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A series of donor-acceptor (D–A) type conjugated molecules was synthesized, with structures D–π–A or A–π–D–π–A where A = N-hexylpyridinium-4-yl and D = electron donor type units, with A and D linked by π-conjugating units: the D–π–A systems are design-testing models for optimizing the A–π–D-π-A systems, where the latter are conjugatively extended variants of viologen. UV–vis absorption spectra and molar absorptivities were determined in varying solvents, and spectra for neat films. The donor units D comprised phenylene, 2,5-thienylene, 3,4-ethylenedioxythiophen-2,5-diyl (EDOT), 1-N-methyl-2,5-pyrrolediyl, and CH CH units. The system with strongest absorptivity at a relatively long absorption maximum and onset was the extended viologen 4-(1-hexyl)pyridinium-CH CH-(2,5-EDOT-CH CH-)-(2,5-thienylene-CH CH )-(2,5-EDOT-CH = CH-)-4-(1-hexyl)pyridinium dibromide, with λmax(methanol) = 576 nm (278,000 M−1 cm−1), low energy onset at 683 nm (1.82 eV). Photoluminescence quantum yields in methanol mostly were quite weak, with solvatochromic behavior indicating strong intramolecular charge transfer character.

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 d...

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-clean...

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 (e = 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.

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] ).

Towards highly luminescent aggregates: A highly twisted triphenylamine displays aggregation-induced enhanced emission. A solvent-specific critical molecular aggregate size, once reached, gives rapid onset of enhanced emission in polar solvent mixtures that favor quenching of solvated individual molecule excited states.

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.

In this contribution, we report on the electronic energy transfer dynamics of bichromophoric systems incorporating two pyrene chromophores tethered by variable-length flexible alkyloxy chains to p-phenylenevinylene oligomers. These were studied using UV-vis absorption and both steady state and time-resolved fluorescence spectroscopy. Time-resolved emission measurements showed an efficient photoinduced energy transfer process in all the multichromophoric systems, which occurs on the time scale of tens of picoseconds after excitation at 265 nm. The energy transfer process is especially efficient in systems where the linker is formed by eight atoms (up to k(ET) ≈ 2.7 × 10(10) s(-1)), which, despite not being the shortest bridge studied, allows the approach of the donor and acceptor chromophores due to an appropriate number of flexible single bonds. Using Förster theory, we calculated the donor-acceptor distance in each triad from the experimental energy transfer rate, finding them to be in the range 8.8-10 Å.

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.

Soluble para-phenylenevinylenes (PPVs) with amino side groups and different spacers have been synthesized. Photophysical properties showed that these compounds emitted light in blue region when dissolved in methanol. They have been designed in order to be platforms for attaching different functional groups to the amino units. Molecule 5 was a model compound which showed a quantum efficiency of 52 % in methanol. Different spacers were added to the model compound to yield molecules 6 and 7 . The preliminary experiment showed that molecule 5 can interact with various metal ions in solution phase. The interaction was observed by the changes of fluorescent emission intensity of the molecule. This molecule will be further functionalized to be more specific in binding certain metal ions.

The photochemistry of azidopyridine 1-oxides was studied using an array of glass and matrix isolation techniques. As with room temperature, the photochemistry of 4-azidopyridine 1-oxide is dominated by triplet nitrene chemistry. However, in the case of the 3-azide, matrix photolysis indicates the formation of diazabicyclo[4.1.0]hepta-2,4,6-triene N-oxide and diazacycloheptatetraene N-oxide intermediates as well as triplet nitrene.

Photolysis of beta-azido propiophenone derivatives, 1, with built-in sensitizer units, leads to selective formation of triplet alkyl nitrenes 2 that were detected directly with laser flash photolysis (lambdamax = 325 nm, tau = 27 ms) and ESR spectroscopy (|D/hc| = 1.64 cm-1, |E/hc| = 0.004 cm-1). Nitrenes 2 were further characterized with argon matrix isolation, isotope labeling, and molecular modeling. The triplet alkyl nitrenes are persistent intermediates that do not abstract H-atoms from the solvent but do decay by dimerizing with another triplet nitrene to form azo products, rather than reacting with an azide precursor. The azo dimer tautomerizes and rearranges to form heterocyclic compound 3. Nitrene 2a, with an n,pi* configuration as the lowest triplet excited state of the its ketone sensitizer moiety, undergoes intramolecular 1,4-H-atom abstraction to form biradical 6, which was identified by argon matrix isolation, isotope labeling, and molecular modeling. beta-Azido-p-methoxy-propiophenone, with a pi,pi* lowest excited state of its triplet sensitizer moiety, does not undergo any secondary photoreactions but selectively yields only triplet alkyl nitrene intermediates that dimerize to form 3b.

