Computer-assisted catalyst development via automated modelling of conformationally complex molecules: application to diphosphinoamine ligands.

Simulation of conformationally complicated molecules requires multiple levels of theory to obtain accurate thermodynamics, requiring significant researcher time to implement. We automate this workflow using all open-source code (XTBDFT) and apply it toward a practical challenge: diphosphinoamine (PNP) ligands used for ethylene tetramerization catalysis may isomerize (with deleterious effects) to iminobisphosphines (PPNs), and a computational method to evaluate PNP ligand candidates would save significant experimental effort. We use XTBDFT to calculate the thermodynamic stability of a wide range of conformationally complex PNP ligands against isomeriation to PPN (ΔGPPN), and establish a strong correlation between ΔGPPN and catalyst performance. Finally, we apply our method to screen novel PNP candidates, saving significant time by ruling out candidates with non-trivial synthetic routes and poor expected catalytic performance.

Coupling Single Giant Nanocrystal Quantum Dots to the Fundamental Mode of Patch Nanoantennas through Fringe Field.

Through single dot spectroscopy and numerical simulation studies, we demonstrate that the fundamental mode of gold patch nanoantennas have fringe-field resonance capable of enhancing the nano-emitters coupled around the edge of the patch antenna. This fringe-field coupling is used to enhance the radiative rates of core/thick-shell nanocrystal quantum dots (g-NQDs) that cannot be embedded into the ultra-thin dielectric gap of patch nanoantennas due to their large sizes. We attain 14 and 3 times enhancements in single exciton radiative decay rate and bi-exciton emission efficiencies of g-NQDs respectively, with no detectable metal quenching. Our numerical studies confirmed our experimental results and further reveal that patch nanoantennas can provide strong emission enhancement for dipoles lying not only in radial direction of the circular patches but also in the direction normal to the antennas surface. This provides a distinct advantage over the parallel gap-bar antennas that can provide enhancement only for the dipoles oriented across the gap.

Quantum Optical Signature of Plasmonically Coupled Nanocrystal Quantum Dots.

Small clusters of two to three silica-coated nanocrystals coupled to plasmonic gap-bar antennas can exhibit photon antibunching, a characteristic of single quantum emitters. Through a detailed analysis of their photoluminescence emissions characteristics, it is shown that the observed photon antibunching is the evidence of coupled quantum dot formation resulting from the plasmonic enhancement of dipole-dipole interaction.

Correlated structural-optical study of single nanocrystals in a gap-bar antenna: effects of plasmonics on excitonic recombination pathways.

We performed time-correlated single-photon counting experiments on individual silica coated CdSe/CdS core/thick-shell nanocrystal quantum dots (a.k.a., giant NQDs [g-NQDs]), placed on the plasmonic gap-bar antennas. Optical properties were directly correlated with the scanning electron microscopy (SEM) images of g-NQD-plasmonic antenna coupled structures. The structures, in which the g-NQDs are located in the gap of the antenna, afford a coupling with up to 9.6 fold enhancement of radiative recombination rates. These coupled g-NQDs are also characterized by a strong enhancement of bi-exciton emission efficiency that increases with their radiative enhancement factor. By analysing these findings with a simple model, we show that the plasmonic field of the antenna does not alter the Auger recombination processes of the bi-exciton states. As a result, enhancements of the single and bi-exciton radiative recombination rates lead directly to bi-exciton emission enhancement. These findings suggest that a plasmonic field can be utilized effectively in achieving a strong bi-exciton emission that is needed for photon pair generation and plasmon-assisted lasing.

Three dimensional time-gated tracking of non-blinking quantum dots in live cells.

Single particle tracking has provided a wealth of information about biophysical processes such as motor protein transport and diffusion in cell membranes. However, motion out of the plane of the microscope or blinking of the fluorescent probe used as a label generally limits observation times to several seconds. Here, we overcome these limitations by using novel non-blinking quantum dots as probes and employing a custom 3D tracking microscope to actively follow motion in three dimensions (3D) in live cells. Signal-to-noise is improved in the cellular milieu through the use of pulsed excitation and time-gated detection.

Plasmonic giant quantum dots: hybrid nanostructures for truly simultaneous optical imaging, photothermal effect and thermometry.

