Multifunctional Vesicles from a Self-assembled Cluster-Containing Diblock Copolymer

We describe a new diblock copolymer composed of two segments with complementary functionalities. One block contains pendent photo-cross-linkable cinnamoyl groups, and the other contains molecular clusters, Co₆Se₈, capable of multielectron redox processes. This multifunctional macromolecule is synthesized by sequential ring-opening metathesis polymerization of monomers constructed using norbornene moieties. Remarkably, the tethered molecular cluster gives access to three different charge states in <i>N</i>,<i>N</i>-dimethylformamide: neutral, +1, and +2. In tetrahydrofuran, by contrast, the charged copolymer self-assembles into vesicles that inhibit the redox reactions. The wall of these vesicles can be cross-linked by exploiting the photoinduced 2 + 2 cycloaddition of the cinnamoyls to form cyclobutane dimers. Moreover, these vesicles can be loaded with molecular cargo and used as cross-linkable containers; we demonstrate this feature by encapsulating the molecular dye methylene blue into the capsules. Our work is the first report of a well-defined block copolymer containing a metal chalcogenide molecular cluster; more generally, it opens the door to new applications of metal-containing polymers

Multifunctional Vesicles from a Self-assembled Cluster-Containing Diblock Copolymer

We describe a new diblock copolymer composed of two segments with complementary functionalities. One block contains pendent photo-cross-linkable cinnamoyl groups, and the other contains molecular clusters, Co₆Se₈, capable of multielectron redox processes. This multifunctional macromolecule is synthesized by sequential ring-opening metathesis polymerization of monomers constructed using norbornene moieties. Remarkably, the tethered molecular cluster gives access to three different charge states in <i>N</i>,<i>N</i>-dimethylformamide: neutral, +1, and +2. In tetrahydrofuran, by contrast, the charged copolymer self-assembles into vesicles that inhibit the redox reactions. The wall of these vesicles can be cross-linked by exploiting the photoinduced 2 + 2 cycloaddition of the cinnamoyls to form cyclobutane dimers. Moreover, these vesicles can be loaded with molecular cargo and used as cross-linkable containers; we demonstrate this feature by encapsulating the molecular dye methylene blue into the capsules. Our work is the first report of a well-defined block copolymer containing a metal chalcogenide molecular cluster; more generally, it opens the door to new applications of metal-containing polymers

Length-Dependent Conductance of Oligothiophenes

We have measured the single-molecule conductance of a family of oligothiophenes comprising 1–6 thiophene moieties terminated with methyl-sulfide linkers using the scanning tunneling microscope-based break-junction technique. We find an anomalous behavior: the peak of the conductance histogram distribution does not follow a clear exponential decay with increasing number of thiophene units in the chain. The electronic properties of the materials were characterized by optical spectroscopy and electrochemistry to gain an understanding of the factors affecting the conductance of these molecules. We postulate that different conformers in the junction are a contributing factor to the anomalous trend in the observed conductance as a function of molecule length

Monoliths of Semiconducting Block Copolymers by Magnetic Alignment

Achieving highly ordered and aligned assemblies of organic semiconductors is a persistent challenge for improving the performance of organic electronics. This is an acute problem in macromolecular systems where slow kinetics and long-range disorder prevail, thus making the fabrication of high-performance large-area semiconducting polymer films a nontrivial venture. Here, we demonstrate that the anisotropic nature of semiconducting chromophores can be effectively leveraged to yield hierarchically ordered materials that can be readily macroscopically aligned. An n-type mesogen was synthesized based on a perylene diimide (PDI) rigid core coupled to an imidazole headgroup <i>via</i> an alkyl spacer. Supramolecular assembly between the imidazole and acrylic acid units on a poly(styrene-<i>b</i>-acrylic acid) block copolymer yielded self-assembled hexagonally ordered polystyrene cylinders within a smectic A mesophase of the PDI mesogen and poly(acrylic acid). We show that magnetic fields can be used to control the alignment of the PDI species and the block copolymer superstructure concurrently in a facile manner during cooling from a high-temperature disordered state. The resulting materials are monoliths, with a single well-defined orientation of the semiconducting chromophore and block copolymer microdomains throughout the sample. This synergistic introduction of both functional properties and the means of controlling alignment by supramolecular attachment of mesogenic species to polymer backbones offer new possibilities for the modular design of functional nanostructured materials

Probing Through-Bond and Through-Space Interactions in Singlet Fission-Based Pentacene Dimers