The photoluminescence (PL) and electroluminescence (EL) of four 2,7-bis(phenylethenyl)fluorenes (OFPVs) and two 2,7-diphenylfluorenes (OFPhs) are compared to evaluate effects of nonconjugating peripheral substitution and conjugation length on their EL emissions. The OFPVs exhibit very similar PL spectra with 460–480 nm emission maxima but show large variation in the organic light-emitting diode (OLED) efficacy: from a material that does not give persistent emission in test OLEDs (9,9-diheptyl substitution on the fluorene ring) to materials with luminance efficiencies of 0.5 cd A–1 and greater (9,9-diethyl substitution on the fluorene ring, methoxy and methoxy/heptyloxy substituents on the phenylethenyl rings). The best OFPV in an ITO/PEDOT:PSS/(emitter)/Ca–Al (ITO: indium tin oxide; PEDOT: poly(ethylenedioxythiophene); PSS: poly(styrene sulfonate)) OLED configuration has 9,9-diethyl substitution and terminal heptyloxy substitution (maximum luminance of 1500 cd m–2 at 12 V). Unlike the OFPVs, the neat OFPhs show not only EL at the desired blue output of ca. 400–410 nm emission maxima but also an undesired green emission component at 500–550 nm. Blending the OFPhs with poly(methyl methacrylate) eliminates the long-wavelength component when the emitter load is 10–25 %, but the OFPh luminance efficiencies, turn-on voltages, and maximum luminance tend to be poorer than those of the OFPVs. The deficiencies of the OFPhs appear to be attributable to thermal degradation and oxidative reactivity, although solid-state annealing and a nonoptimal bandgap match to the OLED device configuration may also contribute.

A set of Don --Acc-- Don multichromophores was synthesized based on a simple design with two pyrene donors (absorbing antennae) connected to a bis-1,4-(3,4,5-trimethoxystyryl)benzene core acceptor using various flexible, nonconjugated tethers. Excitation of the pyrene donors at 276 nm in solution yields near exclusive emission from the core chromophore at 445-450 nm, with energy transfer efficiencies up to 92%, far better than achieved with simple mixtures. The simple tethering design imposes a high "local" concentration of the pyrene near the acceptor core unit that is maintained even at very low multichromophore concentrations. Solvent effects on absorption and emission spectra are very small, except in cases where a pi-conjugating O-C=O moiety of the tethering group is directly attached to the core chromophore, rather than being placed in the middle of the tether. Energy transfer in the systems is effective due to good donor-acceptor energy matching. The optimal energy transfer efficiency was achieved using an eight-atom flexible linker.

New electroluminescent cyano-substituted phenylenevinylene (PV) monomeric and oligomeric chromophores, 1–3, were synthesized by Knoevenagel condensation and investigated in terms of their photoluminescence (PL) and electroluminescence (EL) behavior. All show PL emission maxima between 463 and 471 nm in solution with quantum efficiencies of 0.65, 0.46 and 0.68, respectively. EL maxima in an LED configuration ITO/PEDOT–PSS/1–3/Ca–Al were 507, 518 and 551 nm with luminance efficiencies 0.01, 0.05 and 0.02 cd/A, respectively.

(E,E)-2,7-Bis(3,4,5-trimethoxyphenylethenyl)fluorene, 1, and a segmented copolymer 2 composed of the same chromophore alternated with nonconjugated 1,6-hexanediyl (alt-oligo(2,6-dimethoxylphenylene-4-vinylene-[9,9-diethylfluoren-2-yl-7-vinylene]-3,5-dimethoxy-phenylene-4-[1,6-hexanedioxyl]) were synthesized. They have solution photoluminescence emission maxima at 420−460 nm, with quantum efficiencies of 0.93 and 0.68, respectively, in chloroform. Electroluminescent spectra in an LED configuration ITO/PEDOT-PSS/(1 or 2)/Ca−Al both showed maxima at 470−480 nm, although the spectrum from 2 was significantly broader. The luminance of LEDs with 1 was over 10-fold higher than those with copolymer 2, 0.515 versus 0.040 cd/A, with turn-on voltages of 3 and 5 V, respectively. The crystallography of 1 showed no chromophore π-stacking; this absence should limit tendencies for emission wavelength shifts due to solid state interchromophore interactions. When 1 was heated in air before incorporation into an LED emissiv...

