Tautomerism in Porphycenes: Analysis of Rate-Affecting Factors

Ciąćka, Piotr; Fita, Piotr; Listkowski, Arkadiusz; Kijak, Michał; Nonell, Santi; Kuzuhara, Daiki; Yamada, Hiroko; Radzewicz, Czesław; Waluk, Jacek

doi:10.1021/jp506150r

Cited by 40 publications

(40 citation statements)

References 69 publications

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“…Satisfactory fits were observed for

{\tilde{ν}}_{0}

varying between 2300 and 2600 cm −1 and d 0 around 250 pm. We note that these values are close to those postulated for porphycenes, which form exceptionally strong intramolecular H‐bonds …”

Section: Resultssupporting

confidence: 87%

Parent, Unsubstituted Hemiporphycene: Synthesis and Properties

Nawara

Kijak

Buczyńska

et al. 2016

Chemistry A European J

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Among seven possible nitrogen-in constitutional isomers of porphyrin only one, porphycene, has been obtained so far in the free, unsubstituted form. Herein, the synthesis of another isomer, parent hemiporphycene (HPc), and its thorough structural, spectral, photophysical, electrochemical, and theoretical characterization are reported. Most of the properties of HPc are intermediate between those of porphyrin and porphycene, as evidenced by the values of inner-cavity dimensions, orbital-energy splittings, absorption coefficients, magnetic circular dichroism parameters, NH-stretching frequencies, fluorescence quantum yields, tautomerization rates, and redox potentials. The largest differences arise with respect to tautomerism, due to the low symmetry of HPc and inequivalence of the four nitrogen atoms that define the inner cavity. Two trans tautomers are observed, separated in energy by about 1 kcal mol . Tautomerization from the higher- to the lower-energy form is detected in the lowest-excited singlet state and occurs at a rate that is about four orders of magnitude lower than that observed for porphycene. Hemiporphycene is a very good model for the investigation of inequivalent intramolecular H-bonds present in one molecule; two such bonds in HPc reveal unusual characteristics, and the bond strength results from the interplay between the N⋅⋅⋅N distance and the N-H-N angle.

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“…Satisfactory fits were observed for

{\tilde{ν}}_{0}

varying between 2300 and 2600 cm −1 and d 0 around 250 pm. We note that these values are close to those postulated for porphycenes, which form exceptionally strong intramolecular H‐bonds …”

Section: Resultssupporting

confidence: 87%

Parent, Unsubstituted Hemiporphycene: Synthesis and Properties

Nawara

Kijak

Buczyńska

et al. 2016

Chemistry A European J

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“…Experiments on symmetric medium and strong H-bonded systems are desirable. Some possible candidates are carboxylic acid dimers (R 2.45Å), HCrO 2 (R 2.49Å), porphycenes 51 , and proton sponges 52 , for which the fundamental, first overtone, and isotope effect could be measured and analysed. Slightly asymmetric biomolecular systems with strong H-bonds that could be investigated include mutated GFP 53 , photoactive yellow protein 54 and the enzyme KSI 55 .…”

Section: Summary and Concluding Remarksmentioning

confidence: 99%

Effect of hydrogen bonding on the infrared absorption intensity of OH stretch vibrations

2017

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We consider how the infrared intensity of an O-H stretch in a hydrogen bonded complex varies as the strength of the H-bond varies from weak to strong. We obtain trends for the fundamental and overtone transitions as a function of donor-acceptor distance R, which is a common measure of H-bond strength. Our calculations use a simple two-diabatic state model that permits symmetric and asymmetric bonds, i.e. where the proton affinity of the donor and acceptor are equal and unequal, respectively. The dipole moment function uses a Mecke form for the free OH dipole moment, associated with the diabatic states. The transition dipole moment is calculated using one-dimensional vibrational eigenstates associated with the H-atom transfer coordinate on the ground state adiabatic surface of our model. Over 20-fold intensity enhancements for the fundamental are found for strong H-bonds, where there are significant non-Condon effects. The isotope effect on the intensity yields a nonmonotonic H/D intensity ratio as a function of R, and is enhanced by the secondary geometric isotope effect. The first overtone intensity is found to vary non-monotonically with H-bond strength; strong enhancements are possible for strong H-bonds. Modifying the dipole moment through the Mecke parameters is found to have a stronger effect on the overtone than the fundamental. We compare our findings with those for specific molecular systems analysed through experiments and theory in earlier works. Our model results compare favourably for strong and medium strength symmetric H-bonds. However, for weak asymmetric bonds we find much smaller effects than in earlier work.

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“…It has also been found that the reaction barrier is essentially associated with the hydrogen bond strength (geometry) of the N-H···N moiety in the molecular cavity 39 . The increased barrier for the adsorbed porphycene on Cu(110) could be attributed to the change in the hydrogen bond through the formation of a dative bond of the lone pair orbitals of the imine N atoms with the surface Cu atoms.…”

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confidence: 99%

Force-induced tautomerization in a single molecule

et al. 2016

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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

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Tautomerism in Porphycenes: Analysis of Rate-Affecting Factors

Cited by 40 publications

References 69 publications

Parent, Unsubstituted Hemiporphycene: Synthesis and Properties

Parent, Unsubstituted Hemiporphycene: Synthesis and Properties

Effect of hydrogen bonding on the infrared absorption intensity of OH stretch vibrations

Force-induced tautomerization in a single molecule

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