Dithiophene‐Fused Oxadiborepins and Azadiborepins: A New Class of Highly Fluorescent Heteroaromatics

Abstract

Access to dithiophene‐fused oxadiborepins and the first azadiborepins attained via a modular synthesis route are presented. The new compounds emit intense blue light, some of which demonstrate fluorescence quantum yields close to unity. Cyclic voltammetry (CV) revealed electrochemically reversible one‐electron reduction processes. The weak aromatic character of the novel 1,2,7‐azadiborepin ring is demonstrated with in‐depth theoretical investigations using nucleus‐independent chemical shift (NICS) scans and anisotropy of the induced current density (ACID) calculations.

Fusion of Aza‐ and Oxadiborepins with Furans in a Reversible Ring‐Opening Process Furnishes Versatile Building Blocks for Extended π‐Conjugated Materials

“…The angles between the furan units ( 6 : 0.99°, 7 : 1.12°) and the angles between the plane of the respective B−X−B unit and the best plane through the bifuran moiety are small ( 6 : 0.77°, 7 : 1.05°). The B−O−B binding angle of 136.7(2)° for 6 is slightly smaller than the B−N−B angle for the NH congener 7 (134.9(4)°), as well as in the corresponding dithienodiborepins [13] . Compounds 8 and 9 show slight deviation from planarity, as indicated by the larger angles between the BOB/BNB and the bifuran planes ( 8 : 8.00°, 7.78°, 9 : 8.38°), as well as by the furan–furan angle ( 8 : 2.59°, 4.27°, 9 : 6.93°).…”

“…They show an overall diatropic ring current, which is slightly more pronounced in the bifuran moiety, consistent with the results from the NICS calculations. Overall, the oxa‐ and azadiborepin rings with bifuran fusion show weak aromaticity, similar to their bithiophene congeners, [13] which is a good prerequisite for extended π‐conjugation over these heterocycles.…”

“…In the first step of the synthesis the sterically demanding tert ‐butyldiphenylsilyl (TBDPS) protecting groups were introduced at positions 5 and 5’ of 2,2′‐bifuran ( 1 ) to give 2 (Scheme 1). Following our procedure for the synthesis of the diborepins D and E , [13] our initial plan was then to introduce TMS groups at positions 3 and 3’ of 2 , and to transform them subsequently into boryl groups by Si/B exchange. Surprisingly, upon lithiation and silylation, besides the expected tetrasilylated building block 3 a , compound 3 b , which is an acyclic isomer of 3 a , was formed in addition.…”

“…On the other hand, relatively few BNB‐doped aromatics have been developed and investigated for their optoelectronic properties so far [6,9–11] . The same is true for compounds featuring the structurally and electronically related BOB fragments [6a,12–15] . In 2018, Yan et al.…”

“…Our synthetic route to build up the dithienodiborepin system, however, required blocking of the α‐positions of the thiophene moieties to prevent undesired side reactions, which would otherwise occur at some point. We did this by attaching methyl groups in these positions (i. e., R=Me) [13] …”