Saturated Hydrocarbon Solvents Research Articles

Unusual Solvatofluorochromism of the Asymmetrically Substituted Aromatic Acetylene Derivative CNAacDMA**

AbstractThe photophysical properties of the newly synthesized unsymmetrically substituted aromatic acetylene derivative 9‐(2‐(4‐(N,N‐dimethylamino)phenyl)ethynyl)anthracene‐10‐carbonitrile (CNAacDMA) were investigated with the steady‐state and time‐resolved fluorometry. In saturated hydrocarbon solvents, only fluorescence from a locally‐excited state (LE) is recorded. In more polar solvents however, excitation of this dye leads to a charge transfer state (CT). In moderate polar solvents (ϵ=4–8) dual emission is observed as a result of competition between structural change and intramolecular charge transfer in the excited state. In polar solvents only one emission band, at shorter wavelength than CT emission, is observed, indicating a bidirectional solvatofluorochromism.

The co-catalyst is the key

The synthesis of highly branched low molecular weight products — potential synthetic lubricants — directly from ethylene is challenging when highly efficient early transition metal catalysts are employed. Now, an alkane soluble co-catalyst permits the synthesis of such polyethylenes using a zirconium catalyst in a saturated hydrocarbon solvent, where competitive solvent coordination at the active site of the catalyst is relatively unimportant.

Highly branched polyethylene oligomers via group IV-catalysed polymerization in very nonpolar media

Early transition metal catalysts produce high-density and linear low-density polyethylenes with spectacular efficiency. Nevertheless, these catalysts are ineffective in producing low-density polyethylene homopolymers with large –(CH2)xCH3 branch densities (x ≥ 5) or low-molecular-mass (Mn < 1,200 g mol−1) highly branched polyethylenes (HBPEs). The latter are potential alternative synthetic lubricants that have eluded efficient catalytic synthesis. Here we report the synthesis of low-Mn HBPEs with 61–93 branches per 1,000 carbon atoms from abundant ethylene as the primary feedstock using a soluble, highly active ion-paired organozirconium catalyst in a saturated hydrocarbon solvent. The unprecedented activity and branch selectivity reflect previously unrecognized aspects of the cationic catalyst–counteranion pairing in nonpolar media and are characterized spectroscopically and quantum mechanically. The HBPE products are rheologically and tribologically attractive candidates for synthetic lubricants. Low-molecular-weight, highly branched polyethylenes are attractive candidates for synthetic lubricants, but their efficient production is constrained by a lack of effective catalytic methods. While conventional group IV transition metal catalysts produce diverse polyethylenes on a huge scale, they are unable to produce highly branched polyethylenes. Here a hydrocarbon-soluble organozirconium precatalyst and borate cocatalyst produce the desired polyethylenes with excellent activity and branch selectivity.

A leaning amine-ketone dyad with a nonconjugated linker: solvatofluorochromism and dual fluorescence associated with intramolecular charge transfer.

Dyad 4, comprising a triphenylamine (TPA) electron donor and 1,4-pentadien-3-one (pentadienone) electron acceptor tethered by a nonconjugated linker, displays solvatofluorochromism (SFC) and dual fluorescence associated with intramolecular charge transfer (ICT) in the excited state. While the fluorescence arises from a locally excited state of 4 (LE-4*) in saturated hydrocarbon solvents, the fluorescence from the ICT state of 4 (ICT-4*) occurs in aprotic solvents. ICT-4* has a much greater dipole moment than its corresponding ground state. The results of theoretical calculations suggest that the conversion of LE-4* to ICT-4* involves a unique structural change like a leaning of the pentadienone moiety. Two factors are responsible for the significant SFC displayed by 4, the first being the high electron-donating and -accepting abilities of the respective locally excited TPA and pentadienone moieties in LE-4* and the other being a rigid ethano bridge that links the two moieties in ICT-4*. The former property facilitates photoinduced electron-transfer (PET) and the latter prevents full single electron transfer (SET) by prohibiting direct π-conjugation and the spatial approach of the two dyad components. Consequently, these electronic and geometrical features lead to SFC arising from a large dipole moment change caused by ICT and partial intramolecular SET.

