EXCITED STATES AND REACTIVITY OF CARCINOGENIC BENZPYRENE; A COMPARISON WITH SKIN-SENSITIZING COUMARINS*

Abstract— Significant spectroscopic difference was found between the ³(π, π*) state of coumarins and the ³L_a state of benzpyrene. No analogy of the partially localized triplet state associated with the coumarin chromophore was revealed in the case of the potent carcinogen, benzoa]pyrene. Instead, the ³L_a state of benzoa]pyrene is characterized by more derealization than that of the non–carcinogenic benzoe]pyrene. Therefore, the predominant photoreaction between benzoa]pyrene and DNA bases does not seem to involve cycloaddition in contrast to the coumarin-pyrimidine system. Reactivity indices have been calculated, and results are consistent with recent experimental findings. In addition, spectroscopic properties of benzoa]pyrene and benzoe]pyrene have been comparatively described in terms of relatively high-resolution spectra, polarization measurements, and molecular orbital calculations.