Optical Absorption and Emission of Fully Conjugated Heterocyclic Aromatic Rigid-rod Polymers Containing Sulfonated Pendants

Fully conjugated poly[(1,7-dihydrobenzo[1,2-d:4,5-d']\textit{di}imidazole-2,6-diyl)-2-(2-sulfo)-$p$-phenylene] (sPBI) has a \textit{para}-catenated rod-like backbone, which was synthesized and fabricated for \textit{mono}-layer polymer light-emitting diode (PLED) showing a threshold voltage of 4.5 V and a green light (530 nm) emission. Its SO$_{3}$H moiety attached to the $p$-phenyl ring improved electron \textit{de}localization along the backbone resulted in a \textit{red shift} of absorption spectrum. sPBI was further derivatized for rigid-rod polyelectrolyte sPBI-PS(Li$^{+})$ by attaching propanesulfonated pendants to the heterocyclic moiety of intractable sPBI for water solubility. This fully conjugated polyelectrolyte sPBI-PS(Li$^{+})$ was fabricated for light-emitting electrochemical cells (PLECs) with a dopant of LiCF$_{3}$SO$_{3}$ or LiN(CF$_{3}$SO$_{2})_{2}$ for effects of propanesulfonated pendants and lithium dopants on lunimescent emission and on room-temperature conductivity. sPBI-PS(Li$^{+})$ PLECs doped with 0.41 and 1.01 wt.{\%} of LiN(CF$_{3}$SO$_{2})_{2}$ showed higher green light (514 nm) electroluminescence emission intensity with a threshold voltage of 3.0 V and -4.6 V, respectively. Emission intensity of the sPBI-PS(Li$^{+})$ PLEC did not raise upon increasing the conductivity of the luminescent layer.