Laser Photolysis Studies of Photoinduced Aggregation in Polymers Containing Spiropyran Units

The photoinduced processes which lead to isomerization and complexation in polymers and copolymers containing pendant indolinobenzospiropyran groups are investigated by nanosecond transient absorption spectroscopy techniques. The monomer l-(β-(methacryloxy)ethyl)-3,3-dimethyl-6'-nitrospiro-[indoline-2,2'-[2H-l]benzopyran] (SP) was copolymerized with methyl methacrylate (MMA) to form copolymers (MMA_m-co-SP_n) and homopolymerized PSP. In solution, the copolymer undergoes a photoinduced ring-opening reaction to forpn B, the fully isomerized transform via a short-lived nonplanar ring-opened intermediate (X) on the subnanosecond time scale. A second transient (AB) is formed by a bimolecular reaction between the lowest triplet state of SP (³A^⋆) with a ground-state ring (A) on the same chain. Efficient AB formation is observed even in the copolymer which is most dilute in SP (MMA_99.9-co-SP_0.1). A further red shift occurring at later times is taken to be evidence for higher aggregate formation (AB)_n or (A_mB)_n. The sterically hindered environment of the homopolymer leads to deactivation of X to the ground state rather than isomerization or complexatiqn is observed. Polar environments produce a similar effect. A detailed mechanism for the observed behavior is proposed.