Poly(dimethylsiloxane) as a room-temperature solid solvent for photophysics and photochemistry.

Medium viscosity strongly affects the dynamics of solvated species and can drastically alter the deactivation pathways of their excited states. This study demonstrates the utility of poly(dimethylsiloxane) (PDMS) as a room-temperature solid-state medium for optical spectroscopy. As a thermoset elastic polymer, PDMS is transparent in the near ultraviolet, visible, and near infrared spectral regions. It is easy to mould into any shape, forming surfaces with a pronounced smoothness. While PDMS is broadly used for the fabrication of microfluidic devices, it swells in organic solvents, presenting severe limitations for the utility of such devices for applications employing non-aqueous fluids. Nevertheless, this swelling is reversible, which proves immensely beneficial for loading samples into the PDMS solid matrix. Transferring molecular-rotor dyes (used for staining prokaryotic cells and amyloid proteins) from non-viscous solvents into PDMS induces orders-of-magnitude enhancement of their fluorescence quantum yield and excited-state lifetimes, providing mechanistic insights about their deactivation pathways. These findings demonstrate the unexplored potential of PDMS as a solid solvent for optical applications.

Fluorinated aminoanthranilamides: non-native amino acids for bringing proteomic approaches to charge-transfer systems.

The ability to control charge transfer at molecular and nanometer scales represents the ultimate level of electronic mastery, and its impacts cannot be overstated. As electrostatic analogues of magnets, electrets possess ordered electric dipoles that present key paradigms for directing transduction of electrons and holes. Herein we describe the design and development of fluorinated aminoanthranilamides, derivatives of non-native aromatic beta-amino acids, as building blocks for hole-transfer molecular electrets. A highly regio-selective nucleophilic aromatic substitution of difluorinated nitrobenzoic acid provides the underpinnings for an array of unprecedented anthranilamide structures. Spin density distribution and electrochemical analyses reveal that fluorine induces about 200 mV positive shifts in reduction potentials without compromising the stability of the oxidized residues, making them invaluable building blocks for hole-transfer systems. These findings open unexplored routes to novel amino-acid structures, setting a foundation for bringing principles of proteomics to designs of charge-transfer systems.

Dipole-mediated rectification of intramolecular photoinduced charge separation and charge recombination.

Controlling charge transfer at a molecular scale is critical for efficient light harvesting, energy conversion, and nanoelectronics. Dipole-polarization electrets, the electrostatic analogue of magnets, provide a means for "steering" electron transduction via the local electric fields generated by their permanent electric dipoles. Here, we describe the first demonstration of the utility of anthranilamides, moieties with ordered dipoles, for controlling intramolecular charge transfer. Donor-acceptor dyads, each containing a single anthranilamide moiety, distinctly rectify both the forward photoinduced electron transfer and the subsequent charge recombination. Changes in the observed charge-transfer kinetics as a function of media polarity were consistent with the anticipated effects of the anthranilamide molecular dipoles on the rectification. The regioselectivity of electron transfer and the molecular dynamics of the dyads further modulated the observed kinetics, particularly for charge recombination. These findings reveal the underlying complexity of dipole-induced effects on electron transfer and demonstrate unexplored paradigms for molecular rectifiers.

Anthranilamides as bioinspired molecular electrets: experimental evidence for a permanent ground-state electric dipole moment.

As electrostatic equivalents of magnets, organic electrets offer unparalleled properties for impacting energy conversion and electronic applications. While biological systems have evolved to efficiently utilize protein α-helices as molecular electrets, the synthetic counterparts of these conjugates still remain largely unexplored. This paper describes a study of the electronic properties of anthranilamide oligomers, which proved to be electrets based on their intrinsic dipole moments as evident from their spectral and dielectric properties. NMR studies provided the means for estimating the direction of the intrinsic electric dipoles of these conjugates. This study sets the foundation for the development of a class of organic materials that are de novo designed from biomolecular motifs and possess unexplored electronic properties.

Hybrid low resistance ultracapacitor electrodes based on 1-pyrenebutyric acid functionalized centimeter-scale graphene sheets.

