Highly twisted carbazole-borane derivatives: B–N stereodynamic analysis and consequences on their emission properties

The stereodynamic properties of amino bis-mesityl-boranes bearing carbazole and benzocarbazole as donor heterocycles have been investigated by dynamic NMR analysis and simulated by DFT calculations. The π-contribution to the B–N bond has been estimated to be 24 kcal mol−1 when carbazole is the donor heterocycle, while a value of 21.7 kcal mol−1 has been found for the benzocarbazole series. Two rotational barriers were determined for the B–N bond, the lower one (11.1–16.9 kcal mol−1) leading to conformational enantiomers, and the higher one (21.0–24.0 kcal mol−1) likely being responsible for the E-Z isomerization in compounds bearing different aryl rings bound to the boron atom. It has been shown that both kinds of dynamic rearrangements involve a correlated motion of all the three rings. The difference in the ground state geometries and the different π-contributions led to pronounced variations in the fluorescence spectra, due to different geometric rearrangements in the TICT excited state. Stokes shifts larger than 10 000 cm−1 were observed in the carbazole series, with quantum yields up to 50%. It has been found that the π-contribution to the B–N bond in the excited state is still significant, with B–N isomerism likely not taking place on the ns scale.