The first controlled synthesis of the title radical, 1 , was achieved by oxidation of a hydroxylamine precursor; the radical itself can thus readily be isolated, studied by ESR and UV–vis, and compared to results of computational modeling. The benzothiazole ring induces substantial delocalization of the nitroxide spin density (a(NO) = 9.2 G, a(ring N) = 2.7 G, about 10% more than is seen in the benzimidazole nitroxide analog.

The syntheses and electronic spectroscopy of E,E-2,5-bis[2-hydroxyethoxy]-1,4-bis[2-(3,4,5-trimethoxyphenyl-1,2-ethenediyl]benzene, 3, and alternating block copolymer analogue alt-poly[1,8-octanedioxy-2,6-dimethoxy-1,4-phenylene-1,2-(E)-ethenediyl-2,5-(2-hydroxyethoxy)-1,4-phenylene-1,2-(E)-ethenediyl-3,5-dimethoxy-1,4-phenylene], 2, are described. Polymer 2 has UV−vis absorbances at 320 nm (shoulder) and 389 nm (maximum), with fluorescence emission at 449 nm (maximum) and 480 nm (shoulder) upon excitation at various wavelengths. Model compound 3 shows virtually identical spectral behavior. Neat solid films of polymer 2 show UV−vis absorbance in the 400−450-nm region (broad) and fluorescence at about 490 nm. The 40-nm shift from solution to neat film emission for 2 is attributed to hydrogen-bond-assisted interactions between chromophores in the solid state. The fluorescence quantum yields in chloroform at room temperature (365-nm excitation) for 2 and 3 are 0.36 and 0.74, respectively, relative to quinine...

Model 1,4-bis(styryl)benzene derivatives 1a,b related to poly(2-bromo-5-methoxy-1,4-phenylenevinylene) were synthesized to probe the effect of bromination on excited-state behavior of conjugated phenylenevinylene analogues. They showed solution absorption maxima at about 385 nm, without evidence of excimer formation up to 100 μM. Their fluorescence spectra showed overlapping bands at 435 and 460 nm, with quantum yields in tetrahydrofuran of 0.16 and 0.21 for 1a and 1b, respectively. Excited-state transients were monitored by time-resolved laser flash spectroscopy at room temperature. The triplet states were characterized by absorption maxima at about 520−530 nm with lifetimes of about 0.6 μs, much longer than would be observed for prompt fluorescence states. The quantum yields of singlet to triplet-state intersystem crossing were determined to be 0.45 and 0.28 for 1a and 1b, respectively. The acceleration of triplet transient decay rates under increased oxygen pressure according to the Stern−Volmer law fu...

2,6-Diazidobiphenylene was synthesized and photolyzed in frozen matrix. The resulting electron spin resonance spectrum showed the formation of 2,6-biphenylenedinitrene as a quintet species with zero field splitting (zfs) parameters ∣D/hc∣=0.260±0.002 cm−1, ∣E/hc∣≤0.0005 cm−1. The zfs parameters are in excellent accord with dipolar models for a quintet state produced by interaction between triplet state nitrene sites.

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.

Donor–acceptor molecules incorporating fluoren-9-ylidene malononitrile acceptor units conjugated to trimethoxystyrene and/or diarylamine donor units were synthesized, and their electronic spectral properties and electrochemical behaviors were evaluated by comparison to those of the analogous fluorenones. Frontier energy level and optical transition energy trends are explained based on a quantitative, modular donor–acceptor interaction model. A connectivity effect on absorption transition moment strength is also described.

