Weak Fluorescence Band Research Articles

Synthesis of A2B-type 22-oxacorroles bearing two different five-membered heterocycles at meso positions

A series of 22-oxacorroles containing mixed substituents such as either two five-membered heterocycles and one six-membered aryl group or three five-membered heterocycles at three meso positions were synthesized by 3 + 2 oxidative coupling of appropriate 16-oxatripyrrane and meso-substituted dipyrromethane under mild acid catalyzed conditions. The identities of the 22-oxacorroles were confirmed by the respective molecular ion peaks in HR-MS spectra and the structures were deduced by detailed 1D and 2D NMR spectroscopy. NMR studies clearly showed upfield or downfield shifts of core pyrrole and furan protons and inner NH protons depending on the type of substituent present at the meso positions. All 22-oxacorroles absorb in the 410–655 nm region, and the position of the absorption bands depends on the type of meso substituents. The A2B oxacorroles showed one broad and weak fluorescence band in the 600–700 nm region with low fluorescence quantum yields in the 0.05–0.12 range. Electrochemical studies revealed that the A2B-type 22-oxacorroles are easier to oxidize and also easier to reduce than triphenyl 22-oxacorroles. Thus, the electronic properties of 22-oxacorroles were significantly altered when six-membered aryl groups were replaced with five-membered heterocycles as reflected in spectral and electrochemical measurements.

Cation–Anion Dual Sensing of a Fluorescent Quinoxalinone Derivative Using Lactam–Lactim Tautomerism

A quinoxalinone derivative capable of lactam-lactim tautomerization was designed as a new fluorescence probe for sensing of cation (M(+) = Li(+) and Na(+)) and anion (X(-) = F(-), Cl(-), Br(-), and CH3COO(-)) in organic solvents. In THF, the minor lactam tautomer exhibited a weak fluorescence band at 425 nm with a typical Stokes shift of ∼4400 cm(-1), whereas the major lactim tautomer exhibited an intense fluorescence band at 520 nm with large Stokes shift of ∼8900 cm(-1) due to excited-state intramolecular proton transfer (ESIPT). The presence of either cations or anions was found to promote lactim-to-lactam conversion, resulting in the lowering of the ESIPT fluorescence. The lone pairs on the alkylamide oxygen and the quinoxalinone ring nitrogen of the lactam were found to bind Li(+) to form a 1:2 coordination complex, which was confirmed by single crystal X-ray structural analysis and fluorescent titrations. In addition, the N-H bond of the lactam was able to recognize anions via N-H···X hydrogen bonding interactions. Where X = F(-) or CH3COO(-), further addition of these anions caused deprotonation of the lactam to generate an anionic state, consistent with the crystal structure of the anion prepared by mixing tetrabutylammonium fluoride and the quinoxalinone derivative in THF. Dual cation-anion-sensing responses were found to depend on the ion-recognition procedure. The anionic quinoxalinone derivative and its Li(+) complex, which are formed by the addition of CH3COO(-) and Li(+), respectively, displayed different fluorescence enhancement behavior due to the two anions exchanging with each other.

(1‐Butyl‐4‐methyl‐pyridinium)[Cu(SCN)2]: A Coordination Polymer and Ionic Liquid

The compound (C4C1py)[Cu(SCN)2], (C4C1py = 1-Butyl-4-methyl-pyridinium), which can be obtained from CuSCN and the ionic liquid (C4C1py)(SCN), turns out to be a new organic-inorganic hybrid material as it qualifies both, as a coordination polymer and an ionic liquid. It features linked [Cu(SCN)2](-) units, in which the thiocyanates bridge the copper ions in a μ1,3-fashion. The resulting one-dimensional chains run along the a axis, separated by the C4C1py counterions. Powder X-ray diffraction not only confirms the single-crystal X-ray structure solution but proves the reformation of the coordination polymer from an isotropic melt. However, the materials shows a complex thermal behavior often encountered for ionic liquids such as a strong tendency to form a supercooled melt. At a relatively high cooling rate, glass formation is observed. When heating this melt in differential scanning calorimetry (DSC) and temperature-dependent polarizing optical microscopy (POM), investigations reveal the existence of a less thermodynamically stable crystalline polymorph. Raman measurements conducted at 10 and 100 °C point towards the formation of polyanionic chain fragments in the melt. Solid-state UV/Vis spectroscopy shows a broad absorption band around 18,870 cm(-1) (530 nm) and another strong one below 20,000 cm(-1) (<500 nm). The latter is attributed to the d(Cu(I))→π*(SCN)-MLCT (metal-to-ligand charge transfer) transition within the coordination polymer yielding an energy gap of 2.4 eV. At room temperature and upon irradiation with UV light, the material shows a weak fluorescence band at 15,870 cm(-1) (630 nm) with a quantum efficiency of 0.90(2) % and a lifetime of 131(2) ns. Upon lowering the temperature, the luminescence intensity strongly increases. Simultaneously, the band around 450 nm in the excitation spectrum decreases.

