Aromatic 2-(2′-hydroxyphenyl)benzoxazole esters: a novel class of caged photoluminescent dyes

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Abstract
Esterification of the photoluminescent 2-(2′-hydroxyphenyl)benzoxazole (HPBO) with aromatic acids has led to a novel family of apolar, non-ionic ‘caged’ photoluminescent dyes. The esters are not photoluminescent, but upon exposure to appropriate UV radiation, the ester bond is cleaved and the photoluminescent HPBO is quantitatively restored. This photoactivation process follows first order reaction kinetics, with quantum yields between 7 and 38%, depending on the nature of the photocleavable ester group. The esterification of HPBO induces a significant hypsochromic shift in the absorption spectrum of the chromophore, creating a wavelength region in which the activated HPBO can be excited, while the HPBO-ester remains caged. Thus, the caged/uncaged dye pairs can be developed and detected with high selectivity. HPBO esters further offer the advantage of high thermal stability, which renders them useful for application in melt-processed polymer blends. This possibility was exploited in the production of photoluminescent images in polymer films, and in the visualization of flow patterns in a polymer melt.