Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea.
Phys Chem Chem Phys. 2018 Aug 8;20(31):20688-20694. doi: 10.1039/c8cp03689f.
Cross-conjugated molecules are an interesting class of conjugated systems possessing a spatially separated HOMO and LUMO. Most previous studies have taken advantage of this property by using it in organic semiconductor applications. Herein, we undertake a new investigation on the use of this type of molecule, in particular benzo[1,2-d;4,5-d']bisoxazole (BBO), as a coupler for organic diradicals. BBO has two sites available for adding a substituent and a spin center (SC) which are along its 4,8- and 2,6-axes. Functionalizations using electron donating (ED) and electron withdrawing (EW) groups were imposed to tune its FMOs and it was found that the longer 2,6-axis is an ideal site with a broader LUMO range via substituent effects. Diradicalization of these BBOs using nitronyl nitroxide (NN) and nitroxide (NO) as SCs was done using the remaining available axis. The calculated J values are linearly dependent on the LUMO energy of the coupler, but with 4,8-NH2-2,6-SC as an outlier. This exceptional case is related to 4,8-NH2-2,6-SC having the lowest BBO-NN dihedral angle. Moreover, the diradicals 4,8-X-2,6-SC (with X = H, NH2, CH3) have higher J values than 2,6-X-4,8-SC (with X = H, NH2, CH3), which is counterintuitive because the latter have a shorter coupling path. These diradicals are positioned to the right of the intersection of their trend lines, which implies that diradicals with LUMO values to the right of this intersection have the tendency to attain J values that are higher than those diradicals with a shorter coupling path. 4,8-NH2-2,6-SC even surpasses the projected JMax values which we associate with the highest attainable J values due to LUMO tuning via substituent effects. These results provide useful insights, especially into the interplay between the LUMO and the dihedral angle and how these affect magnetism in diradicals. In conclusion, we found that BBO can be a good candidate as an effective coupler for diradicals with tunable J values via incorporation of ED and EW groups. This first approach to studying the application of cross-conjugated molecules as couplers also paves the way for new candidates for the development of more effective diradical systems.
交叉共轭分子是一类有趣的共轭体系，具有空间分离的 HOMO 和 LUMO。大多数先前的研究都利用了这一特性，将其应用于有机半导体领域。在这里，我们对使用这种类型的分子，特别是苯并[1,2-d;4,5-d']双恶唑（BBO）作为有机双自由基的偶联剂进行了新的研究。BBO 有两个可供添加取代基和自旋中心（SC）的位点，它们沿着其 4,8-和 2,6-轴。通过施加供电子（ED）和吸电子（EW）基团来功能化，以调整其 FMO，并发现较长的 2,6-轴通过取代基效应具有更宽的 LUMO 范围，是理想的位点。使用硝酮氮氧化物（NN）和氮氧化物（NO）作为 SC 对这些 BBO 进行双自由基化，使用剩余的可用轴完成。计算出的 J 值与偶联剂的 LUMO 能量呈线性相关，但 4,8-NH2-2,6-SC 是一个异常值。这种特殊情况与 4,8-NH2-2,6-SC 具有最低的 BBO-NN 二面角有关。此外，4,8-X-2,6-SC（X = H、NH2、CH3）的双自由基的 J 值高于 2,6-X-4,8-SC（X = H、NH2、CH3）的 J 值，这是违反直觉的，因为后者具有更短的耦合路径。这些双自由基位于它们趋势线交点的右侧，这意味着 LUMO 值位于交点右侧的双自由基具有获得高于具有较短耦合路径的双自由基的 J 值的趋势。4,8-NH2-2,6-SC 甚至超过了我们归因于通过取代基效应调谐 LUMO 而获得的最高可达 J 值的预测 JMax 值。这些结果提供了有用的见解，特别是在 LUMO 与二面角之间的相互作用以及它们如何影响双自由基的磁性方面。总之，我们发现 BBO 可以作为一种有效的双自由基偶联剂，通过引入 ED 和 EW 基团来调节 J 值。这种将交叉共轭分子作为偶联剂应用的首次尝试也为开发更有效的双自由基体系开辟了新的候选途径。
