Mechanisms of Energy Transfer and Enhanced Stability of Carbidonitride Phosphors for Solid-State Lighting

Phosphor-converted light emitting diodes (pcLEDs) produce white light through the use of phosphors that convert blue light emitted from the LED chip into green and red wavelengths. Understanding the mechanisms of degradation of the emission spectra and quantum yields of the phosphors used in pcLEDs is of critical importance to fully realize the potential of solid-state lighting as an energy efficient technology. Toward this end, time-resolved photoluminescence spectroscopy was used to identify the mechanistic origins of enhanced stability and luminescence efficiency that can be obtained from a series of carbidonitride red phosphors with varying degrees of substitutional carbon. The increasing substitution of carbon and oxygen in nitrogen positions of the carbidonitride phosphor (Sr₂Si₅N_8-[(4x/3)+z]C_xO_3z/2:Eu²⁺) systematically changed the dimensions of the crystalline lattice. These structural changes caused a red shift and broadening of the emission spectra of the phosphors due to faster energy transfer from higher to lower energy emission sites. Surprisingly, in spite of broadening of the emission spectra, the quantum yield was maintained or increased with carbon substitution. Aging phosphors with lowered carbon content under conditions that accurately reflected thermal and optical stresses found in functioning pcLED packages led to spectral changes that were dependent on substitutional carbon content. Importantly, phosphors that contained optimal amounts of carbon and oxygen possessed luminescence spectra and quantum yields that did not undergo changes associated with aging and therefore provided a more stable color point for superior control of the emission properties of pcLED packages. These findings provide insights to guide continued development of phosphors for efficient and stable solid-state lighting materials and devices.

Reversible Borylene Formation from Ring Opening of Pinacolborane and Other Intermediates Generated from Five-Coordinate Tris-Boryl Complexes: Implications for Catalytic C-H Borylation

Catalytic C-H borylation using the five-coordinate tris-boryl complex (dippe)Ir(Bpin)₃ (5a, dippe = 1,2-bis(diisopropylphosphino)ethane) has been examined using ³¹P{¹H} and ¹H NMR spectroscopy. Compound 5a was shown to react rapidly and reversibly with HBpin to generate a six-coordinate borylene complex, (dippe)Ir(H)-(Bpin)₂(BOCMe₂CMe₂OBpin) (6), whose structure was confirmed by X-ray crystallography. Under catalytic conditions, the H₂ generated from C-H borylation converted compound 6 to a series of intermediates. The first is tentatively assigned from ³¹P{¹H} and ¹H NMR spectra as (dippe)Ir(H₂B₃pin₃) (7), which is the product of formal H₂ addition to compound 5a. As catalysis progressed, compound 7 was converted to a new species with the formula (dippe)Ir(H₃B₂pin₂) (8), which arose from H₂ addition to compound 7 with loss of HBpin. Compound 8 was characterized by ³¹P{¹H} and ¹H NMR spectroscopy, and its structure was confirmed by X-ray crystallography, where two molecules with different ligand orientations were found in the unit cell. DFT calculations support the formulation of compound 8 as an Ir^III agostic borane complex, (dippe)IrH₂(Bpin)(η²-HBpin). Compound 8 was gradually converted to (dippe)Ir(H₄Bpin) (9), which was characterized by ³¹P{¹H} and ¹H NMR spectroscopy and X-ray crystallography. DFT calculations favor its formulation as an agostic borane complex of Ir^III with the formula (dippe)IrH₃(η²-HBpin). Compound 9 reacted further with H₂ to afford the dimeric structure [(dippe)IrH₂(μ₂-H)]₂ (10), which was characterized by ¹H NMR and X-ray crystallography. Compounds 7-10 are in equilibrium when H₂ and HBpin are present.

Electronic effects in iridium C-H borylations: insights from unencumbered substrates and variation of boryl ligand substituents

Experiment and theory favour a model of C-H borylation where significant proton transfer character exists in the transition state.

Getting the sterics just right: a five-coordinate iridium trisboryl complex that reacts with C-H bonds at room temperature

Five-coordinate boryl complexes relevant to Ir mediated C-H borylations have been synthesized, providing a glimpse of the most fundamental step in the catalytic cycle for the first time.

Direct synthesis of mesostructured lamellar molybdenum disulfides using a molten neutral n-alkylamine as the solvent and template

A series of novel mesostructured lamellar molybdenum disulfides with the d spacings from 17 to 30 A can be prepared by the reaction of Mo(CO)6 with elemental sulfur using a molten n-alkylamine as the solvent as well as the template at 140 degrees C. Such intercalated phases can be transformed into mesoporous molybdenum disulfides by slow thermal treatments at 200 degrees C.