STEADY-STATE AND TIME-RESOLVED FLUORESCENCE STUDY OF TRIPLE EXCIPLEX IN THE SYSTEM OF POLYACENAPHTHALENE AND TEREPHTHALIC DIMETHYLESTER

Triple exciplex formed between polyacenaphthalene and terephthalic dimethylester (TDE) has been studied by means of the steady-state and time-resolved fluorescence spectra. The theoretical model of the triple exciplex formation for a flexible polymer chain in a dilute solution has been proposed. The fluorescence decays of the monomer, the exciplex and the triple exciplex obey a double exponential rule in the pbotophysical processes of the triple exciplex formation. The association rate constant of the exciplex formation is k_3=6.0×10~9s~(-1)(mol/L)~(-1); the association rate constant of the triple exciplex formation is k_6=5.2×10~7s~(-1). A mechanism for the triple exciplex formation from the exciplex to the triple exciplex has been proved through the noncrystal films corresponding to the same concentrations of the solution systems and the fluorescence lifetimes of the intramolecular excimer.     

Photoinduced intramolecular electron transfer of fluorescein and violgen, carbazole

A series of polyads consisting of covalently-(CH2)4-linked fluorescein with carbazole and violger.Live been synthesized and characterized The studies of absorption,emission spectra and fluorescence lifetime quenching indicated that the intramolecular fluorescence quenching of fluorescein by violgen is mainly a static process through the formation of non emission complex (fluorescence quenching efficiency φQ=0.97,lifetime quenching efficiency φH 0,quenching efficiency of formation of non-emission complex φC=0.97); while the quenching by carbazole is mainly a dynamic electron transfer process (φQ=0.63,φET=0.63,φC=0).In the violgen-fluorescein-carbazole triads,φQ=0.97,Q ET=0.65,φC=0.32,which suggests that the photoinduced interaction of fluorescein-carbazole pair and that of violgen-fluorescein pair are in a competitive process,the dynamic electron transfer from carbazole to fluorecein is dominant in the process The free energy change of the photoinduced electron transfer and the back reac-tiorns i     

EXCIPLEX FORMATION INFLUENCED BY ISOMERS——EXCIPLEX FORMED FROM DIMETHYL TEREPHTHALATE AND PHTHALATE WITH 9-ETHYL CARBAZOLE

Both dimethyl terephthalate (DMTP) and dimethyl phthalate are able to form exciplex with 9-ethyl carbazole (EtCz) but the fluorescence quenching constant of DMTP to EtCz is larger than that of DMP to EtCz. The peak positions of the fluorescence bands of the exciplex formed from DMTP and DMP with EtCz are also different. The former gives a peak at shorter wavelength region. Either the exciplex formation rate constant k_3 or the exciplex dissociation rate constant k_4 is not alike. Both of these constants of DMTP are larger than those of DMP. All these results are difficult to explain with the established theory, so a new model which could be used to explain the results satisfactorily is supposed.     

EXCIPLEX FORMATION OF A POLYESTER WITH A TEREPHTHALATE MAIN CHAIN HAVING A PENDANT N, N-DIMETHYLAMINO GROUP

The fluorescence behaviours of a new polymer poly [oxy-2 (4-N, N-dimethylaminobenzyl) propane-1 , 3-diyloxy-terephthaloyl] (Ⅰ) in solution were studied. Inter-and intra -molecular exciplex is formed between the singlet excited state of N, N- dimethylanilino group (abbreviated to DMA) and the ground state of terephthalate group (abbreviated to TP). The intensity ratio of the long wavelength exciplex fluorescence to the short wavelength emission of DMA group is used as an index for the inter-and intra-molecular interaction of chromophores. The results are compared with model polymer polyoxy-2-(4-N ,N-dimethylaminobenzyl) propanc-1 , 3-diyloxyadipoyl (Ⅱ), model monomer compound 4-N, N-dimethylaminobenzylmethyl terephthalate(Ⅲ) and diethyl 2-( 4-N, N-dimethylaminobenzyl) malonate (Ⅳ). Polymer association derived by electron donor and acceptor interaction (EDA) is further verified as a key role in the interpolymer exciplex formation . The fluorescence decay time of (Ⅰ) and monomer (Ⅲ) are measured in solutions.     

