Diboron and Triboron Compounds Based on Linear and Star-Shaped Conjugated Ligands with 8-Hydroxyquinolate Functionality: Impact of Intermolecular Interaction and Boron Coordination on Luminescence

New 8-R-quinoline functionalized linear and star-shaped conjugated molecules have been synthesized
using Suzuki−Miyaura coupling methods (R = MeO, L1−L5; R = CH3OCH2O, L1‘−L5‘). When treated
with HCl, L1‘−L5‘ are converted readily to the corresponding 8-hydroxyquinoline compounds L(OH)1−L(OH)5 which react readily with BPh3 in refluxing THF to produce the corresponding polyboron
chelate compounds B1−B5 in good yields. L1−L5 and B1−B5 display similar thermal stability with Td
at ∼300 °C. Experimental and molecular orbital calculation results showed that the chelation by boron
stabilizes the LUMO level of the ligand and narrows the HOMO−LUMO gap, resulting in the blue
emission of the ligands and the green or orange emission of the boron compounds. Crystal structures of
L1, L3, and L5 showed that these molecules have layered arrangements in the solid state with significant
intermolecular π−π interactions. The linear diboron B5 displays concentration and temperature-dependent
emission in solution, attributable to intermolecular interactions. The properties of a monoboron compound
BPh2(5-Ph-8-MeO-q) (B0) and its corresponding free ligand L0 were investigated and compared to the
closely related diboron compound B1 and the ligand L1, which revealed that the increase of the number
of chromophores linked by an aromatic group has a significant impact on thermal stability and the HOMO
and LUMO energy levels.

本研究采用铃木-宫浦偶联反应（Suzuki−Miyaura coupling），成功合成了一系列8-R取代喹啉功能化的线性与星型共轭分子：当R为甲氧基（MeO）时，对应配体为L1−L5；当R为甲氧甲氧基（CH3OCH2O）时，对应配体为L1‘−L5‘。将L1‘−L5‘经氯化氢（HCl）处理后，可便捷转化为对应的8-羟基喹啉类配体L(OH)1−L(OH)5；该类配体可在回流四氢呋喃（THF）体系中与三苯基硼（BPh3）反应，以良好收率得到相应的多硼螯合化合物B1−B5。L1−L5与B1−B5具有相近的热稳定性，其热分解温度（Td）约为300 ℃。实验表征与分子轨道计算结果显示，硼原子的螯合作用可稳定配体的最低未占据分子轨道（Lowest Unoccupied Molecular Orbital, LUMO），并缩窄最高占据分子轨道（Highest Occupied Molecular Orbital, HOMO）与LUMO间的能级差，使得配体呈现蓝色发射，而硼化合物则呈现绿色或橙色发射。对L1、L3与L5的晶体结构解析结果显示，此类分子在固态下呈现层状排列，且存在显著的分子间π-π相互作用。线性二硼化合物B5在溶液中的发射行为呈现浓度与温度依赖性，该现象可归因于分子间相互作用。本研究还对单硼化合物二苯基(5-苯基-8-甲氧基喹啉)硼（BPh2(5-Ph-8-MeO-q)，记为B0）及其对应的游离配体L0进行了性能表征，并与结构相近的二硼化合物B1及配体L1进行对比。结果表明，通过芳香基团连接的生色团数量增加，会对热稳定性以及HOMO、LUMO能级产生显著影响。