Impact of the 2,2′-Bithienyl Framework on the Charge-Transfer Emission of Triarylborane-Based o,o′-Substituted Biaryls

Two new triarylborane-based o,o′-substituted 2,2′-bithienyls, BT-BNMe₂ and BT-BNBn₂, which contain BMes₂ and NMe₂/NBn₂ groups at the 3,3′-positions, have been synthesized. Similar to the o,o′-substituted biphenyl analogues, BP-BNMe₂ and BP-BNBn₂, which contain BMes₂ and NMe₂/NBn₂ groups at the 2,2′-positions, the steric effect of the amino group has significant influence on the conformation of the 2,2′-bithienyl skeleton. The boryl and amino groups are located at the same side of 2,2′-bithienyls axis with a short B···N distance (3.63 Å) for the NMe₂-substituted BT-BNMe₂. On the contrary, the two substituents are arranged on the two different sides of the 2,2′-bithienyls axis for BT-BNBn₂, which is modified with bulky NBn₂. Despite the remarkable differences in the steric structure, the two 2,2′-bithienyls display fluorescence at close wavelengths, which is in sharp contrast to the much red-shifted fluorescence of BP-BNMe₂ than BP-BNBn₂. The theoretical calculations demonstrated that the two 2,2′-bithienyls have close highest occupied molecular orbital–lowest unoccupied molecular orbital gaps in the excited state, which firmly support the experimental results. Thus, the parent main chain framework can exhibit great impact on the charge-transfer emission of o,o′-substituted biaryls.