N-Heterocyclic Carbene-Stabilized Atomically Precise Metal Nanoclusters.

This perspective highlights advances in the preparation and understanding of metal nanoclusters stabilized by organic ligands with a focus on N-heterocyclic carbenes (NHCs). We demonstrate the need for a clear understanding of the relationship between NHC properties and their resulting metal nanocluster structure and properties. We emphasize the importance of balancing nanocluster stability with the introduction of reactive sites for catalytic applications and the importance of a better understanding of how these clusters interact with their environments for effective use in biological applications. The impact of atom-scale simulations, development of atomic interaction potentials suitable for large-scale molecular dynamics simulations, and a deeper understanding of the mechanisms behind synthetic methods and physical properties (e.g., the bright fluorescence displayed by many clusters) are emphasized.

Photophysical investigation into room-temperature emission from xanthene derivatives.

The photophysical consequences of replacing the nitrogen heteroatom in phenothiazine with methylene are investigated for both solutions and crystalline solids. We analysed the excited state dynamics of four xanthene derivatives and observed an anomalous fluorescence from an energy level higher than the S1 state with lifetimes between 2.8 ns and 5.8 ns in solution and as solids. Additionally, the solid-state xanthene derivatives exhibited long-lived emission consistent with a triplet excited state, displaying millisecond lifetimes that ranged from 0.1 ms to 3.4 ms at ambient temperature in air. Our findings were supported by optical studies, crystallographic structural analyses, and DFT computations, which corroborated the photophysical measurements. It was concluded that the presence of the nitrogen atom in phenothiazine is crucial for achieving ultra-long emission lifetimes and that these results contribute to a deeper understanding of excited state dynamics which have potential implications for applications, such as display technologies, anticounterfeiting technologies, and sensors.

Photophysics of Ag and Au alloys of M25(SR)18 clusters.

Superatom clusters, Au25(SR)18, and the silver analog and alloys of the two metals have been extensively investigated for their structure, stability, photoluminescence, and electronic properties. One can readily tune the physicochemical properties by varying the ratio of Au/Ag or the thiol ligand to attain desired properties, such as enhanced emission, increased stability, and catalytic activity. Herein, excitation emission matrix spectroscopy and pump-probe transient absorption spectroscopy are used to show that the excited state dynamics of Au25(SR)18, Ag25(SR)18, and their alloys differ significantly despite having similar structures. State-resolved excited state behavior that is well documented for gold clusters is largely affected by the metal composition, becoming less pronounced for silver analogs, resulting in diversity in terms of their excited state energy and relaxation dynamics and resultant photophysical properties, such as emission.

Photophysics of J-Aggregating Porphyrin-Lipid Photosensitizers in Liposomes: Impact of Lipid Saturation.

Porphyrin aggregates have attractive photophysical properties for phototherapy and optical imaging, including quenched photosensitization, efficient photothermal conversion, and unique absorption spectra. Although hydrophobic porphyrin photosensitizers have long been encapsulated into liposomes for drug delivery, little is known about the membrane properties of liposomes with large amphiphilic porphyrin compositions. In this paper, a porphyrin-lipid conjugate was incorporated into liposomes formed of saturated or unsaturated lipids to study the membrane composition-dependent formation of highly ordered porphyrin J-aggregates and disordered aggregates. Porphyrin-lipid readily phase-separates in saturated membranes, forming J-aggregates that are destabilized during the ripple phase below the main thermal transition. Porphyrin-lipid J-aggregates are photostable with a photothermal efficiency of 54 ± 6%, comparable to gold. Even at high porphyrin-lipid compositions, porphyrin J-aggregates coexist with a minority population of disordered aggregates, which are photodynamically active despite being fluorescently quenched. For photothermal applications, liposome formulations that encourage porphyrin-lipid phase separation should be explored for maximum J-aggregation.

Robust, Highly Luminescent Au13 Superatoms Protected by N-Heterocyclic Carbenes.