Fluorescence following Excited-State Protonation of Riboflavin at N(5)

Excited-state protonation of riboflavin in the oxidized form is studied in water. In the -1 < pH < 2 range, neutral and N(1)-protonated riboflavin coexist in the electronic ground state. Transient absorption shows that the protonated form converts to the ground state in <40 fs after optical excitation. Broadband fluorescence upconversion is therefore used to monitor solvation and protonation of the neutral species in the excited singlet state exclusively. A weak fluorescence band around 660 nm is assigned to the product of protonation at N(5). Its radiative rate and quantum yield relative to neutral riboflavin are estimated. Protonation rates agree with proton diffusion times for H(+) concentrations below 5 M but increase at higher acidities, where the average proton distance is below the diameter of the riboflavin molecule.

Long-Lived π-Shape Platinum(II) Diimine Complexes Bearing 7-Benzothiazolylfluoren-2-yl Motif on the Bipyridine and Acetylide Ligands: Admixing π,π* and Charge-Transfer Configurations

Two π-shape platinum(II) diimine complexes (1 and 2) bearing 7-benzothiazolylfluoren-2-yl motif on the bipyridine and acetylide ligands were synthesized, and their photophysics and reverse saturable absorption were investigated systematically. Both complexes exhibit low-energy, broad, and weak 1MLCT (metal-to-ligand charge transfer)/1LLCT (ligand-to-ligand charge transfer) transitions and intense high-energy bipyridine ligand or acetylide ligand localized 1π,π* transitions mixed with some 1MLCT/1LLCT/1ILCT (intraligand charge transfer) characters in their UV–vis absorption spectra. The emission of 1 and 2 includes a weak fluorescence band and a broad, structureless phosphorescence band emanating from the 3MLCT/3LLCT/3π,π* states for 1 and predominantly from 3π,π* state for 2. Insertion of C≡C bond between the bipyridine and fluorene components induces pronounced red-shifts of the absorption and emission bands because of the extended π-conjugation in the bipyridine ligand. Because of the admixture of the 3π,π* excited state with the charge-transfer excited states, both complexes possess extremely long-lived, broadband (visible to the near-IR region) triplet excited-state absorption, resulting in remarkably strong reverse saturable absorption at 532 nm for nanosecond laser pulses. Therefore, complexes 1 and 2 are excellent candidates for devices that require strong reverse saturable absorption.

Analysis of branched oligophenylene by absorption and fluorescence spectra

A branched oligophenylene has been synthesized based on 1,3,5-tri(4′-bromophenyl)benzene. Absorption and fluorescence spectra were studied and fluorescence quantum yields and lifetimes were measured for the compound in solution. It is demonstrated that the absorption spectrum is a superposition of p-quaterphenyl, p-terphenyl, and biphenyl chromophore absorption bands in a 1:2:1 ratio. The oligomer fluorescence spectrum is found to depend on the excitation wavelength. It is shown that the oligomer fluorescence is determined by two fluorochromic groups, namely fragments with branched p-terphenyl and p-quaterphenyl units. The main fluorescence maxima for these fluorochromic groups coincide with each other and lie in the vicinity of λ = 360 nm. A very weak fluorescence band found in the region 380–440 nm is excited by light with a wavelength lying beyond the oligomer self-absorption region. The reasons for a decrease in fluorescence quantum yields of branched models and the studied oligophenylene as compared with those of linear p-polyphenylene chromophores are discussed.

Detection of picomolar concentrations of lead in water using albumin-based fluorescence sensor

Comprehensive analysis of fluorescence of albumin shows a weak fluorescence band at 430 nm, whose intensity exhibits a remarkable sensitivity to the presence of heavy ions in water. Using this fluorescence as a marker, as low as 10 pM concentration of lead can be routinely detected. Such a great sensitivity is explained in terms of electrostatic interactions in solution, which promote protein agglomeration. The latter is independently confirmed using dynamic light scattering measurements.

The design of a novel fluorescent PET sensor for proton and water: A phenylaminonaphtho[1,2- d]oxazol-2-yl-type fluorophore containing proton donor and acceptor groups

Novel phenylaminonaphtho[1,2- d]oxazol-2-yl-type fluorophores having a diethylamino group as proton binding site and a carboxyl group as proton donating site, for sensing protons and water, have been designed and developed, and their photophysical properties were investigated in solution. In 1,4-dioxane, fluorophores that did not contain a proton binding site exhibited an intense fluorescence band whereas fluorophores that contained a proton binding site displayed only a weak fluorescence band. However, this behavior was reversed when the two types of fluorophore were dissolved in aq. acetic acid solution. The fluorophore that contained both a proton binding site and proton donating site showed weak fluorescence in organic solvents, but very intense fluorescence accompanied an increase in the water content of such solvents. Semi-empirical molecular orbital calculations (AM1 and CNDO/S) and spectral analyses revealed that such fluorophores are capable of sensing protons or water by photo-induced electron transfer.

