Synthesis and Stimuli‐Responsive Luminescent Properties of Phenylene‐Bridged and Non‐Bridged Boron <i>β</i>‐Dialdiminate Dimers

We report synthesis and optical properties of the dimers of boron β-dialdiminate (DAI) complexes with different connection manners. The series of dimers, o-, m-, p- and d-BisDAI, were prepared, and their photophysical properties were compared with the single unit, DAI. It was found that the dimers show characteristic optical properties: o-BisDAI showed bathochromic shifts in absorption and emission spectra and emission quenching because of the through-space interactions, while the photophysical properties of m-BisDAI and DAI were almost identical. The extension of π-conjugation in p-BisDAI could induce not only the bathochromic shifts of its absorption and emission bands but also the solvatochromism originating from the symmetry-breaking charge transfer character. d-BisDAI showed the weak low-energy emission in solution and the mechanochromic luminescence which might stem from the rotational degree of freedom between the adjacent two DAI units.