Triangular Platinum(II) Metallacycles: Syntheses, Photophysics, and Nonlinear Optics

Three triangular platinum­(II) diimine metallacycles incorporating large cyclic oligo­(phenylene-ethynylene) (OPE) bisacetylide ligands are synthesized, and their photophysical properties are studied. Two types of triplet excited states with ligand/metal-to-ligand charge-transfer and acetylide-ligand-centered characteristics respectively, are exhibited by these complexes depending on the size (conjugation length) and electronic features of the cyclic OPE ligands. When the energy levels of the two excited states are close to each other, the lowest triplet state is found to switch between the two in varied solvents, resulting from their relative energy inversion induced by solvent polarity change. Density functional theory and time-dependent density functional theory calculations provide corroborative evidence for such experimental conclusions. More importantly, the designed metallacycles show impressive two-photon absorption (2PA) and two-photon excitation phosphorescing abilities, and the 2PA cross section reaches 1020 GM at 680 nm and 670 GM at 1040 nm by two different metallacycles. Additionally, pronounced reverse saturable absorptions are observed with these metallacycles by virtue of their strong transient triplet-state absorptions

Unusual Temperature-Dependent Photophysics of Oligofluorene-Substituted Tris-Cyclometalated Iridium Complexes

The photophysical properties of a series of tris-cyclometalated Ir­(III) complexes bearing oligofluorene-substituted 2-phenylpyridine (ppy) and/or 1-phenylisoquinoline (piq) ligands were studied at both room temperature and 77 K, for delineating the oligomer-substitution effects on the photophysics in such metal-complex-containing conjugated oligomers/polymers. Unique temperature dependence was observed with the triplet excited-state lifetime of the studied oligomers. Molecules having one of the three ppy ligands substituted with an oligofluorenyl group at varied positions exhibited two distinct types of phosphorescing behaviors. When the oligoflurene group was coupled to ppy in a conjugative fashion (i.e., at 5- or 4′- position), the complexes appeared to emit from a ³MLCT-dominated state perturbed by LC transition, as evidenced by the relatively short lifetimes of phosphorescence as well as hypsochromic shift upon lowering the temperature. Surprisingly, even shorter triplet lifetimes were detected at 77 K for such oligomers. When the oligofluorenyl was tethered to the phenyl ring of ppy meta to pyridine, emission properties were consistent with a ³LC-dominated state, mixed with a certain MLCT component. Uniquely, for these oligomers an evident bathochromic shift of emission with a significantly retarded radiative decay rate was observed at 77 K. Furthermore, when a piq ligand was incorporated, red phosphorescence characteristic of Ir-piq-based ³MLCT transition emerged, disregarding the substation position of the oligofluorene. All these different photophysical behaviors, particularly their unique temperature dependence, were explained by considering an energy transfer process between different triplet states, with dominant MLCT and LC characteristics. In complexes having all ppy-derived ligands, these two states were of similar but different energy. While one played a more important role than the other, both were contributing to the phosphorescence emission. The temperature dependence of the photophysics reflected the equilibrium shifting process. When the ³MLCT-dominated state was lower in energy, faster radiative decay and shorter lifetimes were manifested upon lowering the temperature, as a result of more favored ³MLCT-dominated state. Whereas if the ³LC-dominated state was more stable, slower radiative decay emerged at decreased temperature due to further a reduced MLCT contribution. The bathochromic shift was also a result of equilibrium shifting to the state of lower energy. When the piq ligand was engaged, the emission was governed by the ³MLCT state of the Ir-piq moiety, which had much lower energy compared to the triplet states localized in oligofluorenyl ppy. DFT calculations substantiated the above hypothesis by identifying separate molecular orbitals possessing mixed but imbalanced MLCT and LC components

Water-Soluble Triscyclometalated Organoiridium Complex: Phosphorescent Nanoparticle Formation, Nonlinear Optics, and Application for Cell Imaging

Two water-soluble triscyclometalated organoiridium complexes, <b>1</b> and <b>2</b>, with polar side chains that form nanoparticles emitting bright-red phosphorescence in water were synthesized. The optimal emitting properties are related to both the triscyclometalated structure and nanoparticle-forming ability in aqueous solution. Nonlinear optical properties are also observed with the nanoparticles. Because of their proper cellular uptake in addition to high emission brightness and effective two-photon absorbing ability, cell imaging can be achieved with nanoparticles of <b>2</b> bearing quaternary ammonium side chains at ultra-low effective concentrations using NIR incident light via the multiphoton excitation phosphorescence process