Planarization‐Induced Activation Wavelength Red‐Shift and Thermal Half‐Life Acceleration in Hydrazone Photoswitches

Abstract

The optimization and modulation of the properties of photochromic compounds, such as their activation wavelengths and thermal relaxation half‐lives (τ1/2), are essential for their adaptation in various applications. In this work, we studied the effect of co‐planarization of the rotary fragment of two photochromic hydrazones with the core of the molecule on their switching properties. The Z and E isomers of both compounds exhibit red‐shifted absorption bands relative to their twisted versions, allowing for their photoswitching using longer wavelengths of light. Additionally, the thermal half‐lives of both hydrazones are drastically shortened from hundreds of years to days.

Molecular photoswitches in aqueous environments

“…show good to excellent PSDs 311,312 and fatigue resistance. [311][312][313] Attachment of different substituents, 311,312,314 as well as introduction of strain 314,315 can tune the properties of the (acyl)hydrazones further and results in strongly bathochromically shifted 311,312,314 switches, giving access to a broad variety of thermal lifetimes of the metastable isomer. 315 Recently, compound 3.13 attracted attention, as it could be switched efficiently between both photoisomers with light, it showed strongly switchable fluorescence properties and could be excited both in 1-and 2-photon absorption processes.…”

Hydrazones as New Molecular Tools

“…To further improve on the photophysical properties of hydrazone photoswitch 6, we conducted structure-property analyses 53 that helped us identify (1) a pushpull bistable hydrazone whose Z/E isomerization can be triggered solely with visible light, (2) a NMe 2 -functionalized hydrazone that exhibits ON and OFF emission toggling and two-photon assisted (i.e., near-infrared activation) Z/E isomerization, (3) hydrazones that undergo efficient solid-state Z/E photoisomerization, (4) a number of switchable solid-state hydrazone emitters, and (5) the underlying principles behind the unusual thermal stability 54 and solid-state emission and switching of the switches. In general, the newly developed switches have very high photoswitching efficiencies, i.e., high PSSs and quantum yields and can be switched in various media-from fetal bovine serum buffer to the solid-state and even on metal surfaces.…”

Design of High-Performance Pyridine/Quinoline Hydrazone Photoswitches

“…However, recent efforts to induce a red shift of the absorption maxima of the core ketoester hydrazone structural motif by planarization of the structure resulted in an undesired decrease in the photoswitch thermal stability (from hundreds of years to approximately twenty days). 21 Earlier, Hecht's group also prepared several P-type acylhydrazone photoswitches containing pyridine subunit, although without quantitative determination of the thermal half-lives. 22 Although excellent improvement in absorption red-shift of azoacenes as configurational photoswitches to Vis/NIR region has been achieved by research groups of Herges and Woolley, their half-lives usually do not exceed several hours or days.…”