Free-standing iodine-doped composite samples of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with carbon nanotubes (NTs) showed thermoelectric (TE) power factors (PFs) up to 33 μW·m–1·K–2 after optimizing multiple factors, including: (1) sample fabrication solvent, (2) doping time, (3) average MEH-PPV molecular weight, (4) NT fraction in the composite, and (5) use of single-wall versus multi-wall nanotubes (SWNT and MWNT, respectively). Composite fabrication from halogenated solvents gave the best TE performance after iodine doping times of 2–4 h; performance drops substantially in ∼20 h doped samples. TE performance dropped after at least 24 h of removal from iodine vapor but was fully restored upon re-exposure to the dopant. Longer-chain MEH-PPV gave not only mechanically stronger films but also higher PFs in doped SWNT composites. MWNT composites gave low PFs, attributed to poor NT dispersion. Scanning electron microscopy showed increasingly extensive network formation as NT fraction increased in the composites; this phase separation provides charge transport pathways that improve thermoelectric PFs. The results support a strategy of producing phase-separated materials having both electrical conduction enhanced regions and Seebeck thermopower retaining regions to maximize organic TE response.Keywords: doped polymer thermoelectrics; MEH-PPV; multi-wall carbon nanotubes; organic polymer thermoelectrics; polymer−nanotube composites; scanning electron microscopy; single-wall carbon nanotubes;

The synthesis, crystal structures and magnetic properties of two families of heterospin complexes containing lanthanide ions and a bis(imino nitroxide) biradical (IPhIN = 1-iodo-3,5-bis(4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1H-imidazole-1′-oxyl)benzene) are reported: in [Ln2(hfac)6(IPhIN)(H2O)2] compounds, two lanthanide ions [Ln = GdIII (1) and DyIII (2)] are coordinated to the biradical, and in [Ln(hfac)3(IPhIN)(H2O)] compounds, one lanthanide ion (Ln = TbIII (3), GdIII (4) or DyIII (5)) is coordinated to the biradical. Ferromagnetic intramolecular magnetic interactions between GdIII and the biradical were found for 1 and 4, while intramolecular magnetic interactions between the radicals were ferro- and antiferromagnetic, respectively. Compound 2 shows a field induced slow relaxation of magnetization, which (under an external applied field of 2 kOe) exhibits an activation energy barrier of ΔE/kB = 27 K and a pre-exponential factor of 1.4 × 10−8 s. To support the magnetic characterization of compound 3ab initio calculations were also performed.

The first syntheses of bis(nitronyl nitroxide) and bis(iminoyl nitroxide) (diNN, diIN) biradicals linked through rigid acene core conjugating anthracene (A) and anthraquinone (AQ) units are reported. Computational modeling predicts weak intramolecular exchange in AQ-linked systems, but A-linked biradicals to have ground state multiplicities consistent with the Borden-Davidson disjointness model. Solution electron spin resonance spectra showed inter-radical exchange-coupled triplet states, except for 2,6-AQ biradicals showing isolated spin spectra. Crystallography of the A-linked biradicals shows a key role for inter-radical contacts for molecular packing. DiINs showed lower-dimensional dyad packing with disorder at the radical units: the conformationally more symmetrical diNNs gave staircase one-dimensional or brickwork two-dimensional lattices. Core anthracene unit stacking was only seen in two systems with bromine on the central anthracene ring: the (large) bromine occupies alternate side placement in dyad stacks for the diIN, chain stacks for the diNN. Magnetism of 2,7-A-linked systems showed predominant ferromagnetic intramolecular triplet-singlet splitting of 24–28 K for diNNs and 8 K for diINs, plus weak antiferromagnetic (AFM) interactions from intermolecular contacts. The 2,6-A-linked biradicals showed AFM exchange between spins. Both A and AQ cores offer possibilities for electronic material development, with a combination of multiple radical spins and π-electron-rich acene cores.

