Three boron-pyridyl-isoindoline-1-one based dyes (B1, B2, and B3) with varied side groups were facilely synthesized. Detailed analyses were carried out concerning the crystal conformation, structure dependent photophysical properties and mechanochromic fluorescence (MCF). The MCF behaviours and solid-state emission were demonstrated to be correlated to the different side groups. Upon grinding, the emission of B1 exhibited little changes, whereas reversible MCF with a distinct colour change was observed for B2 and B3. B3 possessing sterically hindered triphenyl amine (TPA) groups showed the most prominent MCF effect. For instance, mechanical grinding resulted in a red-shift of fluorescence from 600 nm to 650 nm. The electronic and steric effects of the donating substituents were found to play important roles in modulating the intramolecular charge-transfer effect and intermolecular interactions. These sensitively and readily tunable mechano-responsive behaviors of the boron-pyridyl-isoindoline-1-one based dyes make them potential candidates for smart fluorescent materials.