Amino proton donors in excited-state intramolecular proton-transfer reactions

Proton transfer involving site-specific hydrogen-bonding interactions is one of the most fundamental and important reactions in chemistry and biology. Deliberately triggering this reaction by photoexcitation enables unique and insightful mechanistic analyses. This Review describes a particularly effective method that involves exciting a photoacid containing both an amine and a basic residue and monitoring the ensuing excited-state intramolecular proton-transfer (ESIPT) reactions. Replacing a H atom on the amine with another substituent R modulates the acidity of the amine and allows for the excited-state hydrogen-bond strength to be tuned over a very broad range. In this way, one can draw empirical correlations between N-H bond distances, acidity, hydrogen-bond strength and the ESIPT kinetics and thermodynamics. For example, stronger intramolecular N-H center dot center dot center dot N hydrogen bonding leads to faster and more exergonic ESIPT. Tuning the amine and basic residues allows one to switch the ESIPT mechanism between the kinetic and thermodynamic regimes, such that molecules can generate ratiometric emission, which is suitable for white-light generation and two-colour imaging. The identity of the amine substituent R not only affects the acidity but can be differentially sensitive towards the local chemical environment. Thus, the R group transduces environmental changes into modified ESIPT rates and/or mechanisms. Such studies open new frontiers in the fundamental aspects of proton transfer in amines, as well as their largely unexplored potential applications.