Assignment of the S1- S0 electronic absorption spectra of 7-azaindole and 1-azaindolizine as [formula omitted] by rotational band contour analysis

Abstract

The 0 0 0 bands of the 289-nm system of 7-azaindole and the 333-nm system of 1-azaindolizine (imidazo[1,2-a]pyridine) have been rotationally analyzed by computer simulation of their rotational contours. Both are shown to be almost pure type A bands and it is shown that each S 1- S 0 system can be assigned as A ̃ 1A′(ππ ∗)- X ̃ 1A′ . The changes of rotational constants from S 0 to S 1 are very different for the two molecules, leading to dissimilar 0 0 0 band contours. It seems likely that, in 7-azaindole, the major geometry change in S 1 is an expansion of the pyridine ring but, in 1-azaindolizine, it may involve contraction of some of the bonds, which are nominally single in S 0, and expansion of other bonds, which are nominally double in S 0, leading to a very small value of the change in the rotational constant A in S 1.