PHOTOCHEMISTRY OF LOCALLY EXCITED AND INTRAMOLECULAR CHARGE-TRANSFER STATES OF (E)-4-CYANO-4'-(PENTAMETHYLDISILANYL)STILBENE

The photophysical properties and photochemistry of (E)-4-cyano-4'-(pen tamethyldisilanyl) stilbene ((E)-3) and the corresponding monosilylate d stilbene (E)-5 have been studied. Solvatochromic plots of (E)-3 show separate slopes for ether solvents and halocarbons. The emission in e thers is assigned to an LE state of similar polarity to that of(E)-5. In halocarbons a more polar intramolecular charge transfer (CT) state apparently contributes to the emission, This component is completely q uenched by nucleophilic ethers, In contrast to (E)-5, Phi(f) and Phi(E ,Z) of (E)-3 strongly decrease in CH3CN, consistent with the solvent p olarity induced LE --> CT process and nucleophilic quenching of the CT state by solvent. Alcohols also quench Phi(f) and Phi(E,Z) with simil ar Stern-Volmer constants, linking the two processes to the same state (LE), Such quenching results in formation of (E)-4'-(hydrodimethylsil yl)-4-cyanostilbene (4) by regioselective nucleophilic cleavage of the Si-Si bond, according to deuterium labeling studies. Double reciproca l plots of Phi(-1)(SiH) vs [ROH](-1) are consistent with both the LE a nd CT states being quenched by ROH with the latter state giving (E)-4, Quadratic behavior for MeOH in CH2Cl2 is ascribed to a solvent polari ty promoted LE --> CT process with a rate constant proportional to [Me OH], in accord with the linear relation of E(T)(30) vs In [MeOH] above 0.3 M MeOH. The quadratic behavior disappears in tert-amyl alcohol, w hich has the same E(T)(30) as CH2Cl2. Results for MeOH in pentane sugg est the LE and CT states have similar energies and interconvert.