Fluorescence in dilute solutions

Harmine, C13H12ON2, crystallises from methyl alcohol in colourless rhombic prisms, m.p, 266°, [a]D 0°. The hydrochloride, m.p. 269-5-270-5° (Chen ), nitrate, platinichloride, m.p. 264-6°, acid chromate and oxalate crystallise well. The salts show a deep blue fluorescence in dilute solution. Harmine behaves as a monoacidic base. It gives a methiodide, from which methylharmine, needles, m.p. 209°, may be prepared, and this in turn yields methylharmine methiodide. On demethylation harmine yields the phenolic base harmol, C12H10ON2, m.p. 321°.i ... 

Comparison of Polymer and Bichromophoric Model Compound Fluorescence in Dilute Solution... 

There are a variety of arylalkyl methacrylate homopolymers which do not exhibit excimer fluorescence in dilute solution. Such polymers containing the phenyl 43,147), carbazolyl196,197), or phenanthryl197) chromophore have been examined. Fluorescence quenching has been studied only in a series of phenyl-containing methacrylate polymers 43), however. 

The fluorescence In dilute solution is measured for five polyesters with terephthalate as the rigid aromatic unit and diols derived from cyclohexane as the flexible spacer, A conformational analysis concludes that the spacers most conducive to excimer formation are the 1,3-c/s-cyclohexanediol and 1,4-e/s-eyclohexanedimethanol. This result from calculations is compatible with experimental results. 

Variables 3 and 4 are found in the familiar relationship, i.e., that fluorescence in dilute solutions is directly proportional to the absorbance of the solute AX at the wavelength selected for excitation, times the quanta of light IoX available at the wavelength selected for excitation, times the fluorescence quantum efficiency X for the solute. Oftentimes the wavelengths available for maximum illumination in an excitation source do not... 

An increase in temperature leads to a decrease in the observed fluorescence in dilute solutions due to increased collisional quenching. Since a rise of one... 

Early on, an attractive feature of PAEs was a marked fluorescence in dilute solution. Emission x and the fluorescence quantum yield depend upon the substituent and the arylene moiety of PAEs. Later, the fluorescence quantum yield of PAEs in solution can reach unity. The rigid character of PAEs seems to make intramolecular dissipation difficult, and insofar fluorescence is efficient. Some PAEs can exhibit strong fluorescence even in the filmy solid state, which opens a way to applications in polymeric light-emitting diodes (PLED) as well as PPV. 

Lavastre et al. [53] explored the synthesis of PAEs in a combichem setup. The diynes 13-20 were coupled in THF utilizing a Sonogashira protocol to the dibromides A-L to give 96 different PAEs (Scheme 6.13). The authors reported that the polymers J14,16, and 20 were fluorescent in thin film preparations, while 20H, J-L were fluorescent in dilute solution. The results for the thin films are not unexpected as it is... 

In Section 2.0 we examine trapping experiments in the aryl vinyl polymers, for which the trap is an excimer forming site (EFS). A one dimensional random walk model is used for analysis of the transient and photostationary fluorescence in dilute solution. In a second study, the three-dimensional results of LAF [5] are applied to trapping experiments in pure solid films of aryl vinyl polymers. 

These polymers are prepared by polycondensation reactions. Polyurethanes containing DCZB moieties in the main chain (26a) were prepared by treating trans-l,2-bis(3-hydroxymethyl-9-carbazolyl)cyclobutane with the corresponding diisocyanate in the presence of dibutyltin dilaurate [204]. The relative molecular mass of the polymer synthesized using hexamethylene diisocyanate as a linking agent was 2700, and that of the polymer prepared with toluylene diisocyanate was 16000. Polymers (26a) exhibit almost exclusively monomer fluorescence in dilute solution, i.e. they practically have no intramolecular excimer-forming sites. Their complexes with TNF show better photoconductivity than PVK-TNF. 

