Vibrations and hydrogen bonding in porphycene

Combined use of IR, Raman, neutron scattering and fluorescence measurements for porphycene isolated in helium nanodroplets, supersonic jet and cryogenic matrices, as well as for solid and liquid solutions, resulted in the assignments of almost all of 108 fundamental vibrations. The puzzling feature of porphycene is the apparent lack of the N–H stretching band in the IR spectrum, predicted to be the strongest of all bands by standard harmonic calculations. Theoretical modeling of the IR spectra, based on ab initio molecular dynamics simulations, reveals that the N–H stretching mode should appear as an extremely broad band in the 2250–3000 cm−1 region. Coupling of the N–H stretching vibration to other modes is discussed in the context of multidimensional character of intramolecular double hydrogen transfer in porphycene. The analysis can be generalized to other strongly hydrogen-bonded systems

Spectroscopic and microscopic investigations of tautomerization in porphycenes: condensed phases, supersonic jets, and single molecule studies

We describe various experimental approaches that have been used to obtain a detailed understanding of double hydrogen transfer in porphycene, a model system for intramolecular hydrogen bonding and tautomerism. The emerging picture is that of a multidimensional tautomerization coordinate, with several vibrational modes acting as reaction-promoters or inhibitors through anharmonic intermode coupling. Tunnelling processes, coherent in the case of isolated molecules and incoherent in condensed phases, are found to play a major role even at elevated temperatures. Single-molecule spectroscopy studies reveal large fluctuations in hydrogen transfer rates observed over time for the same chromophore. Scanning probe microscopy is employed to directly observe the structure and tautomerization dynamics of single molecules adsorbed on metal surfaces and demonstrates how the interactions of the molecules with atoms of the supporting surface affect their static and dynamic properties: different tautomeric forms are stabilized for molecules depending on the surface structure and the reaction mechanism can also change, from a concerted to a stepwise transfer. The scanning probe microscopy studies prove that tautomerization in single molecules can be induced by different stimuli: heat, electron attachment, light, and force exerted by the microscope’s tip. Possible applications utilizing tautomerism are discussed in combination with molecular architectures on surfaces, which could pave the way for the development of single-molecule electronics

Direct observation of double hydrogen transfer via quantum tunneling in a single porphycene molecule on a Ag(110) surface

Quantum tunneling of hydrogen atoms (or protons) plays a crucial role in many chemical and biological reactions. Although tunneling of a single particle has been examined extensively in various one-dimensional potentials, many-particle tunneling in high-dimensional potential energy surfaces remains poorly understood. Here we present a direct observation of a double hydrogen atom transfer (tautomerization) within a single porphycene molecule on a Ag(110) surface using a cryogenic scanning tunneling microscope (STM). The tautomerization rates are temperature-independent below ~10 K and a large kinetic isotope effect (KIE) is observed upon substituting the transferred hydrogen atoms by deuterium, indicating that the process is governed by tunneling. The observed KIE for three isotopologues and density functional theory calculations reveal that a stepwise transfer mechanism is dominant in the tautomerization. It is also found that the tautomerization rate is increased by vibrational excitation via an inelastic electron tunneling process. Moreover, the STM tip can be used to manipulate the tunneling dynamics through modification of the potential landscape

Hot Carrier-Induced Tautomerization within a Single Porphycene Molecule on Cu(111)

Here, we report the study of tautomerization within a single porphycene molecule adsorbed on a Cu(111) surface using low-temperature scanning tunneling microscopy (STM) at 5 K. While molecules are adsorbed on the surface exclusively in the thermodynamically stable trans tautomer after deposition, a voltage pulse from the STM can induce the unidirectional trans → cis and reversible cis ↔ cis tautomerization. From the voltage and current dependence of the tautomerization yield (rate), it is revealed that the process is induced by vibrational excitation via inelastic electron tunneling. However, the metastable cis molecules are thermally switched back to the trans tautomer by heating the surface up to 30 K. Furthermore, we have found that the unidirectional tautomerization can be remotely controlled at a distance from the STM tip. By analyzing the nonlocal process in dependence on various experimental parameters, a hot carrier-mediated mechanism is identified, in which hot electrons (holes) generated by the STM travel along the surface and induce the tautomerization through inelastic scattering with a molecule. The bias voltage and coverage dependent rate of the nonlocal tautomerization clearly show a significant contribution of the Cu(111) surface state to the hot carrier-induced process

Spectral characterization of mesoporous carbons modified by Ag, Au, TiO2 and Fe3O4 nanoparticles

