Electrogenerated Thin Films of Microporous Polymer Networks with Remarkably Increased Electrochemical Response to Nitroaromatic Analytes

Thin films of microporous polymer networks (MPNs) have been generated by electrochemical polymerization of a series of multifunctional carbazole-based monomers. The microporous films show high Brunauer–Emmett–Teller (BET) surface areas up to 1300 m² g^–1 as directly measured by krypton sorption experiments. A correlation between the number of polymerizable carbazole units of the monomer and the resulting surface area is observed. Electrochemical sensing experiments with 1,3,5-trinitrobenzene as prototypical nitroaromatic analyte demonstrate an up to 180 times increased current response of MPN-modified glassy carbon electrodes in relation to the nonmodified electrode. The phenomenon probably involves intermolecular interactions between the electron-poor nitroaromatic analytes and the electron-rich, high surface area microporous deposits, with the electrochemical reduction at the MPN-modified electrodes being an adsorption-controlled process for low scan rates. We expect a high application potential of such MPN-modified electrodes for boosting the sensitivity of electrochemical sensor devices

Thiophene-Based Microporous Polymer Networks via Chemical or Electrochemical Oxidative Coupling

Four thiophene-based monomers have been synthesized by Stille- or Suzuki-type couplings followed by chemical or electrochemical polymerization into microporous polymer networks (MPNs) with high BET surface areas (<i>S</i>_BET). Similar <i>S</i>_BET values of up to 2020 and 2135 m² g^–1 have been determined for tetraphenyl­methane-cored bulk MPN powders and thin films, respectively. Electrochemical polymerization in boron trifluoride diethyl etherate (BFEE)/dichloromethane (DCM) mixtures allows for the generation of MPN films with optimized porosity. Moreover, an interesting effect of boron trifluoride on the connectivity of the monomeric units during electropolymerization is observed for 3-thienyl-based monomers. Finally, the electrochemical reduction of 1,3,5-trinitro­benzene at MPN-modified glassy carbon (GC) electrodes shows increased cathodic responses compared to nonmodified GC electrodes due to interaction between electron-deficient nitroaromatic analyte and electron-rich MPN film. The influence of the specific surface area of MPNs on the electrochemical response is also studied for this class of materials

Silicon- or Carbon-Cored Multifunctional Carbazolyl Monomers for the Electrochemical Generation of Microporous Polymer Films

A series of four tetra- or octacarbazolyl-substituted, tetraphenylmethane/-silane monomers have been oxidatively coupled into microporous polymer networks (MPNs). Chemical polymerization with iron­(III) chloride gives bulk MPNs with BET surface areas (<i>S</i>_BET) of up to 1331 m² g^–1 (for the octacarbazolyl-substituted tetraphenyl­methane monomer). Slightly increased <i>S</i>_BET values result for the materials made from the octacarbazolyl monomers if compared to the tetracarbazolyl analogues, while the exchange of the central carbon by a silicon atom leads to decreased surface areas. The latter phenomenon might be related to electronic interactions of aromatic substituents through the silicon centers. This may cause a reduced reactivity of the carbazoles after the initial oxidative couplings and finally a reduced cross-linking density of the resulting MPNs. Moreover, electrochemical oxidative coupling enables the formation of thin polymer films on the working electrode. These films also show high <i>S</i>_BET values that are only slightly reduced if compared to the corresponding bulk MPNs. Electrochemical quartz microbalance measurements allow for an in-situ characterization of the electrochemical MPN generation. Finally, the electrochemical reduction of a series of nitroaromatic compounds (NACs) on MPN-modified glassy carbon electrodes is studied and applied for high sensitivity NACs detection up to the ppb range

Microporous Polymer Networks Made by Cyclotrimerization of Commercial, Aromatic Diisocyanates

The cyclotrimerization of commercial, aromatic diisocyanates allows for the formation of monolithic, microporous polymer networks with <i>S</i>_BET surface areas up to 1300–1500 m²/g. The process has been up-scaled for production of 100 g batches. The monolithic materials show a promising potential for the removal of lipophilic components from aqueous mixtures

Crystalline and Noncrystalline Forms of Poly(9,9-diheptylfluorene)

The formation of ordered morphologies in poly­(9,9-diheptylfluorene) (PF7) was investigated using X-ray diffraction and grazing incidence X-ray diffraction. Two crystalline phases were found. The α-phase is orthorhombic with <i>a</i> = 2.60 nm, <i>b</i> = 2.25 nm, and <i>c</i> = 3.34 nm, and it is structurally very close to the α-phase in poly­(9,9-dioctylfluorene) (PF8). The γ-phase is monoclinic with <i>a</i> = 2.88 nm, <i>b</i> = 0.96 nm, and <i>c</i> = 1.68 nm, and the oblique angle is close to 90°. The γ-phase is the stable form in the bulk while the α-phase preferentially forms in thin films. Well-ordered and aligned crystalline films were produced from both good (toluene) and moderate (methylcyclohexane, MCH) solvent. Preparing films from MCH without annealing resulted in mesoscopic crystal with decreased order along the <i>a</i>-axis. This mesoscopic structure differs from the β-phase found in PF8 and is more related to the crystalline γ-phase. This difference may explain why mesoscopic PF8 has a phase transition into the α-phase, whereas the mesoscopic PF7 rather into the γ-phase

