Mechanisms of Photochemical Degradation in Xanthene Laser Dyes

摘要

The primary objective of this research program has been the detailed elucidation of the principal photochemical mechanisms which lead to the degradation of xanthene laser dyes. The research performed in the course of this program has led to the discovery of an unprecendent excited state pathway for the photodegradation of Kiton Red S, a sulforhodamine laser dye, in which the reactive, lowest energy excited triplet state of the molecule is populated by preferential intersystem crossing from upper singlet excited states, not from the singlet state nearest in energy to the triplet. The results obtained have also led to a tentative chronology of the first several stages in the photodegradation process, and have revealed the existence of a previously unknown contact charge transfer (CCT) complex between oxygen and methanol. While much remains to be learned about the details of the chemical reactions involved in the photodegradation of kiton Red S and other xanthene dyes under various conditions, the research performed during this program has achieved significant progress toward this goal. In particular, enough has been learned about the degradation of these dyes that it is now possible to suggest new strategies for slowing their degradation under lasing conditions. These include, in particular, the removal of excitation wavelengths below 240 nm by selective filtering, and the use of improved triplet quenchers.