A New Synthetic Procedure to 2,8-Diaminoindeno[1,2-b]fluorene as a Blue Light Emitting Material

Conjugated aromatic compounds are potential candidates as organic semiconductors for use in thin-film transistors (TFTs) and light-emitting diodes (LEDs), among which the fluorene-based compounds (Fig. 1) have received most attention owing to their good availability and processability. Therefore, the synthesis of indenofluorene derivatives draws significant interest for their proper supply in both designing a new derivative and manufacturing. Recently, 2,8-bis(diphenylamino)-6,6,12,12-tetramethyl-6,12-dihydroindeno[1,2b]fluorene (1) has been reported to have a good luminous efficiency and excellent life time, and good operation lifetime of its OELD device. These results prompted us to scale-up the synthesis of 1 for its further applications using the reported procedures as shown in Scheme 1. However we found a couple of problems in the reported synthesis, such as low yield, harsh reaction conditions and the formation of side products. Herein, we report an improved synthetic route to 1 which overcomes the problems found in the reported procedures. When we repeated the reported procedures (Scheme 1), we found three major synthetic problems. They are (i) formation of decarboxylated side products in the cyclization of 5 into 6 for parent indeno[1,2-b]fluorene ring; (ii) use of harsh reaction conditions of Wolff-Kishner reduction of 6 which are not suitable for large-scale preparation; and, (iii) low yield at the tetra-methylation of 7. In particular, when the tetra-methylation reaction of 7 was carried out by using n-BuLi and CH3I condition, tetrabutylindenofluorene was Figure 1. Structures of fluorine (i), indenofluorene (ii), and 2,8diaminoindenofluorene (1).