Triplet sensitization enables bidirectional isomerization of diazocine with 130 nm redshift in excitation wavelengths

Abstract

Diazocines are bridged azobenzenes with phenyl rings connected by a CH₂-CH₂ group. Despite this rather small structural difference, diazocine exhibits improved properties over azobenzene as a photoswitch and most importantly, its <i>Z</i> configuration is more stable than the <i>E</i> isomer. Herein, we reveal yet another unique feature of this emerging class of photoswitches. In striking contrast to azobenzenes and other photochromes, diazocine can be selectively switched in <i>E</i> → <i>Z</i> direction and most intriguingly from its thermodynamically stable <i>Z</i> to metastable <i>E</i> isomer upon successive excitation of two different triplet sensitizers present in solution at the same time. This approach leads to extraordinary large redshift of excitation wavelengths to perform isomerization <i>i.e.</i> from 400 nm blue to 530 nm green light (<i>Z</i> → <i>E</i>) and from 530 nm green to 740 nm far-red one (<i>E</i> → <i>Z</i>), which falls in the near-infrared window in biological tissue. Therefore, this work opens up of potential avenues for utilizing diazocines for example in photopharmacology, smart materials, light energy harvesting/storage devices, and out-of-equilibrium systems.