Rigid–flexible segmented block copolymers were synthesized and characterized as 4.5-oligophenylenevinylene chromophores tethered by flexible, conjugation-interrupting 1,2-ethanedioxy or 1,4-butanedioxy units. The flexible tethers allow the possibility of collapsed order chromophore assemblies within individual polymers by chain folding at specific sites much like an old fashioned, folding carpenter’s rule. Our results indicate that using a short, flexible tether in a rigid–flexible segmented copolymer can result in collapsed rodlike structures as signaled by strongly quenched photoluminescence, even after thermal annealing. Such ability to “program” folding and tertiary structure in conjugated copolymers is important for solid-state organic light emitting materials and understanding of organic chromophore self-assembly.

Polymer solar cells fabricated in air under ambient conditions are of significant current interest, because of the implications in practicality of such devices. However, only moderate performance has been obtained for the air-processed devices. Here, we report that enhanced short circuit current density (JSC) and open circuit voltage (VOC) in air-processed poly(3-hexylthiophene) (P3HT)-based solar cells can be obtained by using a series of donor–acceptor dyes as the third component in the device. Power conversion efficiencies up to 4.6% were obtained upon addition of the dyes which are comparable to high-performance P3HT solar cells fabricated in controlled environments. Multilayer planar solar cells containing interlayers of the donor–acceptor dyes, revealed that along with infrared sensitization, an energy level cascade architecture and Förster resonance energy transfer could contribute to the enhanced performance.Keywords: bulk heterojunction solar cells; donor−acceptor dyes; planar multilayer solar cells; polymer solar cells

2,4-Bis[4′-(N,N-di(4″-hydroxyphenyl)amino)-2′,6′-dihydroxyphenyl]squaraine (Sq-TAA-OH, optical bandgap 1.4 eV, HOMO level −5.3 eV by ultraviolet photoelectron spectroscopy) is used as an active layer material in solution processed, bulk-heterojunction organic photovoltaic cells with configuration ITO/PEDOT:PSS/Sq-TAA-OH:PC71BM/LiF/Al. Power conversion efficiencies (PCEs) up to 4.8% are obtained by a well-reproducible procedure using a mixture of good and poor Sq-TAA-OH solubilizing organic solvents, with diiodooctane (DIO) additive to make a bulk heterojunction layer, followed by thermal annealing, to give optimized VOC = 0.84–0.86 V, JSC = 10 mA cm–2, and FF = 0.53. X-ray diffraction and scattering studies of pristine, pure Sq-TAA-OH solution-cast films show d-spacing features similar to single-crystal packing and spacing. The DIO additive in a good solvent/poor solvent mixture apparently broadens the size distribution of Sq-TAA-OH crystallites in pristine films, but thermal annealing provides a narrower size distribution. Direct X-ray diffraction and scattering morphological studies of “as-fabricated” active layers show improved Sq-TAA-OH/PC71BM phase separation and formation of crystallites, ∼48 nm in size, under conditions that give the best PCE.Keywords: molecule-based solar cells; N,N-diarylanilinosquaraines; organic bulk heterojunctions; solution-processed photovoltaic devices; squaraines; triarylamines

3,6-Bis(N,N-dianisylamino)-fluoren-9-ylidene malononitrile (FMBDAA36) was used as an electron donor material in solution-processed organic photovoltaic devices with configuration ITO/PEDOT:PSS/(1:3[w/w] FMBDAA36:PC71BM)/LiF/Al to give power conversion efficiencies up to 4.1% with open circuit voltage VOC = 0.89 V, short circuit current JSC = 10.35 mA cm–2, and fill factor FF = 44.8%. Conductive atomic force microscopy of the active layer showed granular separation of regions exhibiting easy versus difficult hole transport, consistent with bulk heterojunction type phase separation of FMBDAA36 and PC71BM, respectively. Single-crystal X-ray diffraction analysis showed pure FMBDAA36 to form columnar π-stacks with a 3.3 Å intermolecular spacing.Keywords: conductive atomic force microscopy; fluorenylidene malononitrile; organic photovoltaics; small molecule photovoltaics; solution-processed solar cells

1,3-Bis(4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1′H-imidazol-2′-yl-1′-oxyl)-5-iodobenzene (IPhIN2) forms a co-crystal with dichloromethane (CH2Cl2) and some water, with columnar π-stacking of radical N–O moieties organized into antiferromagnetically exchange-coupled 1-D chains.

