Phosphorescence properties of boron β-diketiminate complexes modulated by spiro structures

Comprehension of the photochemical properties of triplet excited states in organic compounds is of significance for developing advanced organic light-emitting devices, organic photocatalysts, and luminescent probes. However, triplet excited states of typical organic chromophores without heavy atoms are hard to generate because intersystem crossing from singlet to triplet excited states is spin-forbidden. Recently, we demonstrated that efficient crystallization-induced room-temperature phosphorescence is observed in β-diketiminate complexes of group 13 elements bearing heavy halogen substituents on the central atom. In this research, to obtain phosphorescence from β-diketiminate complexes without the use of heavy atoms, twisted donor-acceptor systems were constructed on a central atom through a spiro structure. The resulting biphenylene-based β-diketiminate boron complexes exhibited phosphorescence even at room temperature. On the other hand, diolate-based spiro complexes exhibited only inefficient room-temperature emission. Theoretical calculations suggested that interligand charge-transfer states provided by the spiro structure should facilitate efficient intersystem crossing. In addition, the moderately electron-donating properties of the biphenylene units should be responsible for the room-temperature phosphorescence.