Redox-triggered switch based on platinum(ii) acetylacetonate complexes bearing an isomeric donor–acceptor conjugation ligand shows a high second-order nonlinear optical response

Abstract

Focusing on the remarkably various cyclometalated Pt(II) acetylcetonate complexes bearing an isomeric donor (D)–acceptor (A) conjugation framework ligand by virtue of adjusting the nitrogen atom coordination position, we have systematically investigated the geometric and electronic structures, redox properties, polarizabilities, first hyperpolarizabilities (β_tot), and first hyperpolarizability densities of these intriguing isomeric Pt(II) complexes using density functional theory with the view to rationalizing the nonlinear optical (NLO) structure–function relationships. Our calculations demonstrated that modulating the coordination position that affects the intramolecular D–A interactions gives rise to large second-order NLO responses in the studied complexes, which are qualitatively elucidated in terms of the change in the charge transfer (CT) patterns using time-dependent density functional theory. In particular, for complex 2, coordination with the stronger electron donating capacity of triphenylamine and the electron-withdrawing ability of the [1,2,5]thiadiazolo[3,4-c]pyridine unit produces the highest β_tot value (329 × 10⁻³⁰ esu) owing to the obvious bidirectional CT transition, which is the most fascinating candidate for a NLO material among all of the investigated complexes. Significantly, a noteworthy finding was that remarkable β_tot contrasts were observed for these isomeric Pt(II) complexes upon redox process, illustrating that they could be applicable for efficient redox-triggered NLO switches, especially using an oxidation stimulus with the highest on/off ratio, approaching 168.6 for complex 3. Furthermore, we have also quantitatively investigated the solid-state polarization effects on the β_tot values for neutral complexes using the polarizable continuum model. Therefore, it can be anticipated that the present work may be advantageous to the experimental and theoretical design of novel smart cyclometalated Pt(II) complexes for functional NLO materials.