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Synthesis, structure and properties of semi-internally BN-substituted annulated thiophenes

A series of semi-internally BN-substituted annulated thiophenes were synthesized from easily accessible 2,1-borazaronaphthalenes. Crystal structure studies and DFT calculations on the semi-internally BN-substituted annulated thiophene 3a revealed that the six-membered BN-embedded ring and five-membered BS-embedded ring in 3a had lower aromaticity than the corresponding all-carbon rings in 3a′. 3a possesses quite similar frontier molecular orbital energies and absorption and emission maxima compared to the carbonaceous analogue 3a′. The reactivity of these previously unknown BN-aromatic compounds toward organolithium compounds and Br2 has been studied. The afforded bromo-substituted compounds could be further functionalized via Suzuki couplings. The photophysical properties of these BN-substituted annulated thiophenes were investigated by UV–vis and fluorescence spectroscopy.