Ultrafast Photoswitching Dynamics of Azobenzenes with Intra- and Intermolecular Constraints

Femtosecond time-resolved absorption and emission spectroscopy has been used to study the ultrafast photoinduced processes of azobenzenes (ABs) influenced by intra- and intermolecular constraints. For diazocine, an AB with an intramolecular bridge, accelerated molecular dynamics and increased switching efficiencies demonstrate that favourable pre-orientation and restricted molecular degrees of freedom in rigidified molecules may lead to improved optical switching properties. The significant effects of push-pull substitution were studied for the prototypical AB derivative Disperse Red 1 (DR1), which was shown to undergo sequential electronic deactivation and isomerisation. Finally, dramatic increases of the excited-state lifetimes of AB switches covalently embedded into polymeric micronetworks indicate that the photoisomerisation is severely resisted by mechanical forces inside the polymer environment. AB-functionalised polymer materials should thus avoid cross-linking or entanglement of the chromophores and provide ample free volume. Overall, the results demonstrate that detailed insight into the ultrafast photoswitching dynamics can provide guidelines for the design of photoswitchable systems with better performance.