Excimer geometry

Fluorescence Rise and Decay Curves. Both monomer and excimer fluorescence decay curves of the unirradiated film are nonexponential and the excimer fluorescence shows a slow rise component. This behavior is quite similar to the result reported for the PMMA film doped with pyrene. (23) A delay in the excimer formation process was interpreted as the time taken for the two molecules in the ground state dimer to form the excimer geometry. Dynamic data of the ablated area observed at 375 no (monomer fluorescence) and 500 nm (exciner fluorescence) are shown in Figure 5. When the laser fluence increased, the monomer fluorescence decay became slower. The slow rise of the excimer fluorescence disappeared and the decay became faster. 

Top uiew of the possible excimer geometries and related photoproducts of meso-DlNEE (left) and rac-DlNEE (right) derived from molecular models... 

In the photoinduced singlet dimerization of indene shown in Scheme 55, the endo head-to-head dimer expected from the more favorable excimer geometry is the preferred product. With electron-transfer sensitization, the less sterically hindered exo head-to-head dimer is formed (Farid and Shealer, 1973). 

Fig.i.Space filling representation of the extended (C5) and folded (Cy) conformation of 2ae.In the folded conformation the pyrene group are depicted in the excimer geometry. 

This particular case of the cresols illustrates the point that, in some cases, excimer fluorescence may not be excited at the shorter absorption wavelengths. Emission from the excimer states may require absorption by ground state conformations close to the excimer geometry this is especially true where the lifetime of the monomer is short compared with the time required for diffusional encounter. It seems likely that the humic substances, because they are aggregates, provide many possibilities for achieving these conformations, since chromophore concentration is locally high within the aggregates. 

Ito et al.(1981, 1982) reported also on the photophysical properties of meso and racemic A3. The same observation as before is made when the yield of excimer formation of meso A3 is compared with the yield in racemic A3. Meso attains the excimer geometry more readily, assuming that the radiative rate constant of the excimer and the stabilisation of the excimer are of comparable magnitude. Ito et al. (1981) were able to analyse the fluorescence spectra and decay curves of meso A3 and racemic A3 within scheme I. The activation energy for excimer formation equals 20.1 and 23.0 kJmol for meso A3 and racemic A3 respectively. Large differences in excimer formation rate constants were reported (7.9 10 s 8.7 10 s l at 298 K meso A3 racemic A3). 

These differences were correlated with the conformational changes required for excimer formation (Ito et al. 1982). In meso A3 the excimer conformation is formed via a one rotation process. In racemic A3 more than one rotation takes place before the excimer geometry is reached. 

For both compounds the excimer geometry is supposed to be the full overlap geometry. A more complete analysis of ground state conformations and excited state behaviour of meso and racemic A3 seems necessary when the data on 1,1 -di(2-naphthyl)diethyl ether are considered. 

A conformation is represented with the matrix (0p 0, 0 >-02) The excimer geometries are defined in figure 5. 

Figure 5. Excimer geometries in B3. P partial overlap, E = eclipsed overlap.

These dipeptides are present in two sets of ground state conformations C5(S)which doesf not allow a rotation of the side chains to an excimer geometry within the lifetime of excited pyrene and Cj(f) in which the side chains are at the same side of the molecule enabling excimer (E) formation. 

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