Luminescence studies in polymers—V : Fluorescence spectra of regular condensation copolymers related to poly-α-methylstyrene

The fluorescence spectra of poly-α-methylstyrene (PMS) and of copolymers of general structure

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have been compared. The fluorescence in MTHF at 77°K is identical to that of ethylbenzene for all polymers. At room temperature in the same solvent, excimer fluorescence is not observed for the condensation copolymers, in agreement with the results obtained by Hirayama on α, ω-diphenylalkanes. In PMS films, excimer fluorescence at 330 nm is recorded at room temperature whereas both normal and excimer fluorescence are emitted at 77°K. In copolymer films, excimer fluorescence at 330 nm is never observed. At room temperature, the emission consists of variable proportions of normal fluorescence λ_max = 280–290 nm) and of another fluorescence λ_max = 310 nm) assigned to an energy trap different from the excimer site in PMS and polystyrene. At 77°K, normal fluorescence is the main emission.     

Absorption spectrum,lifetime and reactivity of the excited triplet state of dimesitylcarbene

The first excited triplet state of dimesitylcarbene has been generated in solution at room temperature. It has a lifetime of 60 ns and shows fluorescence with λ_max = 501 nm and absorption with λ_max = 360 nm. This species is quenched by oxygen and carbon tetrachloride with rate constants of (4.4 ± 0.8) × 10¹⁰ and (7.3 ± 0.6) × 10⁹ M⁻¹ s⁻¹, respectively.     

A PREVIOUSLY UNREPORTED INTENSE ABSORPTION BAND AND THE pK, OF PROTONATED TRIPLET METHYLENE BLUE

Abstract— Excitation by a Q-switched giant ruby laser (1.2 J output at 694 nm ?50 ns flash) of 2–10 µM solutions of methylene blue in water, 30% ethanol in water or 50 v/v% water-CH₃CN at pH values in the range 2.0–9.3 converted the dye essentially completely to its T₁ state. The absorption spectrum of T₁ dye was measured in different media at pH 2.0 and 8.2 by kinetic spectrophotometry. Previously reported T-T absorption in the violet in acidic and alkaline solutions and in the near infrared in alkaline solution was confirmed. Values found for these absorptions in the present work with 30% ethanol in water as solvent are λ_max - 370nm, ε_max, - 13,200 M^-1 cm^-1 at pH 2 and λ_max,?420nm, ε_max 9000 M^-1 cm^-1, λ_max, - 840 nm, ε_max - 20,000 m ^-1 cm^-1 at pH 8.2. Long-wavelength T-T absorption in acidic solution is reported here for the first time: λ_max, ? 680 nm, e_max? 19,000 M cm^-1 in 30% ethanol in water at pH 2. Observation of a pH-independent isobestic point ? 720 nm confirms that the long-wavelength absorptions are due to different protonated states of the same species, MB⁺(T₁) and MBH²⁺(T₁). The pK_a of MBH²⁺(T₁) in water was determined from the dependence on pH of absorption at 700 and 825 nm to be 7.1₄± 0.1 and from the kinetics of decay of triplet absorption to be 7.2. The specific rate of protonation of MB⁺(T₁) by H₂PO₄ in water at pH 4.4 was found to be 4.5 ± 0.4 times 10⁸M^-1s^-1.     

Absorption and emission properties of a europium(II) cryptate in aqueous solution

The absorption spectrum of the complex between Eu²⁺ and the [2.2.2] cryptand shows narrow, weak transitions within 4f⁷ superimposed on broad, strong transitions from 4f⁷ to 4f⁶ 5d. Emission from 4f⁶5d to 4f⁷ can be observed at 77 K λ_max = 420 nm, τ = 0.55 μs) and at 293 K (λ_max = 460 nm, τ = 3 ns) in aqueous solution.     

