Imaging of photoinduced tautomerism in single porphyrin molecules

摘要

In this work we present our new experimental and theoretical results upon investigations of the photoinduced tautomerism processes of single metal-free porphyrin-type molecules. During tautomerization a molecule changes its structure, therefore the excitation transition dipole moment (TDM) of the molecule changes its orientation. Using confocal microscopy in combination with azimuthally and radially polarized laser beams we are able to determine the orientation of the TDM as well as the orientation of a single molecule itself. In the case of tautomerism we are able to visualize this process and even the involved isomers separately. The study first focuses on two symmetrical compounds: a phthalocyanine and a porphyrin. Additionally, differences of the single molecules embedded in a polymer matrix or just spin-coated on a glass cover slide and under nitrogen flow are investigated. In the latter case we observe a higher frequency of the change of the TDM orientation. The experimental studies are supplemented by quantum chemical calculations. Variations of the molecular substituents, the environment and excitation wavelength can give new insights into the excited-state tautomerism process of a single molecule. We also introduce some suggestions for future experiments to support the understanding of the photoinduced tautomerism.