Gold superatom nanoclusters stabilized entirely by N-heterocyclic carbenes (NHCs) and halides are reported. The reduction of well-defined NHC-Au-Cl complexes produces clusters comprised of an icosahedral Au13 core surrounded by a symmetrical arrangement of nine NHCs and three chlorides. X-ray crystallography shows that the clusters are characterized by multiple CH-π and π-π interactions, which rigidify the ligand and likely contribute to the exceptionally high photoluminescent quantum yields observed, up to 16.0%, which is significantly greater than that of the most luminescent ligand-protected Au13 superatom cluster. Density functional theory analysis suggests that clusters are 8-electron superatoms with a wide HOMO-LUMO energy gap of 2 eV. Consistent with this, the clusters have high stability relative to phosphine stabilized clusters.

Impact of Ferrocene Substitution on the Electronic Properties of BODIPY Derivatives and Analogues.

Bis(ferrocenyl)-functionalized boron dipyrromethene (BODIPY) compound 1 featuring direct Fc-B bonds was obtained via a "prefunctionalization strategy". UV-vis absorption, electrochemical, and transient absorption experiments were performed on compound 1 and its analogues to examine the impact of ferrocenyl substitution on the electronic properties. The ferrocene units were found to have little impact on the absorption spectrum of the BODIPY unit but significantly change the excited-state dynamics.

A Single Model for the Excited-State Dynamics of Au18(SR)14 and Au25(SR)18 Clusters.

Excited-state properties of photonic materials play an important part in dictating the photocatalytic activity. Thiol-protected gold clusters, like Au18(SR)14 and Au25(SR)18, are an emerging material of interest with unique optical and electronic properties. Au18(SR)14 clusters, in particular, have shown promise as one of the highest efficiency clusters in light harvesting, with a high emission quantum yield. In this work, the excited-state properties of Au18(SR)14 are studied in-depth by ultrafast pump/probe spectroscopy for the first time. A single model describing the optical characteristics of thiol-protected Au18(SR)14 and Au25(SR)18 clusters is offered. Excited-state dynamics analysis suggests that there are state-resolved relaxations due to the presence of multiple excited states. The populations of these excited states are shown to be solvent- and ligand-dependent.

Experimental Evidence for a Triplet Biradical Excited-State Mechanism in the Photoreactivity of N,C-Chelate Organoboron Compounds.

N,C-chelate organoborates represent an emerging class of photoresponsive materials due to their photochromic switching at a boron center. Despite the promising applicability of such systems, little is known about the excited-state processes that lead to their unique photoreactivity, which is detrimental to the design of next-generation smart materials based on boron. As part of our ongoing effort to understand and improve the utility of these organoboron compounds, we report some of the first experimental evidence to support an excited-state mechanism for N,C-chelate organoborates. Femtosecond transient absorption spectroscopy combined with steady-state UV/vis and fluorescence measurements gives direct insight into their underlying photochemical processes, such as the formation of a common triplet charge-transfer state which either relaxes radiatively or undergoes the desired photoisomerization through a biradical intermediate. With this information, a complete mechanistic picture of the excited-state reactivity of N,C-chelate organoborates has been established, which is anticipated to lead to new smart materials with improved performance.

Photochemical synthesis of biocompatible and antibacterial silver nanoparticles embedded within polyurethane polymers.

In situ light initiated synthesis of silver nanoparticles (AgNP) was employed for AgNP incorporation within the polymeric matrices of medical grade polyurethane. The resulting materials showed improved antibacterial and antibiofilm activity against Pseudomonas aeruginosa with negligible toxicity for human primary skin cells and erythrocytes.

Size-dependent excited state behavior of glutathione-capped gold clusters and their light-harvesting capacity.

Glutathione-protected gold clusters exhibit size-dependent excited state and electron transfer properties. Larger-size clusters (e.g., Au25GSH18) with core-metal atoms display rapid (<1 ps) as well as slower relaxation (~200 ns) while homoleptic clusters (e.g., Au(10-12)GSH(10-12)) exhibit only slower relaxation. These decay components have been identified as metal-metal transition and ligand-to-metal charge transfer, respectively. The short lifetime relaxation component becomes less dominant as the size of the gold cluster decreases. The long-lived excited state and ability to participate in electron transfer are integral for these clusters to serve as light-harvesting antennae. A strong correlation between the ligand-to-metal charge-transfer excited state lifetime and photocatalytic activity was evidenced from the electron transfer to methyl viologen. The photoactivity of these metal clusters shows increasing photocatalytic reduction yield (0.05-0.14) with decreasing cluster size, Au25 < Au18 < Au15 < Au(10-12). Gold clusters, Au18GSH14, were found to have the highest potential as a photosensitizer on the basis of the quantum yield of electron transfer and good visible light absorption properties.