A series of squaraine dyes was synthesized with electron-donating (OH, C6H13) and electron-withdrawing (F, CF3) groups allowing for tuning of the optical and electrochemical properties of the dyes. The squaraines exhibited strong absorbance (ε = 104–105 M–1 cm–1) at long wavelengths (λmax = 660–690 nm), attributed to an intramolecular charge transfer (ICT). As the electron-donating character of the dyes increases, the absorbance of the dyes red shifts and the band gap decreases. Interestingly, the ICT band seemed to be strongly dependent on the nature of the solvent, providing insights into the importance of hydrogen-bonding-induced coplanarity in these molecules. The squaraines were investigated for their charge-carrier mobility in FET configuration. Dyes with fluorine functional groups were found to exhibit either ambipolar (−F) or n-type (−CF3) charge-carrier characteristics, although the molecules themselves are made of traditionally p-type triarylamines.

A single hydrogen bonding molecular assembly unit (benzimidazole = BIm) was used for solid state assembly of different types of radicals connected at different sites on the assembly unit. The effects of changing radical spin unit and molecular connectivity were tested by comparing tetramethylnitronylnitroxide (NN), dimethyloxoverdazyl (mV), and diisopropyloxoverdazyl (iV) spin units attached to BIm at either the 2-position or 5/6-position. The mV systems were not isolated with sufficient purity and stability for crystallographic or magnetic analysis, but chain-type hydrogen bonding was found in the crystal structures of all of the other, stable compounds. Close contacts are induced between radical groups in some cases by the hydrogen bonded chains, giving exchange behavior ranging from essentially isolated spin paramagnetism (BIm2iV) to antiferromagnetic interactions of (−)1–3 K.1H-Benzimidazole substituted with nitronylnitroxide (5-position, NN) or oxo-verdazyl (2- or 6-positions, iV) radicals form 1-D hydrogen bonded chains in crystalline solids; the NN variant incorporates H2O in the chains. The chain motifs bring the radical spin units into different degrees of proximity to give 1-D antiferromagnetic exchange interactions up to J/k ≈ (−)2–3 K where 6-iV ⩾ 5-NN > 2-iV in order of magnitude.

Silicon wafers both without and with silicon(IV) oxide surface coverage were covered with benzene solutions of stable organic radical 3-(N-tert-butyl-N-aminoxyl)benzoic acid (mNBA). X-ray photoelectron spectroscopy supported the presence of the radical on both surface-cleaned (oxide-reduced) and oxide-covered surfaces. Optical waveguide spectroscopy showed that the radical retained its structure while adsorbed to the surface of the wafers, without noticeable decomposition. AFM and MFM imaging showed that the radical formed blocky particles with a change in rms roughness from 0.3 nm premodification to 1.7 nm postmodification on the surface-cleaned silicon. Similar experiments using oxide-coated silicon showed that the radical adsorbed to form much smoother layers, with a small change in rms roughness from 0.2 to 0.3 nm. Contact angle measurements of water on the premodified and postmodified samples showed a large, hydrophobic change in the silicon oxide surface but only a modest change in the surface-cleaned silicon surface. Samples of mNBA adsorbed onto silica gel showed strong electron-spin resonance signals from the aminoxyl spin, even years after production. The results demonstrate the prospects for treating and coating oxide-covered silicon wafers and silicon oxide-coated particles with a paramagnetically active organic substrate, without major chemical modification of the pretreatment surface; the resulting organic spin sites can be stable for years.

Two polymorphs of an anthraquinone-nitronylnitroxide radical, AntQNN, were isolated, both with antiferromagnetic (AFM) exchange attributed to chain-type inter-radical contacts: one with J1D/k ≈ −3 K, and one with J1D/k ≈ −17 K.

2-(1′-Pyrenyl)nitronylnitroxide (PyrNN) forms a 2:1:2 co-crystal with octafluoronaphthalene (OFN) and entrained solvent dichloromethane (DCM), in which the radical is “shepherded” into forming chains of radical–radical contacts on the peripheries of (PyrNN–OFN–PyrNN)n π-stacks, giving weak, low dimensional inter-radical antiferromagnetic exchange interactions.

