Synthesis and Photochromic Properties of Diaryl [5]Helicene Derivatives

Photoinduced tuning of aromaticity and correlative molecular properties has attracted enormous interest in recent years both for modulating photochromism properties and designing novel photochromic materials. Here, we report the synthesis and photochemical characterization of diaryl[5]helicene‐based diarylethene molecular switches. 3,4‐Bis(2,4‐dimethyl‐5‐phenylthiophen‐3‐yl)dibenzo[c,g]phenanthrene derivative 1 a showed no photochromic reaction, whereas 3,4‐bis(2‐methyl‐5‐phenylthiophen‐3‐yl)dibenzo[c,g]phenanthrene derivative 2 a and 3,4‐bis(5‐methyl‐2‐phenylthiazol‐4‐yl)dibenzo[c,g]phenanthrene derivative 3 a exhibited reversible photochromism in different aprotic solvents with specific light irradiation. Meanwhile, the diarylethene compounds 2 a and 3 a underwent turn‐off mode fluorescence photoswitching together with photoisomerization upon light irradiation. Remarkably, the photoinduced changes in the aromaticity of [5]helicene as a central ethene bridge along with the relative smaller activation energy and higher frequency factor facilitated the thermal bleaching rates of diarylethene switches 2 and 3 in solution. This research provides new insight for designing aromatic diarylethene photoswitches for reversible fluorescence switching, photoinduced changes in aromaticity and further fast thermal back reactions. Diaryl [5]helicene derivatives demonstrate well‐defined reversible photocyclization reactions with turn‐off mode fluorescent photoswitching behaviors upon photochromic reaction. The loss of aromaticity of the central benzene ring of [5]helicene along with the photocyclization reaction the thermal bleaching rate.