Understanding and Tuning Singlet–Triplet (S 1 –T 1 ) Energy Gaps in Planar Organic Chromophores

Abstract Molecules with large gaps between their first singlet and triplet excited states (Δ E ST ) are key components of various modern technologies, most prominently singlet fission photovoltaics and triplet–triplet annihilation upconversion (TTA‐UC). The design of these molecules is hampered by the fact that only limited rules for maximizing Δ E ST exist, other than increasing the overlap between the frontier molecular orbitals (FMO). Here we suggest a new strategy for tuning and maximizing Δ E ST based on a detailed analysis of the underlying quantum mechanical energy terms. We present a model based on the transition density and derive three straightforward design rules: Δ E ST values can be maximized by (i) minimizing the overall number of π‐electrons, (ii) reducing delocalization, and (iii) optimizing specific geometric interactions. The validity of these rules is first exemplified for a set of 18 hydrocarbon backbones before proceeding to a varied set of dye molecules, highlighting their transferability to realistic settings. We believe that the developed rules will provide an enormous boost to the field, enabling rational design instead of trial‐and‐error screening. More generally, this work demonstrates the power of going beyond the FMO approximation in designing advanced molecular materials.