Like iron rusts when exposed to the elements, daylight fluorescent dyes are at risk of degrading over time. Ironically, the light that provides the energy to initiate fluorescence is responsible for the subsequent degradation of the dye molecule and resulting loss of fluorescence.
Structurally, daylight fluorescent pigments are solutions of daylight fluorescent dyes in highly polar polymer systems. These polymers surround the fluorescent dyes with an electrically charged solvent cage that helps to stabilize the charged dye molecule in the excited state from which fluorescent emission occurs. This excited state results from the absorption of the higher energy frequencies of daylight, much of which is in the ultraviolet region, and then on returning to the unexcited or ground state, the excited dye molecule releases some of that energy as visible radiation (Fig.1).
Because a portion of the absorbed radiation is in the UV and invisible to the human eye while the emitted radiation is in the visible spectrum, the daylight fluorescent process, which converts some of the absorbed invisible radiation, results in a luminous emission of color that far surpasses conventional pigments.
Fig. 1. A Jablonski diagram, illustrates the process of fluorescence at the molecular level. Absorbed Light excites the solvated fluorescent molecule. Some of the absorbed energy is lost as the molecule adopts a new configuration as it readjusts its physical interaction with the pigment matrix. As a result, fluorescent emission occurs at a lower energy/longer wavelength than that of the light absorbed.
Note: The term "Ground State" used in the diagram refers to the lowest electron energy level of a molecule.
Unfortunately, the extra energy that produces the excited state from which daylight fluorescence occurs can also be a significant factor in the deactivation of fluorescence as a result of reaction with oxygen, which permanently transform's the dye molecule into a non-fluorescent molecular structure. In addition to oxygen in the atmosphere, oxygen may be dissolved in the pigment system or become resident when it is let down into a carrier for application. Not only oxygen itself but longer lived energetic free radicals formed by reactions between oxygen and the pigment matrix as well as exposure to shorter, more energetic wavelengths of UV light can all serve to accelerate the denaturing and therefore deactivation of the fluorescent dye molecule.
Subsequent processing of the fluorescent pigment can also have destructive effects on fluorescence. Solvents, depending on their relative polarity, can cause leaching of the dye from the matrix, diminishing the matrix solvating effect required for fluorescence and making the entire system more vulnerable to degradation. Similarly, the use of heat in plastic master batching applications not only causes the fluorescent colorant system to become more labile but the heat itself, depending on temperature and time of exposure can have both immediate as well as long-term degradative effects by introducing both O2 and active free radicals into the system, thereby accelerating fading.
Because of the increased risk of fading due to the nature of fluorescent pigments and their sensitivity to the environment and processing, considerable attention needs to be paid to their chemistry and implementation.
Brilliant offers a range of daylight fluorescent products designed to maximize resistance to fading effects encountered in different applications.