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Title: Distinct properties of the triplet pair state from singlet fission

Singlet fission, the conversion of a singlet exciton (S 1) to two triplets (2 × T 1), 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 S 1 state; both show vibronic progressions of the aromatic ring breathing mode. We assign these near-IR spectra to 1(TT)→S n and S 1→S n' transitions; S n and S n' likely come from the antisymmetric and symmetric linear combinations, respectively, of the S 2 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 tightlymore » bound 1(TT) state also shows chemical reactivity that is distinctively different from that of an individual T 1 state. Using an electron-accepting iron oxide molecular cluster [Fe 8O 4] linked to the pentacene or pentacene dimer (BP0), we show that electron transfer to the cluster occurs efficiently from an individual T 1 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.« less
Authors:
 [1] ;  [1] ; ORCiD logo [1] ; ORCiD logo [1] ; ORCiD logo [1] ; ORCiD logo [2] ;  [1] ;  [1] ;  [1]
  1. Columbia Univ., New York, NY (United States). Department of Chemistry
  2. Brookhaven National Lab. (BNL), Upton, NY (United States). Center for Functional Nanomaterials
Publication Date:
Report Number(s):
BNL-114744-2017-JA
Journal ID: ISSN 2375-2548; KC0403020
Grant/Contract Number:
SC0012704; SC0014563
Type:
Accepted Manuscript
Journal Name:
Science Advances
Additional Journal Information:
Journal Volume: 3; Journal Issue: 7; Journal ID: ISSN 2375-2548
Publisher:
AAAS
Research Org:
Brookhaven National Lab. (BNL), Upton, NY (United States). Center for Functional Nanomaterials (CFN)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Center for Functional Nanomaterials
OSTI Identifier:
1413949