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

We present an ah initio based, scaled quantum mechanical oligomer forc e field (SQMOFF) method for modeling the structure and vibrational spe ctra of doped poly(p-phenylene). By integrating this theoretical metho d and Raman spectroscopic technique, we are able to investigate quanti tatively 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-r ing C-C bond length decreases from 1.501 to 1.45(2) Angstrom upon dopi ng. This structural information, available for the first time, is fund amental in understanding the effects of doping. Additionally, we find that the corresponding force constant increases from 4.573 to 5.475 md yn/Angstrom upon doping. The intensity ratios of the four A(g) modes a re predicted by the SQMOFF method to be primarily dependent on the qui nonoid structure of the doped polymer. The role of charge transfer in this context is primarily to increase the quinonoid character of the s tructure. A discussion on intensity ratios with respect to the effecti ve conjugation coordinates theory is also presented.