Określono możliwość zastosowania technik spektroskopowych do charakterystyki właściwości strukturalnych mezoporowatego węgla otrzymanego metodą miękkiego odwzorowania oraz zmodyfikowanego za pomocą nanocząstek Ag, Au, TiO2 i Fe3O4. Charakterystykę spektralną próbek otrzymanych węgli przeprowadzono techniką spektroskopii IR, FTIR oraz Ramana. Podczas syntezy materiałów węglowych w środowisku kwasowym zastosowano rezorcynol i formaldehyd jako prekursory węglowe w obecności kopolimeru trójblokowego Lutrol F127. Wyniki badań spektralnych wykazały, że zastosowane metody syntezy mezoporowatych węgli pozwoliły na uzyskanie materiałów węglowych o odtwarzalnej budowie. Modyfikacja powierzchni węgla badanymi nanocząstkami spowodowała wyraźne zmiany w widmach Ramana, FTIR i IR kompozytów węglowych, przy czym na zmianę ich właściwości najsilniej wpłynęły tlenki metali. Widma te pokazały również, że w strukturze otrzymanych materiałów węglowych pojawiły się uporządkowane fragmenty grafenowe podobne do tych, jakie występują w nanorurkach węglowych.The structural properties of mesoporous carbons modified by Ag, Au, TiO2 and Fe3O4 nanoparticles were studied using IR, FT-IR and Raman spectroscopy. The carbon materials were prepared by the soft-templating method in an acidic medium, with resorcinol and formaldehyde as the carbon precursors and triblock copolymer Lutrol F127 as a soft template. Spectral analysis has revealed that these carbons are characterized by a repeatable structure. The modification of the carbon surface by the nanoparticles examined has caused some noticeable changes in the Raman, FT-IR and IR spectra of the carbon composites, and most of the influence on the modification of the surface comes from metal oxides. The spectra have also shown that the carbon materials tested contain some ordered fragments of graphene structures similar to those occurring in carbon nanotubes

Force-induced tautomerization in a single molecule

Heat transfer, electrical potential and light energy are common ways to activate chemical reactions. Applied force is another way, but dedicated studies for such a mechanical activation are limited, and this activation is poorly understood at the single-molecule level. Here, we report force-induced tautomerization in a single porphycene molecule on a Cu(110) surface at 5 K, which is studied by scanning probe microscopy and density functional theory calculations. Force spectroscopy quantifies the force needed to trigger tautomerization with submolecular spatial resolution. The calculations show how the reaction pathway and barrier of tautomerization are modified in the presence of a copper tip and reveal the atomistic origin of the process. Moreover, we demonstrate that a chemically inert tip whose apex is terminated by a xenon atom cannot induce the reaction because of a weak interaction with porphycene and a strong relaxation of xenon on the tip as contact to the molecule is formed

Controlling intramolecular hydrogen transfer in a porphycene molecule with single atoms or molecules located nearby

Although the local environment of a molecule can play an important role in its chemistry, rarely has it been examined experimentally at the level of individual molecules. Here we report the precise control of intramolecular hydrogen-transfer (tautomerization) reactions in single molecules using scanning tunnelling microscopy. By placing, with atomic precision, a copper adatom close to a porphycene molecule, we found that the tautomerization rates could be tuned up and down in a controlled fashion, surprisingly also at rather large separations. Furthermore, we extended our study to molecular assemblies in which even the arrangement of the pyrrolic hydrogen atoms in the neighbouring molecule influences the tautomerization reaction in a given porphycene, with positive and negative cooperativity effects. Our results highlight the importance of controlling the environment of molecules with atomic precision and demonstrate the potential to regulate processes that occur in a single molecule

Thermally and Vibrationally Induced Tautomerization of Single Porphycene Molecules on a Cu(110) Surface

We report the direct observation of intramolecular hydrogen atom transfer reactions (tautomerization) within a single porphycene molecule on a Cu(110) surface by scanning tunneling microscopy. It is found that the tautomerization can be induced via inelastic electron tunneling at 5 K. By measuring the bias-dependent tautomerization rate of isotope-substituted molecules, we can assign the scanning tunneling microscopy-induced tautomerization to the excitation of specific molecular vibrations. Furthermore, these vibrations appear as characteristic features in the dI/dV spectra measured over individual molecules. The vibrational modes that are associated with the tautomerization are identified by density functional theory calculations. At higher temperatures above similar to 75 K, tautomerization is induced thermally and an activation barrier of about 168 meV is determined from an Arrhenius plot