Pushing the limits of the electrochemical window with pulse radiolysis in chloroform.

Pulse radiolysis (PR) enables the full redox window of a solvent to be accessed, as it does not require electrodes or electrolyte which limit the potentials accessible in voltammetry measurements. PR in chloroform has the additional possibility to enable reaching highly positive potentials because of its large ionization potential (IP). PR experiments demonstrated the formation of the (deuterated) chloroform radical cation CDCl3+˙, identifying it as the source of the broad absorption in the visible part of the spectrum. Results indicated that solutes with a redox potential up to +3.7 V vs. Fc/Fc+ can be oxidized by CDCl3+˙, which is far beyond what is possible with electrochemical techniques. Oxidation is not efficient because of rapid geminate recombination with chloride counterions, but also due to rapid decomposition of CDCl3+˙ which limits the yield of otherwise longer-lived free ions. The rapid, 6 ± 3 ns, decomposition, confirmed by two independent experiments, means that a solute must be present at a concentration >100 mM to capture >90% of the free holes formed. Addition of ethene removes the broad, overlapping absorptions from ubiquitous (chlorine atom, solute) complexes created by PR in halogenated solvents enabling clear observation of solute cations. The results also unravel the complex radiation chemistry of chloroform including the large reported value G(-CHCl3) = 12 molecules/100 eV for the decomposition of chloroform molecules.

General Method for Determining Redox Potentials without Electrolyte.

A novel method to determine redox potentials without electrolyte is presented. The method is based on a new ability to determine the dissociation constant, K°d, for ion pairs formed between any radical anion and any inert electrolyte counterion. These dissociation constants can be used to determine relative shifts of redox potential as a function of electrolyte concentration, connecting referenced potentials determined with electrochemistry (with 0.1 M electrolyte) to electrolyte-free values. Pulse radiolysis created radical anions enabling determination of equilibrium constants for electron transfer between anions of donor and acceptor molecules as a function of electrolyte concentration in THF. The measurements determined "composite equilibrium constants", KeqC, which contain information about the dissociation constant for the electrolyte cations, X+, with the radical anions of both the donor, K°d(D-•,X+) and the acceptor, K°d(A-•,X+). Dissociation constants were obtained for a selection of radical anions with tetrabutylammonium (TBA+). The electrolyte was found to shift the reduction potentials of small molecules 1-methylpyrene and trans-stilbene by close to +130 mV whereas oligo-fluorenes and polyfluorenes experienced shifts of only (+25 ± 6) mV due to charge delocalization weakening the ion pair. These shifts for reduction of aromatic hydrocarbon molecules are smaller than shifts of +232 and +451 mV seen previously for benzophenone radical anion with TBA+ and Na+ respectively where the charge on the radical anion is localized largely on one C═O bond, thus forming a more tightly bound ion pair.

Dynamic broadening alters triplet extinction coefficients in fluorene oligomers and polymers.

We report Tn ← T1 spectra and extinction coefficients, ε, and other properties as functions of chain length for a series of fluorene oligomers, oFn, and polymers, pFn, with n = 2-84 repeat units. We find that ε increases with length, peaking at 159 400 M-1 cm-1 for oF3 and then decreases for longer chains. ε does not scale with 1/n or e-n to reach a constant value at long length, as predicted by the commonly applied oligomer extrapolation approximation, although spectral shifts, oscillator strengths, and transition dipole moments do reach limiting values for chains near 10 units long. While computations describe the triplet in oF2 and oF3 as having similar geometries with a single flattened dihedral angle between units, computations and simulations suggest that in longer oligomers motion along the chains of the short 2-3 unit, the long T1 state is probably the source of the unusual changes in ε. These occur because hopping along the chain is sufficiently fast that the dihedrals between fluorene units cannot fully relax. At a length near 10 units, hopping and dihedral angle changes produce a steady state distribution of geometries with only small changes from the ground state, which persist for longer chains. Additional decreases in ε from pF28 to pF84 are plausibly due to a small number of chain defects which result in loss of triplets.

Correction: Escape of anions from geminate recombination in THF due to charge delocalization.

Correction for 'Escape of anions from geminate recombination in THF due to charge delocalization' by Hung-Cheng Chen et al., Phys. Chem. Chem. Phys., 2017, 19, 32272-32285.

Escape of anions from geminate recombination in THF due to charge delocalization.

