Fluorescence sensing of microcracks based on cycloreversion of a dimeric anthracene moiety

Novel fluorescent crack sensors have been developed based on dimeric anthracene moiety-containing polymers. Two anthracene derivatives, 9-anthraldehyde (AA) and 9-anthracenecarboxylic acid (AC), were photodimerized to obtain cyclooctane-type dimers. Crack-sensing polymers (Poly-AA and Poly-AC) were prepared by crosslinking of poly(vinyl alcohol) by using the dimers as crosslinkers. The polymers afforded transparent, hard coatings. Upon cracking, the polymers exhibited strong optical absorption and fluorescence emission while the uncracked original polymers did not. This was explained by regeneration of the anthracene moiety by mechanochemical cycloreversion of the cyclooctane dimer structure. It was found that the crack planes emitted fluorescence having emission maxima in the range of 500-600 nm when excited with 330-385 nm UV light. Absolute fluorescence quantum yield measurements indicated that the polymers could have good capability of fluorescence crack sensing. Preliminary evaluation of the crack-sensing ability of Poly-AA and Poly-AC was performed with the polymer films, and fluorescence emission was clearly observed along the crack planes upon excitation with 330-385 nm UV light. Poly-AA and Poly-AC are promising as fluorescent crack sensors because the anthracene moiety regenerated upon cracking has relatively long excitation and emission wavelengths as well as strong fluorescence.