Nature of ground and electronic excited states of higher acenes
Nature of ground and electronic excited states of higher acenes
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DOI：
10.1073/pnas.1606021113
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发表时间：
2016-08-30
影响因子：
11.1
通讯作者：
Yang, Weitao
中科院分区：
文献类型：
--
作者：
Yang, Yang;Davidson, Ernest R.;Yang, Weitao
高碳烯作为一种具有多种电学性质的有机半导体，受到了广泛的关注。然而，它们的基态和电子激发态的性质仍然不完全清楚。它们不寻常的化学反应活性和不稳定性是实验研究的主要障碍，而在如此大的体系中可能需要多参考描述的潜在突出的双自由基特征阻碍了理论研究。在这里，我们用粒子-粒子随机位相近似计算给出了详细的答案。直至十一烯的(1)A(G)基态位于双自由基连续体的闭壳侧，而十一烯和十二烯的基态则更多地向开壳侧倾斜，具有不断增长的多自由基特性。所有苯的基态相对于短轴和长轴都具有共价性。最低的三重态B-3(2U)总是高于单重态基态，即使在聚并苯极限下能隙小得几乎为零。明亮的单重态激发态B-1(2U)是沿短轴方向的两性离子状态。激发态(1)A(G)逐渐从双激发态向另一个两性离子态转变到短轴，但始终保持其对长轴的共价性。在六烯和庚烯之间发生了B-1(2U)和激发(1)A(G)D态之间的能量交叉。进一步的能量考虑表明，较高的苯很可能发生单态分裂，光伏效率较低；然而，如果能够实现单态分裂为多个三态，那么光伏效率可能会提高。
Higher acenes have drawn much attention as promising organic semiconductors with versatile electronic properties. However, the nature of their ground state and electronic excited states is still not fully clear. Their unusual chemical reactivity and instability are the main obstacles for experimental studies, and the potentially prominent diradical character, which might require a multireference description in such large systems, hinders theoretical investigations. Here, we provide a detailed answer with the particle-particle random-phase approximation calculation. The (1)A(g) ground states of acenes up to decacene are on the closed-shell side of the diradical continuum, whereas the ground state of undecacene and dodecacene tilts more to the open-shell side with a growing polyradical character. The ground state of all acenes has covalent nature with respect to both short and long axes. The lowest triplet state B-3(2u) is always above the singlet ground state even though the energy gap could be vanishingly small in the polyacene limit. The bright singlet excited state B-1(2u) is a zwitterionic state to the short axis. The excited (1)A(g) state gradually switches from a double-excitation state to another zwitterionic state to the short axis, but always keeps its covalent nature to the long axis. An energy crossing between the B-1(2u) and excited (1)A(g)D states happens between hexacene and heptacene. Further energetic consideration suggests that higher acenes are likely to undergo singlet fission with a low photovoltaic efficiency; however, the efficiency might be improved if a singlet fission into multiple triplets could be achieved.