Phosphorescence from films

Figure 3. Phosphorescence spectra from films. Intensities are not comparable the unirradiated films give the same spectra, except for intensity, from Kx 260 and kex 380 nm.

One reason for this preoccupation with the physical state concerns the ease with which triplet excimer formation occurs. Phosphorescence from triplet excimers, for example, is common in solid polymeric films but much less common in rigid solutions. For this reason, the interpretation of rate processes involving triplet states tends to be simpler to handle and more susceptible to quantitative treatment for rigid polymer solutions. 

Figure 4.9 Schematic drawing of near-field optical probes. By pulling out an optical fiber and coating with aluminum, an optical probe with a very small aperture (<100 nm) is created. The probe can detect fluorescence or phosphorescence from objects in the near field close to the aperture, such as a thin film or flat surface (a) or from a labeled molecule (b). The probe is relatively insensitive to light in the medium in the far field further from the tip. The excitation light source can be transmitted through the probe or externally.

The modulation technique mentioned above has been used to identify triplet excimers in 1,2-benzanthracene and 1,2 3,4-dibenzanthracene at high solute concentrations167 and the differences between luminescence from naphthalene in fluid solution in the temperature range 353—173 and naphthalene in a rigid solution at 77 have been ascribed to phosphorescence from a triplet excimer.168 Excimer formation in solid poly-(2-vinylnaphthalene) and polystyrene is found to be dependent on the temperature at which the film is cast, and a statistical model based on the rotational isomeric state approximation has been used to formulate an expression for the fraction of excimer sites in the solid systems.168 Kinetic equations for dimer formation and decay, based on the statistical mechanics of ideal gases, have been obtained. These equations, derived from the N-atom von Neumann equation, take into account both bimolecular and termolecular equations.157 158 160... 

On the basis of the overall results, an energy diagram for CT-allowing AN-s-BPDA-AN can be depicted as shown in Fig. 19. In the proposed mechanism, the intermolecular CT fluorescence emission occurs via two different pathways first, the photoinduced electron transfer from a local excited state at the biphenyldiimide unit to the spatially adjacent ground-state PDA residue, and second, the direct excitation at the CT absorption band. The first process is possible even at very low CTC concentration as in the PI film cured at a low temperature such as 200°C. Such a photoinduced electron transfer mechanism will be theoretically discussed again later. In fully aromatic s-BPDA-PDA, both the fluorescence from Si (tt, tt ) and phosphorescence from Ti (tt, tt ) are not observed practically. The results are probably attributed to the considerably fast CT process from Si (it, tt ). 

Merrill and Roberts (Z) have examined both PET films and fibers and have attributed the fluorescence (excitation 342 nm, fmision 388 nm) to a 1(n,n ) transition. They have proposed a (n,ir ) transition, since the observed fluorescence is at lower energy than the observed phosphorescence (excitation 313 nm, emission 452 nm, 1.2 sec), which they have proposed from a (tt,tt ) state. 

FIGURE 3.18 Normalized EL spectra of the white PHOLED at several current densities upper insert Absorption of neat CBP films (100 nm) and lower insert A White PHOLED structure (left) and the chemical structures of phosphorescent dopants (right). (From D Andrade, B.W., Brooks, J., Adamovich, V., Thompson, M.E., and Forrest, S.R., Adv. Mater., 14, 1032, 2002. With permission.)... 

The water lattice may be an important element in forming the ordered thymine structure necessary for dimerization, as pointed out by Beukers and Berends.37 Thymine can crystallize from solution as a monohydrate (a real hydrate)38 in whose crystal lattice one thymine is directly above another. The influence of humidity upon dimer yield in dry films may be connected with monohydrate formation, and monohydrate formation in frozen solutions may be the reason for the almost theoretically maximum quantum yields for dimer formation.31 The possible existence of aggregates in frozen aqueous solutions is supported by a tenfold increase in purine phosphorescence at 44°K produced by the presence of 1% ethanol and by a blue shift of excitation and emission spectra.39... 

Figure 18 Phosphorescence spectra of a 1 1 (by weight) acrylate-based TMPTA/ PMMA 0.25-mm thin film containing an acetophenone photoinitiator and fac-ClRe(CO)3(4,7-Ph2-phen) as a function of UV exposure (A) 0 s, (B) 5 s, (C) 10 s, (D) 20 s, (E) 30 s, and (F) 60 s. Excitation wavelength is 420 nm. (From Ref. 100.)...

Triplet exciton energy transfer from a thin film of phosphorescent molecules (Ir complexes) to a monolayer of CdSe/ZnS core-shell QDs acting as acceptor was... 

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