Zhang Shushu, Qu Zexing, Tao Peng, Brooks Bernard, Shao Yihan, Chen Xiaoyuan, Liu Chungen
Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of the Ministry of Education (MOE), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, China.
J Phys Chem C Nanomater Interfaces. 2012 Jun 14;116(23):12434-12442. doi: 10.1021/jp3027447. Epub 2012 May 21.
Recent experimental investigation (Reitzenstein and Lambert,Macromolecules, 2009, 42, 773) indicated that the quite different optical properties of 2,7- and 3,6-linkage triarylboryl carbazole oligomers may arise from the different nature of their low-lying excited states: a low-lying delocalized within-backbone excitation in longer 2,7-linked oligomers vs a backbone-to-sidechain charge-transfer (CT) excitation independent of the polymerization length in 3,6-linked oligomers. Here in this paper, two long-range corrected functionals, CAM-B3LYP and ωB97X, are applied together with the traditional B3LYP functional in time-dependent density functional theory (TDDFT) calculations to systematically investigate the low-lying electronic excitations in both oligomers. Our calculations indicate that an extensive conjugation exists between monomer molecular orbitals in 2,7-linked oligomers, which is absent in those of 3,6-linked structures, resulting in a considerable narrowing of the HOMO-LUMO gap of their backbone moiety, while having little effect on the side-chains. CAM-B3LYP and ωB97x calculations confirm that the lowest-energy absorption is a within-backbone excitation in longer 2,7-linked oligomers as opposed to a backbone to side-chain charge transfer excitation in 2,7-linked oligmers of shorter length and 3,6-linked oligomers of any length. All these findings are consistent with the experimental findings and the qualitative energy diagram proposed by Reitzenstein and Lambert.
最近的实验研究（Reitzenstein和Lambert，《大分子》，2009年，第42卷，第773页）表明，2,7-和3,6-连接的三芳基硼咔唑低聚物截然不同的光学性质可能源于其低激发态的不同性质：在较长的2,7-连接低聚物中，低激发态是一种在主链内离域的激发，而在3,6-连接低聚物中，是一种与聚合长度无关的主链到侧链的电荷转移（CT）激发。在本文中，两种长程校正泛函CAM-B3LYP和ωB97X与传统的B3LYP泛函一起应用于含时密度泛函理论（TDDFT）计算，以系统地研究这两种低聚物的低激发态电子激发。我们的计算表明，在2,7-连接的低聚物中，单体分子轨道之间存在广泛的共轭，而在3,6-连接结构的低聚物中则不存在，这导致其主链部分的HOMO-LUMO能隙显著变窄，而对侧链影响很小。CAM-B3LYP和ωB97X计算证实，最低能量吸收在较长的2,7-连接低聚物中是主链内激发，而在较短长度的2,7-连接低聚物和任何长度的3,6-连接低聚物中是主链到侧链的电荷转移激发。所有这些发现与实验结果以及Reitzenstein和Lambert提出的定性能量图一致。