Hybrid semiconductor-metal nanoscale constructs are of both fundamental and practical interest. Semiconductor nanocrystals are active emitters of photons when stimulated optically, while the interaction of light with nanosized metal objects results in scattering and ohmic damping due to absorption. In a combined structure, the properties of both components can be realized together. At the same time, metal-semiconductor coupling may intervene to modify absorption and/or emission processes taking place in the semiconductor, resulting in a range of effects from photoluminescence quenching to enhancement. We show here that photostable 'giant' quantum dots when placed at the center of an ultrathin gold shell retain their key optical property of bright and blinking-free photoluminescence, while the metal shell imparts efficient photothermal transduction. The latter is despite the highly compact total particle size (40-60 nm "inorganic" diameter and <100 nm hydrodynamic diameter) and the very thin nature of the optically transparent Au shell. Importantly, the sensitivity of the quantum dot emission to local temperature provides a novel internal thermometer for recording temperature during infrared irradiation-induced photothermal heating.

4-Nitrophenyl- and 4'-nitro-1,1'-biphenyl-4-carboxylates attached to Mo2 quadruple bonds: ground versus excited state M2δ-ligand conjugation.

From the reactions between Mo2(T(i)PB)4, where T(i)PB = 2,4,6-triisopropylbenzoate and two equivalents of the carboxylic acid LH (LH = 4-nitrobenzoic acid and 4'-nitro[1,1'-biphenyl]-4-carboxylic acid) the compounds trans-M2(T(i)PB)2L2 have been prepared: I (L = 4-nitrobenzoate and M = Mo), II (L = 4'-nitro-1,1'-biphenylcarboxylate and M = Mo) and III (L = 4-nitrobenzoate and M2 = MoW). The compounds have been characterized by (1)H NMR, UV-Vis and steady state emission spectroscopy, ns and fs transient absorption spectroscopy and cyclic voltammetry. These data are compared with predictions based on electronic structure calculations on model compounds where T(i)PB is substituted for formate. Together these data indicate stronger ground-state coupling of the Mo2δ and ligand π* systems in I relative to II but this order is reversed in the photo excited S1(1)MLCT state. Attempts to prepare the W2 containing analogs were unsuccessful.

Influence of the core size on biexciton quantum yield of giant CdSe/CdS nanocrystals.

We present a systematic study of photoluminescence (PL) emission intensity and biexciton (BX) quantum yields (QYBX) in individual "giant" CdSe/CdS nanocrystals (g-NCs) as a function of g-NC core size and shell thickness. We show that g-NC core size significantly affects QYBX and can be utilized as an effective tuning parameter towards higher QYBX while keeping the total volume of the g-NC constant. Specifically, we observe that small-core (2.2 nm diameter) CdSe/CdS NCs with a volume of ∼200 nm(3) (shell comprises 4 CdS monolayers) show very low average and maximum QYBX's of ∼3 and 7%, respectively. In contrast, same-volume medium-core (3 nm diameter) NCs afford higher average values of ∼10%, while QYBX's of ∼30% are achieved for same-volume large-core (5.5 nm diameter) CdSe/CdS NCs, with some approaching ∼80%. These observations underline the influence of the g-NC core size on the evolution of PL emissive states in multi-shell NCs. Moreover, our study also reveals that the use of long anneal times in the growth of CdS shells plays a critical role in achieving high QYBX.

3-Dimensional Tracking of Non-blinking 'Giant' Quantum Dots in Live Cells.

While semiconductor quantum dots (QDs) have been used successfully in numerous single particle tracking (SPT) studies due to their high photoluminescence efficiency, photostability, and broad palette of emission colors, conventional QDs exhibit fluorescence intermittency or 'blinking,' which causes ambiguity in particle trajectory analysis and limits tracking duration. Here, non-blinking 'giant' quantum dots (gQDs) are exploited to study IgE-FcεRI receptor dynamics in live cells using a confocal-based 3D SPT microscope. There is a 7-fold increase in the probability of observing IgE-FcεRI for longer than 1 min using the gQDs compared to commercially available QDs. A time-gated photon-pair correlation analysis is implemented to verify that selected SPT trajectories are definitively from individual gQDs and not aggregates. The increase in tracking duration for the gQDs allows the observation of multiple changes in diffusion rates of individual IgE-FcεRI receptors occurring on long (>1 min) time scales, which are quantified using a time-dependent diffusion coefficient and hidden Markov modeling. Non-blinking gQDs should become an important tool in future live cell 2D and 3D SPT studies, especially in cases where changes in cellular dynamics are occurring on the time scale of several minutes.

