The effect of the shell thickness on stimulated emission and photoinduced absorption transitions in CdSe quantum rods is investigated by femtosecond pump and probe spectroscopy. The role of surface/interface defect states, which result resonant to the high energy states and to the midgap, respectively, is pointed out. We find that the density of such states depend on the shell thickness and that the interface defects can be negligible for thin shells. As a result in the latter case stimulated emission can be observed for longer times.