Intramolecular nitro-assisted proton transfer in photoirradiated 2-(2′,4′-dinitrobenzyl)pyridine: Polarized optical spectroscopic study and electronic structure calculations

Pance Naumov, Kenji Sakurai, Tadahiko Ishikawa, Junichi Takahashi, Shin Ya Koshihara, Yuji Ohashi

Research output: Contribution to journalArticle

Abstract

The nitro-assisted proton transfer (NAPT), responsible for the photoactivity of ortho-nitrobenzylpyridines and a model for the nitro-based caged compounds, is studied along with the parent compound 2-(2′,4′- dinitrobenzyl)pyridine (DNBP) with polarized optical spectroscopy and theoretical calculations. The transition dipole moments of a DNBP single-crystal identified oriented molecules of the long-lived enamine tautomer (NH), rather than of the aci-nitro tautomer (OH), as carriers of the photoinduced blue coloration. It is clarified that the blue second singlet transition owes to intramolecular charge transfer from the allyl-pyridinium part to the dinitrophenyl fragment of NH. The theoretical modeling of the ground-state potential energy surface showed that while NH and OH can interconvert by means of direct proton transfer, such a process between the initial form CH and either OH and NH would require significant rotation of the aromatic rings. In the ground state, OH is less stable but the kinetically preferred product over NH. Once created, regardless of whether via ground-state or excited-state routes, the aci-nitro group of OH undergoes energetically inexpensive rotation to deliver the proton to the nitrogen acceptor. The "softening" of the energy surface around OH due to its structural flexibility, that is, mediation of the proton transfer by the nitro group, is crucial to overcome the high barrier for a direct proton jump from CH to NH, even in cases of unfavorable donor-acceptor geometry. The very small structural change experienced by the surrounding of a molecule undergoing NAPT is promising for the design of photoactive systems which retain their crystallinity during a prolonged operation.

Original languageEnglish (US)
Pages (from-to)7264-7275
Number of pages12
JournalJournal of Physical Chemistry A
Volume109
Issue number32
DOIs
StatePublished - Aug 18 2005

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Proton transfer
Electronic structure
pyridines
Ground state
electronic structure
protons
Protons
tautomers
Potential energy surfaces
Molecules
ground state
Dipole moment
Interfacial energy
Excited states
methylidyne
Charge transfer
mediation
Nitrogen
Single crystals
softening

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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Intramolecular nitro-assisted proton transfer in photoirradiated 2-(2′,4′-dinitrobenzyl)pyridine : Polarized optical spectroscopic study and electronic structure calculations. / Naumov, Pance; Sakurai, Kenji; Ishikawa, Tadahiko; Takahashi, Junichi; Koshihara, Shin Ya; Ohashi, Yuji.

In: Journal of Physical Chemistry A, Vol. 109, No. 32, 18.08.2005, p. 7264-7275.

Research output: Contribution to journalArticle

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abstract = "The nitro-assisted proton transfer (NAPT), responsible for the photoactivity of ortho-nitrobenzylpyridines and a model for the nitro-based caged compounds, is studied along with the parent compound 2-(2′,4′- dinitrobenzyl)pyridine (DNBP) with polarized optical spectroscopy and theoretical calculations. The transition dipole moments of a DNBP single-crystal identified oriented molecules of the long-lived enamine tautomer (NH), rather than of the aci-nitro tautomer (OH), as carriers of the photoinduced blue coloration. It is clarified that the blue second singlet transition owes to intramolecular charge transfer from the allyl-pyridinium part to the dinitrophenyl fragment of NH. The theoretical modeling of the ground-state potential energy surface showed that while NH and OH can interconvert by means of direct proton transfer, such a process between the initial form CH and either OH and NH would require significant rotation of the aromatic rings. In the ground state, OH is less stable but the kinetically preferred product over NH. Once created, regardless of whether via ground-state or excited-state routes, the aci-nitro group of OH undergoes energetically inexpensive rotation to deliver the proton to the nitrogen acceptor. The {"}softening{"} of the energy surface around OH due to its structural flexibility, that is, mediation of the proton transfer by the nitro group, is crucial to overcome the high barrier for a direct proton jump from CH to NH, even in cases of unfavorable donor-acceptor geometry. The very small structural change experienced by the surrounding of a molecule undergoing NAPT is promising for the design of photoactive systems which retain their crystallinity during a prolonged operation.",
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