Opto-Electronic Properties Modulation Through Iodine Doping of Imine- and Triphenylamine-Based Oligomers

Conjugated imine derivatives are of particular interest for the opto-electronic field since their opto-electronic properties can be finely tailored by protonation or doping of the imine unit, e.g. by iodine doping. In this context, four triphenylamine-based oligoimines were subjected to iodine doping in the solid state and thoroughly investigated with respect to the electronic structure modulation induced by doping. The iodine doping process was analyzed by several methods, such as cyclic voltammetry, ultraviolet–visible absorption and Fourier-transform infrared (FTIR) spectroscopy. In addition, a four-point probe technique was applied to measure the electrical conductivities of both the doped and undoped oligoimines. We have noticed changes of the electronic absorption spectra that consisted of the appearance of a new absorption band due to dopant-induced states within the highest occupied molecular orbital (HOMO)—lowest unoccupied molecular orbital (LUMO) gap. Due to the p-type doping process, a distinct reduction of the energy bandgap occurred, being contingent on the formation of imine polaron states, according to the proposed mechanisms. This was confirmed by FTIR spectroscopy, which indicated the presence of the quinoid skeleton in polaronic species formed by doping. Nevertheless, the iodine doping process of oligoimines was accompanied by new redox processes in both the anodic and cathodic regions. Electrochemical data indicated that iodine-doped oligoimines exhibit a higher electronic affinity (LUMO) and a slightly lower energy bandgap compared to pristine forms. The electrical conductivity values of the undoped oligoimines were found in the range of 6.4 × 10⁻⁷–8.14 × 10⁻⁸ S/cm and increased eight times after oligoimine iodine-doping only in the case of two oligomers. Since doped oligoimine films display better features compared to their pristine forms, they could find applications in opto-electronic devices.