ROLE OF CHARGE-TRANSFER AND QUINONOID STRUCTURE IN THE RAMAN-SPECTRUM OF DOPED POLY(P-PHENYLENE)

We present an ah 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) Angstrom 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/Angstrom upon doping. The intensity ratios of the four A(g) 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.