Charge Transfer Doping Induced Conformational Ordering of a Non-Crystalline Conjugated Polymer

Charge transfer doping of a nominally disordered conjugated polymer induces long-range conformational ordering (stiffening) of backbone segments. Addition of [2,3-dichloro-5,6-dicyano-<i>p</i>-benzoquinone (DDQ) to dilute solutions of poly­[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) results in quantitative charge transfer in the ground electronic state of the polymer. Following charge (hole) injection, greater MDMO-PPV monomer coplanarity is evident in electronic, Raman, and electron paramagnetic resonance (EPR) spectra over a broad range of dopant loadings. New transitions emerge at lower energies with spectral patterns distinct from pristine materials but closely resemble minority low energy conformers selectively that can be prepared by careful control of processing conditions. We further demonstrate that characteristic Raman patterns of PPV systems actually contain signatures of a minority ordered form that interacts preferentially with the dopant. Subsequent additions of dopant also show that most chains convert into the low energy form. This notion is consistent with greater backbone planarity and, hence, lower torsional reorganization energies required to access the cation form. Preresonant excitation of the emergent red-shifted optical transition reveals long overtone-combination progressions due to enhanced electronic delocalization along planarized backbone segments and diminished coupling the surroundings. We propose that planarity enhancements from doping also lead to the eventual formation of spinless bipolarons, evident from EPR spectra. Facile charge transfer doping induced conversion into the ordered MDMO-PPV conformer suggests that better control of polymer conformations and carrier levels could be harnessed to improve charge and energy transport efficiency in optoelectronic devices

EPR, ENDOR, and Electronic Structure Studies of the Jahn–Teller Distortion in an Fe^V Nitride

The recently synthesized and isolated low-coordinate Fe^V nitride complex has numerous implications as a model for high-oxidation states in biological and industrial systems. The trigonal [PhB­(^<i>t</i>BuIm)₃Fe^VN]⁺ (where (PhB­(^<i>t</i>BuIm)₃^– = phenyltris­(3-<i>tert</i>-butylimidazol-2-ylidene)), (<b>1</b>) low-spin <i>d</i>³ (<i>S</i> = 1/2) coordination compound is subject to a Jahn–Teller (JT) distortion of its doubly degenerate ²E ground state. The electronic structure of this complex is analyzed by a combination of extended versions of the formal two-orbital pseudo Jahn–Teller (PJT) treatment and of quantum chemical computations of the PJT effect. The formal treatment is extended to incorporate mixing of the two <i>e</i> orbital doublets (30%) that results from a lowering of the idealized molecular symmetry from <i>D</i>_3<i>h</i> to <i>C</i>_3<i>v</i> through strong “doming” of the Fe–C₃ core. Correspondingly we introduce novel DFT/CASSCF computational methods in the computation of electronic structure, which reveal a quadratic JT distortion and significant <i>e</i>–<i>e</i> mixing, thus reaching a new level of synergism between computational and formal treatments. Hyperfine and quadrupole tensors are obtained by pulsed 35 GHz ENDOR measurements for the ^14/15N-nitride and the ¹¹B axial ligands, and spectra are obtained from the imidazole-2-ylidene ¹³C atoms that are not bound to Fe. Analysis of the nitride ENDOR tensors surprisingly reveals an essentially spherical nitride trianion bound to Fe, with negative spin density and minimal charge density anisotropy. The four-coordinate ¹¹B, as expected, exhibits negligible bonding to Fe. A detailed analysis of the frontier orbitals provided by the electronic structure calculations provides insight into the reactivity of <b>1</b>: JT-induced symmetry lowering provides an orbital selection mechanism for proton or H atom transfer reactivity

Enhanced Charge Transfer Doping Efficiency in J‑Aggregate Poly(3-hexylthiophene) Nanofibers

