Synthesis and study of new photochromic unsymmetrical bis-spiropyrans with nonequivalent heteroarene fragments conjugated through the common 2H,8H-pyrano[2,3-f]chromene moiety

Abstract

Four novel bis-spiropyrans possessing two different photochromic units based on indoline and 1,3-benzoxazine heterocycles which are connected through the common 2H,8H-pyrano[2,3-f]chromene moiety and modified with electron donating substituents were obtained by multistep synthesis starting with 2,4-dihydroxy-iso-phthalic aldehyde. The structure of the compounds was confirmed by 1H and 13CNMR, FTIR and HRMS. All bis-spirocompounds exhibited photochromic activity at room temperature. Investigation of photochromic properties of bis-spiropyrans revealed that their once-opened merocyanine isomers were more stable than the double-opened ones under experimental conditions. It was also found that modification of the benzoxazine moiety of the molecule with electron donating substituents along with introduction of bulky benzyl group close to the oxazinespiro-center leads to an increase of the merocyanine lifetime up to 148 s.