Photophysical and Redox Properties of Molecule-like CdSe Nanoclusters

Advancing our understanding of the photophysical and electrochemical properties of semiconductor nanoclusters with a molecule-like HOMO–LUMO energy level will help lead to their application in photovoltaic devices and photocatalysts. Here we describe an approach to the synthesis and isolation of molecule-like CdSe nanoclusters, which displayed sharp transitions at 347 nm (3.57 eV) and 362 nm (3.43 eV) in the optical spectrum with a lower energy band extinction coefficient of ∼121 000 M^–1 cm^–1. Mass spectrometry showed a single nanocluster molecular weight of 8502. From this mass and various spectroscopic analyses, the nanoclusters are determined to be of the single molecular composition Cd₃₄Se₂₀(SPh)₂₈, which is a new nonstiochiometric nanocluster. Their reversible electrochemical band gap determined in Bu₄NPF₆/CH₃CN was found to be 4.0 V. There was a 0.57 eV Coulombic interaction energy of the electron–hole pair involved. The scan rate dependent electrochemistry suggested diffusion-limited transport of nanoclusters to the electrode. The nanocluster diffusion coefficient (<i>D</i> = 5.4 × 10 ^–4 cm²/s) in acetonitrile solution was determined from cyclic voltammetry, which suggested Cd₃₄Se₂₀(SPh)₂₈ acts as a multielectron donor or acceptor. We also present a working model of the energy level structure of the newly discovered nanocluster based on its photophysical and redox properties

Effects of Surface-Passivating Ligands and Ultrasmall CdSe Nanocrystal Size on the Delocalization of Exciton Confinement

Here we report an unprecedentedly large and controllable decrease in the optical band gap (up to 107 nm, 610 meV) of molecule-like ultrasmall CdSe nanocrystals (diameters ranging from 1.6 to 2.0 nm) by passivating their surfaces with conjugated ligands (phenyldithiocarbamates, PDTCs) containing a series of electron-donating and -withdrawing functional groups through a ligand-exchange reaction on dodecylamine (DDA)-coated nanocrystals. This band-edge absorption shift is due to the delocalization of the strongly confined excitonic hole from nanocrystals to the ligand molecular orbitals and not from nanocrystal growth or dielectric constant effects. ¹H NMR analysis confirmed that the nanocrystal surface contained a mixed ligation of DDA and PDTC. The effects of the nanocrystal size on the extent of exciton delocalization were also studied and found to be smaller for larger nanocrystals. Modulating the energy level of ligand-passivated ultrasmall nanocrystals and controlling the electronic interaction at the nanocrystal-passivating ligand interface are very important to the fabrication of solid-state devices

Isolation of Bright Blue Light-Emitting CdSe Nanocrystals with 6.5 kDa Core in Gram Scale: High Photoluminescence Efficiency Controlled by Surface Ligand Chemistry

Alkylamine-capped blue light-emitting (CdSe)₃₄ nanocrystals were synthesized via a phosphine-free method and isolated in gram-scale quantity. The exclusive formation of 6.5 kDa core mass was confirmed by combined optical spectroscopy and high resolution mass spectrometry studies. Variable power laser desorption ionization-mass spectrometry further confirmed the formation of the (CdSe)₃₄ core. The surface ligand chemistry was found to be extremely important in enhancing the photoluminescence properties. The nanocrystals were highly stable during the postsynthetic ligand treatment with triphenylphosphine, which increased their fluorescent quantum yield up to 23.6% without compromising the core composition as determined by mass spectrometry. Examination of their ³¹P and ¹H NMR spectra demonstrated the presence of amine and phosphine on the surface of the nanocrystals where phosphines were selectively attached to surface selenium sites that stabilized the nonradiative trap states and increased the fluorescence quantum yield. The gram-scale synthesis and high quantum yield of single-sized nanocrystals should greatly facilitate new and improved semiconductor nanocrystal applications in the field of nanoscience and nanotechnology, resulting in more rapid and less expensive production of future advanced electrochromic and light-emitting devices

“Clicked” Sugar–Curcumin Conjugate: Modulator of Amyloid-β and Tau Peptide Aggregation at Ultralow Concentrations

The synthesis of a water/plasma soluble, noncytotoxic, “clicked” sugar-derivative of curcumin with amplified bioefficacy in modulating amyloid-β and tau peptide aggregation is presented. Curcumin inhibits amyloid-β and tau peptide aggregation at micromolar concentrations; the sugar–curcumin conjugate inhibits Aβ and tau peptide aggregation at concentrations as low as 8 nM and 0.1 nM, respectively. In comparison to curcumin, this conveniently synthesized Alzheimer’s drug candidate is a more powerful antioxidant