Search

Electron-rich heteroarenes stack with perfluoroarene side chains and twist arylene-ethynylene conjugated oligomers, while electron deficient heterocycles do not. These trends correlate with X-ray structures, optical spectra, and computations.

Lanthanum silicate/germanate apatite materials have attracted significant interest as Solid Oxide Fuel Cell electrolytes due to their high oxide ionic conductivity at lower temperatures (500-800 ⁰C). In these structures, oxide ion conduction is due to interstitial pathways associated with the high oxygen excess within the structures. Therefore, cation doped La8+xA2-x(M6-xBxO4)O2+x/2 (A = Ca, Sr, Ba; M = Si and Ge; B = Mg, Ga, Al, Zn, B) have been reported to increase oxide ion conductivity via the introduction of excess oxide ions.[1-3] Whilst apatite silicate has higher conductivities at lower temperatures due to lower activation energies, apatite germanates can achieve higher oxygen excess and therefore higher conductivities at elevated temperatures. However, in increasing oxygen content there is a change in symmetry from hexagonal to triclinic leading to a subsequent reduction in conductivity.[4] Previous studies have shown the issue may be overcome by the incorporation of Y doping e.g La8Y2GeO6O27 which leads to stabilisation of the higher conducting hexagonal phase.[5] Herein, we demonstrate the successful incorporation of borate into La10-xYxGeO6O27 and show that it also stabilises the higher conducting hexagonal form. We show that B can be doped into both the Ge site and the oxide ion channels. Interestingly, the conductivity of samples with borate in the channels (which would be expected to block channel oxide ion conductivity along the c direction) is still reasonably high, which supports suggestions that there is significant conduction perpendicular to the channels in these apatite germanates. [1] A Orera and P R Slater, Chem. Mater 2010 22 675-690 [2] P M Panchmatia, A Orera, G J Rees, M E Smith, J. V. Hanna, P. R. Slater and M. S. Islam, Angew. Chem. Int. Ed, 2011 50 9328-9333 [3] E Kendrick, M S Islam and P R Slater, J. Mater. Chem 2007 17 3104-3111. [4] M S Chambers, P Chater, I R Evans and J S Evans, Inorg. Chem., 2019 58(21)14853-14862, [5] A Najib, J. E. H. Sansom, J. R. Tolchard, P. R. Slater and M. S. Islam, Dalton Trans., vol. 19, pp. 3106-3109, 2004.

Fluorophores that respond to external stimuli on demand have numerous applications in imaging and chemical or biological sensing. In this paper, we describe conjugated polymer nanoparticles (CPNs) ...

Although long acenes remain a key class of π-conjugated molecules for numerous applications, photoinduced oxidation upon exposure of the acene to light, often through sensitization of 1O2, is an important reaction requiring mitigation for most applications. In response to this ongoing challenge, this paper presents a series of four new diarylethynyl-substituted long acenes-three tetracenes and one anthradithiophene-in which the arylene pendants are either benzene, naphthalene, or anthracene. UV/vis and fluorescence spectroscopy reveals that the anthracene-substituted derivatives fluoresce poorly (Φ < 0.01). Although all four long acenes react with 1O2 at expected rates when an external photosensitizer is included and show the expected changes in fluorescence to accompany these reactions, the anthracene-substituted derivatives resist direct photoinduced oxidation. Through a combination of mechanistic experiments, we conclude that rapid nonradiative decay of the anthracene-substituted derivatives, perhaps because of inter-arene torsions that emerge in theoretical geometry optimizations, makes these compounds poor photosensitizers for 1O2 or other reactive oxygen species. This discovery opens new design possibilities for extended acene structures with improved photochemical stability.

Designing strong interactions into π-conjugated materials can direct their typically uncontrolled molecular packing and is an important strategy toward tuning their technologically relevant propert...

This letter describes thiophene-based materials that undergo photoinduced aggregation or precipitation upon irradiation with UV light. The only solubilizing side chains on these materials are photocleavable by connection through photolabile nitrobenzyl esters. While quaterthiophene oligomers yield diacids that remain soluble in dichloromethane at micromolar concentrations upon exposure to ultraviolet light, the polymeric analog shows both red-shifted absorbance and heavily quenched fluorescence, consistent with aggregation due to photochemical cleavage of solubilizing alkyl chains. Thin films of this polymer also resisted dissolution in organic solvent upon irradiation, suggesting applicability in the construction of multilayer solid-state devices.

Progress in photodynamic therapy (PDT) relies on the design and synthesis of photosensitizers that can efficiently sensitize singlet oxygen using visible light irradiation while displaying limited dark toxicity. Here, we highlight the paper by Linker and coworkers published in this issue of Photochemistry and Photobiology, which evaluates the effect of the regiochemistry of pyridinium rings in three isomeric pyridinium alkynylanthracenes on their performance as photosensitizers for PDT in HeLa cells.

