Geminal Dicationic Ionic Liquids (GDILs) and Their Adsorption on Graphene Nanoflakes

In this work, the configuration and stability of 15 geminal dicationic ionic liquids (GDILs) and their adsorption mechanism on the graphene nanoflake (GNF) are investigated using the density functional theory (DFT) method. We find that the interactions of dications ([DAm]⁺, [DIm]⁺, [DImDm]⁺, [DPy]⁺, and [DPyrr]⁺)) are stabilized near the anions ([BF₄]^-, [PF₆]^-, and [Tf₂N]^-) in the most stable configurations of GDILs through electrostatic interactions, van der Waals (vdW) interactions, and hydrogen bonding (H-bonding). Our calculations show that the adsorption of the GDILs on the GNF is consistent with the charge transfer and occurs via X···π (X = N, O, F), C-H···π, and π···π noncovalent interactions, leading to a decrease in the strength of the intermolecular interactions between the dications and anions in the GDILs. The thermochemistry calculations reveal that the formation of GDIL@GNF complexes is an exothermic and favorable reaction. The adsorption energy (E_ads) calculations show that the highest E_ads values for the interaction of GDILs containing [BF₄]^-, [PF₆]^-, and [Tf₂N]^- anions with the GNF are observed for the [DPy][BF₄]@GNF (-23.56 kcal/mol), [DPy][PF₆]@GNF (-29.29 kcal/mol), and [DPyrr][Tf₂N]@GNF (-24.74 kcal/mol) complexes, respectively. Our results show that the adsorption of the GDILs on the GNF leads to the decrease of the chemical potential (μ), chemical hardness (η), and HOMO-LUMO energy gap (E_g) values and an increase in the electrophilicity index (ω) value of the GNF. In addition, the effect of GDIL adsorption on the UV-vis absorption spectrum was studied at the TD-M06-2X/cc-pVDZ level of theory. We find that the adsorption of GDILs results in minimal change in the shape of the main absorption peak (at λ = 363 nm) in the GNF spectrum and only shifts it to higher wavelengths. On the other hand, a new peak appears in the GNF spectrum upon adsorption of [DPy][Y] (Y = [BF₄]^-, [PF₆]^-, and [Tf₂N]^-) due to the relatively strong π···π interactions between the [DPy]⁺ dication and GNF. Finally, the transition density matrix (TDM) heat maps show that electron transfers related to the excitation states in the GDIL@GNF complexes occur mainly through π(C=C) → π*(C=C) transitions in the GNF and the transitions from [DPy]⁺ dication to the GNF.