Charge transfer doping efficiencies of π-stacked poly­(3-hexylthiophene) (P3HT) aggregate nanofibers are studied using spectroscopic and electron microscopy probes. Solution dispersions of self-assembled P3HT nanofibers are doped in the ground electronic state by adding varying amounts (w/w%) of the strong charge transfer dopant 2,3,5,6-tetra­fluoro-7,7,8,8-tetracyano­quino­dimethane (F₄-TCNQ). Careful control of self-assembly conditions allows us to select either the H- and J-aggregate limiting forms, which differ primarily in the degree of purity (i.e., molecular weight fractionation) and nanomorphology. Electron paramagnetic resonance (EPR), electronic absorption, and Raman spectroscopy of F₄-TCNQ^–:P3HT⁺ species are then used to track doping efficiency with dopant loading. J-aggregate nanofibers exhibit over an order of magnitude larger doping efficiencies than polymorphic H-aggregate nanofibers. The higher purity and order of the former promote intrachain polaron delocalization whereas disorder arising from greater molecular weight polydispersity in the latter instead lead to polaron localization resulting in charge transfer complex formation. Interestingly, J-aggregate nanofiber EPR signals decrease significantly after ∼25% F₄-TCNQ loading which we attribute to increased antiferromagnetic coupling between delocalized hole polarons on neighboring P3HT chains leading to spinless interchain bipolarons. Raman spectra excited on resonance with NIR F₄-TCNQ^–:P3HT⁺ absorption transitions also reveal quinoid distortions of the P3HT backbone in J-aggregates. We propose that self-assembly approaches to control aggregate packing and purity can potentially be harnessed to achieve long-range, anisotropic charge transport with minimal losses

Metal Complexes (M = Zn, Sn, and Pb) of 2‑Phosphinobenzenethiolates: Insights into Ligand Folding and Hemilability

The divalent metal complexes M^II{(SC₆H₄-2-PR₂)-κ²S,P}₂ (<b>3</b>–<b>7</b>, and <b>9</b>–<b>11</b>) (M = Zn, Sn, or Pb; R = ^<i>i</i>Pr, ^<i>t</i>Bu, or Ph), the Sn­(IV) complexes Sn­{(SC₆H₄-2-PR₂)-κ²-S,P}­Ph₂Cl (<b>12</b> and <b>13</b>) (R = ^<i>i</i>Pr and ^<i>t</i>Bu), and the ionic Sn­(IV) complexes [Sn­{(SC₆H₄-2-PR₂)-κ²-S,P}­Ph₂]­[BPh₄] (<b>14</b> and <b>15</b>) (R = ^<i>i</i>Pr and ^<i>t</i>Bu) have been prepared and characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction when suitable crystals were afforded. The Sn­(II) and Pb­(II) complexes with R = Ph, ^<i>i</i>Pr, or ^<i>t</i>Bu (<b>5</b>, <b>6</b>, <b>9</b>, and <b>10</b>) demonstrated ligand “folding” hinging on the P,S vectora behavior driven by the repulsions of the metal/phosphorus and metal/sulfur lone pairs and increased M-S sigma bonding strength. This phenomenon was examined by density functional theory (DFT) calculations for the compounds in both folded and unfolded states. The Sn­(IV) compound <b>13</b> (R = ^<i>t</i>Bu) crystallized with the phosphine in an axial position of the pseudotrigonal bipyramidal complex and also exhibited hemilability in the Sn–P dative bond, while compound <b>12</b> (R = ^<i>i</i>Pr), interestingly, crystallized with phosphine in an equatorial position and did not show hemilability. Finally, the crystal structure of <b>15</b> (R = ^<i>t</i>Bu) revealed the presence of an uncommon, 4-coordinate, stable Sn­(IV) cation

Metal Complexes (M = Zn, Sn, and Pb) of 2‑Phosphinobenzenethiolates: Insights into Ligand Folding and Hemilability