Photoluminescence of biphenyl-containing polymers

Poly(4-vinylbiphenyl) and copolymers of methyl methacrylate and 4-vinylbiphenyl show both monomeric (λ_max = 325 nm) and excimer (λ_max = 380 nm) fluorescence. The quantum yield of excimer emission increases and the monomeric emission decreases with increase in the fraction of vinylbiphenyl units in the copolymer. The decrease of the monomeric emission is closely related to a decrease in singlet lifetime. These results are interpreted in terms of a kinetic controlled excimer formation. Comparison of the emission in the homo and copolymers with that of the dimeric model compound shows that excimer formation in the polymer strongly depends upon the possibility of energy migration along sequences of vinylbiphenyl units. This conclusion is considered as of particular relevance due to the change in geometry of the biphenyl unit upon excitation.     

Pulse radiolysis studies of styrene

We have applied the pulse radiolysis technique of studying short-lived, radiation-produced intermediates to a study of pure, dry liquid styrene. We have observed at least three distinct species. The most rapidly decaying species (τ1/2 = 4 μsec., λ_max ≈ 370 mμ) exhibits an apparent first-order decay which is slowed down in the presence of dry oxygen, occurs too rapidly to be measured in the presence of water, and has a temperature coefficient of approximately 1 kcal./mole between 25 and 45°C. We have tentatively identified the species as the styryl anion with a G value for formation of approximately 0.15. A second species (τ1/2 = 220 μsec. λ_max ≈ 320–330 mμ) exhibits a first-order decay which appears to be independent of oxygen and water. From its spectrum, we have tentatively identified it as the styryl radical. The third species (λ_max ≈ 310 and 320 mμ) is relatively stable in the dark, but is radily photolyzed by the analyzing light of the usual experimental set-up for pulse radiolysis studies. The formation of this species appears to be independent of the water and oxygen content of the styrene.     

The fluorescence of biliverdin dimethyl ester

Freshly prepared solutions of biliverdin dimethyl ester ( 2 ) in ethanol showed fluorescence maxima at 710 and 770 nm [Φ_F = 1.1. 10^?4 (room temperature) and 5.0 10^?4 (77 K)]. The maxima of monoprotonated 2 at 77 K were shifted to 725 and 806 nm and the quantum yield was increased to 2.6. 10^?2. This acid effect was reversible by neutralization with base. When a neutral solution was kept standing in the dark at room temperature, or when an acidic solution was neutralized by base, an additional fluorescence maximum at 500 nm with a mirror image excitation spectrum with λ_max = 470 nm developed, which disappeared on addition of acid and which is attributed to a chemical change of 2 .     

Normal and second excimers in macromolecules—I: Poly(1-methoxy-4-vinylnaphthalene) in fluid solution

The emission properties of poly(1-methoxy-4-vinylnaphthalene) (PMVN) in 2-methyltetrahydrofuran solution have been investigated over the range 77–350 K. It exhibits, in addition to the monomer fluorescence (348 and 360 nm), two structureless emissions derived from two different types of excimer, viz. the normal excimer (420 nm) and the second excimer (380 nm), the latter having a partially overlapping structure of aromatic rings. The intensity of second excimer emission of PMVN increases with decreasing temperature, while the normal excimer emission shows a maximum at 215 K. Kinetic analysis of transient decay curves for the fluorescence of PMVN gave results consistent with the previously reported kinetic scheme for the dimer model, 1,3-bis(4-methoxy-1-naphthyl)propane, showing that the second excimer is not formed from or converted to the normal excimer and that two types of excimer are formed independently from the excited monomer.     

Syntheses,Crystal Structures and Photophysical Properties of a Series of Cadmium(II) Coordination Polymers

Three cadmium(II) coordination polymers [Cd(NA)₂(H₂O)₂]_n ( 1 ), {[Cd(NA)(phen)(NO₃)]·(H₂O)_1/2}_n ( 2 ), {[Cd(NA)(CH₃C₆H₄COO)(H₂O)₂]·(CH₃C₆H₄COOH)}_n ( 3 ) (HNA = nicotinic acid, phen = 1, 10‐phenanthroline) have been synthesized by hydrothermal method. Their single‐crystal structures were determined by X‐ray diffractometry. The absorption, excitation and emission spectra were investigated and all the complexes emit strong fluorescence: λ^em_max = 544 nm (λ^ex = 492 nm), 1 ; λ^em_max = 466 nm (λ^ex = 393 nm), 2 ; λ^em_max = 430 nm (λ^ex = 313 nm), 3 . At room temperature in the solid state the fluorescence lifetimes of the complexes were investigated and the relationships between the spectra were discussed as well as the connections of luminescence and crystal structures.     

