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

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.