Excimers emission

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. 

Fig. 7 Fluorescence ratiometry of monomer/excimer emissions in a through space PET system (reprint from ref. [59], Copyright 2005 American Chemical Society)...

A. Weller and K. Zachariasse 157-160) thoroughly investigated this radical-ion reaction, starting from the observation that the fluorescence of aromatic hydrocarbons is quenched very efficiently by electron donors such as N,N diethylaniline which results in a new, red-shifted emission in nonpolar solvents This emission was ascribed to an excited charge-transfer complex 1(ArDD(H )), designated heteroexcimer, with a dipole moment of 10D. In polar solvents, however, quenching of aromatic hydrocarbon fluorescence by diethylaniline is not accompanied by hetero-excimer emission in this case the free radical anions Ar<7> and cations D were formed. 

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

Si - Si Annihilation and Ablation Mechanism. The Si - Si annihilation process is responsible to laser ablation, which was supported by the following experiment. Total fluorescence intensity and the relative intensity of excimer emissions (-15 72 ns gate width) were plotted against the fluence in Figure 3. It is interesting that the relative contribution of excimers showed a similar change to that of total fluorescence intensity. This indicates that the Si - Si annihilation has an important role in the primary processes of laser ablation phenomena, since the relative contribution of excimers is determined by the degree of Si -Si annihilation, and the suppressed fluorescence intensity corresponds to the enhanced ablation. 

The total fluorescence intensity saturated around a few hundreds of mJ/cm2 which corresponds to the irradiation condition where the new plasma-like emission was observed. Above this value fluorescence intensity decreased, which is accompanied with the recovery of the relative intensity of excimer emissions. This means that a quite efficient deactivation channel of excitation intensity opens in this energy range, and the contribution of Si -Si annihilation is depressed. This suggests that fragmentation reactions to diatomic radicals are not induced by the annihilation process. Multi-photon absorption processes via the Si states and chemical intermediates should be involved, although no direct experimental result has as yet been obtained. 

Figure 3. Fluence-dependences of relative intensity of two excimer emissions and fluorescence intensity.

Only excimer emissions were observed during laser pulse, a broad plasma-like emission was detected later, and fragmented radicals became distinct. Ablation behavior can be interpreted in terms of photophysical and photochemical processes, including Si - Si annihilation. 

In a film, however, molecular mobility is severely limited, so that excimer fluorescence must arise mainly from pairs or groups of pyrene molecules that were approximately in the excimer configuration when the film was cast. Thus, the intensity of the excimer emission is also an indication of the local concentration of pyrene in the cast film. If the pyrene aggregates, we expect that the excimer fluorescence would increase with aggregation. This system can be used to look at the aggregation of very low concentrations of a small molecule dye in a polymer film, and potentially detect molecular aggregation before it would be observable by other tech-... 

The energy change associated with the excimer emission is smaller than that for the monomer emission and so the excimer emission will occur at longer wavelengths than the monomer emission. 

Vibrational fine structure is absent from the excimer emission because the Franck-Condon transition is to the unstable dissociative state where the molecule dissociates before it is able to undergo a vibrational transition. In the case of the monomer emission, all electronic transitions are from the v = 0 vibrational level of M to the quantised vibrational levels of M, resulting in the appearance of vibrational fine structure. 

Fluorescent molecule (1) (Figure 6.5) acts as a host for Ca2+ ions. On addition of Ca2+ ions, the flexible polyether chain folds, leading to stacking of the anthracene fluorophores, and monomer emission is replaced by excimer emission. 

Fig. 8 Typical excimer probes utilizing two chelating sites and two fluorophores (20), a flexible central composite receptor site and three fluorophores (21) and a single receptor site with two pendant arms and two fluorophores (22). (a) Na+-induced excimer alignment in 20 and (b) respective spectroscopic response (c) selective probes for Fe3+ (21) and Cu2+ and Ni2+ (22) that show quenching of monomer and excimer emission upon binding. Color code fluorophores in red and atoms responsible for coordination in blue. (Reprinted in part with permission from [83]. Copyright 1995 American Chemical Society)...

Yang JS, Lin CS, Hwang CY (2001) Cu2+-induced blue shift of the pyrene excimer emission a new signal transduction mode of pyrene probes. Org Lett 3 889-892... 

When excited, the molecules of organic dyes tend to form complexes with unexcited molecules like themselves. These excited dimeric complexes are called the excimers. The excimer emission spectrum is easy to observe because it is very different from that of a monomer. It is usually broad and strongly shifted to longer wavelengths, and it does not contain vibrational structure. If the excimer is not formed, we observe emission of the monomer in the fluorescence spectra, and upon its formation there appears a characteristic emission of the excimer. 

A first generation poly(amido amine) dendrimer has been functionalized with three calyx[4]arenes, each carrying a pyrene fluorophore (4) [30]. In acetonitrile solution the emission spectrum shows both the monomer and the excimer emission band, typical of the pyrene chromophore. Upon addition of Al3+ as perchlorate salt, a decrease in the excimer emission and a consequent revival of the monomer emission is observed. This can be interpreted as a change in the dendrimer structure and flexibility upon metal ion complexation that inhibits close proximity of pyrenyl units, thus decreasing the excimer formation probability. 1H NMR studies of dendrimer 4 revealed marked differences upon Al3+ addition only in the chemical shifts of the CH2 protons linked to the central amine group, demonstrating that the metal ion is coordinated by the dendrimer core. MALDI-TOF experiments gave evidence of a 1 1 complex. Similar results have been obtained for In3+, while other cations such as Ag+, Cd2+, and Zn2+ do not affect the luminescence properties of... 

The decay of monomer emission is thus a sum of two exponentials. In contrast, the time evolution of the excimer emission is a difference of two exponentials, the pre-exponential factors being of opposite signs. The time constants are the same in the expressions of iM(t) and iE(t) (/ , and fl2 are the eigenvalues of the system). The negative term in iE (t) represents the increase in intensity corresponding to excimer formation the fluorescence intensity indeed starts from zero because excimers do not absorb light and can only be formed from the monomer (Figure 4.8A). 

E-3 (Figure 10.26) is the first example of an ionophoric calixarene with appended fluorophores, demonstrating the interest in this new class of fluorescent sensors. The lower rim contains two pyrene units that can form excimers in the absence of cation. Addition of alkali metal ions affects the monomer versus excimer emission. According to the same principle, E-4 was designed for the recognition of Na+ the Na+/K+ selectivity, as measured by the ratio of stability constants of the complexes, was indeed found to be 154, while the affinity for Li+ was too low to be determined. 

In calixarene-based compound M-8 (Figure 10.28), bearing four anthracene moieties on the lower rim, some changes in fluorescence intensity were observed on binding of alkali metal ions but no excimer emission was detected. Quenching of the fluorescence by Na+ may arise from interaction of four anthracene residues brought in closer proximity to one another enhancement of fluorescence by K+ is difficult to explain. 

PC3P is a sensitive probe for local viscosity measurement by forming an intramolecular excimer [35,36]. The extent of excimer emission depends upon the rate of conformational change of the chain linking the two... 

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