Effects of Self—coiling of Organic Molecules on Intramolecular Exciplex Formation and Fluoresscence Quenching in DX—H2O Solvent System

Effects of self-coiling of organic molecules on intramolecular exciplex formation of compound I,in which the carbazole chromophore and terephthalic acid methylester acceptor group are linked by one (CH2)10 chain,and the decrease of the fluorescence intensities of compounds Ⅱ，Ⅲ，and Ⅳ,in which the carbazole chromophore and 3,5-dinitrobenzoate are connected by one aliphatic chain of (CH2)10 (Ⅱ），（CH2）12（Ⅲ）,or (CH2)4(Ⅳ）,have been studied in the dioxane (DX)-H2O binary system.The results show that self-coiling of organic molecules in DX-H2O facilitates intramolecular exciplex formation of I and induces the decrease of fluorescence intensities of Ⅱ,bacause of the proximity effect brought about by selfcoiling of organic molecules under hydrophobic-lipophilic interaction(HLI) between the excited carbazole chromophore and the acceptor.Since the similar effects are observed even when the concentration of the probes are less than their CAgCs(critical aggregate concentrations )in the DX-H2O mixture with the same φ values,formation of the intermolecular exciplex has been excluded.The effects are found to be strongly depended on φ values,indication that they are mainly driven by HLI.The properties of the acceptors can also affect the intramolecular exciplex formation.With terephthalic acid methylester moiety as the acceptor,the carbazole chromophore exhibits the fluorescence spectra of the exciplex,while with 3,5-dinitrobenzoate moiety as the acceptor,only the fluorescence spectra of excited carbazolyl chromophore are observed.     

Enhanced phosphorescence in intramolecular exciplex systems

Phosphorescence behavior of the series of compounds, (carbazole)-(CH₂)_n-(terephthalic acid methyl ester), which show intramolecular exciplex emission, has been studied in a rigid medium at 77 K. The phosphorescence of the carbazole moiety is enhanced by electron transfer fluorescence quenching of the carbazole residue in accord with the degree of quenching.     

Magnetic field dependence of intramolecular exciplex formation in polymethyelene-linked A–D systems

An extremely large increase of exciplex fluorescence intensity has been observed with photoexcited compounds pyrene-(CH₂)_n - DMA in acetonitrile when an external magnetic field of several hundred gauss is applied. The effect is readily explained as originating from the radical ion pair which can undergo a magnetic-field-dependent singlet-triplet conversion due to nuclear hyperfine coupling, or an association to form the exciplex. The increasing influence of the exchange interaction in the radical ion pair with decreasing chain length (CH₂)_n is evident.     

Photophysics of intramolecular electron donor—acceptor systems; N-carbazolyl(CH2)ntetrachlorophthalimide

In systems N-carbazolyl—(CH₂)_n—tetrachlorophthalimide (n = 2, 3, 4, 7) virtually complete quenching of the carbazole fluorescence via an intramolecular electron-transfer mechanism occurs, with concomitant formation of an emissive polar excited state. The rate of formation of this polar excited state is found to exceed considerably those reported for formation of intramolecular exciplex states in other non-conjugated bichromophoric systems with a less pronounced electron donor—acceptor character.     

Exciplex formation in aromatic copolymers and photoinduced electron transfer from exciplex to acceptor

The exciplex formation in 9-vinylphenanthrene-p-N,N-dimethylaminostyrene copolymers, its characteristics, and the electron transfer process in polar solvents were studied. The copolymer exhibited a more intense intramolecular exciplex fluorescence than the low-molecular-weight model system, phenanthrene-N,N-dimethylaniline, in which the intermolecular exciplex formation occurred. Intensities of the exciplex fluorescence, which were unchanged regardless of the copolymer composition, led us to speculate that the efficient energy migration takes place from an excited phenanthrene unit to an exciplex forming site on the polymer chain. The electron transfer in the copolymer-p-dicyanobenzene system was studied in polar media. The formation of p-dicyanobenzene anion radical was measured by flash photolysis and electron spin resonance (ESR). p-Dicyanobenzene anion radical was generated by the electron transfer process via exciplex and the direct electron transfer process from the excited phenanthrene unit in the copolymer.     

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.     

Studies in the furan series. 23. Preparation of some new 5-substituted furfurylallylarylamines. Influence of substituents on the intramolecular Diels-Alder (IMDA) reaction

Several new N-allyl-N-(5-substituted)-2-furfuryl-p)-toluidines IIIa-e with Cl, Br, I, NO₂ or CH₃O groups in position 5 of the furan nucleus were prepared by allylation of the corresponding secondary furfurylaryl-amines. Both, electron withdrawing and releasing substituents enhanced the yield of intramolecular [4+2] cycloaddition.     

