Singlet Nitrene Research Articles

Uncatalyzed thermal gas phase aziridination of alkenes by organic azides. Part I: Mechanisms with discrete nitrene species

Alkene aziridination by azides through uncatalyzed thermal gas phase routes has been studied using the DFT B3LYP/6-31G(d,p) method, where the possible role of discrete nitrene intermediates is emphasized. The thermal decomposition of azides is studied using the MP2/aug-cc-pVDZ strategy as well. The MP2 (but not the B3LYP) results discount the existence of singlet alkylnitrenes where the alkyl group has an α-hydrogen. Addition of the lowest lying singlet and triplet nitrenes R-N (R = H, Me, Ac) to four different alkene substrates leading to aziridine formation was studied by the B3LYP method. Singlet nitrenes with alkenes can yield aziridines via a concerted mechanism, where H-N insertion takes place without a barrier, whereas Me-N shows larger barriers than Ac-N. Methyl substitution in the alkene favors this reaction. Triplet nitrene addition to alkenes is studied as a two-step process, where the initially formed diradical intermediates cyclize to form aziridines by ISC (intersystem crossing) and collapse. Scope for C-C bond rotation in the diradical leads to loss of stereochemical integrity for triplet nitrene addition to cis- and trans-2-butenes. Geometries of the transition states in the various reaction steps studied here are described as “early” or “late” in good accordance with the Hammond postulate. The addition of free nitrenes to alkene substrates is studied using DFT and MP2 methods. The B3LYP results predict that concerted azide decomposition can yield singlet nitrenes, unlike the MP2 results. Singlet nitrenes insert into alkenes with a modest or no barrier. Triplet nitrenes employ a step-wise pathway to give aziridines.

Direct Observation of 4-Phenoxyphenylnitrenium Ion: A Transient Absorption and Transient Resonance Raman Study

Femtosecond (fs) and nanosecond (ns) transient absorption (TA) and single pulse transient resonance Raman spectroscopic investigation of the intermediates after laser photolysis of 4-phenoxyphenyl azide in acetonitrile and mixed aqueous solution is reported. fs-TA results show that the singlet 4-phenoxyphenylnitrene was produced immediately after photolysis of the azide. Then, the singlet nitrene underwent intersystem crossing (ISC) and ring expansion to generate triplet nitrene and ketenimine in acetonitrile with t = 346 ps or protonation in mixed aqueous solution with t = 37 ps, respectively, a little slower than the counterparts of the methoxy one (108 and 5.4 ps for ISC and protonation processes, respectively). The transient Raman spectrum combined density functional theory (DFT) calculation predicting the structure and vibrational frequencies suggested that phenoxyphenylnitrenium ion has a comparable quinoidal character to that of methoxy- and ethoxy-phenylnitrenium ions. All of these results indicated that the phenoxy substitution has some impact on the reactivity of phenylnitrene but a slight influence on the structure of phenylnitrenium ion.

Synthesis of reduction-sensitive 1,1-diarylhydrazines from 1,1-diarylamines

1-(2-Nitrophenyl)-1-phenylamine and methyl 4-((2-nitrophenyl)amino)benzoate have been transformed into their corresponding urea derivatives through the action of chlorosulfonyl isocyanate. The initial sulfimidate product from the former reaction has sufficient stability so that it can be isolated and characterized as its disodium salt, and this, as well as three other subsequent products, have been characterized by X-ray diffraction. The corresponding intermediary urea was converted into its hydrazine derivative via a Hofmann rearrangement under oxidative conditions. Density functional theory has been used to examine the nature of the intermediates and transition states for the Hofmann rearrangement. There is little theoretical indication for a cyclic aziridinonium intermediate and the transition state between the urea and the isocyanate corresponds to a reactant-like rotation of the planar singlet nitrene before migration and formation of the new N−N bond.

X-Ray Photoelectron Spectroscopy Investigation of the Nitrogen Species in Photoactive Perfluorophenylazide-Modified Surfaces.

