苯胺引发丙烯腈光聚合的机理研究

苯胺引发丙烯腈(AN)光聚合是通过电荷转移经激基复合物机理进行的。形成激基复合物途径有两条,一是苯胺和AN分子在基态形成的电行转移复合物受光激发;二是苯胺的激发单线态与基态AN分子相互作用。形成的激基复合物再经质子转移产生两个自由基引发聚合。     

STUDIES ON THE MECHANISM OF VINYL PHOTOPOLYMERIZATION INITIATED BY CONDENSED AROMATIC HYDROCARBON-ANILINE SYSTEMS

Initiation mechanism of the photopolymerization of acrylonitrile and methyl methacrylate, usingbinary condensed aromatic hydrocarbon (CAH)-aniline systems as initiator, was studied. The strongerthe electron-donating ability of the aniline derivatives, the faster the the of photopolymerizationwill be. The radical intermediates produced by photoreaction between anthracene and anilinewere detected with ESR spectra. The initiation scheme involves the formation of exciplex betweenthe excited singlet state of CAH and the ground state of aniline as the first step, and then followedby proton transfer, leading to the generation of two kinds of radicals. From the end-group analysisof the polymers obtained, it is suggested that both kinds of radicals are involved in the initiation.     

Lifetimes of partial charge transfer exciplexes of 9-cyanophenanthrene and 9-cyanoanthracene

The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.     

Donor-Acceptor Complexes in Copolymerization. XXXV. Alternating Diene-Dienophile Copolymers. 3. Copolymerization of cis- and trans-1, 3-Pentadiene with Maleic Anhydride and Acrylonitrile… AlEt1.5CI1.5

Abstract

The copolymerization of the cis or trans isomers of 1,3-pentadiene with maleic anhydride in the presence of a peroxide catalyst yields identical equimolar, alternating copolymers in which the pentadiene units have a cis-1, 4 configuration (IR, NMR). The copolymerization of the cis or trans isomers of 1, 3-pentadiene with acrylonitrile in the presence of ethyl aluminum sesquichloride yields identical equimolar, alternating copolymers in which the pentadiene units have a trans-1,4 configuration (IR, NMR). Although the trans isomer forms cyclic adducts with both maleic anhydride and acrylonitrile, the cis isomer does not undergo the Diels-Alder reaction with these dienophlles. The formation of identical copolymers from cis- and trans-1, 3-pentadiene is attributed to isomerization of the diene-dienophile charge transfer complex in the excited state, resulting in the generation of the same homopolymerizable exciplex from both isomers.     

Synthesis and photophysical properties of some N-substituted-1,8-naphthalimides

The synthesis and photophysical properties of some 3-amino-N-substituted-1,8-naphthalimide derivatives are presented. The flourescence behaviour of these compounds in protic and aprotic solutions is consistent with the formation of an exciplex structure. The exciplex formation is due to a proton transfer in the excited state. This exciplex formation eliminates non-radiative channels and therefore an increase in quantum yield is observed.     

An ESR study of the oxygen transfer from excited heteroaromatic N-oxides to triethylamine to form diethyl nitroxide

The photoreaction of systems containing N-oxides and amines has been studied by ESR. The lowest triplet state of the heteroaromatic N-oxide combines with triethylamine to form an exciplex. Oxygen transfer and α-cleavage of the N-C bond in the amine part of the exciplex gave diethyl nitroxide.     

Photo-Induced Intermolecular Electron Transfer-Effect of Acceptor Molecular Structures

Photo-induced electron transfer versus molecular structure of acceptors is investigated using ultrafast time-resolved transient grating spectroscopy. Typical laser dyes Rhodamine 101 (Rh101) and Rhodamine 6G (Rh6G) in electron donor solvent-aniline are adopted as the objects. The forward electron transfer time constant from aniline to the excited singlet state of two Rhodamine dyes and subsequent back electron transfer from two dyes to aniline are measured. The experimental results denote that Rh6G presents faster electron transfer rates with aniline in both forward electron transfer and back electron transfer processes. With chemical calculation and qualitative analysis, it is found that the flexible molecular geometry of Rh6G leads to stronger electron coupling with donor solvent and further gives rise to larger electron transfer rates.     

Magnetic field effects on exciplex-forming systems: the effect on the locally excited fluorophore and its dependence on free energy

This study addresses magnetic field effects in exciplex forming donor-acceptor systems. For moderately exergonic systems, the exciplex and the locally excited fluorophore emission are found to be magneto-sensitive. A previously introduced model attributing this finding to excited state reversibility is confirmed. Systems characterised by a free energy of charge separation up to approximately -0.35 eV are found to exhibit a magnetic field effect on the fluorophore. A simple three-state model of the exciplex is introduced, which uses the reaction distance and the asymmetric electron transfer reaction coordinate as pertinent variables. Comparing the experimental emission band shapes with those predicted by the model, a semi-quantitative picture of the formation of the magnetic field effect is developed based on energy hypersurfaces. The model can also be applied to estimate the indirect contribution of the exchange interaction, even if the perturbative approach fails. The energetic parameters that are essential for the formation of large magnetic field effects on the exciplex are discussed.     