Photochemistry of amphiphilic copolymers: Fluorescence spectra of copolymers of 1-(2-naphthyl)ethyl methacrylate and sodium 2-acrylamido-2-methylpropanesulfonate in solution

Amphiphilic copolymers containing 1-(2-naphthyl)ethyl ester groups were prepared and their fluorescence properties were examined. The fluorescnce spectra of the copolymers with a higher naphthyl content showed predominantly a broad monomer emission with λ_max at 340 nm in methanol, whereas a structured emission with λ_max at 344 and 362 nm was observed in aqueous solution. The former spectra changed rapidly to the latter ones upon UV irradiation. Photolysis of the copolymers and the monomeric model compound in methanol gave 2-vinylnaphthalene, whose fluorescence agreed with those of copolymers in water, together with methyl ether, alkylnaphthalenes, and other products. These results indicate that the copolymers hold vinylnaphthalene-like traps, which may emit preferentially in water because of efficient sensitization. © 1995 John Wiley & Sons, Inc.     

Kinetic and spectral properties of the intermediates formed in the pulse radiolysis of 2-mercaptobenzimidazole

The reactions of primary species of water radiolysis such as e^- _aq, H* and *OH, and some specific one electron reductants and oxidants with 2-mercaptobenzimidazole have been studied at various pHs. *OH radical reaction with MBZ at pH 7 gave a transient species having absorption maxima (λ_max) at 330 and 590 nm. The transient species (pK_a = 3.6) was found to be neutral at this pH and was a mild oxidant. The initial transient species formed by the reaction of MBZ with e^{- aq} at pH 7 and with H atom at pH 0 were found to react with the parent molecule to form another transient species which has an absorption spectrum similar to that obtained by *OH radical reaction with λ_max at 590 nm. However the reaction is not quantitative. The kinetic, spectral, acid-base and redox properties of the transient species are reported.     

Comparison of activities of homogeneous Ti-based catalysts in the presence of methylaluminoxane (MAO) for synthesis of syndiotactic polystyrene by UV/visible spectroscopic study

The catalytic activities in syndiospecific polymerization of styrene in hydrocarbon using homogeneous Ti-based catalysts in the presence of methylaluminoxane (MAO) were investigated through UV/visible spectroscopic analysis. A strong UV absorption band of CpTiCl₃, itself, incipiently appeared at λ_max = 400 nm in toluene, followed by a bathochromic shift with its remarkable decrease by the addition of MAO. The absorption band intensity at λ_max = 400 nm arising from delocalization of π-electrons on the cyclopentadienyl ring decreased by methylation in the presence of MAO with regard to the mechanism for production of an active center (“cation-like”), for example, the change of the ionic nature. The intensity decrease at λ_max = 400 nm was suppressed over 2000 of the [Al]/[Ti] ratio. In the case of Ti(OC₄H₉)₄ having a σ-ligand, new and broad UV absorption bands were developed at λ_max = 340 nm and 410 nm in the presence of MAO in contrast with the CpTiCl₃/MAO system. Comparison between the relative absorption intensities at λ_max = 340 nm and 410 nm led to the determination of a maximum catalytic activity of Ti(OC₄H₉)₄ in the presence of MAO related to the polymerization yield. The maximum polymerization yield was observed with regard to the relative maximum value of the absorption intensity at λ_max = 410 nm with the [Al]/[Ti] ratio (500). From observation for polymorphism of the final products via differential scanning calorimetric analysis (DSC), the thermally unstable β-form seemed to be produced by the CpTiCl₃/MAO system independent of the MAO concentration, the Ti(OC₄H₉)₄/MAO system produced a thermally stable α-form in the low MAO concentration (up to 100 of the [Al]/[Ti] ratio), and a mixture of α- and/or β-forms over 200 of the [Al]/[Ti] ratio under our experimental conditions. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1733–1741, 1998     

Photolysis of Fe(CO)42− in an aqueous NaOH glass at 77 K. formation of solvated electrons

The photolysis of [Fe(CO)₄]²⁻ in 10 M NaOH glass at 77 K leads to the formation of solvated electrons with θ = 0.03 at λ_irr = 254 nm. The solvated electrons, which are stable under these conditions, are identified by their characteristic absorption band at λ_max = 588 nm.     

