Length and Time-Dependent Rates in Diffusion-Controlled Reactions with Conjugated Polymers

Rate constants for diffusion-controlled reactions of solvated electrons with conjugated fluorene oligomers (oF) and polymers (pF) were measured in liquid tetrahydrofuran (THF). Preparative gel permeation chromatography (GPC) was used to separate the polyfluorenes into fractions having narrowed distributions of lengths. Both oF and pF’s were used in determinations of the attachment rate constants kinf as a function of length, where kinf refers to the rate coefficients at long times where they are indeed constant. The results find that in going from oF1 to pF133, kinf increases by a factor of 16, which is much smaller than that of the 133-fold increase in length. The extent of this increase and its change with length are in excellent agreement with published theoretical models that describe diffusion to long thin objects as either prolate spheroids or one-dimensional arrays of spheres. As the concentration of polymer was increased, the effects of large transient terms in the rate constant were observed. As predicted by the Smoluchowski diffusion equation, with modifications by more contemporary theorists, these transient effects are larger and persist to longer times for the larger molecules. For the longest molecule, pF133, k(t) increases by more than a decade at short times. In that case, the “transient term” becomes dominant and the rate coefficient is approximately proportional to the square of the effective reaction radius in contrast to the linear dependence usual for diffusional reactions. The size of these transient effects and their quantitative confirmation are unprecedented

Sudden, “Step” Electron Capture by Conjugated Polymers

Data showing significant time-resolution-limited “step” capture of electrons following radiolysis by 7 – 10 ps electron pulses in a series of different length and different concentration conjugated polyfluorene polymers in tetrahydrofuran (THF) are presented. At the highest concentration, ∼48 mM in repeat units for lengths from 20 to 133 fluorenes, ∼30% of the electrons formed during pulse radiolysis were captured in the step, with a constant efficiency per repeat unit. Step capture per repeat unit (<i>q</i> = 6.9 M^–1) is 60% of the presolvated electron capture efficiency previously reported for biphenyl in THF, giving capture per polymer molecule 12–80 times larger than that for biphenyl at the same concentration. This increase in capture efficiency is large compared to the rate constant per repeat unit for diffusion-limited electron attachment to the same molecules, which is 13% of that of a single unit of fluorene. Plausible mechanisms of this fast capture are explored. It is shown that both capture of quasi-free and localized presolvated electrons can adequately explain the observations. The large yield of radical anions at low concentration of polyfluorene enables observation of subsequent chemistry on the picosecond time scale in these systems, which would otherwise been limited by diffusional attachment to the nanosecond regime

Preparation of Water-Based Alkyl Ketene Dimer (AKD) Nanoparticles and Their Use in Superhydrophobic Treatments of Value-Added Teakwood Products

A process for preparing emulsions of alkyl ketene dimer (AKD) nanoparticles via a nanoemulsion template (emulsion/evaporation) method has been developed. The effects of types and contents of stabilizing agents, i.e., anionic (sodium dodecyl sulfate, SDS), cationic (cetyltrimethylammonium bromide, CTAB), amphoteric (phosphatidylcholine, PC), and polymeric (poly­(vinyl alcohol), PVA), on the colloidal stability and hydrodynamic size of the AKD nanoparticles are investigated. The use of 0.1 wt % anionic SDS as a stabilizer generates nanoparticles with high stability and the smallest average size of 148 ± 5 nm. The environmentally friendly water-based emulsion prepared without halogenated compounds and harsh organic solvents is then applied to enhance the hydrophobicity of teakwood products by a simple dipping process. The properties and structures of the resulting treated woods are examined by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy (SEM), and water contact angle (WCA) measurements. The treated woods show superhydrophobicity with a WCA value of 150 ± 2°, as the emulsion generates a hydrophobic layer covering the wood surfaces due to the β-ketoester bond formation and the arrangement of AKD hydrophobic tails. The nanosized nanoparticles can penetrate the dense structure of the teakwood and form similar bonding for up to a 0.8 mm depth, generating a protective water-repellent layer in the wood structure. The emulsion has high potential for use in the commercial production of value-added teakwood products, with excellent water-resistant properties and high dimensional instability, without altering their physical appearances

