Ground- and excited-state prototropic equilibria of some hydroxylated benzo(a)pyrenes

Fluorescence excitation and emission spectroscopy are used to determine the ground and excited-state acid dissociation constants for 3-OH- and 9-OH-benzo(a)pyrene as well as the 4,5-, 7,8-, and 9,10-dihydrodiol derivatives. These constants and spectral shifts arising from substituent placement are used to show that in the vicinity of the hydroxyl substituents there is a greater charge localization for 9-OH-benzo(a)pyrene than for 3-OH-benzo(a)pyrene in both ground and lowest excited singlet states, and that for 9,10-dihydrodiol there is greater charge localization than for 4,5- and 7,8-dihydrodiol in the ground state. These results are briefly correlated with the carcinogenic potential of benzo(a)pyrenes.