2-(3′,5′-Dimethoxy-4′-hydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (SyrNN) forms a hydrogen bonded network of contacts involving phenolic OH and nitronylnitroxide methyl groups as donors, and nitronylnitroxide NO groups as acceptors. The SyrNN intermolecular contact network is quite similar to that of 2-(4′-hydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (HOPhNN) (Cirujeda et al., J. Mater. Chem. 1995, 5, 243–252), which shows ferromagnetic exchange interactions, but SyrNN has an additional antiparallel stacking contact between nitronylnitroxide NO groups that qualitatively and computationally is expected to induce antiferromagnetic (AFM) exchange between SyrNN molecules. Experimentally, the magnetism of SyrNN shows evidence of multiple exchange interactions with overall low dimensional AFM exchange behavior. Fits of dc magnetic susceptibility versus temperature data for SyrNN to AFM 1-D chain and 2-D square planar models give J/k ∼ (−)1–3 K.

Chemical oxidation of diarylamino-substituted fluorenones gives highly persistent radical cations, some of which can be stored in the solid state for months. Solution UV–vis-NIR spectra show strong monoradical cation absorption bands in the 750–1700 nm range, with minimum energy about 0.73 eV. 2,7-Bis(dianisylamino)fluorenone exhibits strong absorptions at 1670 (monocation) and 873 nm (dication). Fast intervalence electron transfer behavior between amine sites in the monocation is indicated by EPR spectroscopic and absorption spectral Hush analyses.

•1,4-Distyrylbenzene block copolymers (BCPs) with (CH2)n can be thermoelectric.•P-doped BCP films give electrical conductivities of 10−5 to 10−3 S cm−1.•The same BCP films give Seebeck coefficients of 24–2650 S μV K−1.•I2-doped BCP blends with MEH-PPV give Seebeck coefficients of 7–276 S μV K−1.•The same doped MEH-PPV/BCP blends give power factors up to 1 μW m−1 K−2.Segmented block copolymers (BCPs) of conjugated distyrylbenzene conjugated units linked by variable flexible nonconjugated units were p-doped, and their electrical conduction and Seebeck thermoelectric (TE) behaviors evaluated. The doped BCP Seebeck coefficients ranged up about 2600 μV K−1, but with low conductivities giving low TE power factors. BCP blends with MEH-PPV gave much improved conductivity when doped with I2, and retained high enough Seebeck coefficients (up to about 530 μV K−1) to give significantly higher TE power factors compared to the doped BCPs alone.

The X- and Q-band EPR spectra of coordination compounds of 5-(3-N-tert-butyl-N-aminoxylphenyl)pyrimidine (3PPN) with Cu(hfac)2 show peaks in the g ≈ 2.3 region attributable to parallel component transitions of Cu(II) axially coordinated to pyrimidine, and in the g ≈ 2.0–2.2 region attributable to Cu(II) directly coordinated with the aminoxyl NO· radical unit plus perpendicular component transitions of the Cu(II) coordinated to pyrimidine. The spectral region with the Cu(II)–aminoxyl resonance can show temperature dependent behavior attributable to reversible Jahn–Teller distortions, analogous to behavior seen in Ovcharenko’s “breathing crystal” Cu(II) complexes with nitronylnitroxides (e.g., Veber et al. (2008) [5]).X- and Q-band EPR spectroscopy of 0D, 1D, 2D, and 3D networked Cu(hfac)2 complexes with a conformationally flexible, conjugated, aminoxyl radical ligand enables identification of magnetically isolated and exchange-interacting Cu(II) ions, due to different resonance positions and temperature dependence of line shapes. The changes in spectra with temperature and structure can be correlated with the known geometries and Jahn–Teller distortion of the Cu(II) sites in the complexes.