X-ray Photoelectron Spectroscopy (XPS) was used to characterize the nitrogen species in perfluorophenylazide (PFPA) self-assembled monolayers. PFPA chemistry is a novel immobilization method for tailoring the surface properties of materials. It is a simple route for the efficient immobilization of graphene, proteins, carbohydrates and synthetic polymers onto a variety of surfaces. Upon light irradiation, the azido group in PFPA is converted to a highly reactive singlet nitrene species that readily undergoes CH insertion and C=C addition reactions. Here, the challenge of characterizing the PFPA modified surfaces was addressed by detailed XPS experimental analyses. The three nitrogen peaks detected in the XPS N1s spectra were assigned to amine/amide (400.5 eV) and azide (402.1 and 405.6 eV) species. The observed 2:1 ratio of the areas from the 402.1 eV to 405.6 eV peaks suggests the assignment of the peak at 402.1 eV to the two outer nitrogen atoms in the azido group and assignment of the peak at 405.6 eV to the central nitrogen atom in the azido group. The azide decomposition as the function of x-ray exposure was also determined. Finally, XPS analyses were conducted on patterned graphene to investigate the covalent bond formation between the PFPA and graphene. This study provides strong evidence for the formation of covalent bonds during the PFPA photocoupling process.

5-Azido-2-aminopyridine, a New Nitrene/Nitrenium Ion Photoaffinity Labeling Agent That Exhibits Reversible Intersystem Crossing between Singlet and Triplet Nitrenes

The photochemistry of a new photoaffinity labeling (PAL) agent, 5-azido-2-(N,N-diethylamino)pyridine, was studied in aprotic and protic solvents using femtosecond-to-microsecond transient absorption and product analysis, in conjunction with ab initio multiconfigurational and multireference quantum chemical calculations. The excited singlet S1 state is spectroscopically dark, whereas photoexcitation to higher-lying singlet excited S2 and S3 states drives the photochemical reaction toward a barrierless ultrafast relaxation path via two conical intersections to S1, where N2 elimination leads to the formation of the closed-shell singlet nitrene. The singlet nitrene undergoes intersystem crossing (ISC) to the triplet nitrene in aprotic and protic solvents as well as protonation to form the nitrenium ion. The ISC rate constants in aprotic solvents increase with solvent polarity, displaying a "direct" gap effect, whereas an "inverse" gap effect is observed in protic solvents. Transient absorption actinometry experiments suggest that a solvent-dependent fraction from 20% to 50% of nitrenium ions is generated on a time scale of a few tens of picoseconds. The closed-shell singlet and triplet nitrene are separated by a small energy gap in protic solvents. As a result, the unreactive triplet state nitrene undergoes delayed, thermally activated reverse ISC to reform the reactive closed-shell singlet nitrene, which subsequently protonates, forming the remaining fraction of nitrenium ions. The product studies demonstrate that the resulting nitrenium ion stabilized by the electron-donating 4-amino group yields the final cross-linked product with high, almost quantitative efficiency. The enhanced PAL function of this new azide with respect to the widely applied 4-amino-3-nitrophenyl azide is discussed.

Sulfonyl nitrenes from different sources: computational study of formation and transformations

The formation of sulfonyl nitrenes RSO2N (R = Me, p-Tol, CF3) from different sources, such as sulfonyl azides RSO2N3, N-halosulfonamides RSO2NHHal, salts RSO2NNaCl, N-hydroxysulfonamides RSO2NHOH and sulfonylimino-λ3-iodanes ArI = NSO2R or -bromanes, by elimination of neutral molecules (N2, HCl, NaCl, H2O and ArHal, respectively) was studied theoretically at the density functional theory and second-order Møller-Plesset perturbation theory using the 6-311++G(d,p) and cc-pVTZ basis sets. The originally formed singlet nitrenes suffer nonradiative intersystem crossing to the triplet state (S1 ⇝ T1) or, sometimes, undergo spontaneous barrierless pseudo-Curtius rearrangement into the corresponding sulfonylamines RN = SO2. The activation barriers decrease in the order (kcal/mol): RSO2NHHal ~ RSO2NHOH (60–70) ≥ RSO2N(Na)Hal (40–50) > RSO2N3 (35) > RSO2N = IPh (9–13) ≥ RSO2N = BrC6H4CF3 (4–8). Copyright © 2013 John Wiley & Sons, Ltd.

