ELECTRONIC ABSORPTION SPECTRA AND INTERMOLECULAR HYDROGEN BONDING IN MESOGEN–NONMESOGEN SYSTEMS

Abstract

The TD-DFT/B97-D/6-311G** method is used to calculate the electronic absorption spectra of А⋯Х⋯А supramolecules with intermolecular hydrogen bonds (HBs), where А is 4-n-propoxycinnamic acid, Х is a nonmesogen molecule with bifunctional HB acceptors. Nonmesogens Х (B, C, D, E) consist of two pyridyl fragments connected by bridging groups of different natures: 4,4′-bipyridine (В), 1,2-bis(4-pyridyl)ethane (С), 1,2-bis(4-pyridyl)ethylene (D), 4,4′-azopyridine (Е). It is shown that the introduction of bridging groups (–CН₂–CН₂–, –CН=СН–, –N=N–) significantly affects the energy of frontier orbitals and electronic absorption spectra (EAS) of nonmesogens Х in the near-UV region. The EAS of two structural units A⋯В⋯A, A⋯A, which may be formed due to the self-assembly in two-component mesogen А–nonmesogen В systems, are compared. Due to hydrogen bonds (O–H⋯N and O–H⋯O), having different types and strengths in these complexes, the high-intensity band shifts to shorter wavelengths in complex А⋯В⋯А and to longer wavelengths in complex А⋯А with respect to the λ = 333 nm band of monomer А. It is shown that EAS of hydrogen-bonded complexes А⋯Х⋯А are not superpositions of EAS of individual components. The spectra show electronic transitions related to charge transfer between the components of the complexes. Using nonmesogens Х of different nature in supramolecules А⋯Х⋯А will allow one to change purposefully their electro-optical properties and to achieve intense absorption in particular regions of UV spectra.