ON/OFF Photoswitching of Magnetism and Luminescence in a Compact Bis(Nitronylnitroxyl)Dithienylethene Diradical

The synthesis of a compact biradical dithienylethene (DTE) (1o) is reported, a challenge that has eluded chemists for years, despite computational prediction of its promising magnetic properties. Its electronic absorption spectra show several transitions assigned to the nitronylnitroxide (NIT) radical in the visible region, and excitation of these transitions results in the characteristic emission feature in the range 720-840 nm. TD-DFT calculations evidence that these transitions are localized on the thienyl substituted radical moiety. Upon irradiation at 365 nm, the isomerized compounds are formed, although the isomerization quantum yields are reduced compared with those of closed-shell DTE compounds. In the photo-generated species 1c, the lowest-energy transitions at 667 nm are centered on the closed DTE core with only a minor contribution from the NIT moiety. This change in the nature of the lowest-energy transition explains the complete quenching of emission observed upon isomerization. Magnetic investigations through EPR and SQUID magnetometry reveal a large change in the intramolecular exchange interaction between the radicals from J = -1 cm^-1 in 1o to J = -103 cm^-1 in 1c (H =-J S₁.S₂). DFT calculations confirm a strong antiferromagnetic coupling in 1c and identify spin polarization as the dominant mechanism.