Pump-probe spectroscopy after selective excitation of all-trans Cars (n = 9-13) in nonpolar solvent identified a symmetry selection rule of diabatic electronic mixing and diabatic internal conversion, i.e., '1B(u)(+)-to-1B(u)(-) is allowed but 1B(u)(+)-to-3A(g)(-) is forbidden'. Kerr-gate fluorescence spectroscopy showed that this selection rule breaks down, due to the symmetry degradation when the Car molecules are being excited, and, as a result, the 1B(u)(+)-to-3A(g)(-) diabatic electronic mixing and internal conversion become allowed. On the other hand, pump-probe spectroscopy after coherent excitation of the same set of Cars in polar solvent identified three stimulated-emission components (generated by the quantum-beat mechanism), consisting of the long-lived coherent cross term from the 1B(u)(+) + 1B(u)(-) or 1B(u)(+) + 3A(g)(-) diabatic pair and incoherent short-lived 1B(u)(+) and 1B(u)(-) or 3A(g)(-) split incoherent terms. The same type of stimulated-emission components were identified in Cars bound to LH2 complexes, their lifetimes being substantially shortened by the Car-to-BChl singlet-energy transfer. Each diabatic pair and its split components appeared with high intensities in the first component. The low-energy shifts of the 1B(u)(+)(0), 1B(u)(-)(0) and 3A(g)(-)(0) levels and efficient triplet generation were also found.
Mechanistic insight into internal conversion process within Q-bands of chlorophyll a