Undoped Systems

A major achievement towards greater flexibility with respect to structural constraints of the reaction is the discovery of polymerization in Langmuir-Blodgett (LB)-films made by Wegner and coworkers These systems also allow introducing dye 

Compound R C = C-C = C-R Ability to form monomolecular films Multilayer formation Photopolymerization X 300 nm)  

At low conversion the polymerization yield nq ranges between 5 and 10 Anticipating the primary quantum efficiency q to be similar to that in crystalline TS-6, i.e., q 10 , a chain length of order 1000 repeat units would result. No auto-catalytic reaction enhancement is observed. It appears that both the high degree of flexibility of the substituents and the way the molecule pack within the multilayer assembly provide sufficient motional freedom to accomodate strain developing in course of the reaction. 

In course of subsequent work Bubeck, Tieke, and Wegner discovered that the action spectrum for photopolymerization of undoped diacetylene multilayers extends into the visible provided some polymer formed in course of previous UV-irradiation is present. Since obviously excitation of the polymer can sensitize the reaction this effect has been termed self-sensitization. Checking the absorption spectrum of the polymer produced via self-sensitization assured that the final product is identical with the product obtained under UV excitation of the monomer. Later work by Braunschweig and Bassler demonstrated, that the effect is not confined to multilayer systems but is also present in partially polymerized single crystalline TS-6, albeit with lower efficiency. Interestingly, the action spectrum of self-sensitization follows the action spectrum for excitation of an electron from the valence band of the polymer backbone to the conduction band rather than the excitonic absorption spectrum of the polymer which is the dominant spectral feature in the visible (see Fig. 21). The quantum yield is independent of the electric field, whereas in a onedimensional system the yield of free carriers, determined by thermal dissociation of optically produced, weakly bound geminate electron-hole pairs, is an linear function of an applied electric field 29.30,32,129) Apparently, the sensitizing action does not 

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The four Eqs. [(8.3)—(8.6)] are simplified using chemical and physical intuition and appropriate approximations to the electroneutrality Eqs. (8.7) and (8.10). Brouwer diagrams similar to those given in the previous chapter can then be constructed. However, by far the simplest way to describe these equilibria is by way of polynomials. This is because the polynomial appropriate for the doped system is simply the polynomial equation for the undoped system, together with one extra term, to account for the donors or acceptors present. For example, following the procedure described in Section 7.9, and using the electroneutrality equation for donors, Eq. (8.9), the polynomial appropriate to donor doping is ... 

The use ofhydrothermal-like synthesis for the preparation of Mo/V/Al/O catalysts, starting from Anderson-type polyoxometalates, leads to monophasic crystalline compounds, also containing other trivalent elements (Fe, Ga), and dopants [13]. For example, the incorporation of Ti in Mo/V/Al/O gives rise to the formation of a compound having stoichiometry Mo6V2Al1Ti0 5Ox, in which the addition of Ti improves the activity with respect to the undoped system. 

The spectrally narrow triplet PA band of PFO makes it possible to compare PFO triplet-state dynamics in the doped and undoped systems in order to look for evidence of Dexter transfer. The PtOEP triplet lifetime in a PFO matrix was determined by measuring the phosphorescence decay following photoexcitation. The phosphorescence decay of a blend containing 1 wt% PtOEP in PFO is inset in Fig. 10.42. The decay could be fit to a lifetime of 55 2/t,s, in excellent agreement with pulsed EL measurements.71... 

The electrochemical behaviour of pre-treatments obtained with direct addition of inhibitor in sol-gel solution (morphology shown in Fig. 14.7A) is similar or even worse than that for the undoped system. It is likely that the addition of 2-mercaptobenzothiazole to the sol-gel solution negatively affects hydrolysis and condensation reactions involved in the deposition of the sol-gel layer, affecting film continuity. This might be reflected in the electrochemical behaviour. 

In the context of this chapter, we focus on the undoped or lightly doped 7i-conjugated systems that are commonly referred to as organic semiconductors. Conducting polymers, such as PEDOT PSS, plexcore, polyaniline, polypyrrole, and others are not addressed here as their charge transfer mechanisms are rather different and would warrant an article in its own right. 

Hubbard (13) elucidated a mathematical description of the change from one situation to another for the simplest case of a half-filled s band of a solid. His result is shown in Figure 11. For ratios of W/U greater than the critical value of 2/ /3 then a Fermi surface should be found and the system can be a metal. This critical point is associated with the Mott transition from metal to insulator. At smaller values than this parameter, then, a correlation, or Hubbard, gap exists and the system is an antiferromagnetic insulator. Both the undoped 2-1 -4 compound and the nickel analog of the one dimensional platinum chain are systems of this type. At the far left-hand side of Figure 11 we show pictorially the orbital occupancy of the upper and lower Hubbard bands. 

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