Mixtures of 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (F4BImNN) and 2-(benzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (BImNN) crystallize as solid solutions (alloys) across a wide range of binary compositions. (F4BImNN)x(BImNN)(1–x) with x < 0.8 gives orthorhombic unit cells, while x ≥ 0.9 gives monoclinic unit cells. In all crystalline samples, the dominant intermolecular packing is controlled by one-dimensional (1D) hydrogen-bonded chains that lead to quasi-1D ferromagnetic behavior. Magnetic analysis over 0.4–300 K indicates ordering with strong 1D ferromagnetic exchange along the chains (J/k = 12–22 K). Interchain exchange is estimated to be 33- to 150-fold weaker, based on antiferromagnetic ordered phase formation below Néel temperatures in the 0.4–1.2 K range for the various compositions. The ordering temperatures of the orthorhombic samples increase linearly as (1 – x) increases from 0.25 to 1.00. The variation is attributed to increased interchain distance corresponding to decreased interchain exchange, when more F4BImNN is added into the orthorhombic lattice. The monoclinic samples are not part of the same trend, due to the different interchain arrangement associated with the phase change.

2-(1′-Pyrenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (PyrNN) was reacted with M(hfac)2 (M = Mn(II) and Co(II), hfac = hexafluoracetylacetonate) to give two isostructural ML2 stoichiometry M(hfac)2(PyrNN)2 complexes and a ML stoichiometry one-dimensional (1-D) polymer chain complex [Mn(hfac)2(PyrNN)]. The ML2 complexes have similar crystal structures with monoclinic unit cells, in which one NO unit from each PyrNN ligand is bonded to the transition metal on cis vertices of a distorted octahedron. The major magnetic interactions are intracomplex metal-to-radical exchange (J), and intermolecular exchange across a close contact between the uncoordinated NO units (J′). For M = Mn(II) an approximate chain model fit gives g = 2.0, J = (−)125 cm–1 and J′ = (−)49 cm–1; for M = Co(II), g = 2.4, J = (−)180 cm–1, and J′ = (−)70 cm–1. Hybrid density functional theory (DFT) computations modeling the intermolecular exchange by using only the radical units across the close contact are in good accord with the estimated values of J′. The chain type complex structure shows solvent incorporation for overall structure [Mn(hfac)2(PyrNN)]n·0.5(CHCl3)·0.5(C7H16). Both NO groups of the PyrNN ligand are complexed to form helical chains, with very strong metal to radical antiferromagnetic exchange that gives overall ferrimagnetic behavior.

Polymorphism's effect on intermolecular packing is a critical consideration for organic magnetic materials. Magnetostructural studies of pyrene-1-yl (Pyr) bearing nitronylnitroxide (NN) and iminoylnitroxide (IN) radicals found that PyrNN gives two allotropes: one has spin-paired dyads with ΔE = J/k ≈–178 K, and one is half spin-paired with ΔE = J/k ≈ −102 K, and half paramagnetic. PyrIN also gives two allotropes, one an anti conformation that is spin paired with ΔE = J/k = −410 K, and one a paramagnetic system having a syn conformation. PyrNN co-crystallizes with C6F6 in 2:1 ratio with a methyl to nitroxide contact network exhibiting low dimensional 1-D or 2-D antiferromagnetic exchange.

Push–pull organic molecules composed of electron donor diarylamines at the 2- and 2,7-positions of fluorenone exhibit intramolecular charge-transfer behaviour in static absorption and emission spectra. Electrochemical and spectral data combined in a modular electronic analysis model show how the donor HOMO and acceptor LUMO act as major determinants of the frontier molecular orbital energy levels.

2-(Meta-aminophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (mAPN) and 2-(para-aminophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (pAPN) were synthesized and subjected to magnetostructural analysis. Both form extended hydrogen bonding networks involving both amino NH bonds to radical spin-density bearing nitronylnitroxide NO groups. Their crystallographic assembly motifs and magnetic exchange properties are compared to those of tert-butoxylcarbonyl (BOC) and amide derivatives having only one NH bond. The conversion of pAPN to acid salt derivatives gives a solid that is essentially diamagnetic, although dissolution of the solid shows the radical spin units to be preserved.Keywords: hydrogen bonding; molecular magnetism; nitronylnitroxides; radicals;

One pair of reactants, Cu(hfac)2 = M and the hinge-flexible radical ligand 5-(3-N-tert-butyl-N-aminoxylphenyl)pyrimidine (3PPN = L), yields a diverse set of five coordination complexes: a cyclic loop M2L2 dimer; a 1:1 cocrystal between an M2L2 loop and an ML2 fragment; a 1D chain of M2L2 loops linked by M; two 2D M3L2 networks of (M–L)n chains cross-linked by M with different repeat length pitches; a 3D M3L2 network of M2L2 loops cross-linking (M–L)n-type chains with connectivity different from those in the 2D networks. Most of the higher dimensional complexes exhibit reversible, temperature-dependent spin-state conversion of high-temperature paramagnetic states to lower magnetic moment states having antiferromagnetic exchange within Cu–ON bonds upon cooling, with accompanying bond contraction. The 3D complex also exhibited antiferromagnetic exchange between CuII ions linked in chains through pyrimidine rings.

