Title: Influence of the heavy-atom effect on singlet fission: a study of platinum-bridged pentacene dimers

Abstract

The process of singlet fission (SF) produces two triplet excited states (T₁ + T₁) from one singlet excited exciton (S₁) and a molecule in its ground state (S₀). It, thus, possesses the potential to boost the solar cell efficiency above the thermodynamic Shockley–Queisser limit of 33%. A key intermediate in the SF mechanism is the singlet correlated triplet pair state ¹(T₁T₁). This state is of great relevance, as its formation is spin-allowed and, therefore, very fast and efficient. Three fundamentally different pathways to formation of ¹(T₁T₁) have been documented so far. The factors that influence which mechanism is associated with which chromophore, however, remain largely unknown. In order to harvest both triplet excitons independently, a decorrelation of the correlated triplet pair state to two individual triplets is required. This second step of the SF process implies a change in the total spin quantum number. In the case of a dimer, this is usually only possible if the coupling between the two pentacenes is sufficiently weak. In this study, we present two platinum-bridged pentacene dimers in which the pentacenes are coupled strongly, so that spin-decorrelation yielding (T₁ + T₁) was initially expected to be outcompeted by triplet–triplet annihilation (TTA) to themore » ground state. Both platinum-bridged pentacene dimers undergo quantitative formation of the (T₁T₁) state on a picosecond timescale that is unaffected by the internal heavy-atom effect of the platinum. Instead of TTA of (T₁T₁) to the ground state, the internal heavy-atom effect allows for ¹(T₁T₁)–³(T₁T₁) and ¹(T₁T₁)–⁵(T₁T₁) mixing and, thus, triggers subsequent TTA to the (T₁S₀) state and minor formation of (T₁ + T₁). A combination of transient absorption and transient IR spectroscopy is applied to investigate the mechanism of the (T₁T₁) formation in both dimers. Using a combination of experiment and quantum chemical calculations, we are able to observe a transition from the CT-mediated to the direct SF mechanism and identify relevant factors that influence the mechanism that dominates SF in pentacene. Moreover, a combination of time-resolved optical and electron paramagnetic resonance spectroscopic data allows us to develop a kinetic model that describes the effect of enhanced spin–orbit couplings on the correlated triplet pair state.« less

Authors:

Basel, Bettina S. ^[1]; Young, Ryan M.  ^[2];

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Krzyaniak, Matthew D.  ^[2];

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Papadopoulos, Ilias ^[1]; Hetzer, Constantin ^[1]; Gao, Yueze  ^[3];

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La Porte, Nathan T.  ^[2];

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Phelan, Brian T.  ^[2];

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Clark, Timothy  ^[4];

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Tykwinski, Rik R.  ^[3];

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Wasielewski, Michael R.  ^[2];

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Guldi, Dirk M.  ^[1]

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+ Show Author Affiliations

1. Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 91058 Erlangen, Germany
2. Department of Chemistry and Institute for Sustainability and Energy at Northwestern (ISEN), Northwestern University, Evanston, USA
3. Department of Chemistry, University of Alberta, Edmonton, Canada
4. Department of Chemistry and Pharmacy & Computer-Chemistry-Center (CCC), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 91052 Erlangen, Germany

Publication Date:

2019-12-11

Research Org.:

Northwestern Univ., Evanston, IL (United States)

Sponsoring Org.:

USDOE Office of Science (SC)

OSTI Identifier:

1572432

Alternate Identifier(s):

OSTI ID: 1624982

Grant/Contract Number:  

FG02-99ER14999

Resource Type:

Published Article

Journal Name:

Chemical Science

Additional Journal Information:

Journal Name: Chemical Science Journal Volume: 10 Journal Issue: 48; Journal ID: ISSN 2041-6520

Publisher:

Royal Society of Chemistry (RSC)

Country of Publication:

United Kingdom

Language:

English

Subject:

Chemistry