Inherent Stability Limits of Intramolecular Boron Nitrogen Lewis Acid–Base Pairs

The reaction of (C₆F₅)₂BH ( 1 ) with N,N‐dimethylallylamine ( 2 ), N,N‐diethylallylamine ( 3 ) and 1‐allylpiperidine ( 4 ) afforded the five‐membered ring systems (C₆F₅)₂B(CH₂)₃NR₂ (R=Me ( 5 ), Et ( 6 )) and (C₆F₅)₂B(CH₂)₃N(CH₂)₅ ( 7 ) with an intramolecular dative B? N bond. A different product was obtained from the reaction of (C₆F₅)₂BH ( 1 ) with N,N‐diisopropylallylamine ( 8 ), which afforded the seven‐membered ring system (C₆F₅)₂B(CH₂)₃N(iPr)CH(Me)CH₂ ( 9 ) under extrusion of dihydrogen. All compounds were characterised by elemental analysis, NMR spectroscopy and single‐crystal X‐ray diffraction experiments. Density functional theory (DFT) studies were performed to rationalise the different reaction mechanism for the formation of products 6 and 9 . The bonding situation of compound 9 was analysed in terms of its electron density topology to describe the delocalised nature of a borane– enamine adduct.     

A novel fluorescence probe for micellization of amphiphilic molecules in water based on intramolecular exciplex formation

A novel friction‐sensitive fluoresence probe based on intramolecular exciplex formation, (9‐anthrylmethyloxymethyl)pyrene (AMOP), was used to study the micellization of amphiphilic molecules such as sodium dodecyl sulfate (SDS) and hydrocarbon‐modified polyethylene glycol (HM‐PEG) in water. Results demonstrate that AMOP can serve as a sensitive probe for micellization of both SDS and HM‐PEG. In addition, AMOP reveals information about the microviscosity of SDS and HM‐PEG micelles, which is in good agreement with the results obtained from other fluorescence probes based on intramolecular excimer such as dipyrenylpropane (Py(CH₂)₃Py) and bis(1‐pyrenylmethyl) ether (dipyme, PyCH₂OCH₂Py).     

Convenient Synthesis of 3-(1-Carboxyalkyl)pyrido[2,3-d]pyrimidine-2,4-diones

A cheap and safe synthetic route to obtain 3-(1-carboxyalkyl)pyrido[2,3-d]pyrimidinediones (carboxyalkyl = -CHRCO₂H; R = H; CH₂; CH₂Ph; Ph; CH₂ (C₃H₃N₂); (CH₂)₂CO₂H; CH₂CO₂H) starting from 2,3-pyridinedicarboxylic acid is described. A process scheme consistent with empirical observations is proposed.     

Haloacetylated enol ethers. 14 [6]. Reaction of β-alkoxyvinyl trifluoromethyl ketones with N-methylhydroxylamine

The reaction of a series of β-methoxyvinyl trifluoromethyl ketones [CF₃COC(R²)?C(OMe)R¹, where R¹ = Me, -(CH₂)₃-C3, -CH₂)₄-C3, Ph and R² = H, Me, -(CH₂)₃-C4, -(CH₂)₄-C4] with N-methylhydroxylamine is reported. The regiochemistry of the reaction are explained by MO calculation data.     

Kinetic and mechanistic investigation into the influence of chelate substituents on the substitution reactions of platinum(II) terpyridine complexes

The substitution kinetics of the complexes [Pt{4′‐(o‐CH₃‐Ph)‐terpy} Cl]SbF₆ (CH₃PhPtCl(Sb)), [Pt{4′‐(o‐CH₃‐Ph)‐terpy}Cl]CF₃SO₃ (CH₃PhPtCl(CF)), [Pt(4′‐Ph‐terpy)Cl]SbF₆ (PhPtCl), [Pt(terpy)Cl]Cl·2H₂O (PtCl), [Pt{4′‐(o‐Cl‐Ph)‐terpy}Cl]SbF₆ (ClPhPtCl), and [Pt{4′‐(o‐CF₃‐Ph)‐terpy}Cl]SbF₆ (CF₃PhPtCl), where terpy is 2,2′:6′,2″‐terpyridine, with the nucleophiles thiourea (TU), N,N′‐dimethylthiourea (DMTU), and N,N,N′,N′‐tetramethylthiourea (TMTU) were investigated in methanol as a solvent. The substitution reactions of the chloride displacement from the metal complexes by the nucleophiles were investigated as a function of nucleophile concentration and temperature under pseudo‐first‐order conditions using the stopped‐flow technique. The reactions followed the simple rate law k_obs = k₂[Nu]. The results indicate that the introduction of substituents in the ortho position of the phenyl group on the ancillary ring of the terpy unit does influence the extent of π‐backbonding in the terpy ring. This controls the electrophilicity of the platinum center, which in turn controls the lability of the chloro‐leaving group. The strength of the electron‐donating or ‐withdrawing ability of the substituents correlates with the reactivity of the complexes. Electron‐donating substituents decrease the rate of substitution, whereas electron‐withdrawing substituents increase the rate of substitution. This was supported by DFT calculations at the B3LYP/LACVP+** level of theory, which showed that most of the electron density of the HOMO is concentrated on the phenyl ligand rather than on the metal center in the case of the strongest electron‐withdrawing substituent in CF₃PhPtCl. The opposite was found to be true with the strongest electron‐donating substituent in CH₃PhPtCl. Thiourea was found to be the best nucleophile with N,N,N′,N′‐tetramethylthiourea being the weakest due to steric effects. The temperature dependence studies support an associative mode of activation. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 808–818, 2008     

