A Theoretical Study of the Indigoid Dyes and Their Chromophore

Abstract
A theoretical study of the electronic spectra of indigo, bispyrroleindigo, and the H‐chromophore, thought to be the basic structure leading to the intense absorption in the visible spectrum of the indigoid dyes, has been performed by means of the complete active space (CAS) SCF method and the multiconfigurational second‐order perturbation theory (CASPT2). The calculations include excitation energies, oscillator strengths, and transition moment directions for the valence singlet states of the molecules. The assumption that the H‐chromophore is responsible for the absorption in the visible spectrum is quantitatively revisited with the help of the modern ab initio methodologies and the main features of these molecules are theoretically analyzed, some of them for the first time.

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