Conformational change from a twisted figure-eight to an open-extended structure in doubly fused 36π core-modified octaphyrins triggered by protonation: Implication on photodynamics and aromaticity

Ganesan Karthik, Jong Min Lim, A. Srinivasan, C. H. Suresh, Dongho Kim, Tavarekere K. Chandrashekar

Research output: Contribution to journalArticle

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Abstract

Two examples of core-modified 36π doubly fused octaphyrins that undergo a conformational change from a twisted figure-eight to an open-extended structure induced by protonation are reported. Syntheses of the two octaphyrins (in which Ar=mesityl or tolyl) were achieved by a simple acid-catalyzed condensation of dipyrrane unit containing an electron-rich, rigid dithienothiophene (DTT) core with pentafluorobenzaldehyde followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The single-crystal X-ray structure of the octaphyrin (in which Ar=mesityl) shows a figure-eight twisted conformation of the expanded porphyrin skeleton with two DTT moieties oriented in a staggered conformation with a π-cloud distance of 3.7 Å. Spectroscopic and quantum mechanical calculations reveal that both octaphyrins conform to a [4n]π nonaromatic electronic structure. Protonation of the pyrrole nitrogen atoms of the octaphyrins results in dramatic structural change, which led to 1) a large redshift and sharpening of absorption bands in electronic absorption spectrum, 2) a large change in chemical shift of pyrrole β-CH and-NH protons in the 1H NMR spectrum, 3) a small increase in singlet lifetimes, and 4) a moderate increase in two-photon absorption cross-section values. Furthermore, nucleus-independent chemical shift (NICS) values calculated at various geometrical positions show positive values and anisotropy-induced current density (AICD) plots indicate paratropic ring-currents for the diprotonated form of the octaphyrin (in which Ar=tolyl); the single-crystal X-ray structure of the diprotonated form of the octaphyrin shows an extended structure in which one of the pyrrole ring of each dipyrrin subunit undergoes a 180 ° ring-flip. Four trifluoroacetic acid (TFA) molecules are bound above and below the molecular plane defined by meso-carbon atoms and are held by NH×××O, N-H...F, and C-H...F intermolecular hydrogen-bonding interactions. The extended-open structure upon protonation allows π-delocalization and the electronic structure conforms to a [4n]π Hückel antiaromatic in the diprotonated state. Figure-eight to open structure: A major structural change from a figure-eight conformation to an open-extended conformation upon addition of two protons to an expanded 36π fused octaphyrin cavity is reported (see figure). Such a change in conformation results in an increase in singlet lifetime, two-photon absorption cross-section values, and a change in aromaticity from a (4n)π nonaromatic to antiaromatic electronic structure.

Original languageEnglish
Pages (from-to)17011-17020
Number of pages10
JournalChemistry - A European Journal
Volume19
Issue number50
DOIs
Publication statusPublished - 2013 Dec 9

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Protonation
Conformations
Pyrroles
Electronic structure
Chemical shift
Protons
Absorption spectra
Photons
Single crystals
Trifluoroacetic acid
Trifluoroacetic Acid
X rays
Atoms
Porphyrins
Induced currents
Condensation
Hydrogen bonds
Anisotropy
Current density
Nitrogen