Description of Method. Quinine is an alkaloid used in treating malaria (it also is found in tonic water). It is a strongly fluorescent compound in dilute solutions of H2SO4 (f = 0.55). The excitation spectrum of quinine shows two absorption bands at 250 nm and 350 nm, and the emission spectrum shows a single emission band at 450 nm. Quinine is rapidly excreted from the body in urine and is easily determined by fluorescence following its extraction from the urine sample. 

The first chemists to prepare a monocyclic pyrylium salt were von Kostanecki and Rossbach who in 189 6 described the fluorescence in dilute aqueous solution of the reaction product obtained from l,3,5-triphenjdpontane-l,5-dione (benzylidene-diacetophenone) and sulfuric acid. However, they failed to isolate the compound which caused the fluorescence and did not suspect that it was a pyrylium salt. It was only in 1916-1917 that Dilthey recognized that this fluorescence resulted from 2,4,6-triphenylpyrylium (3). 

This experiment33 illustrates how adjustment of pH may be used to control fluorescence and so make the determination more specific. The alkaloids codeine and morphine can be determined independently because whilst both fluoresce strongly at the same wavelength in dilute sulphuric acid solution, morphine gives a generally negligible fluorescence in dilute sodium hydroxide. The fluorescence intensities of the two compounds are assumed to be additive. 

Evidence for the role of a singlet excimer in the formation of (78) was provided by the observation of excimer fluorescence from dilute solutions of coumarin in methylcyclohexane and isopentane glasses at 77°K.(93b)... 

Triplet-triplet annihilation In concentrated solutions, a collision between two molecules in the Ti state can provide enough energy to allow one of them to return to the Si state. Such a triplet-triplet annihilation thus leads to a delayed fluorescence emission (also called delayed fluorescence of P-type because it was observed for the first time with pyrene). The decay time constant of the delayed fluorescence process is half the lifetime of the triplet state in dilute solution, and the intensity has a characteristic quadratic dependence with excitation light intensity. 

Considering that the fluorescence intensities are proportional to the concentrations (which is valid only in dilute solutions), this relationship can be rewritten as... 

Under normal conditions the catacondensed hydrocarbons of molecular formula C4n+2H2n+4 adopt a crystal lattice104 shown schematically as Type A in Figure 13 in which the (rotational and translational) displacement of adjacent molecular planes to produce a symmetric sandwich configuration is prohibited by the interaction of neighboring molecules. These crystals exhibit a structured ( molecular ) fluorescence spectrum red-shifted by 100 cm-1 from the molecular spectrum observed in dilute solutions. 

The fluorescence is measured in dilute solution and in glassy PMMA for polyesters in which 2,6-naphthalene dicarboxylate is the rigid unit, and (CF iy+t is the flexible spacer. The anisotropy in the rigid medium demonstrates the existence of intramolecular energy migration, which becomes more important as y decreases from 5 to 1. The Forster radius is about 12 A in the bichromophoric compounds and 14 A in the polyesters. 

Fluorescence is measured in dilute solution of model compounds for polymers of 2,6-naphthalene dicarboxylic acid and eight different glycols. The ratio of excimer to monomer emission depends on the glycol used. Studies as functions of temperature and solvent show that, in contrast with the analogous polyesters in which the naphthalene moiety is replaced with a benzene ring, there can be a substantial dynamic component to the excimer emission. Extrapolation to media of infinite viscosity shows that in the absence of rotational isomerism during the lifetime of the singlet excited state, there is an odd-even effect In the series in which the flexible spacers differ in the number of methylene units, but not in the series in which the flexible spacers differ in the number of oxyethylene units. 

The fluorescence of the a,co-diesters of HO(CH2)mOH, m 2-6, and 1-naphthoic acid is characterized in dilute solution in solvents of different viscosity. An equilibrium RIS analysis can rationalize the observation of the odd-even effect and the occurrence of the maximum at m = 5. 

On the other hand, electron-withdrawing substituents tend to diminish or inhibit fluorescence completely. In dilute solutions in water, aniline is about 40 times more fluorescent than benzene whereas benzoic acid is nonfluorescent. 

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