Poly(3-decylthiophene) Radical Anions and Cations in Solution: Single and Multiple Polarons and Their Delocalization Lengths in Conjugated Polymers

Optical absorption spectra of anions and cations of poly­(3-decylthiophene) (P3DT) in solution were identified as single polarons. Pulse radiolysis of P3DT in THF determined the spatial extent of one negative polaron to be ∼11.5 thiophene units by observing transient absorption of P3DT–• radical ions, which are prinicpally free ions, at 850 nm with ε = (7.25 ± 0.47) × 104 M–1 cm–1 and bleaching of the neutral absorption band at 450 nm. P3DT–• was formed in a combination of diffusive reactions and fast “step” processes. Similarly, a positive polaron of P3DT was estimated to delocalize over ∼8.7 thiophene units by pulse radiolysis in chloroform. Chemical reduction by sodium and oxidation by FeCl3 injected multiple charges into a single P3DT chain while showing absorption spectra in the early stages of reaction resembling those observed by pulse radiolysis. The results indicated that multiple polarons exist in a single chain of P3DT before coalescing into bipolarons or transforming into other forms of polaron

Role of Bad Dihedral Angles: Methylfluorenes Act as Energy Barriers for Excitons and Polarons of Oligofluorenes

“Defects” are one of the main obstacles for the use of organic conjugated molecules in efficient organic photovoltaics, but the definite origins of these defects are elusive to experiments and even in concept. Bad dihedral angles in conjugated molecules produced by adjacent units are considered to act as defects for excitons and polarons, slowing down their transports. While such defects are discussed, their properties were not well-understood. As a model system for such defects, we synthesized oligofluorenes incorporating methylfluorene(s) that can create large dihedral angles between adjacent fluorenes due to steric hindrance, mimicking bad dihedral angles presumably produced in polyfluorenes. Experimental measurements find that singlet excitons are substantially more sensitive to such bad dihedral angles than triplet excitons or negative or positive polarons. The barrier heights for singlets are about three times higher than the barriers for electrons, holes, or triplets. For all four species, the large dihedrals act as energy barriers, not traps

Direct Enantioselective Organocatalytic Hydroxymethylation of Aldehydes Catalyzed by α,α-Diphenylprolinol Trimethylsilyl Ether

The direct enantioselective hydroxymethylation of aldehydes utilizing α,α-diphenylprolinol trimethylsilyl ether as an organocatalyst is described. The intermediate α-substituted β-hydroxyaldehydes were not isolated but converted to the more readily isolable derivatives. For example, the derived hydroxy acids were isolated in up to 94% yield with excellent enantioselectivity

Rate versus Free Energy Change for Attaching Highly Mobile Electrons to Molecules in Nonpolar Liquids

The inverted region of the Marcus theory, usually absent for bimolecular electron transfer reactions, is clearly observed for electron attachment reactions to molecules in nonpolar fluids. Application of pressure increased the energies of the solvated electron reactants letting us continuously adjust the free energy change. Inverted behavior is enabled by the very high mobilities of the solvated electrons which raise the diffusion-controlled encounter rates so high that they do not limit the reaction rates. The nonpolar media used in these experiments reduce reorganization energies, enhancing inverted behavior. Still, for every case showing an inverted region, the presence of low-lying excited states in the product radical anions led to regions of increasing rate constants that began at the energies of excited states of −0.54 to −1.2 eV. While continuum models predict no solvent reorganization energy in nonpolar liquids, fits to the data found solvent reorganization energies of 0.05–0.4 eV supporting ideas advanced in theories of Matyushov

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

A Highly Enantio- and Diastereoselective Molybdenum-Catalyzed Asymmetric Allylic Alkylation of Cyanoesters

An efficient molybdenum-catalyzed asymmetric allylic alkylation (Mo-AAA) of cyanoester nucleophiles is reported. A number of highly functionalized branched cyanoesters containing a quaternary carbon stereocenter with a vicinal tertiary stereocenter are obtained. This method generates a number of functionalized cyanoesters in excellent yield and chemoselectivity in good to excellent diastereoselectivity and enantioselectivity

