A solid membrane of poly(2-ethylhexyl methacrylate-co-vinylpyridine) containing photochromic 69-nitrospiropyran was prepared. In the rubbery copolymer membrane (T g = 29 uC), 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 21 higher than that of fast decay. Furthermore, the standard enthalpy (DHu) of reversible isomerization (SP P MC) without photolysis in the copolymer membrane was negative, in contrast to positive DHu 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 (l em = 570 nm), while the simple MC form in the homopolymer or pyridine solution showed weak red emission (l 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.

The reversible binding reaction of oxygen to N,N'-ethylene bis(salicylideneiminato) cobalt(II) (CoS), to which a photoisomerizable stilbazole residue of copolymer 1 coordinated, was investigated. The E form of the stilbazole residues coordinating to CoS, (E)-1-CoS, showed photoisomerization into the Z form, (Z)-1-CoS, on direct ultraviolet irradiation and negligible reverse isomerization. The oxygen-binding equilibrium constant (K) values for (E)-1-CoS and (Z)-1-CoS in toluene were 3.3 x 10(-2) mmHg-1 and 4.8 x 10(-3) mmHg(-1), respectively, at 10 degrees C. Although the pK(a) values of the E and Z forms of the stilbazole residue were similar, the oxygen-binding affinity of (Z)-1-CoS was small in terms of the linear correlation of the logarithm of K (ln K) versus the pK(a). Steric hindrance of the polymer chain of 1 on the coordination of the stilbazole residue of (Z)-1 to CoS was thought to cause the small K. A photoresponsive change of apparent oxygen-binding affinity of 1-CoS along with the E/Z isomerization of the stilbazole residues was observed. The ratio of (Z)-1-CoS converted from (E)-1-CoS by the ultraviolet irradiation could be estimated from analysis of absorption spectra for the oxygen binding of a resulting mixture of (E)-1-CoS and (Z)-1-CoS.