PHOTOISOMERIZATION AND PHOTODIMERIZATION IN SELF-ASSEMBLED MONOLAYERSOF CIS-4-CYANO-4'-(10-MERCAPTODECOXY)STILBENE AND TRANS-4-CYANO-4'-(10-MERCAPTODECOXY)STILBENE ON GOLD

Thin films of cis-4-cyano-4'-(10-mercaptodecoxy)stilbene (1) and trans -4-cyano-4'-(10-mercaptodecoxy)stilbene (2) on quartz or self-assemble d monolayers of these substrates on optically transparent gold undergo photoisomerization and photodimerization reactions when irradiated wi th >350 nm visible light. The quantum yield for cyclodimerization in a monolayer of 1 is Phi = 1.0 +/- 0.5 x 10(-6), whereas that observed i n the solid-state thin film was Phi(cis-->trans) = 4 +/- 1 x 10(-3) an d Phi(cis-->dimer) = 6 +/- 2 x 10(-4). Two photodimers are isolated by irradiation of melts of the derivatives cis-4-cyano-4'-(10-(acetylthi o)decoxy)stilbene (3) and trans-4-cyano-4'-(10-(acetylthio)decoxy)stil bene (4). Irradiation of 3 resulted in photoisomerization to 4 as well . as dimerization, whereas irradiation of 4 resulted only in photodime rization. A monolayer of 1 on gold blocks the electrochemical oxidatio n of decamethylferrocene (DMFc) dissolved in CH3CN in the potential re gion from -0.5 to +0.5 V vs Ag wire. After scanning between -0.5 and 1.0 V vs Ag, the monolayer no longer blocks the oxidation of DMFc pres ent in the contacting electrolyte solution. Cycling of the potential o f monolayers of 1 or 2 on Au in 0.1 M KOH between 0 and -1.2 V vs Ag r esults in the reductive stripping of the monolayer from the electrode surface. The cyclic voltammogram of a monolayer of 1 (Gamma = 1 x 10(- 9) mol/cm(2)) has a broad reduction peak, which disappears after the f irst scan, whereas that of 2 (Gamma = 3 x 10(-10) mol/cm(2)) has a sin gle, sharp reduction peak, which persists upon repeated cycling.