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

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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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T2 - modulation of intramolecular charge transfer by intermolecular π - π interactions, kinetic trapping of the aci-nitro group, and reversible molecular locking

AU - Naumov, Pance

AU - Lee, Sang Cheol

AU - Ishizawa, Nobuo

AU - Jeong, Young Gyu

AU - Chung, Ihn Hee

AU - Fukuzumi, Shunichi

PY - 2009/10/22

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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.

AB - 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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