Photophysics and electroluminescence of single nanocrystals of halide perovskites and related nanomaterials

We report simultaneous photoluminescence and electroluminescence single-particle study of nanocrystals of inorganic halide perovskite CsPbBr3, as well as of ternary I-III-IV semiconductor quantum dots

Intrachain Aggregates as the Origin of Green Emission in Polyfluorene Studied on Ensemble and Single-Chain Level

Polyfluorenes are conjugated polymers that show strong blue emission and as such have been explored for potential applications in light-emitting devices. However, heat treatment, prolonged exposure to air, or extended operation in electroluminescent devices can lead to an appearance of parasitic green emission that degrades the material performance. This phenomenon has been extensively studied over the past two decades, and two main and conflicting explanations, i.e., oxidation and formation of fluorenone species on the one hand and inter- or intrachain aggregation on the other, have been put forward. There is abundant experimental evidence to support either of these theories, and the question is far from settled. Here, we aim at getting deeper insight into the problem of the green emission origin using single-molecule spectroscopy performed on individual chains of poly­(9,9-di-<i>n</i>-octylfluorene) (PFO) to resolve the green emission band and reveal its spectral and temporal heterogeneity. We disperse single PFO chains in solid thin-film matrices of polystyrene (PS) and poly­(methyl methacrylate), as well as in solutions of cyclohexane, toluene, or PS/toluene, to simulate good and poor-solvent environments and environments with different permeabilities and diffusions of oxygen, to systematically study the effects of intrachain aggregation as well as oxidation on the appearance and characteristics of the green band. The studies are complemented by direct measurement of individual chain conformation by atomic force microscopy and by bulk measurements of photoluminescence (PL) lifetimes and quantum yield. The single-molecule results reveal two PL spectral forms in the region of the green emission, a vibrationally resolved type located around 500 nm and broad structureless type located toward lower energies, none of them sensitive to the presence of oxygen. These two types are characterized by different lifetimes of 1.4 and 5.1 ns, respectively, and their oscillator strengths are 2 orders of magnitude smaller compared to those of the blue emission band. These results point to two different optical transitions comprising the green band, and these have been assigned to the emission of H-aggregates and charge transfer or indirectly excited excimer states, respectively

Plasmon Enhancement of Triplet Exciton Diffusion Revealed by Nanoscale Imaging of Photochemical Fluorescence Upconversion

Photon upconversion based on the process of triplet–triplet annihilation in a system of organic donor and acceptor molecules has been attracting increasing attention because it can potentially lead to improved efficiency of light energy conversion devices working under sunlight irradiation. Here we aim to gain insight into the effect of localized plasmons of metal nanostructures on the individual photophysical steps involved in the upconversion mechanism. We present an optical microscopy study of the photophysical properties of plasmonic hybrid nanostructures composed of silver nanowires combined with an upconversion system consisting of platinum octaethylporphyrin donor molecules and 9,10-diphenylanthracene acceptor molecules dispersed in poly­(methyl methacrylate) thin film. Using an image-splitting technique, we simultaneously record upconversion and phosphorescence microscopy images and analyze the results to decouple the individual photophysical events. In addition to moderate intensity enhancement on the nanowires, the upconversion emission intensity can be enhanced up to 15-fold in the vicinity of hotspots formed by the silver nanowire junctions, compared with a 4–5-fold enhancement of phosphorescence in the same locations. Furthermore, whereas the phosphorescence enhancement is localized in the hotspots, the upconversion emission is enhanced along micrometer distances on the top of the nanowires. These findings are interpreted in terms of plasmon enhancement of Dexter-type energy transfer between the triplet states of the donor and acceptor as well as between the triplet states of the acceptor molecules. The latter gives rise to the apparent long-distance propagation of the triplet excitons along the nanowires

Sensing Hg(II) <i>in Vitro</i> and <i>in Vivo</i> Using a Benzimidazole Substituted BODIPY

A multisignaling Hg­(II) sensor based on a benzimidazole substituted BODIPY framework was designed, which displays excellent selectively toward Hg­(II) <i>in vitro</i> and <i>in vivo</i>. Optical and fluorogenic measurements in solution reveal that the sensor can detect mercury ions at submicromolar concentrations, with high specificity. The detection of Hg­(II) is associated with a blue-shift in optical spectra and a simultaneous increase in the fluorescence quantum yield of the sensor, which is attributed to a decrease in charge delocalization and inhibition of photoinduced electron transfer upon binding to Hg­(II). Using several spectroscopic measurements, it is shown that the binding mechanism involves two sensor molecules, where lone pairs of the benzimidazole nitrogen coordinate to a single mercury ion. The utility of this BODIPY sensor to detect Hg­(II) <i>in vivo</i> was demonstrated by fluorescence imaging and spectroscopy of labeled human breast adenocarcinoma cells. While average emission intensity of the sensor over a large number of cells increases with incubated mercury concentrations, spatially resolved fluorescence spectroscopy performed on <i>individual cells</i> reveals clear spectral blue-shifts from a subensemble of sensors, corroborating the detection of Hg­(II). Interestingly, the emission spectra at various submicrometer locations within cells exhibited considerable inhomogeneity in the extent of blue-shift, which demonstrates the potential of this sensor to monitor the local (effective) concentration of mercury ions within various subcellular environments

