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Singlet fission has attracted considerable interest for its potential application in organic photovoltaics. However, the underlying microscopic mechanism is not well understood and the molecular parameters that govern SF efficiency remain unclear. We herein study the primary exciton photogeneration and evolution in the thin film of a series of pentacene derivatives (TIPS-Pn and ADPD-Pn) using femtosecond transient absorption spectroscopy. With a favorable "long-edge on" packing motif, the singlet-excited slip-stacked TIPS-Pn and ADPD-Pn molecules undergo ultrafast fission to produce triplet excitonic states with time constants of ∼0.3 ps. More importantly, the ADPD-Pn compound features a considerably higher triplet yield than TIPS-Pn (162 ± 10% vs 114 ± 15%). The enhanced electronic coupling as a result of closer interchromophore distance (3.33 Å for ADPD-Pn vs 3.40 Å for TIPS-Pn) is suggested to account for the much higher triplet yield for ADPD-Pn relative to that for TIPS-Pn, proving SF can be readily modulated by adjusting the intermolecular distance.

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.

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