Fluorescence

Summary

Fluorescence emission is a direct reflection of the properties of excited electronic states of molecules as they return radiatively to the ground electronic state. Fluorescence provides information about the (1) energy of the emitting state relative to the ground state, (2) lifetime of the excited state, (3) orientation of transition dipole moments and (4) symmetry properties of the ground and excited states. Fluorescence is especially valuable as a probe of photosynthetic systems, because it constitutes a sensitive competitive path to photochemical energy conversion, resulting in fluorescence quenching. Fluorescence spectra provide knowledge of the energy levels of different pigment pools in the light-harvesting antenna and reaction center complexes. Steady-state and time-resolved depolarization studies reflect the rapid excitation transfer processes that occur within these multi-pigment arrays in photosynthetic membranes. Using theoretical formulations, such as the Förster inductive resonance transfer mechanism, these transfer rates can be related directly to molecular geometries derived from X-ray crystallography and to fundamental spectroscopic properties of the molecules involved. The consequences of electric fields formed by primary charge separation across photosynthetic membranes can be seen in the influence of an applied electric field on the fluorescence intensity and relaxation kinetics. Much of our current knowledge of the primary processes of photosynthetic energy conversion has derived from fluorescence measurements.