Solvent Selective Hydrogen–Deuterium Exchange on Saturated Polyolefins

A heterogeneous Pt–Re/SiO2 catalyst was used to perform H–D exchange reactions on the linear saturated polyolefins poly(ethylene), poly(ethylene-alt-propylene), and isotactic poly(propylene) in the saturated hydrocarbon solvents decalin, decane, heptane, and isooctane. Exchange reactions using deuterium gas were selective toward the polymer in the case of linear and branched alkane solvents for poly(ethylene) and poly(ethylene-alt-propylene), whereas reactions conducted in decalin result in virtually no isotope exchange with the polymer. Essentially, no exchange occurred with poly(propylene) in either linear (decane) or branched (isooctane) solvents. In all cases, polymer backbone bonds were unperturbed, preserving the molecular weight distribution. These results suggest that competitive adsorption between the polymer and solvent plays a significant role in heterogeneous catalysis of polymers. Furthermore, this strategy provides a simple and efficient method for postsynthesis labeling selected polyolefins...

Hydrogen-Bonded Cyclic Dimer Formation in Temperature-Induced Reversal of Tautomerism of Salicylideneanilines

Tautomeric equilibrium of salicylideneanilines is in favor of the enol form in most organic solvents at room temperature. We have previously reported that the equilibrium in saturated hydrocarbon solvents is reversed at low temperature, being totally in favor of the cis-keto form, which is unfavorable at room temperature. Although this stabilization of the cis-keto form has been explained in terms of aggregation of molecules at low temperature, the structure of the aggregate has remained uncharacterized. In this study, we measured UV-vis absorption spectra of salicylideneanilines including those which have tert-butyl groups at various positions of the benzene rings. The cis-keto form did not appear at low temperature, when salicylideneanilines have a tert-butyl group at a specific position or is restrained to a planar conformation. From these results we propose here that the temperature-induced reversal of the tautomeric equilibrium proceeds through the formation of intermolecular hydrogen-bonded cyclic dimers and the subsequent formation of the higher-order aggregates.

Stoichiometry of Ozonation of Environmentally Relevant Olefins in Saturated Hydrocarbon Solvents

The double bond-to-ozone reaction stoichiometry was quantified for ozonation of several environmentally relevant unsaturated fatty acids and monoterpenes in saturated hydrocarbon solvents. Olefins with initial concentrations from 30 microM to 3mM were injected in a solvent (n-hexadecane, nonane, or cyclohexane) while an ozone-oxygen mixture was slowly bubbled through the solution. The number of ozone molecules consumed by the injection was quantified in the outgoing flow, and the expected 1:1 double bond-to-ozone reaction stoichiometry was observed only under subambient temperature conditions (T < 250 K). At room temperature, the effective number of double bonds oxidized by each ozone molecule increased to 2-5, with a higher degree of oxidation occurring at lower initial olefin concentrations. The observed enhancement in the stoichiometry is consistent with a competition between direct ozonation and free radical initiated oxidation of double bonds, with free radicals being produced by slow reactions between dissolved ozone and solvent molecules.

Temperature-Induced Reversal of Proton Tautomerism: Role of Hydrogen Bonding and Aggregation in 7-Hydroxyquinolines

UV-vis absorption spectra of 7-hydroxyquinolines in saturated hydrocarbon solvents were measured at various temperatures between 293 and 77 K. The tautomeric equilibrium was found to reverse when the temperature was lowered. At 293 K, the enol form was exclusively present. As the temperature was lowered, the enol form decreased substantially, and the keto form became predominant. A close examination of the spectral changes suggests that the reversal of the tautomeric equilibrium at lower temperatures proceeds in two steps: aggregation of the enol forms by intermolecular hydrogen bonding and further aggregation of the hydrogen-bonded aggregates.

