posted on 2021-01-20, 16:08authored byQuan Liu, Frank Wackenhut, Liangxuan Wang, Otto Hauler, Juan Carlos Roldao, Pierre-Michel Adam, Marc Brecht, Johannes Gierschner, Alfred J. Meixner
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.