Tahaoğlu Duygu, Usta Hakan, Alkan Fahri
Department of Nanotechnology Engineering, Abdullah Gül University, Kayseri 38080, Turkey.
J Phys Chem A. 2022 Jul 7;126(26):4199-4210. doi: 10.1021/acs.jpca.2c02435. Epub 2022 Jun 5.
In this study, we performed a detailed investigation of the S potential energy surface (PES) of -carborane-anthracene (-CB-) with respect to the C-C bond length on -CB and the dihedral angle between -CB and moieties. The effects of different substituents (F, Cl, CN, and OH) on carbon- or boron-substituted -CB, along with a π-extended acene-based fluorophore, pentacene, on the nature and energetics of S → S transitions are evaluated. Our results show the presence of a non-emissive S state with an almost pure charge transfer (CT) character for all systems as a result of significant C-C bond elongation (C-C = 2.50-2.56 Å) on -CB. In the case of unsubstituted -CB-, the adiabatic energy of this CT state corresponds to the global minimum on the S PES, which suggests that the CT state could be involved in emission quenching. Despite large deformations on the -CB geometry, predicted energy barriers are quite reasonable (0.3-0.4 eV), and the C-C bond elongation can even occur without a noticeable energy penalty for certain conformations. With substitution, it is shown that the dark CT state becomes even more energetically favorable when the substituent shows -M effects (e.g., -CN), whereas substituents showing +M effects (e.g., -OH) can result in an energy increase for the CT state, especially for partially stretched C-C bond lengths. It is also shown that the relative energy of the CT state on the PES depends strongly on the LUMO level of the fluorophore as this state is found to be energetically less favorable compared to other conformations when anthracene is replaced with π-extended pentacene. To our knowledge, this study shows a unique example of a detailed theoretical analysis on the PES of the S state in -CB- systems with respect to substituents or fluorophore energy levels. Our findings could guide future experimental work in emissive -CB- systems and their sensing/optoelectronic applications.
在本研究中，我们针对碳硼烷 - 蒽（-CB-）的单重态势能面（PES），就 -CB 上的 C-C 键长以及 -CB 与蒽部分之间的二面角进行了详细研究。评估了不同取代基（F、Cl、CN 和 OH）对碳取代或硼取代的 -CB 的影响，以及基于并苯的 π 扩展荧光团并五苯对 S→S 跃迁的性质和能量学的影响。我们的结果表明，由于 -CB 上 C-C 键显著伸长（C-C = 2.50 - 2.56 Å），所有体系中均存在具有几乎纯电荷转移（CT）特征的非发射单重态。对于未取代的 -CB-，该 CT 态的绝热能量对应于单重态 PES 上的全局最小值，这表明 CT 态可能参与发射猝灭。尽管 -CB 几何结构发生了较大变形，但预测的能垒相当合理（0.3 - 0.4 eV），并且对于某些构象，C-C 键伸长甚至可以在没有明显能量损失的情况下发生。通过取代表明，当取代基显示 -M 效应（例如 -CN）时，暗 CT 态在能量上变得更加有利，而显示 +M 效应的取代基（例如 -OH）会导致 CT 态能量增加，特别是对于部分拉伸的 C-C 键长。还表明，PES 上 CT 态的相对能量强烈依赖于荧光团的最低未占分子轨道（LUMO）水平，因为当用 π 扩展的并五苯取代蒽时，发现该态与其他构象相比在能量上不太有利。据我们所知，本研究展示了一个独特的例子，即针对 -CB- 体系中取代基或荧光团能级对单重态 PES 进行详细的理论分析。我们的发现可为发射型 -CB- 体系及其传感/光电子应用的未来实验工作提供指导。
