Benchmark Reference Data and Computational Protocols for Predicting Excitation Energies of Thioindigoid Photoswitches

The accurate prediction of electronically excited states is essential for the computational design of molecular photoswitches but remains challenging for thioindigoid chromophores owing to their complex excited-state electronic structure. In this work, we establish theoretical best estimates for the two lowest singlet and three lowest triplet vertical excitation energies of a chemically diverse set of hemithiophenindigo (HTPI) derivatives using high-level coupled cluster methods (CC3 and CCSDT). The selected model systems retain the characteristic chromophoric core of thioindigo and hemithioindigo photoswitches while remaining accessible to benchmark calculations. Analysis of electron-rich and electron-poor derivatives reveals pronounced position-dependent effects on the low-lying excited states that closely resemble trends reported for larger thioindigoid chromophores. Based on this reference data, we benchmark 65 semiempirical and quantum-chemical methods, including density functional approximations across multiple rungs of Jacob's ladder as well as single- and multireference wave function approaches. To identify computationally efficient workflows for future virtual screening studies, we additionally assess 25 combinations of ground state geometry optimization and excitation-energy methods. The resulting recommendations for singlet-singlet and singlet-triplet excitation energies provide a validated methodological foundation for large-scale in silico exploration of thioindigo and hemithioindigo dyes, thus enabling the screening of tens of thousands of candidate chromophores while retaining high overall accuracy.