No abstract

Carbon nanotubes (CNTs) are appealing candidates for solar and optoelectronic applications. Traditionally used as electron sinks, CNTs can also perform as electron donors, as exemplified by coupling with perylenediimide (PDI). To achieve high efficiencies, electron transfer (ET) should be fast, while subsequent charge recombination should be slow. Typically, defects are considered detrimental to material performance because they accelerate charge and energy losses. We demonstrate that, surprisingly, common CNT defects improve rather than deteriorate the performance. CNTs and other low dimensional materials accommodate moderate defects without creating deep traps. At the same time, charge redistribution caused by CNT defects creates an additional electrostatic potential that increases the CNT work function and lowers CNT energy levels relative to those of the acceptor species. Hence, the energy gap for the ET is decreased, while the gap for the charge recombination is increased. The effect is particularly important because charge acceptors tend to bind near defects due to enhanced chemical interactions. The time-domain simulation of the excited-state dynamics provides an atomistic picture of the observed phenomenon and characterizes in detail the electronic states, vibrational motions, inelastic and elastic electron–phonon interactions, and time scales of the charge separation and recombination processes. The findings should apply generally to low-dimensional materials, because they dissipate defect strain better than bulk semiconductors. Our calculations reveal that CNT performance is robust to common defects and that moderate defects are essential rather than detrimental for CNT application in energy, electronics, and related fields.

The secondary metabolites platensimycin (1) and platencin (2), isolated from the bacterial strain Streptomyces platensis, represent a novel class of natural products exhibiting unique and potent antibacterial activity. Platencin, though structurally similar to platensimycin, has been found to operate through a slightly different mechanism of action involving the dual inhibition of lipid elongation enzymes FabF and FabH. Both natural products exhibit strong, broad-spectrum, Gram-positive antibacterial activity to key antibiotic resistant strains, including methicillin-resistant Staphylococcus aureus, vancomycin-intermediate S. aureus, and vancomycin-resistant Enterococcus faecium. Described herein are our synthetic efforts toward platencin, culminating in both the racemic and asymmetric preparation of the natural product. The syntheses demonstrate the power of the cobalt-catalyzed asymmetric Diels–Alder reaction and the one-pot reductive rearrangement of [3.2.1] bicyclic ketones to [2.2.2] bicyclic olefins.

Colloidal CdSe nanocrystals are often stabilized by organic ligands. The choice of such ligands has tremendous detrimental effects on interparticle charge transfer (CT) dynamics in nanocrystalline thin-film devices. It is evident from the recent experiment that photoexcited hole migrates from CdSe quantum dot (QD) to surface-passivating phenyl chalcogenol ligands (PhEH; E = S, Se, Te) at different time scales. But the backward electron−hole (e−h) recombination at the interface remained unexplored. A deep-level understanding of the mechanism of CT at the interface is therefore required to unravel the key role of chalcogen for the betterment of the device performance. Herein, we have performed timedomain density functional theory calculation along with nonadiabatic molecular dynamics (NAMD) simulation to investigate the photoinduced CT at the CdSe−PhEH interfaces. The simulated time scales for hole transfer (HT) are found to follow the trend PhSH > PhSeH > PhTeH that concur excellently with the experimental observations. We propose that lower electronegativity of the E atom that binds with the CdSe QD facilitates the hole migration. In addition, the delocalized nature of initial donor states, phonon modes, NA coupling, and quantum coherence are the major factors that control the faster HT. Meanwhile, for the first time, we study the linker atom-dependent e−h recombination at such interface. The recombination event is remarkably slower than the HT and occurs at the nanosecond regime. Due to the greater electronegativity of linker atom (E = S), a broad range of phonon vibration and longer-lived quantum coherence expedite the recombination at a higher rate. In contrast, for higher chalcogens with lower electronegativity (Se, Te), the exciton relaxes relatively at a lower rate. We believe our results of atomistic, time-domain methodology provide valuable insight into the exciton relaxation dynamics in CdSe−chalcogenol interface and may be useful for the enhancement of performance of future devices.