Individually Tunable Energy Levels of Oligomers Based on N−B←N Units

Opto-electronic properties and device performance of organic semiconductors are mainly determined by energy levels of their frontier molecular orbitals, e.g. lowest unoccupied molecular orbital (E_LUMO) and highest occupied molecular orbital (E_HOMO) in the ground state, first singlet state (E_S1) and first triplet state (E_T1) in the excited state. These energy levels are always intricately intertwined. Herein, we report a series of monodisperse oligomers based on double B←N bridged bipyridine (BNBP) units. With the increasing number of repeating units, the oligomers exhibit gradually downshifted E_LUMO and nearly unchanged E_HOMO due to the different distribution of the frontier molecular orbitals of the oligomers. Moreover, the oligomers exhibit gradually decreasing E_S1 and nearly unchanged E_T1 because of the different contributions of the charge transfer component in the excited state. This work provides new insight into energy level tuning of organic semiconductors, which is important for high-performance organic opto-electronic devices.