Two-Dimensional Supramolecular Polymerization of a Bis-Urea Macrocycle into a Brick-Like Hydrogen-Bonded Network.

We report on a dendronized bis-urea macrocycle 1 self-assembling via a cooperative mechanism into two-dimensional (2D) nanosheets formed solely by alternated urea-urea hydrogen bonding interactions. The pure macrocycle self-assembles in bulk into one-dimensional liquid-crystalline columnar phases. In contrast, its self-assembly mode drastically changes in CHCl3 or tetrachloroethane, leading to 2D hydrogen-bonded networks. Theoretical calculations, complemented by previously reported crystalline structures, indicate that the 2D assembly is formed by a brick-like hydrogen bonding pattern between bis-urea macrocycles. This assembly is promoted by the swelling of the trisdodecyloxyphenyl groups upon solvation, which frustrates, due to steric effects, the formation of the thermodynamically more stable columnar macrocycle stacks. This work proposes a new design strategy to access 2D supramolecular polymers by means of a single non-covalent interaction motif, which is of great interest for materials development.

Confinement-Driven Photophysics in Hydrazone-Based Hierarchical Materials.

Confinement-imposed photophysics was probed for novel stimuli-responsive hydrazone-based compounds demonstrating a conceptual difference in their behavior within 2D versus 3D porous matrices for the first time. The challenges associated with photoswitch isomerization arising from host interactions with photochromic compounds in 2D scaffolds could be overcome in 3D materials. Solution-like photoisomerization rate constants were realized for sterically demanding hydrazone derivatives in the solid state through their coordinative immobilization in 3D scaffolds. According to steady-state and time-resolved photophysical measurements and theoretical modeling, this approach provides access to hydrazone-based materials with fast photoisomerization kinetics in the solid state. Fast isomerization of integrated hydrazone derivatives allows for probing and tailoring resonance energy transfer (ET) processes as a function of excitation wavelength, providing a novel pathway for ET modulation.

Thermodynamics of nanocrystal-ligand binding through isothermal titration calorimetry.

Manipulations of nanocrystal (NC) surfaces have propelled the applications of colloidal NCs across various fields such as bioimaging, catalysis, electronics, and sensing applications. In this Feature Article, we discuss the surface chemistry of colloidal NCs, with an emphasis on semiconductor quantum dots, and the binding motifs for various ligands that coordinate NC surfaces. We present isothermal titration calorimetry (ITC) as a viable technique for studying the thermodynamics of the ligand association and exchange at NC surfaces by discussing its principles of operation and highlighting results obtained to date. We give an in-depth description of various thermodynamic models that can be used to interpret NC-ligand interactions as measured not only by ITC, but also by NMR, fluorescence quenching, and fluorescence anisotropy techniques. Understanding the complexity of NC surface-ligand interactions can provide a wide range of avenues to tune their properties for desired applications.

Structure-property investigations in urea tethered iodinated triphenylamines.

Herein, we report structural, computational, and conductivity studies on urea-directed self-assembled iodinated triphenylamine (TPA) derivatives. Despite numerous reports of conductive TPAs, the challenges of correlating their solid-state assembly with charge transport properties hinder the efficient design of new materials. In this work, we compare the assembled structures of a methylene urea bridged dimer of di-iodo TPA (1) and the corresponding methylene urea di-iodo TPA monomer (2) with a di-iodo mono aldehyde (3) control. These modifications lead to needle shaped crystals for 1 and 2 that are organized by urea hydrogen bonding, π⋯π stacking, I⋯I, and I⋯π interactions as determined by SC-XRD, Hirshfeld surface analysis, and X-ray photoelectron spectroscopy (XPS). The long needle shaped crystals were robust enough to measure the conductivity by two contact probe methods with 2 exhibiting higher conductivity values (∼6 × 10-7 S cm-1) compared to 1 (1.6 × 10-8 S cm-1). Upon UV-irradiation, 1 formed low quantities of persistent radicals with the simple methylurea 2 displaying less radical formation. The electronic properties of 1 were further investigated using valence band XPS, which revealed a significant shift in the valence band upon UV irradiation (0.5-1.9 eV), indicating the potential of these materials as dopant free p-type hole transporters. The electronic structure calculations suggest that the close packing of TPA promotes their electronic coupling and allows effective charge carrier transport. Our results show that ionic additives significantly improve the conductivity up to ∼2.0 × 10-6 S cm-1 in thin films, enabling their implementation in functional devices such as perovskite or solid-state dye sensitized solar cells.

Coordination of Quantum Dots in a Polar Solvent by Small-Molecule Imidazole Ligands.