Enhancement of intramolecular excimer formation and photodimerization of anthrylmethyl α,ω-alkanedioates via hydrophobic interactions

The fluorescence spectra and photodimerization of anthrylmethyl a,w-alkanedioates (A-Mn-A) both in organic and in aqueous organic mixed solvents have been studied.In aqueous organic mixed solvents strong intramolecular excimer emission is detected and the quantum yield for the intramolecular photodimerization is significantly greater than those in organic solvents.These observations suggest that hydrophobic interactions force A-Mn-A molecule to self-coil.The ratio of the head-to-head to head-to-tail products in the intramolecular photodimers of A-Mn-A depends on the length of the linking chain.This work presents a successful example of application of hydrophobic interactions to enhancement of large-ring formation.     

Photoinduced intramolecular electron transfer between fluorescein and carbazole

Several dyads consisting of a fluoreseein covalently linked with a carhazole at site 2 or site 6 have been synthesized and characterized.Studies of absorption spectra,emission spectra and fluorescence lifetime quern hing Indicate that the ground-state interaction between fluorescein and carhazole in dyads is negligible and the intramolecular electron transfer (ET) reactions are mainly of dynamic process.Moreover,the efficiency and raie conslam of lectron transfer reactions in ZFO4 (carbazole linked at site 2'of fluorescein) are larg er than those in 4FOZ (carbazole linked at site 6 of fluorescein) 0 74; KET 11×108S-1),because the mutual orientation of donor and acceptor in ZFO4 is nearly face-to-face,which is more favorable to the process than the shoulder-to-shoulder mutual orientation in 4FOZ.Estimations are also formed of the free energy change of the photomduced electron transfer and the back reactions in the dyads.     

Photocatalytic decarboxylative alkylations of C(sp~3)–H and C(sp~2)–H bonds enabled by ammonium iodide in amide solvent

A simple ammonium iodide salt in amide solvent catalyzes regioselective decarboxylative alkylation of C(sp~3)-H bonds of Naryl glycine derivatives, of C(sp~2)-H bond of heteroarenes, and cascade radical addition to unsaturated bond followed by intramolecular addition to arene, with a broad scope of N-hydroxyphthalimide derived redox active esters under visible light irradiation. The reactions are suggested to proceed through photoactivation of a transiently assembled chromophore from electron-deficient phthalimide moiety and iodide anion through an anion-π interaction in solvent cage followed by diffusion to generate solvated free radical species to react with C-H substrates. The simplicity, practicality, and broad substrate scope of this method highlight the synthetic power of photocatalysis through transiently assembled chromophore, and will hopefully inspire further developments of low cost photocatalysis based on various non-covalent interactions, which are prevalent in supramolecular chemistry and biosystems, for sustainable organic synthesis.     

TIME-RESOLVED FLUORESCENCE STUDY OF TRIPLE EXCIPLEX FORMED IN THE SYSTEM OF POLY(2-VINYL) NAPHTHALENE AND TEREPHTHALIC DIMETHYLESTER

An intramolecular excimer of poly(2- vinyl) naphthalene was formed by non-adjacentchromophores interaction, then a triple exciplex was formed by interaction with the acceptormolecule further in dilute solution. The lifetime of the intramolecular excimer of poly(2-vinyl) naphthalene, τ_2 = 18.83 ns and the rate constant for the triple exciplex formation,k_7 = 4. 18× 10~9 (mol/L)~(-1)s~(-1), under diffusion-control were measured. An excimer or anexciplex could be an intermediate of the triple exciplex formation. A theoretical model ofthe triple exciplex formation is proposed.     

