Solvent-polarity-regulated ESDPT behaviours for BDIPD derivative: a computational exploration

This work theoretically explores the reaction mechanism of excited-state intramolecular double proton transfer (ESDPT) of the 2,7-Bis-(4,5-diphenyl-1H-imidazol-2-yl)-pyrene-1,6-diol (BDIPD) molecule, and investigates its ESDPT process based on DFT and TDDFT methods. In this work, systematic analyses of S1-state structural variations, vibrational spectra, -valence bifurcation index and hydrogen-bond energies estimated via electron-density topological analysis for S0-state and S1-state all confirm that hydrogen bonds are strengthened upon photoexcitation. In investigating the modulation effect of solvent polarity on ESDPT of the BDIPD molecule, cyclohexane (CYC), toluene (TOL), tetrahydrofuran (THF) and acetonitrile (ACN) solvents were selected as solvents. We found that the intramolecular hydrogen bonds are markedly strengthened as solvent polarity increases. It reveals that higher solvent polarity facilitates the reaction of the BDIPD molecule. Meanwhile, we analysed the frontier molecular orbitals of the BDIPD molecule. The results show that the transition composition is dominated by the HOMO→LUMO contribution, and charge redistribution is one of the key factors affecting the ESDPT reaction. By scanning the potential energy surfaces, we demonstrated that the ESDPT process proceeds along the stepwise I→II→III pathway rather than the concerted direct I→III pathway. Energy-barrier analyses also verify the above conclusion.