【JACS】具有单占据-最高占据轨道能级反转且热力学稳定的π共轭氮氧自由基研究：实现90<10高三线态产率与多重近红外光致变色>#

文章标题：Thermodynamically Stabilized π-Conjugated Nitroxide with SOMO–HOMO Energy Level Inversion
文章作者：Yuka Kuno, Misaki Kuroda, Akihiro Shimizu, Manabu Abe, Ryohei Kishi, Ryo Shintani
文章链接：https://doi.org/10.1021/jacs.6c10503

研究背景与动因#

具有单占据分子轨道与最高占据分子轨道能级反转（简称能级反转） 特性的有机自由基形式上违背了构造原理，展现出提升自由基稳定性、实现高自旋态及非常规光电磁响应的巨大潜力。然而，现有多数能级反转自由基的单占据轨道与最高占据轨道在空间上高度分离，导致单电子氧化生成双自由基阳离子时的分子内自旋交换作用极弱，限制了高自旋态特性的发挥；少数具有显著轨道空间重叠的体系则因热力学稳定性差且缺乏通用分子设计准则，难以进行深入表征与应用。为此，本研究旨在提出明确的分子设计策略，合成并分离出具有显著轨道空间重叠且热力学稳定的π共轭氮氧自由基，以攻克高自旋态调控与多重刺激响应功能材料开发的技术瓶颈。

Fig.1 Electronic configurations of (a) Aufbau-type and SHI radicals. Structures of (b) representative SHI radicals and (c) 1.#

技术路线、实施方案与关键实验发现#

研究团队提出了一种在母体前线轨道特定节面上引入给电子基团的设计策略：以咔唑-9-氧基为核心母体，利用其最高占据轨道在2,7-位具有较大轨道系数、而单占据轨道在相应位置系数几乎为零的特征，首次在2,7-位引入双(4-叔丁基苯基)氨基给电子基团，构建目标自由基分子。密度泛函理论计算表明，引入氨基显著抬升了分子的最高占据轨道能级（阿尔法最高占据轨道为-4.89电子伏，贝塔最高占据轨道为-4.86电子伏），而阿尔法单占据轨道能级（-5.33电子伏）受影响极小，从而成功构建出单占据轨道比最高占据轨道低0.44至0.47电子伏的反转电子结构，并维持了两轨道在咔唑-9-氧基骨架上的显著空间重叠。化学合成路线沿用铑催化氧化同相偶联、双锂化并与亚硝基二苯胺反应得到羟胺中间体，最终经氧化银氧化得到目标分子，该自由基可在空气气氛下通过常规硅胶色谱柱纯化与重结晶分离。

实验表征与性质测试全面验证了该体系的独特性质。电子自旋共振波谱测得其朗德因子值为2.0056，单晶X射线衍射证实了其平面核心骨架与芳香性分布。循环伏安测试显示目标分子在+0.21伏处出现首个可逆氧化峰，显著低于常规咔唑氧基自由基基准分子的+0.68伏，证实首步氧化发生于联苯二胺单元而非氮氧自由基，从实验上确证了能级反转结构。通过电化学单电子氧化制备的双自由基阳离子经变温电子自旋共振测试证实具有三线态基态，在123开尔文下三线态与双线态比例高达90<10>，计算单重态-三重态能隙达+9.2千焦每摩尔。该自由基在二氯甲烷及空气中放置24小时或在70摄氏度苯溶液中加热均未见明显分解，具备极高的溶液热力学稳定性。

基于质子化与氢键相互作用诱导的自旋中心转移机制，目标分子展现了出色的多重近红外刺激响应变色特性。电化学氧化使1018纳米处的弱吸收带转变为488与1512纳米的特征吸收带，并在施加还原电位后完全恢复；加入三氟甲磺酸质子化后体系生成1345纳米近红外吸收峰，加入有机碱后可完全可逆恢复，循环4次以上性能无衰减；在氯仿与六氟异丙醇混合溶剂中，随温度在50至10摄氏度区间变化，氢键结合平衡移动并产生显著的近红外热致变色。

Fig.2 Orbital correlation diagram of 1c from 1a and two di(4-tert-butylphenyl)amines. Orbitals arising from the bonding interactions between HOMOs of 1a and the orbitals of two di(4-tert-butylphenyl)amines are omitted for clarity, as they are significantly mixed with other orbitals.#

