Novel polychromogenic fluorine-substituted spiropyrans demonstrating either uni- or bidirectional photochromism as multipurpose molecular switches

Novel photochromic spiropyrans (SPPs) of the indoline series with fluorine substituents and cationic fragments or formyl groups at positions 6′ and 8′ of the 2H-chromene moiety were synthesized using a simple one-pot method. Molecular structures of 8′-fluorine-substituted compounds were refined by single-crystal X-ray, which revealed that both SPPs exist in their merocyanine forms in a crystalline state. It was established, that structure-related photochromic transformations of studied SPPs include: positive photochromism for SPPs with a vinyl-3H-indolium fragment or a formyl group at position 8′ of the benzopyran moiety; negative photochromism for SPP with a vinyl-3H-indolium fragment at position 6′ of the molecule; both positive and negative photochromism for SPP with a formyl group in position 6′ of the molecule, demonstrating the properties of photochromic “balance”. The polychromogenicity of the obtained compounds allows them to be considered as fluorescent molecular switches and bipolar absorption switches.