Electronic and molecular structures and bulk second–order nonlinear optical properties of ferrocenyl ynones

Abstract

Ferrocenyl ynones FcCOCCR (R = H, TMS, Ph) exhibit moderate second harmonic generation efficiencies in the solid state, whereas the methyl analog proved inactive. Their molecular structures and crystal packings were determined by single crystal X-ray diffraction, confirming that the inactive compound crystallizes in a centrosymmetric space group. The electronic structures of these compounds were studied experimentally (cyclic voltammetry, electronic absorption spectra) and theoretically (DFT and TD-DFT calculations). It appeared that the COCCR groups are stronger electron acceptors than the propionyl (COCH₂CH₃) group. Furthermore, these groups better stabilize metal-centred HOMO-2 to HOMO orbitals and lateral chain-centred LUMO orbitals and decrease HOMO-LUMO gaps. The TDDFT calculation of electronic transitions revealed that lower energy (LE) and higher energy (HE) bands observed in the electronic absorption spectra of compounds under study have more pronounced metal-to-ligand charge transfer character for ferrocenyl ynones than for propionylferrocene. The calculated static quadratic hyperpolarizabililities of ferrocenyl ynones are in the range ∼3 − 6 × 10⁻³⁰ esu.