Ultracapacitors are promising candidates for alternative energy storage applications since they can store and deliver energy at relatively high rates. Here, we present hybrid nanocarbon ultracapacitor electrodes with a low equivalent series resistance (ESR) of 7 ohms. 1-pyrenebutyric acid treated large-area single layer graphene (SLG) sheets covered with shortened multi-walled carbon nanotubes (MWNTs) have been utilized as highly conductive and percolated networks of hybrid carbon nanomaterial composites or thin films as ultracapacitor electrodes. Uniform centimeter scale single layer graphene sheets were produced via low pressure chemical vapor deposition using copper foil substrates and then subsequently modified by 1-pyrenebutyric acid functionalization. Chemically shortened MWNTs ranging in length of 200-500 nm, were deposited by drop casting on 1-pyrenebutyric acid functionalized SLG films. SLG/MWNT nancomposite hybrid films of different thicknesses were obtained by controlling the density of MWNT suspension. Surface morphology and nanostructure of the hybrid nanocomposites indicated relatively dense and homogeneous web-like networks. Specific capacitance values of the hybrid electrodes were substantially increased by 200% compared to those ultracapacitors fabricated using buckypaper electrodes. Average values of specific capacitance and energy density obtained were 140.64 F/g and 21.54 Wh/kg respectively. SLG/MWNT nanocomposite electrodes are very promising for future ultracapacitor devices with their low ESR value that is 95% lower than that of buckypaper based ultracapacitors.

Supercapacitors based on pillared graphene nanostructures.

We describe the fabrication of highly conductive and large-area three dimensional pillared graphene nanostructure (PGN) films from assembly of vertically aligned CNT pillars on flexible copper foils for applications in electric double layer capacitors (EDLC). The PGN films synthesized via a one-step chemical vapor deposition process on flexible copper foils exhibit high conductivity with sheet resistance as low as 1.6 ohms per square and possessing high mechanical flexibility. Raman spectroscopy indicates the presence of multi walled carbon nanotubes (MWCNT) and their morphology can be controlled by the growth conditions. It was discovered that nitric acid treatment can significantly increase the specific capacitance of the devices. EDLC devices based on PGN electrodes (surface area of 565 m2/g) demonstrate enhanced performance with specific capacitance value as high as 330 F/g extracted from the current density-voltage (CV) measurements and energy density value of 45.8 Wh/kg. The hybrid graphene-CNT nanostructures are attractive for applications including supercapacitors, fuel cells and batteries.

Naphthalene-fused metallo-porphyrins--synthesis and spectroscopy.

Oxidative aromatic coupling of meso-substituted porphyrins bearing one electron-rich naphthalene unit has been studied in detail. After thorough optimization of oxidant, naphthalene-fused porphyrins were prepared in high yield without contamination from chlorinated side-products using Fe(ClO(4))(3)·2H(2)O. Copper and nickel complexes were successfully transformed into π-expanded porphyrins in 40-83% yield.

Kinetics of bacterial fluorescence staining with 3,3'-diethylthiacyanine.

For more than a century, colorimetric and fluorescence staining have been the foundation of a broad range of key bioanalytical techniques. The dynamics of such staining processes, however, still remains largely unexplored. We investigated the kinetics of fluorescence staining of two gram-negative and two gram-positive species with 3,3'-diethylthiacyanine (THIA) iodide. An increase in the THIA fluorescence quantum yield, induced by the bacterial dye uptake, was the principal reason for the observed emission enhancement. The fluorescence quantum yield of THIA depended on the media viscosity and not on the media polarity, which suggested that the microenvironment of the dye molecules taken up by the cells was restrictive. The kinetics of fluorescence staining did not manifest a statistically significant dependence neither on the dye concentration, nor on the cell count. In the presence of surfactant additives, however, the fluorescence-enhancement kinetic patterns manifested species specificity with statistically significant discernibility.

Long-lived photogenerated states of alpha-oligothiophene-acridinium dyads have triplet character.

Photoinduced processes, leading to charge-transfer states with extended lifetimes, are of key importance for solar-energy-conversion applications. Utilizing external heavy-atom effect allowed us to photogenerate long-lived transients of electron donor-acceptor dyads. For an electron acceptor and a principal chromophore of the dyads, we selected N-methylacridinium, and for electron donors thiophene, bithiophene, and terthiophene were selected. While the photoinduced charge transfer, mediated by the investigated dyads, occurred in the picosecond time domain, the lifetime of the transients extended to the microsecond time domain. We ascribed the relatively long lifetimes to the triplet character of the observed transients. An increase in the size of the donor lowered the energy of the charge-transfer states of the dyads. When the energy level of the acridinium triplet lies below the energy level of the charge-transfer state, the locally excited triplet accounted for the long-lived transient. For the conjugates with charge-transfer states lying below all other excited states, the long-lived transients were, indeed, the charge-transfer species.

Electrochemical oxidation of ferrocene: a strong dependence on the concentration of the supporting electrolyte for nonpolar solvents.

