Imaging of Tautomerism in a Single Molecule

Abstract

Tautomerization is a fundamental process in chemistry and biology, where it plays a major role in vision and enzymatic reactions. Usually, extensive spectroscopic ensemble studies are required to identify a tautomeric equilibrium. For instance, indirect evidence for the fast motion of the two inner hydrogen atoms between the nitrogen atoms has been deduced for porphycene, 1 a constitutional isomer of porphyrin, from complex NMR 1,2 and fluorescence spectroscopy 3,4 studies in a solid host for both ground and excited singlet states.A fascinating aspect of single-molecule spectroscopy (SMS) is its capability to detect a chemical reaction in a single chromophore. This has been reported, for instance, for electron transfer, 5 conformational changes, 6 or photochromism. 7 These studies recorded variations in fluorescence decay curves, fluorescence intensity trajectories, or fluorescence spectra. The fast exchange of the hydrogens in porphyrin-like molecules, however, leads to a dynamic equilibrium of chemically equivalent structures and hence cannot be inferred from lifetime or spectral measurements. On the other hand, the two structures differ in the orientation of the transition dipole moments of the S 0 -S 1 transition. As determined from bulk fluorescence anisotropy studies, 4 the S 0 -S 1 transition moments in the two trans forms enclose an angle of about 70-80°( Figure 1A). We will exploit this information in the interpretation of the intensity patterns of fluorescence from single porphycene molecules.Single-molecule transition dipole moments have been used previously to probe the electric field distribution in the focal spot of a confocal microscope. [8][9][10] In this report, we show that the analysis of single-molecule fluorescence patterns in real space excited by an azimuthally polarized laser beam ( Figure 1B) provides an important alternative tool for analyzing structural changes in the large class of compounds where fluorescence resonance energy transfer (FRET), fluorescence decay curves, or fluorescence spectra cannot be used. Porphyrins and porphycenes are important representatives of such a reaction. We show for the first time the direct visualization of the two tautomers resulting from double hydrogen transfer in a single molecule of porphycene. Samples of porphycene immobilized in a 10-20-nm-thick PMMA layer have been prepared by spin coating of a 10 -9 M solution in toluene onto a quartz cover slide. Two-dimensional (2D) spatial images of the fluorescence intensities of individual molecules were obtained by raster scanning the sample in 20-nm steps per 5 ms. The fluorescence intensity is proportional to |p‚E| 2 , the square of the projection of the electric field E on the transition dipole moment p. Hence, the observed patterns reveal the orientations of p 8,9 and can be compared with numerical simulations taking into account the experimental parameters of our microscope.We observed a total of 66 spatially separated single-molecule patterns on one sample that revealed characteristic blinking and ...

Vibrations and hydrogen bonding in porphycene

“…An analogous broad feature has been observed for Pc-d 12 , but not for Pc-d 2 . Interestingly, for the latter theory predicts a much narrower band.…”

“…Most importantly, we recorded the spectra of both undeuterated Pc (Pc-d 0 ) and the isotopologue in which the inner protons have been replaced by deuterons (Pc-d 2 ). Finally, we also carried out initial studies on Pc-d 12 , a molecule with all protons except the two internal ones replaced by deuterons. Besides providing information on vibrations involving N-H/N-D bonds, comparison of experimental and theoretical results for isotopologues improves the reliability of the assignments, because the isotopic shifts are usually computed with better accuracy than the absolute energies of vibrational transitions.…”

“…Standard orange cryostats were used for temperature control, the precision being better than AE1 K at all temperatures. The four isotopologues, Pc-d 0 , Pc-d 2 , Pc-d 12 and Pc-d 14 , were measured on both spectrometers. The data treatment was performed using the LAMP program, 49 in particular for the integration of IN4 data to produce generalized densities of states.…”

Electrochemical and Photophysical Properties of Ruthenium(II) Bipyridyl Complexes with Pendant Alkanethiol Chains in Solution and Anchored to Metal Surfaces

“…These dots appeared as concentric rings (Fig. 5) when using the defocus mode (diffraction of the emission of the substrate) [58,59]. The in-homogeneity of the surface coverage was confirmed by the intensity of the emitted light that was not equal over the observed area.…”

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…Unlike the porphyrin case, by perturbing the [20] The inner hydrogen migration between trans structures (see Figure 54, bottom) is common to porphyrin and porphycene [288]. Ground and excited state tautomerism in porphycene has been experimentally investigated by several techniques [323][324][325][326]. While this process has a stepwise mechanism in porphyrin [327], in porphycene the tautomerism has been described as a synchronous double hydrogen tunneling activated by a low frequency mode whose displacement affects the NH· · ·N distances [320].…”