ChemInform Abstract: Chemical Activation in Azide and Nitrene Chemistry: Methyl Azide, Phenyl Azide, Naphthyl Azides, Pyridyl Azides, Benzotriazoles, and Triazolopyridines

AbstractReview: more than 100 refs.

Antiviral Agents Derived from Novel 1-adamantyl Singlet Nitrenes

Amantadine constitutes an interesting, diamond crystal lattice-shaped, antivirally active amine with an inhibitory effect on influenza A viruses causing common 'flu' in humans. Unfortunately, amantadine forfeited most of its therapeutic potential because of resistance development in recent influenza A virus isolates. The antiviral efficacy of amantadine congeners can be chemically modified, resulting in re-constitution, improvement and/or extension of antiviral activities mediated by amino-adamantyls. Newly synthesized compounds were evaluated towards HIV type-1 (HIV-1) replication in primary human lymphocytes. One N-phenacyl amantadine derivative was investigated for inhibiting the in vitro replication of respiratory viruses (influenza A viruses, influenza B virus, human parainfluenza virus type 3 and severe acute respiratory syndrome coronavirus). Two ketone-stabilized 1-adamantyl singlet nitrenes were discovered serendipitously. To our best knowledge these are the first persistently stable nitrenes to be reported. Their structure was proved by determining the X-ray single crystal structure of one hydrolytic elaboration product. This salt adduct revealed an incommensurately modulated crystal structure, which was solved by extensive computational refinement. We could show that ketone-stabilized 1-adamantyl singlet nitrenes are versatile synthons for the synthesis of antiviral drug candidates. An amantadine-folate conjugate was inhibitory on HIV-1 replication in primary human lymphocytes, and one N-phenacyl amantadine derivative was inhibitory towards low pathogenic avian influenza A virus (H5N1) replication in vitro. These results indicate that the aromatic-aliphatic ketone-stabilized 1-adamantyl singlet nitrenes, beyond being of fundamental interest in organic chemistry, represent versatile synthons for the synthesis of new amantadine-related potentially antiviral drugs.

Chemical Activation in Azide and Nitrene Chemistry: Methyl Azide, Phenyl Azide, Naphthyl Azides, Pyridyl Azides, Benzotriazoles, and Triazolopyridines

Chemical activation (the formation of ‘hot’ molecules due to chemical reactions) is ubiquitous in flash vacuum thermolysis (FVT) reactions, and awareness of this phenomenon is indispensable when designing synthetically useful gas-phase reactions. Chemical activation is particularly prevalent in azide chemistry because the interesting singlet nitrenes are high-energy intermediates, and their reactions are highly exothermic. Consequently, chemical activation is observed in the isomerization of methylnitrene CH3N to methylenimine (methanimine) CH2=NH, facilitating the elimination of hydrogen to form HCN or HNC. Rearrangements of phenylnitrene, 1- and 2-naphthylnitrenes, and 2-, 3- and 4-pyridylnitrenes afford cyanocyclopentadiene, 3- and 2-cyanoindenes, and 2- and 3-cyanopyrroles, all showing the effects of chemical activation by undergoing facile interconversion of isomers. Chemical activation can often be reduced or removed entirely by increasing the pressure, thereby promoting collisional deactivation. Larger molecules having more degrees of freedom are better able to dissipate excess energy; therefore the effects of chemical activation are less pronounced or completely absent in the formation of 3-cyanoindole and 1-cyanobenzimidazoles from 3- and 4-quinolylnitrenes and 4-quinazolinylnitrenes, respectively. In compounds possessing nitro groups, chemical activation can cause the loss of the nitro group at nominal temperatures far below those normally needed to cleave the C-NO2 bond.