Neat solids that phosphoresce under ambient conditions are rare due to aggregation-caused quenching. This communication describes a platinum acetylide (PtPE) that phosphoresces as a solid due to programmed aromatic stacking interactions of pendant groups that prevent intermolecular aggregation.

The spectroscopic, electronic, and geometrical properties of acenes have enabled their broad applicability in organic optoelectronics. Beyond these physical characteristics of acenes, acenes also offer characteristic and predictable reaction chemistry, especially their behavior as dienes in cycloaddition reactions. Although these cycloaddition reactions, especially those with singlet oxygen (1O2) as the dienophile, are detrimental for organic electronics, this reactivity has led to several different applications such as sensing of 1O2, the release of cytotoxic reactive oxygen species (ROS), and stimuli-responsive materials for drug delivery. The rational design of acenes in these chemically-responsive applications beyond organic optoelectronics requires an understanding of how chemical structure influences both the physical properties, such as quantum yield of emission, as well as the reactivity of acenes and their cycloadducts. Therefore, the objective of this review is to summarize how cycloaddition reactions of acenes have expanded their applications in different areas of materials chemistry, and in doing so inspire and inform the rational design of acene-based materials with applications beyond organic electronics.

Control over the conformation and packing of conjugated materials is an unsolved problem that prevents the rational design of organic optoelectronics, such as preventing self-quenching of luminescent molecules. Exacerbating this challenge is a general lack of widely applicable strategies for controlling packing with discrete, directional non-covalent interactions. Here, we present a series of conjugated molecules with diverse backbones of three or four arenes that feature pentafluorobenzyl ester substituents. Nearly all the compounds reveal intramolecular stacking interactions between the fluoroarene (ArF) side-chains and non-fluorinated arenes (ArH) in the middle of the chromophores; a twisted PE linkage accompanies each example of this intramolecular ArF–ArH stacking. Furthermore, these molecules can resist dramatic changes to emission upon transition from organic solution to thin film when ArF rings prevent interchromophore interactions. By broadening the structural space of conjugated backbones over which ArF–ArH stacking can twist PE linkages reliably and prevent self-quenching of solids with simple synthetic approaches, this work suggests fluorinated benzyl ester substituents adjacent to phenylene ethynylene linkages as supramolecular synthons for the crystal engineering of organic optoelectronic materials.

Understanding and manipulating crystal polymorphism can provide novel strategies for materials discovery in organic optoelectronics. In this paper, a series of seven ester-terminated three-ring phenylene ethynylenes (PEs) exhibit structure-dependent polymorphism wherein alkyl chain length modulates the propensity to form violet or green fluorescent solid phases, as well as tunable thermal and mechanofluorochromic (MFC) transitions. These compounds harness “soft” non-covalent control to achieve polymorphism: the electronic substituent effect of the ester groups weakens the fluoroarene-arene (ArF-ArH) interactions that typically direct crystal packing of this class of compounds, increasing competitiveness of other interactions. Small structural modifications tip this balance and shift the prevalence of violet- or green-emitting polymorphs. Compounds with short alkyl chain lengths show both violet and various green fluorescent polymorphs, while the violet fluorescent form dominates with alkyl lengths longer than butyl. Further, thermally induced green-to-violet fluorescent crystal-to-crystal transitions occur for single crystals of CO2–1 and CO2–3. Finally, the PEs show reversible violet-to-green mechanofluorochromism (MFC), with temperature required for reversion of this MFC decreasing with alkyl chain length. We therefore present this design of directional but weak interactions as a strategy to access polymorphs and tunable stimuli-responsive behavior in solids.

Engineering the properties of conjugated materials in the solid state is an unsolved, ongoing challenge important to fundamental understanding of how non-covalent interactions dictate packing and key properties, as well as the development of technologies based in organic optoelectronics. The most common design paradigm of such materials divide them into a "main chain" with extended conjugation, the chemical structure of which determines optoelectronic properties, and "side chains" not conjugated to the backbone, which provide solubility when they are long alkyl chains. This paper describes comparisons between phenylene-ethynylene molecules in which slight changes to the structure of "side chains"-swapping hydrogen and fluorine atomic position on an aromatic ring-results in unexpectedly large changes in the solid-state optical properties. In a pair of anisyl-terminated three-ring phenylene-ethynylenes, switching the side chain arenes of benzyl esters from 2,4,6-trifluoro to 2,3,6-trifluoro results in a shift in fluorescence emission spectra of over 100 nm, as well as the opposite direction of force-induced shifting of emission. Through a combination X-ray crystal structures, electronic structure calculations, and comparisons with other derivatives, we describe how the 2,4,6-trifluorinated side chains yield cofacial fluoroarene-arene stacking interactions that twist the PE backbone out of conjugation, while the 2,3,6-trifluoro side chains do not stack, instead yielding more coplanar PE backbones that form intermolecular aggregates. Overall, this work demonstrates how slight modifications to parts of conjugated materials normally considered ancillary to optoelectronic properties can determine their solid-state properties, epitomizing the challenge of rational design but at the same time offering opportunities for materials discovery and improved understanding of non-covalent interactions.

