New type of dual solid-state thermochromism

modulation of intramolecular charge transfer by intermolecular π - π interactions, kinetic trapping of the aci-nitro group, and reversible molecular locking

Pance Naumov, Sang Cheol Lee, Nobuo Ishizawa, Young Gyu Jeong, Ihn Hee Chung, Shunichi Fukuzumi

    Research output: Contribution to journalArticle

    Abstract

    When heated above room temperature, some crystalline polymorphs of the l,3-bis(hydroxyalkylamino)-4,6dinitrobenzenes (BDBn, n = 2-5), bis(hydroxyalkyl) analogues of the intramolecular charge-transfer molecule 1,3-diamino-4,6- dinitrobenzene, exhibit "dual" thermochromism: gradual color change from yellow to orange at lower temperatures, and sharp color change from orange to red at higher temperatures. These two thermochromic changes are related to different solid-state processes. When allowed to cool to room temperature, the yellow color of the thermochromic molecules with different alkyl length (n) is recovered with unexpectedly different kinetics, the order of the respective rate constants ranging from 10-7-10-6 s-1 for BDB2 to about 0.1 s-1 in the case of BDB3. The thermochromic mechanism and the reasons behind the different kinetics were clarified on the basis of detailed crystallographic characterization, kinetic thermoanalysis, and spectroscopic study of eight crystalline forms (seven polymorphs and one solvate). It was found that the polymorphism is due to the possibility of "locking" and "unlocking" of the alkyl arms by formation of a strong intramolecular hydrogen bond between the hydroxyl groups at their hydroxyl termini. The locking of BDB2, with shortest alkyl arms, is reversible and it can be controlled thermally; either of the two conformations can be obtained in the solid state by proper thermal treatment. By use of high temperature in situ single crystal X-ray diffraction analysis of BDB3, direct evidence was obtained that the gradual thermochromic change is related to increased distance and weakened π - π interactions between the stacked benzene rings: the lattice expands preferably in the stacking direction, causing enhanced oscillator strength and red shift of the absorption edge of the intramolecular charge transfer transition. The second, sharp thermochromic change had been assigned previously to solid - solid phase transition triggered by intramolecular proton transfer of one amino proton to the nitro group, whereupon an aci-nitro form is thermally populated. Contrary to the numerous examples of solid thermochromic molecules based on either pericyclic reactions or keto - enol tautomerism, this system appears to be the first organic thermochromic family where the thermochromic change appears as an effect of intermolecular π - π interactions and thermal intramolecular proton transfer to aromatic nitro group.

    Original languageEnglish (US)
    Pages (from-to)11354-11366
    Number of pages13
    JournalJournal of Physical Chemistry A
    Volume113
    Issue number42
    DOIs
    StatePublished - Oct 22 2009

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    locking
    Charge transfer
    trapping
    charge transfer
    Modulation
    solid state
    color
    modulation
    Kinetics
    protons
    kinetics
    Proton transfer
    molecules
    Color
    Polymorphism
    Hydroxyl Radical
    polymorphism
    Molecules
    interactions
    room temperature

    ASJC Scopus subject areas

    • Physical and Theoretical Chemistry

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    New type of dual solid-state thermochromism : modulation of intramolecular charge transfer by intermolecular π - π interactions, kinetic trapping of the aci-nitro group, and reversible molecular locking. / Naumov, Pance; Lee, Sang Cheol; Ishizawa, Nobuo; Jeong, Young Gyu; Chung, Ihn Hee; Fukuzumi, Shunichi.

    In: Journal of Physical Chemistry A, Vol. 113, No. 42, 22.10.2009, p. 11354-11366.