A solvent isotope effect upon exciplex photochemistry

For several naphthylalkylamines, change of solvent from methanol to deuterated methanol (CH₃OD) leads to an increase in quantum yield of intramolecular exciplex fluorescence, an increase in exciplex fluorescence lifetime and an increase in naphthalene triplet yield. The solvent effect is explained by the relative abilities of the two solvents to form hydrogen (deuterium) bonds and to protonate (deuterate).     

Reaction of Carbon Anions With Iron α,β-Unsaturated Ketimine Complexes

Complexes (η⁴-PhCH=CHCR=NPh)Fe(CO)₃, where R=H(1a) and CH₃ were synthesized in excellent yield from the reaction of the corresponding α,β-unsaturated ketimines with excess Fe₂(CO)₉. Reaction of la with (Ph)₂CHLi and (CH₃)₂(NC)CLi at ?78°C or ambient temperature followed by acid quenching gave trans-PhCH=CHCHRNHPh, where R=CHPH₂ and C(CH₃)₂CN, respectively, in good yield. In the presence of 1 atm. of CO the reaction of 1a with (CH₃)₂(NC)CLi, followed by CuCl₂ oxidation resulted in the formation of a carbamyl choride, trans-PhCH=CHCH[C(CH₃)₂CN]N(Ph)COCl. This species was converted to a carbamate compound trans-PhCH=CHCH[C(CH₃)₂ - CN]N(Ph)COOCH₃ in MeOH in the presence of Ag⁺.     

[{LiCH(CH2CH2OMen)SO2Ph}4] (OMen = Menthyl): Synthesis and First Structure of a Chiral Solvent Free Lithiated Sulfone with a Direct Li–C Bond

R*OCH₂CH₂CH₂SO₂Ph (R*OH = MenOH, (–)‐menthol, ( 3a ); BorOH, (1S)‐(–)‐borneol, ( 3b )) were found to react with n‐BuLi in n‐pentane/n‐hexane and toluene/n‐hexane under deprotonation yielding LiCH(CH₂CH₂OR*)SO₂Ph (R* = Men, ( 4a ); Bor, ( 4b )) which reacted with n‐Bu₃SnCl forming the requisite tri(n‐butyl)tin compounds n‐Bu₃SnCH(CH₂CH₂OR*)SO₂Ph (R* = Men, ( 5a ); Bor, ( 5b )) as diastereomeric mixtures. The identities of 5a and 5b were unambiguously proved by ¹H, ¹³C and ¹¹⁹Sn NMR spectroscopic measurements. Solutions of 4a afforded crystals of [{LiCH(CH₂CH₂OMen)SO₂Ph}₄] ( 4a′ ) for which the structure was determined by single‐crystal X‐ray crystallography. Complex 4a′ crystallized in a tetrameric structure without any additional solvent molecules. There were found direct Li–C bonds (Li1–C1/Li2–C20 2.231(9)/2.236(9) Å). The tetrahedral donor set of Li is completed by three oxygen atoms. One oxygen atom comes from the OMen substituent via intramolecular coordination and two oxygen atoms come from SO₂ groups of neighboured LiCH(CH₂CH₂OMen)SO₂Ph moieties. Thus, a heterocubane structure with a Li₄S₄ core is built up.     

Synthesis of N-[chloro(diorganyl)silyl]anilines

A series of N-[chloro(diorganyl)silyl]anilines RR′Si(NR″Ph)Cl (R, R′ = Me, Ph, CH₂=CH, ClCH₂, Cl(CH₂)₃; R″ = H, Me) was prepared via the reaction of diorganyldichlorosilanes with aniline or N-ethylaniline in the presence of triethylamine.     

