A Nontrigonal Tricoordinate Phosphorus Ligand Exhibiting Reversible "Nonspectator" L/X-Switching.

We report here a "nonspectator" behavior for an unsupported L-function σ3 -P ligand (i.e. P{N[o-NMe-C6 H4 ]2 }, 1a) in complex with the cyclopentadienyliron dicarbonyl cation (Fp+ ). Treatment of 1a⋅Fp+ with [(Me2 N)3 S][Me3 SiF2 ] results in fluoride addition to the P-center, giving the isolable crystalline fluorometallophosphorane 1aF ⋅Fp that allows a crystallographic assessment of the variance in the Fe-P bond as a function of P-coordination number. The nonspectator reactivity of 1a⋅Fp+ is rationalized on the basis of electronic structure arguments and by comparison to trigonal analogue (Me2 N)3 P⋅Fp+ (i.e. 1b⋅Fp+ ), which is inert to fluoride addition. These observations establish a nonspectator L/X-switching in (σ3 -P)-M complexes by reversible access to higher-coordinate phosphorus ligand fragments.

Precursor reaction kinetics control compositional grading and size of CdSe1-x S x nanocrystal heterostructures.

We report a method to control the composition and microstructure of CdSe1-x S x nanocrystals by the simultaneous injection of sulfide and selenide precursors into a solution of cadmium oleate and oleic acid at 240 °C. Pairs of substituted thio- and selenoureas were selected from a library of compounds with conversion reaction reactivity exponents (k E) spanning 1.3 × 10-5 s-1 to 2.0 × 10-1 s-1. Depending on the relative reactivity (k Se/k S), core/shell and alloyed architectures were obtained. Growth of a thick outer CdS shell using a syringe pump method provides gram quantities of brightly photoluminescent quantum dots (PLQY = 67 to 90%) in a single reaction vessel. Kinetics simulations predict that relative precursor reactivity ratios of less than 10 result in alloyed compositions, while larger reactivity differences lead to abrupt interfaces. CdSe1-x S x alloys (k Se/k S = 2.4) display two longitudinal optical phonon modes with composition dependent frequencies characteristic of the alloy microstructure. When one precursor is more reactive than the other, its conversion reactivity and mole fraction control the number of nuclei, the final nanocrystal size at full conversion, and the elemental composition. The utility of controlled reactivity for adjusting alloy microstructure is discussed.

A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions near the Homogeneous Limit.

We report a tunable library of N,N,N'-trisubstituted selenourea precursors and their reaction with lead oleate at 60-150 °C to form carboxylate-terminated PbSe nanocrystals in quantitative yields. Single exponential conversion kinetics can be tailored over 4 orders of magnitude by adjusting the selenourea structure. The wide range of conversion reactivity allows the extent of nucleation ([nanocrystal] = 4.6-56.7 µM) and the size following complete precursor conversion (d = 1.7-6.6 nm) to be controlled. Narrow size distributions (σ = 0.5-2%) are obtained whose spectral line widths are dominated (73-83%) by the intrinsic single particle spectral broadening, as observed using spectral hole burning measurements. The intrinsic broadening decreases with increasing size (fwhm = 320-65 meV, d = 1.6-4.4 nm) that derives from exciton fine structure and exciton-phonon coupling rather than broadening caused by the size distribution.

NANOMATERIALS. A tunable library of substituted thiourea precursors to metal sulfide nanocrystals.

Controlling the size of colloidal nanocrystals is essential to optimizing their performance in optoelectronic devices, catalysis, and imaging applications. Traditional synthetic methods control size by terminating the growth, an approach that limits the reaction yield and causes batch-to-batch variability. Herein we report a library of thioureas whose substitution pattern tunes their conversion reactivity over more than five orders of magnitude and demonstrate that faster thiourea conversion kinetics increases the extent of crystal nucleation. Tunable kinetics thereby allows the nanocrystal concentration to be adjusted and a desired crystal size to be prepared at full conversion. Controlled precursor reactivity and quantitative conversion improve the batch-to-batch consistency of the final nanocrystal size at industrially relevant reaction scales.