@article{LiuWackenhutWangetal.2021,
  author    = {Liu, Quan and Wackenhut, Frank and Wang, Liangxuan and Hauler, Otto and Roldao, Juan Carlos and Adam, Pierre-Michel and Brecht, Marc and Gierschner, Johannes and Meixner, Alfred},
  title     = {Direct observation of structural heterogeneity and tautomerization of single hypericin molecules},
  journal   = {The journal of physical chemistry letters},
  volume    = {12},
  number    = {3},
  issn      = {1948-7185},
  doi       = {10.1021/acs.jpclett.0c03459},
  institution = {Life Sciences},
  pages     = {1025 -- 1031},
  year      = {2021},
  abstract  = {Tautomerization is a fundamental chemical reaction which involves the relocation of a proton in the reactants. Studying the optical properties of tautomeric species is challenging because of ensemble averaging. Many molecules, such as porphines, porphycenes, or phenanthroperylene quinones, exhibit a reorientation of the transition dipole moment (TDM) during tautomerization, which can be directly observed in single-molecule experiments. Here, we study single hypericin molecules, which is a prominent phenanthroperylene quinone showing antiviral, antidepressive, and photodynamical properties. Observing abrupt flipping of the image pattern combined with time-dependent density functional theory calculations allows drawing conclusions about the coexistence of four tautomers and their conversion path. This approach allows the unambiguous assignment of a TDM orientation to a specific tautomer and enables the determination of the chemical structure in situ. Our approach can be applied to other molecules showing TDM reorientation during tautomerization, helping to gain a deeper understanding of this important process.},
  language  = {en}
}