In aqueous media, negative photochromism can occur from cationic, protonated merocyanines (MCH+) to spiropyrans (SP) upon exposure to visible light. Herein, we demonstrate the uniquely simple manner in which polymers can tune the key properties of these photochromes through copolymer preparation. Nine water-soluble, SP-containing polymers yield wide ranges of key properties for negative photochromism—accessible MCH+ pKa values spanning 5 pH units, quantum yields of photochemical ring closure up to 0.2, and rates of thermal equilibration ranging 2 orders of magnitude. In buffered solutions, many of these polymers showed photostationary states with greater than 80% conversion to SP and no detectable photochromic fatigue over 15 cycles. These results demonstrate the potential for tuning key properties of the MCH+ photochrome through both the spiropyran pendants themselves and the water-solubilizing comonomers. The dependence of MCH+ pKa on comonomer structure enabled tunable range of pH for reversible photoa...

Seven three-ring phenylene-ethynylene (PE) structural analogs, differing only in the lengths of alkyl chains on terminal aniline substituents, show 50–62 nm bathochromic shifts in emission maxima in response to mechanical force (mechanofluorochromism, MC). These shifts are fully reversible with heat or solvent fuming. Shearing of these solids yields a transition from green-emitting crystalline phases to orange-emitting amorphous phases as established by differential scanning calorimetry and X-ray diffraction. Molecules with shorter alkyl chain lengths required higher temperatures to recover the hypsochromically shifted crystalline phases after grinding, while the recovery with chain lengths longer than butyl occurred at room temperature. In addition to this structure-dependent thermochromism, these compounds retain their MC properties in polymer hosts to various extents. The crystalline phases of these materials have PE chromophores that are twisted due to non-covalent perfluoroarene–arene (ArF–ArH) interactions involving perfluorophenyl pendants and the terminal rings of the PE chromophore, resulting in interrupted conjugation and an absence of chromophore aggregation. The MC behavior of an analog without the perfluoroarene rings is severely attenuated. This work demonstrates the general utility of twisted PEs as stimuli-responsive moieties and reveals clear structure–property relationships regarding the effects of alkyl chain length on these materials.

Purpose/Objective(s) Patients receiving concurrent chemo/radiation therapy (CRT) for non-small cell (NSCLC) or small cell (SCLC) lung carcinoma are at risk for severe acute esophagitis (AE) (≥ grade (G) 3). We sought to compare AE in a cohort of patients treated with an empirically-derived contralateral esophagus sparing technique (CEST) to historical patients treated without this technique. This data will inform nurses and nurse practitioners when anticipating and managing CRT side effects in lung cancer patients. Materials/Methods In this retrospective IRB-approved study, we reviewed the records of patients with locally advanced (stage III by AJCC 7th ed +/- solitary brain metastasis) NSCLC or limited-stage SCLC who had gross tumor within 1 cm of the esophagus. All patients were planned with intensity-modulated radiation therapy (IMRT) to a prescription dose of at least 60 Gy with concurrent chemotherapy. Cohort A represents a secondary analysis of an institutional phase I trial that enrolled patients during 7/2015-2/2019. Cohort B consists of consecutive patients treated with standard of care IMRT during 7/2008-12/2012, before the implementation of CEST at our institution. We tabulated patient, tumor, and treatment characteristics and graded AE using CTCAEv4 from baseline to 3-month post-treatment follow-up appointments. Results We identified a total of 52 CRT patients (A, n = 26; B, n = 26). Median ages for cohorts A and B were 67 and 69.5 years, respectively. In each cohort there were 10 females and 16 males. Stage distribution was: IIIA 50%, IIIB 42%, IV 8% for cohort A and IIIA 65%, IIIB 35% for cohort B. In cohorts A and B, median radiation dose was 70 Gy (range 68-70 Gy) and 68.7 Gy (range 45-74 Gy), respectively. Overall rates of G2+ AE in cohorts A and B were 31% (n = 8) and 58% (n = 15) (P = 0.03), respectively. This difference was evident by week 4 of CRT when for cohorts A and B the rates were 15% and 35%, respectively. By 3 months post-treatment, each cohort had 1 patient with G2+ AE. There were no G4/5 events, and the only G3 AE was in cohort B leading to early termination of treatment at 52.2 Gy. Twelve percent (n = 3) of patients in cohort A and 27% (n = 7) of patients in cohort B required IV fluids. Forty six percent (n = 12) patients in cohort A and 62% (n = 16) of patients in cohort B required diet adjustments, while 27% (n = 7) of patients in cohort A and 73% (n = 19) of patients in cohort B required pain medications. Overall, CEST allowed patients to maintain a more normal diet, spend less time receiving IV fluids, and take fewer pain medications. Conclusion Advances in IMRT-based treatment techniques have enabled a considerable reduction in G2-3 AE rates. Reduced AE is expected to lead to improved quality of life and treatment adherence. Nurses and nurse practitioners can use this data to perform a more focused assessment, create patient-specific interventions, and streamline patient education.

