Self-Assembled Photonic Crystals of Monodisperse Dendritic Fibrous Nanosilica for Lasing: Role of Fiber Density

Photonic crystals are essentially a periodic (“crystalline”) arrangement of dielectric nanoparticles that respond in unison to incident light. They can be used to harvest light in various applications such as photocatalysis, solar cells, and lasing. In this work, we prepared the photonic crystals of dendritic fibrous nanosilica (DFNS) by their self-assembly. Because of the narrow particle size distribution of the as-synthesized DFNS, they readily formed colored photonic crystals. The photonic band gap was found to be tunable by using DFNS of various sizes and fiber densities. Notably, even after having similar particle sizes (but with different fiber densities), they showed different photonic band gaps, indicating that the fiber density plays a role in the band gap of photonic crystals. Such observations have not been reported before. This could have arisen from the difference in their refractive indices because of the difference in their fiber densities and hence the variation in the silica content, leading to a different optical signature

Postsynthetic Systematic Electronic Tuning of Organoplatinum Photosensitizers via Secondary Coordination Sphere Interactions

In this work we show that postsynthetic addition of borane Lewis acids to Lewis base decorated organoplatinum photosensitizers induces significant changes in the optical and electrochemical properties. In particular, the charge transfer (CT) energies of these chromophores are significantly modified by these outer-sphere interactions. The direction of the CT shift depends on the site of Lewis acid binding, which occurs either at the diimine ligand in bipyrazine-linked molecules or at an ancillary acetylide ligand in pyridyl-substituted bis­(acetylide) molecules. The magnitude of the shift depends on the Lewis acidity of the borane and the number of equivalents added and is comparable to the perturbation brought on by covalent substituent modification of supporting ligands in related complexes. This approach offers a new means of tuning the properties of organometallic phosphors that complements the traditional approach of covalent modification and other postsynthetic modification strategies

Postsynthetic Systematic Electronic Tuning of Organoplatinum Photosensitizers via Secondary Coordination Sphere Interactions

In this work we show that postsynthetic addition of borane Lewis acids to Lewis base decorated organoplatinum photosensitizers induces significant changes in the optical and electrochemical properties. In particular, the charge transfer (CT) energies of these chromophores are significantly modified by these outer-sphere interactions. The direction of the CT shift depends on the site of Lewis acid binding, which occurs either at the diimine ligand in bipyrazine-linked molecules or at an ancillary acetylide ligand in pyridyl-substituted bis­(acetylide) molecules. The magnitude of the shift depends on the Lewis acidity of the borane and the number of equivalents added and is comparable to the perturbation brought on by covalent substituent modification of supporting ligands in related complexes. This approach offers a new means of tuning the properties of organometallic phosphors that complements the traditional approach of covalent modification and other postsynthetic modification strategies

Manipulating the Excited States of Cyclometalated Iridium Complexes with β‑Ketoiminate and β‑Diketiminate Ligands

A series of cyclometalated iridium complexes with β-ketoiminate and β-diketiminate ligands are described. Two different cyclometalating (C^N) ligands2-phenylpridine (ppy) and 2-phenylbenzothiazole (bt)are used in concert with three different ancillary (LX) ligandsa phenyl-substituted β-ketoiminate (acNac^Me), a phenyl-substituted β-diketiminate (NacNac^Me), and a fluorinated version of the β-diketiminate (NacNac^CF₃)to furnish a suite of six complexes. The complexes are prepared by metathesis reactions of chloro-bridged dimers [Ir­(C^N)₂(μ-Cl)]₂ with potassium or lithium salts of the ancillary LX ligand. Four of the complexes are characterized by X-ray crystallography, and all six were subjected to in-depth optical and electrochemical interrogation. Cyclic voltammetry shows both reduction and oxidation waves, with the latter strongly dependent on the identity of the LX ligand. The complexes are all luminescent, with the nature of the emissive excited state and the quantum yield (Φ) dependent on the identity of both the C^N and LX ligands. Whereas the complexes Ir­(ppy)₂(NacNac^Me) and Ir­(ppy)₂(acNac^Me) are weakly luminescent (Φ ≈ 0.01), the complexes Ir­(bt)₂(NacNac^Me) and Ir­(bt)₂(acNac^Me) are strongly luminescent, with the latter’s quantum efficiency (Φ = 0.82) among the highest ever observed for cyclometalated iridium complexes. Fluorination of the NacNac ligand gives rise to completely disparate emission behavior suggestive of a NacNac-centered emissive state. The results described here, in comparison with previous groups’ studies on acetylacetonate (acac) analogues, suggest that the weaker-field NacNac and acNac ligands raise the energy of the metal-centered HOMO, with energy of the HOMO increasing in the order NacNac^CF₃ < acNac^Me < NacNac^Me

