Theoretical study on the cascaded double proton transfer mechanism involving two adjacent hydrogen bonds in conjugated organic molecules

Abstract

The cascaded double proton transfer reaction has attracted the interest of many researchers due to its effective six-level system cycle. Herein, the double proton transfer mechanism of naphthalene-based analog NDH and anthracene-based analog ADH in different solvents is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results indicate that the first occurrence of excited-state intramolecular proton transfer (ESIPT) in NDH and ADH is an ultrafast, barrierless process. Additionally, ADH undergoes a second ESIPT reaction, radiating a wider range of fluorescence spectrum. Theoretical studies reveal that this is due to the significantly increased hydrogen bond strength of ADH compared to NDH in the first excited (S1) state, creating favorable conditions for the second ESIPT. Analysis of electronic properties indicates that ADH exhibits enhanced chemical and kinetic activity, with a more pronounced intramolecular charge transfer (ICT), leading to increased stability of the keto structure (KB2*) after the second ESIPT. Finally, we identify the factors influencing the cascaded double proton transfer reaction. The second ESIPT process is promoted by extending the π-conjugated skeleton of the core molecule horizontally and leveraging the driving force provided by the H3 atom near the proton donor O3–H2. It is expected that these studies will provide theoretical guidance and new insights for the design and synthesis of near infrared (NIR) organic materials.