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

Singlet exciton fission photovoltaics 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, 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 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 (K_H,L), which ideally is constant throughout the compound class, providing a constant DE(S₁-T₁). This leads us to a geometric model for identification of singlet fission chromophores, and we explore what factors limit the model. Candidates with calculated E(T₁) of ~1 eV or higher are identified among benzannelated 4npi-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.