Polarized fluorescence decay surface for many-ground- and many-excited-state species in solution

The theory of fluorescence depolarization experiments on many-ground- and many-excited-state species in solution, is presented. In the formalism discussed the state-dependent rotational dynamics of solutes is coupled with the state-to-state kinetic relaxation processes involved. The rotational dynamics of molecules can be different in different excited states due to state-dependent solvation effects (state-dependent solute-solvent interactions) which can modulate the hydrodynamic size and shape of rotating objects. Another reason for the state-dependence of rotational dynamics can be possible changes in the geometries of the solutes in different excited states. Two cases are distinguished: (a) state-dependent geometrical transformations of solutes which do not change the orientation of the principal axes diagonalizing the diffusion tensors in the excited states involved and (b) geometrical transformation of molecules which change the orientation of the principal axes (state-dependent choice for principal axes). It is assumed that in case (b) the change in the molecular geometry is not an extra source for the molecular reorientation or that this effect can be neglected to a first approximation. The case of rigid isotropic systems is considered.