The Quest for the Right Trade-Off for an Efficient Photoclick Monitoring Reaction

Photoinduced cycloaddition of 2,5-diaryltetrazoles with alkenes, to produce 2,5-diarylpyrazolines, has been widely exploited as orthogonal ligation for bio-labeling and cross-linking. This reaction is particularly interesting and useful, as it combines mild activation conditions to the generation of a fluorescent product. However, it suffers from two major drawbacks, as under physiological conditions hydration by-products are formed and the fluorescent quantum yield of the emitting pyrazoline is often reduced in water. In this work, we have thoroughly investigated both the photoreactivity of a small library of water soluble tetrazoles and the optical properties of the resulting pyrazolines, to optimize both chemical selectivity (cycloaddition vs hydration) and photo-physical efficiency. Electron-withdrawing (A) and electron-donating (D) substituents tune pyrazoline reaction yields and their fluorescence quantum yields in opposite directions. The combination of two D-substituents on the tetrazoles ensures quantitative pyrazoline formation with low emission. On the contrary, two A-substituents afforded very emitting pyrazolines in low chemical yield. We have identified the water-soluble tetrazole T7 as the best compromise, able to afford a highly fluorescent pyrazoline, in high yields.