Susan A. Odom

Mechanically-Induced Chemical Changes in Polymeric Materials

Engineering applications of synthetic polymers are widespread due to their availability, processability, low density, and diversity of mechanical properties (Figure 1a). Despite their ubiquitous nature, modern polymers are evolving into multifunctional systems with highly sophisticated behavior. These emergent functions are commonly described as “smart” characteristics whereby “intelligence” is rooted in a specific response elicited from a particular stimulus. Materials that exhibit stimuli-responsive functions thus achieve a desired output (O, the response) upon being subjected to a specific input (I, the stimulus). Given that mechanical loading is inevitable, coupled with the wide range of mechanical properties for synthetic polymers, it is not surprising that mechanoresponsive polymers are an especially attractive class of smart materials. To design materials with stimuli-responsive functions, it is helpful to consider the I-O relationship as an energy transduction process. Achieving the desired I-O linkage thus becomes a problem in finding how to transform energy from the stimulus into energy that executes the desired response. The underlying mechanism that forms this I-O coupling need not be a direct, one-step transduction event; rather, the overall process may proceed through a sequence of energy transduction steps. In this regard, the network of energy transduction pathways is a useful roadmap for designing stimuli-responsive materials (Figure 1b). It is the purpose of this review to broadly survey the mechanical to chemical * To whom correspondence should be addressed. Phone: 217-244-4024. Fax: 217-244-8024. E-mail: jsmoore@illinois.edu. † Department of Chemistry and Beckman Institute. ‡ Department of Materials Science and Engineering and Beckman Institute. § Department of Aerospace Engineering and Beckman Institute. Chem. Rev. XXXX, xxx, 000–000 A

Masked cyanoacrylates unveiled by mechanical force.

Mechanical damage of polymers is often a destructive and irreversible process. However, desirable outcomes may be achieved by controlling the location of chain cleavage events through careful design and incorporation of mechanically active chemical moieties known as mechanophores. It is possible that mechanophores can be used to generate reactive intermediates that can autopolymerize or cross-link, thus healing mechanically induced damage. Herein we report the generation of reactive cyanoacrylate units from a dicyanocyclobutane mechanophore located near the center of a polymer chain. Because cyanoacrylates (which are used as monomers in the preparation of superglue) autopolymerize, the generated cyanoacrylate-terminated polymers may be useful in self-healing polymers. Sonication studies of polymers with the mechanophore incorporated into the chain center have shown that selective cleavage of the mechanophore occurs. Trapping experiments with an amine-based chromophore support cyanoacrylate formation. Additionally, computational studies of small-molecule models predict that force-induced bond cleavage should occur with greater selectivity for the dicyanocyclobutane mechanophore than for a control molecule.

A Self-healing Conductive Ink

Dr. S. A. Odom , O. Zhao , Prof. J. S. Moore Department of ChemistryBeckman Institute for Advanced Science & TechnologyUniversity of Illinois at Urbana-Champaign405 N. Mathews Ave. Urbana, IL 61801, USA E-mail: jsmoore@illinois.edu S. Chayanupatkul , B. J. Blaiszik , A. C. Jackson , Prof. P. Braun , . V Prof. N. R. Sottos Department of Materials Science & EngineeringBeckman Institute for Advanced Science & TechnologyUniversity of Illinois at Urbana-Champaign405 N. Mathews Ave. Urbana, IL 61801, USA Prof. S. R. White Department of Aerospace EngineeringBeckman Institute for Advanced Science & TechnologyUniversity of Illinois at Urbana-Champaign405 N. Mathews Ave. Urbana, IL 61801, USAE-mail: swhite@illinois.edu

Synthesis and Two-Photon Spectrum of a Bis(Porphyrin)-Substituted Squaraine

A chromophore in which zinc porphyrin donors are linked through their meso positions by ethynyl bridges to a bis(indolinylidenemethyl) squaraine core has been synthesized using Sonogashira coupling. The chromophore exhibits a two-photon absorption spectrum characterized by a peak cross section of 11,000 GM and, more unusually, also exhibits a large cross section of >780 GM over a photon-wavelength window 750 nm in width.

Tuning Delocalization in the Radical Cations of 1,4-Bis[4-(diarylamino)styryl]benzenes, 2,5-Bis[4-(diarylamino)styryl]thiophenes, and 2,5-Bis[4-(diarylamino)styryl]pyrroles through Substituent Effects

Radical cations have been generated for 10 bis[4-(diarylamino)styryl]arenes and heteroarenes to investigate the effect of the electron-richness of the terminal groups and of the bridging (hetero)arene on delocalization. The intervalence charge-transfer bands of these radical cations vary from weak broad Gaussians, indicative of localized class-II mixed-valence species, to strong relatively narrow asymmetric bands, characteristic of delocalized class-III bis(diarylamino) species, to narrow symmetric bands in cases where the bridge contribution to the singly occupied molecular orbital is largest. Hush analysis of these bands yields estimates of the electronic coupling varying from 480 cm(-1) (electron-poor bridge, most electron-rich terminal aryl groups) to 1000 cm(-1) (electron-rich bridge, least electron-rich termini) if the diabatic electron-transfer distance, R(ab), is equated to the N-N separation. Computational and electron spin resonance (ESR) evidence for displacement of the diabatic states into the bridge (reduced R(ab)) suggests that these values are underestimates and that even more variation is to be expected through the series. Several dications have also been studied. The vis-NIR absorption of the dication of (E,E)-1,4-bis{4-[bis(4-n-butoxyphenyl)amino]styryl}-2,5-dicyanobenzene is seen at an energy similar to that of the strongest band in the spectrum of the corresponding weakly coupled monocation, with approximately twice the absorptivity, and its ESR spectrum suggests essentially noninteracting radical centers. In contrast, the electronic spectra of class-III monocations show no clear relationship to those of the corresponding dications, which ESR reveals to be singlet species.

