C-Functionalized, Air- and Water-Stable 9,10-Dihydro-9,10-diboraanthracenes: Efficient Blue to Red Emitting Luminophores

Abstract

9,10-Dihydro-9,10-diboraanthracene (DBA) provides a versatile scaffold for the development of boron-doped organic luminophores. Symmetrically C-halogenated DBAs are obtained through the condensation of 4-bromo-1,2-bis(trimethylsilyl)benzene or 4,5-dichloro-1,2-bis(trimethylsilyl)benzene with BBr3 in hexane. Unsymmetrically C-halogenated DBAs are formed via an electrophilic solvent activation reaction if the synthesis is carried out in o-xylene. Mechanistic insight has been achieved by in situ NMR spectroscopy, which revealed C-halogenated 1,2-bis(dibromoboryl)benzenes to be the key intermediates. Treatment of the primary 9,10-dibromo-DBAs with MesMgBr yields air- and water-stable C-halogenated 9,10-dimesityl-DBAs (2-Br-6,7-Me2-DBA(Mes)2; 2,6-Br2-DBA(Mes)2; 2,3-Cl2-6,7-Me2-DBA(Mes)2; 2,3,6,7-Cl4-DBA(Mes)2). Subsequent Stille-type C-C-coupling reactions give access to corresponding phenyl, 2-thienyl, and p-N,N-diphenylaminophenyl derivatives, which act as highly emissive donor-acceptor dyads or donor-acceptor-donor triads both in solution and in the solid state. 2-Thienyl was chosen as a model substituent to show that already a variation of the number and/or the positional distribution of the donor groups suffices to tune the emission wavelength of the resulting benchtop stable compounds from 469 nm (blue) to 540 nm (green). A further shift of the fluorescence maximum to 594 nm (red) can be achieved by switching from 2-thienyl to p-aminophenyl groups. A comparison of the optoelectronic properties of selected C-substituted DBA(Mes)2 derivatives with those of the isostructural anthracene analogues unveiled the following: (i) The DBA core is a much better electron acceptor. (ii) The emission colors of DBAs fall in the visible range of the spectrum (blue to orange), while anthracenes emit exclusively in the near-ultraviolet to blue wavelength regime. (iii) DBAs show significantly higher solid-state quantum yields.