Electron absorption spectra and voltabsorptograms of poly-3-octylthiophene films

Steady-state absorption spectra of poly-3-octylthiophene films in different oxidation states and differential cyclic voltabsorptograms of poly-3-octylthiophene films in 0.1 M LiClO₄ solutions in acetonitrile are studied. Electron spectra of films demonstrate the following three absorption bands: a complex band with a pronounced maximum at λ = 450 nm, which corresponds to π → π* electron transitions in the reduced fragments of poly-3-octylthiophene films and two absorption bands (at λ_max = 780 and λ_max > 1100 nm), which correspond to the oxidized film fragments. It is concluded that two chemically and optically distinguishable oxidation products are formed during the polymer oxidation.     

Detection of point mutations using pyrene-labeled DNA probes

Pyrene-labeled oligodeoxyribonucleotide probes were shown to be suitable for the detection of point mutations. Reagents based on homochiral 2,4-dihydroxybutyramides were used to introduce pyrene residues at the 3"- and 5"- ends of oligonucleotide pairs. The oligonucleotide pair forms a tandem complex with a complementary target, giving rise to an excimer signal (_max 470—490 nm) in the fluorescence spectra when the pyrene residues come into close proximity. The maximum ratio of the intensity of the excimer signal to the monomer signal (_max 380—400 nm) is attained when (S)-N-(1-pyrenylmethyl)-3,3-dimethyl-2,4-dihydroxybutyramide is used to introduce the pyrene residue. The excimer fluorescence completely disappears with an increase in the distance between the pyrene residues (upon the introduction of an additional nucleotide in the target) or in the presence of a mismatch near the contact site of the probes.     

Mechano- and Photoresponsive Behavior of a Bis(cyanostyryl)benzene Fluorophore

The mechanoresponsive behavior and photochemical response of a new bis(cyanostyryl)benzene fluorophore (CSB-5) were investigated. Green fluorescence with λ_em,max of 507 nm was found for CSB-5 in chloroform solution, mirroring the behavior of a previously reported similar dye (CSB-6). Alternatively, crystalline samples of CSB-5 exhibited orange fluorescence with λ_em,max of 620 nm, attributable to excimer emission. Although the emission color change was not clearly noticeable by naked eye, CSB-5 exhibited mechanochromic luminescence, due to transformation into the amorphous state upon grinding the crystalline powder. Interestingly, rubbed films of CSB-5 prepared on glass substrates exhibited a pronounced emission color change from orange to green when exposed to UV light. This response is the result of a photochemical reaction that occurs in the amorphous state and which causes a decrease of the excimer emission sites so that the emission color changes from excimer to monomer. The crystalline material did not display such a photoinduced emission color change and the difference in photochemical reactivity between crystalline and amorphous states was exploited to pattern the emission color of rubbed films.     

Synthesis,structure, and blue photoluminescence of the coordination chain polymer [Cd<Subscript>2</Subscript>(L)Cl<Subscript>4</Subscript>]<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (L = 2-(3,5-dimethylpyrazol-1-yl)-4-methylquinoline)

