Excessive Exoergicity Reduces Singlet Exciton Fission Efficiency of Heteroacenes in Solutions

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• 作者：You-Dan Zhang ; Yishi Wu ; Yanqing Xu ; Qiang Wang ; Ke Liu ; Jian-Wei Chen ; Jing-Jing Cao ; Chunfeng Zhang ; Hongbing Fu ; Hao-Li Zhang
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：June 1, 2016
• 年：2016
• 卷：138
• 期：21
• 页码：6739-6745
• 全文大小：472K
• 年卷期：0
• ISSN：1520-5126

文摘

The energy difference between a singlet exciton and twice of a triplet exciton, ΔE_SF, provides the thermodynamic driving force for singlet exciton fission (SF). This work reports a systematic investigation on the effect of ΔE_SF on SF efficiency of five heteroacenes in their solutions. The low-temperature, near-infrared phosphorescence spectra gave the energy levels of the triplet excitons, allowing us to identify the values of ΔE_SF, which are −0.58, −0.34, −0.31, −0.32, and −0.34 eV for the thiophene, benzene, pyridine, and two tetrafluorobenzene terminated molecules, respectively. Corresponding SF efficiencies of the five heteroacenes in 0.02 M solutions were determined via femtosecond transient absorption spectroscopy to be 117%, 124%, 140%, 132%, and 135%, respectively. This result reveals that higher ΔE_SF is not, as commonly expected, always beneficial for higher SF efficiency in solution phase. On the contrary, excessive exoergicity results in reduction of SF efficiency in the heteroacenes due to the promotion of other competitive exciton relaxation pathways. Therefore, it is important to optimize thermodynamic driving force when designing organic materials for high SF efficiency.