A theoretical elucidation of the mechanism of tuneable fluorescence in a full-colour emissive ESIPT dye

BTImP, 2-(1,3-benzothiazol-2-yl)-4-methoxy-6-(1,4,5-triphenyl-1 H -imidazol-2yl)phenol, a compound showing a very unusual excited state intramolecular proton transfer (ESIPT) process, is theoretically studied. Composed of two ESIPT centres, BTImP presents a switching of the ESIPT from one moiety to the other by acidic stimulation, allowing emission of a large panel of colours. In this work, the switching mechanism and its impact on the structural and optical properties are investigated with time-dependent density functional theory (TD-DFT) and post Hartree-Fock methods [ADC(2) and CC2] including the environment by considering bulk solvation effects and the important impact of the counter-ion (BF 4 − , ClO 4 − , and Cl − ). A special attention is paid to locate all relevant conformers in both the ground and excited states. The possibility of having a doubly protonated structure is investigated as well. Eventually, in connection with experimental data (NMR, X-ray, and fluorescence spectra) a specific structure could be attributed to each emission colour. We reinvestigate with ab initio tools the origin of the diverse colours in a complex multi-ESIPT dye, and we propose a new assignment for the blue fluorescence.