The excited state energy can be dissipated non-radiatively as heat (illustrated by the cyan wavy arrow in Figure 1), the excited fluorophore can collide with another molecule to transfer energy in a second type of non-radiative process (for example, quenching, as indicated by the purple wavy arrow in Figure 1), or a phenomenon known as intersystem crossing to the lowest excited triplet state can occur (the blue wavy arrow in Figure 1). The latter event is relatively rare, but ultimately results either in emission of a photon through phosphorescence or a transition back to the excited singlet.
(B) The fluorescence emission spectrum does not depend on the excitation wavelength. An alternative relaxation process is the transfer of the 1S1 excitation energy to an unexcited molecule of the same or different type. Efficient energy transfer from a donor to an acceptor must be A downhill @. Several types of excitation energy transfer mechanisms have been identified. The A long-range @ Förster process involves a "dipole-dipole" interaction between the excited singlet.
Wilkinson F, Helman WP, Ross AB (1995). "Rate constants for the decay and reactions of the lowest electronically excited singlet state of molecular oxygen in solution. An expanded and revised compilation. ". J. Phys. Chem.