Effect of Oxidation on the Chiroptical Properties of Sulfur-Bridged Binaphthyl Dimers.

A series of axially chiral sulfur-bridged dimers were prepared from 1,1'-binaphthyl-2,2'-diol and subsequently oxidized to the respective sulfones. The chiroptical properties of the chiral chromophores were studied as a function of the oxidation state. Upon oxidation, an increase in quantum yields was observed for directly linked sulfur bridged binaphthyls (0.04 to 0.32), and a modest increase in dissymmetry factor was observed for diphenylsulfide-bridged binaphthyls (-8.9 × 10-4 to -1.4 × 10-3). Computational calculations were used to elucidate the changes in photophysical properties.

An imidazoacridine-based TADF material as an effective organic photosensitizer for visible-light-promoted [2 + 2] cycloaddition.

Energy transfer (EnT) is a fundamental activation process in visible-light-promoted photocycloaddition reactions. This work describes the performance of imidazoacridine-based TADF materials for visible-light mediated triplet-triplet EnT photocatalysis. The TADF material ACR-IMAC has been discovered as an inexpensive, high-performance organic alternative to the commonly used metal-based photosensitizers for visible-light EnT photocatalysis. The efficiency of ACR-IMAC as a photosensitizer is comparable with Ir-based photosensitizers in both intra- and intermolecular [2 + 2] cycloadditions. ACR-IMAC mediated both dearomative and non-dearomative [2 + 2] cycloadditions in good yields, with high regio- and diastereocontrol. Cyclobutane-containing bi- tri- and tetracylic scaffolds were successfully prepared, with broad tolerance toward functional groups relevant to drug discovery campaigns. Fluorescence quenching experiments, time-correlated single-photon counting, and transient absorption spectroscopy were also conducted to provide insight into the reaction and evidence for an EnT mechanism.

Dextran Functionalization of Semiconducting Polymer Dots and Conjugation with Tetrameric Antibody Complexes for Bioanalysis and Imaging.

Brightly fluorescent semiconducting polymer dots (Pdots) are emerging as very useful probes for bioanalysis and imaging. Unfortunately, Pdot materials often suffer from limitations such as poor colloidal and physical stability, nonspecific adsorption, and relatively few reported surface chemistries and bioconjugate chemistries. To help address these limitations, we have developed dextran-functionalized Pdots (Dex-Pdots). This functionalization improves particle stability over a range of pH and at high ionic strength, hinders surface-induced unfolding, and enables the preparation of immunoconjugates via tetrameric antibody complexes (TAC). The utility of TAC-conjugated Dex-Pdots is demonstrated through a proof-of-concept fluorescence-linked immunosorbent assay (FLISA) for human erythropoietin (EPO), and through immunolabeling of human epidermal growth factor receptor 2 (HER2)-positive SK-BR3 breast cancer cells. The conjugates exhibited less nonspecific binding and greater specific binding than Pdots without dextran functionalization. Overall, dextran functionalization is a highly promising surface chemistry for biological applications of Pdots.

Room temperature crystallization of amorphous polysiloxane using photodimerization.

Bulk crystallization in flexible polymeric systems is difficult to control due to the random orientation of the chains. Here we report a photo cross-linking strategy that results in simultaneous cross-linking and crystallization of polysiloxane chains into millimeter sized leaf-like polycrystalline structures. Polymers containing pendant anthracene groups are prepared and undergo [4+4] photocycloaddition under 365 nm irradiation at room temperature. The growth and morphology of the crystalline structures is studied using polarized optical microscopy (POM) and atomic force microscopy and is found to progress through three unique stages of nucleation, growth, and constriction. The mobility of the individual chains is probed using pulsed-field gradient (PFG) NMR to provide insights into the diffusion processes that may govern chain transport to the growing crystal fronts. The room temperature crystallization of this conventionally amorphous polymer system may allow for a new level of morphological control for silicone materials.

Donor-Acceptor Materials Exhibiting Thermally Activated Delayed Fluorescence Using a Planarized N-Phenylbenzimidazole Acceptor.

An N-phenylbenzimidazole constrained in a coplanar fashion with a methylene tether (IMAC) was designed and used to prepare a series of emitters exhibiting thermally activated delayed fluorescence (TADF). Four novel TADF emitters using 9,9-dimethylacridine, phenoxazine, phenothiazine, and bis(di-p-tolylamino)carbazole as the donor group were designed and synthesized using IMAC as the acceptor. Additionally, two deep-blue fluorescent emitters were prepared with carbazole and tercarbazole as the donor moieties. The twisted conformation between donor and acceptor in these molecules resulted in effective spatial separation of the HOMO and LUMO and small singlet-triplet energy gaps. Crystallographic properties, electronic structures, thermal stabilities, photophysical properties, and energy levels were studied systematically. Ultimately, these findings provide a promising opportunity for the design and synthesis of highly efficient TADF materials based on IMAC derivatives.

Aggregation-Induced Energy Transfer in Color-Tunable Multiblock Bottlebrush Nanofibers.

