Poly quinoid structure

CH3 p-quinoid structure p-benzenoid biradical poly(p-xyiyiene)... 

Figure 5,2. A, Poly p-phenylene) and By 12cas-polyacetylene. These compounds arCy respectively y prototypes for polymers with nondegenerate aromatic structure) and degenerate quinoid structure) ground states. The symbol E denotes...

The guided tour, presented here, for the stepwise analysis of the vibrational data recorded for trans-PA, can be and has been, equally applied for the interpretation of the spectra and the structural analysis of other poly conjugated polymers such as polyparaphenylene [44], polypyrrole [45], poly thiophene [46] and their alkyl derivatives [47]. For these substances, more experimental data are needed. Moreover, additional theoretical problems must be considered, due to the fact that the electron delocalisation depends on the torsional angle between the aromatic rings. Moreover, these systems cannot sustain solitonic excitation, but only polarons or bipolarons can be formed with the generation of "quinoid structures."... 

J. Gelan, On the quinoid structure of poly (iso thianaphthene). A vibrational spectroscopic study, Adv. Mater., 7, 1027-1030 (1995). 

Polymers with heteroaromatic units such as PT and poly(pyrrole) and their derivatives are nondegenerate in their ground state and possess two possible types of structure the aromatic and the quinoid structure (Fig. 2). 

The first calculations evaluating the introduction of additional sulfur atoms into isothianaphthene-like ring systems concerned poly(thieno[3,4-c]thiophene). In this polymer, the fused benzene ring of polyisothianaphthene is replaced by a fused thiophene ring. However, the VEH-based calculations [1092] predict a larger bandgap for the aromatic structure of this polymer. Analysis of the HOMO and LUMO band structure showed that the polymer possessed a pronounced quinoid structure. Since the aromatic structure will be characterized by the presence of a diradical, the quinoid structure will be substantially more stable as compared to the aromatic diradical. Calculations by Kertesz and co-workers [1060,1062] and Hong and Marynick [1093] also indicate that this... 

The major bonding environment remains unchanged for both standard and nanofibrous poly aniline structures. The presence of two bands in the vicinity of 1,500 cm and 1,600 cm is assigned to the nonsymmetric C6 ring stretching modes. The higher frequency vibration at 1,600 cm is for the quinoid rings, while... 

The scope of Wessling route has been extended by Mullen and co-workers to develop a soluble precursor route to poly(anthrylene vinyiene)s (PAVs) [51]. It was anticipated that the energy differences between the quinoid and aromatic resonance structures would be diminished in PAV relative to PPV itself. An optical band gap of 2.12 eV was determined for 1,4-PAV 29, some 0.3 eV lower than the value observed in PPV. Interestingly, the 9, lO-b/.v-sulfonium salt does not polymerize, possibly due to stcric effects (Scheme 1-9). 

For instance, poly-p-phenylenes in their doped states manifest high electric conductivity (Shacklette et al. 1980). Banerjee et al. (2007) isolated the hexachloroantimonate of 4" -di(tert-butyl)-p-quaterphenyl cation-radical and studied its x-ray crystal structure. In this cation-radical, 0.8 part of spin density falls to the share of the two central phenyl rings, whereas the two terminal phenyl rings bear only 0.2 part of spin density. Consequently, there is some quinoidal stabilization of the cationic charge or polaron, which is responsible for the high conductivity. As it follows from the theoretical consideration by Bredas et al. (1982), the electronic structure of a lithium-doped quaterphenyl anion-radical also differs in a similar quinoidal distortion. With respect to conformational transition, this means less freedom for rotation of the rings in the ion-radicals of quaterphenyl. This effect was also observed for poly-p-phenylene cation-radical (Sun et al. 2007) and anion-radical of quaterphenyl p-quinone whose C—O bonds were screened by o,o-tert-hutyl groups (Nelsen et al. 2007). 

FIGURE 2.2 (A) Aromatic and quinoid resonance forms of poly(phenylene) (60), poly(p-phenylenevinylene) (10), polythiophene (61), and polyisothianaphthene (62). (B) Chemical structures of 63 and 64. (0 Chemical structures of 65 and 66. (D) Chemical structure of 67 with its resonance structure. 

P. R. Surjdn, M. Kertesz, Electronic structure and optical absorption of poly(biisothianaphthene-methine) and poly(isonaphthothiophene-thiophene) two low-bandgap polymers, J. Am. Chem. Soc., 113, 9865-9867 (1991) (j) J. KUrti, P. R. Suijan, Quinoid vs aromatic structure of polyisothianaphthene, J. Chem. Phys., 92, 3247-3248 (1990) (k) E. Faulques, W. WaUnbfer, H. Kuzmany, Vibrational analysis of heterocyclic polymers a com-... 

These conducting polymers differ from poly acetylene in many ways. Their ground state is nondegenerate as a consequence of the lack of energetic equivalence of their two limiting mesomeric forms, aromatic and quinoid [9], and is generally more stable. Moreover, their structure may be more readily modified, allowing the modulation of their chemical, electronic, and electrochemical properties. 

A poly(heterocycle) PHC can be viewed as a carbon chain with the structure of polyacetylene stabilized by the heteroatom. These conducting polymers differ from polyacetylene by their non-degenerate ground state related to the non-energetic equivalence of their two limiting mesomenc forms, aromatic and quinoid, their higher environmental stability, and their structural versatility which allows modulation of their electronic and electrochemical properties by manipulation of the monomer structure. 

As noticed in the discussion of the VB theoiy, the wave function of poly(p-phenylene), polythiophene, polypyrrole (PP), and polyfuran is principally determined by the aromatic resonance form (C > Cq). In the case of polyisothianaphthene, however, the quinoid resonance form should, according to Kertesz and coworkers, possess a stability such that it has a more important contribution to the ground-state wave function as compared to the aromatic resonance form (C < Cq). Thus, a switchover occurs in energetics from the aromatic structure of the quinoid form at the level of polyisothianaphthene in the series of Figure 105. 

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