２１世紀に向けた結晶学の新たな挑戦 ２． 分子の変化をみる 有機結晶のサーモクロミズム

Reversible change in color of substances with variation of the temperature is known as thermochromism and has attracted much interest from chemist for a long time. Salicylideneanilines belong to a class of the most popular compounds that show thermochromism in the solid state. We recently succeeded for the first time in the observation of the crystal structure change associated with the thermochromism and in the determination of the structure of the colored form. Furthermore, we discovered that salicylideneanilines are generally thermochromic in the fluid solution of a saturated hydrocarbon solvent, although it has been believed that salicylideneanilines are thermochromic only in the solid state. Our study revealed that the association of the molecules plays an essential role in the thermochromism.

High-Pressure Study on the Quenching Mechanism by Oxygen of the Lowest Triplet State in Solution

The mechanism for the quenching by oxygen of the lowest triplet state (T1) of 9-acetylanthracene(ACA) in five saturated hydrocarbon solvents at pressures up to 400 MPa was investigated. The quenching rate constants of the T1 state of ACA, kqT, decrease monotonically with increasing pressure, and the apparent activation volumes for kqT vary in the range 0.8−5.3 cm3/mol. It was also found that the plots of ln kqT against ln η, where η is solvent viscosity, show significant downward curvatures in all the solvents examined. From measurements by the time-resolved thermal lensing at 0.1 MPa, together with measurements of triplet−triplet absorption spectra as a function of pressure, the yields of the T1 state of ACA were found to be approximately unity in the experimental conditions examined. The quenching rate constants, kqS, by oxygen of the lowest excited singlet state (S1) of 9,10-dimethylanthracene (DMEA) whose van der Waals radius is nearly equal to that of ACA decrease strongly with increasing pressure, and the apparent activation volumes for kqS fall in the range of 9.4−14.9 cm3/mol. It was also found that the plots of ln kqS against −ln η are linear, with a slope of 0.59−0.71 depending on solvent. These results of kqS are consistent with our previous conclusion that the oxygen quenching of the S1 state of DMEA is diffusion controlled. The ratio, kqT/kqS, is approximately 1/9 in methylcyclohexane but is less than 1/9 in n-butane, n-pentane, n-hexane, and n-heptane at 0.1 MPa and 25 °C, and the ratio was found to increase over 1/9 with increasing pressure in all the solvents examined. By the bleaching method of DPBF, coupled with time-resolved luminescence measurements, the yields of singlet oxygen (1Δg) formed by the quenching of the T1 state of ACA, ΦΔ, were measured, and the values of ΦΔ were found to be approximately unity. These results were explained by a kinetic model in which the intersystem crossings between encounter complexes with different spin multiplicities are taken into account. From the analysis based on this model, the pressure dependence of kqT/kqS is discussed.

Hydrotreating Reaction of 4,6-Dimethyldibenzothiophene and Carbazole as Petroleum Model Compounds over Ni-Mo/Al2O3 in the Presence of Saturated Hydrocarbon Solvents.

Hydrotreating reaction of 4, 6-dimethyldibenzothiophene (DMDBT) and carbazole (CBZ) as heteroatom-containing petroleum model compounds and biphenyl (BP) was examined without any solvent or with a solvent such as cyclohexane or decalin to obtain basic information on the effect of addition of saturated hydrocarbon solvents on the reaction. The treatment was conducted with a presulfided Ni-Mo/Al2O3 catalyst under 70kg/cm2 of hydrogen (at room temperature) at 300-350°C in a batch autoclave. It was observed that, irrespective of the substrates, both conversion and hydrogen consumption in the reaction increased in the order: no solvent<decalin<cyclohexane. In the treatment of DMDBT in n-paraffins in the range of C6-C12, it was also observed that the reaction was relatively more promoted as their chain lengths became shorter. These results are discussed in terms of diffusivity of the substrates in the reaction medium.

