Intramolecular Nitro-Assisted Proton Transfer in Photoirradiated 2-(2‘,4‘-Dinitrobenzyl)pyridine: Polarized Optical Spectroscopic Study and Electronic Structure Calculations

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.