Fluorescence

Fluorescence the name derives from some forms of the mineral feldspar (calcium fluoride) on which a short-lived luminescence was observed (compare phosphorescence), and was proposed by one of the earliest students of this topic, G G Stoke (‘I am almost inclined to coin a word, and call the appearance fluorescence, from fluor-spar, as the analogous term opalescence is derived from the name of a mineral’) [1]. The fluorescence spectrum normally appears at a slightly lower energy than the longest absorption band, which it roughly mirrors, and in solution, it is shifted toward lower energies in polar solvents (red or Stoke’s shift). Modern use accepts the proposal by GN Lewis, which restricts the application of this term to an emissive vertical transition that maintains the multiplicity of the starting state (in organic chemistry, from the singlet excited state, S[math]_{1}[/math] [2, 3]). Since all of the high-lying excited states undergo internal conversion or intersystem crossing relatively quickly, the only fluorescence observed is that starting from the lowest singlet, and the spectrum observed does not depend on the excitation wavelength (Vavilov/Kasha rule[4]). This rule has many exceptions, due either to emission from a high singlet when S[math]_{2}[/math] and S[math]_{1}[/math] are either distant in energy or of different electronic structure as much as S[math]_{1}[/math] and S[math]_{0}[/math] (examples are azulene and thioketones)[5] or to a dependence on the excitation wavelength changes the emission due to the occurring of ultrafast phenomena (an example is 2-hydroxybenzophenone) [6]. In the biology of several species, fluorescence has an important role, in particular as a signal (think of the green fluorescing protein)[7].

Fluorescence is an excellent choice for analysis, since its short lifetime (typically < 10 ns) limits quenching by impurities and by dissolved oxygen and is intrinsically more easily detected (one photon in a dark room is much more easily detected than are two rays absorbing light at about the same rate as it happens with weak absorptions). Likewise, fluorescence based methods and microscope detections of fluorescence are widely used for fields as diverse as gemology, mineralogy [8], biology [9-12], clinical analyses [13-15], structure demonstration, or material science [15-17], fuel analysis [18], particularly in the solid state [14].