We report a directly photopatternable polythiophene derivative PToNB with o-nitrobenzyl (oNB)-functionalized side chains. PToNB has unique phototunable solubilities programmed through three stages: (i) organic-soluble/aqueous-insoluble, (ii) organic-insoluble/aqueous-insoluble, and (iii) organic-insoluble/aqueous soluble. The capability to control conjugated polymer solubility with spatiotemporal precision and orthogonal developer solvents through three stages allows for direct patterning of this conjugated polymer and provides flexibility to choose between positive and negative tone photolithography. This approach to photomodulate solubility also enables all-solution processing of multilayer stacked conjugated polymer films; we demonstrate here direct two-layer photopatterning with this novel conjugated polymer photoresist.

Although fluorescent probes for the detection of singlet oxygen (1O2) have been an active area of research, most such probes rely upon change in intensity of a single band. Herein, we report a FRET-based, 1O2-sensitive aqueous suspension of conjugated polymer nanoparticles (CPNs) comprising the energy-donating host polymer poly[{9,9-dioctyl-2,7-divinylene-fluorenylene}-alt-co-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene}] doped with an energy acceptor, the 1O2-sensitive thienoacene 5,12-bis(4-methoxyphenyl)-2-butyltetraceno[2,3-b]thiophene. Using a phthalocyanine-based sensitizer, IRDye 700DX, our probe shows a rapid, ratiometric response to photosensitized 1O2 in water in both cuvettes and 96-well plates that compares favorably to the commercial 1O2-sensitive dye, singlet oxygen sensor green. The response to irradiation of even nanomolar concentrations of photosensitizer demonstrates the sensitivity of our ratiometric probe.

We report photolabile polyelectrolyte multilayers (PEMs), produced by the layer-by-layer (LbL) approach, which comprise commercially available photoinert polyanions and photolabile polycations. Photochemical degradation of synthetic photolabile polycations renders these PEMs soluble in near neutral aqueous solutions. Taking advantage of the vast array of available photocleavable chromophores that absorb different wavelengths of light with different quantum yields of photolysis, we designed four photolabile polycations (P1–P4), each containing different photocleavable groups: dialkylaminocoumarin group (P1), different nitrobenzyl groups (P2 and P3), and methoxyphenacyl group (P4). PEMs containing different chromophores dissolved selectively under different irradiation conditions. Sequential photocontrolled dissolution of multilayers from a hybrid, quadruple-compartment LBL film was demonstrated using this approach, as was photolithographic patterning of films at multiple heights using different irradiation...

Layer-by-layer (LbL) self-assembly of polymer coatings is a bottom-up fabrication technique with broad applicability across a wide range of materials and applications that require control over interfacial properties. While most LbL coatings are chemically uniform in directions both tangent and perpendicular to their substrate, control over the properties of surface coatings as a function of space can enhance their function. To contribute to this rapidly advancing field, our group has focused on the top-down spatiotemporal control possible with photochemically reactive LbL coatings, harnessed through charge-shifting polyelectrolytes enabled by photocleavable ester pendants. The photolysis of the photocleavable esters degrades LbL films containing these polyelectrolytes. The chemical structures of the photocleavable groups dictate the wavelengths responsible for disrupting these coatings, ranging from ultraviolet to near-infrared in our work. In addition, spatially segregating reactive groups into "compartments" within LbL films has enabled us to fabricate reactive free-standing polymer films and multiheight photopatterned coatings. Overall, by combining bottom-up and top-down approaches, photoreactive LbL films enable precise control over the interfacial properties of polymer and composite coatings.

