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1.
The photochemistry of 2,6-dimethyl-4-chlorophenol (6) has been studied in methanol and trifluoroethanol (TFE) through product studies and transient absorption spectroscopy. Chloride loss from triplet 6 gave triplet hydroxyphenyl cation 14, which equilibrated with triplet oxocyclohexadienylydene 15 within a few tens of nanoseconds; the cation can, however, be selectively trapped by allyltrimethylsilane (k(ad) = 10(8)-10(9) m(-1) s(-1)) to give a phenonium ion and the allylated phenol. In neat alcohols, 14 and 15 are reduced through different mechanisms, namely by hydrogen transfer through radical cation 17 and via phenoxyl radical 16, respectively. The mechanistic rationalization has been substantiated by the parallel study of an O-silylated derivative. The work shows that the chemistry of the highly (but selectively) reactive phenyl cation 14 can not only be discriminated from that of the likewise highly reactive carbene 15, but also exploited for synthetically useful reactions, as in this case with alkenes. Photolysis of electron-donating substituted halobenzenes may be the method of choice for the mild generation of some classes of phenyl cations.  相似文献   

2.
《Chemphyschem》2004,5(1):47-56
As a powerful natural antioxidant, lipoic acid (LipSS) and its reduced form dihydrolipoic acid (DHLA) exert significant antioxidant activities in vivo and in vitro by deactivation of reactive oxygen and nitrogen species (ROS and RNS). In this study the riboflavin (RF, vitamin B2) sensitized UVA and visible‐light irradiation of LipSS and DHLA was studied employing continuous irradiation, fluorescence spectroscopy, and laser flash photolysis (LFP). Our results indicate that LipSS and DHLA quench both the singlet state (1RF*) and the triplet state (3RF*) of RF by electron transfer to produce the riboflavin semiquinone radical (RFH.) and the radical cation of LipSS and DHLA, respectively. The radical cation of DHLA is rapidly deprotonated twice to yield a reducing species; the radical anion of LipSS (LipSS.?). When D2O was used as solvent, it was confirmed that the reaction of LipSS with 3RF* consists of a simple electron‐transfer step, while loss of hydrogen occurs in the case of DHLA oxidation. Due to the strong absorption of RFH. and the riboflavin ground state, the absorption of the radical cation and the radical anion of LipSS can not be observed directly by LFP. N,N,N′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) and N,N,N′,N′‐tetramethyl benzidine (TMB) were added as probes to the system. In the case of LipSS, the addition resulted in the formation of the radical cation of TMPD or TMB by quenching of the LipSS radical cation. If DHLA is the reducing substrate, no formation of probe radical cation is observed. This confirms that LipSS.+ is produced by riboflavin photosensitization, and that there is no oxidizing species produced after DHLA oxidization.  相似文献   

3.
A new mechanism of anionic polymerization of butadiene is proposed. In the elementary chemical act, the “living” polymer–monomer complex is excited into the low‐lying triplet state. This state has the character of charge (electron) and cation (Li+ or Na+) transfer from the terminal unit of the active center to the monomer molecule. In the framework of this concept, the probability of chemical bond formation is determined by spin density on radical centers of reagent molecules. Semiempirical and ab initio 6‐31G** quantum‐chemical calculations showed stable interaction between components of the complex in the ground electronic state (9–11 kcal/mol) and low energy levels of triplet excited states (<14 kcal/mol). This new approach is shown to be useful in the analysis of polymerization kinetics and the microstructure of polybutadiene depending on the cation type and the ion pair state. The mechanism of cis‐trans isomerization in the terminal unit of the living polymer consists in concerted rotation about the Cβ? Cγ bond and the migration of Li between Cα and Cγ atoms. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002  相似文献   

