Coherences in the time-resolved fluorescence of the TCNE-HMB complex: simulation versus experiment

There are many systems of different complexity ranging from diatomics to biomolecules (the sodium dimer, oxazine dye molecules, the reaction center of purple bacteria, the photoactive yellow protein, etc.) for which coherent oscillatory responses have been observed in the time and frequency gated (TFG) spontaneous emission (SE) spectra. In most cases, these oscillations are characterized by a single well-defined vibrational frequency. It is therefore logical to anticipate that a single optically active mode is responsible for these features, so that the description in terms of few-electronic-states-single-vibrational mode system Hamiltonian may be appropriate. The TCNE-HMB complex, exhibiting an ultrafast electron-transfer (ET) reaction, is well suited for testing this assumption. Although it is formed by relatively large organic molecules with many inter- and intramolecular vibrational modes, underdamped oscillatory responses with only one dominant frequency have recently been clearly observed at an ultrafast time scale via time-resolved fluorescence and pump-probe spectroscopy. To understand and explain the origin of these oscillations, Lin and co-workers performed ab initio molecular orbital calculations, identified all vibrational modes of the electronic ground state, and proposed a new theoretical model of the ET process in TCNE-HMB. This chapter describes briefly the model and presents some of the simulations.