Conjugated Oligomers with Stable Radical Substituents: Synthesis, Single Crystal Structures, Electronic Structure, and Excited State Dynamics
Conjugated Oligomers with Stable Radical Substituents: Synthesis, Single Crystal Structures, Electronic Structure, and Excited State Dynamics
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DOI：
10.1021/acs.chemmater.8b03366
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发表时间：
2018-11-13
影响因子：
8.6
通讯作者：
Egap, Eilaf
中科院分区：
文献类型：
--
作者：
Huang, Yiming;Xu, Zihao;Egap, Eilaf
理解和控制有机半导体中的自旋动力学具有重要的技术意义。我们提出了一个全面的联合实验和计算研究阐明激发态动力学和动力学的寡硫吩共价连接到两个自由基。本文介绍了一系列含硝基氮氧化物（NN）二自由基（RAx和RBx）的寡硫吩的合成、稳态和超快光物理、磁性、计算建模和单晶x射线衍射。我们表明，双自由基的结合导致了一个有趣的分子堆积，这让人想起有机笼，不寻常的激发态动力学，以及有趣的光物理和磁性。我们发现双基环与低硫噻吩核之间的距离和二面角的增加导致弱反铁磁相互作用。单晶x射线衍射和计算模型表明，沿主链的有效共轭导致了有效的自旋极化转移。对苯连接体的插入使低硫噻吩核心与NN自由基组分的平均间距仅为4.3 A，导致发色团与两个NN自由基的单占据分子轨道之间的轨道重叠减少，并导致沿噻吩核心的自旋极化较弱。计算还预测了一个双基态，单线态三重态能隙（DEBT）为0.6 kcal/mol或更小，其中三重态位于单线态之上，这表明在这些分子中，单线态和三重态都是热填充的。综上所述，稳态光学吸收、计算研究和超快瞬态吸收表明，增强的内部转换是RAx和RBx双自由基系列快速衰减的主要途径，原因有两个主要因素：(i)自由基的加入导致新的低空单线态和三重态（SI T1）的形成，这些单线态和三重态充当“陷阱态”；（ii）产生本质上是能量简并的多重单重态和三重态。由于神经网络双基的结合导致了20多个低洼近简并的单重态、三重态和五重态，系统间交叉和内部转换都成为了S和T、、、态衰变回So和to的可行衰变机制。这些结果建立并关联了影响自旋耦合、自旋离域的结构和电子参数，并确定了预测激发态能级的一般趋势。
Understanding and controlling spin dynamics in organic semiconductors is of significant technological interest. We present a comprehensive joint experimental and computational study elucidating excited-state dynamics and kinetics of oligothiophenes covalently linked to two radicals. The synthesis, steady-state, and ultrafast photophysics, magnetic properties, computational modeling, and single crystal X-ray diffraction of a series of oligothiophenes with appended nitronyl nitroxide (NN) diradicals (RAx and RBx) are presented. We show that incorporation of the diradicals results in an intriguing molecular packing that is reminiscent of organic cages, unusual excited-state dynamics, and interesting photo physical and magnetic properties. We find an increase in the distance and dihedral angle between the diradical rings and the oligothiophene core result in weak antiferromagnetic interactions. Single crystal X-ray diffraction and computational modeling suggest that efficient conjugation along the backbone leads to an efficient spin-polarization transfer. Insertion of p-phenylene linkers that separate the oligothiophene core from the NN radical component by an average of only 4.3 A results in a decrease in orbital overlap between the chromophore and singly occupied molecular orbital of the two NN radicals and a weak spin polarization along the thiophene core. Computations also predict a biradical ground state with a small singlet triplet energy gap (DEBT) of 0.6 kcal/mol or less, where the triplet lies above the singlet, suggesting that in some of these molecules both the singlet and triplet states are thermally populated. Together, the steady-state optical absorption, computational study, and ultrafast transient absorption suggest enhanced internal conversion is the dominant pathway for rapid decay in RAx and RBx diradical series due to two major factors: (i) incorporation of the radicals results in new low-lying singlet and triplet states (SI T1) that act as "trap states"; and (ii) generation of multiple singlet and triplet states that are essentially degenerate in energy. Since incorporation of NN diradicals leads to more than 20 low-lying near degenerate singlet, triplet and quintet states, both intersystem crossing and internal conversion become viable decay mechanisms for the decay of the S and T,, states back to the So and To. These results establish and correlate structural and electronic parameters that impact spin coupling, spin delocalization, and determine general trends in predicting energy levels of excited states.