Synthesis and Electronic Spectroscopy of Bromocarbazoles. Direct Bromination of N‐ and C‐Substituted Carbazoles by N‐Bromosuccinimide or a N‐Bromosuccinimide/Silica Gel System

The preparation, isolation and characterization by elemental analysis and ¹H‐NMR, ¹³C‐NMR, and MS data of the bromo derivatives of N‐substituted carbazoles, i.e., of 9‐methyl‐9H‐carbazole ( 1 ), 9‐phenyl‐9H‐carbazole ( 2 ), 9‐benzyl‐9H‐carbazole ( 3 ), 2‐methoxy‐9‐methyl‐9H‐carbazole ( 4 ), and of C‐substituted carbazoles, i.e., of 2‐(acetyloxy)‐9H‐carbazole ( 5 ) and 3‐nitro‐9H‐carbazole ( 6 ), are reported, in part for the first time. As brominating reagents, N‐bromosuccinimide (NBS) or NBS/silica gel in CH₂Cl₂, NBS in AcOH, KBrO₃/KBr in EtOH doped with a catalytic amount of H₂SO₄, or KBrO₃/KBr in AcOH were employed, and their uses were compared. Semi‐empirical PM3 calculations were performed to predict the reactivity of the N‐substituted and C‐substituted carbazoles and of their bromo derivatives and found to verify the experimental results. The UV‐absorption and fluorescence and phosphorescence emission spectra of the bromocarbazole derivatives in MeCN solution at 298 K and in a solid matrix at 77 K were compared with those of the corresponding carbazoles 1 – 6 . The dynamic properties of the lowest excited singlet and triplet states (τ_f, τ_p, ?_f, and ?_p) were measured under the same experimental conditions. The intramolecular spin–orbital‐coupling effect of the Br‐atom and NO₂ group on the spectroscopic data, photophysical parameters, and on the photo reactivity were also briefly analyzed.