Jinquan Chen

Efficient hot-electron transfer by a plasmon-induced interfacial charge-transfer transition

Improving electron harvesting Small metal nanostructures generate electrons from light by creating surface plasmons, which can transfer “hot electrons” to a semiconductor. The efficiency of this process, however, is often low because of electron-electron scattering. Wu et al. demonstrate a pathway that allows the plasmon to directly excite an electron in a strongly coupled semiconductor acceptor (see the Perspective by Kale). Cadmiun selenide nanorods bearing gold nanoparticles on their ends strongly damped plasmons via interfacial electron transfer with a quantum efficiency exceeding 24%. Science, this issue p. 632; see also p. 587 The plasmon of a gold nanoparticle on a cadmium selenide nanorod is strongly damped by interfacial electron transfer. [Also see Perspective by Kale and Christopher] Plasmon-induced hot-electron transfer from metal nanostructures is a potential new paradigm for solar energy conversion; however, the reported efficiencies of devices based on this concept are often low because of the loss of hot electrons via ultrafast electron-electron scattering. We propose a pathway, called the plasmon-induced interfacial charge-transfer transition (PICTT), that enables the decay of a plasmon by directly exciting an electron from the metal to a strongly coupled acceptor. We demonstrated this concept in cadmium selenide nanorods with gold tips, in which the gold plasmon was strongly damped by cadmium selenide through interfacial electron transfer. The quantum efficiency of the PICTT process was high (>24%), independent of excitation photon energy over a ~1–electron volt range, and dependent on the excitation polarization.

Universal Length Dependence of Rod-to-Seed Exciton Localization Efficiency in Type I and Quasi-Type II CdSe@CdS Nanorods.

A critical step involved in many applications of one-dimensional seeded CdSe@CdS nanorods, such as luminescent solar concentrators, optical gains, and photocatalysis, is the localization of excitons from the light-harvesting CdS nanorod antenna into the light-emitting CdSe quantum dot seed. We report that the rod-to-seed exciton localization efficiency decreases with the rod length but is independent of band alignment between the CdSe seed and CdS rod. This universal dependence can be well modeled by the competition between exciton one-dimensional diffusion to the CdSe seed and trapping on the CdS rod. This finding provides a rational approach for optimizing these materials for their various device applications.

Ultrafast Excited-State Dynamics in Hexaethyleneglycol-Linked DNA Homoduplexes Made of A·T Base Pairs

Double-stranded DNA conjugates with the sequence (dA)10·(dT)10 and hexaethylene glycol linkers at one end (hairpin) or both ends (dumbbell) were studied in buffer solution by deep UV femtosecond transient absorption spectroscopy. These covalently constrained duplexes have greatly enhanced thermal stability compared to A·T duplex oligonucleotides that lack linkers. The conjugates eliminate the slipped-strand and end-frayed structures that form readily in unlinked (dA)n·(dT)n sequences, allowing the excited-state dynamics of stacked A·T base pairs to be observed without interference from structures with stacking or pairing defects. Transient absorption signals show that subpicosecond internal conversion to the electronic ground state takes place in addition to the formation of long-lived excited states having lifetimes of approximately 70 ps. Watson-Crick base-pairing slows the rate of vibrational cooling compared to monomeric bases or single-stranded DNA, possibly by reducing the total number of solute-solvent hydrogen bonds. Long-lived excited states in intact A·T base pairs decay several times more quickly than long-lived excited states observed in single-stranded (dA)n sequences. These results show that base-pairing can measurably affect nonradiative decay pathways in A·T duplexes.