Geminate recombination of 24 radical anions (M˙-) with solvated protons (RH2+) was studied in tetrahydrofuran (THF) with pulse radiolysis. The recombination has two steps: (1) diffusion of M˙- and RH2+ together to form intimate (contact and solvent separated) ion pairs, driven by Coulomb attraction; (2) annihilation of anions due to proton transfer (PT) from RH2+ to M˙-. The non-exponential time-dependence of the geminate diffusion was determined. For all molecules protonated on O or N atoms the subsequent PT step is too fast (<0.2 ns) to measure, except for the anion of TCNE which did not undergo proton transfer. PT to C atoms was as slow as 70 ns and was always slow enough to be observable. A possible effect of charge delocalization on the PT rates could not be clearly separated from other factors. For 21 of the 24 molecules studied here, a free ion yield (71.6 ± 6.2 nmol J-1) comprising ∼29% of the total, was formed. This yield of "Type I" free ions is independent of the PT rate because it arises entirely by escape from the initial distribution of ion pair distances without forming intimate ion pairs. Three anions of oligo(9,9-dihexyl)fluorenes, Fn˙- (n = 2-4) were able to escape from intimate ion-pairs to form additional yields of "Type II" free ions with escape rate constants near 3 × 106 s-1. These experiments find no evidence for an inverted region for proton transfer.

Rapid "step capture" of holes in chloroform during pulse radiolysis.

The fundamental process of hole capture in solution was investigated following pulse radiolysis with polyfluorene and 4-cyano-4″-pentyl-p-terphenyl scavengers. Contrary to expectation, a large fraction of holes were captured in experimental time-resolution limited ∼20 ps steps, by a process much faster than diffusion of the initially formed solvent molecular cation. At the highest concentrations, 1.92 mM for a 52 unit long polyfluorene and 800 mM for 4-cyano-4″-pentyl-p-terphenyl, 66% and 99%, respectively, of the initially formed holes were captured by 20 ps, with radiation chemical yield G = 1.2 × 10(-7) and 1.7 × 10(-7) mol J(-1). The data can be explained by capture of presolvated holes, analogous to presolvated electrons, possibly possessing extended wave functions, high mobilities, or excess kinetic energy for the first few picoseconds after their creation. Such a process is not generally known in solution; however, the observed step capture as a function of solute concentration is shown to be well explained by this model. In addition to understanding the capture process in solution, the very large step yields formed in 20 ps will provide the ability to resolve subsequent hole transfer on the polymers with >2 orders of magnitude better time resolution than expected.

Polarons, bipolarons, and side-by-side polarons in reduction of oligofluorenes.

The nature of charge carriers in conjugated polymers was elucidated through optical spectroscopy following single- and multielectron reduction of 2,7-(9,9-dihexylfluorene) oligomers, F(n), n = 1-10, yielding spectra with the two bands typical of polarons upon single reduction. For short oligomers addition of a second electron gave a single band demonstrating the classic polaron-bipolaron transition. However, for long oligomers double reductions yielded spectra with two bands, better described as two polarons, possibly residing side-by-side in the F(n) chains. The singly reduced anions do not appear to delocalize over the entire length of the longer conjugated systems; instead they are polarons occupying approximately four fluorene repeat units. The polarons of F(3) and F(4) display sharp absorption bands, but for longer oligomers the bands broaden, possibly due to fluctuations of the lengths of these unconfined polarons. DFT calculations with long-range-corrected functionals were fully consistent with the experiments describing polarons in anions, bipolarons in dianions of short oligomers, and side-by-side polarons in dianions of long oligomers, while results from standard functionals were not compatible with the experimental results. The computations found F(10)(2-), for example, to be an open-shell singlet ( ≈ 1), with electrons in two side-by-side orbitals, while dianions of shorter oligomers experienced a gradual transition to bipolarons with states of intermediate character at intermediate lengths. The energies and extinction coefficients of each anionic species were measured by ultraviolet-visible-near-infrared absorption spectroscopy with chemical reduction and pulse radiolysis. Reduction potentials determined from equilibria mirrored oxidation potentials reported by Chi and Wegner. Anions of oligomers four or more units in length contained vestigial neutral (VN) absorption bands that arise from neutral parts of the chain. Energies of the VN bands correspond to those of oligomers shorter by four units.

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 (q = 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.

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 k(inf) as a function of length, where k(inf) refers to the rate coefficients at long times where they are indeed constant. The results find that in going from oF(1) to pF(133), k(inf) 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, pF(133), 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.

Electron Transport with Mobility, µ > 86 cm2/(V s), in a 74 nm Long Polyfluorene.

The mobility of charges on conjugated polymers is a fundamentally important feature of these materials, but most fall far short of transport that might lead one to call them "molecular wires". A commonly identified bottleneck is flexible dihedral angles between repeat units. Here we find a very high mobility, µ > 86 cm2/(V s), for electrons attached to polyfluorene polymers in isooctane, despite the presence of varied dihedral angles. The present data suggest that interactions with the surrounding medium may be a principal determinant of charge mobility.

Electron and hole transport to trap groups at the ends of conjugated polyfluorenes.

