Photoisomerization and Photodimerization in Self-Assembled Monolayers of cis- 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-assembled monolayers of these substrates on optically transparent gold undergo photoisomerization and photodimerization reactions when irradiated with >350 nm visible light. The quantum yield for cyclodimerization in a monolayer of 1 is Φ = 1.0 ± 0.5 × 10-6, whereas that observed in the solid-state thin film was Φcis → trans = 4 ± 1 × 10-3 and Φcis → dimer = 6 ± 2 × 10-4. Two photodimers are isolated by irradiation of melts of the derivatives cis-4-cyano-4‘-(10-(acetylthio)decoxy)stilbene (3) and trans-4-cyano-4‘-(10-(acetylthio)decoxy)stilbene (4). Irradiation of 3 resulted in photoisomerization to 4 as well as dimerization, whereas irradiation of 4 resulted only in photodimerization. A monolayer of 1 on gold blocks the electrochemical oxidation of decamethylferrocene (DMFc) dissolved in CH3CN in the potential region 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 present in the contacting electrolyte solution. Cycling of the potential of monolayers of 1 or 2 on Au in 0.1 M KOH between 0 and −1.2 V vs Ag results in the reductive stripping of the monolayer from the electrode surface. The cyclic voltammogram of a monolayer of 1 (Γ = 1 × 10-9 mol/cm2) has a broad reduction peak, which disappears after the first scan, whereas that of 2 (Γ = 3 × 10-10 mol/cm2) has a single, sharp reduction peak, which persists upon repeated cycling.