Visible to near-infrared light responsive halochromic hydrazones

Protonation can introduce an extra handle, besides light, in controlling the properties of photoswitches, opening the way to multi-stimuli responsive functional materials. Here we report on a pair of halochromic quinoline-based hydrazone photoswitches that show divergent photophysical properties based on the protonation site, yielding switches that can be activated using visible to near-infrared (NIR) light. Hydrazone 1 undergoes protonation at the quinolinyl nitrogen, inducing an unusual 160 nm red-shift, allowing its operation with NIR light, and making it one of the most red-shifted protonated switches besides azoniums. On the other hand, the dimethylamine nitrogen is protonated first in hydrazone 2 followed by the quinolinyl site, affording two new switchable systems with operational wavelengths between the visible and the UV regions, and improved switching efficacy and photofatigue resistance. These results underscore the role of protonation as an additional handle in controlling the spectral transitions and photoswitching parameters of hydrazone-based systems. Protonation can introduce an extra handle, besides light, in controlling the properties of photoswitches, opening the way to multi-stimuli responsive functional materials. Here, the authors report a pair of halochromic quinoline-based hydrazone photoswitches that show divergent photophysical properties based on the protonation site, yielding switches that can be activated using visible to near-infrared light.