Fluorination of Cyclometalated Iridium β‑Ketoiminate and β‑Diketiminate Complexes: Extreme Redox Tuning and Ligand-Centered Excited States

In this work we describe a series of bis-cyclometalated iridium complexes with ancillary β-ketoiminate (acNac) and β-diketiminate (NacNac) ligands, prepared by a general synthetic route. Fluorination of these ligands by introducing CF₃ substituents onto the ligand backbone and/or <i>N</i>-aryl substituent(s) leads to pronounced changes in the redox properties and photophysical properties. All of the complexes show a reversible Ir^IV/Ir^III redox couple that is sensitive to the degree of fluorination on the ancillary ligand. Introduction of CF₃ groups at the 3- and 5-positions of the <i>N</i>-aryl substituent shifts the potential positive by ca. 50–70 mV per CF₃ group, whereas fluorination of the acNac or NacNac backbone induces larger shifts of ca. 200–300 mV per CF₃ group. Fluorination of the NacNac backbone gives rise to substantially altered excited-state properties. Complexes with backbone CF₃ groups luminesce in the red and near-infrared regions out of an excited state that is predominately a π → π* NacNac-centered triplet state. A preponderance of evidence supports the assignment of this low-energy feature, including minimal dependence of this emission feature on the identity of the cyclometalating ligand, pronounced vibronic structure, and microsecond lifetimes. These results demonstrate that acNac and NacNac ancillary ligands can engender cyclometalated iridium complexes with desirable and readily tailorable redox and optical properties, motivating continued application of this important ligand class to the design of phosphorescent organometallic molecules

Negative Photochromism Based on Molecular Diffusion between Hydrophilic and Hydrophobic Particles in the Solid State

A colored hybrid based on a merocyanine adsorbed in a nanoporous-silica-composed dendritic fibrous silica was prepared by adsorption onto the nanoporous silica from a spiropyran solution during UV irradiation (photoinduced adsorption). The obtained red hybrid thus exhibited negative photochromism by visible-light irradiation. The hybrid was further combined with an organophilic clay by a solid-state mixing without using solvent to achieve excellent reversibility of the color change, which was thought to be achieved by molecular diffusion through the two materials, where nanoporous silica and organophilic clay accommodated the colored (merocyanine) and colorless (photogenerated spiropyran) isomers, respectively

Bis-Cyclometalated Iridium Complexes with Chelating Dicarbene Ancillary Ligands

In this work, we report that covalent postsynthetic modification can be used for the preparation of a class of bis-cyclometalated iridium complexes featuring Chugaev-type chelating dicarbene ligands. Bis-cyclometalated iridium complexes with electron-deficient aryl isocyanide ancillary ligands react with hydrazine to form neutral dicarbene complexes. The neutral iridium carbene complexes have a basic site that can be protonated by strong acid, permitting access to complexes in two protonation states and allowing an additional layer of control over the key properties. These new Chugaev-type iridium complexes exhibit blue phosphorescence at both room temperature and 77 K. Compared to their bis-isocyanide precursors, the electrochemical and photophysical properties of these new complexes are substantially perturbed, demonstrating the concept that the electronic structure and excited state dynamics can be controlled by ancillary ligand modification. Furthermore, the emission spectra and excited-state dynamics are dependent on the protonation state of the dicarbene ancillary ligand, and we note an ∼2-fold increase in emission quantum yield when the ancillary ligand is protonated. This study demonstrates that ligand-based reactivity can be an alternative method for elaborating the structures of bis-cyclometalated iridium complexes and gives access to structures not readily obtainable by other means