Ultrafast Infrared and UV–Vis Studies of the Photochemistry of Methoxycarbonylphenyl Azides in Solution

The photochemistry of 4-methoxycarbonylphenyl azide (2a), 2-methoxycarbonylphenyl azide (3a), and 2-methoxy-6-methoxycarbonylphenyl azide (4a) were studied by ultrafast time-resolved infrared (IR) and UV-vis spectroscopies in solution. Singlet nitrenes and ketenimines were observed and characterized for all three azides. Isoxazole species 3g and 4g are generated after photolysis of 3a and 4a, respectively, in acetonitrile. Triplet nitrene 4e formation correlated with the decay of singlet nitrene 4b. The presence of water does not change the chemistry or kinetics of singlet nitrenes 2b and 3b, but leads to protonation of 4b to produce nitrenium ion 4f. Singlet nitrenes 2b and 3b have lifetimes of 2 ns and 400 ps, respectively, in solution at ambient temperature. The singlet nitrene 4b in acetonitrile has a lifetime of about 800 ps, and reacts with water with a rate constant of 1.9 × 10(8) L·mol(-1)·s(-1) at room temperature. These results indicate that a methoxycarbonyl group at either the para or ortho positions has little influence on the ISC rate, but that the presence of a 2-methoxy group dramatically accelerates the ISC rate relative to the unsubstituted phenylnitrene. An ortho-methoxy group highly stabilizes the corresponding nitrenium ion and favors its formation in aqueous solvents. This substituent has little influence on the ring-expansion rate. These results are consistent with theoretical calculations for the various intermediates and their transition states. Cyclization from the nitrene to the azirine intermediate is favored to proceed toward the electron-deficient ester group; however, the higher energy barrier is the ring-opening process, that is, azirine to ketenimine formation, rendering the formation of the ester-ketenimine (4d') to be less favorable than the isomeric MeO-ketenimine (4d).

Direct Observation of a Sulfonyl Azide Excited State and Its Decay Processes by Ultrafast Time-Resolved IR Spectroscopy

The photochemistry of 2-naphthylsulfonyl azide (2-NpSO(2)N(3)) was studied by femtosecond time-resolved infrared (TR-IR) spectroscopy and with quantum chemical calculations. Photolysis of 2-NpSO(2)N(3) with 330 nm light promotes 2-NpSO(2)N(3) to its S(1) state. The S(1) excited state has a prominent azide vibrational band. This is the first direct observation of the S(1) state of a sulfonyl azide, and this vibrational feature allows a mechanistic study of its decay processes. The S(1) state decays to produce the singlet nitrene. Evidence for the formation of the pseudo-Curtius rearrangement product (2-NpNSO(2)) was inconclusive. The singlet sulfonylnitrene (1)(2-NpSO(2)N) is a short-lived species (τ ≈ 700 ± 300 ps in CCl(4)) that decays to the lower-energy and longer-lived triplet nitrene (3)(2-NpSO(2)N). Internal conversion of the S(1) excited state to the ground state S(0) is an efficient deactivation process. Intersystem crossing of the S(1) excited state to the azide triplet state contributes only modestly to deactivation of the S(1) state of 2-NpSO(2)N(3).

Time-Resolved Resonance Raman and Computational Investigation of the Influence of 4-Acetamido and 4-N-Methylacetamido Substituents on the Chemistry of Phenylnitrene

