Photochemical Stability in Vapor-deposited Organic Thin Films Can be Modulated by Glass Packing

Photochemically robust materials are desired for organic electronics. While chemical structure is typically altered to improve photostability, we show that photostability of organic amorphous thin films can be significantly improved by packing them more tightly via physical vapor deposition. Disperse Orange 37, a push-pull azobenzene with fast thermal isomerization, can be made 50 times more stable against photoisomerization if deposition temperatures are chosen correctly. Photostability was determined by measuring density and molecular orientation changes during irradiation. We further show that the enhanced photostability in vapor-deposited glasses is a general phenomenon by using a non-push-pull azobenzene, 4,4’-diphenyl azobenzene (DPA). By mixing DPA into the glass host of celecoxib, we directly measure populations of trans and cis via UV-Vis and show that the rate of photoisomerization varies as a function of the substrate temperature. Photostability correlates with density of packing, where the optimum glass is about 30 times more photostable than the liquid-cooled glass. These results provide a molecular explanation for enhanced photostability in glasses, and they provide insight in designing organic electronics by making denser materials to achieve longer lifetimes.