Liaisons between photoconductivity and molecular frame in organometallic Pd(ii) and Pt(ii) complexes

The synthesis and a comprehensive experimental study of molecular structure, electrochemical properties, photoconductivity, absorption and emission properties of two homologous series of square planar cyclometallated Pd(II) and Pt(II) complexes are described. In these complexes, a five member (C,N) metallacycle is formed by either azobenzene, 2-phenylpyridine or benzo[h]quinoline, while the ancillary (O,N) ligand required to complete the metal coordination sphere is the Schiff base resulting from the condensation of 4-n-hexylaniline with 2-hydroxy-4-n-hexyloxybenzaldehyde. The study is complemented by Density Functional Theory calculations of molecular structure in the ground state, frontier orbitals distribution and energies. Absorption energies, intensities for the transitions and composition of the excited states are characterized by Time Dependent Density Functional Theory computations. Results show that for these complexes the HOMO is located on the Schiff base and the LUMO on the cyclometalated ligand. As also indicated by spectroscopic and photoconductivity results, the photogeneration of charge carriers might be associated with the spatial separation of HOMO and LUMO, while an additional contribution could derive from a conformational variation in the excited state. The separation of the frontier orbitals on different ligands provides simple synthetic routes towards band-gap tuning and energy matching with other molecular species and electrodes, both important features for several applications of molecular semiconductors. The results of the present investigation suggest that these square planar cyclometalated Pd(II) and Pt(II) complexes actually form a new class of photoconductors.