The polymers discussed in this contribution consist of phenylenevinylene chromophores linked together across flexible biphenyl “hinges” to shorten the effective conjugation length and to give sufficiently twisted structures that interchain aggregation is limited or prevented. They are poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(4,4‘-dihexyloxy-3,3‘-biphenylenevinylene)] (1), poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2‘-dihexyloxy-3,3‘-biphenylenevinylene)] (2), and poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2‘-biphenylenevinylene)] (3). Absorption spectra in dilute solution and solid states are very similar for 1−3, consistent with the absence of aggregation effects in their ground electronic states. Photoluminescence emission spectra showed substantial red shifts in the solid state relative to dilute solution phase spectra. Solution emission quantum yields ranged from 0.26 to 0.42. LEDs based on 1−3 gave blue-green emission with maxima in the 480−510 nm range. The similarity of the photol...

A series of segmented copolymers containing oxadiazole groups in the conjugated main chain have been synthesized with the objective of raising the electron transport ability. The present copolymers consist of alternating blocks of rigid chromophores containing oxadiazole units together with flexible spacer segments. The effects of chromophore substituents on the optical properties of the copolymers were investigated. Strong solvatochromic effects were observed, indicating intramolecular charge transfer in the excited states. The copolymers not only were used as blue-green electroluminescent materials but also were effective as electron transport/hole blocking layers in polymer light emitting diode architectures as a result of the introduction of electron transporting unit oxadiazole. The quantum efficiency of a single-layer device using PPV was greatly enhanced with the use of a thin film of the oxadiazole copolymer serving as an ETL layer. At 6.8 V, a brightness of 2400 cd/m 2 was achieved with an external quantum efficiency of 0.094%.

The cryogenic, matrix-isolated ESR and electronic spectroscopy of a set of quartet state organic phenylnitreno-radicals are described, using nitronylnitroxide, benzo-nitronylnitroxide, and verdazyl radical spin sites, and both meta and para connectivities. Delocalization of the nitrene unit in the para-linked series is not extensive, judging by the computational results. The ESR spectral results show that zero-field splitting is affected much more by the connectivity in the systems (para versus meta) than by structural variations within the nitreno-radical theme.

A series of new PPV-based alternating copolymers containing conjugated and nonconjugated blocks have been synthesized with the objective of raising the glass transition temperature (Tg) relative to...

Heck carbon−carbon bond-forming polymerization was used to make segmented copolymers based on poly[1,8-octanedioxy-1,4-phenylene-1,2-ethenylene-1,4-phenylene-1,2-ethenylene-1,4-phenylene]. The Heck methodology yielded copolymers with molecular weights, polydispersities, and optical properties that are comparable to or better than those previously obtained for the same or very closely related polymers obtained by Wittig methodology. However, it proved important to have methoxy groups incorporated into the chromophore of the copolymers to get high yields of material with good degrees of polymerization. The obtained yields and purities for the Heck-derived polymers are better, in part due to the direct formation of desirable trans olefin linkages in the polyphenylenevinylene type chromophore units. Differential scanning calorimetric measurements show that placement of isopropyl groups on the central ring of the chromophoric fragment does not alter the polymer glass transition temperature relative to an unsub...

The syntheses of 4-(1H-benzimidazol-2-yl)-2,6-di-tert-butyl-phenoxyl and 4-(1H-5,6-dimethyl-benzimidazol-2-yl)-2,6-di-tert-butyl-phenoxyl radicals (1–2) by oxidation of the corresponding phenols is described. The radicals are stable enough to retain color for weeks in the solid state in air under some conditions. Their hydrogen bonding functionality offers prospects for use in molecular magnetic materials.

The photochemical reactions of cyclooctadienone 6 have been studied as a mode of entry to a cyclopentyl−oxyallyl system. Irradiation of cis,cis-2,7-cyclooctadienone 6 results in isomerization to cis,trans-2,7-cyclooctadienone 24. The strained dienone 24 is stable at −70 °C, but undergoes conrotatory cyclization at temperatures above −30 °C to the cis-bicyclo[3.3.0]oxyallyl system 25. The density functional theory (DFT) structural optimizations for 6, 24, and 25 at the 6-31G* B3LYP level yielded energies, selected geometric parameters, and charge/spin population data. The oxyallyl intermediate reacts with vinyl ethers in a stepwise fashion to give bridged and fused tricyclic products with defined stereochemistry at the ring junctures. Simple alkenes react with 25 in a concerted ene reaction to give bicyclic products. An explanation is offered to harmonize the traditional zwitterionic view of oxyallyl systems with the current computational assessment of oxyallyls as diradical species.

