From molecule to bulk material: Optical properties of hydrogen-bonded dimers [C₁₂H₁₂N₄O₂AgPF ₆]₂ and [C₂₈H₂₈N₆O ₃AgPF₆]₂ depend on the arrangement of the oxime moieties

The dependence of the optical properties of [C₁₂H ₁₂N₄O₂AgPF₆]₂ (dimer-1) and [C₂₈H₂₈N₆O₃AgPF ₆]₂ (dimer-2) on the arrangement of the oxime moieties in the molecule and in bulk crystals was investigated by means of time-dependent density functional theory. Dimer-1 with simple pyridine oxime ligands and a wavy arrangement has a smaller dipole moment and larger transition energy between the two states, and thus smaller third-order polarizabilities and two-photon absorption cross sections. Dimer-2 with extended pyridine oxime ligands and a ladder arrangement has a larger dipole moment and smaller transition energy between the two states, and thus larger third-order polarizabilities and two-photon absorption cross sections. The lowest energy absorption band is redshifted for dimer-2 as compared with dimer-1, due to more pronounced π-π delocalization interactions and weaker hydrogen bonding in dimer-2. The electronic absorption spectra, frequency-dependent third-order polarizabilities, and two-photon absorption cross sections involve significant contributions from charge transfers from π/π* orbitals of the pyridine oxime ligands but no contribution from PF₆^- ions or H₂O molecules in the wavelength range studied for the monomers and dimers of the C₁₂H₁₂N₄O₂AgPF ₆ and C₂₈H₂₈N₆O₃AgPF ₆ molecules. The third-order susceptibilities and two-photon absorption coefficients of bulk solids were estimated on the basis of the optical properties of the corresponding dimers, and the bulk material constructed from dimer-2 has the larger optical parameters of the two.