Disentangling the effects of clustering and multi-exciton emission in second-order photon correlation experiments.

In single particle spectroscopy, the degree of observed fluorescence anti-bunching in a second-order cross correlation experiment is indicative of its bi-exciton quantum yield and whether or not a particle is well isolated. Advances in quantum dot synthesis have produced single particles with bi-exciton quantum yields approaching unity. Consequently, this creates uncertainty as to whether a particle has a high bi-exciton quantum yield or if it exists as a cluster. We report on a time-gated anti-bunching technique capable of determining the relative contributions of both multi-exciton emission and clustering effects. In this way, we can now unambiguously determine if a particle is single. Additionally, this time-gated anti-bunching approach provides an accurate way for the determination of bi-exciton lifetime with minimal contribution from higher order multi-exciton states.

Single-Nanocrystal Photoluminescence Spectroscopy Studies of Plasmon-Multiexciton Interactions at Low Temperature.

Using thick-shell or "giant" CdSe/CdS nanocrystal quantum dots (g-NQDs), characterized by strongly suppressed Auger recombination, we studied the influence of plasmonic interactions on multiexciton emission. Specifically, we assessed the separate effects of plasmonic absorption and plasmonic emission enhancement by a systematic analysis of the pump fluence dependence of low-temperature photoluminescence (low-T PL) derived from individual CdSe/CdS g-NQDs deposited on nanoroughened silver films. Our study reveals that (1) the multiexciton (MX) emissions in g-NQD coupled to silver films were enhanced not only through the creation of more excitons via enhancement of absorption but also through the direct modification of the competition between the radiative and nonradiative recombination processes of MXs; (2) strong enhancement in absorption is not necessary for strong multiexciton emission; and (3) the emission of MXs can become stronger with the increase of multiexciton order. We also exploited the strong enhancement of MX emission to perform second-order photon correlation and cross-correlation experiments using very low pump fluences and observed a strong photon bunching that decays with increasing pump fluence.

Super-Poissonian statistics of photon emission from single CdSe-CdS core-shell nanocrystals coupled to metal nanostructures.

We demonstrate that photon antibunching observed for individual nanocrystal quantum dots (NQDs) can be transformed into photon bunching characterized by super-Poissonian statistics when they are coupled to metal nanostructures (MNs). This observation indicates that, while the quantum yield of a biexciton (Q(2X)) is lower than that of a single exciton (Q(1X)) in freestanding NQDs, Q(2X) becomes greater than Q(1X) in NQDs coupled to MNs. This unique phenomenon is attributed to metal-induced quenching with a rate that scales more slowly with exciton multiplicity than the radiative decay rate and dominates over other nonradiative decay channels for both single excitons and biexcitons.

Lifetime blinking in nonblinking nanocrystal quantum dots.

Nanocrystal quantum dots are attractive materials for applications as nanoscale light sources. One impediment to these applications is fluctuations of single-dot emission intensity, known as blinking. Recent progress in colloidal synthesis has produced nonblinking nanocrystals; however, the physics underlying blinking suppression remains unclear. Here we find that ultra-thick-shell CdSe/CdS nanocrystals can exhibit pronounced fluctuations in the emission lifetimes (lifetime blinking), despite stable nonblinking emission intensity. We demonstrate that lifetime variations are due to switching between the neutral and negatively charged state of the nanocrystal. Negative charging results in faster radiative decay but does not appreciably change the overall emission intensity because of suppressed nonradiative Auger recombination for negative trions. The Auger process involving excitation of a hole (positive trion pathway) remains efficient and is responsible for charging with excess electrons, which occurs via Auger-assisted ionization of biexcitons accompanied by ejection of holes.

New insights into the complexities of shell growth and the strong influence of particle volume in nonblinking "giant" core/shell nanocrystal quantum dots.