Molecular engineering on all ortho-linked carbazole/oxadiazole hybrids toward highly-efficient thermally activated delayed fluorescence materials in OLEDs

The highest efficiency thermally activated delayed fluorescence(TADF) emitters in OLEDs are mostly based on twisted donor/acceptor(D/A) type organic molecules.Herein,we report the rational molecular design on twisted all ortho-linked carbazole/oxadiazole(Cz/OXD) hybrids with tunable D-A interactions by adjusting the numbers of donor/acceptor units and electron-donating abilities.Singlet-triplet energy bandgaps(ΔE_(ST)) are facilely tuned from～0.4,0.15 to～0 eV in D-A,D-A-D to A-D-A type compounds.This variation correlates well with triplet-excited-state frontier orbital spatial separation efficiency.NonTADF feature with solid state photoluminescence quantum yield(PLQY)10% is observed in D-A type2 CzOXD and D-A-D type 4 CzOXD.Owing to the extremely low ΔE_(ST) for efficient reverse intersystem crossing,strong TADF with PLQY of 71%-92% is achieved in A-D-A type 4 CzDOXD and 4 tCzDOXD.High external quantum efficiency from 19.4% to 22.6% is achieved in A-D-A typed 4 CzDOXD and 4 tCzDOXD.     

Metal Ion Enhanced Phosphorescence of 2,3—Naphtho—10—aza—15—crown—5：A Possible Molecular Photonic Operator

A novel crown ether,2,3-naphtho-10-aza-15-crown-5(NAC) was synthesized.This compound does not show strong fluorescence and phosphorescence due to photoinduced intramolecular electron transfer from the nitrogen lone pair of electrons to the singlet excited state of the naphthalene chromophore.Complexation with heavy-metal ions results in the quenching of the fluorescence and enhancement of the phosphorescence significantly.These observations were interpreted in terms of the binding interactions,between the nitrogen lone pair electrons and the metal cation,which prevent photoinduced intramolecular electron transfer,and the heavy-atom effects which induce quenching of the fluorescence and enhancement of the phosphorescence.These azacrown and heavy-metalbased systems could be useful as potential chemical sensors and molecular photonic devices.     

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.     

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.     

Synthesis and characterization of nitrogen‐linked carbazole‐containing fluorescent polymers

We have prepared four light‐emitting polymers bearing a chromophore composed of carbazole and fluorene by the Suzuki coupling polycondensation. Two nonconjugated polymers (P3CzBFXy and P2CzFXy) had a chromophore tethered by the p‐xylylene spacer, whose connection point between carbazole and fluorene in addition to the number of fluorene unit was systematically changed to investigate the emission wavelength and intensity. The red‐shifted absorption and emission maximum wavelengths together with the improved fluorescence quantum yield of polymers P3CzBFXy and P2CzFXy indicate that the increment of the number of para‐connected benzene rings included in the chromophore effectively extends the conjugation length. The fact that polymer P3CzBFXy has longer wavelength absorption and emission spectra also indicates the interaction of the carbazole nitrogen lone pair with the oligophenylene moiety. Other two polymers P3CzFPy and P3CzFPym having the heterocycle directly bound to the carbazole nitrogen were prepared to know the character of the carbazole nitrogen lone pair and their influence on the fluorescence behavior. The fluorescence spectra of polymer P3CzFPym bearing the pyrimidine ring gradually red‐shifted in conjunction with the decrease of fluorescence quantum yield on going from toluene solution to CHCl₃ solution because of the intramolecular charge transfer at the excited state. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3729–3735, 2010     

Energy Transfer and Concentration‐Dependent Conformational Modulation: A Porphyrin‐Containing [3]Rotaxane

