Temperature dependence of the fluorescence quantum yields of diphenylsiloxane‐based copolymers in dilute solutions

The monomer and excimer fluorescence quantum yields of well‐defined poly(dimethylsiloxane‐co‐diphenylsiloxane)s with different diphenylsiloxane (Ph₂SiO) contents have been determined, along with those of 1,1,3,3‐tetraphenyl‐1,3‐dimethyldisiloxane and 1,1,3,3,5,5‐hexaphenyltrisiloxane‐1,5‐diol used as model compounds, in a dilute organic solvent at different temperatures. The measured fluorescence quantum yields of the copolymers are correlated with the fraction of the ? (CH₃)₂SiO? (Ph₂SiO)_n? (CH₃)₂SiO? structures. The monomer fluorescence yield for copolymers with low Ph₂SiO contents is dominated mainly by the isolated ? (CH₃)₂SiO? (Ph₂SiO)? (CH₃)₂SiO? unit, and the apparent mean binding energy of the excimer does not increase significantly with increasing Ph₂SiO content. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 854–861, 2002     

聚醚砜激基缔合物的荧光光谱研究

本文研究了劳环在主链上的刚性高分子聚醚砜在二氯甲烷溶液中的荧光光谱.结果表明在适当的浓度下它也能形成分子间激基缔合物.     

The effect of γ‐cyclodextrin addition in the self‐assembly behavior of pyrene labeled poly(acrylic) acid with different chain sizes

The interaction between poly(acrylic acid) polymers (PAA) of low‐ (2000 g/mol) and high‐ (450,000 g/mol) molecular weight (M_w) hydrophobically modified with pyrene (PAAMePy) and β‐ and γ‐cyclodextrins (β‐CD, γ‐CD) was investigated with fluorescent techniques. The interaction with β‐CD promotes little variation in the spectral and photophysical behavior of the polymer, whereas significant changes are observed upon addition of γ‐CD. The degree of inclusion (between the pyrene groups of the polymer and the cyclodextrins) is followed through the observation of the changes in the absorption, excitation (collected in the monomer and excimer emission regions) and emission (I_E/I_M ratio) spectra and from time‐resolved data. Within the studied range of γ‐CD concentration, the fluorescence decays of the long chain (high M_w) PAAMePy polymers were found tri‐exponential in the monomer and excimer emission regions in agreement with previous studies. In the case of the low M_w PAAMePy polymers, tri‐exponential decays were observed at the monomer and excimer emission wavelengths. However, when a γ‐CD concentration of 0.01 and 0.03 M is reached for, respectively, the low‐ and high‐labeled pyrene short chain (low M_w) polymers, the fluorescence decays in the excimer region become biexponential (two excimers) with no rising component, thus showing that all pyrene groups are encapsulated (and preassociated) into the γ‐CD cavity. In the case of the high M_w polymers, the addition of γ‐CD has been found to change the level of polymer interaction from pure intramolecular (water in the absence of cyclodextrin) to a coexistence of intra‐ with intermolecular interactions. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1402–1415, 2008     

Hydrophobic effects on photophysical and photochemical processes: 1. Aggregation and excimer formation of alkyl β-naphthoates with long chain in poor solvents

The fluorescence spectra of alkyl β-naphthoates with various chain lengths (A_n) in DMSO-H₂O and ethylene glycol-water (EG-H₂O) mixtures were studied. The β-naphthoates with short chain show monomer fluorescence only in both solvent mixtures, while fluorescence spectra of long chain alkyl β-naphthoates are dominated by excimer emission. Addition of long chain hydrocarbon or amylose resulted in the reduction of excimer emission and enhancement of monomer fluorescence. All these experimental results supported intermolecular aggregation of long chain alkyl β-naphthoates in poor solvents. The kinetic parameters of the formation and dissociation of excimer as well as fluorescence polarization in aggregates were measured. These data provided an insight into the characteristics of aggregates.     