Photochemical oxidation of acenes can benefit or impede their function, depending on the application. Although acenes with alkoxy substituents on reactive sites are important for applications as diverse as drug delivery and organic optoelectronics, the influence of chemical structure on their photochemical oxidation remains unknown. This paper therefore describes the synthesis, spectroscopic properties, and reactivity with singlet oxygen (1 O2 ) of a series of dialkoxyacenes that vary in the number and types of fused rings in the (hetero)acene cores. Reductive alkylation of quinone precursors yielded target dialkoxyacenes with fused backbones ranging from benzodithiophene to tetracenothiophene. Trends of their experimental spectroscopic characteristics were consistent with time-dependent density functional theory (TD-DFT) calculations. NMR studies show that photochemically generated 1 O2 oxidizes all but one of these acenes to the corresponding endoperoxides in organic solvent. The rates of these oxidations correlate with the number and types of fused arenes, with longer dialkoxyacenes generally oxidizing faster than shorter derivatives. Finally, irradiation of these acenes in acidic, oxidizing environments cleaves the ether bonds. This work impacts those working in organic optoelectronics, as well as those interested in harnessing photogenerated reactive oxygen species in functional materials.

This article describes several projects in our laboratory relating to stimuli-responsive conjugated materials using functional side chain moieties. Although side chain engineering has become increasingly important in controlling the packing of conjugated materials, these groups have typically been reserved for imparting solubility to otherwise insoluble materials. By incorporating solubilizing side chains through photocleavable nitrobenzyl linkers, new conjugated polymers behave as negative photoresists upon exposure to ultraviolet light with minimal photobleaching. Our approach enables photopatterning and solution-based fabrication of multilayer conjugated polymer films without the use of orthogonally soluble materials. Work to understand the electronic effects of the aromatic side chain substituents in these materials led to the subsequent discovery of aromatic interactions between side chains and main chains that control the conformations of solid-state phenylene-ethynylene oligomers, including highly twisted conjugated backbones. Such materials, when appropriately substituted, display reversible piezochromic and mechanofluorochromic behavior.

We report the synthesis of amphiphilic polymers featuring lipophilic stearyl chains and hydrophilic poly(ethylene glycol) (PEG) polymers that are connected through singlet oxygen-cleavable alkoxyanthracene linkers. These amphiphilic polymers assembled in water to form micelles with diameters of (~)20 nm. Reaction of the alkoxyanthracene linkers with light and O(2) cleaved the ether C-O bonds, resulting in formation of the corresponding 9,10-anthraquinone derivatives and concomitant disruption of the micelles. These micelles were loaded with the chemotherapeutic agent doxorubicin, which was efficiently released upon photo-oxidation. The drug-loaded reactive micelles were effective at killing cancer cells in vitro upon irradiation at 365 nm, functioning through both doxorubicin release and photodynamic mechanisms.

This paper describes two-dimensional conjugated poly(phenylene–ethynylene)s (PPEs) linked to singlet oxygen-reactive diethynyltetracene units through phenylene–ethynylene (PE) bridges that are conjugated to the polymer backbones. Successive Sonogashira coupling reactions enabled the preparation of the tetracene-containing PE pendants, the linking of the pendants to the monomer unit, and the polymerization reactions. Small molecule models of the polymers showed ratiometric responses of fluorescence upon exposure to photogenerated singlet oxygen due to cycloaddition reactions increasing the excited state of individual chromophores. The fluorescent responses of the tetracene-linked PPEs, however, showed markedly different behavior: (i) an initial increase in fluorescence intensity and fluorescence lifetime of the side-chain tetracenes upon exposure to 1O2 indicated an analyte-induced slowing of self-quenching due to high local acene concentration, and (ii) ratiometric blue-shifted response did not occur unti...

This paper describes a series of thiophene-based conjugated polymers that become insoluble upon irradiation with ultraviolet light. Stille or Suzuki reactions of appropriately substituted 2,5-bromothiophene derivatives yielded terthiophene and polythiophene derivatives with either o-nitrobenzyl (ONB) ester or ONB ether photolabile side chains with n-octyl substituents. Light-induced cleavage of these ONB side chains with ultraviolet light at 365 nm cleaves the octyl chains responsible for solubilization of the polymers away from the conjugated main chains, rendering them insoluble. Consistent with the accepted mechanism of ONB photolysis, those structural modifications that would yield a more stable benzylic radical—methyl substitution on the benzylic position, replacement of the ester with an ether, or both—yielded more efficient photolyses as determined by (i) quantum yields of photolysis of ONB-substituted terthiophenes, and (ii) the percentage of polymer that persists in UV-irradiated thin films upon ...

