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> (2019) Strategies for Design of Potential Singlet, Fission Chromophores Utilizing Baird's R
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(2019) Strategies for Design of Potential Singlet Fission Chromophores Utilizing Baird's Rule on Excited State Aromaticity
Источники: https://www.researchgate.net/publication/33...ate_Aromaticity
DOI: 10.26434/chemrxiv.10318043

АВторы: Ouissam El Bakouri, Joshua R. Smith, Henrik Ottosson
- Department of Chemistry - Ångström Laboratory, Uppsala University, Box 523, 751 20 Uppsala, Sweden
- Department of Chemistry, Humboldt State University, One Harpst Street, Arcata, CA 95521, USA

Description
QUOTE
In singlet exciton fission one photon of light is used to create two excitons of triplet multiplicity. This process requires chromophores with their lowest excited states arranged so that 2 E (T<sub>1</sub>) < E (S<sub>1</sub>) and E (S<sub>1</sub>) < E (T<sub>2</sub>). To match different technology platforms there is a high need for new candidate chromophores with the desired excited state orderings. Herein, qualitative theory and quantum chemical calculations are used to develop explicit strategies on how to use Baird’s 4n rule on excited state aromaticity to tailor new potential chromophores for singlet fission.

We first analyze the E (T<sub>1</sub>), E (S<sub>1</sub>) and E (T<sub>2</sub>) of benzene and cyclobutadiene (CBD) as, respectively, excited state antiaromatic and aromatic archetypes, and reveal that CBD fulfils the criteria on the state ordering for a singlet fission chromophore. We then look at fulvenes, a class of compounds that can be tuned from Baird-antiaromatic to Baird-aromatic in T<sub>1</sub> and S<sub>1</sub> by choice of substituents. The T<sub>1</sub> and S<sub>1</sub> states of fulvenes are both described by singly excited HOMO→LUMO configurations, which provides a rational for the simultaneous and similar tuning of E (T<sub>1</sub>) and E (S<sub>1</sub>) along an approximate (anti)aromaticity coordinate. This leads us to a geometric model for identification of singlet fission chromophores. Candidates with calculated E (T<sub>1</sub>) of ~1 eV or higher are also identified among benzannelated 4 n pi-electron compound classes and among siloles influenced to various extents by Baird-(anti)aromaticity in T<sub>1</sub> and S<sub>1</sub>.

Finally, we explore the limitations of the design approach. In brief, it is clarified how Baird’s 4 n rule together with substituent effects (electronic and steric) and benzannelation can be used to tailor new chromophores with potential use in singlet fission photovoltaics.


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Introduction

Research on solar energy harvesting is one of the most active areas within chemistry, and photovoltaics is one of the main directions for turning solar energy into electricity. Today, three different generations of photovoltaics exist.1,2 The third and most recent generation includes materials able to overcome the Shockley-Queisser limit (~33%).3,4 This generation includes singlet exciton fission photovoltaics, or shortly singlet fission, i.e., a process where one photon of light, absorbed by a molecule, is used to create two excitons of triplet multiplicity in two molecules or in two chromophores linked intramolecularly.5,6


QUOTE
References
                                                         
1 Shukla, A. K.; Sudhakar, K.; Baredar, P. A Comprehensive Review on Design of
Building Integrated Photovoltaic System. Energy Build. 2016, 128, 99–110.
2 Ranabhat, K.; Patrikeev, L.; Antal’evna-Revina, A.; Andrianov, K.; Lapshinsky, V.; Sofronova, E. An Introduction to Solar Cell Technology. J. Appl. Eng. Sci. 2016, 14, 481–491.
3 Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of p‐n Junction Solar
Cells. J. Appl. Phys. 1961, 32, 510–519.
4 Nelson, C. A.; Monahan, N. R.; Zhu, X. Y. Exceeding the Shockley–Queisser limit in solar energy conversion. Energy Environ. Sci. 2013, 6, 3508.
5 Casanova, D. Theoretical Modeling of Singlet Fission. Chem. Rev. 2018, 118, 7164–
7207.


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