“Constrained Geometry” Dialkyl Catalysts. Efficient Syntheses, C−H Bond Activation Chemistry, Monomer−Dimer Equilibration, and α-Olefin Polymerization Catalysis

This contribution reports an efficient synthesis of the “constrained geometry” group 4 dibenzyl complexes Me2Si(η5-Me4C5)(tBuN)MR2 (CGCMR2, where R = CH2Ph; M = Ti (1), Zr (2)), as well as the substantially different reaction patterns in the cocatalytic activation of the R = CH2Ph and Me complexes with B(C6F5)3, PBB (tris(2,2‘,2‘‘-perfluorobiphenyl)borane), and Ph3C+B(C6F5)4-. The resulting cationic complexes are highly but not equivalently active for α-olefin polymerization and copolymerization catalysis. The reaction of the neutral free ligand CGCH2 with Ti(CH2Ph)4 in aromatic or saturated hydrocarbon solvents at 60 °C cleanly affords 1 in 90% yield, while the corresponding reaction with Zr(CH2Ph)4 produces 2 in lower yield. When activated with Ph3C+B(C6F5)4- at low temperatures, 2 generates cationic CGCZrCH2Ph+ B(C6F5)4- (4). However, unlike the corresponding metallocene dibenzyl, the cationic derivative of which (Cp2ZrCH2Ph+B(C6F5)4- (3)) can be isolated in quantitative yield, the reaction of 1 with B...

Mechanistic Studies of the Thermolysis of Tetraneopentyltitanium(IV). 1. Solution Evidence That Titanium Alkylidenes Activate Saturated Hydrocarbons

Studies of the thermolysis of Ti(CH2CMe3)4 in solution have been carried out in parallel with studies of the chemical mechanism responsible for its conversion to titanium carbide under CVD conditions. In hydrocarbon solutions, the neopentyl complex thermolyzes to eliminate 2.1 equiv of neopentane as the principal organic product. A deuterium kinetic isotope effect (kR(H)/kR(D) ) 5.2 ( 0.4) upon deuterating the alkyl groups at the R positions provides clear evidence that the initial step in the thermolysis is an R-hydrogen abstraction reaction to form neopentane. The activation parameters for this R-hydrogen abstraction process are ¢H q ) 21.5 ( 1.4 kcal/mol and ¢S q )- 16.6 ( 3.8 cal/(mol K). The titanium-containing product of this reaction is a titanium alkylidene, which in solution activates C-H bonds of both saturated and unsaturated hydrocarbon solvents such as benzene and cyclohexane. No activation of the C-F bonds of hexafluorobenzene is seen, however. Under special circumstances, a second thermolysis pathway for TiNp4 can be detected, A-hydrogen activation, but this pathway is intrinsically about 25 times slower than the R-hydrogen abstraction process.

Sudden polarization in the twisted, phantom state of tetraphenylethylene detected by time-resolved microwave conductivity

Photoexcitation of the symmetrical molecules tetraphenylethylene and tetra-p-methoxyphenylethylene dissolved in saturated hydrocarbon solvents results in a transient increase in the dielectric loss of the solutions as monitored using the nanosecond time-resolved microwave conductivity (TRMC) technique. This provides direct evidence for the dipolar, or [open quotes]zwitterionic,[close quotes] nature of the [sup 1]p* phantom state formed from S[sub 1] by rotation around the central carbon-carbon bond. Dipole relaxation occurs mainly by charge inversion between the two energetically equivalent zwitterionic configurations. Z[sub [+-]], on a timescale of several picoseconds. A minimum dipole moment of ca. 7.5 D for the individual Z[sub [+-]] states is found. The fluorescence of TPE in alkane solvents has two decay components, one with a decay time less than 200 ps and a second with a decay time of 1.9 ns. The former ([lambda][sub max] [approx] 490 nm) is assigned to emission from the partially relaxed S[sub 1] state prior to twisting. The latter ([lambda][sub max] [approx] 540 nm) is assigned to emission from a small, ca. 1%, concentration of the relaxed S[sub 1] state in equilibrium with the [sup 1]p* state in saturated hydrocarbon solvents. 27 refs., 2 figs., 2 tabs.