The growth of ultra-thick inorganic CdS shells over CdSe nanocrystal quantum dot (NQD) cores gives rise to a distinct class of NQD called the "giant" NQD (g-NQD). g-NQDs are characterized by unique photophysical properties compared to their conventional core/shell NQD counterparts, including suppressed fluorescence intermittency (blinking), photobleaching, and nonradiative Auger recombination. Here, we report new insights into the numerous synthetic conditions that influence the complex process of thick-shell growth. We show the individual and collective effects of multiple reaction parameters (noncoordinating solvent and coordinating-ligand identities and concentrations, precursor/NQD ratios, precursor reaction times, etc.) on determining g-NQD shape and crystalline phase, and the relationship between these structural features and optical properties. We find that hexagonally faceted wurzite g-NQDs afford the highest ensemble quantum yields in emission and the most complete suppression of blinking. Significantly, we also reveal a clear correlation between g-NQD particle volume and blinking suppression, such that larger cores afford blinking-suppressed behavior at relatively thinner shells compared to smaller starting core sizes, which require application of thicker shells to realize the same level of blinking suppression. We show that there is a common, threshold g-NQD volume (~750 nm(3)) that is required to observe blinking suppression and that this particle volume corresponds to an NQD radiative lifetime of ~65 ns regardless of starting core size. Combining new understanding of key synthetic parameters with optimized core/shell particle volumes, we demonstrate effectively complete suppression of blinking even for long observation times of ~1 h.

Giant nanocrystal quantum dots: stable down-conversion phosphors that exploit a large stokes shift and efficient shell-to-core energy relaxation.

A new class of nanocrystal quantum dot (NQD), the "giant" NQD (g-NQD), was investigated for its potential to address outstanding issues associated with the use of NQDs as down-conversion phosphors in light-emitting devices, namely, insufficient chemical/photostability and extensive self-reabsorption when packed in high densities or in thick films. Here, we demonstrate that g-NQDs afford significantly enhanced operational stability compared to their conventional NQD counterparts and minimal self-reabsorption losses. The latter results from a characteristic large Stokes shift (>100 nm; >0.39 eV), which itself is a manifestation of the internal structure of these uniquely thick-shelled NQDs. In carefully prepared g-NQDs, light absorption occurs predominantly in the shell but emission occurs exclusively from the core. We directly compare for the first time the processes of shell→core energy relaxation and core→core energy transfer by evaluating CdS→CdSe down-conversion of blue→red light in g-NQDs and in a comparable mixed-NQD (CdSe and CdS) thin film, revealing that the internal energy relaxation process affords a more efficient and color-pure conversion of blue to red light compared to energy transfer. Lastly, we demonstrate the facile fabrication of white-light devices with correlated color temperature tuned from ∼3200 to 5800 K.

'Giant' CdSe/CdS core/shell nanocrystal quantum dots as efficient electroluminescent materials: strong influence of shell thickness on light-emitting diode performance.

We use a simple device architecture based on a poly(3,4-ethylendioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-coated indium tin oxide anode and a LiF/Al cathode to assess the effects of shell thickness on the properties of light-emitting diodes (LEDs) comprising CdSe/CdS core/shell nanocrystal quantum dots (NQDs) as the emitting layer. Specifically, we are interested in determining whether LEDs based on thick-shell nanocrystals, so-called "giant" NQDs, afford enhanced performance compared to their counterparts incorporating thin-shell systems. We observe significant improvements in device performance as a function of increasing shell thickness. While the turn-on voltage remains approximately constant for all shell thicknesses (from 4 to 16 CdS monolayers), external quantum efficiency and maximum luminance are found to be about one order of magnitude higher for thicker shell nanocrystals (≥13 CdS monolayers) compared to thinner shell structures (<9 CdS monolayers). The thickest-shell nanocrystals (16 monolayers of CdS) afforded an external quantum efficiency and luminance of 0.17% and 2000 Cd/m(2), respectively, with a remarkably low turn-on voltage of ~3.0 V.

Furan- and selenophene-2-carboxylato derivatives of dimolybdenum and ditungsten (M[quadruple bond]M): a comparison of their chemical and photophysical properties.

