Pyridinium salts and ylides as partial structures of photoresponsive Merrifield resins

Abstract

Merrifield resin was treated with 4,4′-bipyridine and 2,4′-bipyridine, respectively, to give photochromic materials. On exposure to light, reversible color changes are observable. These resins also serve as indicators because reversible color changes are observable on addition of bases, which deprotonate the benzylic position of the N-benzylpyridinium salts to ylides. These can be detected spectroscopically and by trapping reactions on monomeric model compounds. EPR and HYSCORE measurements in combination with DFT calculations prove that radical species are formed in either case. Evidence is found that the carbanionic centers of the ylide partial structures serve as electron donators in these reversible color changes, thus setting them apart from known photochromic materials.