A zinc porphyrin‐containing [3]rotaxane A was synthesized through a copper(I)‐catalyzed azide–alkyne cycloaddition (CuAAC) reaction. Energy donors and acceptor porphyrin were introduced to dibenzo[24]crown‐8 (DB24C8) and dibenzyl ammonium (DBA) units of [3]rotaxane A to understand the intramolecular energy transfer process. Investigations of the photophysical properties of [3]rotaxane A demonstrated that the intramolecular efficient energy transfer readily occurred from the donors on the wheels to the porphyrin center on the axis. The fluorescence of energy donors in the region of 400 to 450 nm was efficiently absorbed by the porphyrin acceptor under irradiation at 345 nm, and finally a red light emission at about 600 nm was achieved. Further investigation indicated that the conformation of [3]rotaxane A was self‐modulated by changing its concentration in CH₂Cl₂. The triazole groups on the wheel coordinated or uncoordinated to Zn²⁺ through intramolecular self‐coordination with the change in the concentration of [3]rotaxane A in CH₂Cl₂. Therefore, this conformational change was reversible in a non‐coordinating solvent such as CH₂Cl₂ but inhibited in a coordinating solvent such as THF. Such interesting behaviors were rarely observed in porphyrin derivatives. This self‐modulation feature opens up the possibility of controlling molecular conformation by varying concentration.     

Short-chain model studies on emission properties of poly(9-vinylphenanthrene-co-p-N,N-dimethylaminostyrene): Exciplex quenching and subsequent formation of another fluorescent exciplex

The emission properties of 1-(4-dimethylaminophenyl)-3-(9-phenanthryl)propane, Ph(CH₂)₃ DMA, and 1,3-di-(9-phenanthyrl)propane, Ph(CH₂)₃Ph, were studied in comparison with those of poly(9-vinylphenanthrene-co-p-N,N-dimethylaminostyrene). Ph(CH₂)₃DMA showed an intense intramolecular exciplex fluorescence in dioxane. Ph(CH₂)₃Ph did not exhibit a clear intramolecular excimer fluorescence. The quenching of the intramolecular exciplex by several electron acceptors was studied. As a result moderate electron acceptors, such as cyanobenzene, methyl benzoate, and acrylonitrile, selectively quenched the intramolecular exciplex, and in the case of cyanobenzene the subsequent formation of another fluorescent exciplex was observed. The results were discussed in terms of the reduction potentials of electron accepting quenchers.     

Synthesis and Electronic Spectroscopy of Bromocarbazoles. Direct Bromination of N‐ and C‐Substituted Carbazoles by N‐Bromosuccinimide or a N‐Bromosuccinimide/Silica Gel System

The preparation, isolation and characterization by elemental analysis and ¹H‐NMR, ¹³C‐NMR, and MS data of the bromo derivatives of N‐substituted carbazoles, i.e., of 9‐methyl‐9H‐carbazole ( 1 ), 9‐phenyl‐9H‐carbazole ( 2 ), 9‐benzyl‐9H‐carbazole ( 3 ), 2‐methoxy‐9‐methyl‐9H‐carbazole ( 4 ), and of C‐substituted carbazoles, i.e., of 2‐(acetyloxy)‐9H‐carbazole ( 5 ) and 3‐nitro‐9H‐carbazole ( 6 ), are reported, in part for the first time. As brominating reagents, N‐bromosuccinimide (NBS) or NBS/silica gel in CH₂Cl₂, NBS in AcOH, KBrO₃/KBr in EtOH doped with a catalytic amount of H₂SO₄, or KBrO₃/KBr in AcOH were employed, and their uses were compared. Semi‐empirical PM3 calculations were performed to predict the reactivity of the N‐substituted and C‐substituted carbazoles and of their bromo derivatives and found to verify the experimental results. The UV‐absorption and fluorescence and phosphorescence emission spectra of the bromocarbazole derivatives in MeCN solution at 298 K and in a solid matrix at 77 K were compared with those of the corresponding carbazoles 1 – 6 . The dynamic properties of the lowest excited singlet and triplet states (τ_f, τ_p, ?_f, and ?_p) were measured under the same experimental conditions. The intramolecular spin–orbital‐coupling effect of the Br‐atom and NO₂ group on the spectroscopic data, photophysical parameters, and on the photo reactivity were also briefly analyzed.     

