Ionization potential conjugated polymers

LDMS is particularly well suited for the analysis of porphyrins.35-39 The heme molecule—a 22 rc-electron conjugated protoporphyrin system (Figure 8.1)—is an efficient photo-absorber in the visible and near UV (with an absorption maximum—the Soret band—near 400nm). This feature, concurrently with its low ionization potential, warrants that direct LDMS will possess extremely low limits for heme detection. The uses of IR or UV LDMS for structural characterization of natural porphyrins and their metabolites, synthetic monomeric porphyrins (e.g., used in photodynamic therapy), porphyrin polymers, and multimeric arrays, have been well documented.41148 In addition fast atom bombardment MS has been used to characterize purified hemozoin, isolated from the spleens and livers of Plasmodium yoelii infected mice.49... 

Para-phcnylcncs are comparably stable materials under environmental conditions for the degradation and stability of conducting polymers, see Ref. 2. Both theoretical and experimental studies find their oxidation and ionization potentials about 1 eV higher than for conjugated molecules based on pyrrole or thiophene.3... 

Figure 15-14. (a) Energy diagram of a metal l/semiconductor/metal 2 Schottky barrier under open circuit conditions, whereby the metals have different work-functions (j) work-function, /j electron affinity, IP ionization potential band gap, IF depletion width), (b) Charge generation process in single layer conjugated polymer device under short circuit conditions in the MIM model, VB valence... 

The energy barrier for hole injection at the metal-poymer interface is determined by the vacuum work function of the metal contact and the ionization potential Ip of the polymer. For conjugated polymer films spin-coated onto hole injecting metal electrodes, it has been reported that as long as is smaller than a critical value characteristic of the polymer, no interface dipole is formed [104]. In this case, the barrier for hole injection can be estimated simply by aligning the vacuum levels of the metal and the polymer (Mott-Schottky limit) the measured work function of the metal with the polymer deposited on top increases linearly with with a slope of one (see Figure 2.3.11). 

The intrinsic degradation of doped conjugated polymers would be affected by two major factors, the reactivity of the dopant and the reactivity of the polymer backbone. The polyacetylene backbone is comparatively very reactive due to its sufficiently low ionization potential of 4.7 eV [24], so the reactivity of the dopant may be the dominant reaction rate deciding factor in polyacetylene. 

Conjugated polymers have a conjugated n system and n bands. As a result, they have a low ionization potential (usually lower than 6 eV) and/or a high electron affinity (lower than 2 eV). 

Figure 3.2 Ionization potential vs. chain length of poly(3,4-ethylenediox54hiophene (PEDOT) (n = 1 monomer n = large polymer between oligomeric species NDB = number of bonds in the conjugated pathway). Reprinted with permission from Ref. 78. Copyright 2003, AIP Publishing LLC.

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