Organic Photochemistry. Scope and Limitations of Twofold Adiabatic Cis-Trans Isomerizations
Doctoral thesis, 1995

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.


Ingjald Anger

Department of Organic Chemistry

Subject Categories

Chemical Sciences



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 1095

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