AM1 and X‐ray Studies of the Structures and Isomerization Reactions of Indigo Dyes

AbstractAM1‐SCF and AM1‐SDCI calculations have been carried out for indigo and three conformers of N,N′1‐diacetylindigo in their cis and trans configurations. The molecular structure of trans‐ N,N′1‐diacetylindigo was determined by X‐ray diffraction. The calculated ground‐state data obtained within the SCF approximation (relative energies, structural parameters) are in excellent agreement with the corresponding experimental values. Potential energy curves (PEC) for the isomerization reaction cis/trans in several electronic states were obtained from AM1‐SDCI calculations. For N,N′1‐diacetylindigo the ground state PEC has a maximum at a torsion angle about the central CC bond of 90°, whereas the S1‐ and T11‐state PEC's show minima at the corresponding geometry. For indigo itself significant barriers to the photoisomerization in the S1‐ and T11‐excited states are predicted because of increased NH…OC hydrogen bonding. The results provide an explanation for the phototropic behaviour of N,N′1‐diacetylindigo and the photostability of indigo. The calculated enthalpies of activation for the isomerization process cis → trans in the ground state compare well with available experimental data, thus rendering feasible the prediction of the thermal stability of cis isomers not yet synthesized.