Vibrational Stark Effects To Identify Ion Pairing and Determine Reduction Potentials in Electrolyte-Free Environments

A recently developed instrument for time-resolved infrared detection following pulse radiolysis has been used to measure the ν­(CN) IR band of the radical anion of a CN-substituted fluorene in tetrahydrofuran. Specific vibrational frequencies can exhibit distinct frequency shifts due to ion pairing, which can be explained in the framework of the vibrational Stark effect. Measurements of the ratio of free ions and ion pairs in different electrolyte concentrations allowed us to obtain an association constant and free energy change for ion pairing. This new method has the potential to probe the geometry of ion pairing and allows the reduction potentials of molecules to be determined in the absence of electrolyte in an environment of low dielectric constant

A Highly Enantio- and Diastereoselective Molybdenum-Catalyzed Asymmetric Allylic Alkylation of Cyanoesters

An efficient molybdenum-catalyzed asymmetric allylic alkylation (Mo-AAA) of cyanoester nucleophiles is reported. A number of highly functionalized branched cyanoesters containing a quaternary carbon stereocenter with a vicinal tertiary stereocenter are obtained. This method generates a number of functionalized cyanoesters in excellent yield and chemoselectivity in good to excellent diastereoselectivity and enantioselectivity

Kinetics and Energetics of Electron Transfer to Dimer Radical Cations

Spectra of the dimer cations naphthalene (Nap2•+) and ethene (Ethene2•+) were measured in liquid dichloromethane (DCM). The spectra peak at very different energies, 1.2 and 3.3 eV. In DCM dimerization stabilizes Nap2•+ by ΔGd°(Nap2•+) = −218 meV relative to the monomer Nap•+ as determined from the dimerization equilibrium constant. Both dimers can transfer a positive charge to hole acceptor molecules, but for both the rate constants rise more gradually with reaction energetics than do many charge transfer reactions previously studied. A striking observation finds that the rate constant for hole transfer from the Nap2•+ dimer to phenanthrene is smaller by two decades than that from biphenyl•+ monomer to Nap, although both reactions have the same −ΔG° = 0.05 eV. A plausible interpretation for these observations is the presence of an energy of reorganization, λ(M2), for the dimer that involves movement apart of the two partners in the dimer. While the dimerization equilibrium cannot be measured for Ethene2•+, the charge transfer data imply that both ΔGd°(Ethene2•+) and λ(Ethene2•+) are considerably larger, perhaps by factors of 2–4 than for Nap2•+

Identification of Ion-Pair Structures in Solution by Vibrational Stark Effects

Ion pairing is a fundamental consideration in many areas of chemistry and has implications in a wide range of sciences and technologies that include batteries and organic photovoltaics. Ions in solution are known to inhabit multiple possible states, including free ions (FI), contact ion pairs (CIP), and solvent-separated ion pairs (SSIP). However, in solutions of organic radicals and nonmetal electrolytes, it is often difficult to distinguish between these states. In the first part of this work, we report evidence for the formation of SSIPs in low-polarity solvents and distinct measurements of CIP, SSIP, and FI, by using the ν­(CN) infrared (IR) band of a nitrile-substituted fluorene radical anion. Use of time-resolved IR detection following pulse radiolysis allowed us to unambiguously assign the peak of the FI. In the presence of nonmetal electrolytes, two distinct red-shifted peaks were observed and assigned to the CIP and SSIP. The assignments are interpreted in the framework of the vibrational Stark effect (VSE) and are supported by (1) the solvent dependence of ion-pair populations, (2) the observation of a cryptand-separated sodium ion pair that mimics the formation of SSIPs, and (3) electronic structure calculations. In the second part of this work, we show that a blue-shift of the ν­(CN) IR band due to the VSE can be induced in a nitrile-substituted fluorene radical anion by covalently tethering it to a metal-chelating ligand that forms an intramolecular ion pair upon reduction and complexation with sodium ion. This adds support to the conclusion that the shift in IR absorptions by ion pairing originates from the VSE. These results combined show that we can identify ion-pair structures by using the VSE, including the existence of SSIPs in a low-polarity solvent