An efficient approach to diarylethene-amino acid photochromic fluorescent hybrids

Abstract

• An effective approach to creating photo-controlled diarylethene-amino acid hybrids. • X-ray diffraction study, spectral luminescent and photochromic properties. • Thermally and photochemically reversible intramolecular photocyclization. • Dependence of the photoinduced form stability on the structure. • Modulation of emission under successive irradiation with light of different wavelengths. An effective approach to the synthesis of diarylethene-amino acid hybrids DE-Gly, DE-AABA and DE-DAA ( 5a-d - 7-a-d ) was developed via condensation of furan-2,5-dione-based diarylethenes (DE) and ethyl esters of glycine (Gly), α-aminobutyric acid (AABA) and D-aspartic acid (DAA) with moderate to good yields. According to X-ray diffraction data, DE-Gly hybrid 5a exists in an antiparallel conformation with a distance of 4.549 Å between the reactive carbon atoms C(1)-C(11), potentially capable of forming a single bond, which is suitable for the conrotatory photocyclization reaction allowed by the Woodward-Hoffman rules. The structures of the obtained compounds were proved by 1 H, COZY, HSQC, HMBC and 13 C NMR spectroscopy. The hybrids DE-Gly, DE-AABA and DE-DAA absorb at 443–456 nm, which corresponds to their existence in a ring-open form O, and display fluorescence at 531–612 nm. Irradiation with light of 436 nm results in their rearrangement into ring-closed colored nonfluorescent isomers C. Backwards re-opening occurs under the action of visible light (λ > 500 nm). Spectral kinetic investigation of 5a,c revealed that the efficiency of photocyclization, as well as the emission quantum yield, decreases with the growth of solvent polarity. The ring-closed isomer C of sterically hindered 5a (R 2 = R 3 = Me) is stable at room temperature, whereas for 5c (R 3 = H), ring opening readily occurs with the speed increasing with the polarity of the solvent. The internal emission of sterically hindered hybrids 5a, 6a and 7a (R 2 = R 3 = Me) is reversibly modulated in a binary response with good fatigue resistance under successive irradiation with light of 436 and 540 nm.