A time-resolved resonance Raman (TR(3)) and computational investigation of the photochemistry of 4-acetamidophenyl azide and 4-N-methylacetamidophenyl azide in acetonitrile is presented. Photolysis of 4-acetamidophenyl azide appears to initially produce singlet 4-acetamidophenylnitrene which undergoes fast intersystem crossing (ISC) to form triplet 4-acetamidophenylnitrene. The latter species formally produces 4,4'-bisacetamidoazobenzene. RI-CC2/TZVP and TD-B3LYP/TZVP calculations predict the formation of the singlet nitrene from the photogenerated S(1) surface of the azide excited state. The triplet 4-acetamidophenylnitrene and 4,4'-bisacetamidoazobenzene species are both clearly observed on the nanosecond to microsecond time-scale in TR(3) experiments. In contrast, only one species can be observed in analogous TR(3) experiments after photolysis of 4-N-methylacetamidophenyl azide in acetonitrile, and this species is tentatively assigned to the compound resulting from dimerization of a 1,2-didehydroazepine. The different photochemical reaction outcomes for the photolysis of 4-acetamidophenyl azide and 4-N-methylacetamidophenyl azide molecules indicate that the 4-acetamido group has a substantial influence on the ISC rate of the corresponding substituted singlet phenylnitrene, but the 4-N-methylacetamido group does not. CASSCF analyses predict that both singlet nitrenes have open-shell electronic configurations and concluded that the dissimilarity in the photochemistry is probably due to differential geometrical distortions between the states. We briefly discuss the probable implications of this intriguing substitution effect on the photochemistry of phenyl azides and the chemistry of the related nitrenes.

Photochemistry of 2-Naphthoyl Azide. An Ultrafast Time-Resolved UV–Vis and IR Spectroscopic and Computational Study

The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resolved transient absorption spectroscopy with IR and UV-vis detection. The experimental findings were interpreted with the aid of computational studies. Using polar and nonpolar solvents, the formation and decay of the first singlet excited state (S(1)) was observed by both time-resolved techniques. Three processes are involved in the decay of the S(1) excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S(1) state decreases significantly as the solvent polarity increases. In all solvents studied, isocyanate formation correlates with the decay of the azide S(1) state. Nitrene formation correlates with the decay of the relaxed S(1) state only upon 350 nm excitation (S(0) → S(1) excitation). When S(n) (n ≥ 2) states are populated upon excitation (λ(ex) = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S(1) and higher singlet excited states (S(n)) of 2-naphthoyl azide. The data correlate with the results of electron density difference calculations that predict nitrene formation from the higher-energy singlet excited states, in addition to the S(1) state. For all of these experiments, no recovery of the ground state was observed up to 3 ns after photolysis, which indicates that both internal conversion and fluorescence have very low efficiencies.

Ultrafast Spectroscopy and Computational Study of the Photochemistry of Diphenylphosphoryl Azide: Direct Spectroscopic Observation of a Singlet Phosphorylnitrene

The photochemistry of diphenylphosphoryl azide was studied by femtosecond transient absorption spectroscopy, by chemical analysis of light-induced reaction products, and by RI-CC2/TZVP and TD-B3LYP/TZVP computational methods. Theoretical methods predicted two possible mechanisms for singlet diphenylphosphorylnitrene formation from the photoexcited phosphoryl azide. (i) Energy transfer from the (π,π*) singlet excited state, localized on a phenyl ring, to the azide moiety, thereby leading to the formation of the singlet excited azide, which subsequently loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. (ii) Direct irradiation of the azide moiety to form an excited singlet state of the azide, which in turn loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. Two transient species were observed upon ultrafast photolysis (260 nm) of diphenylphosphoryl azide. The first transient absorption, centered at 430 nm (lifetime (τ) ∼ 28 ps), was assigned to a (π,π*) singlet S(1) excited state localized on a phenyl ring, and the second transient observed at 525 nm (τ ∼ 480 ps) was assigned to singlet diphenylphosphorylnitrene. Experimental and computational results obtained from the study of diphenyl phosphoramidate, along with the results obtained with diphenylphosphoryl azide, supported the mechanism of energy transfer from the singlet excited phenyl ring to the azide moiety, followed by nitrogen extrusion to form the singlet phosphorylnitrene. Ultrafast time-resolved studies performed on diphenylphosphoryl azide with the singlet nitrene quencher, tris(trimethylsilyl)silane, confirmed the spectroscopic assignment of singlet diphenylphosphorylnitrene to the 525 nm absorption band.

