Yahagh Aida, Kaswan Ram R, Kazemi Shahrzad, Karr Paul A, D'Souza Francis
Department of Chemistry, University of North Texas 1155 Union Circle #305070 Denton TX 76203-5017 USA
Department of Physical Sciences and Mathematics, Wayne State College 111 Main Street Wayne NE 68787 USA.
Chem Sci. 2023 Dec 15;15(3):906-913. doi: 10.1039/d3sc05034c. eCollection 2024 Jan 17.
Symmetry breaking charge transfer is one of the important photo-events occurring in photosynthetic reaction centers that is responsible for initiating electron transfer leading to a long-lived charge-separated state and has been successfully employed in light-to-electricity converting optoelectronic devices. In the present study, we report a newly synthesized, far-red absorbing and emitting BODIPY-dimer to undergo symmetry-breaking charge transfer leading to charge-separated states of appreciable lifetimes in polar solvents. Compared to its monomer analog, both steady-state and time-resolved fluorescence originating from the S state of the dimer revealed quenching which increased with an increase in solvent polarity. The electrostatic potential map from DFT and the time-dependent DFT calculations suggested the existence of a quadrupolar type charge transfer state in polar solvents, and the singlet excited state to be involved in the charge separation process. The electrochemically determined redox gap being smaller than the energy of the S state supported the thermodynamic feasibility of the envisioned symmetry-breaking charge transfer and separation. The spectrum of the charge-separated state arrived from spectroelectrochemical studies, revealing diagnostic peaks helpful for transient spectral interpretation. Finally, ultrafast transient pump-probe spectroscopy provided conclusive evidence of diabatic charge separation in polar solvents by far-red pulsed laser light irradiation. The measured lifetime of the final charge-separated states was found to be 165 ps in dichlorobenzene, 140 ps in benzonitrile, and 43 ps in dimethyl sulfoxide, revealing their significance in light energy harvesting, especially from the less-explored far-red region.
对称破缺电荷转移是光合反应中心发生的重要光事件之一，它负责启动电子转移，导致形成长寿命的电荷分离态，并且已成功应用于光电转换光电器件。在本研究中，我们报道了一种新合成的、吸收和发射远红光的硼二吡咯二聚体，它在极性溶剂中经历对称破缺电荷转移，导致具有可观寿命的电荷分离态。与其单体类似物相比，源自二聚体S态的稳态荧光和时间分辨荧光均显示猝灭，且随着溶剂极性的增加而增强。密度泛函理论（DFT）和含时DFT计算得到的静电势图表明，在极性溶剂中存在一种四极型电荷转移态，且单重激发态参与电荷分离过程。电化学测定的氧化还原能隙小于S态的能量，这支持了所设想的对称破缺电荷转移和分离的热力学可行性。光谱电化学研究得到的电荷分离态光谱显示出有助于瞬态光谱解释的诊断峰。最后，超快瞬态泵浦 - 探测光谱通过远红光脉冲激光照射，为极性溶剂中的非绝热电荷分离提供了确凿证据。在二氯苯中测得的最终电荷分离态的寿命为165 ps，在苄腈中为140 ps，在二甲基亚砜中为43 ps，这表明它们在光能收集方面具有重要意义，特别是在探索较少的远红光区域。