Crystallographic lattice compression and magnetic variations in the quasi-1-D ferromagnet 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (F4BImNN) were studied under hydrostatic pressure for polycrystalline samples. The crystallographic c-axis—along which F4BImNN forms hydrogen bonds—was compressed by 3% at 10 kbar and by 4% at 17.8 kbar. The overall lattice volume contracts by 12% from ambient pressure to 17.8 kbar. The axis of the hydrogen bonded chain propagation is compressed, since the bent F4BImNN hydrogen bonds accommodate geometric compression. The magnetic susceptibility measured over 1.8–300 K showed an increase in ferromagnetic exchange interactions as pressure increased. Curie–Weiss 1/χ vs T analyses showed an increase in Weiss constant from (+)10.4 K to (+)15.3 K from ambient pressure to 9.85 kbar. Fitting of the χT vs T data indicated predominantly 1-D Heisenberg chain type behavior at all pressures, increasing from J/k = (+)19 K at room pressure to (+)25 K at 4.77 kbar and (+)30 K at 9.85 kbar. The increase in exchange strength is attributed to pressure-increased overlap of spin orbitals in the hydrogen bonded chains, which favors 1-D ferromagnetic interaction. There was no indication of a phase change, either crystallographically or in discontinuities of the magnetic behavior.

Q-band 34 GHz EPR spectra are reported for quartet state 2-(para-nitrenophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl and 3-(para-nitrenophenyl)-1,5,6-triphenylverdazyl reactive intermediates generated from the corresponding azido precursors under frozen matrix photochemical conditions, in situ in a Q-band resonator. Comparison of the Q-band spectra to those generated under conventional X-band (9–10 GHz) conditions shows the much superior resolution of transitions in the g > 2 region of the former. Spectral transitions assigned by line shape simulation yield the zero field splittings for the nitreno-radical species.

9-(N,N-Dianisylamino)anthracene (9DAAA), 9-(N,N-dianisylamino)dinaphth([1,2-a:2′-1′-j]-anthracene (9DAAH), and 9,10-bis(N,N-dianisylamino)anthracene (910BAA) were synthesized as highly twisted triarylamines with potential for photoexcited internal charge transfer. Crystallography of 9DAAA shows its dianisylamino group to be twisted nearly perpendicular to its anthracene unit, similar to a report for 910BAA. The solution fluorescence spectra show strong bathochromic shifts for each of the three molecular systems with strongly decreased quantum efficiency in higher polarity solvents. Solution-phase (ensemble) time-resolved photoluminescence measurements show up to 4-fold decreases in fluorescence lifetime in acetonitrile compared to hexane. The combined results are consistent with photoinduced, transient intramolecular charge-transfer from the bis-anisylamine unit to the polycyclic aromatic unit. Computational modeling is in accord with intramolecular transfer of electron density from the bis-anisylamino unit to the anthracene, based on in comparisons of HOMO and LUMO.

Stable radicals 2-(6-uradinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (Ur6IN) and 4-(p-tert-butylaminoxylphenyl)-2,6-di(propylamido)pyridine (DAPPN) form heterospin radical pair complexes due to complementary multi-point hydrogen-bonds.

The heat capacity of the title organic free radical, PhBABI, was measured over 0.3−300 K by adiabatic calorimetry and relaxation methods in the presence of external magnetic fields up to 9 T. A hump in the magnetic heat capacity was observed with a maximum at about 15 K in zero field, which did not shift at fields up to 9 T. The experimental magnetic entropy was in good agreement with the theoretical value of R ln 2 (= 5.76 J K−1 mol−1) for S = 1/2 systems. The higher temperature, field-insensitive feature was fitted to several antiferromagnetic Heisenberg models. The best fits were obtained using spin ladder and coupled spin bilayer models.

