Tryptanthrin (T) and 2-fluorotryptanthrin (T2F) exhibit no fluorescence in the solid state. The presumption exists that this is due to the strong π-π stacking interaction resulting from the planar structure. However, the analogs, DCM-T and DCM-T2F, in which the 6-position carbonyl group is replaced with a stronger electron-withdrawing group, the dicyanomethylene group, exhibit fluorescence in the solid state despite the planar structure. The reasons for this were analyzed and discussed based on the results of X-ray crystal structure analysis.
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Kawakami et al. (2021) studied this question.
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