Two types of radical pairs, stable at room temperature, were found in monocrystals of 2,6-di-tert-butyl-4-methylphenol doped with bis(2,6-di-tert-butyl-4-methoxyphenoxy) phosphine azide after photoirradiation using a Xe arc lamp. One pair consists of two 2,6-di-tert-butyl-4-methoxyphenoxyls (RP1), the other of 2,6-di-tert-butyl-4-methoxyphenoxyl and 2,6-di-tert-butyl-4-methylphenoxyl (RP2), as indicated by their EPR hyperfine splitting. In the temperature range of 120–200 K RP1 accumulates 7 times faster than at 77 K. Explanations for this photoaccumulation as well as a mechanism of RP formation are proposed.

Photolysis of naphtho[b]cyclopropene in hydrocarbons leads to products derived from hydrogen abstraction by a diradical. Photolysis in a cryogenic matrix allows ESR detection of this diradical, which has zero field spliting parameters D/hc = 0.057, E/hc −1 .

Three fluorine-containing polymers, poly(2-fluoro-1,4-phenylene vinylene), poly(2,5-difluoro-1,4-phenylene vinylene), and poly(2[5]-(n-hexyloxy)-5[2]-fluoro-1,4-phenylene vinylene) (1−3, respectively) have been synthesized by the soluble precursor method and were used to fabricate light emitting diode (LED) devices. Monosubstituted 1 yielded an electroluminescent emission peaking at 560 nm (green-yellow); polymers 2 and 3 emitted in the 600−610 nm region (red). The result for 1 represents a blue shift in emission relative to emission from hompolymeric unsubstituted poly(1,4-phenylene vinylene) (PPV) LEDs fabricated by similar techniques, while the results for 2 and 3 represent a substantial red shift. These results may be compared to those obtained from LEDs fabricated from copolymers of PPV and poly(2,3,5,6-tetrafluoro-1,4-phenylene vinylene), which show a slightly blue shifted emission relative to homopolymeric PPV (Benjamin, I.; Faraggi, E. Z.; Avny, Y.; Davidov, D.; Neumann, R. Chem. Mater. 1996, 8, 3...

A photochemical precursor to a pendant conjugated polyradical has been synthesized, poly[3,5-di-tert-butyl-4-[(2,4,6-tri-tert-butylphenyl)oxalato] phenylacetylene], 3. Irradiation of 3 at 77 K in the solid state at < 300 nm yielded poly(3-5-di-tert-butyl-2-oxyphenyl acetylene), 2, with 30-40% of the expected number of radical spins. Spin yields on the surface of solid samples appears to be considerably higher. Electron spin resonance experiments showed no evidence of cooperative exchange interaction between the pendant spins. Computational modeling indicated that a major reason for the failure of this and other polyphenylacetylenes to show ferromagnetic exchange between spins is the substantial twisting of the polyacetylene backbone required by steric interactions, leading to deconjugation and a loss of exchange interaction between pendant radicals along the chain.

The diazoketone derived from tricyclo[4.1.0.0 1,3 ]heptan-4-one on photolysis in dimethylamine leads to a novel product that appears to be formed via a cyclopropylcarbene rearrangement followed by dimethylamine addition and the cleavage of one of the cyclobutane rings. In a matrix at 15K, photolysis led to the loss of the 2085 cm −1 diazo band and the formation of a new band at 2117 cm −1 which is characteristic of a small ring ketene such as cyclopropylketene.

The “push−pull” electronically substituted systems poly(2[5]-chloro-5[2]-(n-hexyloxy)-1,4-phenylenevinylene) and poly(2[5]-bromo-5[2]-(n-hexyloxy)-1,4-phenylenevinylene)1 and 2, respectivelywere synthesized by a modified soluble precursor route. Thermal elimination of polyether precursors to 1 and 2 yields the final conjugated polymers as free-standing, flexible red films. The polyether precursors may be film-cast from methanol to make light emitting diode (LED) devices which have red light emission in the 620 nm region. The ease of synthesis and processibility of 1 and 2 make these promising candidates for red light emitting LEDs.

