Reversible photo-switching interaction between spiropyrans and polymer pyridine residues in a solid polymer membrane

Abstract

A solid membrane of poly(2-ethylhexyl methacrylate-co-vinylpyridine) containing photochromic 6′-nitrospiropyran was prepared. In the rubbery copolymer membrane (T_g = −9 °C), the merocyanine (MC) form of the spiropyran isomerized into the spiro (SP) form by thermal decay after irradiation with UV light. The resulting decay showed two types of first-order kinetics, fast and slow. The activation energy for slow decay in the copolymer membrane was about 4 kcal mol⁻¹ higher than that of fast decay. Furthermore, the standard enthalpy (ΔH°) of reversible isomerization (SP ⇄ MC) without photolysis in the copolymer membrane was negative, in contrast to positive ΔH° values obtained for a homopolymer membrane of poly(2-ethylhexyl methacrylate) and for a liquid pyridine solution of spiropyrans. It was deduced that some of MC molecules in the copolymer membrane were thermodynamically stabilized. Additionally, the stabilized MC form in the copolymer membrane emitted intense green fluorescence (λ_em = 570 nm), while the simple MC form in the homopolymer or pyridine solution showed weak red emission (λ_em = 620–630 nm). The green fluorescence of the copolymer membrane was photo-reversible. These results suggest the existence of a photo-switchable interaction between spiropyrans and polymer pyridine residues in the solid copolymer membrane.