New Photoswitching Unit for Magnetic Interaction: Diarylethene with 2,5-Bis(arylethynyl)-3-thienyl Group

Photoswitching of the intramolecular magnetic interaction was demonstrated using diarylethenes
with 2,5-bis(arylethynyl)-4-methyl-3-thienyl side group. Two nitroxide radicals were placed at each end of
the 2,5-bis(arylethynyl)-4-methyl-3-thienyl group. Three kinds of aryl groups, 2,5-thienylene, p-phenylene,
and m-phenylene groups, were used in the arylethynyl moiety. The diarylethene photoswitching units have
an extended π-conjugated chain on one side of the diarylethene. The photochromic reactivity was dependent
on the arylethynyl group. Diarylethenes with m-phenylene group showed an efficient photochromic reactivity.
Along with the photochromic reaction the diarylethenes showed photoswitching of an ESR spectrum
originating from the change in the magnetic interaction between two unpaired electrons. The open-ring
isomer showed stronger exchange interaction than the photogenerated closed-ring isomer. The magnetic
interaction between two radicals via the π-conjugated chain was altered by photocyclization due to the
change of the hybrid orbital at the 2-position of the thiophene ring from sp2 to sp3.