Conjugated “Molecular Wire” for Excitons

We have synthesized new conjugated, rigid rod oligomers of fluorene, F_<i>n</i>(C₆₀)₂, <i>n</i> = 4, 8, 12, and 16. These pure compounds have F_<i>n</i> chains up to 140 Å long. The C₆₀ groups covalently attached at both ends serve as traps for excitons created in the F_<i>n</i> chains. Excitons created in the chains by photoexcitation reacted rapidly with the C₆₀ groups with decays described well by the sum of two exponentials. Mean reaction times were 2.3, 5.5, and 10.4 ps for <i>n</i> = 8, 12, and 16. In F₁₆(C₆₀)₂, the 10.4 ps reaction time was 40 times faster than that found in earlier reports on molecules of slightly longer length. The simplest possible model, that of one-dimensional diffusion of excitonic polarons that react whenever they encounter the end of a chain, fits the results to obtain diffusion coefficients. Deviations of those fits from the data may point to the need for alternative pictures or may just indicate that diffusion is not ideal. The definite lengths of these molecules enable a stringent test for theories. These results reveal that exciton transport can be much faster than previously believed, a finding that could, along with appropriate nanoassembly, enable new kinds of high-efficiency organic photovoltaics

Chain Length Dependence of Energies of Electron and Triplet Polarons in Oligofluorenes

Bimolecular equilibria measured the one-electron reduction potentials and triplet free energies (Δ<i>G</i>°_T) of oligo­(9,9-dihexyl)­fluorenes and a polymer with lengths of <i>n</i> = 1–10 and 57 repeat units. Accurate one-electron potentials can be measured electrochemically only for the shorter oligomers. Starting at <i>n</i> = 1 the free energies change rapidly with increasing length and become constant for lengths longer than the delocalization length. Both the reduction potentials and triplet energies can be understood as the sum of a free energy for a fixed polaron and a positional entropy. The positional entropy increases gradually with length beyond the delocalization length due to the possible occupation sites of the charge or the triplet exciton. The results reinforce the view that charges and triplet excitons in conjugated chains exist as polarons and find that positional entropy can replace a popular empirical model of the energetics

Triplet Transport to and Trapping by Acceptor End Groups on Conjugated Polyfluorene Chains

Triplet excited states created in polyfluorene (pF) molecules having average lengths up to 170 repeat units were transported to and captured by trap groups at the ends in less ∼40 ns. Almost all of the triplets attached to the chains reached the trap groups, ruling out the presence of substantial numbers of defects that prevent transport. The transport yields a diffusion coefficient D of at least 3 × 10–4 cm2 s–1, which is 30 times typical molecular diffusion and close to a value for triplet transport reported by Keller (J. Am. Chem. Soc. 2011, 133, 11289–11298). The triplet states were created in solution by pulse radiolysis; time resolution was limited by the rate of attachment of triplets to the pF chains. Naphthylimide (NI) or anthraquinone (AQ) groups attached to the ends of the chains acted as traps for the triplets, although AQ would not have been expected to serve as a trap on the basis of triplet energies of the separate molecules. The depths of the NI and AQ triplet traps were determined by intermolecular triplet transfer equilibria and temperature dependence. The trap depths are shallow, just a few times thermal energy for both, so a small fraction of the triplets reside in the pF chains in equilibrium with the end-trapped triplets. Trapping by AQ appears to arise from charge transfer interactions between the pF chains and the electron-accepting AQ groups. Absorption bands of the end-trapped triplet states are similar in peak wavelength (760 nm) and shape to the 760 nm bands of triplets in the pF chains but have reduced intensities. When an electron donor, N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), is added to the solution, it reacts with the end-trapped triplets to remove the 760 nm bands and to make the trapping irreversible. New bands created upon reaction with TMPD may be due to charge transfer states

Biocompatible Degradable Hollow Nanoparticles from Curable Copolymers of Polylactic Acid for UV-Shielding Cosmetics

Hollow polymeric nanoparticles have attracted vast attention as UV-shielding materials in personal care products due to their excellent light scattering characteristics and low density. In this work, a process for fabricating biocompatible/degradable poly­(lactic acid-co-glycidyl methacrylate), P­(LA-co-GMA), hollow nanoparticles via one-step phase inversion emulsification is examined, to gain insights into their formation mechanisms and optimization of the process parameters. The migration of poly­(vinyl alcohol) (PVA) (stabilizing agent) from the oil droplet to the oil/water interface while entangled with cross-linked P­(LA-co-GMA) chains and the fast evaporation rate of the chloroform solvent play an essential role in the hollow structure formation. Under optimum conditions, monodispersed hollow nanoparticles, with an average size of 500–700 nm and good colloidal stability, are obtained. The as-prepared hollow nanoparticles exhibit high UV shielding capabilities and low toxicity. The nanoparticles show high stability under UV exposure but can be completely degraded within 24 weeks under accelerated hydrolysis conditions. The materials have a high potential for use as environmental-friendly UV-shielding additives in cosmetic applications

