Tautomerization is an intramolecular chemical reaction defined as structural interconversion of a molecule between two isomers separated by a low energy barrier. Tautomerization in free base phthalocyanines, porphyrins or porphycenes has fascinated and attracted chemists and physicists alike over the last decades for its fundamental importance in various chemical and biological systems and many applications in technology [1,2]. Particularly interesting is the reaction rate which can range over orders of magnitude depending on the temperature and the local environment. In solution, this reaction in porphycenes occurs usually in several picoseconds or even faster, as determined via anisotropy studies by ultrafast transient absorption spectroscopy [3] and in porphyrins at cryogenic temperatures it can last for hours or days as determined form the live time of narrow spectral holes photochemically burned in the inhomogeneously broadened absorption band with a narrow band laser [4]. However, a spectacular and unexpected observation has been made by optical single molecule imaging and spectroscopy [5]. Examining a large number of single molecules embedded at low concentration in various polymer matrices at room temperature has revealed unquestionably that the rate in these molecules can vary dramatically over time such that tautomerization can be frozen for seconds and longer under ambient conditions[6]. The interconversion of the inner protons of phthalocyanine changes the orientation of the transition dipole moment by 90^o degrees with respect to the molecular structure. Hence, if these molecules are embedded with a fixed orientation in a transparent solid polymer film at high dilution and tautomerization is frozen, the orientation of their transition dipole moments can be determined by polarization spectroscopy.

Here we report on our recent experimental and theoretical studies of monitoring tautomerization by fluorescence imaging of single molecules of phthalocyanines, porphyrins, or porphycenes excited in the tightly focused azimuthally and radially polarized laser beam with a sample scanning confocal optical microscope equipped with single photon counting electronics and a fluorescence spectrometer. This technique has been used to determine the transition dipole moment orientation of single molecules, the polarizability of plasmonic nanoparticles or the dimensionality of single quantum dots. Different diffraction limited fluorescence intensity patterns are observed for molecules that undergo tautomerization and for those that do not. The latter corresponds to a molecule with a fixed transition dipole moment direction, whereas the tautomerizing molecule can be treated as the superposition of two emitters with different transition dipole moment orientations.

Previous studies have shown that tautomerization in porphyrins or phthalocyanines can occur via ground state tunneling and is frozen at low temperatures, particularly at cryogenic temperatures. The structural heterogeneity in a polymer matrix leads to a distribution of asymmetrically distorted ground state double well potential and hence can lead to long residence times in the deeper energy minimum. However, even at helium temperatures tautomerization can be induced by electronic excitation to the first excited state, making photochemical spectral hole-burning possible. Recently we could demonstrate in a single-molecule study that the tautomerization rate of phthalocyanine tetrasulfonate embedded in a thin PVA-film at room temperature could be controlled in a tunable λ/2 Fabry Perot resonator by weak coupling between the electronically excited state and the vacuum electromagnetic field of a resonant cavity mode.

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