Effect of the Imidazole π-Extension on TADF Emitters in Electrochemiluminescence

Already known molecules which exhibit good electrochemiluminescence (ECL) efficiencies and high photoluminescence quantum yields (PLQY) have been structurally modified in order to increase their performance. The followed strategy is to stiffen the structures to limit the rotational and vibrational freedom degrees and favour radiative decay processes once excited. Molecules under investigation consist of donor-acceptor systems in which the acceptor fraction is a benzonitrile with an imidazole in para position, while the donor fraction consists of four diphenylamine (NPh₂) or 3,6-di(tert-butyl)-9H-carbazole (^t-BuCz) groups in the remaining positions on the central benzene ring. Therefore, in order to stiffen these systems and restrict the intramolecular rotations (RIR), the imidazole in the para position has been replaced with more extended π-systems, i. e., benzimidazole and phenanthro[9,10-d]imidazole. The restriction of the intramolecular rotation can be clearly observed by ¹H NMR analysis. We expected to observe an increase in ECL efficiency and PLQY with the rigidity. Surprisingly, we observed a generally opposite trend: molecules with the smallest imidazole fraction showed the best performance in ECL and higher PLQY. Notably, NPh₂ derivatives with benzimidazole and phenanthro[9,10-d]imidazole showed an hypsochromic shift of the emission spectra with concomitant increase of the PLQY as the solvent polarity increases.