Remarkable Aurophilicity and Photoluminescence Thermochromism in a Homoleptic Cyclic Trinuclear Gold(I) Imidazolate Complex

A new aurophilically-bonded cyclic trinuclear gold­(I) complex, tris­[μ₂-(1-ethylimidazolato-N³,C²)­gold­(I)] ([Au₃(EtIm)₃], <b>1</b>), has been synthesized and characterized by temperature-dependent crystallographic and photophysical investigations. The crystal packing of <b>1</b> reveals two independent molecules in the unit cell, signifying two distinct pairs of dimer-of-trimer units convened by pairwise intermolecular Au···Au interactions of 3.0662(3) and 3.1407(3) Å at 100 K, representing the shortest pairwise intermolecular aurophilic interactions among all cyclic trimetallic gold­(I) complexes to date. Remarkably, crystals of <b>1</b> exhibit gigantic photoluminescence thermochromism of 10164 cm^–1from violet to red!attributed to internal conversion between a higher-energy (T₂ → S₀; λ_max ∼409 nm) and lower-energy (T₁ → S₀; λ_max ∼700 nm) phosphorescent band below and above 200 K, respectively, likely representing an excited-state phase change

Polymorphic, Porous, and Host–Guest Nanostructures Directed by Monolayer–Substrate Interactions: Epitaxial Self-Assembly Study of Cyclic Trinuclear Au(I) Complexes on HOPG at the Solution–Solid Interface

Synthesis, crystallographic characterization, and molecular self-assembly of two novel cyclotrimeric gold­(I) complexes, Au₃[3,5-(COOEt)₂Pz]₃ (Au₃Pz₃) and Au₃[(<i>n</i>-Pr–O)­CN­(Me)]₃ (Au₃Cb₃) was studied. Single crystal X-ray crystallography data reveal that both gold­(I) complexes have one-dimensional stacking patterns caused by intermolecular Au­(I)···Au­(I) aurophilic interactions. The Au₃Pz₃ trimer units stack with two alternate and symmetrical Au­(I)···Au­(I) interactions while the Au₃Cb₃ units have three alternating and nonsymmetrical Au­(I)···Au­(I) interactions. Molecular self-assembly of the gold­(I) complexes on the 1-phenyloctane/highly ordered pyrolytic graphite (HOPG) (0001) solution–solid interface is studied with scanning tunneling microscopy (STM). The gold­(I) cyclotrimers form epitaxial nanostructures on the HOPG surface. At a concentration of ∼1 × 10^–4 M, Au₃Pz₃ complexes exhibit a single morphology, while Au₃Cb₃ complexes exhibit polymorphology. Two polymorphs, one nonporous and the other porous, are observed at 22.0 ± 2.0 °C for Au₃Cb₃ complexes. A nonporous, low-surface-density (0.82 molecules/nm²) Au₃Cb₃ nanostructure forms first and then transforms into a high-density (1.43 molecules/nm²) porous nanostructure. This is the first time any porous surface nanostructure is reported for an organometallic system. The porous structure is thought to be stabilized by a combination of hydrogen bonding and monolayer–substrate interactions. These pores are utilized to incorporate pyrene into the film, rendering this the first organometallic host–guest system imaged at the solid–solution interface. Molecular and periodic density functional theory (DFT) calculations shed light on the two-dimensional topography and polymorphic self-assembly revealed by STM; these calculations suggest significant electronic hybridization of the Au₃ trimer orbitals and HOPG. The multiple-technique approach used herein provides insights concerning molecule–substrate and molecule–molecule interactions

Mechanistic Investigations of Photoinduced Oxygenation of Ru(II) Bis-bipyridyl Flavonolate Complexes

We previously reported that a Ru-bound flavonolate model of flavonol dioxygenases, [Ru^II(bpy)₂(3-hydroxyfla)]­[PF₆], photochemically reacts with dioxygen in two different manners. Broad-band excitation generates mixtures of products characteristic of 1,3-addition of dioxygen across the central pyrone ring, as is observed in enzymatic reactions. However, low temperature excitation at wavelengths longer than 400 nm generates a unique Ru-bound 2-benzoatophenylglyoxylate product resulting from a 1,2-dioxetane intermediate. Herein, we investigate this reactivity in a series of Ru­(II)­bis-bipyridyl flavonolate complexes [Ru^II(bpy)₂(3-hydroxyfla^R)]­[PF₆] (bpy = 2,2′-bipyridine; fla = flavonolate; R = <i>p</i>-OMe (<b>1</b>), <i>p</i>-Me (<b>2</b>), <i>p</i>-H (<b>3</b>), <i>p</i>-Cl (<b>4</b>)), and [Ru^II(bpy)₂(5-hydroxyfla)]­[PF₆] (<b>5</b>). The complexes’ structures, photophysical and electrochemical properties, and photochemical reactivity with oxygen were investigated in detail. Two different reaction product mixtures, from 1,2- and 1,3-additions of dioxygen, are observed by illumination into distinct excitation/emission manifolds. By analogy to previous reports of excited state intramolecular proton transfer, the two manifolds are attributed to tautomeric diradicals that predict the observed reactivity patterns