Low-Bandgap Conjugated Polymers. A Joint Experimental and Theoretical Study of the Structure of Polyisothianaphthene

We investigated the structure of a low-bandgap conjugated polymer, polyisothianaphthene (PITN), with a joint experimental and theoretical approach. On the one hand, ¹³C NMR measurements are performed on the polymer and on a series of model isothianaphthene molecules. These molecular compounds are designed and synthesized to represent either aromatic or quinoid segments of the polymer chain. Combining cross-polarization magic angle spinning (CP/MAS), cross-depolarization (CDP), and proton-dephasing (PDP) experiments as a function of the polarization and the depolarization time allows determination of the chemical shifts of the four carbon of PITN. These values are compared to the data obtained on the model molecules and discussed in terms of the ground-state structure (aromatic or quinoid) of the polymer. On the other hand, quantum-chemical calculations using the Austin model 1 (AM1) semiempirical Hamiltonian are performed on ITN oligomers of various lengths. The relative stabilities of the aromatic and quinoid valence bond isomers are estimated in relation to the corresponding values for polythiophene. Finally, the electronic properties (bandgap, ionization potential) of aromatic and quinoid PITN are evaluated with the valence effective Hamiltonian method and compared to the experimental data.