Ionization of aniline and its N-methyl and N-phenyl substituted derivatives by (free) electron transfer to n-butyl chloride parent radical cations

The electron transfer from aniline and its N-methyl as well as N-phenyl substituted derivatives (N-methylaniline, N,N-dimethylaniline, diphenylamine, triphenylamine) to parent solvent radical cations was studied by electron pulse radiolysis in n-butyl chloride solution. The ionization results in the case of aniline (ArNH2) and the secondary aromatic amines (Ar2NH, Ar(Me)NH) in the synchronous and direct formation of amine radical cations, as well as aminyl radicals, in comparable amounts. Subsequently, ArNH2*+ deprotonates in a delayed reaction with the present nucleophile Cl-, and forms further ArNH*. In contrast, tertiary aromatic amines such as triphenylamine and dimethylaniline yield primarily the corresponding amine radical cations Ar3N*+ or Ar(Me2)N*+, only. The persistent Ar3N*+ forms a charge transfer complex (dimer) with the parent amine molecule, whereas Ar(Me2)N*+ deprotonates to carbon-centered radicals Ar(Me)NCH2*.     

Femtodynamics reflected in nanoseconds: bimolecular free electron transfer in nonpolar media

The (free) electron transfer (FET) from electron donor molecules to parent solvent radical cations of alkanes and alkyl chlorides exhibits mechanistic peculiarities that are conditioned by the low polarity of these solvents. Because of the negligible solvation of ions in such systems and the almost complete lack of an activation barrier, the electron jump takes place at the very first encounter of the reactants and, as such, in extremely short times of 相似文献   

Ionization of aromatic sulfides in nonpolar media: free vs reaction-controlled electron transfer

The primary products of the bimolecular free electron transfer (FET) from aromatic sulfides (PhSCH2Ph, PhSCHPh2, PhSCPh3) to n-butyl chloride radical cations are two radical cation conformers: a dissociative and a metastable one. In analogy with formerly studied donor systems, this result seems to reflect femtosecond oscillations in the ground state of the sulfides such as torsion motions around the Ar-S bond. This motion is accompanied by a marked electron fluctuation within the HOMO (or the n) orbitals. The FET products observed in the nanosecond time scale such as the metastable sulfide radical cations (Ar-S-CR3*+), the dissociation products R3C+; and R3C*, and their (experimentally) nondetectable counterparts Ar-S* as well as Ar-S+ can be understood with the simplified assumption of two extreme conformations, namely a planar and a twisted donor molecule. Using mediator radical cations (benzene, butylbenzene, biphenyl), the stepwise reduction of the free energy of the electron transfer from -DeltaH = 2.5 to 相似文献   

Production of free radicals and triplets from contact radical pairs and from photochemically generated radical ions

The quantum yields of triplets and free radicals (or radical ions) that escaped recombination in photochemically created primary radical pairs (or radical ion pairs) are calculated. As the products of monomolecular photodissociation, the neutral radicals appear at contact, while the ions are initially distributed over the space due to distant photoionization (bimolecular electron transfer) in the liquid solution. The diffusional dependence of the quantum yields is shown to be different when recombination starts from contact or from separated reactants. The experimental data for recombination of ionized perylene with aromatic amine counterions is well fitted with the noncontact initial distribution provided the recombination is also noncontact and even more distant than ionization.     

A computational study of remote C-H activation by amine radical cations: implications for the photochemical reduction of carbon dioxide

A computational study, using density functional theory calibrated against higher-level methods, has been undertaken to evaluate tertiary amines whose radical cations might lose hydrogen atoms from positions other than the alpha carbons. The purpose was to find photochemically activated reducing agents for carbon dioxide that could be regenerated in a separate photochemical reaction. The calculations have revealed two reactions that might be suitable for this purpose. In one, the nitrogen of the radical cation makes a bond to a remote carbon with simultaneous displacement of a hydrogen atom. In the other, a remote hydrogen atom is transferred to the nitrogen, thereby creating a distonic radical cation that can lose a hydrogen atom beta to the radical site. The latter reaction is found to be particularly favorable since it apparently involves a surface crossing that allows the amine radical cation and CO2 radical anion to transform smoothly to a ground-state formate ion and an alkene. A number of structural motifs are investigated for the amines. The lower ionization potential of aromatic amines, compared to their aliphatic analogues, is desirable in that it could permit the use of longer wavelength light to drive the reaction. However, a thermochemical cycle shows that the reduction in ionization potential must be matched by an increase in proton affinity of the amine if the intramolecular hydrogen transfer is to be exothermic. Most aromatic amines do not satisfy this criterion and, hence, would have to rely on the displacement reaction for hydrogen-atom release if they were to be used as renewable reagents for CO2 reduction. Examples of specific aromatic and aliphatic tertiary amines that should be suitable for the purpose are presented, and their relative merits and weaknesses are discussed.     

