The opposite and amplifying effect of B ← N coordination on photophysical properties of regioisomers with an unsymmetrical backbone

1,3-Dipolar cycloaddition of pyrido[2,1-a]isoindole with internal alkynes functionalized by a BMes₂ph and an N-aromatic heterocycle leads to the formation of two types of regioisomers (major a and minor b) that have distinct physical and photophysical properties. Examination on 5 pairs of regioisomers unveils that the major isomers consistently have a smaller optical energy gap and emission energy than the corresponding minor isomers, which is greatly amplified by the formation of an internal B ← N bond. The regioisomers with a B ← N bond display contrasting temperature-dependent structural dynamics and response to fluoride ions, owing to an entropy-driven or fluoride initiated B ← N bond rupture/ring-opening process and the different B ← N bond strength. The opposite inductive effect and the Lewis pair properties of the dichotomic substituent units are responsible for the contrasting properties of the regioisomers in this system.