Selective access to either a doubly boron-doped tetrabenzopentacene or an oxadiborepin from the same precursor.

The well-known red emitter tetrabenzo[de,hi,op,st]pentacene (TBPA) has been transformed into a bright blue emitter (B2-TBPA; λem = 472 nm; c-hexane) via substitutional doping with two boron atoms. In contrast to the electron-rich TBPA, which forms endo-peroxides with O2 under daylight, the benchtop-stable B2-TBPA is a good electron acceptor and undergoes reversible reduction at a moderate half-wave potential of E 1/2 = -1.73 V (vs. FcH/FcH+; THF). Although the size of B2-TBPA falls within the nanoscale, the helically twisted compound readily dissolves in c-hexane and does not require solubilizing substituents. The synthesis of B2-TBPA is based on the nickel-mediated Yamamoto-type dehalogenation of tetrabrominated 9,10-di(naphth-1-yl)-9,10-dihydro-9,10-diboraanthracene. This intramolecular C-C heterocoupling reaction shows a remarkable solvent dependence: B2-TBPA forms only in pyridine (79% yield), whereas an oxadiborepin is obtained from THF solutions (ODBE, 81%; the reaction mixture is quenched with air in both cases). Insight into the corresponding reaction mechanism was gained from the isolation of intermediates and an investigation of their chemical properties. ODBE is an interesting blue emitter in its own right. Furthermore, it can be ring-opened with excess BBr3 at the B-O-B moiety to afford a dimeric borabenzo[de]anthracene.

Facile Route to Quadruply Annulated Borepins.

A two-step synthesis sequence furnishes quadruply annulated borepins in high yields. The first step involves a nucleophilic substitution reaction between aryl-BF3K salts (aryl = mesityl, phenyl) and lithiated bromonapthalene derivatives LiNaphBr,R (HNaphBr,R = 8-bromonaphthalene (a), 5-bromoacenaphthene (b), 5-bromoacenaphthylene (c)). In the second step, the resulting heteroleptic triarylboranes aryl-B(NaphBr,R)2 (3a-c) are subjected to an intramolecular Ni-mediated Yamamoto reaction to close the seven-membered rings and create the borepins 4a-c. Only in the case of 3b is the Yamamoto reaction accompanied by a C-H activation reaction furnishing the 7-hydro-7-borabenzo[de]anthracene derivative 5. The product ratio 4b/5 can be influenced by control of the local Ni(0) concentration. The borepins 4a-c are benchtop stable and highly soluble even in hexane. Compounds 4a-c undergo reversible one-electron reduction; 4c is also able to accept a second electron in a reversible manner and already at moderate potential values (E1/2 = -1.49 V and -1.84 V (vs FcH/FcH+)). 4a, 4b, and 5 show photoluminescence in the blue-green region of the spectrum, while 4c is nonfluorescent, which is likely attributable to an intramolecular charge-transfer transition.

A boron-doped helicene as a highly soluble, benchtop-stable green emitter.

The high-yield synthesis of a boron-doped [4]helicene was achieved through a Ni-mediated Yamamoto C-C-coupling reaction; the moderate distortion of the molecular scaffold confers excellent solubility to the air- and water-stable green luminophore.