芳香叔胺引发丙烯腈光聚合研究

我们发现芳香叔胺是丙烯腈聚合极有效的光引发剂。芳香叔胺苯环上氢被正性基取代时,引发能力增加,被负性基取代时,引发能力下降。下列胺的活性次序是:N,N-二甲基对甲苯胺(DMT)>N,N-二羟乙基对甲苯胺(DHET)>N,N-二甲基苯胺(DMA)>N,N-二甲胺基苯甲醛(DMB)>N,N-二甲基对硝基苯胺(DNA)。芳香叔胺引发丙烯腈光聚合属自由基机构,聚合速度与叔胺浓度的0.66次方成正比。在叔胺浓度为10~(-2)—10~(-4)M范围内,聚合物平均聚合度的倒数与叔胺浓度的0.5次方成正比。初步认为在紫外光激发下,芳香叔胺与丙烯腈分子生成激发态电子转移络合物,再分解产生自由基,引发丙烯腈聚合。     

Competitive consecutive electron transfer in determination of ionization potentials: ketene derivatives

Kinetics of competitive consecutive electron transfer was used to determine ionization potentials of transient species. Kinetics of two-stage electron transfer reactions in aprotic solvent was studied using 355 nm laser flash photolysis. The concentrations of transients produced by the laser flash photolysis were monitored by their light absorption. Triplet-excited tetrachloro-p-benzo-quinone (p-chloranil) generated by a 355 nm laser flash oxidized diethyl ketene, diphenyl ketene, or phenyl ethyl ketene to form radical cations. The ketene radical cations, in turn, oxidized tertiary amine, forming ground state ketene and ammonium radical cation. The kinetics of the disappearance of ketene radical cations (and/or appearance of ammonium radical cations) due to consecutive, competitive electron transfer to ketene and p-chloranil radical cations was monitored. By monitoring kinetics in the presence of tertiary amines with different oxidation potentials, it was established that in acetonitrile the oxidation potential of diethyl ketene was 5.4 eV; for phenyl ethyl ketene, it was approximately 4.8 eV; and for diphenyl ketene, it was 4.6 eV. The results were in agreement with the oxidation potentials of ketenes computed using published data.     

Quenching of singlet and triplet excited aromatic hydrocarbons by sulphides :- the amine and sulphide enhanced photo-induced degradation of chloro- and cyanoaromatic hydrocarbons

Sulphides have been shown to be capable of quenching the excited singlet and triplet states of several aromatic hydrocarbons and their derivatives. The quenching is proposed to involve an electron transfer mechanism. The use of sulphides and amines to generate aromatic hydrocarbon radical cations from the excited states of these compounds has been utilised in order to carry out reductive decyanation reactions of cyanoaromatic hydrocarbons.     

Ionization of amino-, thio- and hydroxy-naphtalenes via free (unhindered) electron transfer

The electron transfer from various monosubstituted naphthyl derivatives (naphtols, NpOH; naphtylamines, NpNH2; and thionaphtols, NpSH) to parent n-BuCl radical cations was studied by means of pulse radiolysis. The experiments reveal the synchronous and direct formation of two types of transients: the metastable solute radical cation (NpXH(*+), X = heteroatom) and the corresponding heteroatom-containing radical (NpX(*)) in comparable amounts. This is explained in terms of the free (unhindered) electron transfer in nonpolar solvents, which is a bimolecular process reflecting femtosecond time scale events of intramolecular dynamic motions accompanied by significant changes of the electron distribution within the donor molecule.     

