Polarons in π-conjugated ladder-type polymers: a broken symmetry density functional description
Polarons in π-conjugated ladder-type polymers: a broken symmetry density functional description
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DOI：
10.1039/c9tc03283e
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发表时间：
2019-11-07
影响因子：
6.4
通讯作者：
Negri, Fabrizia
中科院分区：
文献类型：
--
作者：
Fazzi, Daniele;Fabiano, Simone;Negri, Fabrizia
电子带电态（即极化子）在有机功能材料的电荷转移、自旋、热电和氧化还原机制中起着至关重要的作用。要理解它们的响应和输运性质，必须在量子化学水平上进行准确的描述。我们报道了一种高电子导电性（n型）偶联阶梯型聚合物，即聚苯并咪唑苯并菲罗啉（BBL）的极化子性质的综合计算研究。我们展示了自旋极化无限制密度泛函理论（UDFT）和限制（RDFT）方法如何导致非最稳定的极化子和双极化子电子波函数的解。这方面可以追溯到电子带电态波函数的多构型特征。我们展示了破对称DFT （BS-UDFT）如何绕过这个问题，从自旋密度和结构变形的角度很好地描述了极化子/双极化子的局域化，从而提供了对电子传递参数（如重组能）的正确评估，否则在UDFT/RDFT水平上计算是错误的。通过比较掺杂BBL薄膜上极化态物质的红外光谱与实验光谱，进一步验证了我们的计算。我们的研究要求对高性能π共轭材料（如阶梯型聚合物和其他供体-受体衍生物）中的电子带电态（如极化子、双极化子等）进行紧急和仔细的计算评估，以正确理解它们的电荷、热量和自旋输运机制。
Electronic charged states (i.e., polarons) play a crucial role in governing charge transfer, spin, thermo-electric and redox mechanisms in organic functional materials. An accurate description at the quantum-chemical level is mandatory to understand their response and transport properties. We report a comprehensive computational investigation concerning the polaron properties of a high electron conductivity (n-type) pi-conjugated ladder-type polymer, namely polybenzimidazobenzophenanthroline (BBL). We show how spin polarized unrestricted Density Functional Theory (UDFT) and restricted (RDFT) methods can lead to solutions of the polaron and bipolaron electronic wavefunctions which are not the most stable ones. This aspect can be traced back to the multiconfigurational character of the electronic charged states' wavefunction. We demonstrate how broken symmetry DFT (BS-UDFT) can circumvent this issue, well describing the polaron/bipolaron localization in terms of spin densities and structural deformations, thus providing a correct assessment of the electron transport parameters (e.g., reorganization energy), otherwise incorrectly computed at the UDFT/RDFT levels. Our calculations are further validated by comparing the IR spectra of polaronic species with the experimental one, as measured on doped BBL films. Our study calls for an urgent and careful computational assessment of the electronic charged states (e.g., polaron, bipolaron, etc.), in high performance pi-conjugated materials, such as ladder-type polymers and other donor-acceptor derivatives, for a correct understanding of their charge, heat, and spin transport mechanisms.