Excimer emission from crystals

Figure 6.6 Excimer emission from crystalline planar aromatic hydrocarbons and types of crystal lattice.

FIGURE 10. Schematic diagram showing the source of the difference in energies of excimer emissions from pair (P) and stack (S) crystal structures. 

Fig. 6.19 Excimer emission from a pyrene crystal. At 297 K, in the region of the absorption edge of the crystal at 26 595 cm , one can see a weak signal which corresponds to the 0,0 transition of the monomeric exciton emission of pyrene. From [42].

Birks 68) has proposed that the only change between the unexcited and excited pyrene pair is a reduction in the interplanar distance from 3.53 to 3.37 A, i.e. that the pyrene excimer is not a completely eclipsed sandwich pair either in solution or in the crystal. This proposal is consistent with the observed similarity of the excimer band position for the crystal and solution environment, and with the emission of excimer fluorescence from the crystal even at 4 K. For naphthalene, the greater separation and the nonparallel structure of nearest-neighbor pairs in the crystal apparently prohibits the formation of the sandwich excimer during the naphthalene singlet monomer lifetime. Thus, no excimer fluorescence is observed from defect-free naphthalene crystals. 

The geometrical requirements for excimer emission are stringent. In crystals the molecules must be stacked in columns or arranged in pairs with an interplanar distance of less than 3.5 A.31 Chandross and Dempster carried out an elegant investigation of the geometrical requirements for excimer formation in solution. They studied the fluorescence of compounds 6-10 in methylcyclohexane solution and found that only compounds 7 and 9 exhibit strong emission from an intra-... 

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. 

Excimer formation has been studied in polystyrene and poly(a-alkylstyrenes)189 (PS), poly(vinylcarbazole),139>140 poly-(2-vinylnaphthalene), and poly-(4-vinyl-biphenyl).141 For polystyrene films, David et a/.189 showed that the fluorescence yield increased with increasing crystallinity, at both ambient temperature and 77 K. The contribution of excimer fluorescence yield increased in the sequence atactic (0.7) < atactic oriented (0.60) < isotactic amorphous (0.28) < isotactic crystallized (0.01), with normal yields relative to excimer given in parentheses. Similar results were obtained for poly(vinylcarbazole), PVCZ, although the contribution of excimer fluorescence at 77 was independent of crystallinity. The results can be interpreted in terms of electronic energy migration to low-energy defect sites from which excimer emission can occur. In PVCZ copolymers with fumaronitrile (10), diethyl fumarate (11), and diethyl maleate (12) (Scheme 6),... 

We can therefore conclude that such flexible molecules are intrinsically capable of forming excimers. Further, from studies of the exclmer emissions from these crystals we can derive such basic parameters of the excimer as its binding energy, and can obtain information on its geometry, as has been done for pyrene and perylene. 

Fig. 6.18 Potential curves and an emission transition for excimers in a crystal. Absorption occurs at the equilibrium spacing in the ground state and leads to the excited state S-. From there, a radiationless relaxation into the excimer state with the smaller equilibrium spacing tex takes place. The excimer emission...

Excimer emission is observed from many crystals when they are subjected to pressure or when they are strongly deformed in some other manner. In the deformed crystals, molecular configurations are produced which favour excimer emission. Through annealing, these defects can often be removed again. Also in vapour-deposited films which are initially amorphous at low temperatures, excimer structures can frequently be formed by a suitable annealing process. 

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.

The information on the formation of a polaron or an excimer is derived from the low temperature electronic absorption and emission spectra of the reactive crystals. The strong electron-phonon coupling in the reactive state manifests itself as a very strong phonon-side band In the liquid helium temperature spectra. 

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