Chain Dynamics, Relaxation Times, and Conductivities of Bithiophene--Acene Copolymers Measured Using High Frequency Saturation Transfer EPR.

We present a study to probe the formation of localized aromatic sextets and their effects on the charge transport properties in polymers with acene cores. Bithiophene-acene copolymers containing benzene, naphthalene, or anthracene as acene cores were synthesized using Yamamoto polymerization. Drop-casted polymer films were chemically doped and analyzed using high frequency saturation transfer EPR (HF ST-EPR), a method which has proven useful in the study of conducting polymers. The spin-spin and spin-lattice relaxation times were determined for these polymers at low temperatures (4 to 20 K) and used to obtain inter- and intrachain spin diffusion rates and conductivities. Similar interchain spin diffusion rates were seen across all polymer systems; however, anthracene containing polymer poly(hexylTTATT) was found to have the largest intrachain spin diffusion rate. The poly(hexylTTATT) intrachain spin diffusion rate may be artificially high if the anthracene ring restricts the diffusion of spin to the hexylated quaterthiophene segment in poly(hexylTTATT) whereas the spins diffuse through the acene cores in the benzene and naphthalene derivatives. Alternatively, as both the spin diffusion rates and conductivities vary unpredictably with temperature, it is possible that the π-electron localization previously seen in the anthracene core could be relieved at lower temperatures.

Direct, catalytic monofluorination of sp³ C-H bonds: a radical-based mechanism with ionic selectivity.

Recently, our group unveiled a system in which an unusual interplay between copper(I) and Selectfluor effects mild, catalytic sp(3) C-H fluorination. Herein, we report a detailed reaction mechanism based on exhaustive EPR, (19)F NMR, UV-vis, electrochemical, kinetic, synthetic, and computational studies that, to our surprise, was revealed to be a radical chain mechanism in which copper acts as an initiator. Furthermore, we offer an explanation for the notable but curious preference for monofluorination by ascribing an ionic character to the transition state.

Charge delocalization through benzene, naphthalene, and anthracene bridges in π-conjugated oligomers: an experimental and quantum chemical study.

To understand the influence of orthogonal conjugation pathways fused directly to π-conjugated polymer backbones, we synthesized and studied three series of thiophene-based model compounds containing benzene, naphthalene, and anthracene peri-substituted central cores as representative acenes. These models were functionalized with methyl groups at the reactive thiophene positions in order to generate and observe oxidized species without complications from follow-up polymerization. The neutral monomers and their oxidized charged counterparts were subjected to cyclic voltammetry, spectroelectrochemistry, and EPR spectroscopy as appropriate, and these results were further corroborated with thorough density functional theory studies. This joint experimental and theoretical analysis allowed us to determine that benzene-based conjugated linkers led to more delocalized charge carriers on account of the quinoidal character maintained within the benzene core. In contrast, anthracene-based linkers displayed very localized carriers due to torsional strain between the adjacent aryl groups and to the local evolution of formal aromatic sextets on the benzo-fused rings orthogonal to the backbone in the quinoidal state. In some cases, the electronics of the thiophene-based substituent dominated the electronic properties of the oxidized species regardless of the nature of the central acene linker. These results highlight the dramatic influence that orthogonal conjugation pathways can exert on the electronic properties of π-conjugated materials.

Generation of oxynitrenes and confirmation of their triplet ground states.

New sulfoximine- and phenanthrene-based photochemical precursors to oxynitrenes have been developed. These precursors have been used to examine the chemistry and spectroscopy of oxynitrenes. The first EPR spectra of oxynitrenes are reported and are consistent with their triplet ground states. Additional support for the triplet ground state of oxynitrenes is provided by trapping and reactivity studies, nanosecond time-resolved IR investigations, and computational studies.

DNA oxidation in anionic reverse micelles: ruthenium-mediated damage at Guanine in single- and double-stranded DNA.

One-electron guanine oxidation in DNA has been investigated in anionic reverse micelles (RMs). A photochemical method for generating Ru3+ from the ruthenium polypyridyl complex tris(2-2'-bipyridine)ruthenium(II) chloride ([Ru(bpy)3]Cl2) is combined with high-resolution polyacrylamide gel electrophoresis (PAGE) to quantify piperidine-labile guanine oxidation products. As characterized by emission spectroscopy of Ru(bpy)3(2+), the addition of DNA to RMs containing Ru(bpy)3(2+) does not perturb the environment of Ru(bpy)3(2+). The steady-state quenching efficiency of Ru(bpy)3(2+) with K3[Fe(CN)6] in buffer solution is approximately 2-fold higher than that observed in RMs. Consistent with the difference in quenching efficiency in the two media, a 1.5-fold higher yield of piperidine-labile damage products as monitored by PAGE is observed for duplex oligonucleotide in buffer vs RMs. In contrast, a 13-fold difference in the yield of PAGE-detected G oxidation products is observed when single-stranded DNA is the substrate. Circular dichroism spectra showed that single-stranded DNA undergoes a structural change in anionic RMs. This structural change is potentially due to cation-mediated adsorption of the DNA phosphates on the anionic headgroups of the RMs, leading to protection of the guanine from oxidatively generated damage.

Catalytic, enantioselective alkylation of alpha-imino esters: the synthesis of nonnatural alpha-amino acid derivatives.

Methodology for the practical synthesis of nonnatural amino acids has been developed through the catalytic, asymmetric alkylation of alpha-imino esters and N,O-acetals by enol silanes, ketene acetals, alkenes, and allylsilanes using chiral transition metal-phosphine complexes as catalysts (1-5 mol %). The alkylation products, which are prepared with high enantioselectivity (up to 99% ee) and diastereoselectivity (up to 25:1/anti:syn), are protected nonnatural amino acids that represent potential precursors to natural products and pharmaceuticals. A kinetic analysis of the catalyzed reaction of alkenes with alpha-imino esters is presented to shed light on the mechanism of this reaction.

Radical Nature of Pathways to Alkene and Ester from Thermal Decomposition of Primary Alkyl Diacyl Peroxide.

Thermal decomposition of a primary alkyl diacyl peroxide 2 is investigated. Dependence of product yields on temperature, viscosity, and solvent polarity is examined in a variety of media. The excess of the alkene disproportionation product 4 and the presence of ester 3 and acid 5 is argued to demonstrate the existence of a discrete acyloxy-alkyl geminate radical pair. Stereoselective deuterium labeling of 2 and subsequent (1)H-NMR analysis of the resulting isotopomers of 4 confirm the radical nature of detected decomposition products.