Kinetic Investigation of Oxidation of Aromatic Anils by Potassium Peroxymonosulfate in Aqueous Acidic Medium

The kinetics of oxidation of aromatic anils to benzaldehyde and azobenzene by potassium peroxymonosulfate has been studied in aqueous acetic acid medium. The low dielectric constant of the medium facilitates the reactivity. It has been found that the variation in the ionic strength of the reaction has a negligible effect on the rate. Similarly, polymerization was not observed when acrylonitrile was added to the reaction mixture. This observation rules out the formation of any free radical in the reaction. The added Mn(II) increases the rate of the reaction, which indicates the involvement of two‐electron transfer. Highly negative ?S^# values indicate a structured transition state. The deviation of the Hammett plot was noted, and a concave downward curve was obtained for the anils with substituents in the aniline moiety. The observed break in the log k_obs versus σ is attributed to the transition state whereas the concave upward curve was observed for the substituents in the benzaldehyde moiety and in the combination of aniline and benzaldehyde moieties, and a suitable mechanism was proposed.     

双枝芳醚树枝形聚合物构象研究

合成了外围只以一个芘基团修饰、核心为苯胺的双枝芳醚树枝形聚合物Py-[Gn]2-NPh (n＝1～2), 利用分子内电子转移和激基复合物的形成对其折叠构象和折叠程度进行了研究. 二氯甲烷溶液中选择性激发芘基团, 树枝形聚合物Py-[Gn]2-NPh分子内发生从苯胺到芘基团之间的电子转移, 观察到了分子内外围芘基团和核心苯胺基团之间形成激基复合物的发光, 为芳醚树枝形聚合物折叠构象的存在给出了直接实验观察. 二氯甲烷溶液中1～2代Py-[Gn]2-NPh分子内电子转移效率分别为0.87和0.81, 速率常数分别为2.3×108和1.5×108 s－1. 利用电子转移速率常数估算得到1～2代Py-[Gn]2-NPh分子内给、受体之间的距离分别为0.79和0.81 nm, 说明双枝芳醚树枝形聚合物与单枝结构类似, 其外围基团也可以折叠到达分子内部接近核心的位置.     

Time-resolved fluorescence studies on the formation and decomposition of the aromatic hydrocarbon-aliphatic amine exciplex in solution

Fluorescence rise and decay processes were measured and rate parameters were determined for the pyrene-tri-n-butylamine and pyrene-N,N-diethylaniline exciplex system in various solvents. An additional activation energy over that for the diffusional motion appears necessary for both exciplex formation and the deactivating quenching processes in the case of the pyrene-tri-n-butylamine system. The rate-determining step for these processes is electron transfer in the encounter collision leading to the nonrelaxed electron transfer state.     

Ab initio QM/MM study of excited state electron transfer between pyrene and 4,4′‐bis(dimethylamino)‐diphenylmethane with different solvent systems: Role of hydrogen bonding within solvent molecules

The exciplex is a charge transfer species formed in the process of electron transfer between an electron donor and an electron acceptor and hence is very sensitive to solvent polarity. In order to understand the role of solvent in exciplex formation between pyrene (PY) and 4,4′‐bis(dimethylamino)diphenylmethane (DMDPM), we used two types of solvent approximations: an implicit solvent model and an explicit solvent model. The difference in energies between the excited and the meta‐stable Frank–Condon state (ΔE) of the structures were assumed to correspond to the emission maximum of the exciplex in different solvents. The ΔE values show the trend of stabilization of the exciplex with an increase in solvent polarity. This trend in stabilization is substantially more prominent in the explicit solvent model than that with the implicit solvent model. The ΔE value obtained in methanol reflects equal stabilization compared to that in a more polar solvent, N,N‐dimethylformamide. This extra stabilization of the exciplex may be explained on the basis of the H‐bonding capability of the protic solvent, methanol. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Bonded exciplexes. A new concept in photochemical reactions

Charge-transfer quenching of the singlet excited states of cyanoaromatic electron acceptors by pyridine is characterized by a driving force dependence that resembles those of conventional electron-transfer reactions, except that a plot of the log of the quenching rate constants versus the free energy of electron transfer is displaced toward the endothermic region by 0.5-0.8 eV. Specifically, the reactions with pyridine display rapid quenching when conventional electron transfer is highly endothermic. As an example, the rate constant for quenching of the excited dicyanoanthracene is 3.5 x 10(9) M(-1)s(-1), even though formation of a conventional radical ion pair, A*-D*+, is endothermic by approximately 0.6 eV. No long-lived radical ions or exciplex intermediates can be detected on the picosecond to microsecond time scale. Instead, the reactions are proposed to proceed via formation of a previously undescribed, short-lived charge-transfer intermediate we call a "bonded exciplex", A- -D+. The bonded exciplex can be formally thought of as resulting from bond formation between the unpaired electrons of the radical ions A*- and D*+. The covalent bonding interaction significantly lowers the energy of the charge-transfer state. As a result of this interaction, the energy decreases with decreasing separation distance, and near van der Waals contact, the A- -D+ bonded state mixes with the repulsive excited state of the acceptor, allowing efficient reaction to form A- -D+ even when formation of a radical ion pair A*-D*+ is thermodynamically forbidden. Evidence for the bonded exciplex intermediate comes from studies of steric and Coulombic effects on the quenching rate constants and from extensive DFT computations that clearly show a curve crossing between the ground state and the low-energy bonded exciplex state.     