Sensing Hg(II) <i>in Vitro</i> and <i>in Vivo</i> Using a Benzimidazole Substituted BODIPY

A multisignaling Hg­(II) sensor based on a benzimidazole substituted BODIPY framework was designed, which displays excellent selectively toward Hg­(II) <i>in vitro</i> and <i>in vivo</i>. Optical and fluorogenic measurements in solution reveal that the sensor can detect mercury ions at submicromolar concentrations, with high specificity. The detection of Hg­(II) is associated with a blue-shift in optical spectra and a simultaneous increase in the fluorescence quantum yield of the sensor, which is attributed to a decrease in charge delocalization and inhibition of photoinduced electron transfer upon binding to Hg­(II). Using several spectroscopic measurements, it is shown that the binding mechanism involves two sensor molecules, where lone pairs of the benzimidazole nitrogen coordinate to a single mercury ion. The utility of this BODIPY sensor to detect Hg­(II) <i>in vivo</i> was demonstrated by fluorescence imaging and spectroscopy of labeled human breast adenocarcinoma cells. While average emission intensity of the sensor over a large number of cells increases with incubated mercury concentrations, spatially resolved fluorescence spectroscopy performed on <i>individual cells</i> reveals clear spectral blue-shifts from a subensemble of sensors, corroborating the detection of Hg­(II). Interestingly, the emission spectra at various submicrometer locations within cells exhibited considerable inhomogeneity in the extent of blue-shift, which demonstrates the potential of this sensor to monitor the local (effective) concentration of mercury ions within various subcellular environments

Bioactive Polymersomes Self-Assembled from Amphiphilic PPO-<i>Glyco</i>Polypeptides: Synthesis, Characterization, and Dual-Dye Encapsulation

Glycopolypeptide-based polymersomes have promising applications as vehicles for targeted drug delivery because they are capable of encapsulating different pharmaceuticals of diverse polarity as well as interacting with specific cell surfaces due to their hollow structural morphology and bioactive surfaces. We have synthesized glycopolypeptide-<i>b</i>-poly­(propylene oxide) by ROP of glyco-<i>N</i>-carboxyanhydride (NCA) using the hydrophobic amine-terminated poly­(propylene oxide) (PPO) as the initiator. This block copolymer is composed of an FDA-approved PPO hydrophobic block in conjugation with hydrophilic glycopolypeptides which are expected to be biocompatible. We demonstrate the formation of glycopolypeptide-based polymersomes from the self-assembly of glycopolypeptide-<i>b</i>-poly­(propylene oxide) in which the presence of an ordered helical glycopolypeptide segment is required for their self-assembly into spherical nanoscale (∼50 nm) polymersomes. The polymersomes were characterized in detail using a variety of techniques such as TEM, AFM, cryo-SEM, and light-scattering measurements. As a model for drugs, both hydrophobic (RBOE) and hydrophilic (calcein) dyes have been incorporated within the polymersomes from solution. To substantiate the simultaneous entrapment of the two dyes, spectrally resolved fluorescence microscopy was performed on the glycopeptide polymersomes cast on a glass substrate. We show that it is possible to visualize individual nanoscale polymersomes and effectively probe the dyes’ colocalization and energy-transfer behaviors therein as well as investigate the variation in dual-dye encapsulation over a large number of single polymersomes. Finally, we show that the galactose moieties present on the surface can specifically recognize lectin RCA₁₂₀, which reveals that the polymersomes’ surface is indeed biologically active

Single-Particle Tracking To Probe the Local Environment in Ice-Templated Crosslinked Colloidal Assemblies