4.
With the target to design and develop new functionalized green triplet light emitters that possess distinctive electronic properties for robust and highly efficient phosphorescent organic light‐emitting diodes (PHOLEDs), a series of bluish–green to yellow–green phosphorescent tris‐cyclometalated homoleptic iridium(III) complexes [Ir(ppy‐X)3] (X=SiPh3, GePh3, NPh2, POPh2, OPh, SPh, SO2Ph, Hppy=2‐phenylpyridine) have been synthesized and fully characterized by spectroscopic, redox, and photophysical methods. By chemically manipulating the lowest triplet‐state character of Ir(ppy)3 with some functional main‐group 14–16 moieties on the phenyl ring of ppy, a new family of metallophosphors with high‐emission quantum yields, short triplet‐state lifetimes, and good hole‐injection/hole‐transporting or electron‐injection/electron‐transporting properties can be obtained. Remarkably, all of these IrIII complexes show outstanding electrophosphorescent performance in multilayer doped devices that surpass that of the state‐of‐the‐art green‐emitting dopant Ir(ppy)3. The devices described herein can reach the maximum external quantum efficiency (ηext) of 12.3 %, luminance efficiency (ηL) of 50.8 cd A?1, power efficiency (ηp) of 36.9 Lm W?1 for [Ir(ppy‐SiPh3)3], 13.9 %, 60.8 cd A?1, 49.1 Lm W?1 for [Ir(ppy‐NPh2)3], and 10.1 %, 37.6 cd A?1, 26.1 Lm W?1 for [Ir(ppy‐SO2Ph)3]. These results provide a completely new and effective strategy for carrier injection into the electrophosphor to afford high‐performance PHOLEDs suitable for various display applications.  相似文献   

5.
Chichibabin's and Müller's hydrocarbons are classical open‐shell singlet diradicaloids but they are highly reactive. Herein we report the successful synthesis of their respective stable analogues, OxR‐2 and OxR‐3 , based on the newly developed oxindolyl radical. X‐ray crystallographic analysis on OxR‐2 reveals a planar quinoidal backbone similar to Chichibabin's hydrocarbon, in accordance with its small diradical character (y0=11.1 %) and large singlet–triplet gap (ΔES‐T=−10.8 kcal mol−1). Variable‐temperature NMR studies on OxR‐2 disclose a slow cis/trans isomerization process in solution through a diradical transition state, with a moderate energy barrier (ΔG298K=15–16 kcal mol−1). OxR‐3 exhibits a much larger diradical character (y0=80.6 %) and a smaller singlet–triplet gap (ΔES‐T=−3.5 kcal mol−1), and thus can be easily populated to paramagnetic triplet diradical. Our studies provide a new type of stable carbon‐centered monoradical and diradicaloid.  相似文献   

6.
para‐Phenylene‐bridged spirobi(triarylamine) dimer 2 , in which π conjugation through four redox‐active triarylamine subunits is partially segregated by the unique perpendicular conformation, was prepared and characterized by structural, electrochemical, and spectroscopic methods. Quantum chemical calculations (DFT and CASSCF) predicted that the frontier molecular orbitals of 2 are virtually fourfold degenerate, so that the oxidized states of 2 can give intriguing electronic and magnetic properties. In fact, the continuous‐wave ESR spectroscopy of radical cation 2 .+ showed that the unpaired electron was trapped in the inner two redox‐active dianisylamine subunits, and moreover was fully delocalized over them. Magnetic susceptibility measurements and pulsed ESR spectroscopy of the isolated salts of 2 , which can be prepared by treatment with SbCl5, revealed that the generated tetracation 2 4+ decomposed mainly into a mixture of 1) a decomposed tetra(radical cation) consisting of a tri(radical cation) moiety and a trianisylamine radical cation moiety (≈75 %) and 2) a diamagnetic quinoid dication in a tetraanisyl‐p‐phenylendiamine moiety and two trianisylamine radical cation moieties (≈25 %). Furthermore, the spin‐quartet state of the tri(radical cation) moiety in the decomposed tetra(radical cation) was found to be in the ground state lying 30 cal mol?1 below the competing spin‐doublet state.  相似文献   

