Photoswitching in Polymers with Photochromic Dipolar Species
Petr Toman, Stanislav Nespurek, Martin Weiter, Martin Vala, Juliusz Sworalowski, Wojciech Bartkowiak, and Miroslav Mensk
Experimental and quantum mechanical study of the charge carrier mobility photo-switching in poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene] (MEH–PPV) doped with the photochromic spiropyran 6-nitro-1,3,3-trimethylspiro[2H-1-benzopyran-2, 2-indoline] was performed. The photochromic additive undergoes a reversible ring-opening reaction resulting in a significant increase of its dipole moment. Due to charge-dipole interactions, dipolar traps are formed on the polymer chain in the vicinity of dispersed polar species. The charge carrier trap formation was checked by photocurrent kinetics measurements. At the same time the capacitance of the system was found to increase due to the formation of dipolar species. The calculations show that the presence of dipolar species in the vicinity of an MEH-PPV chain modifies the on-chain site energies and consequently increases of the width of distribution of hopping transport states. The influence of various levels of energetic disorder of chain hopping states on the charge carrier mobility was described by means of the tight–binding approximation model. It was shown that the increasing disorder destroys the resonance between charge carrier energies on adjacent sites, and therefore limits the diffusive charge carrier motion. Thus, the transport properties of a polymer chain can be reversibly changed by a photochromic reaction of the additive.