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Organic Chemistry

Cite this

@article{42c3a9a90d0445d2b3b8dcf92a44dcba,
title = "Conformational change from a twisted figure-eight to an open-extended structure in doubly fused 36π core-modified octaphyrins triggered by protonation: Implication on photodynamics and aromaticity",
abstract = "Two examples of core-modified 36π doubly fused octaphyrins that undergo a conformational change from a twisted figure-eight to an open-extended structure induced by protonation are reported. Syntheses of the two octaphyrins (in which Ar=mesityl or tolyl) were achieved by a simple acid-catalyzed condensation of dipyrrane unit containing an electron-rich, rigid dithienothiophene (DTT) core with pentafluorobenzaldehyde followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The single-crystal X-ray structure of the octaphyrin (in which Ar=mesityl) shows a figure-eight twisted conformation of the expanded porphyrin skeleton with two DTT moieties oriented in a staggered conformation with a π-cloud distance of 3.7 {\AA}. Spectroscopic and quantum mechanical calculations reveal that both octaphyrins conform to a [4n]π nonaromatic electronic structure. Protonation of the pyrrole nitrogen atoms of the octaphyrins results in dramatic structural change, which led to 1) a large redshift and sharpening of absorption bands in electronic absorption spectrum, 2) a large change in chemical shift of pyrrole β-CH and-NH protons in the 1H NMR spectrum, 3) a small increase in singlet lifetimes, and 4) a moderate increase in two-photon absorption cross-section values. Furthermore, nucleus-independent chemical shift (NICS) values calculated at various geometrical positions show positive values and anisotropy-induced current density (AICD) plots indicate paratropic ring-currents for the diprotonated form of the octaphyrin (in which Ar=tolyl); the single-crystal X-ray structure of the diprotonated form of the octaphyrin shows an extended structure in which one of the pyrrole ring of each dipyrrin subunit undergoes a 180 ° ring-flip. Four trifluoroacetic acid (TFA) molecules are bound above and below the molecular plane defined by meso-carbon atoms and are held by N{\"i}£¿H×××O, N-H...F, and C-H...F intermolecular hydrogen-bonding interactions. The extended-open structure upon protonation allows π-delocalization and the electronic structure conforms to a [4n]π H{\"u}ckel antiaromatic in the diprotonated state. Figure-eight to open structure: A major structural change from a figure-eight conformation to an open-extended conformation upon addition of two protons to an expanded 36π fused octaphyrin cavity is reported (see figure). Such a change in conformation results in an increase in singlet lifetime, two-photon absorption cross-section values, and a change in aromaticity from a (4n)π nonaromatic to antiaromatic electronic structure.",
author = "Ganesan Karthik and Lim, {Jong Min} and A. Srinivasan and Suresh, {C. H.} and Dongho Kim and Chandrashekar, {Tavarekere K.}",
year = "2013",
month = "12",
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Conformational change from a twisted figure-eight to an open-extended structure in doubly fused 36π core-modified octaphyrins triggered by protonation : Implication on photodynamics and aromaticity. / Karthik, Ganesan; Lim, Jong Min; Srinivasan, A.; Suresh, C. H.; Kim, Dongho; Chandrashekar, Tavarekere K.

In: Chemistry - A European Journal, Vol. 19, No. 50, 09.12.2013, p. 17011-17020.

Research output: Contribution to journalArticle

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T1 - Conformational change from a twisted figure-eight to an open-extended structure in doubly fused 36π core-modified octaphyrins triggered by protonation

T2 - Implication on photodynamics and aromaticity

AU - Karthik, Ganesan

AU - Lim, Jong Min

AU - Srinivasan, A.

AU - Suresh, C. H.

AU - Kim, Dongho

AU - Chandrashekar, Tavarekere K.