Self-assembled dipole nanolasers.

Visible light excitation of silver nanoparticles in the presence of polymerizable monomers and selected dyes triggers the self-assembly of nanolasers in a synthetically simple process. The new nanolasers incorporate a thin, fully organic gain medium that allows the tuning of the core absorption to a selected dye, or of the dye to a preselected core material. This versatile synthesis of surface plasmon lasers, or "spasers", has unique simplicity and enables spatial and temporal control of the nanolaser fabrication process.

Sensitized excited free-radical processes as read-write tools: impact on non-linear lithographic processes.

A prefluorescent radical probe in which a 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) unit is sensitized with visible light via energy transfer from a tethered coumarin dye has been synthesized. The excited TEMPO moiety undergoes hydrogen abstraction from a polymer with concomitant functionalization of the polymeric matrix; this writing process can be reversed thermally. The fluorescent probes can be bleached under high intensity illumination, making these new materials suitable for dual-tone lithography with potential applications in sub-diffraction lithographic imaging.

Tuning the Functionalization of Graphite for Hydrovoltaic Power Generation.

The recent discovery of hydrovoltaic devices for power generation has led to a rapid growth into new materials for harvesting energy specifically for this research field. Of the materials investigated, carbon materials have dominated, and graphene oxide (GO) has emerged as the leader. While graphite is conductive, it does not have functional groups to strongly interact with water, and highly functionalized GO forms strong interaction with water to generate necessary surface charges but does not typically have high conductivity. Herein, we report the fabrication and functionalization of a graphite-based structure, controlling the extent of oxidation to balance the effects of conductivity and functionalization to achieve high power outputs in hydrovoltaics. Devices prepared using the functionalized graphite achieve a power output of 53.3 µW/g. High power output and good film stability are key advances toward the practical application of hydrovoltaic devices for renewable energy.

Photochemical synthesis of fluorescent Au16(RGDC)14 and excited state reactivity with molecular oxygen.

Aqueous metal nanoclusters have emerged as effective materials for biomedical imaging and therapy. Among them, gold nanoclusters (AuNCs) have been widely studied due to their unique electronic structures. These nanoclusters are often optically impure, comprising a mixture of fluorescent clusters with different metal/ligand compositions. The polydispersity of nanoclusters makes it challenging to isolate the most stable structure, and poses further risks for eventual clinical applications. Herein, Au16L14 clusters are reported which are optically pure as assessed by fluorescence excitation-emission matrix (EEM) spectroscopy and parallel factor (PARAFAC) analysis. The reactivity of their excited state with molecular oxygen was also probed, demonstrating that the Au16L14 clusters generate type I reactive oxygen species (ROS), which can make them effective sensitizers for photodynamic therapy.

Hybrid plasmonic metasurface as enhanced Raman hot-spots for pesticide detection at ultralow concentrations.

A surface-enhanced Raman scattering (SERS) active metasurface composed of metallic nanohole arrays and metallic nanoparticles is developed. The metasurface can operate in aqueous environments, achieves an enhancement factor of 1.83 × 109 for Rhodamine 6G, and enables the detection of malachite green at a concentation of 0.46 ppb.

Accelerated size-focusing light activated synthesis of atomically precise fluorescent Au22(Lys-Cys-Lys)16 clusters.

Atomically precise metal nanoclusters are promising candidates for various biomedical applications, including their use as photosensitizers in photodynamic therapy (PDT). However, typical synthetic routes of clusters often result in complex mixtures, where isolating and characterizing pure samples becomes challenging. In this work, a new Au22(Lys-Cys-Lys)16 cluster is synthesized using photochemistry, followed by a new type of light activated, accelerated size-focusing. Fluorescence excitation-emission matrix spectroscopy (EEM) and parallel factor (PARAFAC) analysis have been applied to track the formation of fluorescent species, and to assess optical purity of the final product. Furthermore, excited state reactivity of Au22(Lys-Cys-Lys)16 clusters is studied, and formation of type-I reactive oxygen species (ROS) from the excited state of the clusters is observed. The proposed size-focusing procedure in this work can be easily adapted to conventional cluster synthetic methods, such as borohydride reduction, to provide atomically precise clusters.

