Photochemistry of locally excited and intramolecular charge transfer states of a disilane bearing a cyclic arylethenyl electron acceptor substituent

Article abstract of DOI:10.1021/ja00082a014

1-(p-cyanophenyl)-2-(pentamethyldisilanyl)cyclopentene (1) exhibits a fluorescence at λ > 400 nm, which strongly shifts to the red with increasing polarity of the solvent. The corresponding solvatochromic plot is indicative of an emissive, lowest energy charge-transfer (CT) state. Stern-Volmer quenching of the fluorescence by dilute methanol in pentane gives a linear plot with a unit intercept and a slope K_Qτ_ct = 0.70. Quenching is accompanied by formation of (arylcyclopentenyl)dimethylsilane 7, the major product observed upon photolysis of 1 in the presence of methanol. This is evidenced by the linear double-reciprocal plot of quantum yields of 7 versus methanol concentration, which gives a k_Qτ_ct value in agreement with fluorescence quenching. In addition to nucleophilic attack by methanol at the terminal trimethylsilyl group to give 7, two minor photoproducts, (arylcyclopentenyl)trimethylsilane 6 and a tricyclic product 8 are observed, which are attributable to silylene extrusion and to homolytic Si-Si bond cleavage, the latter process giving a silyl radical pair that cyclizes onto the ortho position of the aryl group with subsequent disproportionation. The Stern-Volmer plot of Φ°/Φ versus [MeOH] for 8 is curved, suggesting that both the locally excited (LE) and CT states are involved in the formation of this product. In contrast, quantum yields of 6 do not vary with alcohol concentration, consistent with the LE state as the origin of this product.