Twofold π-Extension of Polyarenes via Double and Triple Radical Alkyne peri-Annulations: Radical Cascades Converging on the Same Aromatic Core.

A versatile synthetic route to distannyl-substituted polyarenes was developed via double radical peri-annulations. The cyclization precursors were equipped with propargylic OMe traceless directing groups (TDGs) for regioselective Sn-radical attack at the triple bonds. The two peri-annulations converge at a variety of polycyclic cores to yield expanded difunctionalized polycyclic aromatic hydrocarbons (PAHs). This approach can be extended to triple peri-annulations, where annulations are coupled with a radical cascade that connects two preexisting aromatic cores via a formal C-H activation step. The installed Bu3Sn groups serve as chemical handles for further functionalization via direct cross-coupling, iodination, or protodestannylation and increase solubility of the products in organic solvents. Photophysical studies reveal that the Bu3Sn-substituted PAHs are moderately fluorescent, and their protodestannylation results in an up to 10-fold fluorescence quantum yield enhancement. DFT calculations identified the most likely possible mechanism of this complex chemical transformation involving two independent peri-cyclizations at the central core.

Solid State Multicolor Emission in Substitutional Solid Solutions of Metal-Organic Frameworks.

Preparing crystalline materials that produce tunable organic-based multicolor emission is a challenge due to the inherent inability to control the packing of organic molecules in the solid state. Utilizing multivariate, high-symmetry metal-organic frameworks, MOFs, as matrices for organic-based substitutional solid solutions allows for the incorporation of multiple fluorophores with different emission profiles into a single material. By combining nonfluorescent links with dilute mixtures of red, green, and blue fluorescent links, we prepared zirconia-type MOFs and found that the bulk materials exhibit features of solution-like fluorescence. Our study found that MOFs with a fluorophore link concentration of around 1 mol % exhibit fluorescence with decreased inner filtering, demonstrated by changes in spectral profiles, increased quantum yields, and lifetime dynamics expected for excited-state proton-transfer emitters. Our findings enabled us to prepare organic-based substitutional solid solutions with tunable chromaticity regulated only by the initial amounts of fluorophores. These materials emit multicolor and white light with high quantum yields (∼2-14%), high color-rendering indices (>93), long shelf life, and superb hydrolytic stability at ambient conditions.

Visible-Light-Activated Quinolone Carbon-Monoxide-Releasing Molecule: Prodrug and Albumin-Assisted Delivery Enables Anticancer and Potent Anti-Inflammatory Effects.

The delivery of controlled amounts of carbon monoxide (CO) to biological targets is of significant current interest. Very few CO-releasing compounds are currently known that can be rigorously controlled in terms of the location and amount of CO released. To address this deficiency, we report herein a new metal-free, visible-light-induced CO-releasing molecule (photoCORM) and its prodrug oxidized form, which offer new approaches to controlled, localized CO delivery. The new photoCORM, based on a 3-hydroxybenzo[ g]quinolone framework, releases 1 equiv of CO upon visible-light illumination under a variety of biologically relevant conditions. This nontoxic compound can be tracked prior to CO release using fluorescence microscopy and produces a nontoxic byproduct following CO release. An oxidized prodrug form of the photoCORM is reduced by cellular thiols, providing an approach toward activation in the reducing environment of cancer cells. Strong noncovalent affinity of the nonmetal photoCORM to albumin enables use of an albumin:photoCORM complex for targeted CO delivery to cancer cells. This approach produced cytotoxicity IC50 values among the lowest reported to date for CO delivery to cancer cells by a photoCORM. This albumin:photoCORM complex is also the first CO delivery system to produce significant anti-inflammatory effects when introduced at nanomolar photoCORM concentration.

Examining the role of acceptor molecule structure in self-assembled bilayers: surface loading, stability, energy transfer, and upconverted emission.

Self-assembly of sensitizer and acceptor molecules has recently emerged as a promising strategy to facilitate and harness photon upconversion via triplet-triplet annihilation (TTA-UC). In addition to the energetic requirements, the structure and relative orientation of these molecules can have a strong influence on TTA-UC rates and efficiency. Here we report the synthesis of five different acceptor molecules composed of an anthracene core functionalized with 9,10- or 2,6-phenyl, methyl, or directly bound phosphonic acid groups and their incorporation into self-assembled bilayers on a ZrO2 surface. All five films facilitate green-to-blue photon upconversion with Φuc as high as 0.0023. The efficiency of TTA, and not triplet energy transfer, fluorescence, or losses via FRET, was primarily responsible for dictating the Φuc emission. Even for molecules having similar photophysical properties, variation in the position of the phosphonic acid resulted in dramatically different ΦTTA, Ith values, γTTA, and D. Interestingly, we observed a strong linear correlation between ΦTTA and the Ith value but the cause of this relationship, if any, is unclear.

Examining the influence of bilayer structure on energy transfer and molecular photon upconversion in metal ion linked multilayers.

Metal ion linked multilayers is a unique motif to spatially control and geometrically restrict molecules on a metal oxide surface and is of interest in a number of promising applications. Here we use a bilayer composed of a metal oxide surface, an anthracene annihilator molecule, Zn(II) linking ion, and porphyrin sensitizers to probe the influence of the position of the metal ion binding site on energy transfer, photon upconversion, and photocurrent generation. Despite being energetically similar, varying the position of the carboxy metal ion binding group (i.e. ortho, meta, para) of the Pt(II) tetraphenyl porphyrin sensitizer had a large impact on energy transfer rates and upconverted photocurrent that can be attributed to differences in their geometries. From polarized attenuated total reflectance measurements of the bilayers on ITO, we found that the orientation of the first layer (anthracene) was largely unperturbed by subsequent layers. However, the tilt angle of the porphyrin plane varies dramatically from 41° to 64° to 57° for the para-, meta-, and ortho-COOH substituted porphyrin molecules, which is likely responsible for the variation in energy transfer rates. We go on to show using molecular dynamics simulations that there is considerable flexibility in porphyrin orientation, indicating that an average structure is insufficient to predict the ensemble behavior. Instead, even a small subset of the population with highly favorable energy transfer rates can be the primary driver in increasing the likelihood of energy transfer. Gaining control of the orientation and its distribution will be a critical step in maximizing the potential of the metal ion linked structures.

Enantioenrichment of racemic BINOL by way of excited state proton transfer.

Here we report a method for enantioenriching BINOL using a chiral auxiliary and an excited state proton transfer (ESPT) event. Regardless of the starting enantiomeric excess (ee), after irradiation the solution reaches a photostationary state whose ee is dependent solely on the identity of the chiral auxiliary group. The enantioenriched BINOL is easily recovered by cleaving the auxiliary group in mild conditions.

Enantioselective Protonation of Silyl Enol Ether Using Excited State Proton Transfer Dyes.

Enantiopure excited state proton transfer (ESPT) dyes were used for the asymmetric protonation of silyl enol ether. Under 365 nm irradiation, with 3,3'-dibromo-VANOL as the ESPT dye, up to 49% enantioselectivity with a 68% yield of product was observed at room temperature. The reaction is effective with a range of silyl enol ethers and can also be achieved with visible light upon the addition of triplet sensitizer. The relatively low ee of the protonated product is due to the racemization/decomposition of the ESPT dye in the excited state as indicated by circular dichroism, HPLC, and UV-vis spectroscopy.