Colloidal quantum dots (QDs) are attractive fluorophores for bioimaging and biomedical applications because of their favorable and tunable optoelectronic properties. In this study, the native hydrophobic ligand environment of oleate-capped sphalerite CdSe/ZnS core/shell QDs was quantitatively exchanged with a set of imidazole-bearing small-molecule ligands. Inductively coupled plasma-optical emission spectroscopy and 1H NMR were used to identify and quantify three different ligand exchange processes: Z-type dissociation of the Zn(oleate)2, L-type association of the imidazole, and X-type anionic exchange of oleate with Cl-, all of which contributed to the overall ligand exchange.

Photoresponsive frameworks: energy transfer in the spotlight.

In this paper, spiropyran-containing metal- and covalent-organic frameworks (MOFs and COFs, respectively) are probed as platforms for fostering photochromic behavior in solid-state materials, while simultaneously promoting directional energy transfer (ET). In particular, Förster resonance energy transfer (FRET) between spiropyran and porphyrin derivatives integrated as linkers in the framework matrix is discussed. The photochromic spiropyran derivatives allow for control over material optoelectronic properties through alternation of excitation wavelengths. Photoinduced changes in the material electronic profile have also been probed through conductivity measurements. Time-resolved photoluminescence studies were employed to evaluate the effect of photochromic linkers on material photophysics. Furthermore, "forward" and "reverse" FRET processes occurring between two distinct chromophores were modeled, and the Förster critical radii and ET rates were estimated to support the experimentally observed changes in material photoluminescence.

A p-type PbS quantum dot ink with improved stability for solution processable optoelectronics.

A highly stable p-type PbS-QDs ink is prepared using a single-step biphasic ligand exchange route, overcoming instability encountered in previous reports. Chemical characterization of the ink reveals 3-mercaptopriopionic acid (MPA) capped QDs stable in benzylamine solvent over a period of weeks or longer. The film resistivity, 1.45 kΩ cm, is an order magnitude lower and surface roughness, ∼ 0.5 nm, is superior vs. PbS films reported so far, and proof of concept photovoltaic devices showed efficiency > 5.5%.

Heterometallic Actinide-Containing Photoresponsive Metal-Organic Frameworks: Dynamic and Static Tuning of Electronic Properties.

Acquiring fundamental knowledge of properties of actinide-based materials is a necessary step to create new possibilities for addressing the current challenges in the nuclear energy and nuclear waste sectors. In this report, we established a photophysics-electronics correlation for actinide-containing metal-organic frameworks (An-MOFs) as a function of excitation wavelength, for the first time. A stepwise approach for dynamically modulating electronic properties was applied for the first time towards actinide-based heterometallic MOFs through integration of photochromic linkers. Optical cycling, modeling of density of states near the Fermi edge, conductivity measurements, and photoisomerization kinetics were employed to shed light on the process of tailoring optoelectronic properties of An-MOFs. Furthermore, the first photochromic MOF-based field-effect transistor, in which the field-effect response could be changed through light exposure, was constructed. As a demonstration, the change in current upon light exposure was sufficient to operate a two-LED fail-safe indicator circuit.

Connecting Wires: Photoinduced Electronic Structure Modulation in Metal-Organic Frameworks.

Electronic structure modulation of metal-organic frameworks (MOFs) through the connection of linker "wires" as a function of an external stimulus is reported for the first time. The established correlation between MOF electronic properties and photoisomerization kinetics as well as changes in an absorption profile is unprecedented for extended well-defined structures containing coordinatively integrated photoresponsive linkers. The presented studies were carried out on both single crystal and bulk powder with preservation of framework integrity. An LED-containing electric circuit, in which the switching behavior was driven by the changes in MOF electronic profile, was built for visualization of experimental findings. The demonstrated concept could be used as a blueprint for development of stimuli-responsive materials with dynamically controlled electronic behavior.

Purification and In Situ Ligand Exchange of Metal-Carboxylate-Treated Fluorescent InP Quantum Dots via Gel Permeation Chromatography.

Recently the addition of M2+ Lewis acids (M = Cd, Zn) to InP quantum dots (QDs) has been shown to enhance the photoluminescence quantum yield (PL QY). Here we investigate the stability of this Lewis acid layer to postsynthetic processing such as purification and ligand exchange. We utilize gel permeation chromatography to purify the quantum-dot samples as well as to aid in the ligand-exchange reactions. The Lewis-acid-capped particles are stable to purification and maintain the enhanced luminescence properties. We demonstrate successful ligand exchange on the quantum dots by switching the native carboxylate ligands to phosphonate ligands. Changes in the optical spectra after exposure to ambient environment indicate that both carboxylate- and phosphonate-capped QDs remain air-sensitive.

Hierarchical Corannulene-Based Materials: Energy Transfer and Solid-State Photophysics.

