Molecular and crystal structure of the merocyanine form of 1′-hydroxyethyl-3′,3′-dimethyl-6-nitro-8-methoxyspiro(indoline-2,2′-[2H-1]benzopyran)

Conclusions

• 1. An x-ray diffraction structural analysis has established the structure of the merocyanine form of 1′-hydroxyethyl-3′,3′-dimethyl-6-nitro-8-methoxyspiro(indoline-2,2′-[2H-1]-benzopyran). The introduction of an 8-OCH₃ substituent into the benzopyran fragment reduces the zwitter-ionic nature of its complex hybrid structure and increases the contribution of the less polar quinoid form, leading to increased stability of the photochromic systems derived from this type of spiropyrans relative to side-reactions.

• 2. The molecules in this crystal structure form strong chains due to Van der Waals interactions, which are linked by hydrogen bonds. The formation of such chains and their mutual association largely determines the structure of the absorption spectra of this compound in the solid phase and solution.