Organic Photochemistry. Scope and Limitations of Twofold Adiabatic Cis-Trans Isomerizations

To investigate the scope and limitations of twofold adiabatic cis/trans photoisomerizations, a number of bis-stilbenes and bisstyryl compounds have been prepared and studied in the excited singlet or triplet states. Their structures vary according to the size of the .pi.-systems and the efficiency of the conjugation between the styryl groups or stilbene units. Each type of compound exists as three different <i>E/Z</i> isomers, most of which have been prepared by twofold Wittig reactions followed by efficient purification using preparative HPLC.<p /> Twofold adiabatic <i>cis-trans</i> photoisomerizations are not favored in the excited singlet states of a number of bis-stilbenes where conjugation between the two stilbene units is broken by saturated ethylene bridges. Thus, on direct excitation, the bis-stilbenes undergo stepwise diabatic <i>E/Z</i> isomerizations similar to stilbene. However, the proximity of the two stilbene units to one another allows for significant excited state interactions between the two stilbene chromophores in both bis-stilbenes. For example, excimer formation was observed on direct excitation of an <i>E,E</i>-stilbenophane. To our knowledge, this is the first example of an excimer between two stilbene units.<p /> On triplet sensitization of a 1,5-bisstyrylcyclooctatetraene, the <i>Z,Z</i> isomer undergoes stepwise isomerizations with low quantum yields via the <i>E,Z</i> isomer to the <i>E,E</i> isomer in a one-way manner. The inefficiency of the isomerizations allows for observation of the intrinsic T₁-T_n absorption spectra of the three <i>E/Z</i> isomers. This is explained by high energy barriers and/or by low pre-exponential factors for isomerization between the planar triplet species.<p /> In contrast to the bisstyrylcyclooctatetraene, twofold adiabatic isomerizations are favored in triplet excited bisstyrylarenes. In a series of bisstyrylarenes, the <i>Z,Z</i> isomers were found to undergo exclusively adiabatic isomerization to the <i>E,Z</i> geometry, while the second double bond was isomerized by a combination of diabatic and adiabatic mechanisms in varying proportions. The adiabatic part of the second cis-trans isomerization increases as the .pi.-system grows in size and the triplet energy of the central aromatic unit decreases. The triplet state of these compounds consists of an equilibrium between a twisted species, ³<i>E</i>p*, and a planar species ³<i>EE</i>*. This equilibrium is shifted towards the planar triplet species as the size of the .pi.-system increases and the triplet energy of the central aromatic unit decreases.<p /> In accordance with the findings on the bisstyrylarenes, Z,Z-1,6-bisstyrylcycloheptatriene undergoes very efficient twofold adiabatic isomerization to the <i>E,E</i> isomer on triplet sensitization