This paper describes a series of four ortho-nitrobenzyl substituted bis(norbornene) crosslinkers that are suitable for the preparation of photoresponsive organogels and hydrogels through ring-opening metathesis polymerization. As measured by several techniques—visual inspection, rheology, and release of network-bound polymers—organogels preapred using these four crosslinkers varied in their sensitivity to UV irradiation by about two orders of magnitude. The reactivity of the gels show qualitative correlation with the stability of the intermediate benzylic radicals. Gels with larger crosslink densities required longer UV irradiation time to dissolve than gels with smaller crosslink densities. Together, these results demonstrate how rational changes to the structures of crosslinkers and gels can tune photosensitivity. Finally, we show a proof-of-concept photochemical release experiment of a physically entrapped fluorescent polymer from a photolabile ROMP hydrogel.

This paper describes polyelectrolyte multilayer films prepared by the layer-by-layer (LbL) technique capable of undergoing dissolution upon exposure to either ultraviolet or near-infrared light. Film dissolution is driven by photochemical deprotection of a random methacrylic copolymer with two types of side chains: (i) 6-bromo-7-hydroxycoumarinyl esters, photocleavable groups that are known to have substantial two-photon photolysis cross sections, and (ii) cationic residues from the commercially available monomer N,N-dimethylaminoethyl methacrylate (DMAEMA). In addition, the dependence of stability of both unirradiated and irradiated films on pH provides experimental evidence for the necessity of disrupting both ion-pairing and hydrophobic interactions between polyelectrolytes to realize film dissolution. This work therefore provides both new fundamental insight regarding photolabile LbL films and expands their applied capabilities to nonlinear photochemical processes.

May the force be with you: Xia, Burns, Martinez, and co-workers harnessed the ring strain of ladderenes to enable both their polymerization and mechanochemical unzipping to yield semiconducting polyacetylene-based block copolymers. These materials have promise as functional polymers for applications such as detection of physical stress.

Although understanding the conformations and arrangements of conjugated materials as solids is key to their prospective applications, predictive power over these structural factors remains elusive. In this work, substituent effects tune non-covalent interactions between side-chain fluorinated benzyl esters and main-chain terminal arenes, in turn controlling the conformations and interchromophore aggregation of three-ring phenylene-ethynylenes (PEs). Cofacial fluoroarene-arene (ArF-ArH) interactions cause twisting in the PE backbone, interrupting intramolecular conjugation as well as blocking chromophore aggregation, both of which prevent the typically observed bathochromic shift observed upon transitioning PEs from solution to solid. This work highlights two structural factors that determine whether the ArF-ArH interactions, and the resulting twisted, unaggregated chromophores, occur in these solids: (i) the electron-releasing characteristic of substituents on ArH, with more electron-releasing character favoring ArF-ArH interactions, and (ii) the fluorination pattern of the ArF ring, with 2,3,4,5,6-pentafluorophenyl favoring ArF-ArH interactions over 2,4,6-trifluorophenyl. These trends indicate that considerations of electrostatic complementarity, whether through a polar-π or substituent-substituent mechanism, can serve as an effective design principle in controlling the interaction strengths, and therefore the optoelectronic properties, of these molecules as solids.

In this study, we present a novel self-cleaning, photoresponsive membrane that is capable of removing predeposited foulant layers upon changes in surface morphology in response to UV or visible light irradiation while maintaining stable pore size and water permeance. These membranes were prepared by creating thin film composite (TFC) membranes by coating a porous support membrane with a thin layer of novel comb-shaped graft copolymers at two side-chain lengths featuring polyacrylonitrile (PAN) backbones and photoreactive side chains, synthesized by atom transfer radical polymerization (ATRP). Photoregulated control over membrane properties is attained through a light-induced transition, where the side chains switch between a hydrophobic spiropyran (SP) state and a zwitterionic, hydrophilic merocyanine (MC) state. The light-induced switch between the SP and MC forms changes surface hydrophilicity and causes morphological changes on the membrane surface as evidenced by atomic force microscopy (AFM). Before any phototreatment, the as-coated membrane surface comprises mostly hydrophobic SP groups that allow the adsorption of organic solutes such as proteins the membrane surface, reducing flow rate. Once exposed to UV light, conversion of the SP groups to hydrophilic MC groups leads to the release of adsorbed molecules and the full recovery of the initial water flux. A fouled membrane in the more hydrophilic MC form is also capable of self-cleaning upon conversion to the less hydrophilic SP form by visible light irradiation. The self-cleaning behavior observed for this system, where the surface became less hydrophilic but also experienced a morphological change, demonstrates a novel mechanism that has a mechanical component in addition to the changes in hydrophilicity. It is also the first report, to our knowledge, of self-cleaning performance accompanied by a decrease in hydrophilicity.

