Syntheses of three-dimensional catenanes under kinetic control.

Although catenanes comprising two ring-shaped components can be made in large quantities by templation, the preparation of three-dimensional (3D) catenanes with cage-shaped components is still in its infancy. Here, we report the design and syntheses of two 3D catenanes by a sequence of SN2 reactions in one pot. The resulting triply mechanically interlocked molecules were fully characterized in both the solution and solid states. Mechanistic studies have revealed that a suit[3]ane, which contains a threefold symmetric cage component as the suit and a tribromide component as the body, is formed at elevated temperatures. This suit[3]ane was identified as the key reactive intermediate for the selective formation of the two 3D catenanes which do not represent thermodynamic minima. We foresee a future in which this particular synthetic strategy guides the rational design and production of mechanically interlocked molecules under kinetic control.

Alkoxy-Substituted Quadrupolar Fluorescent Dyes.

Polar and polarizable π-conjugated organic molecules containing push-pull chromophores have been investigated extensively in the past. Identifying unique backbones and building blocks for fluorescent dyes is a timely exercise. Here, we report the synthesis and characterization of a series of fluorescent dyes containing quadrupolar A-D-A constitutions (where A = acceptor and D = donor), which exhibit fluorescence emission at a variety of different wavelengths. We have investigated the effects of different electron-withdrawing groups, located at both termini of a para-terphenylene backbone, by steady-state UV/vis and fluorescence spectroscopy. Pyridine and substituted pyridinium units are also introduced during the construction of the quadrupolar backbones. Depending on the quadrupolarity, fluorescence emission wavelengths cover from 380 to 557 nm. Time-resolved absorption and emission spectroscopy reveal that the photophysical properties of those quadrupolar dyes result from intramolecular charge transfer. One of the dyes we have investigated is a symmetrical box-like tetracationic cyclophane. Its water-soluble tetrachloride, which is non-cytotoxic to cells up to a loading concentration of 1 µM, has been employed in live-cell imaging. When taken up by cells, the tetrachloride emits a green fluorescence emission without any hint of photobleaching or disruption of normal cell behavior. We envision that our design strategy of modifying molecules through the functionalization of the quadrupolar building blocks as chromophores will lead to future generations of fluorescent dyes in which these A-D-A constitutional fragments are incorporated into more complex molecules and polymers for broader photophysical and biological applications.

Cyclophane-Sustained Ultrastable Porphyrins.

We report the encapsulation of free-base and zinc porphyrins by a tricyclic cyclophane receptor with subnanomolar binding affinities in water. The high affinities are sustained by the hydrophobic effect and multiple [CH···π] interactions covering large [π···π] stacking surfaces between the substrate porphyrins and the receptor. We discovered two co-conformational isomers of the 1:1 complex, where the porphyrin is orientated differently inside the binding cavity of the receptor on account of its tricyclic nature. The photophysical properties and chemical reactivities of the encapsulated porphyrins are modulated to a considerable extent by the receptor. Improved fluorescence quantum yields, red-shifted absorptions and emissions, and nearly quantitative energy transfer processes highlight the emergent photophysical enhancements. The encapsulated porphyrins enjoy unprecedented chemical stabilities, where their D/H exchange, protonation, and solvolysis under extremely acidic conditions are completely blocked. We anticipate that the ultrahigh stabilities and improved optical properties of these encapsulated porphyrins will find applications in single-molecule materials, artificial photodevices, and biomedical appliances.

'Reverse' Hofmeister effects on the sol-gel transition rates for an α-helical peptide-PEG bioconjugate.

We examine the dynamics of the sol-gel transition for end-functionalized linear- and 4-arm-peptides bioconjugated to poly-ethylene glycol (PEG) in aqueous environments with increasingly chaotropic (Cl- < Br- < I-) anions. A 23-amino acid peptide sequence is rationally designed to self-assemble upon folding into the ordered α-helical conformation due to the hydrophobic effect. We use Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) to quantify the ensemble average reversible secondary structure transitions as a function of electrolyte concentration and specific ion effects along the Hofmeister series. Subsequently, microrheology is used to quantify the kinetics of the gelation process, as it relates to folding and specific ion interactions. Our key findings were non-intuitive. We observe the faster evolution of the gel transitions in systems with more chaotropic anions. For our peptides in aqueous solution, "water-structuring" ions yield faster assembly behavior with a viscoelastic exponent, n, closer to unity representing self-assemblies that are Rouse-like. In contrast, ions that are "water-breaking" resulted in smaller viscoelastic exponents where self-assembly dynamics result in a viscoelastic exponent that suggests polymer entanglements.