2,7-Bis(1′-pyrenylvinylene)-9,9-diethylfluorene (1) and 2,7-bis(1′-pyrenylvinylene-4″-phenylenevinylene)-9,9-diethylfluorene (2) were synthesized and their static luminescence behavior assessed. They show solution photoluminescence (PL) maxima in chloroform at 475 nm and 467 nm, with quantum efficiencies of 54% and 52%, respectively. Double-layer LED devices with configuration ITO/PEDOT-PSS/(1 or 2)/Ca–Al emitted blue-green light with turn-on voltages of 2.5 V and emission maxima at 500 nm (2.48 eV); their luminance efficiencies were 0.36 and 0.30 cd A−1, respectively. Blending of 1 at 20% (w/w) in PVK improved the luminance efficiency to 1.81 cd A−1 for the same device configuration, with only a small increase in turn-on voltage to 3 V; the emission maximum was 497 nm (blue-green emission), the maximum luminance 7600 cd m−2 with CIE color coefficients of (0.12, 0.45). For a 10% (w/w) blend of 2 in PVK using the same device configuration, the luminance efficiency was 1.47 cd A−1, turn-on voltage 3 V, maximum luminance 2600 cd m−2 with CIE coefficients of (0.13, 0.45). Simple π-MO calculations show that structural extension of the nominal conjugation length in 2 does not significantly decrease the effective band gap relative to 1, consistent with the observed lack of red shift in 2.

Stable radical 2-(6-uradinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (1) binds to hydrogen-bonding complement 2,6-di(propylamido)pyridine (DAP) in chloroform with Ka = 220 M−1 at 33 °C; ESI-MS shows not only 1∶DAP complementary dyad formation, but also 1∶(DAP)2 formation at higher concentrations of DAP.

Radicals 1–2 form networks of OH⋯ON and OH⋯Me(nitroxide) interactions. In 2, a frustrated network forms with insufficient N–O units to form extended chain interactions. The magnetism of 1 fits a 1-D Heisenberg model with J/k = −25 J mol−1, while 2 shows more complex exchange behaviour consistent with its disordered crystal lattice.

Stable radical 2-(6-uradinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl shows antiferromagnetic spin pairing with 2J/k= −14 K, attributable to a close contact between unpaired spin density on the imidazole-type nitrogen atoms; hydrogen bonds aid dimer formation, but do not appear to play an eletronic role in the magnetic behaviour.

Poly(para-phenylenevinylene-alt-3-tert-butyl-meta-phenylenevinylene) was synthesized by Wittig condensation with varying ratio of cis and trans olefin bonds. The luminescent properties of the unisomerized and trans-isomerized polymers (1 and 2, respectively) were compared. Nearly identical emission spectra were observed for both polymers in dilute solutions, but the emission intensity of 1 is lower. This is attributed to the cis vinylic moieties in 1, which favor nonradiative excited state decay processes by limiting formation of a planar excited state. Double layer LEDs with the configuration ITO/PPV/(1 or 2)/Ca–Al were fabricated. Substantially better efficiencies and luminances were observed for devices made with polymer 2 as the emissive layer, by comparison to 1. The results show that the elimination of cis olefin defects in these polymers is important for the optimization of LEDs.

Mn(hfac)2 and Cu(hfac)2 form 1∶1 complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine that exhibit strong metal–nitroxide exchange; spin polarization models do not explain the antiferromagnetic exchange behavior between spin sites in these complexes.

The EPR spectroscopy, crystallography, and magnetic susceptibility of tetrakis(N-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)silane and tetrakis(4-N-tert-butyl-N-aminoxylphenyl)silane show that silicon acts as a weak intramolecular exchange linker for polynitroxides, although both tetraradicals show onset of inter-spin exchange at reduced temperatures.

A detailed crystal structure analysis for 2-(3′,5′-difluorophenyl)nitronylnitroxide (F2PhNN) elucidates the major exchange interactions that control the previously reported magnetic behavior of this radical (Y. Hosokoshi et al., Physica B, 1994, 201, 497). The relationship between crystal structure and magnetic behavior can be interpreted in terms of overlap between sites of opposite spin density, due to the isolation of spin orbital overlap interactions in one type of dimer in this crystal lattice. Density functional computations of model dimers are consistent with analysis based on spin orbital overlap in F2PhNN.

