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