Photoisomerization of a sterically constrained merocyanine dye

Abstract

A particular concern regarding the photophysical properties of merocyanine 540 derivatives, and related cyanine dyes, involves identifying which double bond in the central polyenic bridge undergoes light-induced isomerization. In order to address this issue we have synthesized a novel merocyanine dye in which the first double bond is built into a cyclic 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 the triplet manifold are kept small. For this dye, isomerization must take place at the central double bond. It is further shown that the strategy of inserting bulky or constraining groups into the polyenic bridge is not a viable approach for the development of highly fluorescent merocyanine dyes.

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