7.
Photosensitization mechanisms in photopolymer coating film containing an aminochalcone‐type dye sensitizer and a radical generating reagent, sensitizer dyes, (E)‐3‐(9‐julolidinyl)‐1‐phenyl‐2‐propen‐1‐one (A), (E)‐2‐(9‐julolidinyl)‐methylene‐1‐indanone (B), 9‐benzoyl‐2,3,6,7‐tetrahydro‐1H,5H‐benzo[i,j]‐furano‐[3,2‐g]quinolizine (C), 4‐(dimethylamino) chalcone (D) and a radical‐generating reagent, 2,4,6‐tris (trichloromethyl)‐1,3,5‐triazine (TCT), were investigated by laser flash photolysis using a total reflection cell. Weak fluorescence and strong broad triplet absorption were detected. The fluorescence was statically quenched by TCT at quenching distances (Rf) of 15, 14, 20 and 14 Å for A, B, C and D as well as the triplet initial absorption, at quenching distances (Rt) of 16, 16, 16 and 14 for A, B, C and D, similar to the fluorescence quenching distances. The triplet decay time of the dyes was inefficiently quenched by TCT with the rate constants (k q) of 1.9, 3.1, 0.7 and 1.0×105 mol−1/dm3/s for A, B, C and D. The sensitivity of photopolymers containing a sensitizer dye and a TCT was obtained at an excitation of 488 nm corresponding to the emission peaks of argon ion laser of 1.1, 0.2, 0.54 and 9.1 mJ cm2 for A, B, C and D. The results indicated that the static sensitization process from the fluorescent singlet excited state of the dyes to the ground state of TCT was predominant, and the high sensitivity for A and B was caused by the high absorbance at 488 nm and that for C by the high fluorescent quenching distance. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

8.
A series of sterically encumbered [Pt( L )(σ‐acetylide)2] complexes were prepared in which L , a dendritic polyaromatic diimine ligand, was held constant ( L =1‐(2,2′‐bipyrid‐6‐yl)‐2,3,4,5‐tetrakis(4‐tert‐butylphenyl)benzene) and the cis ethynyl co‐ligands were varied. The optical properties of the complexes were tuned by changing the electronic character, extent of π conjugation and steric bulk of the ethynyl ligands. Replacing electron‐withdrawing phenyl‐CF3 substituents ( 4 ) with electron‐donating pyrenes ( 5 ) resulted in a red shift of both the lowest‐energy absorption (ΔE=3300 cm?1, 61 nm) and emission bands (ΔE=1930 cm?1, 64 nm). The emission, assigned in each case as phosphorescence on the basis of the excited‐state lifetimes, switched from being 3MMLL′CT‐derived (mixed metal–ligand‐to‐ligand charge transfer) when phenyl/polyphenylene substituents ( 3 , 4 , 6 ) were present, to ligand‐centred 3ππ* when the substituents were more conjugated aromatic platforms [pyrene ( 5 ) or hexa‐peri‐hexabenzocoronene ( 7 )]. The novel PtII acetylide complexes 5 and 7 absorb strongly in the visible region of the electromagnetic spectrum, which along with their long triplet excited‐state lifetimes suggested they would be good candidates for use as singlet‐oxygen photosensitisers. Determined by in situ photooxidation of 1,5‐dihydroxynaphthalene (DHN), the photooxidation rate with pyrenyl‐ 5 as sensitiser (kobs=39.3×10?3 min?1) was over half that of the known 1O2 sensitiser tetraphenylporphyrin (kobs=78.6×10?3 min?1) under the same conditions. Measured 1O2 quantum yields of complexes 5 and 7 were half and one‐third, respectively, of that of TPP, and thus reveal an efficient triplet–triplet energy‐transfer process in both cases.  相似文献   

9.
Upon laser photolysis of chlorophyll-quinone solutions in ethanol, transients due to the chlorophyll triplet state (Ct), the chlorophyll cation radical (C+) and the semiquinone radical (Q-) can be observed. The rise of Q- parallels the decay of Ct. demonstrating the precursor role of the triplet. The decay of C+ is second order, consistent with reverse electron transfer, and has a rate constant which is independent of quinone potential, and an activation energy of 14kJ/mol due mainly to the temperature dependence of solvent viscosity. Triplet quenching and C+ yield are found to decrease with decreasing quinone potential.  相似文献   

