Benzene monomer fluorescence

TABLE R. B. Cundall, et al. 8. Monomer fluorescence yields for benzene in solution. 

TABLE 9. solution. Monomer fluorescence emission lifetimes for benzene in... 

Figure 6.8 Excimer formation (2). Relative intensities of pyrene fluorescence in benzene as a function of pyrene concentration c in M. Open circles represent relative intensity of monomer fluorescence (///fl) filled circles represent relative intensity of dimer fluorescence (/ //q). From Ref. [22,d).

Conjugated boron polymers containing platimnn or palladium atom in the main chain were also prepared by hydroboration polymerization between tetrayne/ metal complex monomers and tripylborane (scheme 16).30 From gel permeation chromatographic analysis [THF, polystyrene (PSt) standards], the number-average molecular weights of the polymers obtained were found to be 9000. The polymers were soluble in common organic solvents such as THF, chloroform, and benzene. The absorption peaks due to tt-tt transition were observed around 390 nm in the UV-vis spectra of these polymers. The fluorescence emission spectra exhibited intense peaks at 490 nm in chloroform. 

Poly(styrene) and PMMA were synthesized from their respective monomers using azobisisobutyronitrile-initiated radical polymerization in benzene. Four freeze-pump-thaw cycles were used to degas the monomer solutions and polymerization was carried out for 48 hours at 60°C. The polymers were purified by multiple reprecipitations from dichloromethane into methanol. Films of these polymers were prepared and found to be free of any fluorescent impurity. 

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. 

We can exclude a predissociation process [39] responsible for the decrease of the lifetime for three reasons, (i) Dispersed emission spectra did not show any indication of emission from the fragment monomer [40]. Thus no dissociation occurs on the time scale of the fluorescence emission, (ii) The additional excitation of the van der Waals stretching vibration in benzene-Ar does not lead to a further decrease of the lifetime, (iii) The stronger decrease of the lifetime of the 61 state in benzene-Kr would not be expeced for a predissociation process since the benzene-Kr complex is more strongly bound and has only a slightly higher density of states since the frequencies of the three van der Waals modes do not differ very much from that of benzene-Ar [41]. 

Leismann et al.[182] have recognized this problem in their publication of 1984, in which they describe a thorough and detailed investigation of the kinetics of formation and deactivation of exciplexes of. S) benzene or toluene and 1,3-diox-ole, 2,2-dimethyl-l,3-dioxole, and 2,2,4-trimethyl-l,3-dioxole. The evolution in time of monomer and exciplex fluorescence after excitation using a nanosecond flash lamp was analyzed, and again it was concluded that the formation of exciplexes is diffusion controlled their decay proceeds mainly (>90%) via radiationless routes. The polar solvent acetonitrile enhances radiationless deactivation, possibly by promoting radical ion formation. Because decay of benzene fluorescence is essentially monoexponential, dissociation of the exciplex into Si benzene and dioxole is negligible. 

Fluorescence detected magnetic resonance effects observed during the pulse radiolysis of anthracene-d10 in the presence of tetramethylethene portray an additional facet of the cyclobutane radical cation system [345, 346]. The spectra [eight ( ) equivalent methyl groups, ad = 8.2 G approximately one half of the monomer splitting, am = 17.1 G] are compatible with a dimer cation. In analogy to the benzene dimer radical cation [347, 348] they were interpreted as evidence for a sandwich , one molecule above the other [346],... 

The quantum yield of fluorescence of many fluorescent substances in solution decreases with increasing concentration. In some cases, e.g., with aqueous solutions of thionine and Methylene Blue, this selfquenching is due to formation of stable dimers in equilibrium with the monomer.78 In other cases, e.g., with solutions of anthracene, perylene, and coronene in solvents such as benzene, chloroform, and kerosene, Bowen and co-workers have shown that quenching takes place by collision17 the self-quenching rate constants obtained are very close to those given in Table I (see also ref. 19). 

The formation of a benzene dimer from association of a ground state monomer and an excited monomer is a well-established observation of this species was first noted by Dammers de Klerk (270) and Ivanova and co-workers (271) in a study of the effects of concentration on the fluorescence spectrum of benzene. Broad structureless emission on the long wavelength side of monomer emission at room temperature is clearly observed at concentrations... 

Since this fluorescent labeling methodology is a living functionalization reaction, the resulting living fluorescent-labeled polymers can be used to initiate the polymerization of a second monomer to produce a block copolymer with the label at the block interface as discussed previously. For example, this procedure has been used to prepare polystyrene-Wock-poly(ethylene oxide) copolymers with both pyrene (60) (see Scheme 23) and naphthalene fluorescent groups at the interface between the two blocks [180-182]. Lithium was used as the counterion to prepare well-defined, quantitatively-ethylene oxide-functionalized polystyrenes in benzene solution [183]. However, under these conditions, it is not possible to polymerize ethylene oxide [183]. Therefore, it was necessary to add either dimethylsulfoxide [180, 181] or a potassium alkoxide [182] to promote ethylene oxide block formation as shown in Scheme 23. These diblock copolymers were fractionated to obtain pure diblock copolymer... 

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