Mechanisms for Adsorption of Methyl Viologen on CdS Quantum Dots

This paper describes the surface composition-dependent binding of the dichloride salt of methyl viologen (MV²⁺) to CdS quantum dots (QDs) enriched, to various degrees, with either Cd or S at the surface. The degree of enrichment is controlled synthetically and by postsynthetic dilution of the QDs in their solvent, THF. NMR shows the Cd-enriched QDs to contain a relatively dense (2.8 ligands/nm²) surface layer of oleic acid, in the form of Cd-oleate, and S-enriched QDs to contain relatively sparse (1.0 ligands/nm²) surface density of native ligands containing both oleic acid and octadecene. Electron transfer-mediated photoluminescence quenching of the QDs by MV²⁺ serves as a probe for the binding affinity of MV²⁺ for the surfaces of the QDs. Diluting Cd-enriched QDs removes Cd-oleate from the surface, exposing the stoichiometric CdS surface beneath and increasing the quenching efficiency of MV²⁺, whereas diluting S-enriched QD does not change their surface chemistry or the efficiency with which they are quenched by MV²⁺. The photoluminescence quenching data for all of the surface chemistries we studied fit well to a Langmuir model that accounts for binding of MV²⁺ through two reaction mechanisms: (i) direct adsorption of MV²⁺ to exposed stoichiometric CdS surfaces (with an equilibrium adsorption constant of 1.5 × 10⁵ M^–1), and (ii) adsorption of MV²⁺ to stoichiometric CdS surfaces upon displacement of weakly bound Cd-oleate complexes (with an equilibrium displacement constant of 3.5 × 10³ M^–1). <i>Ab initio</i> calculations of the binding energy for adsorption of the dichloride salt of MV²⁺ on Cd- and S-terminated surfaces reveal a substantial preference of MV²⁺ for S-terminated lattices due to alignment of the positively charged nitrogens on MV²⁺ with the negatively charged sulfur. These findings suggest a strategy to maximize the adsorption of redox-active molecules in electron transfer-active geometries through synthetic and postsynthetic manipulation of the inorganic surface

Light-Activated Protein Inhibition through Photoinduced Electron Transfer of a Ruthenium(II)–Cobalt(III) Bimetallic Complex

We describe a mechanism of light activation that initiates protein inhibitory action of a biologically inert Co­(III) Schiff base (Co­(III)-sb) complex. Photoinduced electron transfer (PET) occurs from a Ru­(II) bipyridal complex to a covalently attached Co­(III) complex and is gated by conformational changes that occur in tens of nanoseconds. Reduction of the Co­(III)-sb by PET initiates displacement of the inert axial imidazole ligands, promoting coordination to active site histidines of α-thrombin. Upon exposure to 455 nm light, the rate of ligand exchange with 4-methylimidazole, a histidine mimic, increases by approximately 5-fold, as observed by NMR spectroscopy. Similarly, the rate of α-thrombin inhibition increases over 5-fold upon irradiation. These results convey a strategy for light activation of inorganic therapeutic agents through PET utilizing redox-active metal centers

Subpicosecond Photoinduced Hole Transfer from a CdS Quantum Dot to a Molecular Acceptor Bound Through an Exciton-Delocalizing Ligand

This paper describes the enhancement of the rate of hole transfer from a photoexcited CdS quantum dot (QD), with radius <i>R</i> = 2.0 nm, to a molecular acceptor, phenothiazine (PTZ), by linking the donor and acceptor through a phenyldithiocarbamate (PTC) linker, which is known to lower the confinement energy of the excitonic hole. Upon adsorption of PTC, the bandgap of the QD decreases due to delocalization of the exciton, primarily the excitonic hole, into interfacial states of mixed QD/PTC character. This delocalization enables hole transfer from the QD to PTZ in <300 fs (within the instrument response of the laser system) when linked by PTC, but not when linked by a benzoate group, which has a similar length and conjugation as PTC but does not delocalize the excitonic hole. Comparison of the two systems was aided by quantification of the surface coverage of benzoate and PTC-linked PTZ by ¹H NMR. This work provides direct spectroscopic evidence of the enhancement of the rate of hole extraction from a colloidal QD through covalent linkage of a hole acceptor through an exciton-delocalizing ligand

Measurement of Wavelength-Dependent Polarization Character in the Absorption Anisotropies of Ensembles of CdSe Nanorods

Transient absorption (TA) and photoluminescence excitation (PLE) anisotropy measurements were used to investigate the polarization of band-edge and above-band-edge excitonic states in ensembles of CdSe nanocrystals with aspect ratios of 1:1, 3:1, and 10:1, dispersed in hexanes. The 1:1 nanocrystals (quantum dots) are isotropic absorbers and emitters. The 10:1 nanorods have a nonzero but featureless anisotropy spectrum above the band edge due to heterogeneity in the crystal structure and, therefore, electronic structure within single nanorods. The nanocrystals with an aspect ratio of 3:1, which are largely single crystals, have PLE and TA anisotropy spectra with features that correspond to those in the absorption spectrum. Direct measurement of the TA anisotropy spectrum of the nanorods and comparison with the PLE anisotropy spectrum reveal that the band-edge absorptive and emissive transitions contain both linear (<i>z</i>) and planar (<i>xy</i>) character. The degree of planar character at the band-edge states, modulated by classical local field effects arising from the dielectric contrast between the nanorod and the solvent, limits the degree of photoselection at this wavelength. The variation in the magnitude of the <i>xy</i> projection of the absorptive transitions within states above the band edge is responsible for the wavelength dependence of the absorption and emission anisotropies

Enhancement of the Yield of Photoinduced Charge Separation in Zinc Porphyrin–Quantum Dot Complexes by a Bis(dithiocarbamate) Linkage

This paper describes the use of a phenyl bis­(dithiocarbamate) (PBTC) linker to enhance the quantum yield of photoinduced electron transfer (eT) from a zinc porphyrin (ZnP) molecule (donor) to a CdSe quantum dot (QD) (acceptor), where quantum yield is defined as the fraction of photoexcited ZnP molecules in the sample that donate an electron to the QD. The PBTC ligand links the ZnP to the QD by coordinating to Cd²⁺ on the surface of the QD and the Zn metal center in ZnP via its dithiocarbamate groups. Compared with the donor–acceptor complex formed in the absence of PBTC linkers, where the ZnP molecule adsorbs to the QD through its carboxylate moiety, the PBTC linkage increases the binding affinity between ZnP molecules and QDs by an order of magnitude, from 1.0 × 10⁵ ± (0.7 × 10⁴) M^–1 to 1.0 × 10⁶ ± (1.0 × 10⁵) M^–1, and thereby increases the eT quantum yield by, for example, a factor of 4 (from 8% to 38%) within mixtures where the molar ratio ZnP:QD = 1:1