Strategies for Design of Potential Singlet Fission Chromophores Utilizing Baird's Rule on Excited State Aromaticity

In singlet exciton fission one photon of light is used to create two excitons of triplet multiplicity. This process requires chromophores with their lowest excited states arranged so that 2E(T₁) < E(S₁) and E(S₁) < E(T₂). To match different technology platforms there is a high need for new candidate chromophores with the desired excited state orderings. 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 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 from Baird-antiaromatic to Baird-aromatic in T₁ and S₁ by choice of substituents. The T₁ and S₁ states of fulvenes are both described by singly excited HOMO→LUMO configurations, which provides a rational for the simultaneous and similar tuning of E(T₁) and E(S₁) along an approximate (anti)aromaticity coordinate. This leads us to a geometric model for identification of singlet fission chromophores. Candidates with calculated E(T₁) of ~1 eV or higher are also identified among benzannelated 4npi-electron compound classes and among siloles influenced to various extents by Baird-(anti)aromaticity in T₁ and S₁. Finally, we explore the limitations of the design approach. In brief, it is clarified how Baird’s 4n rule together with substituent effects (electronic and steric) and benzannelation can be used to tailor new chromophores with potential use in singlet fission photovoltaics.