Control of the association of indolylfulgimide with bis(acylamino)pyridine by photochromism

While the association constant of the colored form of a thermally irreversible photochromic indolylfulgimide with 2,6-bis(octanoylamino)pyridine in toluene at 21 degreesC was 156 +/- 11 mol(-1) dm(3) through triplex hydrogen bonding, that of its colorless form, generated by visible-light irradiation, was increased to 885 +/- 63 mol(-1) dm(3). This result was supported by PM3 semiempirical molecular-orbital calculations that the difference of the association constants between the C-form and the E-form was mainly due to differences in the structures and conformations of the imide moiety.