Fluoride-Triggered Ring-Opening of Photochromic Diarylpyrans into Merocyanine Dyes: Naked-Eye Sensing in Subppm Levels

The
fluoride-mediated desilylation reaction has been exploited,
for the first time, to trigger ring-opening of photochromic diarylbenzo-/naphthopyrans
into highly colored anionic merocyanine dyes with high molar absorptivities
to permit naked-eye sensing. The absorption spectral shifts, i.e.,
differences in the absorption maxima of colorless and colored forms,
observed for a rationally designed set of silyloxy-substituted diarylpyrans
subsequent to fluoride-induced ring opening are remarkably high (330–480
nm), and are unknown for any colorimetric probe. In particular, the
disilyloxy-substituted diphenylnaphthopyran and its analog, in which
the diphenyl groups are fused in the form of fluorene, allows “naked-eye”
detection of fluoride in subppm levels (<1.0 ppm) in THF as well
as in DMSO-H2O. The sensing is specific for fluoride among
various other anions. This approach for colorimetric sensing of fluoride
by ring-opening of the otherwise photochromic benzo-/naphthopyrans
is heretofore unprecedented.