Very stable open form of 2H-naphthopyran: A theoretical study

Abstract

Using DFT calculations, it was shown that the intermediate stacked form of the ground state of 2H-naphthopyran can be formed not only during the relaxation of the intramolecular S1 excimer but also during the thermal recovery of the closed form from the trans–trans and cis–trans isomers of the open form. The potential well corresponding to the intermediate form is an additional factor in the stability of the open form, since from this well the molecule can return to the open cis–trans form using thermal motion. The cis–trans and trans–trans isomers of the open form are separated from each other by potential barriers with a height of 1.25 eV. Their overcoming by thermal motion is less likely than the transition of the cis–trans form to the intermediate stacked form. The absorption of conformers in the visible region, which determines the color of the open form, was calculated using TD-DFT; the resulting vibronic absorption band of the trans–trans form coincided well with the experimental one.