EFFECT OF THE BROMINE ATOM UPON THE DISSOCIATION-CONSTANTS OF BROMOPYRIDINES, BROMOQUINOLINES AND 4-BROMOISOQUINOLINE IN THEIR LOWEST EXCITED SINGLET AND TRIPLET-STATES

Abstract— The lowest excited singlet‐state dissociation constants (pKSa) of bromosubstituted pyridines, quinolines, and isoquinolines were determined from the pH‐dependent shifts in their electronic absorption spectra. The lowest excited triplet‐state dissociation constants (pKTa) of bromosubstituted quinolines and 4‐bromoisoquinoline were obtained from the shifts of the 0–0 phosphorescence bands measured in rigid aqueous solution at 77 K. The pKSa values indicate that the basicity of these brominated nitrogen heterocycles is increased in the lowest excited singlet state by 2 to 10 orders of magnitude as compared with the ground state. The pKTa values are found to be significantly different from the corresponding ground‐state pKa values, indicating that the basicity of bromoquinolines is increased in the lowest excited triplet state by 1.7 to 3.0 pK units. The enhancement of the excited singlet‐and triplet‐state basicity of brominated nitrogen heterocycle derivatives as compared with the unsuhstituted parent compounds is attributed to the increased electron‐donor conjugative interactions of the bromine atom pπ orbitals with π orbitals in the lowest excited singlet and triplet state. Copyright © 1979, Wiley Blackwell. All rights reserved