The chemical and molecular structure of a resin has the greatest effect on fluorescence and emission intensity. Previous studies have revealed that only a very small proportion of organic compounds exhibit fluorescence, but the most common chemical structures that lead to fluorescence are found in hydrogenated hydrocarbon resins.

Figure 1 illustrates:

• The relationship of the observed colors of fluorescent emission with emission wavelength.
• Emission intensity is related to the chemical structure.
• The degree of hydrogenation for Regalite resins.

Figure 1: Fluorescence Emission Colors and Relative Intensities of Regalite Partially Hydrogenated Hydrocarbon Resins with Varying Degrees of Hydrogenation

The three Regalite hydrogenated hydrocarbon resins are differentiated only by their degree of hydrogenation (UVa value).

• UVa = 2.90 corresponds to about 50% hydrogenated
• UVa = 2.16 corresponds to about 70% hydrogenated
• UVa = 0.61 corresponds to about 90% hydrogenated

Regalite S5100 hydrocarbon resin has nearly five times more emission intensity than Regalite R9100 hydrocarbon resin.

A molecule can emit radiation by the fluorescence process only when it can absorb UV radiation. Once absorbed, it can emit fluorescence at high or low emission intensity. This intensity is dependent on the fluorescence efficiency, _f, which describes the relationship between absorption of UV radiation and intensity of fluorescence emitted whereby:

f =

Number of photons emitted Number of photens absorbed

=

Intensity of fluorescence Intensity of absorption

Prediction of Fluorescence from Molecular Structure
High _f values are associated with molecules possessing an extensive delocalized system of conjugated bonds (for example aromatic compounds), resulting in a relatively rigid molecular structure. This is why compounds such as fluorescein, anthracene, and other condensed ring aromatic structures fluoresce. It also accounts for the high absorption of these compounds.

Low Fluorescence Intensity (See Figure 1)
It is possible for a structure to have a high absorption but very low emission intensities of fluorescence. In this case the excitation energy must be lost in another way, known as radiationless transfer of energy. The conversion of excitation (radiant) energy to kinetic (vibrational) energy is much easier for relatively flexible molecules, since they are freer to convert radiant energy to motion. The result is much less fluorescence. This is the case with Regalite R9100 hydrogenated hydrocarbon resin. It has absorbed similar excitation energy to the other samples, but the emission intensity is significantly lower.

A rigid molecule cannot make efficient use of internal conversion of energy to return to the ground state, therefore it must emit photons resulting in high fluorescence intensity. Thus a rigid molecular structure will emit more fluorescence than a flexible molecular structure.