Duoduo Bao

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.

Long-Lived Photogenerated States of α-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 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.

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.

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 310− and α‐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 α‐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 α‐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 π‐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 π‐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.

Electrochemical supercapacitor based on flexible pillar graphene nanostructures

Here we report the fabrication of high conductive and large surface-area 3D pillar graphene nanostructures (PGN) films from assembly of vertically aligned CNT pillars on flexible copper foils and directly employed for the application in electrochemical double layer capacitance (EDLC) supercapacitor. The fabricated supercapacitor based on PGN films with excellent mechanical flexibility and electrical conductivity has high energy storage capability. The PGN films which were one-step synthesized on flexible copper foil (25 um) by CVD process exhibit high conductivity with sheet resistance as low as 1.6 ohm per square and high mechanical flexibility. The fabricated EDLC supercapacitor based on high surface-area PGN electrodes (563m2/g) shows high performance with high specific capacitance of 330F/g and energy density as high as 45.8Wh/kg. All of these make this 3D graphene/CNTs hybrid carbon nanostructures highly attractive material for high performance supercapacitor and other energy storage material.