Describing Excited State Intramolecular Proton Transfer (ESIPT) in Dual Emissive Systems: A Density Functional Theory Based Analysis

The excited state intramolecular proton transfer (ESIPT) reaction taking place within 2-(2-hydroxyphenyl) benzoxazole (HBT) and two recently experimentally characterised napthalimide derivatives – known as N-1 and N-4 – has been investigated in order to identify and test a possible protocol for the description and complete mechanistic and electronic characterisation of the reaction at the excited state. This protocol is based on density functional theory (DFT), time-dependent density functional theory (TD-DFT) and a recently proposed electron density based index (DCT). This method is able to identify all stable species involved in the reaction; discriminate between possible reaction pathways over potential energy surfaces (PES), which are intrinsically very flat and difficult to characterise, and quantitatively measure the excited state charge transfer character throughout the reaction. The photophysical properties of the molecules (i.e. absorption and emission wavelength) are also quantitatively determined via the implicit inclusion of solvent effects in the case of toluene and, more polar, tetrahydrofuran (THF). The accuracy obtained with this protocol then opens up the possibility of the ab initio design of molecules exhibiting ESIPT for tailored applications such as highly selective molecular sensors