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-BNMe 2 and BT-BNBn 2 , which contain BMes 2 and NMe 2 /NBn 2 groups at the 3,3'-positions, have been synthesized. Similar to the o,o'-substituted biphenyl analogues, BP-BNMe 2 and BP-BNBn 2 , which contain BMes 2 and NMe 2 /NBn 2 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 NMe 2 -substituted BT-BNMe 2 . On the contrary, the two substituents are arranged on the two different sides of the 2,2'-bithienyls axis for BT-BNBn 2 , which is modified with bulky NBn 2 . 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-BNMe 2 than BP-BNBn 2 . 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.