Exitons

Baldeck and Andraud provide a chapter entitled Exitonic Coupled Oligomers and Dendrimers for Two-Photon Absorption, wherein the concepts of exci-tonic coupling are developed and their relevance to multi-photon absorption processes are described. 

The powerful role of the exitonic migration was proved on the basis of the luminescence and photosensitivity investigations [270]. The preliminary ultraviolet illumination of PAC increases the photosensitivity and decreases the luminescence. The experimental data are given in Fig. 40. One can see the redistribution of the maxima intensity in the spectra without changing their positions. Apparently ultra violet illumination promotes the photolysis of the weak coordination bonds. This leads to the changing of the polymer homolog content. Stimulated exciton dissociation on the ruptured bonds results in an increase in the photosensitivity and a luminescence decrease. The experiments carried out at 77 K show that in the luminescence spectrum of irradiated frozen PAC a new maximum appears with a position close to the phosphorescence maximum of diphenylbutadiene. So the rupture of weak coordination bonds under ultraviolet irradiation was proved. 

Huang, W.-T. (1973 ). Binding energy of exitonic molecules in isotropic semiconductors. Phys. Stat. Sol. B 60 309-317. 

In materials where the photogeneration involves the surface-enhanced dissociation of an exciton, as is generally the case for the phthalocanines, the photogeneration efficiency defined by Kanemitsu and Imamura represents the fraction of photons that create exitons that diffuse to the interface between the generation and transport layers. The injection efficiency then represents the fraction of pairs that dissociate into free electrons and free holes. The field dependence of the photogeneration efficiency was described by the Onsager theory. A primary quantum yield of 0.50 was reported. Values of the thermalization distance and the injection efficiency were not cited. 

Oresyl violet (Exiton) was used as received. Spectrograde acetonitrile, methanol and ethanol were used without further purification. Two or three samples of each solvent with different dye concentration in the order of 10" M were used for the measurements. No concentration effect on the time correlation function was observed. 

An interesting feature of system 3.123 is the finding that the two porphycene rings are well separated. As a result, there is no apparent barrier to rotation about the piperazine bridge. Further, the visible absorption spectrum of 3.123 ( max = 380 nm and 618 nm in CH2CI2) is rather unperturbed relative to the monomer 3.119 and displays no evidence of appreciable exitonic coupling between the rings." ... 

Enhancement of two-photon cross-sections by two-dimensional and three-dimensional arrangements of monomers has been demonstrated with fluorene V-shapes and dendrimeric structures. Such multidimensional structures lead to lower two-photon absorptions than Hnear oUgomers, but they have better one-photon transparencies. An accurate calculation of large exitonic systems is obtained by diagonalizing the Hamiltonian operator on a reduced basis set. 

The values of A in Table 23 show a maximum at about 13 e V, which can be attributed to exitonic absorption [49] other electronic processes in the ultra-violet spectral range are also described in [49],... 

Circular Dichroic Spectroscopy - Exiton Coupling in Organic Chemistry, University Press Oxford, 1983. 

Vossmeyer T et al 1994 CdS nanoclusters synthesis, characterization, size dependent oscillator strength, temperature shift of the exitonic transition energy and reversible absorbance shift J. Phys. Chem. 98 7665... 

Table 1. The summary of bleaching decay measurements for the PbS-doped phosphate glasses with mean diameters D of nanoerystals. ob is the background absorption of the samples oo is the absorption coefficient at the exitonic maxima (for the rest notations see the text).

The reduction of the inhomogeneous linewidth follows from Van Vleck s calculation of the second moment (AB ). It states that (AB > is proportional to N is here the number of nuclear spins. In an A - B pair, the number of nuclear spins is doubled in comparison to an isolated molecule. Therefore, the inhomogeneous linewidth is reduced by the factor 1 /-/z. The experimental values in naphthalene confirm this interpretation in the immediate neighbourhood of the A-M - B crossing points (a = 120° in Fig. 7.4), the experimental value for the ratio of the linewidths of the mini-exitons to those of the isolated molecules is ABm/ABab = 1/1.6. The width AB is thus an additional confirmation of the model for a mini-exciton it consists of the two molecules in a unit cell. At higher N-hg concentrations x = 10%), A-A-B and A-B-B mini-excitons have also been observed [4]. 

Kopeknan, R. The exitons dynamics resembling fractal one geometrical and energetic disorder. In book Fractals in Physics. Ed. Pietronero, L. Tosatti E. Amsterdam, Oxford, New York, Tokyo, North-Holland, 1986, 524-527. 

Similar through-space asymmetric polymerization from achiral mono-, di-, or tri-thiophenes and pyrrole monomers was also achieved by the use of cholesteric liquid crystals as an asymmetric reaction solvent [19]. As no reaction occurred between the molecules of the liquid crystal and the monomers, the chiral morphology of the polymers (which have no chiral substituent) is considered to derive from the asymmetry produced by the chiral liquid crystal medium during polymerization. Heat treatment of the polymer causes disaggregation and a loss of chirality, and polymers prepared in this way exhibit an exiton splitting signal in the circular dichroism spectra in the absorbance region of the polymeric backbone they also display a circular polarized luminescence. A representative example is shown in Scheme 8.2 [19]. 

Nakanishl and co-workers have continued to develop the exiton chirality method. The remarkable additivity relationship in the coupled c.d. of pyranose p-methoxycarbonylbenzyl ethers has permitted the identification of constituent sugars and their glycoside... 

Kepler carried out fluorescence quenching studies and concluded that the migration of the exiton is the most probable energy transfer mechanism of poly(vinyl carbazole). He, furthermore, suggested that the exiton can visit one thousand monomer units during its lifetime. This is a distance of about... 

K. Nakanishi and N. Harada, Circular Dichroism Spectroscopy Exiton Coupling in Organic Stereochemistry, University Science Books, Mill Valley, California, 1983 D. N. Kirk, Tetrahedron 42, 111 (1986). 

The macromolecular crystals of interest to the present review are largely polyethylene-like, so that electronic defects, such as electron holes, interstitial electrons, and exitons, need not be considered. 

Fig. 4.17. Variation of the bandgap (exiton energy) of CdS nanocrystals as a function of size compared with results from effective mass and tight binding approximations (reproduced with permission from [763])...

Figure 5 Structural analysis of supamolecules by using the exciton chirality method. The directions of the chromophores for the exiton chirality method (in bine) are represented by the doubleheaded arrows.

To conclude the discussion of exciton theory, we present a brief account of the application of PCM-based exiton methods to studies of spectral shifts in bacteriochlorin (BC) dimers [57, 66]. This work was motivated by earlier exciton studies of a model methyl bacteriopheophorbide a (Me-BPheo-a) dimer reported by Scherz and Parson [67]. Using the point-dipole model to approximate monomeric charge and transition densities [54-56], they obtained a large (1330 cm" ) red shift of the band in a dimer whose geometry was defined as a translation of one monomer relative to the other by Ax = 0.5 A, Ay = 3.5 A, and Az = 3.6 A along the axes in Fig. 4. 

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