Synthesis and Optoelectronic Properties of Hexahydroxylated 10‐O‐R‐Substituted Anthracenes via a New Modification of the Friedel–Crafts Reaction Using O‐Protected ortho‐Acetal Diarylmethanols

A new modification of the Friedel–Crafts type intramolecular cyclization involving O‐protected ortho‐acetal diarylmethanols as a new type of reactant, was carried out for the first time in a medium containing a large amount of water at room temperature and enabled synthesis of a series of electron‐rich, hexahydroxylated 10‐O‐R‐substituted anthracenes, where R is an alkyl (Me, nBu, n‐C₁₆H₃₃) or arylalkyl group (CH₂Ph, CH₂‐2‐Napht, CH₂C₆H₄CH₂OAr) and also evaluation of their electronic and optoelectronic properties in solution, crystal, and solid thin film. In this transformation, a central 10‐O‐R‐substituted benzene ring was formed, fused to rings originating from two independent aromatic aldehydes. The reaction proceeded via two identified mechanisms involving acetal and/or free aldehyde groups. The acid sensitive acetal and dibenzyl alkoxy functions have never been used together in the intramolecular Friedel–Crafts type cyclization. The new compounds revealed deep blue fluorescence and quantum yields in solution around 0.3. The electrical properties investigated for thin films obtained by vacuum deposition on glass were 10‐O‐R‐substituent dependent and showed much faster transient current decay in the case of the 10‐O‐CH₂Ph derivative than for the material with a 10‐O‐Me substituent (the lifetime of charge carriers was 25 times shorter in this case). The AFM images of thin films, Stokes shifts, and X‐ray analysis of π‐stacking interactions in crystals of the new materials have been also obtained.     

EXCIPLEX STUDIES–V. ELECTRON EJECTION FROM INDOLE AND METHYL-INDOLE DERIVATIVES*

Abstract— Electron-scavenging experiments with N₂O as scavenger demonstrate at least two electron-producing reactions of the excited singlet states of the exciplex species formed by indole or 1 -methyl-indole with water. Most electrons reacting with N₂O result from collision of the scavenger with a metastable state formed from the initial exciplex state but finite electron yields from indole and 1-methyl-indole at limiting scavenger concentrations suggest that the intermediate states also eject electrons directly into the solvent. The formation of the first metastable state from the fluorescent exciplex state has an activation energy, E_M, estimated to be about 13 kcal/mole for both indole and 1 -methyl-indole water exciplexes. The E_M values for 1-methyl-indole from fluorescence and electron yields are the same, Indicating that at neutral and alkaline pH fluorescence quenching and electron extraction are both being controlled by the formation of the first metastable intermediate. Observed electron yields from indole-water and indole-methanol exciplexes are less than predicted using fluorescence data, although E_M values of 1 kcal/mole are obtained for the indole-methanol exciplex by both methods. At pH 12·0 and 28°C the total electron yields for indole-water and 1 -methyl-indole-water exciplexes are 0·30 and 0·25, respectively. The residual yields attributed to outright formation of hydrated electrons from the initial exciplex excited stateare 0·11 and 0·05, respectively. Electron yields from the indole-water exciplex are strongly pH dependent only near pH 1 where the fluorescence yields as well as the electron yields decrease rapidly with increasing acidity. The 1-methyl-indole-water exciplex shows an additional pH dependence which is first-order in hydrogen-ion activity and has an effective pK_a of about 11·5. Comparable yields for indole and 1-methyl-indole are found only above pH 12. High electron yields are found with indole in the exciplex-forming solvent dioxane and in the non-exciplex forming solvent cyclohexane. For the latter system electrons are probably derived only from the lowest excited state of indole on collision with N₂O.     

The lead tetraacetate oxidation of bisacetylhydrazones of α-dicarbonyl compounds

The oxidative cyclization of the title compounds results in generally two different kinds of products. The first, 1-(N,N-bisacetylamino)-1,2,3-triazole 7 (R³ = CH₃) is the primary product, while the second, 1-N-acetylamino-1,2,3-triazole 8 (R³ = CH₃), when observed, is obtained via hydrolysis from the former during work-up and separation of the reaction mixture. The primary products are considered as resulting from intramolecular nucleophilic attack on the acetyl group, of the presumed zwitterionic intermediate 5 (R³ = CH₃), by the N of the ambident N-acetylimine site of 5 .     

Intramolecular diels-alder reactions. XI. Modal selectivity in the syntheses of some parent cyclolignan lactones

Ph(C₂)CH₂Cl and Ph(C₂)'CO₂Na [where (C₂) = -C=C- or trans-CH=CH-, (C₂)' = -C=C- or cis-CH=CH-] reacted in refluxing dimethylformamide to yield unsaturated esters Ph(C₂)'CO₂-CH₂(C₂)Ph and/or their intramolecular Diels-Alder cyclization products (cyclolignan lactones). It was found that modal selectivity for cyclization in DMF sometimes varies from that found previously with acetic anhydride as solvent. Two new parent tetrahydrocyclolignan lactones were synthesized.