Excited States in DNA Strands Investigated by Ultrafast Laser Spectroscopy

Ultrafast laser experiments on carefully selected DNA model compounds probe the effects of base stacking, base pairing, and structural disorder on excited electronic states formed by UV absorption in single and double DNA strands. Direct π-orbital overlap between two stacked bases in a dinucleotide or in a longer single strand creates new excited states that decay orders of magnitude more slowly than the generally subpicosecond excited states of monomeric bases. Half or more of all excited states in single strands decay in this manner. Ultrafast mid-IR transient absorption experiments reveal that the long-lived excited states in a number of model compounds are charge transfer states formed by interbase electron transfer, which subsequently decay by charge recombination. The lifetimes of the charge transfer states are surprisingly independent of how the stacked bases are oriented, but disruption of π-stacking, either by elevating temperature or by adding a denaturing co-solvent, completely eliminates this decay channel. Time-resolved emission measurements support the conclusion that these states are populated very rapidly from initial excitons. These experiments also reveal the existence of populations of emissive excited states that decay on the nanosecond time scale. The quantum yield of these states is very small for UVB/UVC excitation, but increases at UVA wavelengths. In double strands, hydrogen bonding between bases perturbs, but does not quench, the long-lived excited states. Kinetic isotope effects on the excited-state dynamics suggest that intrastrand electron transfer may couple to interstrand proton transfer. By revealing how structure and non-covalent interactions affect excited-state dynamics, on-going experimental and theoretical studies of excited states in DNA strands can advance understanding of fundamental photophysics in other nanoscale systems.

Size-Independent Exciton Localization Efficiency in Colloidal CdSe/CdS Core/Crown Nanosheet Type-I Heterostructures

CdSe/CdS core/crown nanoplatelet type I heterostructures are a class of two-dimensional materials with atomically precise thickness and many potential optoelectronic applications. It remains unclear how the precise thickness and lack of energy disorder affect the properties of exciton transport in these materials. By steady-state photoluminescence excitation spectroscopy and ultrafast transient absorption spectroscopy, we show that in five CdSe/CdS core/crown structures with the same core and increasing crown size (with thickness of ∼1.8 nm, width of ∼11 nm, and length from 20 to 40 nm), the crown-to-core exciton localization efficiency is independent of crown size and increases with photon energy above the band edge (from 70% at 400 nm to ∼100% at 370 nm), while the localization time increases with the crown size. These observations can be understood by a model that accounts for the competition of in-plane exciton diffusion and selective hole trapping at the core/crown interface. Our findings suggest that the exciton localization efficiency can be further improved by reducing interfacial defects.

Engineering an N-doped Cu2O@N–C interface with long-lived photo-generated carriers for efficient photoredox catalysts

Enhancing the separation efficiency of electrons and holes plays an important role in improving photocatalysis. One of the most effective methods for this is to engineer suitable interface materials. We develop a simple two-step strategy to engineer a N-doped Cu2O@N–C interface using N-rich MOFs (NTU-105) as templates. The presence of N-rich ligands (H6−1) in well-defined cubic MOFs allows the formation of a N-doped porous C matrix hosting well-dispersed N-doped Cu2O. This nanostructure provides several favorable features for photocatalysis: (1) the porous C matrix substrate effectively stabilizes the small Cu2O nanoparticles, preventing their aggregation; (2) the N doping of Cu2O and the C substrate increases their conductivity, which can enhance electron and hole transfer properties; and (3) the uniform distribution of N-doped Cu2O nanoparticles provides abundant highly active catalytic sites. As a result, the N-doped Cu2O@N–C (MCNC) nanoparticles exhibit extraordinary photoredox catalysis in C–C bond forming reactions. Femtosecond transient absorption spectroscopy was used to trace the charge carrier dynamics in the N-doped Cu2O@N–C nanoparticles, revealing the fact that high photocatalytic performance relies on long-lived holes.

A Water-Soluble, Green-Light Triggered, and Photo-Calibrated Nitric Oxide Donor for Biological Applications

Nitric oxide (NO) is a versatile endogenous molecule, involved in various physiological processes and implicated in the progression of many pathological conditions. Therefore, NO donors are valuable tools in NO related basic and applied applications. The traditional spontaneous NO donors are limited in scenarios where flux, localization, and dose of NO could be monitored. This has promoted the development of novel NO donors, whose NO release is not only under control, but also self-calibrated. Herein, we reported a phototriggered and photocalibrated NO donor (NOD565) with an N-nitroso group on a rhodamine dye. NOD565 is nonfluorescent and could release NO efficiently upon irradiation by green light. A bright rhodamine dye is generated as a side-product and its fluorescence can be used to monitor the NO release. The potentials of NOD565 in practical applications are showcased in in vitro studies, e.g., platelet aggregation inhibition and fungi growth suppression.