Chemical oxidation of diarylamino-substituted fluorenones gives highly persistent radical cations, some of which can be stored in the solid state for months. Solution UV–vis-NIR spectra show strong monoradical cation absorption bands in the 750–1700 nm range, with minimum energy about 0.73 eV. 2,7-Bis(dianisylamino)fluorenone exhibits strong absorptions at 1670 (monocation) and 873 nm (dication). Fast intervalence electron transfer behavior between amine sites in the monocation is indicated by EPR spectroscopic and absorption spectral Hush analyses.Download full-size image

The synthesis, crystal structures and magnetic properties of two families of heterospin complexes containing lanthanide ions and a bis(imino nitroxide) biradical (IPhIN = 1-iodo-3,5-bis(4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1H-imidazole-1′-oxyl)benzene) are reported: in [Ln2(hfac)6(IPhIN)(H2O)2] compounds, two lanthanide ions [Ln = GdIII (1) and DyIII (2)] are coordinated to the biradical, and in [Ln(hfac)3(IPhIN)(H2O)] compounds, one lanthanide ion (Ln = TbIII (3), GdIII (4) or DyIII (5)) is coordinated to the biradical. Ferromagnetic intramolecular magnetic interactions between GdIII and the biradical were found for 1 and 4, while intramolecular magnetic interactions between the radicals were ferro- and antiferromagnetic, respectively. Compound 2 shows a field induced slow relaxation of magnetization, which (under an external applied field of 2 kOe) exhibits an activation energy barrier of ΔE/kB = 27 K and a pre-exponential factor of 1.4 × 10−8 s. To support the magnetic characterization of compound 3ab initio calculations were also performed.

Two polymorphs of an anthraquinone-nitronylnitroxide radical, AntQNN, were isolated, both with antiferromagnetic (AFM) exchange attributed to chain-type inter-radical contacts: one with J1D/k ≈ −3 K, and one with J1D/k ≈ −17 K.

Stable radicals 2-(6-uradinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (Ur6IN) and 4-(p-tert-butylaminoxylphenyl)-2,6-di(propylamido)pyridine (DAPPN) form heterospin radical pair complexes due to complementary multi-point hydrogen-bonds.

Push–pull organic molecules composed of electron donor diarylamines at the 2- and 2,7-positions of fluorenone exhibit intramolecular charge-transfer behaviour in static absorption and emission spectra. Electrochemical and spectral data combined in a modular electronic analysis model show how the donor HOMO and acceptor LUMO act as major determinants of the frontier molecular orbital energy levels.

2,7-Bis(1′-pyrenylvinylene)-9,9-diethylfluorene (1) and 2,7-bis(1′-pyrenylvinylene-4″-phenylenevinylene)-9,9-diethylfluorene (2) were synthesized and their static luminescence behavior assessed. They show solution photoluminescence (PL) maxima in chloroform at 475 nm and 467 nm, with quantum efficiencies of 54% and 52%, respectively. Double-layer LED devices with configuration ITO/PEDOT-PSS/(1 or 2)/Ca–Al emitted blue-green light with turn-on voltages of 2.5 V and emission maxima at 500 nm (2.48 eV); their luminance efficiencies were 0.36 and 0.30 cd A−1, respectively. Blending of 1 at 20% (w/w) in PVK improved the luminance efficiency to 1.81 cd A−1 for the same device configuration, with only a small increase in turn-on voltage to 3 V; the emission maximum was 497 nm (blue-green emission), the maximum luminance 7600 cd m−2 with CIE color coefficients of (0.12, 0.45). For a 10% (w/w) blend of 2 in PVK using the same device configuration, the luminance efficiency was 1.47 cd A−1, turn-on voltage 3 V, maximum luminance 2600 cd m−2 with CIE coefficients of (0.13, 0.45). Simple π-MO calculations show that structural extension of the nominal conjugation length in 2 does not significantly decrease the effective band gap relative to 1, consistent with the observed lack of red shift in 2.