DFT-guided molecular design of tetrameric self-assemblies based on curved bicyclo[3.3.1]nonane monomers with isocytosine tautomeric edges.

In this work, the geometrical, energetic and electronic properties of tetrameric self-assemblies formed by the monomers involving various isocytosine tautomers were investigated in both the gas phase and chloroform using the SMD solvation model at the M06-2X/6-311++G(d,p) level of theory. The results demonstrate that the thermodynamic stability order of the monomers containing different isocytosine tautomers is dependent on the phase in which they are investigated. Moreover, the calculated energies indicate that heteroleptic and homoleptic tetrameric structures composed of isocytosine units in the keto-amine tautomeric form are energetically preferred over those containing the keto-imine and enol-amine tautomers. Furthermore, homoleptic tetrameric assemblies show greater energetic preference than their heteroleptic counterparts. Several electronic properties, including band gap, first ionization energy, electron affinity, chemical potential, electrophilicity index, hardness, and softness were evaluated. The reduced band gap values observed for the heteroleptic macrocycles suggest their potential for conductivity-related applications, warranting further investigation of their charge-transport properties. The AIM and NBO analyses provided detailed insights into the strength and nature of hydrogen bonding interactions within the tetramers. Overall, these findings highlight the crucial influence of tautomeric forms on the structure, energetics, and electronic features of isocytosine-based assemblies, suggesting useful guidelines for designing functional nucleic acid analogues and advanced supramolecular materials.