The mechanism of an unusual benzo/benzo photocycloaddition reaction : photophysical and photochemical investigation of [3.3] orthocyclophanes with two face-to-face preoriented benzenoid chromophores

The clean and reversible [6pi+6pi] photocycloaddition (= PCA) in a [3.3] orthocyclophane (= OCP), in which two benzene chromophores are rigidly held in an unusually proximate, strongly interacting, face-to-face orientation, is the key step in the Pagodane - Dodecahedrane - Scheme. Due to the intriguing synthetic potential of the benzo/benzo (= B/B) dimer thus generated, the „scope and limitation“ of this photoreaction (= PR) was of utmost importance. Even though limited functionalization is tolerated, minor skeletal changes have severe consequences. This structural dependency asked for a detailed photomechanistic study, the more so, since B/B interactions have become a topic of great interest in structural protein chemistry. For three structurally closely related [3.3] OCPs the photophysical - absorption, transient absorption; fluorescence emission, excitation, lifetimes, quantum yields; phosphorescence - and photochemical properties - photoreaction quantum yields, triplet sensitization - have been investigated. The analysis of the data allowed the formulation of kinetic models and potential energy surface (= PES) diagrams. The PR was found to be extremely complex and to proceed via three reaction pathways: S1, S2 and T1. For the [3.3] OCP with the best possible geometry and highest skeletal rigidity, the occurrence of an excimer (= EX) with an unusually short lifetime of 184 ps was established. The interpretation of the PES justifies the conclusion that given a shortest B/B distance of less than 3.2 Å and near to parallel orientation of the benzene units, the tendency to undergo [6pi+6pi] PCA depends on the equilibrium geometry of the EX state and can be reliably judged by the 0-0 transition energy as a measure for the EX stabilization: for the investigated [3.3] OCs 12 kcal/mol were found to be necessary. The mechanism of the photocycloreversion was found to be less complex and to be one of the few reactions known to follow an adiabatic pathway.

Author dc:creator

• Tonne, Jörn

Contributors dc:contributor

• Prinzbach, Horst

• pi-pi-interactions
• charge-transfer-absorption
• face-to-face oriented benzenoid chromophores
• excited state photochemistry
• photoluminescence

Repository record source_url

https://freidok.uni-freiburg.de/data/188

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:188