Interpretation of the raman and IR spectra of poly-p-phenylenes: undoped, doped, and planarized

Summary form only given. Interpretation of IR and Raman spectra of poly(p-phenylene) can be used to gain insight into the conformation of undoped PPP and its structural change upon doping. We use the scaled quantum mechanical oligomer force field (SQMOFF) method which has reproduced very good vibrational frequencies and relative intensities of a number of polymers. This method is based on ab initio force constants of an oligomer, which are empirically scaled. Scaling factors are transferred from similar molecules, usually a monomer and/or dimer. The conformation of undoped PPP is studied with three models of infinite chain whose inter-ring torsional angles are 0/spl deg/, 20/spl deg/, and 50/spl deg/. On the basis of good agreement between the 20/spl deg/ model and experimental data, we conclude that PPP is non-planar, having a inter-ring torsional angle of about 20/spl deg/. Upon heavy doping, the inter-ring C-C bond shortens to 1.45(2) from 1.501 /spl Aring/. The corresponding force constant increases to 5.475 from 4.573 mdyne//spl Aring/. The newly synthesized PPP, planarized with methylene bridging groups (Scherf aid Mullen, 1992), is predicted to have a much higher inter-ring stretching frequency even though the inter-ring bond length (1.475 /spl Aring/) is only slightly shorter than that of undoped PPP. This large increase can be attributed to the increase in the rigidity of the backbone due to the methylene bridge. The IR spectrum of planarized PPP is predicted to show CH/sub 2/ peaks, two new peaks in the 1350 - 1450 cm/sup -1/ region, and a strong peak at 844 cm/sup -1/ which is consistent with the frequency range of 1,2,4,5-tetrasubstituted phenyl rings.