Characterization of N-vinyl carbazole/vinyl 4-tert-butyl-benzoate copolymers of several molar compositions: SEC, DSC and intramolecular excimers in dilute solutions

Several N-vinylcarbazole/vinyl p-tert-butyl-benzoate copolymers of different molar monomer compositions and their respective homopolymers have been synthesized by free-radical polymerization. Their solution properties were characterized by size-exclusion chromatography (SEC) coupled with a differential refractive index (RI) and multiangle light scattering (MALS). Differential scanning calorimetry (DSC) was also used to investigate the glass transition temperatures, T_g. In addition, steady-state fluorescence emission spectra in a dilute solution of several fluid and non-fluid solvents were obtained to study the influence of monomer composition on the intramolecular carbazole excimer formation. The amount of excimers strongly depends on the solvent nature and copolymer composition. Molecular Dynamics (MD) simulations on iso- and syndiotactic homopolymer and copolymer fragments were used to obtain the probability of the conformations that satisfy excimer requirements. The analysis of the MD trajectories allows us to evaluate the amount and types of intramolecular excimers for the different systems.     

Three rare heteromultinuclear 3d-4f salamo-like complexes constructed from auxiliary ligand 4,4′-bipy: Syntheses,structural characterizations,fluorescence and antimicobial properties

Three rare heteromultinuclear complexes, _∞[NiL(4,4′-bipy)Pr (NO₃)₃]·(CH₃)₂CHOH ( 1 ), [{CuLSm (NO₃)₃}₂(4,4′-bipy)]·CH₃OH ( 2 ) and [{CuL (CH₃CH₂OH)Eu (NO₃)₃] ( 3 ) with a symmetrical salamo-like hexadentate ligand H₂L have been synthesized, and characterized by FT-IR, UV–vis and X-ray crystallography. Complex 1 is a 1D coordination polymer constructed from heterobimetallic [Ni(L)Pr (NO₃)₃] units which are connected by the exo-dentate ligand 4,4′-bipy bearing nitrogen-donor atoms. Complex 2 is a heterotetranuclear dimer based on [Cu(L)Sm (NO₃)₃] moieties which are linked through the exo-dentate 4,4′-bipy hasing nitrogen-donor atoms. Complex 3 is a heterodinuclear structure, Cu (II) atom is five-coordinate possessing a distorted square pyramidal geometry, and Eu (III) atom is a deca-coordinate adopting a distorted bicapped square antiprism. In addition, fluorescence and antimicobial properties of the ligand H₂L and its complexes 1 – 3 have also been discussed.     

Ratiometry of monomer/excimer emissions of dipyrenyl calix[4]arene in aqueous media

Variations in the ratio of monomer/excimer emissions from pyrenyl groups bound to a calix[4]arene through facing carboxamidomethyl substituents have been investigated in H₂O/CH₃CN mixtures. Above a level of 50% H₂O, monomer emission declines and the excimer emission concomitantly increases. DFT calculations support the argument that disruption of intramolecular NHO bonds by water results in a geometry, which favors contact of the pyrene units and thus a strong excimer emission. Addition of Fe(III) to a H₂O/CH₃CN (4:1, v/v) solution of 1 at pH 6.1 quenches both monomer and excimer emissions through electron transfer (eT) from excited pyrene to the metal ion.     

Synthesis,fluorescence properties,and conformational analysis of ether-linked (1,8)pyrenophanes

Mono-, di- and oligo-ether linked (1,8)pyrenophanes 1–7 were synthesized, and their fluorescence and conformational properties in the absence and presence of metal ions were elucidated. Fluorescence spectra of 1.0 × 10⁻⁵ M solutions of the mono- and di-ether linked pyrenophanes 1–5 were comprised of only monomer emission bands, while those of the oligoethylene glycol linked analogs 6 and 7 contained both monomer and intramolecular excimer emission bands. Addition of perchlorate salts of Ba²⁺, Na⁺ and Li⁺ to 1:1 v/v CH₃CN:CH₂Cl₂ solutions of 6 and 7 caused decreases in the intensities of the corresponding intramolecular excimer emission bands and, in some cases, increases in the intensities of the monomer emission. Monomer and intramolecular excimer emission from the (1,8)pyrenophanes are suggested to arise from the respective anti and syn conformers, whose ratios are dependent on solvent polarity, temperature and kinds of added metal ions.     

