Solvent polarity-regulated excited state double-proton-transfer behaviours for pyrene-tethered bis-salicylidene Schiff-base NBPP: a DFT/TD-DFT computational exploration

Symmetric pyrene-linked bis-salicylidene Schiff-base NBPP bears dual phenolic-imine intramolecular hydrogen bonds. DFT and TD-DFT computations were performed to decode its excited-state intramolecular proton-transfer (ESIPT) behaviours across cyclohexane, chloroform and acetonitrile. Geometric parameters, simulated IR spectra, QTAIM-BCP and ELF-CVB analyses consistently confirm that elevated solvent polarity strengthens excited-state hydrogen-bond interactions. Frontier-orbital results reveal dominant π→π* electronic excitations. Two-dimensional potential-energy-surface calculations identify sequential proton-transfer routes: NBPP → NBPP-PT1 → NBPP-DPT. Energy-barrier evaluations indicate that non-polar cyclohexane suppresses double-proton transfer, while polar acetonitrile lowers kinetic barriers and enables stepwise double-proton translocation. This work offers mechanistic insights for designing polarity-responsive ESIPT fluorophores.