A novel photochromic hetarylalkylideneisocromandione system

A novel photochromic system based on an alkylideneisochromanedione core with heterocyclic substituents was synthesized, and its spectral luminescent and photoinduced properties were studied. The obtained isochromandiones containing the hexatriene fragment exist in a ring-open E-form, confirmed by the data of 1H, 13C NMR spectroscopy and X-ray diffraction studies. The observed photoinitiated processes depend fundamentally on the nature of the heterocyclic substituents. Thiophenyl, benzo[b]thiophenyl and 1H-indolyl derivatives 5c-f are non-photochromic in the solid state, and in solutions their transformations are limited by thermally reversible E/Z-photoisomerization. Compound 5a with a furanyl substituent is photochromic in crystalline form due to the light-induced rearrangement of the ring-open hexatriene isomer into the ring-closed cyclohexadiene structure. Upon irradiation with light of 436 nm, the yellow crystals of 5a turn dark blue-green. The reverse reaction of the cycle opening and restoration of the original color of the crystals occurs extremely easily upon irradiation with visible light of 578 nm. In contrast to crystals, solutions of compounds 5a and 5b with furanyl substituents in hexane, 2-propanol, acetonitrile and DMSO are discolored when irradiated with light of 365 nm. The irreversible photorearrangement 5a,b → 6a,b includes the stages of cyclization, the opening of the furan cycle and the formation of highly fluorescent naphthalic anhydride derivatives 6 (the quantum yield of fluorescence reaches 0.69 in hexane)), which can be used for the production of optical memory media for archive 3D discs.