Control of the properties of molecules, for example, their magnetic, [1] electrical, [2] or optical [3] properties, through an external trigger signal is a very interesting design feature of new promising functional systems and smart materials. Indeed, switchable materials have been explored during the last decade within materials science for a wide range of applications, such as micropumps or autonomous valves, [4] photoactive polymers that mimic cilia movement, [5] artificial musclelike actuators, [6] molecular rotary motors, [7] and photooscillators. [8] Furthermore, the precise spatial and temporal control of the information transmission of organisms through the application of suitable input energies has become an important topic in cell biology. [9] Light-responsive materials are a very attractive possibility, since optical triggering has many advantages over both chemical and electrical stimulation. Light is a clean, fast, and environmentally friendly energy source, which can be projected onto a specific position of the target system with great accuracy in a wireless, noninvasive fashion. These useful features have inspired the development of many photoswitching systems whose temporal response range is limited only by the kinetics of the chromophore that acts as a molecular switch.Azobenzene is doubtless the most widely used organic chromophore for photoswitching applications in biological systems, since it can be successfully incorporated into all types of biopolymers (including peptides, proteins, sugars, and DNA). [10] Azobenzene derivatives can be photoisomerized cleanly to their metastable cis form with the appropriate light source in just a few nanoseconds or even more rapidly. Isomerization induces a pronounced change in the optical properties of the system. The reverse process can be induced either by visible light or thermally. [11] Thermal induction of the reverse process is preferred for applications that require real-time information transmission, as it avoids the use of a second optical stimulus for active regeneration of the sample. For this goal to be met, it is essential that the azo dye exhibit a fast thermal cis-to-trans back conversion, preferably on a sub-microsecond time scale.Azo dyes that combine a strong push-pull configuration with the ability to establish an azo-hydrazone tautomeric equilibrium are promising chromophores for this purpose, since they show very fast thermal cis-to-trans isomerization kinetics at room temperature. [12] Specifically, we recently described the use of hydroxyazopiridinium salts for photoswitching applications; these compounds exhibit relaxation times as low as 33 ms at 298 K. [13] Nevertheless, it remains a challenge to increase the isomerization rate of azophenols sufficiently for their isomerization to occur on a sub-microsecond time scale.Herein we report the synthesis and thermal cis-to-trans isomerization kinetics of a new highly biocompatible pyrimidine-based azophenol (Scheme 1) that exhibits thermalrelaxation times on the nanosecond time scale ...