Crystallographic report: Cis‐[Zn(3,5‐dinitrobenzoato)2(1,10‐phenanthroline)2]·CH3CH2OH

Cis‐[Zn(3,5‐dinitrobenzoato)₂(1,10‐phenanthroline)₂]·CH₃CH₂OH features unidentate and cis‐disposed 3,5‐dinitrobenzoate ligands and chelating 1,10‐phenanthroline ligands so that a distorted octahedral N₄O₂ coordination geometry results. Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis and fluorescence behavior of 3‐methacryamide‐9‐carbazole and its polymer 1

A novel acrylic monomer‐bearing carbazole chromophore, 3‐methacrylamide‐9‐ethyl‐carbazole and its model compound 3‐isobutyramide‐9‐ethylcarbazole were synthesized by reaction of 3‐amino‐9‐ethyl carbazole and the corresponding acyl chloride in the presence of triethylamine. It can be polymerized easily by using azo‐bisisobutyronitrile as an initiator or photopolymerized without any sensitizer. The photochemical behavior of 3‐methacrylamide‐9‐ethyl‐carbazole, its polymer and 3‐isobutyramide‐9‐ethylcarbazole were investigated by recording the fluorescence spectra in N,N‐dimethylformamide. It was found that the fluorescence intensity of the monomer is dramatically lower than those of its polymer and the model compound in the same chromophore concentration. This phenomenon, termed as the ‘structural self‐quenching effect’, was commonly observed for acrylic monomers bearing chromophore moieties and ascribed to the coexistence of the electron‐donating chromophore and the electron‐accepting double bond within one molecule. The strong fluorescence of the polymer can be quenched by adding electron‐deficient monomers having no chromophore moieties such as methyl methacrylate and acrylonitrile, and the Stern–Volmer constants were determined. It is observed that the higher the electron deficiencies of the quenchers, the higher the Stern–Volmer constants, implying a stronger quenching effect.Copyright © 2000 John Wiley & Sons, Ltd.     

Fluorescence Quenching by Intercalation of a Pyrene Group Tethered to an N4‐modified Cytosine in Duplex DNA

A series of duplex DNA oligomers was prepared that contain a pyrene chromophore linked by a trimethylene chain (‐(CH₂)₃‐) to N⁴ of a cytosine. The pyrene group stabilizes the DNA as evidenced by an increase in melting temperature. The absorption spectrum of the linked pyrene chromophore shows a temperature‐dependent shift and there is also a strong induced circular dichroism spectrum attributed to the pyrene group. The fluorescence of the pyrene chromophore is strongly quenched at room temperature by linkage to the DNA, but it increases above the melting temperature. We attribute these observations to intramolecular intercalation of the pyrene group at a base pair adjacent to its linkage site at cytosine.     

Light‐driven and aggregation‐induced emission from side‐chain azoindazole polymers

The well‐defined azoindazole‐containing homopolymer, poly(6‐{6‐[(4‐dimethylamino) phenylazo]‐indazole}‐hexyl methacrylate) (PDHMA), and amphiphilic diblock copolymer, poly({6‐[6‐(4‐dimethylamino)phenylazo]‐indazole}‐hexyl methacrylate)‐b‐poly(2‐(dimethylamino)ethylmethacrylate) (PDHMA_m‐b‐PDMAEMA_n), were successfully prepared via reversible addition‐fragmentation chain transfer polymerization technique. The homopolymer and amphiphilic diblock copolymer in CH₂Cl₂ exhibited intense fluorescence emission accompanied by trans–cis photoisomerization of azoindazole group under UV irradiation. The experiment results indicated that the intense fluorescence emission may be attributed to an inhibition of photoinduced electron transfer of the cis form of azoindazole. On the other hand, the intense fluorescence emission of amphiphilic diblock copolymers in water‐tetrahydrofuran mixture was observed, which increased with the volume ratio of water in the mixed solvent. The self‐aggregation behaviors of three amphiphilic diblock copolymers were examined by transmission electron microscopy, laser light scattering, and UV–vis spectra. The restriction of intramolecular rotation of the azoindazole groups in aggregates was considered as the main cause of aggregation‐induced fluorescence emission. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.