Stabilisation of a heptamethine cyanine dye by rotaxane encapsulation

The crystal structure of a cyanine dye rotaxane shows that the cyclodextrin is tightly threaded round the polymethine bridge of the dye; encapsulation dramatically increases the kinetic chemical stability of the radicals formed on oxidation and reduction of the dye, making it possible to observe the rotaxane radical dication by ESR and UV-vis-NIR spectroscopy.

High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries

Non-aqueous redox flow batteries (NAqRFBs) employing redox-active organic molecules show promise to meet requirements for grid energy storage. Here, we combine the rational design of organic molecules with flow cell engineering to boost NAqRFB performance. We synthesize two highly soluble phenothiazine derivatives, N-(2-methoxyethyl)phenothiazine (MEPT) and N-[2-(2-methoxyethoxy)ethyl]phenothiazine (MEEPT), via a one-step synthesis from inexpensive precursors. Synthesis and isolation of the radical-cation salts permit UV-vis decay studies that illustrate the high stability of these open-shell species. Cyclic voltammetry and bulk electrolysis experiments reveal the promising electrochemical properties of MEPT and MEEPT under dilute conditions. A high performance non-aqueous flow cell, employing interdigitated flow fields and carbon paper electrodes, is engineered and demonstrated; polarization and impedance studies quantify the cells low area-specific resistance (3.2–3.3 Ω cm2). We combine the most soluble derivative, MEEPT, and its tetrafluoroborate radical-cation salt in the flow cell for symmetric cycling, evincing a current density of 100 mA cm−2 with undetectable capacity fade over 100 cycles. This coincident high current density and capacity retention is unprecedented in NAqRFB literature.

Linear and nonlinear spectroscopy of a porphyrin-squaraine-porphyrin conjugated system.

The linear and nonlinear absorption properties of a squaraine-bridged porphyrin dimer (POR-SQU-POR) are investigated using femto-, pico-, and nanosecond pulses to understand intramolecular processes, obtain molecular optical parameters, and perform modeling of the excited-state dynamics. The optical behavior of POR-SQU-POR is compared with its separate porphyrin and squaraine constituent moieties. Linear spectroscopic studies include absorption, fluorescence, excitation and emission anisotropy, and quantum yield measurements. Nonlinear spectroscopic studies are performed across a wide range (approximately 150 fs, approximately 25 ps, and approximately 5 ns) of pulsewidths and include two-photon absorption (2PA), single and double pump-probe, and Z-scan measurements with detailed analysis of excited-state absorption induced by both one- and two-photon absorption processes. The 2PA from the constituent moieties shows relatively small 2PA cross sections; below 10 GM (1 GM = 1 x 10(-50) cm4 s/photon) for the porphyrin constituent and below 100 GM for the squaraine constituent except near their one-photon resonances. In stark contrast, the composite POR-SQU-POR molecule shows 2PA cross sections greater than 10(3) GM over most of the spectral range from 850 to 1600 nm (the minimum value being 780 GM at 1600 nm). The maximum value is approximately 11,000 GM near the Nd:YAG laser wavelength of 1064 nm. This broad spectral range of large 2PA cross sections is unprecedented in any other molecular system and can be explained by intramolecular charge transfer. A theoretical quantum-chemical analysis in combination with different experimental techniques allows insight into the energy-level structure and origin of the nonlinear absorption behavior of POR-SQU-POR.

Autonomic restoration of electrical conductivity using polymer-stabilized carbon nanotube and graphene microcapsules

We report the use of microcapsules containing suspensions of polymer-stabilized carbon nanotubes and/or graphene flakes for the autonomic restoration of conductivity in fractured gold lines. Multilayered samples were prepared in which microcapsules were embedded in layers of epoxy above and below a glass slide patterned with gold lines. Upon sample fracture, conductivity was lost as a crack formed in the gold line. Simultaneous release of carbon nanotubes and/or graphene suspensions from capsule cores restored conductivity in minutes. We suggest a healing mechanism in which the released carbon nanomaterials bridge gaps in the gold lines.

Visual indication of mechanical damage using core-shell microcapsules

We report a new core-shell microcapsule system for the visual detection of mechanical damage. The core material, 1,3,5,7-cyclooctatetraene, is a conjugated cyclic olefin and a precursor to intensely colored polyacetylene. A combination of poly(urea-formaldehyde) and polyurethane is required to effectively encapsulate the volatile core material. Increasing the outer shell wall thickness and including a core-side prepolymer improves the thermal stability and free-flowing nature of these capsules, which tend to leach and rupture with thinner shell walls. Capsules ruptured in the presence of the Grubbs-Love ruthenium catalyst show immediate color change from nearly colorless to red-orange and dark purple over time, and color change in thin films resulted from scratch damage.