We report the first example of a donor-acceptor corannulene-containing hybrid material with rapid ligand-to-ligand energy transfer (ET). Additionally, we provide the first time-resolved photoluminescence (PL) data for any corannulene-based compounds in the solid state. Comprehensive analysis of PL data in combination with theoretical calculations of donor-acceptor exciton coupling was employed to estimate ET rate and efficiency in the prepared material. The ligand-to-ligand ET rate calculated using two models is comparable with that observed in fullerene-containing materials, which are generally considered for molecular electronics development. Thus, the presented studies not only demonstrate the possibility of merging the intrinsic properties of π-bowls, specifically corannulene derivatives, with the versatility of crystalline hybrid scaffolds, but could also foreshadow the engineering of a novel class of hierarchical corannulene-based hybrid materials for optoelectronic devices.

Fulleretic Well-Defined Scaffolds: Donor-Fullerene Alignment Through Metal Coordination and Its Effect on Photophysics.

Herein, we report the first example of a crystalline metal-donor-fullerene framework, in which control of the donor-fullerene mutual orientation was achieved through chemical bond formation, in particular, by metal coordination. The (13) C cross-polarization magic-angle spinning NMR spectroscopy, X-ray diffraction, and time-resolved fluorescence spectroscopy were performed for comprehensive structural analysis and energy-transfer (ET) studies of the fulleretic donor-acceptor scaffold. Furthermore, in combination with photoluminescence measurements, the theoretical calculations of the spectral overlap function, Förster radius, excitation energies, and band structure were employed to elucidate the photophysical and ET processes in the prepared fulleretic material. We envision that the well-defined fulleretic donor-acceptor materials could contribute not only to the basic science of fullerene chemistry but would also be used towards effective development of organic photovoltaics and molecular electronics.

Gel permeation chromatography as a multifunctional processor for nanocrystal purification and on-column ligand exchange chemistry.

This article illustrates the use of gel permeation chromatography (GPC, organic-phase size exclusion chromatography) to separate nanocrystals from weakly-bound small molecules, including solvent, on the basis of size. A variety of colloidal inorganic nanocrystals of different size, shape, composition, and surface termination are shown to yield purified samples with greatly reduced impurity concentrations. Additionally, the method is shown to be useful in achieving a change of solvent without requiring precipitation of the nanocrystals. By taking advantage of the different rates at which small molecules and nanoparticles travel through the column, we show that it is furthermore possible to use the GPC column as a multi-functional flow reactor that can accomplish in sequence the steps of initial purification, ligand exchange with controlled reactant concentration and interaction time, and subsequent cleanup without requiring a change of phase. This example of process intensification via GPC is shown to yield nearly complete displacement of the initial surface ligand population upon reaction with small molecule and macromolecular reactants to form ligand-exchanged nanocrystal products.

Surface labeling of enveloped virus with polymeric imidazole ligand-capped quantum dots via the metabolic incorporation of phospholipids into host cells.

We report a general method for the preparation of quantum dot-labeled viruses through a strain-promoted azide-alkyne cycloaddition (SPAAC) reaction. The quantum dot sample was functionalized with methacrylate-based polymeric imidazole ligands (MA-PILs) bearing dibenzocyclooctyne groups. Enveloped measles virus was labeled with azide groups through the metabolic incorporation of a choline analogue into the host cell membrane, and then linked with the modified QDs. The virus retained its infectious ability against host cells after the modification with MA-PIL capped QDs.

A Bio-inspired Approach for Chromophore Communication: Ligand-to-Ligand and Host-to-Guest Energy Transfer in Hybrid Crystalline Scaffolds.

Efficient multiple-chromophore coupling in a crystalline metal-organic scaffold was achieved by mimicking a protein system possessing 100% energy-transfer (ET) efficiency between a green fluorescent protein variant and cytochrome b562. The two approaches developed for ET relied on the construction of coordination assemblies and host-guest coupling. Based on time-resolved photoluminescence measurements in combination with calculations of the spectral overlap function and Förster radius, we demonstrated that both approaches resulted in a very high ET efficiency. In particular, the observed ligand-to-ligand ET efficiency value was the highest reported so far for two distinct ligands in a metal-organic framework. These studies provide important insights for the rational design of crystalline hybrid scaffolds consisting of a large ensemble of chromophore molecules with the capability of directional ET.

A methacrylate-based polymeric imidazole ligand yields quantum dots with low cytotoxicity and low nonspecific binding.

This paper assesses the biocompatibility for fluorescence imaging of colloidal nanocrystal quantum dots (QDs) coated with a recently-developed multiply-binding methacrylate-based polymeric imidazole ligand. The QD samples were purified prior to ligand exchange via a highly repeatable gel permeation chromatography (GPC) method. A multi-well plate based protocol was used to characterize nonspecific binding and toxicity of the QDs toward human endothelial cells. Nonspecific binding in 1% fetal bovine serum was negligible compared to anionically-stabilized QD controls, and no significant toxicity was detected on 24h exposure. The nonspecific binding results were confirmed by fluorescence microscopy. This study is the first evaluation of biocompatibility in QDs initially purified by GPC and represents a scalable approach to comparison among nanocrystal-based bioimaging scaffolds.