    Research output: Contribution to journalArticle

    Naumov, Pance ; Lee, Sang Cheol ; Ishizawa, Nobuo ; Jeong, Young Gyu ; Chung, Ihn Hee ; Fukuzumi, Shunichi. / New type of dual solid-state thermochromism : modulation of intramolecular charge transfer by intermolecular π - π interactions, kinetic trapping of the aci-nitro group, and reversible molecular locking. In: Journal of Physical Chemistry A. 2009 ; Vol. 113, No. 42. pp. 11354-11366.
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    abstract = "When heated above room temperature, some crystalline polymorphs of the l,3-bis(hydroxyalkylamino)-4,6dinitrobenzenes (BDBn, n = 2-5), bis(hydroxyalkyl) analogues of the intramolecular charge-transfer molecule 1,3-diamino-4,6- dinitrobenzene, exhibit {"}dual{"} thermochromism: gradual color change from yellow to orange at lower temperatures, and sharp color change from orange to red at higher temperatures. These two thermochromic changes are related to different solid-state processes. When allowed to cool to room temperature, the yellow color of the thermochromic molecules with different alkyl length (n) is recovered with unexpectedly different kinetics, the order of the respective rate constants ranging from 10-7-10-6 s-1 for BDB2 to about 0.1 s-1 in the case of BDB3. The thermochromic mechanism and the reasons behind the different kinetics were clarified on the basis of detailed crystallographic characterization, kinetic thermoanalysis, and spectroscopic study of eight crystalline forms (seven polymorphs and one solvate). It was found that the polymorphism is due to the possibility of {"}locking{"} and {"}unlocking{"} of the alkyl arms by formation of a strong intramolecular hydrogen bond between the hydroxyl groups at their hydroxyl termini. The locking of BDB2, with shortest alkyl arms, is reversible and it can be controlled thermally; either of the two conformations can be obtained in the solid state by proper thermal treatment. By use of high temperature in situ single crystal X-ray diffraction analysis of BDB3, direct evidence was obtained that the gradual thermochromic change is related to increased distance and weakened π - π interactions between the stacked benzene rings: the lattice expands preferably in the stacking direction, causing enhanced oscillator strength and red shift of the absorption edge of the intramolecular charge transfer transition. The second, sharp thermochromic change had been assigned previously to solid - solid phase transition triggered by intramolecular proton transfer of one amino proton to the nitro group, whereupon an aci-nitro form is thermally populated. Contrary to the numerous examples of solid thermochromic molecules based on either pericyclic reactions or keto - enol tautomerism, this system appears to be the first organic thermochromic family where the thermochromic change appears as an effect of intermolecular π - π interactions and thermal intramolecular proton transfer to aromatic nitro group.",
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    AU - Naumov, Pance

    AU - Lee, Sang Cheol

    AU - Ishizawa, Nobuo

    AU - Jeong, Young Gyu

    AU - Chung, Ihn Hee

    AU - Fukuzumi, Shunichi

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    N2 - When heated above room temperature, some crystalline polymorphs of the l,3-bis(hydroxyalkylamino)-4,6dinitrobenzenes (BDBn, n = 2-5), bis(hydroxyalkyl) analogues of the intramolecular charge-transfer molecule 1,3-diamino-4,6- dinitrobenzene, exhibit "dual" thermochromism: gradual color change from yellow to orange at lower temperatures, and sharp color change from orange to red at higher temperatures. These two thermochromic changes are related to different solid-state processes. When allowed to cool to room temperature, the yellow color of the thermochromic molecules with different alkyl length (n) is recovered with unexpectedly different kinetics, the order of the respective rate constants ranging from 10-7-10-6 s-1 for BDB2 to about 0.1 s-1 in the case of BDB3. The thermochromic mechanism and the reasons behind the different kinetics were clarified on the basis of detailed crystallographic characterization, kinetic thermoanalysis, and spectroscopic study of eight crystalline forms (seven polymorphs and one solvate). It was found that the polymorphism is due to the possibility of "locking" and "unlocking" of the alkyl arms by formation of a strong intramolecular hydrogen bond between the hydroxyl groups at their hydroxyl termini. The locking of BDB2, with shortest alkyl arms, is reversible and it can be controlled thermally; either of the two conformations can be obtained in the solid state by proper thermal treatment. By use of high temperature in situ single crystal X-ray diffraction analysis of BDB3, direct evidence was obtained that the gradual thermochromic change is related to increased distance and weakened π - π interactions between the stacked benzene rings: the lattice expands preferably in the stacking direction, causing enhanced oscillator strength and red shift of the absorption edge of the intramolecular charge transfer transition. The second, sharp thermochromic change had been assigned previously to solid - solid phase transition triggered by intramolecular proton transfer of one amino proton to the nitro group, whereupon an aci-nitro form is thermally populated. Contrary to the numerous examples of solid thermochromic molecules based on either pericyclic reactions or keto - enol tautomerism, this system appears to be the first organic thermochromic family where the thermochromic change appears as an effect of intermolecular π - π interactions and thermal intramolecular proton transfer to aromatic nitro group.

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