Blue-Emitting <i>N</i>,<i>O</i>-Coordinated Boron Difluoride Complexes with Benzochalcogenazole-Containing Donor-Acceptor Frameworks Featuring Amplified Spontaneous Emission and Delayed Fluorescence.

Furthermore, a pronounced trend is observed in the C15–Ch–C14 angle (Ch = chalcogen) of the benzochalcogenazole framework: this angle reduces markedly from 105.2–105.3° in the benzoxazole derivatives to 90.3° in the benzothiazole analogue and 85.7–86.0° in the benzoselenazole derivatives ( Table S7 in the SI), reflecting the increasing covalent radii and decreasing (X)­np-π­(C) ( n = 2 for X = O, n = 3 for X = S, n = 4 for X = Se) orbital overlap of the heavier chalcogens.

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These results indicate a clear trend of increasing delayed fluorescence lifetime with the heavier chalcogen atom, suggesting that the incorporation of selenium enhances the intersystem crossing and stabilizes the triplet states, thus favoring DF (e.g., through TTA).

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