Theoretical approach to weakly bound triplet-triplet multiexciton in intramolecular singlet fission chromophores

该奖项支持在有机材料中将太阳能转化为电能的物理过程的理论和计算研究。太阳能电池吸收阳光并产生电能，在减少对化石燃料衍生能源的需求方面正在并将继续发挥重要作用。由有机分子或聚合物构成的太阳能电池之所以引起人们的兴趣，是因为它们价格低廉、易于加工、重量轻且灵活。由于热力学的限制，今天的有机太阳能电池产生的电力是有限的。目前，科学界正在努力克服这些限制，制造出功率更高的太阳能电池。一种很有前途的方法是使用单线态裂变，这是一种光物理过程，其中光吸收产生的电流原则上增加了一倍。单线态裂变的实际应用需要进一步的研究。电荷的产生分两步进行，虽然在发色团的发展中取得了令人印象深刻的成功，其中第一步非常有效，但在几乎所有情况下，第二步花费的时间太长，电荷和能量都损失在了重组过程中。PI将进行量子化学计算，这将导致新的分子材料，其中单线态裂变的第二步，即电荷分离，将超快地发生，从而导致具有更高量子效率的有机太阳能电池。这项研究的很大一部分是与专门从事单线态裂变实验研究的小组合作进行的。这项工作的广泛影响包括对一名研究生和一名博士后的跨学科培训，在缺乏服务的群体中培养对化学和物理的兴趣，以及提高科学素养和公众意识。该奖项支持有机太阳能电池材料单线态裂变过程的理论和计算研究和教育。直到最近，对单线态裂变的研究都集中在设计能使单线态到三重态-三重态快速转换的材料上。快速三胞胎-三胞胎生成；然而，这是强的发色团间耦合的结果，也导致强的三重-三重态多激子结合和快速的三重态重组，而不是产生自由的三重态。克服这一僵局需要清楚地了解键基序与产生具有弱结合能的三重态-三重态多激子之间的关系，这种关系最近在分子内单线态裂变材料的不同家族中被发现。PI将使用图解激子基在半经验的pariser - parr - people哈密顿量中执行高阶多参考构型相互作用计算，从而产生具有预测能力的精确结构-性质相关性。这项研究的目标是达到在超快时间内快速产生自由三重子的结构特征。这项研究为太阳能和其他技术提供了基础知识，特别是那些可能基于有机材料的技术。参与该项目的初级研究人员正在获得最先进的计算方法的经验，包括量子多体物理和化学。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NONTECHNICAL SUMMARYThis award supports theoretical and computational research on physical processes to convert solar energy to electricity in organic materials. Solar cells, which absorb sunlight and generate electricity, are playing and will continue to play a strong role in reducing the demand for fossil-fuel derived energy. Solar cells constructed out of organic molecules or polymers are of interest because they are inexpensive, easy to process, lightweight and flexible. The electric power generated by today’s organic solar cells is limited because of thermodynamic constraints. There is currently intense scientific effort to overcome these constraints and to construct solar cells that yield higher power. A promising approach is to use singlet fission, a photophysical process within which the electrical current generated by light absorption is in principle doubled. Practical utilization of singlet fission requires further research. Charge generation here occurs in two steps, and while impressive success has been reached in the development of chromophores in which the first step is very efficient, in nearly all cases the second step takes too long, and electric charge, and so, energy, is lost to the process of recombination. The PI will perform quantum chemical calculations that will lead to new molecular materials in which the second step of singlet fission, charge separation, will occur ultrafast, leading to organic solar cells with higher quantum efficiencies. A significant portion of the research is in collaboration with groups that specialize in experimental research on singlet fission. The broader impacts of this work include interdisciplinary training of a graduate student and a postdoctoral fellow, outreach efforts fostering interest in chemistry and physics among members of underserved groups, and increased scientific literacy and public awareness.TECHNICAL SUMMARY This award supports theoretical and computational research and education on the process of singlet fission in organic solar cell materials. Research on singlet fission until recently focused on designing materials that give rapid singlet to triplet-triplet conversion. Rapid triplet-triplet generation; however, is a consequence of strong interchromophore coupling that also leads to strong triplet-triplet multiexciton binding and fast triplet recombination instead of generation of free triplets. Overcoming this impasse requires clear understanding of the relationship between bonding motifs and the generation of triplet-triplet multiexcitons with weak binding energy, that have been discovered recently in different families of intramolecular singlet fission materials. The PI will perform high order multiple reference configuration interaction-calculations within the semiempirical Pariser-Parr-Pople Hamiltonian using a diagrammatic exciton basis that will yield precise structure-property correlations with predictive capability. The goal of this research is to arrive at structural features that will give fast generation of free triplets in ultrafast times. The research contributes to fundamental knowledge that underlies solar energy and other technologies, particularly those that could be based on organic materials. Junior researchers involved in this project are gaining experience in state-of-the-art computational methods, including quantum many-body physics and chemistry.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.