The divalent metal complexes M^II{(SC₆H₄-2-PR₂)-κ²S,P}₂ (<b>3</b>–<b>7</b>, and <b>9</b>–<b>11</b>) (M = Zn, Sn, or Pb; R = ^<i>i</i>Pr, ^<i>t</i>Bu, or Ph), the Sn­(IV) complexes Sn­{(SC₆H₄-2-PR₂)-κ²-S,P}­Ph₂Cl (<b>12</b> and <b>13</b>) (R = ^<i>i</i>Pr and ^<i>t</i>Bu), and the ionic Sn­(IV) complexes [Sn­{(SC₆H₄-2-PR₂)-κ²-S,P}­Ph₂]­[BPh₄] (<b>14</b> and <b>15</b>) (R = ^<i>i</i>Pr and ^<i>t</i>Bu) have been prepared and characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction when suitable crystals were afforded. The Sn­(II) and Pb­(II) complexes with R = Ph, ^<i>i</i>Pr, or ^<i>t</i>Bu (<b>5</b>, <b>6</b>, <b>9</b>, and <b>10</b>) demonstrated ligand “folding” hinging on the P,S vectora behavior driven by the repulsions of the metal/phosphorus and metal/sulfur lone pairs and increased M-S sigma bonding strength. This phenomenon was examined by density functional theory (DFT) calculations for the compounds in both folded and unfolded states. The Sn­(IV) compound <b>13</b> (R = ^<i>t</i>Bu) crystallized with the phosphine in an axial position of the pseudotrigonal bipyramidal complex and also exhibited hemilability in the Sn–P dative bond, while compound <b>12</b> (R = ^<i>i</i>Pr), interestingly, crystallized with phosphine in an equatorial position and did not show hemilability. Finally, the crystal structure of <b>15</b> (R = ^<i>t</i>Bu) revealed the presence of an uncommon, 4-coordinate, stable Sn­(IV) cation

Ligand Control of Donor–Acceptor Excited-State Lifetimes

Transient absorption and emission spectroscopic studies on a series of diimineplatinum­(II) dichalcogenolenes, LPtL′, reveal charge-separated dichalcogenolene → diimine charge-transfer (LL′CT) excited-state lifetimes that display a remarkable and nonperiodic dependence on the heteroatoms of the dichalcogenolene ligand. Namely, there is no linear relationship between the observed lifetimes and the principle quantum number of the E donors. The results are explained in terms of heteroatom-dependent singlet–triplet (S–T) energy gaps and anisotropic covalency contributions to the M–E (E = O, S, Se) bonding scheme that control rates of intersystem crossing. For the dioxolene complex, <b>1-O,O′</b>, <i>E</i>(T₂) > <i>E</i>(S₁) and rapid nonradiative decay occurs from S₁ to S₀. However, <i>E</i>(T₂) ≤ <i>E</i>(S₁) for the heavy-atom congeners, and this provides a mechanism for rapid intersystem crossing. Subsequent internal conversion to T₁ in <b>3-S,S</b> produces a long-lived, emissive triplet. The two LPtL′ complexes with mixed chalcogen donors and <b>5-Se,Se</b> show lifetimes intermediate between those of <b>1-O,O′</b> and <b>3-S,S</b>

Synthesis and Characterization of the Actinium Aquo Ion

Metal aquo ions occupy central roles in all equilibria that define metal complexation in natural environments. These complexes are used to establish thermodynamic metrics (i.e., stability constants) for predicting metal binding, which are essential for defining critical parameters associated with aqueous speciation, metal chelation, <i>in vivo</i> transport, and so on. As such, establishing the fundamental chemistry of the actinium­(III) aquo ion (Ac-aquo ion, Ac­(H₂O)_<i>x</i>³⁺) is critical for current efforts to develop ²²⁵Ac [<i>t</i>_1/2 = 10.0(1) d] as a targeted anticancer therapeutic agent. However, given the limited amount of actinium available for study and its high radioactivity, many aspects of actinium chemistry remain poorly defined. We overcame these challenges using the longer-lived ²²⁷Ac [<i>t</i>_1/2 = 21.772(3) y] isotope and report the first characterization of this fundamentally important Ac-aquo coordination complex. Our X-ray absorption fine structure study revealed 10.9 ± 0.5 water molecules directly coordinated to the Ac^III cation with an Ac–O_H2O distance of 2.63(1) Å. This experimentally determined distance was consistent with molecular dynamics density functional theory results that showed (over the course of 8 ps) that Ac^III was coordinated by 9 water molecules with Ac–O_H2O distances ranging from 2.61 to 2.76 Å. The data is presented in the context of other actinide­(III) and lanthanide­(III) aquo ions characterized by XAFS and highlights the uniqueness of the large Ac^III coordination numbers and long Ac–O_H2O bond distances