A series of m,n′-diazidodiphenyl ethers (m ≤ n, m = 3,4; n = 3,4) was photolyzed at 77 K in frozen, glassy 2-methyltetrahydrofuran matrices to generate the corresponding diphenyl ether m,n′-dinitrenes for study by electron spin resonance (ESR) spectroscopy. 3,4′-Diazidodiphenyl ether gave an ESR spectrum dominated by a mononitrene peak with ∣D/hc∣ = 0·972 cm−1, and also showed a weak dinitrene quintet spectrum with ∣D/hc∣ = 0·162 cm−1 having ESR spectral intensity vs temperature dependence (Curie law) consistent with either a high-spin ground state or a very small singlet–quintet gap. Di(3-azidophenyl) ether gave a strong mononitrene peak with ∣D/hc∣ = 0·996 cm−1 and a quintet dinitrene ESR spectrum (∣D/hc∣ = 0·162 cm−1) which exhibited non-linear Curie law intensity behavior consistent with the quintet being a thermally populated excited state 40 cal mol−1 above a singlet ground state. Di(4-azidophenyl) ether gave a strong mononitrene peak with ∣D/hc∣ = 0·961 cm−1, but no observable spectrum related to a high-spin open-shell dinitrene. The results are consistent with oxygen being a weak exchange coupling linker in pi-conjugated open-shell molecules. The observed ground-state spin multiplicities are in accord with qualitative superexchange and connectivity models, despite any perturbations due to resonance effects between the oxygen linker and p-nitrene sites.

Photolysis of sterically hindered bis(aryloxy)phosphine azides in solution, frozen matrix, or neat solid states leads to facile unimolecular production of aryloxyl radicals.

Synthesis of poly(4′-vinyl-2,4,6-tri-tert-butyl-diphenyloxalate) and its 65 : 35 and 19 : 81 copolymers with styrene are described. Rigid phase broad-band photolysis of the homopolymer with a quartz filtered xenon arc at 77 K results in production of up to 25% of the theoretical number of spins/mol expected for quantitative production of pendant phenoxyl radicals, and shows no major loss of radical signal in the ESR at temperatures below 100 K. Curie law analysis of the temperature dependence of the ESR radical signal intensity for the neat photolyzed homopolymer 1 shows curvature consistent with antiferromagnetic pairing of radical spins at low temperatures. Since through-bond conjugation of radical spins is not possible in this system, the antiferromagnetic interaction is interpreted in terms of intrachain and/or interchain through-space exchange interactions.

Selective photolysis of the α-azido group in 2,4-diazidopyridines has been determined by EPR spectroscopy and theoretically treated in terms of non-adiabatic photodissociation processes.

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 color purity of blue polyfluorene fluorescence emission is often adversely contaminated by green emission bands. The origin of this green emission has been attributed to various factors including on-chain oxidation of fluorene leading to monomeric fluorenone emission and inter/intra-chain interaction between segments of fluorenone units leading to excimer formation on the polyfluorene backbone. We report here emission properties at the bulk and single molecule level of a molecular fluorene derivative and compare them to those of its oxidative fluorenone derivative. Whereas the bulk fluorescence emission of 2,7-bis(3,4,5-trimethoxyphenylethenyl)-9,9-diethyl-9H-fluorene (OFPV) exhibits a blue to blue-green emission at about 480 nm, 2,7-bis(3,4,5-trimethoxyphenylethenyl)-9,9-diethyl-9H-fluorenone (OFOPV) shows red luminescence with a peak centered at about 630 nm. However, the peak position for OFOPV shifts to higher energies (∼540 nm) upon dilution or dispersion in polymer matrices like PMMA or Zeonex. Single molecule measurements of OFOPV show fluorescence spectra dominated by peaks around 540 nm, with a small minority at longer wavelengths that are attributed to emission from dimers or higher aggregates. This distribution indicates that monomeric emission of OFOPV is green, consistent with green emission bands seen in polyfluorenes.

UV−vis photolysis of the film of a bisaryloxyphosphineazide derivative of poly(4-oxyphenyl-1,2-phenylenevinylene) generates the phenoxy radicals attached to the π-conjugated linear backbone. The product polyradical is in a high-spin ground state with the averaged exchange coupling constant (2J) of ca. 50 cm-1.

