Theoretical study of stepwise excited-state intramolecular double proton transfer in two 1,8-dihydroxynaphthalene derivatives

Stepwise excited-state intramolecular double proton transfer (ESIDPT) was theoretically investigated in two similar 1,8-dihydroxynaphthalene derivatives in methanol. A stepwise ESIDPT leads to existence of three different molecular forms in the excited state, commonly denoted as normal form, tautomer A and tautomer B. Absorption and emission spectral characteristics were determined for different forms of both compounds. In this work, it was shown that the investigated molecules can potentially belong to the class of stepwise ESIDPT-active systems. It was also shown that ESIDPT process proceeds efficiently even under specific solvation (in methanol) and yields a spectroscopically heterogeneous system with tunable emission properties.