Luminescence excimer emission

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). 

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...

Studies continue on the effects of a polymer host matrix on the excited-state properties of guest molecules. For example, the lifetime of excited singlet-state species may be greatly prolonged through restrictions of molecular motions (Gusten and Meisner, inter alia). Accurate information on the blend miscibility of polymers is provided by studies of excimer emission (Mikes et al.), and George et al. claim that the service life of many polymers can be predicted from their luminescence properties see also Martin. 

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. 

Optical temperature sensor (opt(r)odes) based on the viscosity-dependent intramolecular excimer formation of l,3bi(l-pyrenyl)propane in [C4mypr][Tf2N] have been developed [33], The relative intensity of the excimer emission was found to gain in intensity with higher temperature. This has been attributed to the generally low viscosity of the ionic liquid. The working temperature of this luminescence thermometer is between 25°C and about 150°C. 

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. 

Pyrene photophysics has produced the usual, and now to be expected, crop of papers on diverse topics. Aggregation in concentrated solutions has been evidenced by the Shpol ski effect and two photon excitation spectra have yielded new electronic state assignments. The maximum entropy method, mentioned earlier, for the determination of fluorescence lifetimes shows that in dipyrenylpropane the data for luminescence decay do not fit a simple 3-state model.This study adds more information upon a system hitherto open to much dispute. It is unlikely that the problem will be considered as solved. The presence of a ground state dimer of pyrene moieties has been shown by NMR in the bichromophoric molecules of (pyrenylcarboxyl) alkanesand also with racemic and meso dipyrenyl alkanes. Strong circular polarization in excimer emission has been detected from pairs of pyrene groups linked to a polypeptide chain. ... 

Many other piezochromic or mechanochromic organic materials are now known, some of which (23, 24, 25 and 26 ) are shown in Scheme 7. It is worth noting that the mode of applied stress can be distinguished by the different luminescent response. The yellow luminescence (2em = 556 nm) of a tetrathiazolylthiophene (27) crystal is blue-shifted to green (Agm = 490 nm) by anisotropic grinding but is red-shifted to orange (Agm = 609nm at 3.2 GPa) under isotropic compression, which is explained by decreased and increased contributions of excimer emission, respectively. 

Figure 19 Excimer emission of pyrene. The upper curves show the potential surfaces of the ground and the luminescent excited electronic state (Reproduced with permission from Turro NJ (1978) Modem Molecular Photochemistry, p 141. Menlo Park Benjamin/Cummings.

Figure 21 Potential energy diagram of the ground and the first excited electronic states of [Ag(CN)32 (eclipsed configuration) as plotted from extended Huckel calculations. The excimer [Ag(CN)32 corresponds to the potential minimum of the excited state. The optical transitions shown are (a) excimer emission, (b) solid state excitation and (c) dilute solution absorption. (Reproduced with permission from Omary MA and Patterson HH (1998) Luminescent homoatomic exciplexes in dicyanoargentate 0) ions doped in alkali halide crystals 1. Exciplex tuning by site-selective excitation. Journal of the American Chemical Society 120 7606-7706.

Triplet states can also be detected by excimer emission, and the study of luminescence transients [3]. 

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