Interfacial Clustering-Triggered Fluorescence–Phosphorescence Dual Solvoluminescence of Metal Nanoclusters

The fluorescence-phosphorescence dual solvoluminescence (SL) of water-soluble metal nanoclusters (NCs) at room temperature was successfully achieved by a simple solvent-stimulated strategy. The strong interaction between carboxylate ligands and the metal core at the nanoscale interface not only induces rigid conformations of carbonyl groups but also affords a perfect carbonyl cluster that acts as an exact chromophore of metal NCs for aggregation-induced emission (AIE) mechanics. The clustering of carbonyl groups bearing on the polymer backbone chain is promoted by newly discovered n → π* noncovalent interactions. The efficient delocalization of electrons in overlapped C═O double bonds between neighboring carbonyl groups triggered by strong n → π* interactions in the polymer cluster accounts for its unique SL properties, especially the abnormal phosphorescence. This was further confirmed by controlled experiments for the presence of intersystem crossing (ISC) transitions. The results provide novel insights for understanding the complex SL process and open up a new way to study the inherent mechanism of SL by broadening the application of metal NCs.

Engineering an N-doped TiO2@N-doped C butterfly-like nanostructure with long-lived photo-generated carriers for efficient photocatalytic selective amine oxidation

The photocatalytic efficiency of titanium oxide (TiO2) has been limited by the degree of visible-light absorption, electron/hole separation and surface reactivity. In this article, N-doped porous carbon incorporating N-doped TiO2 with a butterfly-like structure has been synthesized by using NH2-MIL-125 (Ti), a Ti-based metal–organic framework, as a hard template. The SEM, TEM and AFM observations indicate that the as-synthesized nanocomposites preserve the butterfly-like morphology of the MOF template, which are assembled with two-dimensional (2D) corrugated nanosheets. The EDX and XPS results prove that the composites of the product are small N doped TiO2 particles distributed in the N doped C matrix. The N2 adsorption and desorption isotherms, UV-Vis spectra and photocurrent density measurements indicate that the N-doped porous carbon incorporating N-doped TiO2 nanoparticles provide several favorable features for photocatalysis: a large BET surface area, visible-light absorption and high charge separation efficiency. As a result, the nanocomposites exhibit excellent photocatalytic activity toward selective oxidation of amines to imines. Density functional calculations, electron spin resonance (ESR) spectroscopy and transient absorption spectroscopy were used to reveal the mechanism of the photocatalytic process.

A stable electron-deficient metal–organic framework for colorimetric and luminescence sensing of phenols and anilines

A 3D metal–organic framework (LVMOF-1) with unique electron-deficient channels was synthesized and its sensing properties for electron-rich benzene derivatives were demonstrated. The MOF is built of robust [Eu(OH)(COO)2]n columns and tetratopic viologen-based crosslinkers and shows excellent chemical stability. The structure integrates Eu(III) centers to luminesce and viologen moieties to accept electrons, and most notably, the electron-deficient viologen moieties, like those in box-like diviologen cyclophanes, are ideally spaced for sandwiching electron-rich aromatic rings. The MOF shows a bimodal response (color and luminescence) to phenols, anilines, benzenediols and aminophenols, with excellent selectivity against a wide range of other organic molecules. The chromogenic phenomena allow facile, quick and naked-eye test-paper detection of these priority contaminants in water, while the luminescence response affords very fast and sensitive quantitative detection. In particular, the detection limits for anilines and benzenediols are as low as 1–9 ppb. The charge transfer and energy transfer mechanisms for the sensing properties were elucidated on the basis of X-ray crystallography after single-crystal-to-single-crystal adsorption and orbital energy analyses according to electrochemical and spectroscopic data and also DFT calculations. The MOF bridges the gap between discrete cyclophanes functioning in solution and extended porous lattices in the solid state and can provide a blueprint for further development of sensory MOFs.