Blue emitting Butterfly-Shaped 1,3,5,9-Tetraarylpyrenes: Synthesis, Structures, and Physical Properties

Novel butterfly-shaped, highly fluorescent stable monomers of the type 7-tert-butyl-1,3,5,9-tetrakisarylpyrene were synthesized by the Suzuki-Miyaura coupling reaction of the new bromopyrene derivative 7-tert-butyl-1,3, 5,9-tetrabromopyrene with the corresponding arylboronic acids. The design and synthesis of efficient electroluminescent materials based on polyaromatic hydrocarbons (PAH) has been under investigation for well over a decade. Pyrene and its derivatives are important members of the polyaromatic hydrocarbon family (PAHs) that have exhibited several advantages: (1) solution processable, (2) good thermal stability, (3) enhanced charge carrier mobility, and (4) intense luminescence efficiency. Indeed, polyaromatics have been employed in organic light-emitting diodes (OLEDs) as well as in other optoelectronic devices, and as fluorescence probes. However, the use of pyrene as an efficient emitting material in OLED applications has been somewhat limited, primarily because the planar structure has a tendency to form π-aggregates/excimers thereby quenching the fluorescence in concentrated solution or in the solid state. To suppress the aggregation, many research groups have focused on exploring the availability of methods for the functionalization of the pyrene core. In general, the 1-, 3-, 6-, and 8-positions of pyrene preferentially undergo electrophilic aromatic substitution (SEAr) reactions. Thus, various pyrene derivatives (see SI) can be easily accessed depending on the experimental conditions. On the other hand, the 4-, 5-, 9and 10positions of the parent pyrene are susceptible to bromination in the presence of iron powder, in the presence of sterically bulky tert-butyl groups located at the 2and 7-positions as positional protective groups. Even on extending the reaction time, pentabromide pyrene derivative would be obtained. Besides electrophilic substitution of pyrene, Harris showed an efficient, one-step synthetic approach to catalyze the oxidation of the K-region of pyrene using ruthenium chloride. Müllen et. al. have developed an asymmetric functionalization method to direct bromine atoms to the K-region without the need for tert-butyl groups. Additionally, in our laboratory, we also reported the selective formation of the 5-monoand 5,9-di-substitution products from 7-tert-butyl-1,3-dimethylpyrene by formylation and acetylation depending on the type of Lewis acid catalyst deployed. 12 More recently, the 1and 3positions of pyrene were brominated using 2-tert-butylpyrene; the tert-butyl group on the pyrene ring protects the ring from electrophilic attack at the 6,8-positions. Based on the previously mentioned research, we decided to exploit a new intermediate in order to develop a series of pyrene related materials for further applications. Our initial efforts attempted to synthesize 7-tert-butyl-1,3,4,5,9,10-hexabromo-pyrene (3) using iron powder to catalyze its formation from 2-tert-butylpyrene in different solvents, however efforts using CH2Cl2, nitrobenzene, and benzene, all failed. A bromo atom substituted at the 1and 3positions of pyrene would sterically hinder the 4and 10positions, thereby enabling regioselective substitution at the 5and 9positions. Since the 7-position of the pyrene core has been protected, four bromines atoms can be introduced at the 1-, 3-, 5-, 9positions by electrophilic bromination of 2-tert-butylpyrene in the presence of iron powder. The 7-tert-butyl-1,3,5,9-tetrabromopyrene (2) was prepared and purified successfully in excellent yield. To the best of our knowledge, this is the first example of a method to halogenate pyrene not only in the activated sites but also in the K-region. The intermediate 2 has two advantages: (1) the active sites at the 1and 3positions could give C-functionalized pyrene by Suzuki/Sonogashira coupling reactions to avoid aggregation; (2) the K-region (5-, 9positions) affords a strategy to extended conjugated systems to larger PAHs by cyclization reaction. Scheme 2. Synthetic route to compound 4, 5