A fluorescence emission spectrum arises from transitions between the lowest vibrational level of the
first excited electronic state and different vibrational levels of the ground state. Excited species return to
the ground state very rapidly after excitation, i.e. within about 10–^8 s, so that emission of radiation ceases
as soon as the exciting radiation is removed. In solution, due to collisional broadening, one or more
fairly broad overlapping bands are observed. The overall appearance is an approximate mirror image of
the absorption or excitation spectrum of the species but displaced to longer wavelengths due to the
initial rapid loss of energy by vibrational relaxation which precedes the fluorescent emission. The
shortest wavelength (highest energy) emission, which arises from the transition to the lowest vibrational
level in the ground electronic state, is called resonance fluorescence. Solute-solvent interactions usually
result in the position of the resonance band being displaced to a slightly longer wavelength than the
corresponding excitation band, although in theory they should coincide being due to transitions between
the same vibrational levels. Phosphorescence can be observed only after a molecular system has
undergone radiationless relaxation by intersystem crossing. It is of longer wavelength (lower energy)
than fluorescence and persists for seconds or even minutes after excitation. Analytical methods based
on phosphorescence will not be discussed further.
The number of molecular species capable of relaxing by fluorescence is limited mainly to rigid organic
and mostly aromatic structures for which radiationless relaxation mechanisms are often comparatively
slow. The intensity of fluorescent emission is dependent on a quantum efficiency factor or yield, ΦF,
which can vary between zero (no fluorescence) and unity (all excited molecules relax by fluorescence).
Quantum efficiency is determined by a number of structural factors, including the presence and
positions of heteroatoms in the molecule, and the solvent used. It generally increases with degree of
rigidity and extended conjugation especially in
Figure 9.11(b)
Effect of structural rigidity on
quantum yield, ΦF.