The experimental and theoretical characterisation of the phenyl-perfluorophenyl π-π and hydrogen bond interactions in the aldimine co-crystal

The experimental and theoretical characterisation of optical (UV-Vis and FTIR) properties of aldimine co-crystal complex (system C) formed from C6H5CHNC6H5 (subsystem A) and C6F5CHNC6F5 (subsystem B) molecules is presented. As a result of intermolecular interaction, the non-additive intensive band at 987 cm−1 is observed in the finger-print region of infrared spectrum. In the liquid state, the existence of this complex has been suggested on the basis of electrochemical measurements. These experiments reveal a slight decrease in cathodic and anodic peak potentials for the C complex comparing to the non-interacting molecules A and B. The supermolecular interaction energy calculated at MP2/6-31 + G(d) level with the basis set superposition error correction for the twisted head-to-tail orientation has the value 55 kJ mol−1. Finally, the analysis of the frontier molecular orbitals based on the TDDFT method shows for the experimental geometry that the origin of the lowest excitation energy (404 nm), with very small oscillator strength (0.0001) is composed only of mono-excitations from HOMO (localised on system A) to LUMO (localised on system B) orbitals. However, the TDDFT approach could yield to underestimated energies for long-range CT states as this additional low-lying CT state was not observed in the corresponding experimental UV-Vis spectrum.