Photochemical and photophysical properties were investigated for poly(arylenevinylene)s containing a flexible biphenyl "hinge" unit by applying one-photon (OP) and two-photon (TP) excitation to explore excited-state properties. The poly(arylenevinylene)s were poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(4,4'-dihexyloxy-3,3'-biph enylenevinylene)] (1), poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-dihexyloxy-3,3'-biph enylenevinylene)] (2), and poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-biphenylene-vinylene )] (3). Effective emission quantum yields and related photonic properties were evaluated on a realistic per-chromophore basis using effective conjugation lengths based on the Strickler-Berg relationship. intramolecular photocyclization was deduced to occur in the one case where the biphenyl molecular connectivity permitted the reaction, based on matrix-assisted loser desorption/ionization time-of-flight (MALDI-TOF), heteronuclear multiple-quantum coherence (HMQC)-NMR, and gel-permeation chromatography (GPC) results. The various photoprocesses could be induced by either OP or TP excitation, though the first excited singlet state is the photoactive state. The higher excitation energy 1 of the TP excited state favors indirect population of the S, state by electronic coupling between the TP and OP excited states [lambda(max)(TPE) (nm): 726; delta (GM)([9]): 1 = 229, 2 = 215, 3 = 109). Photochemical processes occurring from the lowest OP excited state (S-1) could therefore also be indirectly induced by TP excitation.

A number of persistent 2,6-di-tert-butylphenoxyl based radicals were synthesized as models for unpaired spin delocalization as a function of conjugation pathway, and were investigated by ESR and UV-vis spectroscopy. Phenoxyl spin delocalizes significantly onto a para-phenyl ring, but further delocalization through a meta-vinyl or meta-carbonyl linkage is not detectable by ESR hyperfine coupling. UV- vis spectra do show a red shift of the longest wavelength transition for a carbonyl-type substituent by comparison to a vinyl substituent in the meta-position of a para-phenyl group. By comparison, a para-styryl substituent on the phenoxyl causes very large spin delocalization from the phenoxyl unit, with the largest hyperfine coupling being found on the ethenyl unit of the assemblage. This large delocalization accounts for the reactivity of radicals that incorporate such units.

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.

Phenoxyl radicals were synthesized having heterocyclic substituents in the 4-position. Electron spin resonance spectroscopy shows spin density can be modulated by delocalization onto the heterocycle structure, to the extent that some of the largest spin density bearing sites in 3 and 4 lies in the heterocycle, not in the phenoxyl ring.

The use of aryloxy oxalyl chlorides (AOCs), aryloxy oxalyl tert-butyl peroxides (AOBs), and diaryl oxalates (DAOs) for unimolecular generation of phenoxyl-based radicals under solution and rigid matrix conditions is described. AOCs are usable for photochemical generation of phenoxyl radicals, but are only conveniently stable as precursors when 2,6-di-tert-butylated derivatives are used. AOBs may be used as thermal precursors to aryloxyl radicals, since they typically decompose within 2-3 h at 60-85 degrees C to give phenols. (1)H-NMR solution kinetic studies find that DeltaH() = 31 kcal/mol, and DeltaS() = +3.4 cal/mol-K for decomposition of phenoxyoxalyl tert-butyl peroxide, consistent with substantial concertedness in peroxide bond cleavage. AOBs and the more stable DAOs are also convenient photochemical phenoxyl radical precursors. AOBs yield phenoxyl radicals more readily by photolysis than do corresponding DAOs, but the DAOs have fewer side reactions that can quench the product phenoxyl radicals.

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 poly(4'-vinylphenoxy) nonconjugated polyradical was generated by photolysis of an oxalate-functionalized poly(4-hydroxystyrene). Spin-counting experiments in the electron spin resonance spectrum (ESR) showed efficient production of radicals with g = 2.0041, consistent with the expected pendant phenoxyl radicals. The thermal behavior of the ESR signal intensity indicated through-space antiferromagnetic coupling of the pendant radicals. The similarity of the behavior of this nonconjugated system to that of nominally conjugated polyradical systems synthesized by others suggests that geometric effects leading to deconjugation may be a major consideration in the synthesis of organic ferromagnetically exchange coupled polyradicals.