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.

Solvent Effect on Morphology and Optical Properties of Poly(3-hexylthiophene):TIPS-Pentacene Blends

“…40 For the P3HT:TP blend films obtained in toluene and trichlorobenzene, the last absorption peak showed a red-shift compared with the same peak for the P3HT:TP film obtained in chloroform, indicating a narrower highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap for both P3HT:TP films (obtained in toluene and trichlorobenzene). 41 PL spectra of the neat P3HT film (in chloroform) and P3HT:TP blend films for the three different solvents are presented in Fig. 8.…”

Triplet transport in thin films: fundamentals and applications

“…Current investigations into the intermediates in singlet fission, e.g. bound triplet pair excimers 99 or strongly interacting triplet pairs, 136 etc., enabled by various monomer or dimer configurations 97,[137][138][139][140][141][142] will shed light on the dielectric environment needed to stabilize triplet excitons. Such interdisciplinary approaches will reveal the fundamental structure-property relationships affecting triplet diffusion in thin films, and ultimately enable the engineering of optoelectronic devices making use of multi-excitonic processes.…”

Singlet fission in a hexacene dimer: energetics dictate dynamics

“…The rate constant decreases considerably in hexacene, where DE S-TT is on the order of several molecular vibrations. 36,45,46 The important role of vibrations in iSF has also been explored. 17 However, no coherent generation of triplet pairs has been reported to date.…”