Examining the Effects of Ligand Variation on the Electronic Structure of Uranium Bis(imido) Species

Arylazide and diazene activation by highly reduced uranium­(IV) complexes bearing trianionic redox-active pyridine­(diimine) ligands, [Cp^PU­(^MesPDI^Me)]₂ (<b>1-Cp</b>^<b>P</b>), Cp*U­(^MesPDI^Me)­(THF) (<b>1-Cp*</b>) (Cp^P = 1-(7,7-dimethylbenzyl)­cyclopentadienide; Cp* = η⁵-1,2,3,4,5-pentamethylcyclopentadienide), and Cp*U­(^<i>t</i>Bu-^MesPDI^Me) (THF) (<b>1-</b>^{<i><b>t</b></i>}<b>Bu</b>) (2,6-((Mes)­NCMe)­2-<i>p</i>-R-C₅H₂N, Mes = 2,4,6-trimethylphenyl; R = H, ^MesPDI^Me; R = C­(CH₃)₃, ^<i>t</i>Bu-^MesPDI^Me), has been investigated. While <b>1-Cp*</b> and <b>1-Cp</b>^<b>P</b> readily reduce N₃R (R = Ph, <i>p</i>-tolyl) to form <i>trans</i>-bis­(imido) species, Cp^PU­(NAr)₂(^MesPDI^Me) (Ar = Ph, <b>2-Cp</b>^<b>P</b>; Ar = <i>p</i>-Tol, <b>3-Cp</b>^<b>P</b>) and Cp*U­(NPh)₂(^MesPDI^Me) (<b>2-Cp*</b>), only <b>1-Cp*</b> can cleave diazene NN double bonds to form the same product. Complexes <b>2-Cp*</b>, <b>2-Cp</b>^<b>P</b>, and <b>3-Cp</b>^<b>P</b> are uranium­(V) <i>trans</i>-bis­(imido) species supported by neutral [^MesPDI^Me]⁰ ligands formed by complete oxidation of [^MesPDI^Me]^3– ligands of <b>1-Cp</b>^<b>P</b> and <b>1-Cp*</b>. Variation of the arylimido substituent in <b>2-Cp*</b> from phenyl to <i>p</i>-tolyl, forming Cp*U­(NTol)₂(^MesPDI^Me) (<b>3-Cp*</b>), changes the electronic structure, generating a uranium­(VI) ion with a monoanionic pyridine­(diimine) radical. The <i>tert</i>-butyl-substituted analogue, Cp*U­(NTol)₂(^<i>t</i>Bu-^MesPDI^Me) (<b>3-</b>^{<i><b>t</b></i>}<b>Bu</b>), displays the same electronic structure. Oxidation of the ligand radical in <b>3-Cp*</b> and <b>3-</b>^{<i><b>t</b></i>}<b>Bu</b> by Ag­(I) forms cationic uranium­(VI) [Cp*U­(NTol)₂(^MesPDI^Me)]­[SbF₆] (<b>4-Cp*</b>) and [Cp*U­(NTol)₂(^<i>t</i>Bu-^MesPDI^Me)]­[SbF₆] (<b>4-</b>^{<i><b>t</b></i>}<b>Bu</b>), respectively, as confirmed by metrical parameters. Conversely, oxidation of pentavalent <b>2-Cp*</b> with AgSbF₆ affords cationic [Cp*U­(NPh)₂(^MesPDI^Me)]­[SbF₆] (<b>5-Cp*</b>) from a metal-based U­(V)/U­(VI) oxidation. All complexes have been characterized by multidimensional NMR spectroscopy with assignments confirmed by electronic absorption spectroscopy. The effective nuclear charge at uranium has been probed using X-ray absorption spectroscopy, while structural parameters of <b>1-Cp</b>^<b>P</b>, <b>3-Cp*</b>, <b>3-</b>^{<i><b>t</b></i>}<b>Bu</b>, <b>4-Cp*</b>, <b>4-</b>^{<i><b>t</b></i>}<b>Bu</b>, and <b>5-Cp*</b> have been elucidated by X-ray crystallography

