Kristian M. Roth

Studies related to the design and synthesis of a molecular octal counter

An approach to the storage of multiple bits of information at the molecular level employs molecules with a large number of distinct oxidation states. Europium triple-decker sandwich molecules composed of porphyrins and phthalocyanines afford four cationic states and are very attractive for molecular information-storage applications. A larger number of states can be achieved by combinations of triple deckers that afford interleaved oxidation potentials. In order to identify suitable candidates for effective interleaving of oxidation potentials, a library of 19 new triple-decker complexes was prepared. Electron-donating groups have been attached to the porphyrin and/or phthalocyanine moieties in order to achieve oxidation states in the low potential regime. The triple deckers are of three different types: (Pc)Eu(Pc)Eu(Por), (Pc)Eu(Por)Eu(Pc), and (Por)Eu(Pc)Eu(Por). The solution electrochemistry of each member of the library was examined. These studies revealed suitable pairs of triple deckers that provide effective interleaving of oxidation potentials. Six triple deckers of type (Pc)Eu(Pc)Eu(Por) were derivatized with a thioacetyl or thiocyanate group on the porphyrin unit for attachment to an electroactive surface. Each of the S-(acetylthio)-derivatized triple deckers forms a self-assembled monolayer (SAM) on Au viain situ cleavage of the thiol protecting group. The SAM of each triple decker is electrochemically robust and exhibits four, well-resolved reversible oxidation waves. Upon disconnection from the source of applied potential, the triple-decker SAMs retain charge for tens to hundreds of seconds. The exact value of the charge-retention time depends on the specific porphyrin/phthalocyanine in the triple decker and the particular oxidation state of the molecules in the SAM (e.g., mono- vs. di- vs. tri- vs. tetracation). For all of the triple-decker SAMs, the charge-retention time monotonically increases as the oxidation state of the molecules in the SAM increases. Collectively, the studies suggest that the triple-decker complexes are excellent candidates for multibit molecular information storage elements.

Kinetically controlled vapor-diffusion synthesis of novel nanostructured metal hydroxide and phosphate films using no organic reagents

Nanostructured Co5(OH)8Cl2·3H2O, Co5(OH)8(NO3)2·2H2O, Co5(OH)8SO4·2H2O, Zn5(OH)8(NO3)2·2H2O, Cu2(OH)3(NO3) and Mn3(PO4)2·7H2O thin films have been prepared using a kinetically controlled vapor-diffusion method. Vectorial control by diffusion of ammonia as a base catalyst into an aqueous metal salt solution yields large area (2 cm2) metal hydroxide and metal phosphate films with unique structures. No supporting substrate for growth of the films is necessary in this approach. The films were characterized using X-ray powder diffraction and scanning electron microscopy. The cobalt containing films were studied in more detail using transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption near edge structure and various chemical analysis techniques. For the first time the electronic properties and crystal structure of these materials could be studied in thin films not influenced by the presence of an underlying substrate. For Co5(OH)8(NO3)2·2H2O films, which crystallize in a layered hydrotalcite-like structure that is homogeneous from the nanoscale to the macroscale, unprecedented photoconductivity properties were observed. Resistivity measurements show that this material is a p-type semiconductor with an unusually long minority carrier lifetime and high carrier density.