Excimer emission

Different aromatic hydrocarbons (naphthalene, pyrene and some others) can form excimers, and these reactions are accompanying by an appearance of the second emission band shifted to the red-edge of the spectrum. Pyrene in cyclohexane (CH) at small concentrations 10-5-10-4 M has structured vibronic emission band near 430 nm. With the growth of concentration, the second smooth fluorescence band appears near 480 nm, and the intensity of this band increases with the pyrene concentration. At high pyrene concentration of 10 2 M, this band belonging to excimers dominates in the spectrum. After the act of emission, excimers disintegrate into two molecules as the ground state of such complex is unstable. 

Inclusion complexes have also been shown to aid excimer formation provided that the cavity dimensions and polarity are appropriate. Thus Ueno et al. (1980) have shown that the lipophilic cavity of y-cyclodextrin can accommodate two molecules of sodium (l-naphthyl)methyl acetate and that this leads to enhanced excimer emission. Excimer formation by diaryl-methylammonium salts is promoted by y-cyclodextrin but not by a- and P-cyclodextrins (Emert et al., 1981a). The best results were obtained with compounds having fairly large aryl groups e.g. 4-biphenylyl and 1-naphthyl. The hydrophobic character of these groups helped to solubilise the compounds in the interior of the cyclodextrin. 

In addition to the antennae effect, dendrimers are also effective insulators and exhibit the shell effect. In providing a dense shell around the incorporated chromophores, dendrimers effectively prevent aggregation which leads to non-emissive excimers and self-quenching that occurs when chromophores with small Stokes shifts are within short distances of one another. This shell effect allows for increased photoluminescence efficiency of the enclosed chromophore, which is important for optoelectronic devices. 

Some aromatic hydrocarbons, for example pyrene and perylene, also crystallise in a herringbone pattern however, the structural units of these lattices are pairs of molecules which form sandwich-like dimers (see Figs. 2.12 and 2.13 as well as Table 2.4). The two partners are bound together only weakly and absorb light like monomers, but they flouresce as dimers. This interesting phenomenon is called excimer emission (excimer stands for excited dimer ) more will be said about this topic later in Chap. 5. By the way, perylene (see also Table 2.4) crystallises not only in the dimer-like a phase but also in a jS phase (Fig. 2.13). In the latter, which... 

Transient absorption measurements of the TIPS-tetracene solutions revealed the presence of three distinct states in concentrated solutions. An intermediate was observed that displayed a lifetime identical to the emissive excimer from PL. Formation of the intermediate quenched the singlet exciton emission with a 70 ps lifetime and led to subsequent triplet exciton formation in a 120% yield. The free triplet exciton population rose over the first ten nanoseconds, consistent with thermal dissociation of the excimer intermediate over the endothermic energy barrier. 

The low quantum yields in planar conjugated PHT can be explained by classical concentration quenching effects which arise from non-emissive excimer complexes intermolecular decay channels) [100],... 

Excimer formation has been found to be most prominent for small organic aromatic molecules such as benzene [13],p-xylene [14], naphthalene [15], anthracene [16], pyrene [17], perylene [18], stilbene [19], and others [20]. Some polymers that contain aromatic groups, such as polystyrene [21] and poly (ethylene terephthalate) (PET) [22], and polynucleotides such as cytosine and thymine [23] have also been shown to exhibit excimer emission. Excimer emission is, in fact, widely observed in aromatic hydrocarbons [20]. 

The potential of LA-based techniques for depth profiling of coated and multilayer samples have been exemplified in recent publications. The depth profiling of the zinc-coated steels by LIBS has been demonstrated [4.242]. An XeCl excimer laser with 28 ns pulse duration and variable pulse energy was used for ablation. The emission of the laser plume was monitored by use of a Czerny-Turner grating spectrometer with a CCD two-dimensional detector. The dependence of the intensities of the Zn and Fe lines on the number of laser shots applied to the same spot was measured and the depth profile of Zn coating was constructed by using the estimated ablation rate per laser shot. To obtain the true Zn-Fe profile the measured intensities of both analytes were normalized to the sum of the line intensities. The LIBS profile thus obtained correlated very well with the GD-OES profile of the same sample. Both profiles are shown in Fig. 4.40. The ablation rate of approximately 8 nm shot ... 

The excimer emission occurs from an excited associated complex (D ) formed between a species in the excited singlet state (5 ) and a similar ground-state (So) species. The excimer is also called a dimer and is shortlived. 

The electroluminescence spectra of the single-layer devices are depicted in Figure 16-40. For all these OPV5s, EL spectra coincided with the solid-state photoluminescence spectra, indicating that the same excited states are involved in both PL and EL. The broad luminescence spectrum for Ooct-OPV5-CN" is attributed to excimer emission (Section 16.3.1.4). 

The presence of a critical St content in ASt-x can also be seen in fluorescence spectra [29], This copolymer in aqueous solution shows an excimer emission peaking at 325 nra. As shown in Fig. 8, the intensity of the excimer emission increases, while the monomer emission decreases, with increasing St content. Eventually the excimer dominates the monomer emission at an St content of 72 mol%. The excimer emission becomes apparent at an St content of about 50 mol%, which agrees with the critical St content estimated by viscometry and NMR spectroscopy. The existence of the critical St content suggests the hydro-phobic self-aggregation to be a cooperative process. 

EL), conjugated polymers are also of interest as materials for optically or electrically pumped stimulated emission. For effects of this type, the ratio of stimulated emission to photoinduced absorption (PA) is of particular interest for conjugated polymers. In this context, the orign of the PA is controversial the PA can be a result of the formation of either charge-separated polaron pair -states or excimers. Initial experiments support the conjecture that LPPP 26 is significantly superior [49], as the stimulated emission of LPPP 26 is markedly more intense than that of PPV under comparable conditions. 

Additional support for this disassembly mechanism was obtained by monitoring the release of the pyrene tail units by fluorescence spectroscopy. The confined proximity of the pyrene units in the dendritic molecule results in formation of excimers. The excimer fluorescence generates a broad band at a wavelength of 470 nm in the emission spectrum of dendron 31 (Fig. 5.25). Upon the release of the pyrene units from the dendritic platform, the 470 nm band disappeared from... 

Excimers are often characterized by a broad emission band containing no vibrational structure, occurring at longer wavelengths than emission corresponding to the monomeric singlet state/41,87-89,71-73 ... 

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