Ultraviolet spectra of coumarins and psoralens

The π → π^* transitions of coumarins in the ultraviolet region have been described on the basis of fluorescence polarization, dichroic spectra, and SCF MO CI results. The two lowest π → π^* transitions in coumarin, 4-hydroxy-coumarin, and psoralen were found to be polarized nearly parallel to each other, probably along the long molecular axis. The two lowest π → π^* transition moments in 8-methoxypsoralen were found to be oriented with a substantial angle between them, in contrast to the case of coumarin and psoralen. It was not possible to identify any n → π^* bands as they are probably obscured by the strong π → π^* bands.