Excited state intermolecular hydrogen bond’s effect on the luminescent behaviour of the 2D covalent organic framework (PPy-COF): A TDDFT insight

The theoretical investigation of electronically excited stated intermolecular hydrogen bonding dynamics of the 2D luminescent polypyrene covalent organic framework and methanol molecule (PPy-COF-MeOH) was performed using the density functional theory (DFT) and time-dependent (TD-DFT) method. The strengthening of Hydrogen bonds C-H---O-H and B-O---H-O upon photoexcitation was confirmed via comparison of geometric structures, electronic transition energies, 1H-NMR, binding energies, UV-Vis and infrared spectra in S0 and S1 states. Frontier molecular orbitals (MOs) analysis, electronic configuration, Mulliken charge analysis; and the charge density variation in hydrogen bonding proximity demonstrated that the strengthened hydrogen bonds facilitate the nonradiative path which may consequently proceed the luminescence quenching. Hence, the molecular material property prediction package (MOMAP) programme verified the fluorescence quenching because PPy-COF-MeOH complex showed a lower fluorescent rate constant compared to isolated PPy-COF fragment. The S1-T1 energy gap analysis also revealed the possibility of the Intersystem crossing (ISC). Above results significantly highlighted the role of the hydrogen bonding dynamics on luminescence property of the PPy-COF.