BODIPY-Based Polymers of Intrinsic Microporosity for the Photocatalytic Detoxification of a Chemical Threat.

Effective heterogeneous photocatalysts capable of detoxifying chemical threats in practical settings must exhibit outstanding device integrity. We report a copolymerization that yields robust, porous, processible, chromophoric BODIPY (BDP; boron-dipyrromethene)-containing polymers of intrinsic microporosity (BDP-PIMs). Installation of a pentafluorophenyl at the meso position of a BDP produced reactive monomer that when combined with 5,5,6,6-tetrahydroxy-3,3,3,3-tetramethyl-1,1-spirobisindane (TTSBI) and tetrafluoroterephthalonitrile (TFTPN) yields PIM-1. Postsynthetic modification of these polymers yields Br-BDP-PIM-1a and -1b─polymers containing bromine at the 2,6-positions. Remarkably, the brominated polymers display porosity and processability features similar to those of H-BDP-PIMs. Gas adsorption reveals molecular-scale porosity and Brunette-Emmet-Teller surface areas as high as 680 m2 g-1. Electronic absorption spectra reveal charge-transfer (CT) bands centered at 660 nm, while bands arising from local excitations, LE, of BDP and TFTPN units are at 530 and 430 nm, respectively. Fluorescence spectra of the polymers reveal a Förster resonance energy-transfer (FRET) pathway to BDP units when TFTPN units are excited at 430 nm; weak phosphorescence at room temperature indicates a singlet-to-triplet intersystem crossing. The low-lying triplet state is well positioned energetically to sensitize the conversion of ground-state (triplet) molecular oxygen to electronically excited singlet oxygen. Photosensitization capabilities of these polymers toward singlet-oxygen-driven detoxification of a sulfur-mustard simulant 2-chloroethyl ethyl sulfide (CEES) have been examined. While excitation of CT and LEBDP bands yields weak catalytic activity (t1/2 > 15 min), excitation to higher energy states of TFTPN induces significant increases in photoactivity (t1/2 ≅ 5 min). The increase is attributable to (i) enhanced light collection, (ii) FRET between TFTPN and BDP, (iii) the presence of heavy atoms (bromine) having large spin-orbit coupling energies that can facilitate intersystem crossing from donor-acceptor CT-, FRET-, or LE-generated BDP singlet states to BDP-related triplet states, and (iv) polymer triplet excited-state sensitization of the formation of CEES-reactive, singlet oxygen.

Supramolecular Porous Organic Nanocomposites for Heterogeneous Photocatalysis of a Sulfur Mustard Simulant.

Efficient heterogeneous photosensitizing materials require both large accessible surface areas and excitons of suitable energies and with well-defined spin structures. Confinement of the tetracationic cyclophane (ExBox4+ ) within a nonporous anionic polystyrene sulfonate (PSS) matrix leads to a surface area increase of up to 225 m2 g-1 in ExBox•PSS. Efficient intersystem crossing is achieved by combining the spin-orbit coupling associated to Br heavy atoms in 1,3,5,8-tetrabromopyrene (TBP), and the photoinduced electron transfer in a TBP⊂ExBox4+ supramolecular dyad. The TBP⊂ExBox4+ complex displays a charge transfer band at 450 nm and an exciplex emission at 520 nm, indicating the formation of new mixed-electronic states. The lowest triplet state (T1 , 1.89 eV) is localized on the TBP and is close in energy with the charge separated state (CT, 2.14 eV). The homogeneous and heterogeneous photocatalytic activities of the TBP⊂ExBox4+ , for the elimination of a sulfur mustard simulant, has proved to be significantly more efficient than TBP and ExBox+4 , confirming the importance of the newly formed excited-state manifold in TBP⊂ExBox4+ for the population of the low-lying T1 state. The high stability, facile preparation, and high performance of the TBP⊂ExBox•PSS nanocomposites augur well for the future development of new supramolecular heterogeneous photosensitizers using host-guest chemistry.