10.
The cation of the title compound, C12H15N2O+·CF3SO3?, exists as an E‐configured hydroxy­imino derivative conjugated with a nearly planar iminium system. The twist angle between the phenyl ring and the oxime group is 72.2 (2)°. An O—H?O hydrogen bond links the oxime group of the cation to the anion.  相似文献   

11.
A new series of (E)‐1‐(4‐((1‐benzyl‐1H‐1,2,3‐triazol‐4‐yl)methoxy)phenyl)‐3‐phenylprop‐2‐en‐1‐one 1a (4‐((1‐benzyl‐1H‐1,2,3‐triazol‐4‐yl) methoxy)phenyl)‐1‐phenylprop‐2‐en‐1‐one 1b – 15b were designed, synthesized based on click chemistry, and biologically evaluated for their activity on tyrosinase. The result showed that most of prepared compounds 1a – 15a have potent activating effect on tyrosinase, especially for 3a , 8a – 10a and 14a – 15a . Among them, compounds 10a and 14a demonstrated the best activity with EC50=1.71 and 5.60 µmol·L?1 respectively, even better than the positive control 8‐MOP (EC50=14.8 µmol·L?1). Conversely, compounds 3b , 5b – 6b , 9b – 10b , and 15b induced enzymatic inhibition on tyrosinase.  相似文献   

12.
The photochemistry of six N3‐(3‐fluoro‐4‐dialkylaminophenyl)‐oxazolidinones known for their antimicrobial activity has been examined. All of these compounds are defluorinated in water (Φdec ≈ 0.25) and in methanol (Φdec ≈ 0.03), reasonably via the triplet. The chemical processes observed are reductive defluorination and solvolysis, depending on the structural variation introduced (thus, tethering the dialkylamino group to the aromatic ring and introducing a highly polar group in the oxazolidinone moiety have an effect). A likely mechanism involves the fragmentation of the C–F bond yielding the corresponding triplet phenyl cation. This intermediate either is reduced or, under appropriate conditions, intersystem crosses to the singlet state that adds the solvent. These data demonstrate a sizeable photodecomposition of these drugs that causes a decrease in the therapeutic activity. Furthermore, the likely formation of phenyl cations may cause a photogenotoxic effect.  相似文献   

13.
The synthesis and the solid state magnetic properties of (nitronyl nitroxide)‐substituted trioxytriphenylamine radical cation tetrachlorogallate, NNTOT+·GaCl4? , are reported. In the temperature region between 300 and 3 K, the magnetic behavior is characterized by the strong intramolecular ferromagnetic interaction (J/kB=+400 K) between the radical ( NN ) and the radical cation ( TOT +) and the weak intermolecular antiferromagnetic interaction (J/kB=?1.9 K) between NNTOT+ ions. Below 3 K, a 3D‐type long‐range magnetic ordering into a weak ferromagnet was observed (TN=2.65 K). The magnetic entropy (Smag=8.97 J K?1 mol?1) obtained by the heat capacity measurement is in good agreement with the theoretical value of R ln3=9.13 J K?1 mol?1 based on the S=1 state.  相似文献   

14.
Salts that contain radical cations of benzidine (BZ), 3,3′,5,5′‐tetramethylbenzidine (TMB), 2,2′,6,6′‐tetraisopropylbenzidine (TPB), and 4,4′‐terphenyldiamine (DATP) have been isolated with weakly coordinating anions [Al(ORF)4]? (ORF=OC(CF3)3) or SbF6?. They were prepared by reaction of the respective silver(I) salts with stoichiometric amounts of benzidine or its alkyl‐substituted derivatives in CH2Cl2. The salts were characterized by UV absorption and EPR spectroscopy as well as by their single‐crystal X‐ray structures. Variable‐temperature UV/Vis absorption spectra of BZ . +[Al(ORF)4]? and TMB . +[Al(ORF)4]? in acetonitrile indicate an equilibrium between monomeric free radical cations and a radical‐cation dimer. In contrast, the absorption spectrum of TPB . +SbF6? in acetonitrile indicates that the oxidation of TPB only resulted in a monomeric radical cation. Single‐crystal X‐ray diffraction studies show that in the solid state BZ and its methylation derivative (TMB) form radical‐cation π dimers upon oxidation, whereas that modified with isopropyl groups (TPB) becomes a monomeric free radical cation. By increasing the chain length, π stacks of π dimers are obtained for the radical cation of DATP. The single‐crystal conductivity measurements show that monomerized or π‐dimerized radicals (BZ . +, TMB . +, and TPB . +) are nonconductive, whereas the π‐stacked radical (DATP . +) is conductive. A conduction mechanism between chains through π stacks is proposed.  相似文献   

