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.

A vinylogous Norrish reaction as a strategy for light-mediated ring expansion.

The reactions of bicyclic divinyl ketones display wavelength-dependent changes in product formation. UV irradiation results in the formation of competitive [6,3,5] and [7,3,5] tricyclic unsaturated ketones that subsequently undergo ring expansion and reaction with a range of nucleophiles. DFT calculations and transient absorption experiments were completed that are consistent with a vinylogous Type II Norrish pathway.

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.

Exciting clusters, what does off-resonance actually mean?

Noble metal clusters have unique photophysical properties, especially as a new class of materials for multiphoton biomedical imaging. The previously studied Au25SR18 exhibits "giant" two-photon absorbance cross sections. Herein, we investigate the origins of the large two photon absorption for Au25SR18, as well as 10 other Au and Ag clusters using femtosecond pump/probe transient absorption spectroscopy (fsTAS). Excited state absorbance (ESA) ubiquitous to thiolated Au and Ag clusters is used herein as an optical signature of two-photon absorbances of the 11 different Au and Ag clusters, which does not require high quantum yields of emission. The large selection of clusters, studied with a single laser system, allows us to draw conclusions on the role of the particular metal, cluster size/structure, and the effects of the ligands on the ability to absorb multiple NIR photons. The use of a laser with a 1028 nm excitation also allows us to investigate the dramatic effect of excitation wavelength and explain why laser wavelength has led to large variances in the non-linear responses reported for clusters to date. We discuss the double resonance mechanism, responsible for giant two photon absorbance cross-sections, helping match properties of metal clusters with experimental conditions for maximizing signal/response in multiphoton applications.

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.

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.

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.

Effect of nanosilver surfaces on peptide reactivity towards reactive oxygen species.

The interaction of a terminal tryptophan residue within collagen mimetic peptides when tethered to nanometric silver surfaces was studied using a combination of steady state spectroscopy, ultrafast spectroscopy, and molecular dynamics experiments. Our findings indicate that the effective interaction between the tryptophan and the metal surface occurs in short-time scales (ps) and it is responsible for improving the colloidal stability of the nanoparticles exposed to free radicals. The extent and efficiency of the interaction depends on factors beyond the peptide length that include conformation and distance from the terminal tryptophan to the metal surface.