Factors influencing vapor‐phase photografting of styrene/acrylonitrile monomer mixtures on polymer films

AbstractAn accelerating effect due to a styrene (St)/acrylonitrile (AN) monomer mixture was investigated in vapor‐phase photografting on polymer films, on which a photoinitiator was coated. A maximum percent grafting was observed at a certain St concentration of monomer feed irrespective of the nature of photoinitiator, such as benzoyl peroxide, benzophenone, and α,α′‐azobisisobutylonitrile, and the type of polymer flim, such as low‐ and high‐density polyethylene, polypropyrene, and nylon 6. The same effect due to the monomer mixture was recorded for liquid‐phase photografting in a water‐ethanol mixture solvent. With photoinduced copolymerization of the St/AN mixture using benzoyl peroxide, conversion of AN was sharply reduced by the addition of St to the AN system using DMF, DMSO, methanol, and ethyl acetate solvents. In case of saturated hydrocarbon solvents such as n‐hexane, cyclohexane, n‐hexane, cyclohexane, n‐octane, and n‐decane, on the other hand, a maximum conversion was observed at a certain St concentration of monomer feed. The accelerating effect due to the St/AN mixture was discussed in terms of copolymerizability of monomers and a gel effect.

Temperature Dependence of the Fluorescence Intensity of 1-Benzyloxy-2-pyridone and Related Compounds

Abstract A quantitative analysis of the temperature dependence of fluorescence intensity of the title compounds in saturated hydrocarbon solvents has revealed that the relevant thermally-activated radiationless process from singlet manifold involves a rotation about the N–O bond in these compounds.

Charge recombination kinetics of giant dipoles in saturated hydrocarbon solvents

Long-distance, intramolecular charge recombination following photoexcitation of a series of rigid donor-insulator-acceptor molecules dissolved in n-hexane, cyclohexane and trans-decalin has been monitored using the time-resolved microwave conductivity (TRMC) technique. Two recombination pathways are found to be operative; a direct transition to the ground state which is temperature and solvent independent and an indirect route via the local excited donor which is thermally activated and dependent on the polarisability of the medium. The latter pathway becomes important when the charge-separated state and the local excited state are close to being degenerate.

Scintillation process in three-component systems: mechanism of the luminescence minimum

Three-component systems consisting of a saturated hydrocarbon solvent, C, an aromatic solvent, B, and a fluorescent solute, T, are excited either optically (in the absorption band of C and below its ionization potential) or with 0.67-MeV ..beta../sup -/ particles. The luminescence of T is detected and studied as a function of the B/C concentration ratio. Systems studied are C = cyclohexane, trans-decalin, methylcyclohexane, bicyclohexyl, n-heptane, or 2,3-dimethylbutane; B - benzene or toluene; and T = N,N,N',N'-tetramethylphenylenediamine (TMPD), p-terphenyl, or 2,5-diphenyloxazole (PPO). The luminescence behavior is observed under both aerated and nitrogenated conditions. For B = benzene, the luminescence of T is depressed by the replacement of C with B, at low B/C concentration ratios. This occurs for both modes of excitation for all C except bicyclohexyl and 2,3-dimethylbutane. At higher B/C concentration ratios, the luminescence of T recovers and ultimately exceeds its intensity in pure C + T. Thus there is observed a luminescence minimum. At low T concentrations the position and depth of the minimum are very sensitive to the presence of O/sub 2/, but at higher concentrations this sensitivity is lost. For B = toluene, the luminescence minimum is only observed under ..beta../sup -/ particle excitation conditions. Amore » mechanism is developed to accommodate these observations. Its analysis indicates that production of S/sub 1/ states of B (i.e., B*) via either energy transfer from C* or via charge transfer from C/sup +/ followed by the geminate recombination B/sup +/ + e/sup -/ ..-->.. B* is intrinsically inefficient in dilute cyclohexane solutions. For B = benzene this inefficiency resides mainly in the electronic energy transfer process and for toluene in the ion-recombination process. Also, the analysis indicates that there must be two states of C that can transfer energy to B and/or T.« less