15.
The crystal structures of sodium 4‐({4‐[N,N‐bis(2‐hydroxy­ethyl)­amino]­phenyl}diazenyl)­benzoate 3.5‐hydrate, Na+·C17H18N3O4?·3.5H2O, (I), and potassium 4‐({4‐[N,N‐bis(2‐hydroxy­ethyl)­amino]­phenyl}diazenyl)­benzoate dihydrate, K+·C17H18N3O4?·2H2O, (II), are described. The results indicate an octahedral coordination around sodium in (I) and a trigonal prismatic coordination around potassium in (II). In both cases, coordination around the metal cation is achieved through O atoms of the water mol­ecules and hydroxy groups of the chromophore. The organic conjugated part of the chromophore is approximately planar in (I), while a dihedral angle of 30.7 (2)° between the planes of the phenyl rings is observed in (II).  相似文献   

16.
A kinetic study of the nucleophilic substitution reaction of Y‐substituted phenyl diphenylphosphinothioates 2 a – g with alkali‐metal ethoxides (MOEt; M=Li, Na, K) in anhydrous ethanol at (25.0±0.1) °C is reported. Plots of pseudo‐first‐order rate constants (kobsd) versus [MOEt], the alkali ethoxide concentration, show distinct upward (KOEt) and downward (LiOEt) curvatures, respectively, pointing to the importance of ion‐pairing phenomena and a differential reactivity of dissociated EtO? and ion‐paired MOEt. Based on ion‐pairing treatment of the kinetic data, the kobsd values were dissected into k and kMOEt, the second‐order rate constants for the reaction with the dissociated EtO? and ion‐paired MOEt, respectively. The reactivity of MOEt toward 2 b (Y=4‐NO2) increases in the order LiOEt?NaOEt>KOEt>EtO?. The current study based on Yukawa–Tsuno analysis has revealed that the reactions of 2 a – g (P?S) and Y‐substituted phenyl diphenylphosphinates 1 a – g (P?O) with MOEt proceed through the same concerted mechanism, which indicates that the contrasting selectivity patterns are not due to a difference in reaction mechanism. The P?O compounds 1 a – g are approximately 80‐fold more reactive than the P?S compounds 2 a – g toward the dissociated EtO? (regardless of the electronic nature of substituent Y) but are up to 3.1×103‐fold more reactive toward ion‐paired LiOEt. The origin of the contrasting selectivity patterns is further discussed on the basis of competing electrostatic effects and solvational requirements as a function of anionic electric field strength and cation size (Eisenman’s theory).  相似文献   

17.
AZMAT Rafia  UDDIN Fahim 《中国化学》2009,27(7):1237-1243
Photo decoloration of the methylene blue (MB) with reducing sugar, ribose (RH), was investigated on an especially designed optical processor using monochromatic radiation of 661 nm through a red filter. The dye molecule gets excited into triplet transient species (MBT) during flushing with lifetime of 10.1 ms into acetate buffered aqueous alcoholic medium, which later on reduces to protonated leuco dye (MBH). Photolysis of the aqueous alcholic medium generated highly reactive oxygen radical (O-•) with the production of 2e-, which led to probable oxidation of the ribose into respective acid while hydrogen abstraction and 2e- reduced the dye (MB) into MBH by following reaction  相似文献   

