Role of Dark States and Stokes Shift Simulations for Tetraphenylpyrazine Compared to Other Donor-Acceptor Photosensitizers.

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.