The role of statistics and microenvironment for the photoresponse in multi-switch architectures: The case of photoswitchable oligoazobenzene foldamers

Systems containing multiple photochromic units possibly display a synergistic interplay of individual switching events and hence potentially give rise to unprecedented photoresponsive behavior. Among such systems photoswitchable foldamers are attractive as the photoisomerization events are coupled to the helix-coil conformational transition. To gain comprehensive insight into the role of the number of switching units (statistics) as well as their specific location and relative orientation in the helix backbone, several series of foldamers have been synthesized and characterized. In these series of foldamers, the local environment of the photoswitchable units was precisely tuned as pi,pi-stacking interactions were enforced to occur between specific pairs, i.e. azobenzene-azobenzene, azobenzene-tolane, or phenylene-phenylene units. These particular arrangements are reflected not only in the stability of the helical conformation, but also affect the photoresponsive behavior, i.e. the rate of photoisomerization and extent of denaturation. Furthermore, determining the intramolecular spin-spin distance in a series of TEMPO-labeled foldamers with variable chain lengths and different spatial locations of the spin-labels deduced an independent verification of the photoinduced helix-coil transition by ESR spectroscopy. Quantitative analysis of the corresponding ESR spectra shows an excellent correlation of the extent of intramolecular spin-spin coupling and the intensity of the Cotton effect in CD spectroscopy. From all of these results an unusual relationship between the rate of photoisomerization and the extent of photoinduced denaturation could be unraveled, as they are not going hand-in-hand but compete with each other, i.e. the easier the individual switching event is, the harder it becomes to achieve a high degree of unfolding. This insight into the effect of microenvironment on the ease of individual switching events and the role of statistics on the resulting degree of the overall conformational transition is of general interest for the design of multi-switch architectures with improved photoresponse.