Advancing Understanding of the +4 Metal Extractant Thenoyltrifluoroacetonate (TTA^–); Synthesis and Structure of M^IVTTA₄ (M^IV = Zr, Hf, Ce, Th, U, Np, Pu) and M^III(TTA)₄^– (M^III = Ce, Nd, Sm, Yb)

Thenoyltrifluoroacetone (HTTA)-based extractions represent popular methods for separating <i>micro</i>scopic amounts of transuranic actinides (i.e., Np and Pu) from <i>macro</i>scopic actinide matrixes (e.g. bulk uranium). It is well-established that this procedure enables +4 actinides to be selectively removed from +3, + 5, and +6 f-elements. However, even highly skilled and well-trained researchers find this process complicated and (at times) unpredictable. It is difficult to improve the HTTA extractionor find alternativesbecause little is understood about why this separation works. Even the identities of the extracted species are unknown. In addressing this knowledge gap, we report here advances in fundamental understanding of the HTTA-based extraction. This effort included comparatively evaluating HTTA complexation with +4 and +3 metals (M^IV = Zr, Hf, Ce, Th, U, Np, and Pu vs M^III = Ce, Nd, Sm, and Yb). We observed +4 metals formed neutral complexes of the general formula M^IV(TTA)₄. Meanwhile, +3 metals formed anionic M^III(TTA)₄^– species. Characterization of these M­(TTA)₄^<i>x</i>– (<i>x</i> = 0, 1) compounds by UV–vis–NIR, IR, ¹H and ¹⁹F NMR, single-crystal X-ray diffraction, and X-ray absorption spectroscopy (both near-edge and extended fine structure) was critical for determining that Np^IV(TTA)₄ and Pu^IV(TTA)₄ were the primary species extracted by HTTA. Furthermore, this information lays the foundation to begin developing and understanding of why the HTTA extraction works so well. The data suggest that the solubility differences between M^IV(TTA)₄ and M^III(TTA)₄^– are likely a major contributor to the selectivity of HTTA extractions for +4 cations over +3 metals. Moreover, these results will enable future studies focused on explaining HTTA extractions preference for +4 cations, which increases from Np ^IV to Pu^IV, Hf^IV, and Zr^IV

Covalency in Americium(III) Hexachloride

Developing a better understanding of covalency (or orbital mixing) is of fundamental importance. Covalency occupies a central role in directing chemical and physical properties for almost any given compound or material. Hence, the concept of covalency has potential to generate broad and substantial scientific advances, ranging from biological applications to condensed matter physics. Given the importance of orbital mixing combined with the difficultly in measuring covalency, estimating or inferring covalency often leads to fiery debate. Consider the 60-year controversy sparked by Seaborg and co-workers (Diamond, R. M.; Street, K., Jr.; Seaborg, G. T. J. Am. Chem. Soc. 1954, 76, 1461) when it was proposed that covalency from 5<i>f</i>-orbitals contributed to the unique behavior of americium in chloride matrixes. Herein, we describe the use of ligand K-edge X-ray absorption spectroscopy (XAS) and electronic structure calculations to quantify the extent of covalent bonding inarguablyone of the most difficult systems to study, the Am–Cl interaction within AmCl₆^3–. We observed both 5<i>f</i>- and 6<i>d</i>-orbital mixing with the Cl-3<i>p</i> orbitals; however, contributions from the 6<i>d</i>-orbitals were more substantial. Comparisons with the isoelectronic EuCl₆^3– indicated that the amount of Cl 3<i>p</i>-mixing with Eu^III 5d-orbitals was similar to that observed with the Am^III 6<i>d</i>-orbitals. Meanwhile, the results confirmed Seaborg’s 1954 hypothesis that Am^III 5<i>f-</i>orbital covalency was more substantial than 4<i>f</i>-orbital mixing for Eu^III