The influence of the excitation spectrum on the quantum yield is investigated for organic molecules used in Light emitting diodes (LEDs). The significance of the competition between radiative and nonradiative recombination channels is pointed out. A system of master equations is proposed to describe the relaxation of lower levels of the excited molecule, and the solution for the fluorescence yield is obtained assuming relatively weak vibronic coupling. The results are used to interpret the experimental data in oligothiophenes, and general approaches are proposed to increase the relative weight of the radiative decay channel and correspondingly enhance the working properties of organic Light emitting diodes. In particular, the fluorescence quantum yield tan exceed the simple estimate of 0.25.