Effect of α‑Substitution on Dibenzoylmethanatoboron Difluoride on Evaporative Crystallization

• Published in: Journal of physical chemistry. C 2024-01, Vol.128 (3), p.1469-1476
• Main Authors: Ito, Fuyuki, Mochiduki, Yoshifumi, Fujimoto, Yushi, Kitagawa, Daichi, Kobatake, Seiya
• Format: Article
• Language: English
• Subjects: C: Physical Properties of Materials and Interfaces
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.3c07353

Organic crystals dominated by weak intermolecular interactions are formed in various molecular packings with different physical properties, especially fluorescence properties, also known as mechanochromic luminescence. In this study, evaporative crystallization was investigated using aggregation-dependent fluorescence properties. Dibenzoylmethanatoboron difluoride derivatives (BF2DBM) exhibited large Stokes shifts (orange emission) both in the intermediate liquid-like cluster state during solvent evaporation and in the amorphous state due to smearing. This behavior is most likely related to specific intermolecular interactions. Herein, we investigate the α-substitute dependence of BF2DBM with methoxy substituents at the dibenzoyl parts (2a) to clarify the intermolecular interaction. The large Stokes shift was observed only for 1,3-bis­(4-methoxyphenyl)­methanatoboron difluoride (2aBF 2 ). The methyl-substituted 2a at the α-position (2amBF 2 ) exhibited aggregation-induced fluorescence and weak emission in the intermediates during solvent evaporation without a large Stokes shift. The difluoroboron 2-benzoyl-1-tetralone derivative (2atBF 2 ) also showed a small Stokes shift compared to 2aBF 2 . These findings suggest that the hydrogen atom at the α-position acts as an intermolecular interaction with another F atom by hydrogen bonding, leading to a larger Stokes shift originating from dimeric aggregation, such as J-aggregates. Apparently, both 2atBF 2 and 2amBF 2 cannot form the J-aggregates mediated by the H···F hydrogen bonding because these two molecules do not consist of an H atom at the α-position. The strength of the intermolecular interactions affects the sequential and stepwise aggregation, as revealed by the phase- and aggregation-dependent fluorescence properties.