Luminescence monomer emission

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... 

Figure 7.32 Kinetics of luminescence of pyrene following laser flash excitation. L, laser pulse profile M, monomer emission, E, excimer emission rise and decay. Horizontal axis, time in ns vertical axis, light intensity in arbitrary units. The three kinetic curves are normalized to a common maximum...

Kozhevnikov et al. observed that the luminescence colour of vitrified mesophase of liquid-crystalline N,C,N-coordinated platinum(II) complexes (Figure 2.32) is different from that observed for a film of the same compound obtained by fast cooling from the isotropic phase. The samples that were fast cooled from the liquid crystal phase displayed monomer emission, whereas the samples fast cooled from the isotropic state showed excimer-like emission. Spin-coated thin films exhibited excimer-like emission, whereas heat treatment of the sample to 110 °C followed by cooling to room temperature resulted in a drastic change of the luminescence colour from the red of the excimer to the yellow of a mixture of the monomer and the excimer. However, rubbing of the heat treated film resulted in a return of the red excimer emission. 

Organic exciplexes are best identified by studying the variation of their emission spectra with concentration. This is illustrated in Figure 19 for pyrene (py). At low pyrene concentrations (<10 M), the major luminescence band is due to the monomer. The characteristics of the monomer emission include (i) vibronic structure is present, and (ii) the emission profile is independent of concentration at concentrations <10 M. As the pyrene concentration is increased above 10 M, the monomer emission is quenched and a new lower-energy emission band appears due to the formation of a [py-py] exdmer. The intensity of the excimer band increases with a concentration increase. The characteristics of the... 

In most cases, the interaction between excited and ground state components in a supramolecular system, and even more so in an encounter, is weak. When the interaction is strong, new chemical species, which are called excimers (from excited dimers) or exciplexes (from excited complexes), depending on whether the two interacting units have the same or different chemical nature. The scheme shown in Fig. 2.3 refers to a supramolecular system, but it holds true also for species in an encounter complex. It is important to notice that excimer and exciplex formation are reversible processes and that both excimers and exciplexes sometimes can give luminescence. Compared with the monomer emission, the... 

Figure 3.1 Analytical working curve for a self-indexed luminescent thermometer based on the ratio between the measured excimer (E, 475 nm) and monomer (M, 375 nm) emission bands of l,3-b/s(l-pyrenyl)propane in [C4Cjpyr][Tf2Nj. The optical thermometer is perfectly reversible in the temperature range shown and highly precise, with the measured uncertainties in the ratio (1 /1 ) falling well within the symbol dimensions. The dashed curve represents the temperature uncertainty predicted from explicit differentiation of a sigmoidal fit to the calibration profile 5T = 0T/0R 5R where R = I /Iu- (Reprinted from Baker, G.A., Baker, S.N., and McCleskey, T.M., Chem. Commun., 2932-2933, 2003. Copyright 2003 Royal Society of Chemistry. With permission.)...

Weller and Zachariasse thoroughly investigated exciplex formation and luminescence for donor acceptor systems in THF [18]. A particularly interesting result from their work came from an examination of the temperature dependence of radiative charge recombination between 9,10-dimethylanthracene anion (DMA") and TPTA+ in THF [19]. They found that both exciplex emission and fluorescence from DMA were observed in solution at low temperature (ca. —50°C). As the solution temperature is raised, the excimer emission decreases in relative intensity, and at room temperature the emission is nearly completely DMA fluorescence. The monomer-to-exciplex emission intensity ratio as a function of temperature follows Arrhenius kinetic behavior and yields an activation barrier that is nearly the same as the energy gap between the exciplex and the DMA states. Thus, their model consisted of reaction of the solvent-separated ions to form an intimate emissive ion pair which could dissociate to yield the singlet anthracene derivative. 

The preceding analysis and review of the literature indicate a need for additional types of experiments to study triplet mobility in polymers. One experiment which has been particularly useful in studies of singlet energy migration in polymers, involves determination of the quenching of donor emission by a known mole fraction of a copolymerized luminescent quencher (26). We have extended this approach to the study of triplet states. The polymers chosen for study are homopolymers of isomeric acetonaphthyl methacrylates (aceto-NMA, 1). A related monomer, 2,4-diaceto-l-naphthyl... 

The formation of a luminescent exciplex between fumarate and phenanthrene, recently questioned, has been confirmed.181 Quenching of the exciplex emission by electron donors without quenching of the phenanthrene monomer fluorescence and a parallel quenching of cycloadduct formation confirm the role of the exciplex in product formation. 

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