Low-lying excitations of polydiacetylene - art. no. 035208

The Pariser-Parr-Pople Hamiltonian is used to calculate and identify the na ture of the low-lying vertical transition energies of polydiacetylene. The model is solved using the density matrix renormalization group method for a fixed acetylenic geometry for chains of up to 102 atoms. The nonlinear opt ical properties of polydiacetylene are considered, which are determined by the third-order susceptibility. The experimental 1B(u) data of Giesa and Sc hultz are used as the geometric model for the calculation. For short chains , the calculated E(1B(u)) agrees with the experimental value, within solvat ion effects (similar to0.3 eV). The charge gap is used to characterize boun d and unbound states. The nB(u) state is above the charge gap and hence a c ontinuum state; the 1B(u), 2A(g), and mA(g) states are not and hence are bo und excitons. For large chain lengths, the nB(u) state tends towards the ch arge gap as expected, strongly suggesting that the nB(u) state is the condu ction band edge. The conduction band edge for polydiacetylene is agreed in the literature to be similar to3.0 eV. Accounting for the strong polarizati on effects of the medium and polaron formation gives our calculated E-infin ity(nB(u)) similar to3.6 eV, with an exciton binding energy of similar to1. 0 eV. The 2A(g) state is found to be above the 1B(u) state, which does not agree with relaxed transition experimental data. However, this could be res olved by including explicit lattice relaxation in the Pariser-Parr-Pople-Pe ierls model. Particle-hole separation data further suggest that the 1B(u), 2A(g), and mA(g) states are bound excitons, and that the nB(u) is an unboun d exciton.