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。     

Exciplexes photonics on the silica surface

Processes of photoinduced electron transfer and formation of bimolecular complexes on the surface of nonporous dispersed silica and aerosil have been explored. Complexes of the two types, with a dissociative (acene + electron donor) and a stable (acene + acceptor) ground state have been obtained. Formation of the anthracene-N,N-diethylaniline exciplex on the surface proceeds in a diffusion mode. Increasing the quencher concentration to σ = 1% is accompanied by exciplex formation. Formation and stabilization of Mullican’s (dark) complexes has been noted with σ = 10%. The reaction of complexation on the surface is stimulated by heating. A suggestion of CTC localization in places of the highest energetic uniformity of the surface has been made.     

Water-induced fluorescence quenching of aniline and its derivatives in aqueous solution

Nonradiative deactivation processes of excited aniline and its derivatives in aqueous solution were investigated by steady-state and time-resolved fluorescence measurements to reveal characteristic solvent effects of water on the relaxation processes of excited organic molecules. The magnitude of nonradiative rate (k_nr) of excited aniline derivatives increased significantly in water compared to that in organic solvents (cyclohexane, ethanol, and acetonitrile). The fluorescence lifetime measurements in organic solvent/H₂O mixed solvents suggested that the fluorescence quenching in water was not due to exciplex formation but due to interactions with a water cluster. From temperature effect experiments on the fluorescence lifetime and quantum yield of aniline, N-methylaniline, and N,N-dimethylaniline, the apparent activation energies for the nonradiative deactivation rate in water were determined as 21, 30, and 41 kJ mol^-1, respectively. Upon substitution of hydrogen atoms in the aromatic ring of aniline derivatives for deuterium atoms resulted in normal deuterium isotope effect in cyclohexane, i.e. k_nr decreased by deuterium substitution, while in water the same deuterium substitution led to an increase in k_nr (the inverse isotope effect). The inverse isotope effects implied that a direct internal conversion to vibrationally higher excited states in the electronically ground state is not a dominant mechanism but the transition to a close-lying energy level, e.g. the relaxation to charge transfer to solvent (ctts) state, would be associated with the quenching mechanism in water.     

电荷转移聚合研究——苯肼及其衍生物引发丙烯腈聚合

研究了苯肼(PHZ)及其衍生物引发丙烯腈(AN)聚合以及苯肼作为电子给体与醌作为电子受体形成的电荷转移复合物(CTC)对丙烯腈的引发聚合作用。测定了光聚合时的引发剂、单体指数和聚合活化能分别为0.66、2.1和33.4千焦耳/摩尔。光聚合机理认为是PHZ与AN在光照下形成激基复合物(exciplex),它分解产生自由基引发单体聚合。而暗聚合的机理认为是PHZ与醌形成CTC,CTC再分解引发单体聚合。exciplex与CTC分别已由荧光光谱和紫外吸收光谱检出。     

Effects of conformation and solvent polarity on intramolecular charge transfer: a picosecond laser study

Picosecond dynamics of the intramolecular exciplex anthracene-(CH₂)₃-N,N-diniethylaniline have been measured in acetonitrile. The results indicate two processes. First, very rapid (7 ± 1 ps) electron transfer for molecules in extended conformations, producing solvated ion pairs without passing through the exciplex state. Second, folded conformers yield exciplexes within 2 ps, having a lifetime of 580 ± 30 ps.     

Magnetic field effect on fluorescence in a mixture of N-ethylcarbazole and dimethyl terephthalate in a polymer film in the presence of electric fields

Magnetic field effects on the fluorescence spectrum and on the electrofluorescence spectrum (plots of the electric field-induced change in fluorescence intensity as a function of wavelength) have been examined in electron donor and acceptor pairs of N-ethylcarbazole (ECZ) and dimethyl terephthalate (DMTP) in polymer films at different ratios of donor/acceptor concentration. In the mixture having a high concentration of ECZ, electric field-induced quenching of the exciplex fluorescence originating from the photoinduced electron transfer becomes less efficient in the presence of a magnetic field. In the mixture having a low concentration of ECZ, on the other hand, no magnetic field effect was observed in the electrofluorescence spectrum, indicating that the hole carrier plays an important role in synergy effects of magnetic and electric field effects on exciplex fluorescence. In the absence of the applied electric field, the magnetic field does not affect either exciplex fluorescence with a peak at 450 nm or LE fluorescence emitted from the locally excited state of ECZ but enhances the broad emission with a peak at approximately 380 nm, probably assigned to the fluorescence of another type of exciplex between ECZ and DMTP. Thus, two kinds of magnetic field effects on fluorescence have been observed in a mixture of ECZ and DMTP in a polymer film.