Modulation of Triarylhydrazone Photoswitching Efficiency Through Heteroaryl Variations

Triarylhydrazones (TAHZs) complement the mosaic of recently discovered P-type diarylhydrazone photoswitches. They typically exhibit higher molar absorptivities and operate under light of longer wavelengths, while preserving the attractive features of diarylhydrazones such as excellent addressability and extraordinary thermal stability of both isomers. However, TAHZs often suffer from low quantum yields of the Z-to-E photoisomerization due to a relatively strong excited-state intramolecular hydrogen bonding (IHB) in the Z isomer. Although the presence of IHB ensures the large separation of absorption maxima, it simultaneously leads to competing processes that reduce the efficiency of E-to-Z photoisomerization. Here, we present a strategy of reducing the IHB strength through a suitably chosen variation of heteroaryl, which not only significantly increases the efficiency of the Z-to-E photoreaction but also allows for the maintenance of sufficient addressability due to the reduced intensity of the S0-S1 transition accompanied with the emergence of bright higher excited states of the E isomer.