Small Is Different: N, N-Chelated Organoboron Complexes with Seven-Membered Rings

Abstract

Two seven-membered N,N-chelated organoboron complexes were synthesized. A small structural change led to a significant difference in molecular geometry-one was saddle-shaped, the other approximately planar. This change strongly impacted their optical properties; the planar complex had red-shifted absorption and emisson but a much lower fluorescence quantum yield.Organoboron compounds have attracted significant attention in materials science and medicinal chemistry owing to their unique electronic structures, tunable properties, and compatibility with diverse molecular frameworks. [1][2][3] Incorporation of boron into π-conjugated systems offers access to functionally versatile compounds with promising applications in optoelectronics, molecular sensing, and biomedical fields. 3 This multifunctionality primarily arises from the electrondeficient nature of the boron center, which allows for finetuning of photophysical properties through interactions with appropriately designed donor backbones. According to Baeyer's strain theory, 4 five-and six-membered boron-containing rings are thermodynamically favored due to their low ring strain. Among them, six-membered BODIPY dyes represent a prototypical class, well-known for their high molar extinction coefficients, efficient photothermal conversion, and easily modifiable photophysical behavior. [5][6][7][8][9] These features have enabled their applications in bioimaging, anti-counterfeiting, photodynamic therapy, and photothermal therapy. [10][11][12][13][14][15][16] In contrast, the development of seven-membered organoboron complexes remains limited. Synthetic methods are relatively scarce, and structure-property relationships remain poorly understood, highlighting the need for systematic exploration of this class of compounds.