Long-range photoinduced through-bond electron transfer and radiative recombination via rigid nonconjugated bridges: distance and solvent dependence

A series of molecules 1 was synthesized containing a 1,4-dimethoxynaphthalene donor (D) an a 1,1-dicyanoethylene acceptor (a) interconnected by five different, rigid, nonconjugated bridges. The length of the bridges varies with increments of two sigma-bonds from four in 1(4) to 12 sigma-bonds in 1(12), to provide donor-acceptor center-to-center separations (R/sub c/) ranging from 7.0-14.9 A. In solvents of medium and high polarity, excitation of the donor D is followed by rapid intramolecular electron transfer. The rate constant (k/sub et/) shows only small dependence upon the solvent polarity (a factor of 2-3 between benzene and acetonitrile, for example) but decreases with increasing separation ranging from >10/sup 11/ s/sup -1/ for a four-bond separation to approx. =4 x 10/sup 8/ s/sup -1/ for a 12-bond separation. In saturated hydrocarbon solvents photoinduced electron transfer is not observed for 10 and 12-bond separations, while it is not significantly decreased for the shorter homologues. Therefore the absence of electron transfer at 10- and 12-bond separations in saturated hydrocarbon solvents is attributed to a thermodynamic rather than to a kinetic effect. In solvents where electron transfer is thermodynamically feasible, its rate is considerably greater than that found from various other experimental studies where either different bridgesmore » were used or intermolecular electron transfer was studied. Through-bond interaction involving sigma/..pi.. interaction between the bridge and the donor-acceptor pair is proposed to explain the very high electron transfer rates observed in 1; this is qualitatively correlated with independent information about this coupling derived from both theory and experiment (photoelectron spectroscopy).« less

On the formation and structure of self-assembling monolayers: III. Time of formation, solvent retention, and release

FTIR spectroscopy (in the ATR mode) and contact angle measurements are used to determine the characteristic time scales of formation of some representative oleophobic monolayers produced on polar solids by spontaneous adsorption from organic solutions, and to investigate the mechanis of solvent incorporation into such organized films during their formation and its subsequent release. The more general question of adsorption of low-energy surfaces, not wetted by the bulk liquid adsorbate, is also addressed, in connection with the present observations of solvent retention on surfaces coated with nonwettable oleophobic monolayers. Two amphiphile/solid systems and two saturated hydrocarbon solvents were selected for this study, with the purpose of providing examples for different modes of binding to the surface and different geometrical relationships between solvent and amphiphile molecules. Ionic binding (arachidic acid/ZnSe) is found to yield practically complete monolayers within immersoin times not longer than 15 s, whereas several minutes are required for the formation of complete monolayers involving covalent binding to the surface ( n-octadecyltrichlorosilane/Si). Solvent incorporation into the films during their formation is facilitated by geometrical matching between the solvent and the non polar portion of the amphiphile, as indicated by the observed differences between n-hexadecane and bicyclohexyl in this respect. Most of the solvent is usually squeezed out of the inner core of the film and replaced by amphiphile, with the completion of a compact and continuous monolayer of the latter. However, in some of the studied systems, variable amounts of solvent were found to be retained on top of the outer monolayer surface after its separation from the bulk liquid phase. Liquid retention on such nonwettable surfaces appears to involve adhesion at microscopic defect sites in the film. It is, finally, demonstrated that high qualtiy, solvent-free monolayers may be obtained by adsorption from organic solutions, provided geometrical matching between solvent and amphiphile is avoided and amphiphiles capable of irreversible binding to the surface are employed.