Date of Award
Fall 1996
Document Type
Dissertation - Restricted
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Steinmetz, Mark G.
Second Advisor
Cremer, Sheldon E.
Third Advisor
McKinney, Michael A.
Abstract
The photochemistry of locally excited and intramolecular charge transfer states of (E)-4-cyano-4$\sp\prime$-pentamethyldisilanylstilbene: direct photolysis of (E)-4-cyano-4$\sp\prime$-pentamethyldisilanylstilbene results in efficient E,Z isomerization. Both the E,Z photoisomerization and the fluorescence of (E)-4-cyano-4$\sp\prime$-pentamethyldisilanylstilbene are quenched with similar Stern-Volmer constants by MeOH and by tert-amyl alcohol in pentane and $\rm CH\sb2Cl\sb2$, suggesting that both photoprocesses are linked to the same excited state, possibly a locally excited (LE) state associated with the $\pi$ system. The LE assignment is consistent with the structured fluorescence of (E)-4-cyano-4$\sp\prime$-pentamethyldisilanylstilbene at 382 nm in pentane, which shifts to longer wavelengths in more polar solvents and retains vestiges of structure at 423 nm in $\rm CH\sb2Cl\sb2$. In the presence of alcohols (E)-4$\sp\prime$-hydrodimethylsilyl-4-cyallostilbene is an additional photoproduct, produced via regioselective nucleophilic Si-Si cleavage by the alcohol, as shown by deuterium labeling utilizing MeOD. Quantum yields of (E)-4$\sp\prime$-hydrodimethylsilyl-4-cyanostilbene increase with increasing alcohol concentration. With $\rm CH\sb2Cl\sb2$ as the solvent double reciprocal plots $\Phi\sp{-1}$ vs. (ROH) $\sp{-1}$ are consistent with (E)-4$\sp\prime$-hydrodimethylsilyl-4-cyanostilbene deriving from an intramolecular charge transfer (CT) state or twisted CT (TICT) state rather than the LE state. The quadratic plot for binary mixtures of MeOH in $\rm CH\sb2Cl\sb2$ is interpreted in terms of an LE to CT conversion that is promoted by increasing in polarity of the solvent with increasing (MeOH).
Quenching of aromatic ketone triplet excited states by polysilanes: polysilane oligomers quench the triplet excited states of m-chloro- and p-chlorotri- fluoroacetophenones, p-cyanoacetophenone, acetophenone, and benzophenone in benzene. Phosphorescence lifetimes $\tau$ were measured by laser flash photolyses for varying concentrations of polysilane quencher Q, where $\rm Q=Me\sb3Si(SiMe\sb2)\sb{n-2}SiMe\sb3$ (n = 2-4, 6) and $\rm (Me\sb2Si)\sb6$. From $\tau\sp{-1}$ versus (Q) bimolecular rate constants for quenching k$\sb{\rm q}$ were obtained. Quenching by polysilanes exhibited a continuous "S-shaped" profile of log k$\sb{\rm q}$ versus $\rm\Delta G\sb{et}$, the free energy for electron transfer. In the exergonic region and for $\rm\Delta G\sb{et}