From the reactions between M(2)(T(i)PB)(4), where T(i)PB = 2,4,6-triisopropylbenzoate and two equivalents each of 2-furan carboxylic acid, FuCO(2)H, and 2-selenophene carboxylic acid, SpCO(2)H in toluene, the new compounds trans-M(2)(T(i)PB)(2)(O(2)CFu)(2) (1a M = Mo, 2a M = W) and trans-M(2)(T(i)PB)(2)(O(2)CSp)(2) (1b M = Mo, 2b M = W) were formed. These new compounds have been characterized by (1)H NMR, steady-state UV-Vis-NIR absorption and emission spectroscopy, cyclic and differential pulse voltammetry, and fs and ns transient absorption spectroscopy. The compound Mo(2)(T(i)PB)(2)(O(2)CSp)(2) (1b) has been characterized by single crystal X-ray crystallography. These data are compared with those previously reported for related 2-thiophene carboxylate derivatives: M(2)(T(i)PB)(2)(O(2)CTh)(2). The physico-chemical data correlate well with electronic structure calculations performed on model compounds. All compounds have detectible S(1) photoexcited states with lifetimes that vary from ∼5 ps to < 1 ps. The molybdenum compounds have T(1) states with microsecond lifetimes that are assigned as MMδδ* whereas the T(1) states for tungsten are (3)MLCT with lifetimes on the order of nanoseconds. In all cases, shorter lifetimes were seen in complexes containing heavier atoms.

Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots.

Photoluminescence blinking--random switching between states of high (ON) and low (OFF) emissivities--is a universal property of molecular emitters found in dyes, polymers, biological molecules and artificial nanostructures such as nanocrystal quantum dots, carbon nanotubes and nanowires. For the past 15 years, colloidal nanocrystals have been used as a model system to study this phenomenon. The occurrence of OFF periods in nanocrystal emission has been commonly attributed to the presence of an additional charge, which leads to photoluminescence quenching by non-radiative recombination (the Auger mechanism). However, this 'charging' model was recently challenged in several reports. Here we report time-resolved photoluminescence studies of individual nanocrystal quantum dots performed while electrochemically controlling the degree of their charging, with the goal of clarifying the role of charging in blinking. We find that two distinct types of blinking are possible: conventional (A-type) blinking due to charging and discharging of the nanocrystal core, in which lower photoluminescence intensities correlate with shorter photoluminescence lifetimes; and a second sort (B-type), in which large changes in the emission intensity are not accompanied by significant changes in emission dynamics. We attribute B-type blinking to charge fluctuations in the electron-accepting surface sites. When unoccupied, these sites intercept 'hot' electrons before they relax into emitting core states. Both blinking mechanisms can be electrochemically controlled and completely suppressed by application of an appropriate potential.

Electron delocalization in the S1 and T1 metal-to-ligand charge transfer states of trans-substituted metal quadruply bonded complexes.

The singlet S(1) and triplet T(1) photoexcited states of the compounds containing MM quadruple bonds trans-M(2)(T(i)PB)(2)(O(2)CC(6)H(4)-4-CN)(2), where T(i)PB = 2,4,6-triisopropylbenzoate and M = Mo (I) or M = W (I(')), and trans-M(2)(O(2)CMe)(2)((N[(i) Pr ])(2)CC ≡ CC(6)H(5))(2), where M = Mo (II) and M = W (II(')), have been investigated by a variety of spectroscopic techniques including femtosecond time-resolved infrared spectroscopy. The singlet states are shown to be delocalized metal-to-ligand charge transfer (MLCT) states for I and I(') but localized for II and II(') involving the cyanobenzoate or amidinate ligands, respectively. The triplet states are MoMoδδ* for both I and II but delocalized (3)MLCT for I(') and localized (3)MLCT for II('). These differences arise from consideration of the relative orbital energies of the M(2)δ or M(2)δ* and the ligand π(∗) as well as the magnitudes of orbital overlap.

Effect of shell thickness and composition on blinking suppression and the blinking mechanism in 'giant' CdSe/CdS nanocrystal quantum dots.

We recently developed an inorganic shell approach for suppressing blinking in nanocrystal quantum dots (NQDs) that has the potential to dramatically improve the utility of these fluorophores for single-NQD tracking of individual molecules in cell biology. Here, we consider in detail the effect of shell thickness and composition on blinking suppression, focusing on the CdSe/CdS core/shell system. We also discuss the blinking mechanism as understood through profoundly altered blinking statistics. We clarify the dependence of blinking behavior and photostability on shell thickness, as well as on interrogation times. We show that, while the thickest-shell systems afford the greatest advantages in terms of enhanced optical properties, thinner-shell NQDs may be adequate for certain applications requiring relatively shorter interrogation times. Shell thickness also determines the sensitivity of the NQD optical properties to aqueous-phase transfer, a critical step in rendering NQDs compatible with bioimaging applications. Lastly, we provide a proof-of-concept demonstration of the utility of these unique NQDs for fluorescent particle tracking.