Decoding the ECD Spectra of Poly(phenylacetylene)s: Structural Significance

The role of the main dihedral angles in the electronic circular dichroism (ECD) spectra of poly-(phenylacetylene)s (PPAs) was estimated by using time-dependent density functional theory (TD-DFT) for oligo-(phenylacetylene)s (n = 12). These studies reveal that in cis-transoidal arrangements, the first Cotton effect is dominated by excitations involving molecular orbitals (MOs) mainly related to the polyene backbone. Hence, for this scaffold, the +/- sign of the first Cotton effect reflects the P/M helical sense of the internal helix of the polymer. However, in cis-cisoidal arrangements, contribution of MOs in the polyene and the aryl rings of the PPA backbone produce the first Cotton effect band. As a result, two different ECD signatures with three or four alternating Cotton effects can be produced depending on the sign of the omega(1) and omega(3) dihedral angles which determine the helical sense of the polyene (omega(1)) and the relative orientation of the aryl ring toward the polyene (omega(3)), respectively. Thus, on the one hand, if omega(1) and omega(3) rotate in opposite directions, a CD with three alternating Cotton effects is observed, where the sign of first Cotton correlates with the P/M helical sense of the polyene. On the other hand, if omega(1) and omega(3) rotate in the same direction, a CD signature with four alternating Cotton effects is produced where the information relative to the P/M helical sense of the polyene is contained in the second Cotton effect.