Modulation of Solvent Polarity on Excited State Intramolecular Double Proton Transfer of 8‐( <scp>1H</scp> ‐Benzo[d]imidazol‐2‐yl)quinolin‐7‐ol

ABSTRACT This study used density functional theory (DFT) and time‐dependent density functional theory (TDDFT) to investigate how solvent polarity affects the excited‐state double proton transfer (ESDPT) behavior of 8‐(1H‐benzimidazol‐2‐yl)quinolin‐7‐ol (HQB) compound. Cyclohexane, toluene, chloroform, and acetonitrile were selected as model solvents with increasing polarity. Structural, charge density topology, and molecular orbital analyses were conducted to examine the impact of solvent polarity on hydrogen bonding and ESDPT kinetics. Solvent polarity was found to differentially influence two intramolecular hydrogen bonds, O 1 –H 2 ···N 3 and N 4 –H 5 ···N 6 . The strengthening of N 4 –H 5 ···N 6 was identified as a key factor in initiating excited‐state proton transfer (ESPT). With decreasing solvent polarity, hydrogen bond lengths shortened and vibrational frequencies red‐shifted, indicating stronger hydrogen bonding. This was attributed to more localized charge distribution and stronger electrostatic interactions in non‐polar environments. Charge density topology parameters ( ρ ( r ), V ( r )) and binding energy calculations confirmed that the proton transfer energy barrier was lower in low‐polarity solvents. In polar solvents, the HOMO‐LUMO gap of HQB increased, and intramolecular charge transfer (ICT) was enhanced. This altered the electron density at hydrogen bond sites and reduced the energy barrier for proton transfer. Analysis of the S 1 potential energy surface (PES) showed that ESDPT mainly follows a stepwise mechanism: the first proton transfers along N 4 –H 5 ···N 6 to form intermediate III, followed by the second proton transfer along O 1 –H 2 ···N 3 to yield product IV. Non‐polar solvents favored the first step, while polar solvents lowered the energy barrier of the second step by stabilizing intermediate III.