Ratiometry of monomer/excimer emissions of dipyrenyl thiacalix[4]arene for Cu detection: a potential Cu and K switched INHIBIT logic gate with NOT and YES logic functions

A new thiacalix[4]arene derivative 2 of 1,3-alternate conformation possessing two pyrene groups has been synthesized and examined for its cation recognition abilities towards different cations such as lithium, sodium, potassium, nickel, zinc, cadmium, silver, mercury, lead and copper by fluorescence spectroscopy. In CH₃CN/CH₂Cl₂ (1:1), the presence of Cu(II) induces the formation of a 1:2 ligand/metal complex, which exhibits increasing monomer emission at 376 nm at the expense of the fluorescent excimer emission of 2 centered at 476 nm. In the presence of K⁺, the intensity of the excimer emission increases along with the formation of a new blue shifted band at 435 nm which corresponds to a static dimer. The compound behaves as a fluorescent molecular switch upon chemical input of Cu²⁺ and K⁺.     

Immobilization of pyrene on quartz plate surface via a flexible long spacer and its sensing properties to dicarboxylic acids

It is known that polycyclic aromatic hydrocar-bons (PAHs) form excimer easily, and show both monomer and excimer emission in polar medium[1]. This character comes from their hydrophobic nature and plane structures. The ratio of the intensity of ex-cimer emission to that of monomer emission is a func-tion of distance between neighbouring PAHs in a sys-tem. It is not difficult to understand that many factors, such as the concentration of PAHs, temperature, pres-ence and concentration of othe…     

Configurations conducive to the formation of intramolecular excimers in poly(N‐vinyl carbazole) and its copolymers

The ratios of the intensity of excimer and monomer emissions, denoted I_E/I_M, in poly(N‐vinyl carbazole) and copolymers of N‐vinyl carbazole and methyl methacrylate were measured with steady‐state fluorescence. Measurements were performed in dilute solutions of several fluid solvents at 25 °C and in a solid matrix of poly(methyl methacrylate) at room temperature. The values of I_E/I_M depended on the nature of the solvent, the emission wavelength, and the copolymer composition. Molecular dynamics simulations were performed for diastereoisomers of 2,4‐di(N‐carbazolyl)pentane and for isotactic and syndiotactic trichromophoric copolymer fragments to assist in the identification of the thermally accessible conformations capable of forming intramolecular excimers and the configurational relationship of the carbazole units in these complexes. Nearest neighbor carbazole groups made the dominant contribution to the excimers. Excimers were more likely in isotactic sequences than in syndiotactic sequences, as was also the case for the low‐energy excimer arising from the complete overlap of two carbazole units. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1272–1281, 2001     

Correlation of architecture with excimer emission in 100% pyrene‐labeled self‐assembled polymers

Pyrene was incorporated as pendant unit to side‐chain urethane methacrylate polymers having a short ethyleneoxy or a long polyethyleneoxy spacer segment. The short‐spacer pyrene urethane methacrylate was also incorporated either as block or random copolymer (1:9) along with polystyrene. The excimer emission was observed to be different for different polymers with the random copolymer exhibiting the lowest efficiency. But, the total quantum yield was highest (? = 0.58) for random copolymer due to the high emission coefficient of monomer compared to that of excimer. The polymer dynamics were compared by steady state emission and fluorescence decay in THF or THF/water (9:1) solvent mixture and films. The solid state decay profile showed decay without a rise time indicating presence of ground state aggregates. In THF/water (9:1), the decay profile at the excimer emission (500 nm) showed a rise time indicating dynamic excimers. The evolution of excimeric emission centred ～430 or ～480 nm as a function of temperature was also studied in THF/water (9:1). The I_E/I_M ratio for the λ_{343 nm} excitation exhibited steady increase with temperature with the block copolymer PS‐b‐PIHP exhibiting the highest ratio and highest rate of increase; whereas, the random copolymer PS‐r‐PIHP had the lowest I_E/I_M ratios. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Aggregation and Photodimerization of Areno‐Condensed Annulenes

[18]‐ and [24]Annulenes condensed with three pyrene or phenanthrene ring systems show, in solution, especially in apolar solvents, a strong aggregation evidenced, for example, by concentration‐dependent ¹H‐NMR spectra. However, the fluorescence bands and the fluorescence lifetimes of these compounds depend only to a minor extent on the concentration and correspond essentially to the measurements obtained for the monomer. Among several models discussed, one is favored in which aggregates can be excited, but the emission is due to an almost undisturbed monomer. Even for the pyrene system, an excimer fluorescence cannot be detected. Nevertheless, short‐lived excimers are responsible for the regio‐ and stereoselective photodimerization to belt cyclophanes.     