The synthesis of multicomponent nanoscale structures with precisely addressable function is critical to the discovery of both new phenomena and new applications in nanotechnology. Though self-assembly offers low-cost routes to many such materials, these methods often require building blocks with particular structural motifs, thus limiting the scope of nanomaterials that can be prepared in these ways. Herein we use a bottom-up approach based on covalent chemistry to synthesize a series of bottlebrush copolymers from red, green, and blue luminescent macromonomers, which were then used to prepare multiblock organic nanofibers structurally analogous to nanoscale RGB pixels. Efficient energy transfer from a blue fluorophore to red and green phosphors can be modulated, using the solvent polarity as a stimulus, to give aggregation-induced changes in emission color. Aggregation was also accompanied by changes in the emission lifetime of the nanofiber from the nanosecond to microsecond regime. Additionally, changes in energy transfer efficiency and interchromophore distance were quantified using a FRET model. Preliminary demonstration of these materials as polarity-sensitive inks for encryption and encoding were also demonstrated using a red/blue fluorescence switch upon exposure to solvent. Finally, the potential complexity of optoelectronic materials accessible with these methods was demonstrated by combining these building blocks with charge-transporting materials to give organic nanofibers with ordered structures mimicking that of multilayer white OLEDs. Ultimately this work describes the preparation of robust, multicomponent nanofibers from general building blocks, combining their optoelectronic properties in ways that can be both reversibly switched and temporally resolved.

Self-assembly of giant bottlebrush block copolymer surfactants from luminescent organic electronic materials.

Bottlebrush copolymers have shown promise as building blocks for self-assembled nanomaterials due to their reduced chain entanglement relative to linear polymers and their ability to self-assemble with remarkably low critical micelle concentrations (CMCs). Concurrently, the preparation of bottlebrush polymers from organic electronic materials has recently been described, allowing multiple optoelectronic functions to be incorporated along the length of single bottlebrush strands. Here we describe the self-assembly of bottlebrush surfactants containing soluble n-butyl acrylate blocks and carbazole-based organic semiconductors, which self-assemble in selective solvent to give spherical micelles with CMCs below 54 nM. These narrowly dispersed structures were stable in solution at high dilution over periods of months, and could further be functionalized with fluorescent dyes to give micelles with quantum yields of 100%. These results demonstrate that bottlebrush-based nanostructures can be formed from organic semiconductor building blocks, opening the door to the preparation of fluorescent or redox-active micelles from giant polymeric surfactants.

Multiblock Bottlebrush Nanofibers from Organic Electronic Materials.

Methods are described for the preparation of fiber-like nanomaterials that mimic the multilayer structure of organic electronic devices on individual polymer chains. By combining Cu(0) reversible-deactivation radical polymerization (RDRP) and ring-opening metathesis polymerization (ROMP), multiblock bottlebrush copolymers are synthesized from ordered sequences of organic semiconductors. Narrowly dispersed fibers are prepared from materials commonly used as the hole transport, electron transport, and host materials in organic electronics, with molecular weights exceeding 2 × 106 Da and dispersities as low as 1.12. Diblock nanofibers are then synthesized from pairs of semiconducting building blocks, giving nanostructures analogous to p- n junctions that exhibit the reversible electrochemistry of their individual parts. Finally, this strategy is used to construct nanofibers with the structure of phosphorescent organic light-emitting diodes (OLEDs) on single macromolecules, such that the photophysical properties of each component of an OLED can be independently observed. These multiblock nanofibers can be formed from arbitrary organic semiconductors without the need for crystallinity, selective solvation, or supramolecular interactions, providing powerful methods for the miniaturization of materials for organic devices.

Group 6 metal pentacarbonyl complexes of air-stable primary, secondary, and tertiary ferrocenylethylphosphines.

The synthesis and characterization of a series of Group 6 metal pentacarbonyl complexes of air stable primary, secondary, and tertiary phosphines containing ferrocenylethyl substituents are reported [M(CO)5L: M = Cr, Mo, W; L = PH2(CH2CH2Fc), PH(CH2CH2Fc)2, P(CH2CH2Fc)3]. The structure and composition of the complexes were confirmed by multinuclear NMR spectroscopy, IR and UV-Vis absorption spectroscopy, mass spectrometry, X-ray crystallography, and elemental analysis. The solid-state structural data reported revealed trends in M-C and M-P bond lengths that mirrored those of the atomic radii of the Group 6 metals involved. UV-Vis absorption spectroscopy and cyclic voltammetry highlighted characteristics consistent with electronically isolated ferrocene units including wavelengths of maximum absorption between 435 and 441 nm and reversible one-electron (per ferrocene unit) oxidation waves between 10 and -5 mV relative to the ferrocene/ferrocenium redox couple. IR spectroscopy confirmed that the σ donating ability of the phosphines increased as ferrocenylethyl substituents were introduced and that the tertiary phosphine ligand described is a stronger σ donor than PPh3 and a weaker σ donor than PEt3, respectively.

Polymers containing nickel(II) complexes of Goedken's macrocycle: optimized synthesis and electrochemical characterization.

The synthesis and characterization of a new class of nickel-containing polymers is described. The optimized copolymerization of alkyne-bearing nickel(II) complexes of Goedken's macrocycle (4,11-dihydro-5,7,12,14-tetramethyldibenzo[b,i][1,4,8,11]tetraazacyclotetradecine) and brominated 9,9-dihexylfluorene produced polymers with potential application as functional redox-active materials. The title polymers exhibit electrochemically reversible, ligand-centered oxidation events at 0.24 and 0.73 V versus the ferrocene/ferrocenium redox couple. They also display exceptional thermal stability and interesting absorption properties due to the presence of the macrocyclic nickel(II) complexes and π-conjugated units incorporated in their backbones.