with a Jahn–Teller conical intersection is investigated. Two vibrational modes are considered: a tuning mode corresponding to a vibrational motion that reduces the symmetry, thus removing the degeneration between the electronic states and a coupling mode, which is responsible of the electronic coupling. While the above scheme may be quite general, we consider in particular a charge-transfer system, in which the charge (an electron) is exchanged between identical moieties and the hopping term is null for the reference geometry. A typical realization may be that in biphenyl-like compounds in which the two rings are held at 90° by bulky substituents. We perform calculations of the time-dependent populations of the two diabatic (charge separated) states, assuming that the system is initially distorted, being trapped in one of the two minima, while a short pulse excites the wave-packet vertically. We not only discuss the ordinary situation in which we deal with the response of an ensemble of molecules, but also the one in which an individual molecule is interrogated. In the latter case, mimicking what may happen in a molecular electronic device grounded on a single molecule, quantum jumps come out. The role of adding energy quanta in the coupling mode is also investigated, showing that this gives rise to an accelerated charge-transfer dynamics. From the observation that no electronic coherence is produced between diabatic states during the time evolution, we are led to try with a very simple model (in the spirit of surface hopping but working with diabatic surfaces) which is shown to reproduce quite well the exact results.