Microporous Polymer Networks Made by Cyclotrimerization of Commercial, Aromatic Diisocyanates

The cyclotrimerization of commercial, aromatic diisocyanates allows for the formation of monolithic, microporous polymer networks with <i>S</i>_BET surface areas up to 1300–1500 m²/g. The process has been up-scaled for production of 100 g batches. The monolithic materials show a promising potential for the removal of lipophilic components from aqueous mixtures

Room-Temperature Exciton-Polariton Condensation in a Tunable Zero-Dimensional Microcavity

We create exciton-polaritons in a zero-dimensional (0D) microcavity filled with organic ladder-type conjugated polymer in the strong light–matter interaction regime. Photonic confinement at wavelength scale is realized in the longitudinal direction by two dielectric Bragg mirrors and laterally by a submicron Gaussian-shaped defect. The cavity is separated into two parts, allowing nanometer position control and enabling tuning of the exciton and photon fractions of the polariton wave function. Polariton condensation is achieved with nonresonant picosecond optical excitation under ambient conditions and evidenced by a threshold behavior with a nonlinear increase in the emission intensity, line narrowing, and a blue shift in the emission peak. Furthermore, angular emission spectra show that condensation occurs in the ground state of the 0D cavity, and first-order coherence measurements reveal the coherent nature. These experiments open the door for polariton quantum fluids in complex external potentials at room temperature

Excited State Characterization and Energy Transfer in Hyperbranched Polytruxenes and Polytruxene-<i>block</i>-Polythiophene Multiblock Copolymers

A comprehensive investigation of the excited state characteristics of two hyperbranched truxene polymers [one end-terminated with poly­(3-hexylthiophene) blocks, P3HT] and a bistruxene model compound has been undertaken aiming to rationalize its inherent photophysical properties, including the energy transfer processes between the truxene (donor) and P3HT (acceptor) moieties. The study comprises qualitative absorption, emission, and triplet-singlet difference spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion and singlet oxygen formation) and fluorescence decay times. From the time-resolved data in solvents of different viscosity and as a function of temperature, it was established that with the P3HT-terminated hyperbranched polytruxene, the excited state deactivation mainly results from energy transfer and that conformational relaxation is absent in these systems, which gives further support for the rigidity of these polymers both in the ground and excited state. An energy transfer efficiency of 91% was obtained at room temperature. From a qualitative analysis of the data, it was also seen that radiationless processes (particularly the S₁∼∼→S₀ internal conversion channel) mainly contribute to the excited state deactivation of the hyperbranched polytruxenes, a behavior that is in contrast to what was observed for the bistruxene model compound. Spectral and fluorescence time-resolved data in thin films was also obtained and compared with the solution data

The Impact of Driving Force and Temperature on the Electron Transfer in Donor–Acceptor Blend Systems

We discuss whether electron transfer from a photoexcited polymer donor to a fullerene acceptor in an organic solar cell is tractable in terms of Marcus theory, and whether the driving force Δ<i>G</i>₀ is crucial in this process. Considering that Marcus rates are presumed to be thermally activated, we measured the appearance time of the polaron (i.e., the radical-cation) signal between 12 and 295 K for the representative donor polymers PTB7, PCPDTBT, and Me-LPPP in a blend with PCBM as acceptor. In all cases, the dissociation process was completed within the temporal resolution of our experimental setup (220–400 fs), suggesting that the charge transfer is independent of Δ<i>G</i>₀. We find that for the PCPDTBT:PCBM (Δ<i>G</i>₀ ≈ −0.2 eV) and PTB7:PCBM (Δ<i>G</i>₀ ≈ −0.3 eV) the data is mathematically consistent with Marcus theory, yet the condition of thermal equilibrium is not satisfied. For MeLPPP:PCBM, for which electron transfer occurs in the inverted regime (Δ<i>G</i>₀ ≈ −1.1 eV), the dissociation rate is inconsistent with Marcus theory but formally tractable using the Marcus–Levich–Jortner tunneling formalism which also requires thermal equilibrium. This is inconsistent with the short transfer times we observed and implies that coherent effects need to be considered. Our results imply that any dependence of the total yield of the photogeneration process must be ascribed to the secondary escape of the initially generated charge transfer state from its Coulomb potential

Correlation between the Open Circuit Voltage and the Energetics of Organic Bulk Heterojunction Solar Cells

A detailed investigation of the open circuit voltage (<i>V</i>_OC) of organic bulk heterojunction solar cells comprising three different donor polymers and two different fullerene-based acceptors is presented. Bias amplified charge extraction (BACE) is combined with Kelvin Probe measurements to derive information on the relevant energetics in the blend. On the example of P3HT:PC₇₀BM the influence of composition and preparation conditions on the relevant transport levels will be shown. Moderate upward shifts of the P3HT HOMO depending on crystallinity are observed, but contrarily to common believe, the dependence of <i>V</i>_OC on blend composition and thermal history is found to be largely determined by the change in the PCBM LUMO energy. Following this approach, we quantified the energetic contribution to the <i>V</i>_OC in blends with fluorinated polymers or higher adduct fullerenes