Role of dark states and Stokes shift simulations for tetraphenylpyrazine compared to other donor‐acceptor photosensitizers

Abstract An excellent agreement for simulated and measured absorption and emission spectra is found for four donor‐acceptor aromatic molecules (tetraphenylpyrazine, tetraphenylethene, distirylanthracene and hexaphenylsilole) whose derivatives serve as solid state photosensitizers. After comparing several hybrid TDDFT functionals, EOM‐CCSD, and experiments, the best agreement was found with TD‐B3LYP and double zeta basis sets (6‐31G** and def2‐SVP) for one molecule in gas phase. A full characterisation of twelve to twenty electronic excited states was performed in every system. Symmetry‐forbidden bands are found in the absorption spectra by sampling fifty to hundred geometries from a Wigner distribution. The density of states in the region 2–6 eV was also analysed, showing a very packed region of excited states and suggesting that dark electronic states may play a role in the dynamics of some of the photoexcited systems. Further calculations were done with QM/xTB at geometries extracted from previously published X‐ray data to evaluate the influence of the environment on the excitations of the four aggregated molecular crystals.