Strategies for Design of Potential Singlet Fission Chromophores Utilizing a Combination of Ground State and Excited State Aromaticity Rules

Abstract

Singlet exciton fission photovoltaics requires chromophores with their lowest excited states arranged so that 2<i>E</i>(T<sub>1</sub>) < <i>E</i>(S<sub>1</sub>) and <i>E</i>(S<sub>1</sub>) < <i>E</i>(T<sub>2</sub>). Herein, qualitative theory and quantum chemical calculations are used to develop explicit strategies on how to use Baird’s 4<i>n</i> rule on excited state aromaticity, combined with Hückel’s 4<i>n</i>+2 rule for ground state aromaticity, to tailor new potential chromophores for singlet fission. We first analyze the <i>E</i>(T<sub>1</sub>), <i>E</i>(S<sub>1</sub>) and <i>E</i>(T<sub>2</sub>) of benzene and cyclobutadiene (<b>CBD</b>) as, respectively, excited state antiaromatic and aromatic archetypes, and reveal that <b>CBD </b>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 by choice of substituents from Baird-antiaromatic to Baird-aromatic in T<sub>1</sub> and S<sub>1</sub>, and from Hückel-aromatic to Hückel-antiaromatic in S<sub>0</sub>. The T<sub>1</sub> and S<sub>1</sub> states of most substituted fulvenes (159 of 225) are described by singly excited HOMO→LUMO configurations, providing a rational for the simultaneous tuning of <i>E</i>(T<sub>1</sub>) and <i>E</i>(S<sub>1</sub>) along an approximate (anti)aromaticity coordinate. Key to the tunability is the exchange integral (K<sub>H,L</sub>), which ideally is constant throughout the compound class, providing a constant D<i>E</i>(S<sub>1</sub>-T<sub>1</sub>). This leads us to a geometric model for identification of singlet fission chromophores, and we explore what factors limit the model. Candidates with calculated <i>E</i>(T<sub>1</sub>) of ~1 eV or higher are identified among benzannelated 4<i>n</i>pi-electron compound classes and siloles. In brief, it is clarified how the joint utilization of Baird’s 4<i>n</i> and Hückel’s 4<i>n</i>+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.<br>