18.
Cyclometalated IrIII complexes with acetylide ppy and bpy ligands were prepared (ppy=2‐phenylpyridine, bpy=2,2′‐bipyridine) in which naphthal ( Ir‐2 ) and naphthalimide (NI) were attached onto the ppy ( Ir‐3 ) and bpy ligands ( Ir‐4 ) through acetylide bonds. [Ir(ppy)3] ( Ir‐1 ) was also prepared as a model complex. Room‐temperature phosphorescence was observed for the complexes; both neutral and cationic complexes Ir‐3 and Ir‐4 showed strong absorption in the visible range (ε=39600 M ?1 cm?1 at 402 nm and ε=25100 M ?1 cm?1 at 404 nm, respectively), long‐lived triplet excited states (τT=9.30 μs and 16.45 μs) and room‐temperature red emission (λem=640 nm, Φp=1.4 % and λem=627 nm, Φp=0.3 %; cf. Ir‐1 : ε=16600 M ?1 cm?1 at 382 nm, τem=1.16 μs, Φp=72.6 %). Ir‐3 was strongly phosphorescent in non‐polar solvent (i.e., toluene), but the emission was completely quenched in polar solvents (MeCN). Ir‐4 gave an opposite response to the solvent polarity, that is, stronger phosphorescence in polar solvents than in non‐polar solvents. Emission of Ir‐1 and Ir‐2 was not solvent‐polarity‐dependent. The T1 excited states of Ir‐2 , Ir‐3 , and Ir‐4 were identified as mainly intraligand triplet excited states (3IL) by their small thermally induced Stokes shifts (ΔEs), nanosecond time‐resolved transient difference absorption spectroscopy, and spin‐density analysis. The complexes were used as triplet photosensitizers for triplet‐triplet annihilation (TTA) upconversion and quantum yields of 7.1 % and 14.4 % were observed for Ir‐2 and Ir‐3 , respectively, whereas the upconversion was negligible for Ir‐1 and Ir‐4 . These results will be useful for designing visible‐light‐harvesting transition‐metal complexes and for their applications as triplet photosensitizers for photocatalysis, photovoltaics, TTA upconversion, etc.  相似文献   

19.
The structural information gained from the study of the chiral building block (R)‐(?)‐4‐(3,4‐di­chloro­phenyl)‐4‐(2‐pyridyl)­butanoic acid–l ‐(?)‐ephedrine [methyl(1‐hydroxy‐1‐phenyl­prop‐2‐yl)ammon­ium 4‐(3,4‐di­chloro­phenyl)‐4‐(2‐pyrid­yl)but­an­oate], C10H16NO+·C15H12Cl2NO2?, can be used to deduce the absolute configuration of highly potent arpromidine‐type hist­amine H2 receptor agonists, as the chiral butanoic acid can be converted to (R)‐(?)‐3‐(3,4‐di­chloro­phenyl)‐3‐(2‐pyridyl)­propyl­amine and to the corresponding R‐configured arpromidine analogue.  相似文献   

20.
Cyamemazine (CMZ) is a neuroleptic drug that mediates cutaneous phototoxicity in humans. Here, the photobehavior of CMZ has been examined within α1‐acid glycoproteins, β‐ and γ‐cyclodextrins and SDS micelles. In all these microenvironments, CMZ emission was enhanced and blue‐shifted, and its lifetime was longer. Irradiation of the entrapped drug at 355 nm, under air; led to the N,S‐dioxide. Within glycoproteins or SDS micelles the reaction was clearly slower than in phosphate buffered solution (PBS); protection by cyclodextrins was less marked. Transient absorption spectroscopy in PBS revealed formation of the triplet state (3CMZ*) and the radical cation (CMZ+?). Upon addition of glycoprotein, the contribution of CMZ+? became negligible, whereas 3CMZ* dominated the spectra; in addition, the triplet lifetime became considerably longer. In cyclodextrins, this occurred to a lower extent. In all microheterogeneous systems, quenching by oxygen was slower than in solution; this was most remarkable inside glycoproteins. The highest protection from photooxidation was achieved inside SDS micelles. The results are consistent with photooxidation of CMZ through photoionization and subsequent trapping of the resulting radical cation by oxygen. This reaction is extremely sensitive to the medium and constitutes an appropriate probe for localization of the drug within a variety of biological compartments.  相似文献   

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