Structure and linear spectroscopic properties of near IR polymethine dyes

We performed a detailed experimental investigation and quantum-chemical analysis of a new series of near IR polymethine dyes with 5-butyl-7,8-dihydrobenzo[ cd]furo[2,3- f]indolium terminal groups. We also synthesized and studied two neutral dyes, squaraine and tetraone, with the same terminal groups and performed a comparison of the spectroscopic properties of this set of “near IR” dyes (polymethine, squaraine, and tetraone) with an analogous set of “visible” dyes with simpler benzo[ e]indolium terminal groups. From these measurements, we find that the dyes with dihydrobenzo[ cd]furo[2,3- f]indolium terminal groups are characterized by a remarkably large shift ≈300 nm (≈200 nm for tetraone) of their absorption bands towards the red region. We discuss the difference in electronic structure for these molecules and show that the “near IR” dyes are characterized by an additional weak fluorescence band from the higher lying excited states connected with the terminal groups. Absorption spectra for the longest polymethines are solvent-dependent and are characterized by a broadening of the main band in polar solvents, which is explained by ground state symmetry breaking and reduced charge delocalization within the polymethine chromophore. The results of these experiments combined with the agreement of quantum chemical calculations moves us closer to a predictive capability for structure–property relations in cyanine-like molecules.

PHOTOCHEMISTRY OF TYPE I ACID-SOLUBLE CALF SKIN COLLAGEN: DEPENDENCE ON EXCITATION WAVELENGTH

Although previous studies have demonstrated that the predominant photochemistry of type I collagen under 254 nm irradiation may be attributed either to direct absorption by tyrosine/phenylalanine or to peptide bonds, direct collagen photochemistry via solar UV wavelengths is much more likely to involve several age- and tissue-related photolabile collagen fluorophores that absorb in the latter region. In this study, we compare and contrast results obtained from irradiation of a commercial preparation of acid-soluble calf skin type I collagen in solution with UVC (primarily 254 nm), UVA (335-400 nm) and broad-band solar-simulating radiation (SSR; 290-400 nm). Excitation spectroscopy and analysis of photochemically induced disappearance of fluorescence (fluorescence fading) indicates that this preparation has at least four photolabile fluorescent chromophores. In addition to tyrosine and L-3,4-dihydroxyphenylalanine, our sample contains two other fluorophores. Chromophore I, with emission maximum at 360 nm, appears to be derived from interacting aromatic moieties in close mutual proximity. Chromophore II, with broad emission at 430-435 nm, may be composed of one or more age-related molecules. Collagen fluorescence fading kinetics are sensitive to excitation wavelength and to conformation. Under UVC, chromophore I fluorescence disappears with second-order kinetics, indicating a reaction between two proximal like molecules. Adherence to second-order kinetics is abrogated by prior denaturation of the collagen sample. A new broad, weak fluorescence band at 400-420 nm, attributable to dityrosine, forms under UVC, but not under solar radiation. This band is photolabile to UVA and UVB wavelengths.(ABSTRACT TRUNCATED AT 250 WORDS)

Excimer emission in protonated pyridine systems. 1. Fluorescence spectroscopy of protonated pyridine and its methyl derivatives in rigid glass solution at 77 K

The emission properties of pyridine and mono- and dialkylpyridines have been studied in solution in the presence of trifluoroacetic acid at room temperature and 77 K. At room temperature, mono- and dialkylpyridines exhibit a weak and broad fluorescence band with a peak at about 300 nm except for pyridine and 4-n-alkyl- and 2-methylpyridines. This fluorescence originates from a (..pi../sup -/*) state of protonated mono- and dialkylpyridines. However, they exhibit no excimer fluorescence even in a highly concentrated system. At 77 K, in the mixed solvent of tetrahydrofuran, methanol, and methyltetrahydrofuran (4:3:1 by volume) in the presence of trifluoroacetic acid, mono- and dialkylpyridines exhibit a broad and structureless fluorescence band at about 325 nm, in addition to the normal fluorescence band. 4-n-Alkyl- and 2-methylpyridines apparently exhibit only a very weak fluorescence band at about 325 nm, but pyridine is nonfluorescent even at 77 K. It is concluded from the observations of absorption and fluorescence excitation spectra and the fluorescence characteristics that this broad and structureless band is ascribed to a particular excimer (termed dimerlike excimer fluorescence for convenience) which originates from the interaction between protonated monoalkylpyridines (or dialkylpyridines). The analysis of temperature and solvent dependence of fluorescence spectra and the phasemore » transition of the mixed solvent show that the cage of the mixed solvent plays an important role in the dimerlike excimer formation. Further, on the basis of a four-electron ASMO approximation, the dimerlike excimer fluorescence is assigned to result from the in-plane twisted and plane parallel configuration of a compact pair of protonated pyridines.« less