The complex of CdCl₂ with 2-(3,5-dimethylpyrazol-1-yl)-4-methylquinoline (L), Cd₂(L)Cl₄ (I), is synthesized. According to the X-ray diffraction data, the structure of coordination polymer I is formed by chains in which two crystallographically independent Cd atoms linked by the bridging Cl atoms have different coordination numbers (5 and 6) and different environments. Five Cl atoms form the coordination polyhedron of the Cd(2) atom: a square pyramid (Cd-Cl 2.554(3)–2.689(3) Å) completed to a bipyramid by the CI atom at a substantially longer distance of 2.865(4) Å. Two Cl atoms and the N atom of quinoline fragment L are arranged in the equatorial plane of the coordination polyhedron of the Cd(1) atom, which is a distorted trigonal bipyramid. The axial vertices are occupied by the N atom of the pyrazole cycle of L and the Cl atom. Five-membered metallocycles CdCN₃ are formed upon the coordination of bidentate chelating ligands L. The planes of L of two adjacent chains are parallel (the distances between them are ～3.5 Å), which suggests the π-π interaction between the planes. The luminescence excitation spectrum of compound I has a broad band with λ_max = 350 nm. The spectrum of bright blue photoluminescence of compound I (300 K, λ_exc = 340 nm) contains an intense band with λ_max = 420 nm.     

Highly sensitive fluorescent sensor targeting CuCl2 based on thiophene attached anthracene compound (TA)

A novel thiophene attached anthracene (TA) based fluorescent compound was designed and synthesized. The TA showed a high quantum yield (Qy = 0.34) in regard to fluorescence. We applied this TA compound to detect specific metal compound and found that it could identify CuCl₂ from other metals through dramatic fluorescence change at λ_max = 460 nm. It showed strong quenching fluorescence property with CuCl₂ while with other metal compounds it exhibited strong blue fluorescence emission. UV/Vis absorption spectroscopy clearly demonstrated that the quenching property of TA at λ_max = 460 nm was due to overlapping of the fluorescence peak of TA at λ_max = 460 nm and the absorption band of CuCl₂ (from 190 nm to 525 nm). Binding constant (K′), which was 0.0895 mM^?2, indicated a complexation ratio between TA and CuCl₂ as 1:2 and this interaction induced quenching property.     

Liquid chromatographic determination of ornithine and lysine based on intramolecular excimer-forming fluorescence derivatization.

A highly sensitive and selective fluorometric determination method for ornithine and lysine has been developed. This method is based on an intramolecular excimer-forming fluorescence derivatization with a pyrene reagent, 4-(1-pyrene)butyric acid N-hydroxysuccinimide ester (PSE), followed by reversed-phase liquid chromatography (LC). The analytes, containing two amino moieties in a molecule, were converted to the corresponding dipyrene-labeled derivatives by reaction with PSE. The derivatives afforded intramolecular excimer fluorescence (450-550 nm) which can clearly be discriminated from the normal fluorescence (370-420 nm) emitted from PSE and monopyrene-labeled derivatives of monoamines. The structures of the derivatives and the emission of excimer fluorescence were confirmed by LC with mass spectrometry and with three-dimensional fluorescence detection system, respectively. The PSE derivatives of ornithine and lysine could be separated by reversed-phase LC on ODS column with isocratic elution. The detection limits (signal-to-noise ratio = 3) for ornithine and lysine were 3.5 and 3.7 fmol, respectively, for a 20-microl injection. Furthermore, this method had enough selectivity and sensitivity for the determination of ornithine and lysine in normal human urine.     

Liquid chromatographic determination of dicarboxylic acids based on intramolecular excimer-forming fluorescence derivatization

A highly sensitive and selective fluorimetric determination method for dicarboxylic acids (C5-C12) has been developed. This method is based on an intramolecular excimer-forming fluorescence derivatization with a pyrene reagent, 4-(1-pyrene)butyric acid hydrazide (PBH), followed by reversed-phase liquid chromatography (LC). The carboxylic acids were converted to the corresponding dipyrene-labeled derivatives by reaction with PBH in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The derivatives afforded intramolecular excimer fluorescence (450-550 nm) which can clearly be discriminated from the normal fluorescence (370-420 nm) emitted from PBH and monopyrene-labeled derivatives of monocarboxylic acids. The structures of the derivatives and the emission of excimer fluorescence were studied by LC with mass spectrometry and with spectrofluorimetry, respectively. The PBH derivatives of the carboxylic acids could be separated by reversed-phase LC on an ODS column with isocratic elution. The detection limits (signal-to-noise ratio = 3) were 1.3 fmol to undetectable for a 20-microl injection.