The estimation of the driving force for photoinduced charge-transfer processes, using the Rehm-Weller equation, requires the employment of redox and spectroscopic quantities describing the participating electron donor and acceptor. Although the spectroscopic data are usually obtained from diluted solutions, the redox potentials are most frequently obtained from electrochemical measurements conducted in concentrated electrolyte solutions. To correct for the differences in the media, in which the various types of measurements are conducted, a term, based on the Born equation for solvation energy of ions, is introduced in the Rehm-Weller equation. The Born correction term, however, requires a prior knowledge of the dielectric constants of the electrolyte solutions used for the redox measurements. Because of limited information for such dielectrics, the values for the dielectric constants of electrolyte solutions are approximated to the values of the dielectric constants of the corresponding neat solvents. We examined the validity of this approximation. Using cyclic voltammetry, we recorded the first one-electron oxidation potential of ferrocene for three different solvents in the presence of 1-500 mM supporting electrolyte. The dielectric constants for some of the electrolyte solutions were extracted from fluorescence measurements of a dimethylaminonaphthalimide chromophore that exhibits pronounced solvatochromism. The dielectric constants of the concentrated electrolyte solutions correlated well with the corresponding oxidation potentials. The dependence of the oxidation potential of ferrocene on the electrolyte concentration for different solvents revealed that the abovementioned approximation in the Born correction term indeed introduces a significant error in the estimation of the charge-transfer driving force from redox data collected using relatively nonpolar solvents.

How Do Amides Affect the Electronic Properties of Pyrene?

The electronic properties of amide linkers, which are intricate components of biomolecules, offer a wealth of unexplored possibilities. Herein, we demonstrate how the different modes of attaching an amide to a pyrene chromophore affect the electrochemical and optical properties of the chromophore. Thus, although they cause minimal spectral shifts, amide substituents can improve either the electron-accepting or electron-donating capabilities of pyrene. Specifically, inversion of the amide orientation shifts the reduction potentials by 200 mV. These trends indicate that, although amides affect to a similar extent the energies of the ground and singlet excited states of pyrene, the effects on the doublet states of its radical ions are distinctly different. This behavior reflects the unusually strong orientation dependence of the resonance effects of amide substituents, which should extend to amide substituents on other types of chromophores in general. These results represent an example where the Hammett sigma constants fail to predict substituent effects on electrochemical properties. On the other hand, Swain-Lupton parameters are found to be in good agreement with the observed trends. Examination of the frontier orbitals of the pyrene derivatives and their components reveals the underlying reason for the observed amide effects on the electronic properties of this polycyclic aromatic hydrocarbon and points to key molecular-design strategies for electronic and energy-conversion systems.

Photoinduced dynamics of a cyanine dye: parallel pathways of non-radiative deactivation involving multiple excited-state twisted transients.

Cyanine dyes are broadly used for fluorescence imaging and other photonic applications. 3,3'-Diethylthiacyanine (THIA) is a cyanine dye composed of two identical aromatic heterocyclic moieties linked with a single methine, -CH[double bond, length as m-dash]. The torsional degrees of freedom around the methine bonds provide routes for non-radiative decay, responsible for the inherently low fluorescence quantum yields. Using transient absorption spectroscopy, we determined that upon photoexcitation, the excited state relaxes along two parallel pathways producing three excited-state transients that undergo internal conversion to the ground state. The media viscosity impedes the molecular modes of ring rotation and preferentially affects one of the pathways of non-radiative decay, exerting a dominant effect on the emission properties of THIA. Concurrently, the polarity affects the energy of the transients involved in the decay pathways and further modulates the kinetics of non-radiative deactivation.

Correction: Photoinduced dynamics of a cyanine dye: parallel pathways of non-radiative deactivation involving multiple excited-state twisted transients.

[This corrects the article DOI: 10.1039/C4SC02881C.].

Permanent electric dipole moments of carboxyamides in condensed media: what are the limitations of theory and experiment?

Electrostatic properties of proteins are crucial for their functionality. Carboxyamides are small polar groups that, as peptide bonds, are principal structural components of proteins that govern their electrostatic properties. We investigated the medium dependence of the molar polarization and of the permanent dipole moments of amides with different state of alkylation. The experimentally measured and theoretically calculated dipole moments manifested a solvent dependence that increased with the increase in the media polarity. We ascribed the observed enhancement of the amide polarization to the reaction fields in the solvated cavities. Chloroform, for example, caused about a 25% increase in the amide dipole moments determined for vacuum, as the experimental and theoretical results demonstrated. Another chlorinated solvent, 1,1,2,2-tetrachloroethane, however, caused an "abnormal" increase in the experimentally measured amide dipoles, which the theoretical approaches we used could not readily quantify. We showed and discussed alternatives for addressing such discrepancies between theory and experiment.

Electrochemical reduction of quinones: interfacing experiment and theory for defining effective radii of redox moieties.

