No abstract

A simple method, using density functional theory (DFT), of predicting spin-state in advance of synthesis is reported. Specifically, an excellent correlation is observed between the switching temperatures (T) in CDCl solution of five spin-crossover (SCO)-active [Fe(L)(NCBH)] complexes and the DFT-calculated (and observed) N NMR chemical shift (δ) of the five different azine-substituted 1,2,4-triazole ligands employed, L = 4-(4-methylphenyl)-3-phenyl-5-(azine)-1,2,4-triazole, where azine = pyridine, pyridazine, 4-pyrimidine, pyrazine, and 2-pyrimidine. To test the generality of this finding, DFT was also employed to readily predict the δ values for a family of 16 literature ligands, known as bpp [X,Y-substituted 2,6-(pyrazol-1-yl)pyridines], which have produced 16 SCO-active [Fe(bpp)](Z) complexes (Z = BF or in one case PF) in (CD)CO solution: again an excellent correlation was found between the computed δ and the observed T. These correlations represent a key advance in the field, as they allow a simple DFT calculation on a modified ligand to be used to reliably predict, before synthesis of the ligand or complex, the T that would result from that modification. Achieving such easily predictable tuning of T, and hence of spin-state, is a significant step forward in the field of SCO and also has big implications in many other fields in which spin-state is key, including catalysis, metallo-enzyme modeling studies, and host-guest chemistry.

Digestive ripening (DR) represents a powerful strategy for improving the size homogeneity of colloidal nanostructures. It relies on the ligand-mediated dissolution of larger nanoparticles in favor of smaller ones and is often considered to be the opposite of Ostwald ripening. Despite its successful application to size-focusing of metal colloids, digestive ripening of semiconductor nanocrystals has received little attention to date. Here, we explore this synthetic niche and demonstrate that ligand-induced ripening of semiconductor nanocrystals exhibits an unusual reaction path. The unique aspect of the DR process in semiconductors lies in the thermally activated particle coalescence, which leads to a significant increase in the nanocrystal size for temperatures above the threshold value (T th = 200–220 °C). Below this temperature, nanoparticle sizes focus to an ensemble average diameter just like in the case of metal colloids. The existence of the thermal threshold for coalescence offers an expedient strategy for controlling both the particle size and the size dispersion. Such advanced shape control was demonstrated using colloids of CdS, CdSe, CsPbBr3, and CuZnSnS4, where monodisperse samples were obtained across broad diameter ranges. We expect the demonstrated approach to be extended to other semiconductors as a simple strategy for tuning the nanoparticle morphology.

The emerging generation of quantum dot optoelectronic devices offers an appealing prospect of a size-tunable band gap. The confinement-enabled control over electronic properties, however, requires nanoparticles to be sufficiently small, which leads to a large area of interparticle boundaries in a film. Such interfaces lead to a high density of surface traps which ultimately increase the electrical resistance of a solid. To address this issue, we have developed an inverse energy-gradient core/shell architecture supporting the quantum confinement in nanoparticles larger than the exciton Bohr radius. The assembly of such nanostructures exhibits a relatively low surface-to-volume ratio, which was manifested in this work through the enhanced conductance of solution-processed films. The reported core/shell geometry was realized by growing a narrow gap semiconductor layer (CdSe) on the surface of a wide-gap core material (CdS) promoting the localization of excitons in the shell domain, as was confirmed by ultrafast transient absorption and emission lifetime measurements. The band gap emission of fabricated nanoshells, ranging from 15 to 30 nm in diameter, has revealed a characteristic size-dependent behavior tunable via the shell thickness with associated quantum yields in the 4.4-16.0% range.