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Strategies for Design of Potential Singlet Fission Chromophores Utilizing a Combination of Ground-State and Excited-State Aromaticity Rules
journal contribution
posted on 2020-03-13, 16:07 authored by Ouissam El Bakouri, Joshua R. Smith, Henrik OttossonSinglet exciton fission photovoltaic
technology requires chromophores
with their lowest excited states arranged so that 2E(T1) < E(S1) and E(S1) < E(T2).
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, combined with Hückel’s
4n + 2 rule for ground-state aromaticity, to tailor
new potential chromophores for singlet fission. We first analyze the E(T1), E(S1), and E(T2) of benzene and cyclobutadiene (CBD) as
excited-state antiaromatic and aromatic archetypes, respectively,
and reveal that CBD fulfills the criteria on the state ordering for
a singlet fission chromophore. We then look at fulvenes, a class of
compounds that can be tuned by choice of substituents from Baird-antiaromatic
to Baird-aromatic in T1 and S1 and from Hückel-aromatic
to Hückel-antiaromatic in S0. The T1 and
S1 states of most substituted fulvenes (159 of 225) are
described by singly excited HOMO → LUMO configurations, providing
a rational for the simultaneous tuning of E(T1) and E(S1) along an approximate
(anti)aromaticity coordinate. Key to the tunability is the exchange
integral (KH,L), which ideally is constant
throughout the compound class, providing a constant ΔE(S1 – T1). This leads us to
a geometric model for the identification of singlet fission chromophores,
and we explore what factors limit the model. Candidates with calculated E(T1) values of ∼1 eV or higher are identified
among benzannelated 4nπ-electron compound classes
and siloles. In brief, it is clarified how the joint utilization of
Baird’s 4n and Hückel’s 4n + 2 rules, 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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HusubstituentLUMOT 2T 1excited-statesinglet fission photovoltaicsBairdCBDquantum chemical calculationsHOMOaromaticityS 1 statesbenzannelated 4 n π- electron compound classesfulveneExcited-State Aromaticity Rules Singlet exciton fission photovoltaic technologysinglet fission chromophoresS 1Potential Singlet Fission Chromophores Utilizingsinglet fission chromophoremodel
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