Toward Designed Singlet Fission: Electronic States and Photophysics of 1,3-Diphenylisobenzofuran

Single crystal molecular structure and solution photophysical properties are reported for 1,3-diphenylisobenzofuran (1), of interest as a model compound in studies of singlet fission. For the ground state of 1 and of its radical cation (1+•) and anion (1−•), we report the UV−visible absorption spectra, and for neutral 1, also the magnetic circular dichroism (MCD) and the decomposition of the absorption spectrum into purely polarized components, deduced from fluorescence polarization. These results were used to identify a series of singlet excited states. For the first excited singlet and triplet states of 1, the transient visible absorption spectra, S1 → Sx and sensitized T1 → Tx, and single exponential lifetimes, τF = ∼5.3 ns and τT = ∼200 μs, are reported. The spectra and lifetimes of S1 → S0 fluorescence and sensitized T1 → Tx absorption of 1 were obtained in a series of solvents, as was the fluorescence quantum yield, ΦF = 0.95−0.99. No phosphorescence has been detected. The first triplet excitation energy of solid 1 (11 400 cm−1) was obtained by electron energy loss spectroscopy, in agreement with previously reported solution values. The fluorescence excitation spectrum suggests an onset of a nonradiative channel at ∼37 000 cm−1. Excitation energies and relative transition intensities are in agreement with those of ab initio (CC2) calculations after an empirical 3000 cm−1 adjustment of the initial state energy to correct differentially for a better quality description of the initial relative to the terminal state of an absorption transition. The interpretation of the MCD spectrum used the semiempirical PPP method, whose results for the S0 → Sx spectrum require no empirical adjustment and are otherwise nearly identical with the CC2 results in all respects including the detailed nature of the electronic excitation. The ground state geometry of 1 was also calculated by the MP2, B3LYP, and CAS methods. The calculations provided a prediction of changes of molecular geometry upon excitation or ionization and permitted an interpretation of the spectra in terms of molecular orbitals involved. Computations suggest that 1 can exist as two nearly isoenergetic conformers of C2 or Cs symmetry. Linear dichroism measurements in stretched polyethylene provide evidence for their existence and show that they orient to different degrees, permitting a separation of their spectra in the region of the purely polarized first absorption band. Their excitation energies are nearly identical, but the Franck−Condon envelopes of their first transition differ to a surprising degree

Toward Designed Singlet Fission: Electronic States and Photophysics of 1,3-Diphenylisobenzofuran

Single crystal molecular structure and solution photophysical properties are reported for 1,3-diphenylisobenzofuran (1), of interest as a model compound in studies of singlet fission. For the ground state of 1 and of its radical cation (1+•) and anion (1−•), we report the UV−visible absorption spectra, and for neutral 1, also the magnetic circular dichroism (MCD) and the decomposition of the absorption spectrum into purely polarized components, deduced from fluorescence polarization. These results were used to identify a series of singlet excited states. For the first excited singlet and triplet states of 1, the transient visible absorption spectra, S1 → Sx and sensitized T1 → Tx, and single exponential lifetimes, τF = ∼5.3 ns and τT = ∼200 μs, are reported. The spectra and lifetimes of S1 → S0 fluorescence and sensitized T1 → Tx absorption of 1 were obtained in a series of solvents, as was the fluorescence quantum yield, ΦF = 0.95−0.99. No phosphorescence has been detected. The first triplet excitation energy of solid 1 (11 400 cm−1) was obtained by electron energy loss spectroscopy, in agreement with previously reported solution values. The fluorescence excitation spectrum suggests an onset of a nonradiative channel at ∼37 000 cm−1. Excitation energies and relative transition intensities are in agreement with those of ab initio (CC2) calculations after an empirical 3000 cm−1 adjustment of the initial state energy to correct differentially for a better quality description of the initial relative to the terminal state of an absorption transition. The interpretation of the MCD spectrum used the semiempirical PPP method, whose results for the S0 → Sx spectrum require no empirical adjustment and are otherwise nearly identical with the CC2 results in all respects including the detailed nature of the electronic excitation. The ground state geometry of 1 was also calculated by the MP2, B3LYP, and CAS methods. The calculations provided a prediction of changes of molecular geometry upon excitation or ionization and permitted an interpretation of the spectra in terms of molecular orbitals involved. Computations suggest that 1 can exist as two nearly isoenergetic conformers of C2 or Cs symmetry. Linear dichroism measurements in stretched polyethylene provide evidence for their existence and show that they orient to different degrees, permitting a separation of their spectra in the region of the purely polarized first absorption band. Their excitation energies are nearly identical, but the Franck−Condon envelopes of their first transition differ to a surprising degree