Is it possible to switch ESIPT-channel of hydroxyanthraquinones with the strategy of modifying electronic groups?

In this work, to unveil the substituent effect on the excited-state intramolecular proton transfer (ESIPT) reaction of 1,8-Dihydroxyanthraquinone (DHAQ), four derivatives DHAQ-Me, DHAQ-NH2, DHAQ-COOH, and DHAQ-CN were considered using quantum chemistry methods in the acetonitrile (ACN) phase. The excited-state intramolecular hydrogen bonds (IHBs) (O1-H2⋯O3 and O5-H4⋯O3) strengthening mechanism was confirmed by computing primary geometric parameters, infrared (IR) spectra and reduced density gradient (RDG) scatter plots. The strength relationship of the dual IHBs within all the investigated compounds was established based on the calculated binding energies. Charge-recombination triggered ESIPT mechanism has been demonstrated by the analysis of frontier molecular orbitals (FMOs) and natural bond orbital (NBO) population. Besides, the hole-electron analysis indicates that a remarkable intramolecular charge transfer (ICT) behavior has induced the fluorescence intensity to decrease significantly for DHAQ-NH2 in K1 form. DHAQ derivatives with electron-withdrawing groups substituted (DHAQ-COOH and DHAQ-CN) have exhibited a larger Stokes shift, which is a desirable property for manufacturing luminescent materials. From the located transition state (TS) structures and the calculated potential energy curves (PECs), it is noticeable that the dominant channel for ESIPT reactions is always the O1-H2⋯O3 hydrogen bonding wire, which is irrelevant to the types of substituents introduced, although O5-H4⋯O3 is the stronger one for DHAQ-Me and DHAQ-NH2. This theoretical research may well deepen the comprehension of the relationship between IHB and ESIPT behavior and thereby provide valuable guidance to synthesize efficacious ESIPT-based organic chromophores experimentally.