Fluorescence spectra aromatic molecules

Readily measurable fluorescence intensities are found for molecules having aromatic and heteroaromatic rings, in particular when annulated rings are present, and in the case of conjugated 7x-electron systems. If the polymer molecules contain such fluorescence-active subunits they can be characterized by this technique, either directly via their fluorescence spectrum or via fluorescence quenching experiments (for polymers with appropriate quencher groups). It is... 

With an interplanar separation of 3.73 A, 4,4 -paracyclophane is the lowest member of the series to exhibit an alkylbenzene absorption spectrum and the broad structureless fluorescence spectrum of this molecule with a peak intensity at 3400 A is by definition an excimer band further separation of the aromatic rings in 4,5 and 6,6 -paracyclophanes restores the fluorescence spectrum to that of the alkylbenzenes. These observations by Rice et al.115 illustrate the critical nature of the interplanar separation in determining the extent of interaction between -electron systems in the ground and excited configurations. 

The photodissociation of aromatic molecules does not always take place at the weakest bond. It has been reported that in a chlorobenzene, substituted with an aliphatic chain which holds a far-away Br atom, dissociation occurs at the aromatic C-Cl bond rather than at the much weaker aliphatic C-Br bond (Figure 4.30). This is not easily understood on the basis of a simple picture of the crossing to a dissociative state, and it is probable that the reaction takes place in the tt-tt Si excited state which is localized on the aromatic system. There are indeed cases in which the dissociation is so fast (< 10-12 s) that it competes efficiently with internal conversion. 1-Chloromethyl-Np provides a clear example of this behaviour, its fluorescence quantum yield being much smaller when excitation populates S2 than when it reaches Figure 4.31 shows a comparison of the fluorescence excitation spectrum and the absorption spectrum of this compound. This is one of the few well-documented examples of an upper excited state reaction of an organic molecule which has a normal pattern of energy levels (e.g. unlike azulene or thioketones). This unusual behaviour is related of course to the extremely fast dissociation, within a single vibration very probably. We must now... 

While luminescence in vapor-deposited matrices accordingly should be a powerful technique for detection and quantitation of subnanogram quantities of PAH in complex samples, it suffers from two major limitations. First, it is obviously limited to the detection of molecules which fluoresce or phosphoresce, and a number of important constituents of liquid fuels (especially nitrogen heterocyclics) luminesce weakly, if at all. Second, the identification of a specific sample constituent by fluorescence (or phosphorescence) spectrometry is strictly an exercise in empirical peak matching of the unknown spectrum against standard fluorescence spectra of pure compounds in a hbrary. It is virtually impossible to assign a structure to an unknown species a priori from its fluorescence spectrum qualitative analysis by fluorometry depends upon the availabihty of a standard spectrum of every possible sample constituent of interest. Inasmuch as this latter condition cannot be satisfied (particularly in view of the paucity of standard samples of many important PAH), it is apparent that fluorescence spectrometry can seldom, if ever, provide a complete characterization of the polycyclic aromatic content of a complex sample. 

Intramolecular Excimer Fluorescence Studies in Polymers Carrying Aromatic Side Chains. Some years ago, it was shown that certain excited aromatic molecules may form a complex with a similar molecule in the ground state, which is characterized by a structureless emission band red-shifted relative to the emission spectrum of the monomer. The formation of such complexes, called "exclmers", requires the two chromophores to lie almost parallel to one another at a distance not exceeding about 3.5A° (11). Later, it was found that Intramolecular excimer formation is also possible. In a series of compounds of the type C5H (CH2)jiC H5, excimer fluorescence, with a maximum at 340nm, was observed only for n 3 -all the other compounds had emission spectra similar to toluene, with a maximum at about 280nm (12). Similar behavior was observed in polystyrene solutions, where the phenyl groups are also separated from one another by three carbon atoms (13). 

The intensity of fluorescence observable from a given molecular species depends on the molar absorptivity of the transition excited and the quantum yield of fluorescence. The molar absorptivity determines the number of molecules ultimately populating the lowest excited singlet state. For most aromatic molecules, the rc, 7i absorption bands lying in the near-ultraviolet and visible regions of the spectrum have molar absorptivities of 1000-10,000, so that the proper choice of the transition to excite can affect the intensity of fluorescence by about one order of magnitude. 

The fine and resolved vibronic structure of the pyrene emission spectrum coupled with the low water-solubility of this aromatic molecule, make pyrene a cheap, efficient, and polymer-cost effective fluorescent probe for characterizing micellar cores (eg, dielectric constants). 

Chapter 7 introduces the reader to solutions of many selected problems in molecular physics. In particular, the following important problems are studied in detail the fluorescence spectrum ofp-terphenyl crystal, the vibrational fine structure of the spin-allowed absorption band of rans-[Co(CN)2(f )2]Cl3H20, and transport phenomena of electronic excitation in pentacene-doped molecular crystals. It is followed by an analysis of phosphorescence and radiationless transition in aromatic molecules with nonbonding electrons as well as predissociation of the 82 state of H2O+ by nonadiabatic interaction via conical intersection. 

The most common feature of a fluorescent molecule resulting from complexation with CD or non-fluorescent CD derivatives in aqueous media is intensity enhancement and spectral blue shift of its fluorescence spectrum. This is mainly due to the hydrophobic nature of CD cavity the feature is similar to that observed when the solvent medium is changed from water to less polar solvent. The reduced micropolarity of the fluorescent molecule inside a CD cavity also results in variations of relative intensities of the vibronic fine structures of the fluorescence spectra. Though numerous aromatic hydrocarbons exhibit this effect, the most intensively studied system is pyrene. The peak ratio (III/I) and fluorescence lifetime of pyrene increase when pyrene molecule moves from water to CD cavity [12]. 

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