Mechanism for the reduction of the mixed-valent MnIIIMnIV[2-OHsalpn]2+ complex by tertiary amines

The mixed-valent dimanganese(III/IV) complex MnIIIMnIV(2-OHsalpn)2+, 1, is cleanly reduced in acetonitrile by aliphatic tertiary amines to give the dimanganese(III) product MnIII2(2-OHsalpn)2, 2. Thorough characterization of the organic reaction products shows that tributylamine is converted to dibutylformamide and propionaldehyde. Kinetic studies and radical trapping experiments suggest that this occurs via initial single-electron transfer from the amine to 1 coupled with C-H alpha proton transfer from the oxidized amine. EPR spectroscopy and base inhibition studies indicate that coordination of the amine to 1 is a critical step prior to the electron transfer step. Rate data and its dependence on the amine indicate that the ability of the amine to reduce 1 is correlated to its basicity rather than to its reduction potential. Weakly basic amines were unable to reduce 1 irrespective of their reduction potential. This was inferred to indicate that proton transfer from the amine radical cation is also important in the reduction of 1 by tertiary amines. Comparison of the activation energy with reaction thermodynamics indicates that proton transfer and electron transfer must be concerted to explain the rapidity of the reaction. The fate of the amine radical is dependent on the presence of oxygen, and labeling studies show that oxygen in the organic products arises from dioxygen, although incorporation from trace water was also observed. These data indicate that inhibition of the hydrolytic quenching of the amine radical in an aprotic solvent results in a different fate for the amine radical when compared to amine oxidation reactions in aqueous solution. The proposed mechanism gives new insight into the ability of amines with high reduction potential to reduce metal ions of lower potential. In particular, these data are consistent with the ability of small amines and certain amine-containing buffers to inhibit manganese-dependent oxygen evolution in photosynthesis, which arises in some cases as a result of manganese reduction and its concomitant loss from the PS II reaction center.     

Ruthenium-phenothiazine electron transfer dyad with a photoswitchable dithienylethene bridge: flash-quench studies with methylviologen

A molecular ensemble composed of a phenothiazine (PTZ) electron donor, a photoisomerizable dithienylethene (DTE) bridge, and a Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) electron acceptor was synthesized and investigated by optical spectroscopic and electrochemical means. Our initial intention was to perform flash-quench transient absorption studies in which the Ru(bpy)(3)(2+) unit is excited selectively ("flash") and its (3)MLCT excited state is quenched oxidatively ("quench") by excess methylviologen prior to intramolecular electron transfer from phenothiazine to Ru(III) across the dithienylethene bridge. However, after selective Ru(bpy)(3)(2+1)MLCT excitation of the dyad with the DTE bridge in its open form, (1)MLCT → (3)MLCT intersystem crossing on the metal complex is followed by triplet-triplet energy transfer to a (3)π-π* state localized on the DTE unit. This energy transfer process is faster than bimolecular oxidative quenching with methylviologen at the ruthenium site (Ru(III) is not observed); only the triplet-excited DTE then undergoes rapid (10 ns, instrumentally limited) bimolecular electron transfer with methylviologen. Subsequently, there is intramolecular electron transfer with PTZ. The time constant for formation of the phenothiazine radical cation via intramolecular electron transfer occurring over two p-xylene units is 41 ns. When the DTE bridge is photoisomerized to the closed form, PTZ(+) cannot be observed any more. Irrespective of the wavelength at which the closed isomer is irradiated, most of the excitation energy appears to be funneled rapidly into a DTE-localized singlet excited state from which photoisomerization to the open form occurs within picoseconds.     

The Interaction of Ground and Excited States of Lumichrome with Aliphatic and Aromatic Amines in Methanol

