Effect of Aromatic Ring Number and Substituent Arrangement on the Properties of Phenanthro[9,10-d]imidazole Derivatives for OLEDs

Abstract

This study investigates whether the number, arrangement, and attachment position of aromatic rings in C2 substituents influence the properties and OLED applicability of phenanthro[9,10-d]imidazole derivatives. We obtained ten derivatives via a condensation reaction, differing in only their C2 aromatic substituents: phenyl (0A), biphenyl-1-yl (0B), naphthalen-1-yl (0C), naphthalen-2-yl (0D), anthracen-9-yl (0E), phenanthren-9-yl (0F), anthracen-2-yl (0G), triphenylen-2-yl (0H), pyren-1-yl (0I), and perylen-3-yl (0J). All derivatives exhibit thermal stability above 290 °C, though this property does not directly correlate with the number of aromatic rings. Electrochemical studies showed that the redox behavior is strongly modulated by the aromatic substituent at the C2 position, with the most extended substituents exhibiting the easiest reduction; this trend is consistent with DFT results indicating substantial LUMO localization on the C2 aryl fragment. The absorption spectra of 0A–0D, 0F, and 0H are largely independent of the aromatic substituent, whereas 0E, 0G, and 0J show absorption features characteristic of anthracene or perylene substituents. The emission spectra of 0A–0D, 0H, and 0F indicate emission from a locally excited singlet emission state (1LE), while compounds 0E, 0G, 0I, and 0J exhibit stronger substituent-dependent emission behavior. Electroluminescence studies have shown that the emission color and efficiency can be fine-tuned by changing the aromatic substituent at the C2 position. Compounds 0E, 0I, and 0J combine favorable energetic parameters and strong solid-state emission, with 0J achieving the best device performance in the series, reaching an EQE of 4.29% and a luminance of 8962 cd/m2.