Fig.3 (a) ESR spectra of 1c in CH2Cl2 at room temperature (red: experiment, black: simulation). (b) Spin densities of 1c (blue and green surfaces represent α and β spin densities, respectively) determined based on the simulation (red) and calculated (black) at the BLYP/6–311G(d,p)//B3LYP/6–311G(d,p) level of approximation. (c) ORTEP drawing of top view of 1c at 113 K with HOMA values (bold, calculated using the X-ray crystal structure) and NICS(1)zz values in ppm (italic, calculated using the optimized structure). Displacement ellipsoids are drawn at the 50% probability level.#

Fig.4 (a) Cyclic voltammograms of 1c (red), 1b (blue), (27) and 5 (black) (V vs Fc/Fc+, in 0.1 M Bu4NBF4/CH2Cl2, scan rate 100 mV/s). (b) Structure of 5. (c) UV–vis–NIR absorption spectra of 1c (red), and 1b (blue) (27) in CH2Cl2.#

Fig.5 (a) ESR spectra of 1c+·B(C6F5)4– in CH2Cl2 of ΔMs = ±1 at 123 K (red: experiment, black: simulation with a 90<10> ratio of triplet to doublet species). (b) Variable-temperature ESR spectra of 1c+·B(C6F5)4– in CH2Cl2 of ΔMs = ±2 from 150 to 5.0 K. Inset is the plot of the temperature dependence of the triplet signal intensity versus T–1 from 150 to 5.0 K. (c) α-SOMO and α-SOMO–1 calculated at the B3LYP/6–311G(d,p) level of approximation and spin densities of 1c+ (blue and green surfaces represent α and β spin densities, respectively) calculated at the BLYP/6–311G(d,p)//B3LYP/6–311G(d,p) level of approximation.#

Fig.6 (a) ESR spectra of 1c2+ 2PF6– in CH2Cl2 at room temperature (red: experiment, black: simulation). (b) Spin densities of 1c2+ determined based on the simulation (red) and calculated (black) at the BLYP/6–311G(d,p)//B3LYP/6–311G(d,p) level of approximation.#

Fig.7 ESR spectra of (a) 1c·HBF4 and (c) 5+·PF6– in CH2Cl2 at room temperature (red: experiment, black: simulation). Spin densities of (b) 1c·H+ and (d) 5+ (blue and green surfaces represent α and β spin densities, respectively) determined based on the simulation (red) and calculation (black) at the BLYP/6–311G(d,p)//B3LYP/6–311G(d,p) level of approximation.#

Fig.8 (a) UV–vis–NIR absorption spectra of 1c·HBF4 (red) and 5+·PF6– (blue) in CH2Cl2. Because the molar absorption coefficient (ε) of 5+·PF6– could not been determined, its spectrum was scaled to match the maximum peak height of 1c·HBF4 in the NIR region for comparison. β-HOMO and β-SUMO of (b) 1c·H+ and (c) 5+.#

Fig.9 (a) Spin-center transfer of 1c induced by hydrogen-bonding interaction with HFIP. (b) ESR and (c) UV–vis–NIR absorption spectra of 1c in CH2Cl2 = 10<0>, 9<1>, 8<2>, and 4<6>. (d) Cyclic voltammograms of 1c in CH2Cl2 and CH2Cl2 = 4<6> and 1c·HBF4 in CH2Cl2 (V vs Fc/Fc+, in 0.1 M Bu4NBF4, scan rate 100 mV/s).#

Fig.10 (a) NIR-electrochromism, halochromism, and thermochromism of 1c. UV–vis–NIR spectra of 1c with Bu4NB(C6F5)4 as supporting electrolyte with applied potentials of (b) +0.19 V and (c) −0.06 V (vs Fc/Fc+) in CH2Cl2, (d) with increasing the amount of TfOH and (e) with increasing the amount of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) after the addition of TfOH in CH2Cl2, and (f) with lowering the temperature from 50 to 10 °C in CHCl3 = 8<2>.#

核心创新与性能提升#

本研究的核心创新在于确立了“在最高占据轨道大系数且单占据轨道零系数位点修饰强给电子基团”的通用能级反转分子设计法则，成功合成了兼具显著轨道空间重叠与优异热力学稳定性的共轭氮氧自由基，彻底解决了前人体系“稳定性极差”与“轨道重叠微弱导致高自旋态不稳定”的双重痛点。实验实现了低至+0.21伏的低电位可逆氧化、90%高纯度三线态双自由基阳离子的稳定生成，以及电致、酸碱与温致三重可逆近红外吸收切换，为开发多功能有机自由基光电与自旋电子学材料提供了全新平台。

【JACS】具有单占/最高占据轨道能级反转的热力学稳定π共轭氮氧自由基及其多重近红外变色响应