The Hydrogen Bonding in Electronically Excited States of the Luminescent Metal-Organic Frameworks Containing H₂O: Time-Dependent Density Functional Theory Study

We have theoretically investigated two luminescent metal-organic frameworks, (a) [Zn(cin)₂ (L¹] · H₂O and (b) [Cd(cin)₂(L¹] · H₂O, where cin = cinnamate anion and L¹ = 1,1 -(1,4-butanediyl)bis(2-methylbenzimidazole), and have explored their hydrogen bonding and luminescent properties using time-dependent density functional theory (TDDFT). The calculated excitation maxima are in good agreement with experimental results. By examining frontier molecular orbitals and electron configurations, we have found that the origin of luminescence is dominated by the ligand-to-ligand charge transfer (LLCT) transition. In addition, we investigated the excitation energy changes between the isolated monomers and the hydrogen-bonded complexes, and concluded that intermolecular hydrogen bonds in the S ₁ states of the two polymers are all weakened compared with those in the ground states. Further observation revealed that the excited-state hydrogen bond weakening can enhance the radiative deactivation of the fluorescence. Accordingly, both of the coordination polymers a and b are promising as multifunctional luminescent materials.