Optical limiting effects of 1,10-phenanthroline functionalized heterometallic Sn–Ti oxo clusters with distinct π⋯π interactions

Abstract

The optical performance of metal oxo clusters has attracted increasing research attention. Here, we investigate the photophysical properties of heterometallic Sn–Ti oxo clusters incorporating conjugated ligands. Three unprecedented dual-layered Sn–Ti oxo clusters were prepared using 1,10-phenanthroline as a functional ligand with formulas: [Sn^II₂Ti^IV₁₂(μ₃-O)₆(μ₂-O)₁₂(phen)₈(Pr)₁₀Cl₂]·(Sn^IICl₃)₂·(Pr)₂·(CH₃CN)₂ (TOC-61, HPr = propionic acid; phen = 1,10-phenanthroline), [Sn^II₂Ti^IV₁₂(μ₃-O)₈(μ₂-O)₁₀(phen)₈(IPA)₄(Pr)₈Cl₂]·(Sn^IICl₃)₂ (TOC-62, HIPA = isopropanol), and [Sn^II₆Ti^IV₁₄(μ₃-O)₉(μ₂-O)₁₄(phen)₆(Pr)₁₄Cl₆]·Sn^IICl₃·Pr (TOC-63). The distances between the 1,10-phenanthroline ligands in the three structures are in the ranges of 3.636–4.139, 3.692–4.388 and 3.422–4.001 Å, respectively, so that they all have strong intramolecular π⋯π interactions. Moreover, TOC-62 exhibits stronger intermolecular π⋯π interactions than TOC-61 and TOC-63. The optical gap values of TOC-61 to TOC-63 are 2.04, 1.95 eV, and 2.27 eV, respectively. The open aperture (OA) Z-scan measurements show that the three compounds all exhibit an obvious third-order nonlinear optical (NLO) response, as well as laser irradiation stability and repeatability. Interestingly, with the enhancement of the π⋯π interactions in these heterometallic Sn–Ti oxo clusters, the general trend of the optical gaps is gradually reduced and optical limiting (OL) effects are gradually enhanced. This work offers a route for preparing heterometallic Sn–Ti compounds with interesting optical properties, and provides a platform for studying the structure–performance relationship of heterometallic materials at the molecular level.