Modulation of triarylhydrazone photoswitching efficiency through heteroaryl variations

Triarylhydrazones (TAHZs) complement the mosaic of the 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 the excellent addressability and extraordinary thermal stability of both of its isomers. However, TAHZs often suffer from low quantum yields of <i>Z</i>-to-<i>E</i> photoisomerization due to the relatively strong excited-state intramolecular hydrogen bonding (IHB) in the <i>Z</i> isomer. Although the presence of IHB ensures the large separation of the absorption maxima, it simultaneously leads to competing processes that reduce the efficiency of <i>Z</i>-to-<i>E</i> photoisomerization. Here, we present a strategy for reducing the IHB strength through varying the heteroaryls, which significantly increases the efficiency of <i>Z</i>-to-<i>E</i> photoisomerization, preserving sufficient addressability owing to the reduced intensity of the S₀-S₁ transition accompanied by the emergence of bright higher excited states of the <i>E</i> isomer.