Role of charge transfer and quinonoid structure in the Raman spectrum of doped poly(p-phenylene)

Abstract

We present an ab initio based, scaled quantum mechanical oligomer force field (SQMOFF) method for modeling the structure and vibrational spectra of doped poly(p-phenylene). By integrating this theoretical method and Raman spectroscopic technique, we are able to investigate quantitatively the structural evolution of poly(p-phenylene) upon doping. On the basis of our periodic quinonoid model and the observed inter-ring stretching frequency, we find heavily doped PPP to have only about 30% quinonoid character on the average. Accordingly, the average inter-ring C-C bond length decreases from 1.501 to 1.45(2) Á upon doping. This structural information, available for the first time, is fundamental in understanding the effects of doping. Additionally, we find that the corresponding force constant increases from 4.573 to 5.475 mdyn/A upon doping. The intensity ratios of the four Ag modes are predicted by the SQMOFF method to be primarily dependent on the quinonoid structure of the doped polymer. The role of charge transfer in this context is primarily to increase the quinonoid character of the structure. A discussion on intensity ratios with respect to the effective conjugation coordinates theory is also presented.

High‐Pressure Chemistry and the Mechanochemical Polymerization of [5]‐Cyclo‐p‐phenylene

“…These rates are presented as the slopes of the pressure dependence of the Raman frequencies in Table . The frequency increase effect of these BLA mode is correlated to the decrease of the bond length alternation in Figure a, indicating a direction of the structural transformation from aromatic to quinonoid …”

Properties of Sizeable [n]Cycloparaphenylenes as Molecular Models of Single‐Wall Carbon Nanotubes Elucidated by Raman Spectroscopy: Structural and Electron‐Transfer Responses under Mechanical Stress

“…Angewandte Zuschriften 7154 www.angewandte.de bands resemble that of G + band in armchair SWCNTs. [18] Such a correlation between Raman frequencies and aromatic-quinonoid character is further corroborated by the oxidation of [10]CPP to its radical cation ([10]CPPC + ; Figure 5), which unequivocally provokes ring quinoidization, and shows the main Raman band at 1569 cm À1 . [17] These modes derive from the 8a mode in benzene, which allows us to analyze the benzoquinonoid nature of such molecular structures.…”

Combined ab initio and density functional study on polaron to bipolaron transitions in oligophenyls and oligothiophenes

“…For example, Ehrendorfer and Karpfen investigated positively charged bipolaronic defects in a series of oligothiophenes [24][25][26][27] and oligophenyls. 28 Kertesz et al 29 presented the structures and vibrational spectra of dications and dianions of terphenyl, and the structures of doubly positive charged closed-shell and open-shell defects in oligothiophenes were analyzed by Tol. 14,15 Common to these investigations on charged defects without counterions is the fact that bipolaronic defects extend over about six monomer units in the case of oligophenyls and by nine to eleven monomer units in the case of oligothiophenes, whereas polaronic defects were found to be less extended, with defect lengths of about three monomer units in oligothiophenes.…”