ChemInform Abstract: Thermal Reactivity of 2-Azido- and 3-Azido-benzo(b)thiophene with Alkenes.

Thermal decomposition of 2-azidobenzo[b]thiophene (2-BTA) in the presence of various (E)-and (Z)-alkenes, at room temperature, affords thiochroman-4-carbonitriles resulting from cycloaddition of an ortho-quinoidal enethione intermediate (2) to the olefin double bonds and/or 1-(2-benzothienyl)aziridines which generally occur in a non-stereospecific fashion. In one case, i.e. with diethyl fumarate, clear-cut spectroscopic and chemical evidence for the intermediacy of a triazoline adduct in the formation of the observed trans-and cis-aziridines has been obtained. In the presence of 1-pyrrolidinyl-cyclopentene or -cyclohexene the azide furnishes an isolated triazoline in quantitative yield, whereas with methyl (E)-3-(N-pyrrolidinylacrylate leads to methyl 1-(2-benzothienyl)triazole-4-carboxylate arising from an intermediate triazoline by readily occurring elimination of pyrrolidine. Results suggest that 2-BTA generally undergoes cycloaddition reactions to give triazoline adducts, from which aziridines can be eventually produced, in competition with unimolecular ring-cleavage fragmentation leading to the enethione (2) probably via concerted ring opening and nitrogen extrusion. Suitable support has been provided by the finding that 3-azidobenzo[b]thiophene (3-BTA) can exhibit analogous cycloaddition reactions with alkenes under the same reaction conditions. The present evidence contradicts our previous claim that a singlet nitrene should be an intermediate in the formation of aziridine and ring-cleavage products arising from decomposition of 2-BTA in the presence of alkenes.

Photochemistry of 7-azide-1-ethyl-3-carboxylate-6,8-difluoroquinolone: a novel reagent for photoaffinity labeling

A novel reagent for photoaffinity labeling, 7-azido-1-ethyl-3-carboxylate-6,8-difluoroquinolone, was obtained by nucleophilic substitution of 1-ethyl-3-carboxylate-6,7,8-trifluoroquinolone. Photochemical reaction of this azide with diethyl amine gave 7-hydrazino-derivative as the major product. This compound was generated by singlet nitrene N–H insertion. In addition, 7-amino-1-ethyl-3-carboxylate-6,8-difluoroquinolone was also obtained.

Understanding the rate of spin-forbidden thermolysis of HN3 and CH3N3

The pyrolysis of the simplest azides HN(3) and CH(3)N(3) has been studied computationally. Nitrogen extrusion leads to the production of NH or CH(3)N. The azides have singlet ground states but the nitrenes CH(3)N and NH have triplet ground states. The competition between spin-allowed decomposition to the excited state singlet nitrenes and the spin-forbidden N(2) loss is explored using accurate electronic structure methods (CASSCF/cc-pVTZ and MR-AQCC/cc-pVTZ) as well as statistical rate theories. Nonadiabatic rate theories are used for the dissociation leading to the triplet nitrenes. For HN(3), (3)NH formation is predicted to dominate at low energy, and the calculated rate constant agrees very well with energy-resolved experimental measurements. Under thermal conditions, however, the singlet and triplet pathways are predicted to occur competitively, with the spin-allowed product increasingly favored at higher temperatures. For CH(3)N(3) thermolysis, spin-allowed dissociation to form (1)CH(3)N should largely dominate at all temperatures, with spin-forbidden formation of (3)CH(3)N almost negligible. Singlet methyl nitrene is very unstable and should rearrange to CH(2)NH immediately upon formation, and the latter species may lose H(2) competitively with vibrational cooling, depending on temperature and pressure.

