Biphenylene-based all-carbon analogues of dithienylethene photoswitches

Abstract

We report the exploration of novel diarylethene photoswitches constructed through the successive incorporation of the biphenylene ring system at key structural positions. The low aromatic character of the benzene rings in biphenylene enables observable photoswitching even when all three double bonds that are involved in the electrocyclization reaction are embedded within biphenylene subunits. Replacing a single thienyl group in the well-known dithienylethene containing a perfluorocyclopentene bridge with a biphenylene unit leads to a P-type photoswitch that responds to longer-wavelength light and exhibits improved fatigue resistance compared to the parent photoswitch. Incorporation of two biphenylene units as the aryl groups provides a thermally stable switch (t_1/2 = 780 h), while the terbiphenylene analogue is a T-type system that undergoes rapid thermal ring opening (t_1/2 ≈ 20 min) at room temperature. The changes in the aromatic character of the biphenylene units associated with photochemical ring closing were quantified by NICS calculations for each photoswitch in both the open and closed forms of the molecules.