Isomeric naphthalimides bearing pyran units: Insight into mutual relation between structure and photochromic properties

• Novel naphthopyrans bearing naphthalimide units were synthesized.
• The mutual position of pyran and naphthalimide units effect on the photochromism.
• Long-lived open form of photochrome is due to TT isomer and is thermally stable.
• The fluorescence of naphthalimide is switched by photochromic conversion.

Two novel isomeric photochromic naphthopyrans (1 and 2) containing naphthalimide moieties were prepared and studied. In the compound 1, O-atom of pyran cycle is at C-3 position of naphthalene ring, whereas, in compound 2, O-atom of pyran cycle is at C-4 position. In the compound 2 due to para-position O-atom of pyran photochrome cycle is involved into the conjugated naphthalimide system. The variety in mutual position of pyran and naphthalimide units leads to remarkable difference in photochromic characteristics. Both compounds demonstrate the switching of the fluorescence by photoinduced conversion between the closed and open forms.

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