This paper describes the synthesis and systematic study of substituted acenes that have differences in conjugation both along their long axes (by the number of fused benzene or thiophene rings) and along their short axes (by the number of arylethynyl substituents). These acenes include what we believe to be the first reported examples of five new subclasses of substituted acenes. Systematic analyses of data obtained using absorbance and fluorescence spectroscopies, cyclic voltammetry, and DFT calculations reveal clear correlations between these common structural perturbations to acene structure and the key parameters, such as HOMO-LUMO gap, frontier molecular orbital energies, and reactivity with singlet oxygen.

Controlling the optical properties of conjugated materials, especially their bandgaps, is critical to nearly all of applications of these materials. The most prevalent strategy involves changes to the structures of conjugated backbones, while side chains are generally reserved for imparting solubility. This paper, using a series of donor–acceptor conjugated oligo- and poly(arylene–ethynylene)s that have terephthalate units as the electron-deficient unit, demonstrates examples of how the structures of side chains that are not formally part of the conjugated backbone can have significant effects on bandgaps of these materials. In organic solution, changing alkoxy substituents on the terephthalate unit yields changes in absorbance onsets of, in some cases, greater than 20 nm; the position of absorbance spectra of these materials correlates with the Taft σ* values of the ester alkoxy groups, consistent with the side chains inductively altering the electron-accepting nature of the terephthalate ring. This stru...

This paper describes the reactivity of unsymmetrically substituted pentacenes to photochemical oxidation. Acenes in general, and pentacenes in particular, are a key family of compounds for a variety of organic electronics applications. The instability of many pentacene derivatives, particularly to oxidation, is an important restriction in their applicability. Several substitution strategies for decreasing the reactivity of pentacene exist, but these almost always involve symmetrically substituted derivatives, restricting the chemical space of structures from which to choose. In this paper, we demonstrate that combining electronic and steric effects yields highly stable unsymmetrically substituted pentacenes.

This paper describes photosensitive polyelectrolyte multilayers (PEMs), constructed using layer-by-layer (LbL) self-assembly, which contain polymers that comprise combinations of photocleavable methacrylate esters and photoinert cationic methacrylate esters. The solubility of these polymers in aqueous base depends upon irradiation with either ultraviolet or visible light. PEMs with dialkylaminocoumarin groups dissolve upon irradiation with visible light, while analogous films with nitrobenzyl ester-substituted polymers dissolve upon irradiation with UV light. Wavelength-selective release of fluorescent polymers as guests is demonstrated using this approach.

This paper describes a series of three-ring phenylene-ethynylenes (PEs) in which specific, non-covalent arene–arene interactions control conformation in the solid-state. As determined by single crystal X-ray structures, edge-face interactions between benzyl ester side chains and conjugated main chains are observed. In contrast, perfluorobenzyl ester side chains interact cofacially with main chains, resulting in ∼60° torsional angles between neighboring aryl rings in crystalline PEs. Absorbance and fluorescence spectra of films of these compounds reflect these conformational effects, with the spectra of perfluorobenzyl-substituted compounds shifting hypsochromically from solution- to solid-state. In a demonstration of how balancing non-covalent interactions can open the way to new responsive materials, a main chain twisted derivative with octyloxy substituents displayed significant piezochromic behavior.

This paper describes conjugated polymers with 2,5-diarylfuran moieties as nonconjugated pendants that respond to singlet oxygen (1O2) by fluorescence quenching. By oxidizing the diarylfurans to more electron-poor moieties such as enol esters, singlet oxygen causes poly(fluorene-co-phenylene) conjugated backbones to donate excited electrons to the oxidized pendants, resulting in quenching of up to 93% of the initial fluorescence of the polymer, while an analogous furan-substituted poly(phenylene–ethynylene) showed no quenching upon furan–1O2 reaction. All furan derivatives reacted rapidly with 1O2 (k = 107–108 M–1 s–1), with more electron-poor furans reacting more slowly, but yielding more efficient fluorescence quenching, than less electron-poor furans.

Combining the numerous advantages of using light as a stimulus, simple free radical random copolymerization, and the easy, all-aqueous preparation of polyelectrolyte complexes (PECs), we prepared photolabile PEC nanoparticles and demonstrated their rapid degradation under UV light. As a proof of concept demonstration, the dye Nile Red was encapsulated in the PECs and successfully released into the surrounding solution as the polyelectrolyte nanocomplex carriers dissolved upon light irradiation.