Polyfluorenes (pF) were synthesized having anthraquinone (AQ) or naphtylimide (NI) end caps that trap electrons or di- p-tolylaminophenyl (APT2) caps that trap holes. The average lengths of the pF chains in these molecules varied from 7 to 30 nm. End capping was found not to be complete in these molecules so that some were without caps. Electrons or holes were injected into these polymers in solution by pulse radiolysis. Following attachment, the charges migrated to the end cap traps in times near 2 ns in pF12AQ or 5 ns in pF35NI. From these observations, electron mobilities for transport along single chains to the end caps in THF solution were determined to be smaller by a factor of 100 than those observed by microwave conductivity. Despite this, the mobilities were sufficiently large to provide encouragement to the use of such single chains in solar photovoltaics. Most charges were observed to transport over substantial distances in these polymers, but 23, 18, and 37% of the charges attached to pFNI, pFAQ, and pFAPT2, respectively, were trapped in the pF chains and decayed by slower bimolecular reactions. For pFAQ and pFAPT2, all of the trapped charges were accounted for by estimates of the fraction of molecules having no end cap traps. For pF35NI, 23% of the attached electrons were found to be trapped in the chains, but only 4% of chains were expected to have no end caps. This could indicate some trapping by kinks or other defects but may just reflect uncertainties in the capping of this long polymer. When the charges reach the trap groups, their spectra have no features of pF(*-) or pF(*+), nor do the principal bands of the trapped ions resemble spectra of the radical ions of isolated trap molecules. The optical absorption spectra are rather dominated by new bands identified as charge-transfer transitions, which probably reinject electrons or holes into the pF chains. The energies of those bands correlate well with measured redox potentials.

Entrainer effect on photochirogenesis in near- and supercritical carbon dioxide: dramatic enhancement of enantioselectivity.

Diethyl ether added as an entrainer (cosolvent) to near- and supercritical CO2 significantly enhanced the enantioselectivity of photocyclization of 5,5-diphenyl-4-penten-1-ol sensitized by saccharide naphthalenedicarboxylate to give a cyclization product in enantiomeric excesses much larger than those obtained in conventional organic solvents, revealing the unique features of nc- and sc-CO2 as well as the critical role of entrainer clustering to the intervening diastereomeric exciplex pair.

Transport of Triplet Excitons along Continuous 100 nm Polyfluorene Chains.

Triplet excitons created in poly-2,7-(9,9-dihexyl)fluorene (pF) chains with end trap groups in solution are efficiently transported to and captured by the end groups. The triplets explore the entire lengths of the chains, even for ∼100 nm long chains, enabling determination of the completeness of end-capping. The results show that the chains are continuous: they may contain transient barriers or traps, such as those from fluctuations of dihedral angles, but they are free of major defects that stop motion of the triplets. Quantitative determinations are aided by the addition of a strong electron donor, TMPD, which removes absorption bands of the end-trapped triplets. For chains having at least one end trap, triplet capture is quantitative on the 1 µs time scale imposed by the use of the donor. Fractions of chains having no end traps were 0.15 for pF samples with anthraquinone (AQ) end traps and 0.063 with naphthylimide (NI) end traps. These determinations agreed with measurements by NMR for short (<40 polymer repeat units (PRU)) chains, where NMR determinations are accurate. The results find no evidence for traps or barriers to the transport of triplets, and places limits on the possible presence of defects as impenetrable barriers to less than one per 300 PRU. The present results present a paradigm different from the current consensus, derived from observations of singlet excitons, that conjugated chains are divided into "segments," perhaps by some kind of defects. For the present pF chains, the segmentation either does not apply to triplet excitons or is transient so that the defects are healed or surmounted in times much shorter than 1 µs. Triplets on chains without end trap groups transfer to chains with end traps on a slower time scale. Rate constants for these bimolecular triplet transfer reactions were found to increase with the length of the accepting chain, as did rate constants for triplet transfer to the chains from small molecules like biphenyl. A second set of polyfluorenes with 2-butyloctyl side chains was found to have a much lower completeness of end-capping.

Charge Transfer Fluorescence and 34 nm Exciton Diffusion Length in Polymers with Electron Acceptor End Traps.