Unraveling the Formation Mechanism of Dendritic Fibrous Nanosilica

We studied the formation mechanism of dendritic fibrous nanosilica (DFNS) that involves several intriguing dynamical steps. Through electron microscopy and real-time small-angle X-ray scattering studies, it has been demonstrated that the structural evolution of bicontinuous microemulsion droplets (BMDs) and their subsequent coalescence, yielding nanoreactor template, is responsible for to the formation of complex DFNS morphology. The role of cosurfactant has been found to be quite crucial, which allowed the understanding of this intricate mechanism involving the complex interplay of self-assembly, dynamics of BMDs formation, and coalescence. The role of BMDs in formation of DFNS has not been reported so far and the present work allows a deeper molecular-level understanding of DFNS formation

Steric and Electronic Influence of Aryl Isocyanides on the Properties of Iridium(III) Cyclometalates

Cyclometalated iridium complexes with efficient phosphorescence and good electrochemical stability are important candidates for optoelectronic devices. Isocyanide ligands are strong-field ligands: when attached to transition metals, they impart larger HOMO–LUMO energy gaps, engender higher oxidative stability at the metal center, and support rugged organometallic complexes. Aryl isocyanides offer more versatile steric and electronic control by selective substitution at the aryl ring periphery. Despite a few reports of alkyl isocyanide of cyclometalated iridium­(III), detailed studies on analogous aryl isocyanide complexes are scant. We report the synthesis, photophysical properties, and electrochemical properties of 11 new luminescent cationic biscyclometalated bis­(aryl isocyanide)­iridium­(III) complexes. Three different aryl isocyanides2,6-dimethylphenyl isocyanide (CNAr^dmp), 2,6-diisopropylphenyl isocyanide (CNAr^dipp), and 2-naphthyl isocyanide (CNAr^nap)were combined with four cyclometalating ligands with differential π–π* energies2-phenylpyridine (ppy), 2,4-difluorophenylpyridine (F₂ppy), 2-benzothienylpyridine (btp), and 2-phenylbenzothiazole (bt). Five of them were crystallographically characterized. All new complexes show wide redox windows, with reduction potentials falling in a narrow range of −2.02 to −2.37 V and oxidation potentials spanning a wider range of 0.97–1.48 V. Efficient structured emission spans from the blue region for [(F₂ppy)₂Ir­(CNAr)₂]­PF₆ to the orange region for [(btp)₂Ir­(CNAr)₂]­PF₆, demonstrating that isocyanide ligands can support redox-stable luminescent complexes with a range of emission colors. Emission quantum yields were generally high, reaching a maximum of 0.37 for two complexes, whereas btp-ligated complexes had quantum yields below 1%. The structure of the CNAr ligand has a minimal effect on the photophysical properties, which are shown to arise from ligand-centered excited states with very little contribution from metal-to-ligand charge transfer in most cases

Fluoride Complexes of Cyclometalated Iridium(III)

Many electroluminescent devices rely on cyclometalated iridium­(III). Their advancement depends on access to reactive starting materials because of the inertness of Ir­(III). Notably, fluoride complexes of bis­(cyclometalated) Ir­(III) are scarce. Syntheses of bridged and terminal fluorides are reported here. New compounds are luminescent and thermally reactive; they are characterized by ground-state and optical methods. Crystal structures were determined for one bridging and one terminal fluoride complex. The terminal fluoride shows intramolecular hydrogen bonding to an adjacent 3,5-dimethylpyrazole ligand; a lesser interaction may occur between F and a nearby aromatic C–H bond. Terminal fluoride complexes react with carbon-, silicon-, and sulfur-based electrophiles. The new complexes phosphoresce with microsecond lifetimes at 77 and 298 K. Density-functional theory calculations indicate triplet states with little contribution from fluoride. The compounds herein are versatile phosphors having the ground-state reactivity of late transition metal fluorides