PY - 2013/12/9

Y1 - 2013/12/9

N2 - Two examples of core-modified 36π doubly fused octaphyrins that undergo a conformational change from a twisted figure-eight to an open-extended structure induced by protonation are reported. Syntheses of the two octaphyrins (in which Ar=mesityl or tolyl) were achieved by a simple acid-catalyzed condensation of dipyrrane unit containing an electron-rich, rigid dithienothiophene (DTT) core with pentafluorobenzaldehyde followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The single-crystal X-ray structure of the octaphyrin (in which Ar=mesityl) shows a figure-eight twisted conformation of the expanded porphyrin skeleton with two DTT moieties oriented in a staggered conformation with a π-cloud distance of 3.7 Å. Spectroscopic and quantum mechanical calculations reveal that both octaphyrins conform to a [4n]π nonaromatic electronic structure. Protonation of the pyrrole nitrogen atoms of the octaphyrins results in dramatic structural change, which led to 1) a large redshift and sharpening of absorption bands in electronic absorption spectrum, 2) a large change in chemical shift of pyrrole β-CH and-NH protons in the 1H NMR spectrum, 3) a small increase in singlet lifetimes, and 4) a moderate increase in two-photon absorption cross-section values. Furthermore, nucleus-independent chemical shift (NICS) values calculated at various geometrical positions show positive values and anisotropy-induced current density (AICD) plots indicate paratropic ring-currents for the diprotonated form of the octaphyrin (in which Ar=tolyl); the single-crystal X-ray structure of the diprotonated form of the octaphyrin shows an extended structure in which one of the pyrrole ring of each dipyrrin subunit undergoes a 180 ° ring-flip. Four trifluoroacetic acid (TFA) molecules are bound above and below the molecular plane defined by meso-carbon atoms and are held by NH×××O, N-H...F, and C-H...F intermolecular hydrogen-bonding interactions. The extended-open structure upon protonation allows π-delocalization and the electronic structure conforms to a [4n]π Hückel antiaromatic in the diprotonated state. Figure-eight to open structure: A major structural change from a figure-eight conformation to an open-extended conformation upon addition of two protons to an expanded 36π fused octaphyrin cavity is reported (see figure). Such a change in conformation results in an increase in singlet lifetime, two-photon absorption cross-section values, and a change in aromaticity from a (4n)π nonaromatic to antiaromatic electronic structure.

AB - Two examples of core-modified 36π doubly fused octaphyrins that undergo a conformational change from a twisted figure-eight to an open-extended structure induced by protonation are reported. Syntheses of the two octaphyrins (in which Ar=mesityl or tolyl) were achieved by a simple acid-catalyzed condensation of dipyrrane unit containing an electron-rich, rigid dithienothiophene (DTT) core with pentafluorobenzaldehyde followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The single-crystal X-ray structure of the octaphyrin (in which Ar=mesityl) shows a figure-eight twisted conformation of the expanded porphyrin skeleton with two DTT moieties oriented in a staggered conformation with a π-cloud distance of 3.7 Å. Spectroscopic and quantum mechanical calculations reveal that both octaphyrins conform to a [4n]π nonaromatic electronic structure. Protonation of the pyrrole nitrogen atoms of the octaphyrins results in dramatic structural change, which led to 1) a large redshift and sharpening of absorption bands in electronic absorption spectrum, 2) a large change in chemical shift of pyrrole β-CH and-NH protons in the 1H NMR spectrum, 3) a small increase in singlet lifetimes, and 4) a moderate increase in two-photon absorption cross-section values. Furthermore, nucleus-independent chemical shift (NICS) values calculated at various geometrical positions show positive values and anisotropy-induced current density (AICD) plots indicate paratropic ring-currents for the diprotonated form of the octaphyrin (in which Ar=tolyl); the single-crystal X-ray structure of the diprotonated form of the octaphyrin shows an extended structure in which one of the pyrrole ring of each dipyrrin subunit undergoes a 180 ° ring-flip. Four trifluoroacetic acid (TFA) molecules are bound above and below the molecular plane defined by meso-carbon atoms and are held by NH×××O, N-H...F, and C-H...F intermolecular hydrogen-bonding interactions. The extended-open structure upon protonation allows π-delocalization and the electronic structure conforms to a [4n]π Hückel antiaromatic in the diprotonated state. Figure-eight to open structure: A major structural change from a figure-eight conformation to an open-extended conformation upon addition of two protons to an expanded 36π fused octaphyrin cavity is reported (see figure). Such a change in conformation results in an increase in singlet lifetime, two-photon absorption cross-section values, and a change in aromaticity from a (4n)π nonaromatic to antiaromatic electronic structure.

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