Distinct properties of the triplet pair state from singlet fission

Authors

M. Tuan Trinh, Columbia University
Andrew Pinkard, Columbia University
Andrew B. Pun, Columbia University
Samuel N. Sanders, Columbia University
Elango Kumarasamy, Columbia University
Matthew Y. Sfeir, CUNY Graduate CenterFollow
Luis M. Campos, Columbia University
Xavier Roy, Columbia University
X.-Y. Zhu, Columbia University

Document Type

Article

Publication Date

7-14-2017

Abstract

Singlet fission, the conversion of a singlet exciton (S₁) to two triplets (2 × T₁), may increase the solar energy conversion efficiency beyond the Shockley-Queisser limit. This process is believed to involve the correlated triplet pair state ¹(TT). Despite extensive research, the nature of the ¹(TT) state and its spectroscopic signature remain actively debated. We use an end-connected pentacene dimer (BP0) as a model system and show evidence for a tightly bound ¹(TT) state. It is characterized in the near-infrared (IR) region (~1.0 eV) by a distinct excited-state absorption (ESA) spectral feature, which closely resembles that of the S₁ state; both show vibronic progressions of the aromatic ring breathing mode. We assign these near-IR spectra to ¹(TT)→S_n and S₁→S_n′ transitions; S_n and S_n′ likely come from the antisymmetric and symmetric linear combinations, respectively, of the S₂ state localized on each pentacene unit in the dimer molecule. The ¹(TT)→S_n transition is an indicator of the intertriplet electronic coupling strength, because inserting a phenylene spacer or twisting the dihedral angle between the two pentacene chromophores decreases the intertriplet electronic coupling and diminishes this ESA peak. In addition to spectroscopic signature, the tightly bound ¹(TT) state also shows chemical reactivity that is distinctively different from that of an individual T₁ state. Using an electron-accepting iron oxide molecular cluster [Fe₈O₄] linked to the pentacene or pentacene dimer (BP0), we show that electron transfer to the cluster occurs efficiently from an individual T₁ in pentacene but not from the tightly bound ¹(TT) state. Thus, reducing intertriplet electronic coupling in ¹(TT) via molecular design might be necessary for the efficient harvesting of triplets from intramolecular singlet fission.

Comments

This article was originally published in Science Advances, available at https://doi.org/10.1126/sciadv.1700241

This work is distributed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).