Hydrocarbons, aromatic phosphorescence emissions

Of particular interest in the application of cyclodextrins is the enhancement of luminescence from molecules when they are present in a cyclodextrin cavity. Polynuclear aromatic hydrocarbons show virtually no phosphorescence in solution. If, however, these compounds in solution are encapsulated with 1,2-dibromoethane (enhances intersystem crossing by increasing spin-orbit coupling external heavy atom effect) in the cavities of P-cyclodextrin and nitrogen gas passed, intense phosphorescence emission occurs at room temperature. Cyclodextrins form complexes with guest molecules, which fit into the cavity so that the microenvironment around the guest molecule is different from that in... 

In aromatic hydrocarbons, the radiationless transitions from the triplet to the ground state are dominated by CH stretching vibrational modes. In these hydrocarbons perdeuteration reduces kJsc and consequently enhances kp, the rate constant for phosphorescence emission. The lifetime rp, is considerably reduced. The reduction of kjsc (T, 5,) which... 

McGlynn Mid Boggus describe the phenomenon thus absorption in the charge transfer bMid is followed either by the converse emission or by intersystem crossing (according to Kasha [124]) to a dissociative level of the complex which yields the aromatic in its first excited triplet state. The aromatic hydrocarbon then phosphoresces. 

An elegant method in induce phosphorescence emission from a guest exploits the heavy-atom effect of heptakis(6-Br-6-deoxy- -CD). In nitrogen-purged dimethylformamide/water (4/1), solution phosphorescence emission from five polynuclear aromatic hydrocarbons (PNA), diben-zofuran, and dibenzothiophene was readily observed in the presence of 10" Br-functionalized P-CD [191]. 

Figures 1.6 and 1.7 show fluorescence and phosphorescence of polyolefins, respectively [69]. Figure 1.6 shows that fluorescence of polyolefins cannot be attributed to the presence of polynuclear aromatic hydrocarbons (e.g. naphthalene), because the positions of emitted bands differ significantly. On the other hand phosphorescence emission (Fig. 1.7) shows the presence of a,)S-unsaturated carbonyl groups. These results indicate the presence in polyolefins of enone and/or dienone chromophoric groups which are responsible for the observed luminescence.

P-type delayed fluorescence is so called because it was first observed in pyrene. The fluorescence emission from a number of aromatic hydrocarbons shows two components with identical emission spectra. One component decays at the rate of normal fluorescence and the other has a lifetime approximately half that of phosphorescence. The implication of triplet species in the mechanism is given by the fact that the delayed emission can be induced by triplet sensitisers. The accepted mechanism is ... 

In aromatic hydrocarbons, short-lived ](it, it ) is the lowest excited state and energy gap between (n, it ) and 8(it, it ) states is large. Both these factors are conducive to fluorescence emission and in general aromatic hydrocarbons are good fluorescer. Sometimes, the prediction may not come true if a higher triplet state T2 is available near the St state such as in anthracene. In such cases, fluorescence and phosphorescence both are observed at low temperatures in suitable solvent medium specially when S, and T, are states of different symmetry type. Some data correlating AEst and 4[Pg.148]

For a complete picture, we should briefly refer to the influence of the chemical structure of the emitting molecule. As with fluorescence, aromatic compounds are most important because they may yield phosphorescence. A shift of this emission toward longer wavelengths with an increase in the number of condensed benzene rings is observed. Introduction of a heteroatom into the aromatic electron system (e.g., in a polyaromatic hydrocarbon) or substitution by atoms of high atomic number (e.g., the internal heavy-atom effect of bromine or iodine) usually... 

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