title: Excessive Exoergicity Reduces Singlet Exciton Fission Efficiency of Heteroacenes in Solutions.
abstract: The energy difference between a singlet exciton and twice of a triplet exciton, ΔESF, provides the thermodynamic driving force for singlet exciton fission (SF). This work reports a systematic investigation on the effect of ΔESF 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 ΔESF, 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 ΔESF 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.
authors.affiliation: State Key Laboratory of Applied Organic Chemistry (SKLAOC), Key Laboratory of Special Function Materials and Structure Design (MOE), College of Chemistry and Chemical Engineering, Lanzhou University , Lanzhou 730000, P. R. China.
authors.forename: You-Dan
authors.initials: YD
authors.lastname: Zhang
authors.affiliation: Beijing National Laboratory for Molecules Science (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, and Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, P. R. China.
authors.forename: Yishi
authors.initials: Y
authors.lastname: Wu
authors.affiliation: National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University , Nanjing 210093, China.
authors.forename: Yanqing
authors.initials: Y
authors.lastname: Xu
authors.affiliation: State Key Laboratory of Applied Organic Chemistry (SKLAOC), Key Laboratory of Special Function Materials and Structure Design (MOE), College of Chemistry and Chemical Engineering, Lanzhou University , Lanzhou 730000, P. R. China.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Wang
authors.affiliation: State Key Laboratory of Applied Organic Chemistry (SKLAOC), Key Laboratory of Special Function Materials and Structure Design (MOE), College of Chemistry and Chemical Engineering, Lanzhou University , Lanzhou 730000, P. R. China.
authors.forename: Ke
authors.initials: K
authors.lastname: Liu
authors.affiliation: Beijing National Laboratory for Molecules Science (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, and Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, P. R. China.
authors.forename: Jian-Wei
authors.initials: JW
authors.lastname: Chen
authors.affiliation: State Key Laboratory of Applied Organic Chemistry (SKLAOC), Key Laboratory of Special Function Materials and Structure Design (MOE), College of Chemistry and Chemical Engineering, Lanzhou University , Lanzhou 730000, P. R. China.
authors.forename: Jing-Jing
authors.initials: JJ
authors.lastname: Cao
authors.affiliation: National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University , Nanjing 210093, China.
authors.forename: Chunfeng
authors.initials: C
authors.lastname: Zhang
authors.affiliation: Beijing National Laboratory for Molecules Science (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, and Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, P. R. China.
authors.forename: Hongbing
authors.initials: H
authors.lastname: Fu
authors.affiliation: State Key Laboratory of Applied Organic Chemistry (SKLAOC), Key Laboratory of Special Function Materials and Structure Design (MOE), College of Chemistry and Chemical Engineering, Lanzhou University , Lanzhou 730000, P. R. China.
authors.forename: Hao-Li
authors.initials: HL
authors.lastname: Zhang