Conformationally Driven Dual Fluorescence Properties of Higher Heteroacenes With Periodically Incorporated Boron Atoms

Abstract

Hexaboraheptacene (B6-hept), featuring a periodic arrangement of boron atoms, together with its shorter homologues tetraborapentacene (B4-pent) and diboraanthracene (B2-ant), were synthesized via cyclocondensation of o-bis(dihydroboryl)arenes. The reactive boron centers are effectively protected by 2,4,6-triisopropylphenyl (Tip) groups, enabling handling under ambient conditions. As the number of boron bridges increases, the electron-accepting ability as well as the multistep reversible redox properties are progressively enhanced. B6-hept exhibits pronounced dual fluorescence in solution, in which relative intensities depend on the excitation wavelength and solvent viscosity. DFT calculations suggest that this phenomenon originates from two interconvertible conformers, namely, the bent-zigzag and twisted forms, which result from a delicate balance between the extended π-system and the steric demands of the Tip groups. This interpretation is supported by the fact that B4-pent displays only weak dual emission, while B2-ant shows none. This study not only introduces a new class of heteroacenes with potential applications in optoelectronics and electrocatalysis, but also provides insight into steric-design strategies for tuning the conformational landscapes of boron-containing π-systems.