Dimers conjugated chains

In the following section, step by step a qualitative picture is formed describing the impact of intcrmolecular interactions oil the absorption and luminescence of organic conjugated chains. The present calculations do not distinguish between dimers and aggregates (for which the wavefunctions of adjacent chains interact in the ground state, due to, for instance, solid-state effects) and excimers (where overlap occurs only upon photoexcitation) [29]. 

Early LEED studies [97] indicated that the basic structure of a clean 100 surface was 2x1. (For a discussion of the nomenclature of surface crystallography, see ref. 113.) Three different models have been proposed to account for this form of reconstruction the vacancy model [78, 79, 114] assumes that half the surface atoms are missing, while the other two suggestions invoke surface dimerization [80, 114] and the formation of complex conjugated chain structures [115, 116], respectively. We will consider each of these in a little more detail. 

The vacancy, dimer and conjugated chain models are illustrated in Fig. 8, by reference to a non-reconstructed 100 surface [Fig. 8(a)], where, in each case, the top two layers of atoms are shown. The two-fold periodicity then results either from removal of alternate rows of first layer atoms, which can be parallel [Fig. 8(b)] or perpendicular [Fig. 8(c)] to the plane of the first-to-second layer bonds, or by displacement of adjacent rows of first layer atoms in opposite directions [Fig. 8(d)]. The conjugated chain model [Fig. 8(e)] described by Jona et al. [116] derives from an idea proposed by Seiwatz [115] in which each atom forms a bonds with neighbouring atoms in the chain, a bent bond with the nearest... 

Fig. 8. Models of Si l00 surfaces. Surface unit mesh shown by broken lines, (a) Unreconstructed surface (b) vacancy model (i) (c) vacancy model (ii) (d) dimer model (e) conjugated chain model.

This result underlines the major impact of the sulfur atoms of the alkylsulfanyl groups on the steric distortion of the jt-conjugated chain [69]. Further work based on crystallographic, electrochemical and spectroscopic analysis of bis-EDOT, bis-EDST and mixed EDOT-EDST dimers has confirmed this fact. Whereas the solid-state structures of bis-EDOT and EDOT-EDST are planar and almost planar, respectively, bis-EDST presents a median torsion angle of 45° [70]. 

Figure 4-6. Evolution of the INDO/SCI-calculalcd. splitting between the lowest two oplieal transitions of cofacial dimers formed by two PPV chains as a function of the inverse number of bonds (1/u) along the conjugated backbone of the oligomer. The theoretical results are reported for interchain distances of 4 A (open circles) and C> A (tilled circles).

The introduction of substituents, which have a mesomeric effect and extend the chain of conjugation, at the a-C atom of nitronates leads to a shift of the absorption 260 to a maximum of 320 nm. It should be noted that the UV spectra of dimers of dialkylboron nitronates (entry 8) are virtually identical to those of other types of nitronates. 

Deng, L, Wang, C., Spencer, E., Yang, L, Braun, A., You, f., Slaughter, C., Pickart, C., and Chen, Z. J. Activation of the IkappaB kinase complex by TRAE6 requires a dimeric ubiquitin-conjugating enzyme complex and a unique polyubiquitin chain, Cdl 2000, 103, 351—361. 

Deng, L. et al. Activation of the IkappaB kinase complex by TRAF6 requires a dimeric ubiquitin-conjugating enzyme complex and a unique polyubiquitin chain. Cell 2000, 103, 351-361. 

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