Turning‐on the Quenched Fluorescence of Azomethines through Structural Modifications

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Abstract
A series of fluorene azomethines were prepared to examine the effect of structural modification on the absolute fluorescence quantum yields (Φfl). The change in fluorescence occurring upon reversing the orientation of the azomethine bond, the number of fluorene units, and alkylation at the 9,9′‐positions of fluorene was examined. It was found that Φfl could be tailored with these structural modifications with 0.05 ≤ Φfl ≤ 0.45. The highest Φfl was measured for the trimer derived from 2,7‐diamino fluorene with each fluorene being alkylated at the 9,9′‐positions. The fluorescence quantum yields did not increase at 77 K, implying the absence of fluorescence quenching modes involving internal conversion. Fluorescence enhancement was also not observed upon protonating the imine with trifluoroacetic acid. This confirms that Waldon inversion and isomerization are not fluorescence deactivation modes. Additionally, no photoisomerization was observed either at room temperature or at 77 K when the fluorene derivatives were irradiated at 354 nm. In contrast, the Z photoisomer of a N‐benzylideneaniline analogue was spectroscopically detected both by absorbance and by 1H NMR spectroscopy.

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