Reversed photoswitching of intraniolecular magnetic interaction using a photochromic bis(2-thienyl)ethene spin coupler

Bis(3-thienyl)- and bis(2-thienyl)ethenes having nitronyl nitroxide radicals at both ends of the molecules were synthesized. Bis(3-thienyl)ethene diradicals underwent a reversible photochromic reaction by irradiation with UV and visible light. Photoswitching of intramolecular magnetic interaction was proved by EPR spectroscopy. By choosing an appropriate spacer, the intramolecular exchange interaction in the closed-ring isomer was detected to be more than 150 times larger than that in the open-ring isomer. In the case of bis(2-thienyl)ethene diradicals, the photocyclization reaction was prohibited, and only the cycloreversion reaction took place by irradiation with visible light. The closed-ring isomers of the diradicals were synthesized from the closed-ring isomers of the precursors. From the analysis of the EPR spectra of the system with an appropriate spacer, it was found that on/off switching behavior was reversed by changing the substitution position of the thiophene rings to the ethene moiety from the 3- to the 2-position. The open-ring isomer had a larger interaction than the closed-ring isomer. These results were rationally explained by the switching of the connectivity of the pi bonds.