PHOTOISOMERIZATION OF A STERICALLY CONSTRAINED MEROCYANINE DYE

A particular concern regarding the photophysical properties of merocya nine 540 derivatives, and related cyanine dyes, involves identifying w hich double bond in the central polyenic bridge undergoes light-induce d isomerization. In order to address this issue we have synthesized a novel merocyanine dye in which the first double bond is built into a c yclic structure that prevents isomerization at this site. Contrary to expectations, the dye photoisomerizes with reasonable efficiency, such that the quantum yields of fluorescence and intersystem crossing to t he triplet manifold are kept small. For this dye, isomerization must t ake place at the central double bond. It is further shown that the str ategy of inserting bulky or constraining groups into the polyenic brid ge is not a viable approach for the development of highly fluorescent merocyanine dyes.