Dual pyrene-labeled pyrrolidinyl peptide nucleic acid as an excimer-to-monomer switching probe for DNA sequence detection

The unique ability of pyrene to form excimers with distinct emission characteristic from monomer offers an attractive means to signal the interactions between biomolecules. In this work, dual pyrene-labeled pyrrolidinyl peptide nucleic acid probe with a d-prolyl-2-aminocyclopetanecarboxylic acid α,β-dipeptide backbone (acpcPNA) was designed as an excimer-to-monomer switching probe for DNA sequence detection. In single stranded state, the excimer emission at 470 nm was mainly observed in the fluorescence spectrum. In the presence of DNA target, the hybridization resulted in separation of the two pyrene units, therefore the spectrum displayed increased monomer emission at 380 nm with concomitant decreased excimer emission. Switching ratio, which is defined as the ratio of the monomer to excimer in the double stranded form [F₃₈₀/F₄₇₀(ds)] divided by the same value obtained from the single stranded form [F₃₈₀/F₄₇₀(ss)], was used to describe the performance of the probes. Switching ratios in the range of 5–30 were observed with various dual pyrene-labeled acpcPNA probes bearing pyrenebutyryl label attached five-base apart. Practically no excimer-to-monomer switching behavior was observed with DNA targets carrying a single mismatched base as shown by the small switching ratios of ∼1.     

Anilinolysis of nitro‐substituted diphenyl ethers in acetonitrile: The effect of some ortho‐substituents on the mechanism of SNAr reactions

Rate data are reported for the reactions of a series of X‐phenyl 2,4,6‐trinitrophenyl ethers 1a–e [X = H, 4‐NO₂, 2‐NO₂, 2,4‐(NO₂)₂, or 2,6‐(NO₂)₂] with substituted anilines 2a–e [Y = H, 2‐CH₃, 2,4‐(CH₃)₂, 2,6‐(CH₃)₂, or N‐CH₃] in acetonitrile as solvent. For individual amine, kinetic data show that there is little steric hindrance to attack at the 1‐position of the parent molecules, even in the presence of di‐ortho substitution. With each substrate, however, there is evidence for significant steric interactions; such effects leading to rate retardation were very severe for 2,6‐dimethylaniline 2d (2,6‐(CH₃)₂) and N‐methylaniline 2e (Y = N‐CH₃), the deactivating effect of N‐CH₃ in most cases is slightly higher than that of 2,6‐(CH₃)₂. However, the reactions with 2e are base catalyzed whereas those of 2d are not. The corresponding reactions with aniline 2a (Y = H) and mono‐ortho methyl‐substituted aniline 2b (Y = CH₃) are wholly base catalyzed. Only with the dinitro substrates, an uncatalyzed reaction is observed and when X = 2,6‐(NO₂)₂ this pathway takes all the reaction flux. A rationale is provided for the dichotomy of amine effects observed in this investigation. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 37–49, 2010     

Star‐shaped polymer PFStODO by atom transfer radical polymerization: Its synthesis,characterization, and fluorescence property