The reaction of the ground and excited states of lumichrome (=7,8‐dimethylalloxazine=7,8‐dimethylbenzo[g]pteridine‐2,4(1H,3H)‐dione) with aliphatic and aromatic amines was investigated in MeOH. In the presence of aliphatic amines of high basicity, new bands are observed in the absorption and fluorescence spectra. These bands arise in a proton‐transfer reaction from lumichrome, in the ground and in the singlet excited states, to the amine. On the other hand, amines with lower basicity such as triethanolamine (=2,2′,2″‐nitrilotris[ethanol]) and aromatic amines are not able to deprotonate lumichrome, and hence a quenching of the fluorescent emission takes place without changes in the spectral shape. In this case, bimolecular‐quenching rate constants were determined for the excited singlet and triplet states. Based on laser‐flash‐photolysis experiments, an electron‐transfer mechanism is proposed. Aliphatic amines yield lower rate constants than the aromatic ones for the same driving force. A notable difference arises in the limiting value reached by the singlet and triplet quenching rate constants by aromatic amines. For the singlet quenching, the limit is coincident with a diffusion‐controlled reaction, while those for triplet quenching reach a lower constant value, independent of the driving force. This is explained by an electron‐transfer mechanism, with a lower frequency factor for the triplet‐state process.     

A study on one-electron oxidation of phenothiazine derivatives BV piperidine oxoammonium ion in SDS micelle

N-Substituted phenothiazines were oxidized by 2,2,6,6-tetramethy-4-acetyloxypiperidine oxoammonium hexachloroantimonate (TAPO) to the corresponding radical cations in sodium dodecyl sulfate (SDS) micellar solution. The radical cations of phenothiazine derivatives were generated and stabilized by the anionic micelle in queous solution. An electron transfer mechanism has been proposed.     

Solvent‐Free Photooxidation of Alkanes by Dioxygen with 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone via Photoinduced Electron Transfer

Photooxidation of alkanes by dioxygen occurred under visible light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) which acts as a super photooxidant. Solvent‐free hydroxylation of cyclohexane and alkanes is initiated by electron transfer from alkanes to the singlet and triplet excited states of DDQ to afford the corresponding radical cations and DDQ^??, as revealed by femtosecond laser‐induced transient absorption measurements. Alkane radical cations readily deprotonate to produce alkyl radicals, which react with dioxygen to afford alkylperoxyl radicals. Alkylperoxyl radicals abstract hydrogen atoms from alkanes to yield alkyl hydroperoxides, accompanied by regeneration of alkyl radicals to constitute the radical chain reactions, so called autoxidation. The radical chain is terminated in the bimolecular reactions of alkylperoxyl radicals to yield the corresponding alcohols and ketones. DDQ^??, produced by the photoinduced electron transfer from alkanes to the excited state of DDQ, disproportionates with protons to yield DDQH₂.     

Excited states quenching of phenosafranine dye by electron donors

The quenching of the excited singlet and triplet states of phenosafranine by aromatic amines, methoxybenzenes and triethanolamine was investigated in acetonitrile and methanol. The rate constants for the aromatic quenchers present a typical dependence of an electron transfer process with the one-electron redox potential of the donor. A Rehm–Weller correlation is obtained with the driving force. The fitting parameters are very similar in both solvents. The electron transfer nature of the quenching reaction is further confirmed by the detection of the radical cations of the quenchers and the semireduced form of the dye in laser flash photolysis experiments. The absorption coefficients of the transient species were estimated, and the quantum yield of the charge separation process was determined.     

芳香叔胺引发烯类单体聚合研究——单体结构与聚合的关系

不同烯类单体在芳香叔胺存在下的聚合机构不一样。甲基丙烯酸甲酯等有α-甲基的烯类单体在不照光的条件下即可被芳香叔胺引发聚合,其聚合机构认为是首先α—甲基被胺-氧复合物氧化,生成单体过氧化物。再与胺形成氧化还原体系,分解产生自由基。 丙烯酸酯,丙烯腈等没有α-甲基的单体,只有光照时才被芳香叔胺氧化聚合,不光照时完全不聚合。这是因为这些单体不被胺-氧复合物氧化。光照下聚合的机构认为是因光的激发,这些单体与芳香叔胺形成电子转移激发络合物,再分解产生自由基。 研究了单体结构,胺结构对光聚合速度的影响。不同单体的活性次序是: AN>MA>VA>St 不同芳香叔胺的活性次序是: DMT>DMA>DMB>DNA 表明单体双键电子云密度越小,芳香叔胺氮原子上电子云密度越大,越容易形成激发态电子转移络合物,从而越容易聚合。     

