Tautomerism process of single fluorescent molecules was studied by means of confocal microscopy in combination with azimuthally or radially polarized laser beams. During a tautomerism process the transition dipole moment (TDM) of a molecule changes its orientation which can be visualized by the fluorescence excitation image of the molecule. We present experimental and theoretical studies of two porphyrazine-type molecules and one type of porphyrin molecule: a symmetrically substituted metal-free phthalocyanine and porphyrin, and nonsymmetrically substituted porphyrazine. In the case of phthalocyanine the fluorescence excitation patterns show that the angle between the transition dipole moments of the two tautomeric forms is near 90°, in agreement with quantum chemical calculations. For porphyrazine we find that the orientation change of the TDM is less than 60° or larger than 120°, as theoretically predicted. Most of the porphyrin molecules show no photoinduced tautomerization, while for 7% of the total number of investigated molecules we observed excitation patterns of two different trans forms of the same single molecule. We demonstrate for the first time that a molecule, undergoing a tautomerism process stays in one tautomeric trans conformation during a time comparable with the acquisition time of one excitation pattern. This allowed us to visualize the existence of each of the two trans forms of one single porphyrin molecule, as well as the sudden switching between these tautomers.
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