Quantum yield regeneration: influence of neutral ligand binding on photophysical properties in colloidal core/shell quantum dots.

This article describes an experiment designed to identify the role of specific molecular ligands in maintaining the high photoluminescence (PL) quantum yield (QY) observed in as-synthesized CdSe/CdZnS and CdSe/CdS quantum dots (QDs). Although it has been possible for many years to prepare core/shell quantum dots with near-unity quantum yield through high-temperature colloidal synthesis, purification of such colloidal particles is frequently accompanied by a reduction in quantum yield. Here, a recently established gel permeation chromatography (GPC) technique is used to remove weakly associated ligands without a change in solvent: a decrease in ensemble QY and average PL lifetime is observed. Minor components of the initial mixture that were removed by GPC are then added separately to purified QD samples to determine whether reintroduction of these components can restore the photophysical properties of the initial sample. We show that among these putative ligands trioctylphosphine and cadmium oleate can regenerate the initial high QY of all samples, but only the "L-type" ligands (trioctyphosphine and oleylamine) can restore the QY without changing the shapes of the optical spectra. On the basis of the PL decay analysis, we confirm that quenching in GPC-purified samples and regeneration in ligand-introduced samples are associated chiefly with changes in the relative population fraction of QDs with different decay rates. The reversibility of the QY regeneration process has also been studied; the introduction and removal of trioctylphosphine and oleylamine tend to be reversible, while cadmium oleate is not. Finally, isothermal titration calorimetry has been used to study the relationship between the binding strength of the neutral ligands to the surface and photophysical property changes in QD samples to which they are added.

Mimic of the green fluorescent protein ß-barrel: photophysics and dynamics of confined chromophores defined by a rigid porous scaffold.

Chromophores with a benzylidene imidazolidinone core define the emission profile of commonly used biomarkers such as the green fluorescent protein (GFP) and its analogues. In this communication, artificially engineered porous scaffolds have been shown to mimic the protein ß-barrel structure, maintaining green fluorescence response and conformational rigidity of GFP-like chromophores. In particular, we demonstrated that the emission maximum in our artificial scaffolds is similar to those observed in the spectra of the natural GFP-based systems. To correlate the fluorescence response with a structure and perform a comprehensive analysis of the prepared photoluminescent scaffolds, (13)C cross-polarization magic angle spinning solid-state (CP-MAS) NMR spectroscopy, powder and single-crystal X-ray diffraction, and time-resolved fluorescence spectroscopy were employed. Quadrupolar spin-echo solid-state (2)H NMR spectroscopy, in combination with theoretical calculations, was implemented to probe low-frequency vibrational dynamics of the confined chromophores, demonstrating conformational restrictions imposed on the coordinatively trapped chromophores. Because of possible tunability of the introduced scaffolds, these studies could foreshadow utilization of the presented approach toward directing a fluorescence response in artificial GFP mimics, modulating a protein microenvironment, and controlling nonradiative pathways through chromophore dynamics.

Energy transfer on demand: photoswitch-directed behavior of metal-porphyrin frameworks.

In this paper, a photochromic diarylethene-based derivative that is coordinatively immobilized within an extended porphyrin framework is shown to maintain its photoswitchable behavior and to direct the photophysical properties of the host. In particular, emission of a framework composed of bis(5-pyridyl-2-methyl-3-thienyl)cyclopentene (BPMTC) and tetrakis(4-carboxyphenyl)porphyrin (H4TCPP) ligands anchored by Zn(2+) ions can be altered as a function of incident light. We attribute the observed cyclic fluorescence behavior of the synthesized porphyrin-BPMTC array to activation of energy transfer (ET) pathways through BPMTC photoisomerization. Time-resolved photoluminescence measurements show a decrease in average porphyrin emission lifetime upon BPMTC insertion, consistent with an ET-based mechanism. These studies portend the possible utilization of photochromic ligands to direct chromophore behavior in large light-harvesting ensembles.

Metabolic tumor profiling with pH, oxygen, and glucose chemosensors on a quantum dot scaffold.

Acidity, hypoxia, and glucose levels characterize the tumor microenvironment rendering pH, pO2, and pGlucose, respectively, important indicators of tumor health. To this end, understanding how these parameters change can be a powerful tool for the development of novel and effective therapeutics. We have designed optical chemosensors that feature a quantum dot and an analyte-responsive dye. These noninvasive chemosensors permit pH, oxygen, and glucose to be monitored dynamically within the tumor microenvironment by using multiphoton imaging.