Electronic tera-order stabilization of photoinduced metastable species: Structure of the photochromic product of spiropyran determined with in situ single crystal X-ray photodiffraction

Pance Naumov, Pei Yu, Kenji Sakurai

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Abstract

The extraordinary stability of the photoinduced red form of a cationic spiropyran (k ≈ 10-6 s-1 in water and ∼10 -6 to less than 10-8 s-1 in the solid state) was employed to obtain in situ X-ray diffraction evidence of its molecular structure. By UV excitation under selected experimental conditions, on average, approximately one third of the cations in a single crystal of spiropyran iodide salt was converted and retained as the red form during the experiment. According to the structure of the mixed crystal, the ring opening, which is due to increased distance between the spiro oxygen and carbon atoms, is associated with slight molecular flattening caused by concurrent out-of-plane shift (11.2(5)°) of the pyranopyridinium half and in-plane shift (4.8(7)°) of the indoline half. The overall geometry change of the cation fits the steric requirements imposed by the ion packing in the crystal and can be viewed as molecular flattening caused by breaking of the spiroconjugation. The structure of the cation confirms that (at least in the case of cationic spiropyrans) the product is confined in the crystal mainly as a zwitterionic resonance structure in cis configuration similar to the (early) transition state. Although the positive charge of the closed form facilitates the ring-opening reaction by moving the reactant closer to the transition state, neither the weakening of the spiropyran C-O bond nor the space provided by the iodide alone can account for the stability of the product. Instead, the density functional theory calculations indicate that the stabilization of the red form of the cationic relative to the neutral spiropyran is thermodynamically controlled, probably through compensation of the charge within the zwitterion by the methylpyridinium group.

Original languageEnglish (US)
Pages (from-to)5810-5814
Number of pages5
JournalJournal of Physical Chemistry A
Volume112
Issue number26
DOIs
StatePublished - Jul 3 2008

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Stabilization
stabilization
Single crystals
flattening
cations
X rays
iodides
single crystals
Cations
products
electronics
Iodides
zwitterions
x rays
rings
shift
Crystals
mixed crystals
crystals
oxygen atoms

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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title = "Electronic tera-order stabilization of photoinduced metastable species: Structure of the photochromic product of spiropyran determined with in situ single crystal X-ray photodiffraction",
abstract = "The extraordinary stability of the photoinduced red form of a cationic spiropyran (k ≈ 10-6 s-1 in water and ∼10 -6 to less than 10-8 s-1 in the solid state) was employed to obtain in situ X-ray diffraction evidence of its molecular structure. By UV excitation under selected experimental conditions, on average, approximately one third of the cations in a single crystal of spiropyran iodide salt was converted and retained as the red form during the experiment. According to the structure of the mixed crystal, the ring opening, which is due to increased distance between the spiro oxygen and carbon atoms, is associated with slight molecular flattening caused by concurrent out-of-plane shift (11.2(5)°) of the pyranopyridinium half and in-plane shift (4.8(7)°) of the indoline half. The overall geometry change of the cation fits the steric requirements imposed by the ion packing in the crystal and can be viewed as molecular flattening caused by breaking of the spiroconjugation. The structure of the cation confirms that (at least in the case of cationic spiropyrans) the product is confined in the crystal mainly as a zwitterionic resonance structure in cis configuration similar to the (early) transition state. Although the positive charge of the closed form facilitates the ring-opening reaction by moving the reactant closer to the transition state, neither the weakening of the spiropyran C-O bond nor the space provided by the iodide alone can account for the stability of the product. Instead, the density functional theory calculations indicate that the stabilization of the red form of the cationic relative to the neutral spiropyran is thermodynamically controlled, probably through compensation of the charge within the zwitterion by the methylpyridinium group.",
author = "Pance Naumov and Pei Yu and Kenji Sakurai",
year = "2008",
month = "7",
day = "3",
doi = "10.1021/jp711922k",
language = "English (US)",
volume = "112",
pages = "5810--5814",
journal = "Journal of Physical Chemistry A",
issn = "1089-5639",
publisher = "American Chemical Society",
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}

TY - JOUR

T1 - Electronic tera-order stabilization of photoinduced metastable species

T2 - Structure of the photochromic product of spiropyran determined with in situ single crystal X-ray photodiffraction

AU - Naumov, Pance

AU - Yu, Pei

AU - Sakurai, Kenji

PY - 2008/7/3

Y1 - 2008/7/3

N2 - The extraordinary stability of the photoinduced red form of a cationic spiropyran (k ≈ 10-6 s-1 in water and ∼10 -6 to less than 10-8 s-1 in the solid state) was employed to obtain in situ X-ray diffraction evidence of its molecular structure. By UV excitation under selected experimental conditions, on average, approximately one third of the cations in a single crystal of spiropyran iodide salt was converted and retained as the red form during the experiment. According to the structure of the mixed crystal, the ring opening, which is due to increased distance between the spiro oxygen and carbon atoms, is associated with slight molecular flattening caused by concurrent out-of-plane shift (11.2(5)°) of the pyranopyridinium half and in-plane shift (4.8(7)°) of the indoline half. The overall geometry change of the cation fits the steric requirements imposed by the ion packing in the crystal and can be viewed as molecular flattening caused by breaking of the spiroconjugation. The structure of the cation confirms that (at least in the case of cationic spiropyrans) the product is confined in the crystal mainly as a zwitterionic resonance structure in cis configuration similar to the (early) transition state. Although the positive charge of the closed form facilitates the ring-opening reaction by moving the reactant closer to the transition state, neither the weakening of the spiropyran C-O bond nor the space provided by the iodide alone can account for the stability of the product. Instead, the density functional theory calculations indicate that the stabilization of the red form of the cationic relative to the neutral spiropyran is thermodynamically controlled, probably through compensation of the charge within the zwitterion by the methylpyridinium group.

AB - The extraordinary stability of the photoinduced red form of a cationic spiropyran (k ≈ 10-6 s-1 in water and ∼10 -6 to less than 10-8 s-1 in the solid state) was employed to obtain in situ X-ray diffraction evidence of its molecular structure. By UV excitation under selected experimental conditions, on average, approximately one third of the cations in a single crystal of spiropyran iodide salt was converted and retained as the red form during the experiment. According to the structure of the mixed crystal, the ring opening, which is due to increased distance between the spiro oxygen and carbon atoms, is associated with slight molecular flattening caused by concurrent out-of-plane shift (11.2(5)°) of the pyranopyridinium half and in-plane shift (4.8(7)°) of the indoline half. The overall geometry change of the cation fits the steric requirements imposed by the ion packing in the crystal and can be viewed as molecular flattening caused by breaking of the spiroconjugation. The structure of the cation confirms that (at least in the case of cationic spiropyrans) the product is confined in the crystal mainly as a zwitterionic resonance structure in cis configuration similar to the (early) transition state. Although the positive charge of the closed form facilitates the ring-opening reaction by moving the reactant closer to the transition state, neither the weakening of the spiropyran C-O bond nor the space provided by the iodide alone can account for the stability of the product. Instead, the density functional theory calculations indicate that the stabilization of the red form of the cationic relative to the neutral spiropyran is thermodynamically controlled, probably through compensation of the charge within the zwitterion by the methylpyridinium group.

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U2 - 10.1021/jp711922k

DO - 10.1021/jp711922k

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