Triplet-Sensitized Bidirectional Isomerization of Bridged Azobenzene

Diazocine is a bridged azobenzene with both phenyl rings connected by a CH2-CH2 group. Despite this rather small structural difference, diazocine exhibits improved properties over azobenzene as a photoswitch, such as high switching efficiencies, very high quantum yields, switching wavelengths in the visible range, and most importantly, the fact that it is more stable in the Z configuration, which is particularly expedient in photopharmacology and mechanophore applications. According to our studies presented here, another advantage over conventional azobenzene is now added. In contrast to azobenzenes and other photochromes, diazocine can be switched with two different triplet sensitizers present at the same time in both directions: Z→E as well as E→Z. Experimental and theoretical (CASPT2) studies of triplet excitation energies provide an explanation for this fact. The triplet energies in Z and E azobenzene are almost equal, which prevents selective sensitization of either isomer. In diazocine, the two excitation energies are well-separated, so they can be accessed selectively. Besides offering fundamental physical insight to diazocines, an emerging class of photoswitches, our work opens up a number of potential avenues for utilizing them for example in photopharmacology and smart materials design due to the significant redshift of excitation wavelengths to from blue to green (Z→E) and green to far-red (E→Z), which triplet sensitization offers.