Early Events in the Photochemistry of 2-Naphthyl Azide from Femtosecond UV/Vis Spectroscopy and Quantum Chemical Calculations: Direct Observation of a Very Short-Lived Singlet Nitrene

Exposure of 2-naphthyl azide in acetonitrile at ambient temperature to femtosecond pulses of 266 nm light produces a transient absorption with maxima at 350 and 420 nm. The carrier of the 350 nm band decays more rapidly than that of the 420 nm band which has a lifetime of 1.8 ps. Analogous experiments with 1-chloro-2-naphthyl azide in methanol allow the assignment of the 350 nm band to a singlet excited state of 2-naphthyl azide and the carrier of the 420 nm band to singlet 2-naphthylnitrene. This reactive intermediate has the shortest lifetime of any singlet nitrene observed to date and is a true reactive intermediate. Computational studies at the RI-CC2 level of theory support these conclusions and suggest that initial excitation populates the S2 state of 2-naphthyl azide. The S2 state, best characterized as a pi --> (pi*, aryl) transition, has a geometry similar to S0. S2 of 2-naphthyl azide can then populate the S1 state, a pi --> (in-plane, pi*, azide) excitation, and in the S1 state, electron density is depleted along the proximal N-N bond. S1 is dissociative along that N-N coordinate to form the singlet nitrene, and with a barrier of only approximately 5 kcal/mol for N2 extrusion.

General Photo‐Patterning of Polyelectrolyte Thin Films via Efficient Ionic Bis(Fluorinated Phenyl Azide) Photo‐Crosslinkers and their Post‐Deposition Modification

AbstractWe have extended the well known bisfluorinated(phenyl azide) (bisFPA) methodology to develop an ionic bisFPA process suitable for photo‐crosslinking a wide variety of polyelectrolyte thin films. The crosslinking efficiencies (0.1–1.0 crosslink per photo‐reaction) are sufficiently high for the gel fraction to exceed 80 % for crosslinker concentrations of only a few weight %. This method is based on the photo‐induced formation of singlet nitrenes from FPAs and their insertion into unactivated C–H or other bonds, which thus general and not dependent on the presence of specific chemical functional groups. By derivatizing with ionic charge groups, we obtained ionic bisFPAs that can be properly dispersed into polyelectrolyte thin films. The sorbed moisture always present in these films however severely limits the photo‐crosslinking efficiency, apparently through nitrene protonation and intersystem crossing. This can be avoided by dehydration of the films, in some cases, to 130 °C for 10 min in nitrogen before photo‐exposure. We found that efficient photo‐crosslinking can then be achieved for polyelectrolytes even when they have nucleophilic groups. These include poly(styrenesulfonic acid) and their salts, poly(acrylic acid) and their salts, poly(dimethyldiallylammonium salts), as well as the electrically‐conducting poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonic acid) complex (PEDT:PSSH). We further demonstrate using this ionic bisFPA methodology both photo‐patterning and post‐deposition chemical modifications of polyelectrolyte thin films. This opens broad new possibilities in membrane, sensor and actuator technologies, as well as for organic semiconductor plastic electronics (such as field‐effect transistors) and polyelectrolyte‐based devices.

Molecular structure and shear rheology of long chain branched polypropylene formed by light cross‐linking of a linear precursor with 1,3‐benzenedisulfonyl azide

AbstractLinear isotactic metallocene based polypropylene (Mw = 125,000 g/mol, Mn = 61,000 g/mol) was lightly crosslinked by small amounts of 1,3‐benzenedisulfonyl azide to form long chain branches (LCB). Crosslinking was carried out by reactive extrusion at 200°C in a small scale extruder. At the processing temperature, the two azide groups decompose to nitrenes that work as crosslinkers for PP. The crosslinking reaction occurs primarily by insertion of singlet nitrenes into CH bonds. 1,3‐benzenedisulfonyl azide concentrations of, respectively, 518, 806, 1029, and 2038 ppm were used. SEC revealed formation of a high molecular weight LCB fraction. No degradation was observed by SEC. The introduction of LCB gave rise to rheological behavior typical of LCB as increased zero shear viscosity, accompanied by increased shear thinning, increased flow activation energy and a phase angle plateau. However, the rheological impact and crosslinking efficiency observed were limited compared to earlier studies on PE. Possible reasons for this are discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007