Kinetic and energetic analysis of the free electron transfer

In this paper, the bimolecular free (unhindered) electron transfer (FET) from various trityl-containing compounds to the solvent radical cations of n-BuCl is described. In good agreement with the previously studied cases, the FET involving trityl-derived compounds results in the formation of two different types of the radical cation, which undergo the subsequent fragmentation via two alternative reaction channels. This unusual effect is caused by the intramolecular rotational motion in the ground-state molecules around the arrow-marked bond Ar-//-X-CPh 3 (Ar = aromatic moiety; X = S, O, NH, CH 2), since such oscillations are directly connected with the electron distribution within the molecule. An unhindered electron jump from the donor trityl compound to the solvent radical cation, taking place in the subfemtosecond time range, generates the solute radical cation with the inherited geometry and the electron distribution of its precursor. Among the whole variety of produced radical cations, two extreme conformer states can be distinguished, namely, a planar and a twisted state. The planar type represents the structures with minimum energy, whereas the twisted type is destabilized by the increased value of the rotational barrier in the ionized state. The difference in the energetic profiles between planar and twisted radical cations plays a crucial role in their subsequent fragmentation. The planar radical cation follows the thermodynamically favored pathway generating ArX (*) and Ph 3C (+). A distinct part of the twisted radical cation dissociates faster than it relaxes into the more preferable planar conformation and, therefore, produces a thermodynamically unfavorable couple of products: ArX (+) and Ph 3C (*). This fragmentation channel is exclusively caused by FET. The undertaken quantum chemical calculations enable the judgment of the energetics of the different dissociation channels of the radical cations of the trityl derivatives.     

Diphenyldiselenide As Novel Non–salt Photoinitiator for Photosensitized Cationic Polymerization of N-Vinyl Carbazole

Photoinitiated cationic polymerization of N-vinyl carbazole (NVC) with diphenyldiselenide (DPDS) as an initiator and aromatic nitriles such as dicyanonaphthalene (DCN) and dicyanoanthracene (DCA) as sensitizer was studied at λ > 290 nm in CH₂Cl₂ solvent using single electron transfer (SET) reactions in this investigation. Aromatic nitriles were found to be effective photosensitizer in initiating polymerization at wavelength λ > 290 nm where DPDS (λ_max = 246 nm) do not absorb. However, the DPDS works well as an initiator with both DCN and DCA. An initiation mechanism is also proposed that involves the formation of selenium radical cation as actual initiating species. Upon photoirradiation, these radical cations are formed by single electron transfer between the excited singlet state sensitizer and the ground state of DPDS.     

Near‐Infrared Free‐Radical and Free‐Radical‐Promoted Cationic Photopolymerizations by In‐Source Lighting Using Upconverting Glass

A method is presented for the initiation of free‐radical and free‐radical‐promoted cationic photopolymerizations by in‐source lighting in the near‐infrared (NIR) region using upconverting glass (UCG). This approach utilizes laser irradiation of UCG at 975 nm in the presence of fluorescein (FL) and pentamethyldiethylene triamine (PMDETA). FL excited by light emitted from the UCG undergoes electron‐transfer reactions with PMDETA to form free radicals capable of initiating polymerization of methyl methacrylate. To execute the corresponding free‐radical‐promoted cationic polymerization of cyclohexene oxide, isobutyl vinyl ether, and N ‐vinyl carbazole, it was necessary to use FL, dimethyl aniline (DMA), and diphenyliodonium hexafluorophosphate as sensitizer, coinitiator, and oxidant, respectively. Iodonium ions promptly oxidize DMA radicals formed to the corresponding cations. Thus, cationic polymerization with efficiency comparable to the conventional irradiation source was achieved.     

ESR study of the mechanism of radical formation during liquid-and solid-phase radiolyses of ethylamines

The formation of radicals during the liquid-phase radiolysis of ethylamine, diethylamine, and triethylamine was studied by means of the spin trapping technique. The radicals produced in ion-molecule reactions and in the rearrangement and fragmentation reactions of the primary radical cations of the amines were identified. The structure and reactions of the primary radical cations were studied in a low-temperature CFCl₃ freonic matrix in which amine radical cations were generated via charge transfer from matrix radical cations to amines during freon irradiation. The results of experiments in the liquid and solid phases are consistent with one another. The structure of neutral radicals and radical cations of the ethylamines was corroborated by quantum-chemical calculations.