Interchromophoric interactions such as Coulombic coupling and exchange interactions are crucial to the functional properties of numerous π-conjugated systems. Here, we use magnetic circular dichroism (MCD) spectroscopy to investigate interchromophoric interactions in singlet fission relevant pentacene dimers. Using a simple analytical model, we outline a general relationship between the geometry of pentacene dimers and their calculated MCD response. We analyze experimental MCD spectra of different covalently bridged pentacene dimers to reveal how the molecular structure of the “bridge” affects the magnitude of through-space Coulombic and through-bond exchange interactions in the system. Our results show that through-bond interactions are significant in dimers with conjugated molecules as bridging units and these interactions promote the overall electronic coupling in the system. Our generalized approach paves the way for the application of MCD in investigating interchromophoric interactions across a range of π-conjugated systems

Fast Singlet Exciton Decay in Push–Pull Molecules Containing Oxidized Thiophenes

A common synthetic strategy used to design low-bandgap organic semiconductors employs the use of “push–pull” building blocks, where electron -rich and electron-deficient monomers are alternated along the π-conjugated backbone of a molecule or polymer. Incorporating strong “pull” units with high electron affinity is a means to further decrease the optical gap for infrared optoelectronics or to develop n-type semiconducting materials. Here we show that the use of thiophene-1,1-dioxide as a strong acceptor in “push–pull” oligomers affects the electronic structure and carrier dynamics in unexpected ways. Critically, the overall excited-state lifetime is reduced by several orders of magnitude relative to unoxidized analogs due to the introduction of low-energy optically dark states and low-energy triplet states that allow for fast internal conversion and intramolecular singlet fission. We found that the electronic structure and excited-state lifetime are strongly dependent on the number of sequential thiophene-1,1-dioxide units. These results suggest that both the static and dynamical optical properties are highly tunable via small changes in chemical structure that have drastic effects on the optoelectronic properties, which can impact the types of applications that involve these materials

Influence of Nanostructure on the Exciton Dynamics of Multichromophore Donor–Acceptor Block Copolymers

We explore the synthesis and photophysics of nanostructured block copolymers that mimic light-harvesting complexes. We find that the combination of a polar and electron-rich boron dipyrromethene (BODIPY) block with a nonpolar electron-poor perylene diimide (PDI) block yields a polymer that self-assembles into ordered “nanoworms”. Numerical simulations are used to determine optimal compositions to achieve robust self-assembly. Photoluminescence spectroscopy is used to probe the rich exciton dynamics in these systems. Using controls, such as homopolymers and random copolymers, we analyze the mechanisms of the photoluminescence from these polymers. This understanding allows us to probe in detail the photophysics of the block copolymers, including the effects of their self-assembly into nanostructures on their excited-state properties. Similar to natural systems, ordered nanostructures result in properties that are starkly different than the properties of free polymers in solution, such as enhanced rates of electronic energy transfer and elimination of excitonic emission from disordered PDI trap states

Hierarchically Ordered Nanopatterns for Spatial Control of Biomolecules

The development and study of a benchtop, high-throughput, and inexpensive fabrication strategy to obtain hierarchical patterns of biomolecules with sub-50 nm resolution is presented. A diblock copolymer of polystyrene-<i>b</i>-poly(ethylene oxide), PS-<i>b</i>-PEO, is synthesized with biotin capping the PEO block and 4-bromostyrene copolymerized within the polystyrene block at 5 wt %. These two handles allow thin films of the block copolymer to be postfunctionalized with biotinylated biomolecules of interest and to obtain micropatterns of nanoscale-ordered films <i>via</i> photolithography. The design of this single polymer further allows access to two distinct superficial nanopatterns (lines and dots), where the PEO cylinders are oriented parallel or perpendicular to the substrate. Moreover, we present a strategy to obtain hierarchical mixed morphologies: a thin-film coating of cylinders both parallel and perpendicular to the substrate can be obtained by tuning the solvent annealing and irradiation conditions

Exciton Correlations in Intramolecular Singlet Fission

We have synthesized a series of asymmetric pentacene–tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photoexcitations in conjugated dimers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay channels, including delayed fluorescence via triplet–triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated triplets increases with bridge length. Overall, these constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases

Nanopatterning Biomolecules by Block Copolymer Self-Assembly

The fabrication of sub-100 nm features with bioactive molecules is a laborious and expensive process. To overcome these limitations, we present a modular strategy to create nanostructured substrates (ca. 25 nm features) using functional block copolymers (BCPs) based on poly­(styrene-<i>b</i>-ethylene oxide) to controllably promote or inhibit cell adhesion. A single type of BCP was functionalized with a peptide, a perfluorinated moiety, and both compounds, to tune nanoscale phase separation and interactions with NIH3T3 fibroblast cells. The focal adhesion formation and morphology of the cells were observed to vary dramatically according to the functionality presented on the surface of the synthetic substrate. It is envisioned that these materials will be useful as substrates that mimic the extracellular matrix (ECM) given that the adhesion receptors of cells can recognize clustered motifs as small as 10 nm, and their spatial orientation can influence cellular responses