Deciphering the excited state intramolecular charge-coupled double proton transfer in an asymmetric quinoline–benzimidazole system

Abstract

8-(1H-Benzo[d]imidazol-2-yl)quinolin-7-ol (HQB) has structural asymmetry and contains double hydrogen bonds (HBs), which undergo excited state intramolecular charge transfer and double proton transfer (ESICT/ESIDPT). HQB exhibits three absorption peaks at 360, 315, and 305 nm. On photoexcitation, HQB displays dual emission at 420 and 570 nm, with Stokes’ shifts of 60 and 210 nm, respectively. The dual emission with large Stokes’ shifts and growth of an absorption shoulder peak at ∼400 nm in high polarity environments depict excited state intramolecular charge transfer (ESICT) coupled with excited state intramolecular proton transfer (ESIPT). The calculated Franck–Condon (FC) vertical excitation peaks and shifts in electron density in the frontier molecular orbitals (FMOs) involved in the transition endorse the observed absorption peaks and ESICT phenomenon. Atoms in molecules (AIM) calculations establish dual HBs in the form of O₁₆–H₁₇⋯N₂₉ and N₃₀–H₂₈⋯N₁₅. The four tautomeric forms (HQB, SPT(OH), SPT(NH) and DPT) establish an energy relationship of SPT(OH) > DPT > SPT(NH) > HQB for the S₀ state and HQB > SPT(OH) > DPT > SPT(NH) for the S₁ state. On S₀ → S₁, HQB displays a red-shift of 212 cm⁻¹ for N₃₀–H₂₈ vibrational stretching, which favors the tautomeric conversion HQB* → SPT(NH)*. Once HQB* → SPT(NH)* conversion takes place, the O–H vibrational stretching displays a red-shift of 178 cm⁻¹, which strengthens the O₁₆–H₁₇⋯N₂₉ intramolecular HBs and thus promotes dual proton transfer in the S₁ state. A small energy barrier in the proton transfer (PT) pathway and resemblance of predicted emission peaks with experimental results support the ESIDPT phenomenon.