| The effects of altering the bridge structure and length from 5 to 17 bonds in aryl-bridged dihydrazine mixed-valence (MV) systems were investigated to probe valence-localized to valence-delocalized characteristics.Hush Theory was applied to a series of valence-localized compounds that contain a 9 to 13 bond-bridge and was shown to predict electron transfer (ET) rates quite well for these aryl substituted dihydrazine MV systems. Additionally, our findings suggest that the stilbene bridged MV systems exhibit greater ET rates than the corresponding tolane bridged MV systems because the more electronegative and cylindrically symmetrical tolane bridge has a larger solvation energy, lambdasThe electronic coupling for a series of 5 bond-bridged systems were found to decrease in the order benzene > naphthalene > anthracene, which is contrary to what is predicted because an anthracene bridge is expected to have a low-lying bridge oxidation state. The severe steric constraint introduced by the double ortho-benzo groups of anthracene that inhibit ample overlap of the charge-bearing unit and bridge orbitals, decrease the electronic coupling.The dications of tetramethyl and tetra-i-propyl substituted p-phenylenediamine dimers, held together by two, three carbon alkyl chains between the nitrogens, were found to have singlet ground states but consist of different spin state ratios. Using the Evans magnetic susceptibility method, it was found that the ratio of singlet to triplet spin states at room temperature were 97:3 and 80:20 for the tetramethyl and tetra-i-propyl substituted systems, respectively. This is presumably due to the large i-propyl groups that make the a stacking conformation less accessible.These studies provide utility for the use of Hush Theory to predict thermal ET rates for bridges up to 13 bonds in length, despite the simplicity of the calculations, suggesting its use as a means to estimate the electronic communication in such molecular wires. |