Using cyclic voltammetry, we examined the dependence of the reduction potentials of six quinones on the concentration of the supporting electrolyte. An increase in the electrolyte concentration, resulting in an increase in the solution polarity, caused positive shifts of the reduction potentials. We ascribed the observed changes in the potentials to the dependence of the solvation energy of the quinones and their anions on the media polarity. Analysis of the reduction potentials, using the Born solvation energy equation, yielded unfeasibly small values for the effective radii of the quinone species, that is, the experimentally obtained effective radii were up to 4-fold smaller than the radii of the solvation cavities that we calculated for the quinones. The nonspherical shapes of the quinones, along with the uneven charge density distribution in their anions, encompassed the underlying reasons for the discrepancies between the obtained experimental and theoretical values for the radii of these redox species. The generalized Born approach, which does not treat the solvated species as single spheres, provided means for addressing this discrepancy and yielded effective radii that were relatively close to the measured values.

Print-and-peel fabrication for microfluidics: what's in it for biomedical applications?

This article reviews the development and the advances of print-and-peel (PAP) microfabrication. PAP techniques provide means for facile and expedient prototyping of microfluidic devices. Therefore, PAP has the potential for broadening the microfluidics technology by bringing it to researchers who lack regular or any accesses to specialized fabrication facilities and equipment. Microfluidics have, indeed, proven to be an indispensable toolkit for biological and biomedical research and development. Through accessibility to such methodologies for relatively fast and easy prototyping, PAP has the potential to considerably accelerate the impacts of microfluidics on the biological sciences and engineering. In summary, PAP encompasses: (1) direct printing of the masters for casting polymer device components; and (2) adding three-dimensional elements onto the masters for single-molding-step formation of channels and cavities within the bulk of the polymer slabs. Comparative discussions of the different PAP techniques, along with the current challenges and approaches for addressing them, outline the perspectives for PAP and how it can be readily adopted by a broad range of scientists and engineers.

Theoretical design of bioinspired macromolecular electrets based on anthranilamide derivatives.

Polypeptide helices possess considerable intrinsic dipole moments oriented along their axes. While for proline helices the dipoles originate solely from the ordered orientation of the amide bonds, for 3(10-) and alpha-helices the polarization resultant from the formation of hydrogen-bond network further increases the magnitude of the macromolecular dipoles. The enormous electric-field gradients, generated by the dipoles of alpha-helices (which amount to about 5 D per residue with 0.15 nm residue increments along the helix), play a crucial role in the selectivity and the transport properties of ion channels. The demonstration of dipole-induced rectification of vectorial charge transfer mediated by alpha-helices has opened a range of possibilities for applications of these macromolecules in molecular and biomolecular electronics. These biopolymers, however, possess relatively large bandgaps. As an alternative, we examined a series of synthetic macromolecules, aromatic oligo-ortho-amides, which form extended structures with amide bonds in ordered orientation, supported by a hydrogen-bond network. Unlike their biomolecular counterparts, the extended pi-conjugation of these macromolecules will produce bandgaps significantly smaller than the polypeptide bandgaps. Using ab initio density functional theory calculations, we modeled anthranilamide derivatives that are representative oligo-ortho-amide conjugates. Our calculations, indeed, showed intrinsic dipole moments oriented along the polymer axes and increasing with the increase in the length of the oligomers. Each anthranilamide residue contributed about 3 D to the vectorial macromolecular dipole. When we added electron donating (diethylamine) and electron withdrawing (nitro and trifluoromethyl) groups for n- and p-doping, respectively, we observed that: (1) proper positioning of the electron donating and withdrawing groups further polarized the aromatic residues, increasing the intrinsic dipole to about 4.5 D per residue; and (2) extension of the pi-conjugation over some of the doping groups narrowed the band gaps with as much as 1 eV. The investigated bioinspired systems offer alternatives for the development of broad range of organic electronic materials with nonlinear properties.

Print-and-peel fabrication of microelectrodes.

We describe a facile and expedient approach for the fabrication of arrays of microelectrodes on smooth substrates. A sequence of print-and-peel procedures allowed for the microfabrication of capacitance microsensors using office equipment and relatively simple wet chemistry. Microfluidic assemblies with reversibly adhered elastomer components allowed for the transfer of patterns of metallic silver, deposited via Tollens' reaction, onto the substrate surfaces. Electroplating of the silver patterns produced an array of micrometer-thick copper electrodes. Capacitance sensors were assembled by placing nonlithographically fabricated flow chambers over the microelectrode arrays. Triangular-waveform current-voltage (I/V) measurements showed a linear correlation between the capacitance of the print-and-peel fabricated devices and the dielectric constant of the samples injected into their flow chambers.