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

Singlet exciton fission photovoltaic technology requires chromophores with their lowest excited states arranged so that 2E(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 4n rule on excited-state aromaticity, combined with Huckel's 4n + 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 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 T-1 and S-1 and from Huckel-aromatic to Huckel-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,K-L), which ideally is constant throughout the compound class, providing a constant.E(S-1 - T-1). 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(T-1) values of similar to 1 eV or higher are identified among benzannelated 4npelectron compound classes and siloles. In brief, it is clarified how the joint utilization of Baird's 4n and Huckel'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.