Interplay between size, composition, and phase transition of nanocrystalline Cr(3+)-doped BaTiO3 as a path to multiferroism in perovskite-type oxides.

Multiferroics, materials that exhibit coupling between spontaneous magnetic and electric dipole ordering, have significant potential for high-density memory storage and the design of complex multistate memory elements. In this work, we have demonstrated the solvent-controlled synthesis of Cr(3+)-doped BaTiO(3) nanocrystals and investigated the effects of size and doping concentration on their structure and phase transformation using X-ray diffraction and Raman spectroscopy. The magnetic properties of these nanocrystals were studied by magnetic susceptibility, magnetic circular dichroism (MCD), and X-ray magnetic circular dichroism (XMCD) measurements. We observed that a decrease in nanocrystal size and an increase in doping concentration favor the stabilization of the paraelectric cubic phase, although the ferroelectric tetragonal phase is partly retained even in ca. 7 nm nanocrystals having the doping concentration of ca. 5%. The chromium(III) doping was determined to be a dominant factor for destabilization of the tetragonal phase. A combination of magnetic and magneto-optical measurements revealed that nanocrystalline films prepared from as-synthesized paramagnetic Cr(3+)-doped BaTiO(3) nanocrystals exhibit robust ferromagnetic ordering (up to ca. 2 µ(B)/Cr(3+)), similarly to magnetically doped transparent conducting oxides. The observed ferromagnetism increases with decreasing constituent nanocrystal size because of an enhancement in the interfacial defect concentration with increasing surface-to-volume ratio. Element-specific XMCD spectra measured by scanning transmission X-ray microscopy (STXM) confirmed with high spatial resolution that magnetic ordering arises from Cr(3+) dopant exchange interactions. The results of this work suggest an approach to the design and preparation of multiferroic perovskite materials that retain the ferroelectric phase and exhibit long-range magnetic ordering by using doped colloidal nanocrystals with optimized composition and size as functional building blocks.

Dual-stage lithography from a light-driven, plasmon-assisted process: a hierarchical approach to subwavelength features.

A hierarchy of lithographic-type imaging generating 3 µm lines incorporating subdiffraction limit features was obtained through a novel two-step reaction process. Photochemically generated ketyl radicals were used to make defined lines of silver nanoparticles. The excitation of nanoparticle surface plasmons was then used to generate highly localized heat that causes polymerization selectively on the surfaces of excited particles. The nylon-6 polymer that is generated serves as a solubility switch used to retain the features on the substrate selectively; various imaging techniques were used to establish the nature of the nylon shells. This work shows that the heat generated by plasmon excitation can be exploited to generate negative-type lithographic features with dimensions well below the diffraction limit.

FDTD Analysis of Hotspot-Enabling Hybrid Nanohole-Nanoparticle Structures for SERS Detection.

Metallic nanoparticles (MNPs) and metallic nanostructures are both commonly used, independently, as SERS substrates due to their enhanced plasmonic activity. In this work, we introduce and investigate a hybrid nanostructure with strong SERS activity that benefits from the collective plasmonic response of the combination of MNPs and flow-through nanohole arrays (NHAs). The electric field distribution and electromagnetic enhancement factor of hybrid structures composed of silver NPs on both silver and gold NHAs are investigated via finite-difference time-domain (FDTD) analyses. This computational approach is used to find optimal spatial configurations of the nanoparticle positions relative to the nanoapertures and investigate the difference between Ag-NP-on-Ag-NHAs and Ag-NP-on-Au-NHAs hybrid structures. A maximum GSERS value of 6.8 × 109 is achieved with the all-silver structure when the NP is located 0.5 nm away from the rim of the NHA, while the maximum of 4.7 × 1010 is obtained when the nanoparticle is in full contact with the NHA for the gold-silver hybrid structure. These results demonstrate that the hybrid nanostructures enable hotspot formation with strong SERS activity and plasmonic enhancement compatible with SERS-based sensing applications.