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 E/Z isomers, most of which have been prepared by twofold Wittig reactions followed by efficient purification using preparative HPLC.
Twofold adiabatic cis-trans 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 E/Z 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 E,E-stilbenophane. To our knowledge, this is the first example of an excimer between two stilbene units.
On triplet sensitization of a 1,5-bisstyrylcyclooctatetraene, the Z,Z isomer undergoes stepwise isomerizations with low quantum yields via the E,Z isomer to the E,E isomer in a one-way manner. The inefficiency of the isomerizations allows for observation of the intrinsic T1-Tn absorption spectra of the three E/Z isomers. This is explained by high energy barriers and/or by low pre-exponential factors for isomerization between the planar triplet species.
In contrast to the bisstyrylcyclooctatetraene, twofold adiabatic isomerizations are favored in triplet excited bisstyrylarenes. In a series of bisstyrylarenes, the Z,Z isomers were found to undergo exclusively adiabatic isomerization to the E,Z 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, 3Ep*, and a planar species 3EE*. 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.
In accordance with the findings on the bisstyrylarenes, Z,Z-1,6-bisstyrylcycloheptatriene undergoes very efficient twofold adiabatic isomerization to the E,E isomer on triplet sensitization.