A novel monomer bearing with pyrazoline chromophore, 3‐(4‐fluorophenyl)‐1‐phenyl‐5‐(4‐(4‐vinylbenzyloxy)phenyl‐4,5‐dihydro‐1H‐pyrazole (FStODO), was synthesized, and its atom transfer radical polymerization initiated by a tetrafunctional initiator, pentaerythritol tetrakis(2‐bromoisobutyrate) (4Bri‐Bu), was studied in detail and characterized by ¹H NMR, GPC, and TG‐DSC. The solution, film luminescence of monomer, and its polymer were studied in detail. Compared with that of monomer, both luminescence emission intensity and quantum yield of star‐shaped polymer PFStODO in DMF solution are decreased. However, the two‐photon absorption (TPA) spectra in DMF solution measured by a femtosecond laser show an entirely different result that the maxima TPA cross‐section value of polymer reaches to 203 GM, better than that of the monomer itself (13 GM). More interestingly, the film of polymer shows surprisingly white emission ranging from 400 to 700 nm assigned to the excimer formation. We attribute this formation of excimers to the ordered chromophore aggregates in film, which is further verified by X‐ray diffraction. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Interpenetrating Coordination Polymers from CuI or AgI and Flexible Ligands: 2D Polyrotaxanes and Interpenetrating Grids

The reaction of [Cu(CH₃CN)₄]NO₃ or AgNO₃ with flexible ligands 1,4‐bis(4‐pyridylmethyl)‐2,3,5,6‐tetrafluorobenzene (bpf) or 4,4′‐bis(4‐pyridylmethyl)‐2,2′,3,3′,5,5′,6,6′‐octafluorobiphenyl (bpfb) afforded two types of interpenetrating coordination polymers. The structures of [Cu₂(bpf)₃(NO₃)₂]_n and [Ag₂(bpf)₃‐(NO₃)₂]_n are 2D polyrotaxane networks in which 1D polymeric chains are aligned in a grid. The structure of {[Cu(bpfb)₂]NO₃}_n is a 2D grid polymer with two‐fold parallel interpenetration.     

Computer simulation study on the structural-optical related properties of a pyrene-functionalized fluorescent film

Molecular dynamics simulations were carried out to investigate the structural properties and the structure related optical properties of a pyrene-functionalized film and also the film in the presence of nitrobenzene. It has been shown that at equilibrium (1) the pyrene molecules of the film, of which the fluorophore molecules have been attached to a glass plate surface via relatively long flexible spacers, adopt quasi-coplanar structures; (2) the distance between a pair of pyrene rings populates from 4 A to longer than 10 A, but quite a large number of pyrene molecules populate within distances between 4 and 5 A, indicating that the fluorescence of the film should be characterized by both monomer emission and excimer emission; (3) introduction of nitrobenzene into the system results in a decrease of the population of pyrene molecules within the distances suitable for the formation of excimers, suggesting that excimer emission of the film would be decreased; and (4) the incoming nitrobenzene molecules insert themselves in between the previously formed coplanar structures of pyrene rings and form a complex with one of them. Considering that nitrobenzene is an electron-poor compound and nonfluorescent and that pyrene is an electron-rich one, it is expected that the formation of the complex must result in an excitation transfer from pyrene to nitrobenzene, provided that the distance between them and the orientations of them are reasonable. This indicates that the introduction of nitrobenzene not only decreases the number of excimers of pyrene but also quenches the monomer emission of the fluorophore. All the expectations from the simulation studies are basically consistent with the experimental observations.     

Cholic acid-based high sensitivity fluorescent sensor for α,ω-dicarboxylate: an intramolecular excimer emission quenched by complexation

Fluorescent receptors (1 and 2) bearing two binding units on C3 and C24 and two signal display units on C7 and C12 of cholic acid, respectively, were designed and synthesized. Both 1 and 2 emit a much weaker fluorescence than that of the control compound 3 lacking of the binding units reflecting that a PET process originated from the C-3 thiourea group to the plural pyrenyl pendant groups is operative. Addition of terminal dicarboxylate anions to the CH₃CN solution of 1 or 2 enhances the PET process, which leads to a significant and highly sensitive fluorescence response, resulting in an almost complete quenching of the excimer emission of the signal units. To maximize the interaction of the host and the guest, carboxylates of more than five carbon chains could penetrate through the space between the two pyrenyl pendants of the host, triggering a considerable conformational change of the fluorophores. As a result, an enhancement of the monomer emission at the expense of the excimer emission will take place. The binding properties and mechanism of 1 and 2 to dicarboxylates in CH₃CN were manifested by the combined fluorescence, UV-vis, and ¹H NMR spectroscopic method.     

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.