Carbazole Compounds as Host Materials for Triplet Emitters in Organic Light-Emitting Diodes:  Tuning the HOMO Level without Influencing the Triplet Energy in Small Molecules

Abstract

A series of novel carbazole compounds was synthesized and tested for their suitability as host for triplet emitters in an organic-light emitting diode (OLED). In these compounds, a carbazole unit is either connected to other carbazole units to form carbazole dimers and trimers or to fluorene and oxadiazole derivatives to form mixed compounds. The HOMO level of carbazole compounds can be tuned by substitution at the 3, 6, and/or 9 positions. Making oligomers by connecting carbazole molecules via their 3 (3') positions shifts the HOMO level to higher energy, while replacing alkyl groups at the 9 (9') positions by aryl groups shifts the HOMO level to lower energy. Furthermore, it has been found that the triplet energy of these compounds is determined by the presence of poly(p-phenyl) chains in the molecular structure. By identifying the longest poly(p-phenyl) chain, one can predict whether a compound will be a suitable host for a high-energy triplet emitter. An overview of HOMO levels, singlet and triplet levels, and exchange energies is given for all carbazole compounds synthesized. Finally, OLEDs employing two selected carbazole compounds as host and fac-tris(2-phenylpyridine)-iridium (Ir(ppy)(3)) as guest were constructed and characterized electrically.

Theoretical Rationalization of the Singlet–Triplet Gap in OLEDs Materials: Impact of Charge-Transfer Character

“…95 This energy gap is known to be a very sensitive property depending on the chargetransfer nature of the underlying excitations. …”

(More) About biphenyl first excited triplet state energy

“…Biphenyl has a renewed significance today, since it is the repeating unit of polyphenyl films with potential application as conducting polymers [13,14] and as light emitting diodes [15,16]. The triplet energy of biphenyl tabulated in reference databases [17,18] is still its spectroscopic energy (E T (biphenyl) = 65.5 kcal mol −1 ).…”

Carbazole based electrochromic polymers with benzoazole units: Effect of heteroatom variation on electrochromic performance

“…Nevertheless, organic optoelectronic applications of poly-3,6-carbazoles are limited because of both poor conjugation within the polymer backbone and also not obtaining high molecular weights. Therefore, poly-3,6-carbazoles are still used as host materials because of their high ability of hole transporting [16][17][18]. Besides, these polymers demonstrate remarkable electrochromic properties due to the conjugation broken on the carbazyl nitrogen.…”