ABSTRACT
The ease of the ESIPT process is key to the stability of these absorbers. The energy difference between S~ and S~ is thought to be sufficiently small that the excitation energy can be rapidly and efficiently dissipated by internal conversion (14); that is, by losing it as heat. The heat energy so released is a very small fraction of the total heat load on a sample (most coming from absorbed visible and infrared [IR] wavelengths) and does not contribute to thermal degradation of the material. Compounds with ethers in place of the hydroxyl groups cannot undergo ESIPT and generally show strong fluorescence, indicating that the energy dissipation mechanism has been disrupted. They are not photostable. In apolar media, good UVAs generally exhibit little or no fluorescence or phosphorescence. However, polar aprotic solvents are capable of disrupting the internal hydrogen bond of these compounds, resulting in fluorescence and photoreactivity. Any factors that affect the strength of the intramolecular hydrogen bond, such as polarity and basicity of the matrix, are likely to affect the photostability of the UVA.
