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 2 E (T 1 ) < E (S 1 ) and E (S 1 ) < E (T 2 ). Herein, qualitative theory and quantum chemical calculations are used to develop explicit strategies on how to use Baird’s 4 n rule on excited state aromaticity, combined with Hückel’s 4 n +2 rule for ground state aromaticity, to tailor new potential chromophores for singlet fission. We first analyze the E (T 1 ), E (S 1 ) and E (T 2 ) 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 T 1 and S 1 , and from Hückel-aromatic to Hückel-antiaromatic in S 0 . The T 1 and S 1 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 (T 1 ) and E (S 1 ) 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 D E (S 1 -T 1 ). 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 (T 1 ) of ~1 eV or higher are identified among benzannelated 4 n pi-electron compound classes and siloles. In brief, it is clarified how the joint utilization of Baird’s 4 n and Hückel’s 4 n +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.