This account describes progress in the author’s laboratory in the area of new fluorescent polymers that respond to the reactive oxygen species singlet oxygen (1O2). Key to the development of these materials are the [4+2] cycloaddition reactions between singlet oxygen and dienes such as acenes and furans. When covalently bound to conjugated polymer backbones, cycloadditions of these dienes with singlet oxygen can yield dramatic changes in the wavelength and intensity of luminescence: three such examples are given here. The account also summarizes our work to understand how changing the chemical structures of acenes affects reactivity with singlet oxygen as well as the cycloreversion of the resulting endoperoxides. 1 Introduction 2 Motivation 3 Organic Soluble Diene-Linked Conjugated Polymers 4 Red-Shifting Woes Lead to Reversibility 5 Building a Database 6 Integrating Other Acenes 7 Conclusion and Looking Forward

This article describes progress in the author's laboratory in two new areas of photoresponsive polymers: (1) photochemical control of electrostatics, and (2) conjugated materials with photocleavable pendants. In both cases, incorporation of o-nitrobenzyl esters (NBEs) imparts this photoreactivity. NBE-derivatized, ring-opening metathesis polymerization (ROMP)-derived polycations form positive photoresist layer-by-layer (LbL) films that dissolve in base upon irradiation. NBE-derivatized conjugated oligomers show photoactivated fluorescence, whereas NBE-substituted polythiophenes are negative photoresists due to photolysis of solubilizing alkyl chains.

The hydrophobic effect—a rationalization of the insolubility of nonpolar molecules in water—is centrally important to biomolecular recognition. Despite extensive research devoted to the hydrophobic effect, its molecular mechanisms remain controversial, and there are still no reliably predictive models for its role in protein–ligand binding. Here we describe a particularly well-defined system of protein and ligands—carbonic anhydrase and a series of structurally homologous heterocyclic aromatic sulfonamides—that we use to characterize hydrophobic interactions thermodynamically and structurally. In binding to this structurally rigid protein, a set of ligands (also defined to be structurally rigid) shows the expected gain in binding free energy as hydrophobic surface area is added. Isothermal titration calorimetry demonstrates that enthalpy determines these increases in binding affinity, and that changes in the heat capacity of binding are negative. X-ray crystallography and molecular dynamics simulations are compatible with the proposal that the differences in binding between the homologous ligands stem from changes in the number and organization of water molecules localized in the active site in the bound complexes, rather than (or perhaps in addition to) release of structured water from the apposed hydrophobic surfaces. These results support the hypothesis that structured water molecules—including both the molecules of water displaced by the ligands and those reorganized upon ligand binding—determine the thermodynamics of binding of these ligands at the active site of the protein. Hydrophobic effects in various contexts have different structural and thermodynamic origins, although all may be manifestations of the differences in characteristics of bulk water and water close to hydrophobic surfaces.

This paper describes controlled ring-opening metathesis polymerizations (ROMP) of norbornene derivatives that bear photoreactive o-nitrobenzyl ester (NBE) moieties. Termination reactions have frustrated previous reported attempts to execute controlled radical polymerizations with nitrobenzyl ester-containing acrylate derivatives. We therefore prepared new NBE-containing norbornene derivatives, and found that polymerization with Grubbs’s ruthenium carbene catalysts led to complete consumption of monomer in minutes to hours and yielded polymers with predictable molecular weights. We also demonstrate the preparation of NBE-containing block copolymers and show through UV irradiation of these polymers in solution and in thin film that the NBE moieties remain photoreactive.

An infofuse is a combustible fuse in which information is encoded through the patterning of metallic salts, with transmission in the optical range simply associated with burning. The constraints, advantages and unique error statistics of physical chemistry require us to rethink coding and decoding schemes for these systems. We take advantage of the non-binary nature of our signal with a single bit representing one of N =7 states to produce a code that, using a single or pair of intensity thresholds, allows the recovery of the intended signal with an arbitrarily high recovery probability, given reasonable assumptions about the distribution of errors in the system. An analysis of our experiments with infofuses shows that the code presented is consistent with these schemes, and encouraging for the field of chemical communication and infochemistry given the vast permutations and combinations of allowable non-binary signals.

This paper describes a method for determining the location of contact electrification-induced electrical discharges detected in a system comprising a steel sphere rolling in a circular path on an organic insulator. The electrode of the “rolling sphere tool” monitors, in real time, the separation of charge between the sphere and the organic insulator and the resultant electrostatic discharges. For every revolution of the sphere, the electrometer records a peak, the height of which represents the amount of charge on the sphere. As the charge on the sphere accumulates, the resulting electric field at the surface of the sphere eventually exceeds the breakdown limit of air and causes a discharge. The position of this discharge can be inferred from the relative amplitudes and positions of the peaks preceding and following the discharge event. We can localize each discharge event to one of several zones, each of which corresponds to a geometrically defined fraction of the circular path of the sphere. The fractio...