3.5 Fluorescence

Fluorescence from organic materials is a very important phenomenon that is exploited in a broad range of application areas. These include their use in the dyeing of synthetic textile fibres, as daylight fluorescent pigments in a wide variety of media, including inks, paints and plastics, in detection of flaws in materials, in dye lasers, as solar collectors, in many very important analytical methodologies and in biological and medical applications in both detection and diagnosis of diseases, in assays and in display applications such as electroluminescence (see section 3.8) and in molecular electronics.17

Some of the theoretical aspects of fluorescence have already been discussed alongside phosphorescence in section 3.2 and shown schematically in Figure 3.1. In organic fluorophores, especially dyes, excitation from the ground level S ₀ to the first excited state S ₁ is generally a ?- ?* electronic transition. During the extremely short time (1 10 10 ^?9 s) that the excited molecule spends in the various higher vibrational levels of the excited S ₁ state it loses energy by various actions in its molecular environment. If the molecule returns to the ground state emitting the radiation as visible light it exhibits fluorescence, which, due to the loss of vibrational energy, is at a longer wavelength than the initially absorbed light. As already stated this difference in wavelength between absorption and emission is known as the Stokes shift. In general the Stokes shift is in the region of 30 70 nm but can...