Photo-Activation of 2,2’-Bipyridine-Sulfur Oxides Donor-Acceptor Complex

The work presented in this thesis discusses the synthesis of a novel photochromic molecule which is induced by a bipyridine-sulfur dioxide (SO2) charge transfer mechanism. SO2 is produced in large quantities by many industrial processes, and great effort is spent to reduce its emission. SO2 is an underutilized feedstock for chemical reactions and functional materials alike, so great interest is applied to new uses of this byproduct.Photochromic bipyridines typically appear N, N’-disubstituted-4,4’-bipyridines, known as viologens. Following exposure to UV light, viologens undergo a one electron reduction to a blue colored radicle cation. This radicle cation is easily detectable by EPR spectroscopy. The photochromic bipyridine presented in this work undergoes a color change from light yellow to magenta upon irradiation, resulting in a new absorption peak at 550 nm. The EPR spectroscopy of the irradiated colored product does not detect any radicles, which suggests a new mechanism different from viologens. The present bipyridine containing system exhibits photochromic response for small molecule solids, small molecule solutions, and polymeric systems. Each of these environments exhibit tunable color change. Small molecule solutions and polymeric systems exhibit t-type photochromic behavior. DFT calculations were performed on the photochromic small molecule complexed with SO2. The results of these calculations indicate that photochromism is attributed to a UV-induced bipyridine-SO2 bond strengthening. The decrease of N-S bond length lowers the energy of the of the charge transfer band into the visible range. This represents a new class of photo-responsivity which can be applied to various applications common to photo switching materials. The concluding remarks suggest future directions of mechanistic studies as well as implementation of the photoproduct in film fabrication, and as a light induced mechano-responsive material