We use single-particle tracking to investigate colloidal dynamics in hybrid assemblies comprising colloids enmeshed in a crosslinked polymer network. These assemblies are prepared using ice templating and are macroporous monolithic structures. We investigate microstructure-property relations in assemblies that appear chemically identical but show qualitatively different mechanical response. Specifically, we contrast elastic assemblies that can recover from large compressive deformations with plastic assemblies that fail on being compressed. Particle tracking provides insights into the microstructural differences that underlie the different mechanical response of elastic and plastic assemblies. Since colloidal motions in these assemblies are sluggish, particle tracking is especially sensitive to imaging artifacts such as stage drift. We demonstrate that the use of wavelet transforms applied to trajectories of probe particles from fluorescence microscopy eliminates stage drift, allowing a spatial resolution of about 2 nm. In elastic and plastic scaffolds, probe particles are surrounded by other particlesthus, their motion is caged. We present mean square displacement and van Hove distributions for particle motions and demonstrate that plastic assemblies are characterized by significantly larger spatial heterogeneity when compared with the elastic sponges. In elastic assemblies, particle diffusivities are peaked around a mean value, whereas in plastic assemblies, there is a wide distribution of diffusivities with no clear peak. Both elastic and plastic assemblies show a frequency independent solid modulus from particle tracking microrheology. Here too, there is a much wider distribution of modulus values for plastic scaffolds as compared to elastic, in contrast to bulk rheological measurements where both assemblies exhibit a similar response. We interpret our results in terms of the spatial distribution of crosslinks in the polymer mesh in the colloidal assemblies

1‑, 3‑, 6‑, and 8‑Tetrasubstituted Asymmetric Pyrene Derivatives with Electron Donors and Acceptors: High Photostability and Regioisomer-Specific Photophysical Properties

The systematic synthesis of five 1-, 3-, 6-, and 8-tetrasubstituted asymmetric pyrenes with electron donor and acceptor moieties is presented, together with an examination of their photophysical properties. Pyrene derivative <b>PA1</b>, containing one formyl and three piperidyl groups, showed bright solvatochromic fluorescence from green (λ_em = 557 nm, Φ_FL = 0.94 in hexane) to red (λ_em = 648 nm, Φ_FL = 0.50 in methanol), suggesting potential applications for <b>PA1</b> as an environmentally responsive probe. Although the synthesis of simple 1- and 3-disubstituted pyrene derivatives is considered difficult, <b>PA13</b>, with two formyl groups at the 1- and 3-positions and two piperidyl groups at the 6- and 8-positions, could be synthesized successfully. <b>PA13</b> exhibited less pronounced solvatochromism, but displayed a narrow fluorescent band with high Φ_FL in all solvents (Φ_FL > 0.75). Moreover, its absorption band displayed an exceptional bathochromic shift compared to the other derivatives (e.g., λ_abs = 480 and 522 nm in ethanol for <b>PA1</b> and <b>PA13</b>, respectively), suggesting that such modifications of pyrene may be quite important for the modulation of its energy gap. Additionally, all compounds exhibited exceptionally high photostability, which highlights the advantage of these new dyes and provides new insights on the design of photostable fluorophores

Plasticization of Poly(vinylpyrrolidone) Thin Films under Ambient Humidity: Insight from Single-Molecule Tracer Diffusion Dynamics

Studies on diffusion dynamics of single molecules (SMs) have been useful in revealing inhomogeneity of polymer thin films near and above the glass-transition temperature (<i>T</i>_g). However, despite several applications of polymer thin films where exposure to solvent (or vapor) is common, the effect of absorbed solvent molecules on local morphology and rigidity of polymer matrices is yet to be explored in detail. High-<i>T</i>_g hydrophilic polymers such as poly­(vinylpyrrolidone) (PVP) are used as pharmaceutical coatings for drug release in aqueous medium, as they readily absorb moisture, which results in effective lowering of the <i>T</i>_g and thereby leads to plasticization. The effect of moisture absorption on swelling and softening of PVP thin films was investigated by visualizing the diffusion dynamics of rhodamine 6G (Rh6G) tracer molecules at various ambient relative humidities (RH). Wide-field epifluorescence microscopy, in conjunction with high-resolution SM tracking, was used to monitor the spatiotemporal evolution of individual tracers under varied moisture contents of the matrix. In the absence of atmospheric moisture, Rh6G molecules in dry PVP films are translationally inactive, suggestive of rigid local environments. Under low moisture contents (RH 30–50%), translational mobility remains arrested but rotational motion is augmented, indicating slight swelling of the polymer network which marks the onset of plasticization. The translational mobility of Rh6G was found to be triggered only at a threshold ambient RH, beyond which a large proportion of tracers exhibit extensive diffusion dynamics. Interestingly, SM tracking data at higher moisture contents of the film (RH ≥ 60%) reveal that the distributions of dynamic parameters (such as diffusivity) are remarkably broad, spanning several orders of magnitude. Furthermore, Rh6G molecules display a wide variety of translational motion even at a fixed ambient RH, clearly pointing out the extremely inhomogeneous environment of plasticized PVP network. Intriguingly, it is observed that a majority of tracers undergo anomalous subdiffusion even under high moisture contents of the matrix. Analyses of SM trajectories using velocity autocorrelation function reveal that subdiffusive behaviors of Rh6G are likely to originate from fractional Brownian motion, a signature of tracer dynamics in viscoelastic medium