Photoexcitation of conjugated poly-2,7-(9,9-dihexylfluorene) polyfluorenes with naphthylimide (NI) and anthraquinone (AQ) electron-acceptor end traps produces excitons that form charge transfer states at the end traps. Intramolecular singlet exciton transport to end traps was examined by steady state fluorescence for polyfluorenes of 17-127 repeat units in chloroform, dimethylformamide (DMF), tetrahydrofuran (THF), and p-xylene. End traps capture excitons and form charge transfer (CT) states at all polymer lengths and in all solvents. The CT nature of the end-trapped states is confirmed by their fluorescence spectra, solvent and trap group dependence, and DFT descriptions. Quantum yields of CT fluorescence are as large as 46%. This strong CT emission is understood in terms of intensity borrowing. Energies of the CT states from onsets of the fluorescence spectra give the depths of the traps which vary with solvent polarity. For NI end traps, the trap depths are 0.06 (p-xylene), 0.13 (THF), and 0.19 eV (CHCl3). For AQ, CT fluorescence could be observed only in p-xylene where the trap depth is 0.27 eV. Quantum yields, emission energies, charge transfer energies, solvent reorganization, and vibrational energies were calculated. Fluorescence measurements on chains >100 repeat units indicate that end traps capture ∼50% of the excitons, and that the exciton diffusion length is LD = 34 nm, which is much larger than diffusion lengths reported in polymer films or than previously known for diffusion along isolated chains. The efficiency of exciton capture depends on chain length but not on trap depth, solvent polarity, or which trap group is present.

Discontinuous pressure effect upon enantiodifferentiating photosensitized isomerization of cyclooctene.

A hydrostatic pressure of up to 750 MPa induced discontinuous changes in the enantiomeric excess of the (E)-isomer obtained in the enantio-differentiating photoisomerization of (Z)-cyclooctene and (Z,Z)-cycloocta-1,5-diene, sensitized by chiral benzene-1,2,4,5-tetracarboxylates; indicating a switching of the enantio-differentiation mechanism, which is attributable to dramatic conformational changes of chiral alkoxycarbonyl auxiliaries at a specific pressure.

Fast Controlled Living Polymerization of Arylisocyanide Initiated by Aromatic Nucleophile Adduct of Nickel Isocyanide Complex.

The fast living polymerization of chiral arylisocyanide in the presence of the aromatic nucleophile adduct of tetra(t-butylisocyano)nickel(II) complex as an initiator gave the predominantly one-handed helical polyisocyanide with narrow polydispersity. X-ray crystal structures of initiators and MALDI-TOF MS and NMR studies of the polymer products elucidated the key role of the aromatic substituents in the initiator and monomer achieving narrow polydispersity. The aromatic groups in the initiator and monomer stabilized the electronic structure of the carbene-like ligand to suppress dissociation of the active nickel complex that leads to chain transfer and termination. The aromatic groups also controlled the reactivity of the active site for initiation and propagation.

Mechanistic study on the enantiodifferentiating anti-Markovnikov photoaddition of alcohols to 1,1-diphenyl-1-alkenes in near-critical and supercritical carbon dioxide.

Enantiodifferentiating anti-Markovnikov photoaddition of alcohol (methanol, ethanol, 2-propanol, and tert-butanol) to aromatic alkene (1,1-diphenylpropene and 1,1-diphenyl-1-butene), sensitized by optically active alkyl and saccharide naphthalene(di)carboxylates, was investigated in supercritical carbon dioxide at varying pressures to elucidate the effects of clustering on photosensitization and enantiodifferentiation behavior, in particular on the product's enantiomeric excess (ee). For all the alkene/alcohol/chiral sensitizer combinations examined, a sudden change in the product's ee was consistently observed near the critical density, which is attributable to the critical pressure dependence of clustering around the intervening exciplex intermediate.

Nature and energies of electrons and holes in a conjugated polymer, polyfluorene.

Electrons and holes were injected selectively into poly-2,7-(9,9-dihexylfluorene) (pF) dissolved in a tetrahydrofuran (THF) and a 1,2-dichloroethane (DCE) solution, respectively, using pulse radiolysis. Transient absorption spectra of monoions of both signs revealed two bands attributable to formation of polarons, one in the visible region (pF+* at 580 nm, pF-* at 600 nm) and another in the near-IR region. Additional confirmation for the identification of pF+* and pF-* comes from bimolecular charge-transfer reactions, such as bithiophene-* + pF --> pF-* or pF+* + TTA --> +TTA+* (TTA = tri-p-tolylamine), in which known radical ions transfer charge to pF or from pF. Difference absorption spectra of pF chemically reduced by sodium in THF provided a ratio of absorbance of anions formed to bleaching of the neutral band at 380 nm. In conjunction with pulse-radiolysis results, the data show that each polaron occupies 4.5 +/- 0.5 fluorene units, most probably contiguous units. Extensive reduction of pF by sodium also revealed resistance to formation of bipolarons: excess electrons reside as separate polarons when two or more electrons were injected. Redox equilibria with pyrene and terthiophene by pulse radiolysis established reversible one-electron redox potentials of E0(pF+/0) = +0.66 V and E0(pF0/-) = -2.65 V vs Fc+/0. Together with the excited-state energy, these results predict a singlet exciton binding energy of 0.2 eV for pF in the presence of 0.1 M tetrabutylammonium tetrafluoroborate. This binding energy would increase substantially without an electrolyte.