Quantum Chemical Investigations of the Thermal and Photoinduced Proton-Transfer Reactions of 2-(2‘,4‘-Dinitrobenzyl)pyridine

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.

Mechanochromism of Piroxicam Accompanied by Intermolecular Proton Transfer Probed by Spectroscopic Methods and Solid-Phase Changes

Structural and solid-state changes of piroxicam in its crystalline form under mechanical stress were investigated using cryogenic grinding, powder X-ray diffractometry, diffuse-reflectance solid-state ultraviolet-visible spectroscopy, variable-temperature solid-state (13)C nuclear magnetic resonance spectroscopy, and solid-state diffuse-reflectance infrared Fourier transform spectroscopy. Crystalline piroxicam anhydrate exists as colorless single crystals irrespective of the polymorphic form and contains neutral piroxicam molecules. Under mechanical stress, these crystals become yellow amorphous piroxicam, which has a strong propensity to recrystallize to a colorless crystalline phase. The yellow color of amorphous piroxicam is attributed to charged piroxicam molecules. Variable-temperature solid-state (13)C NMR spectroscopy indicates that most of the amorphous piroxicam consists of neutral piroxicam molecules; the charged species comprise only about 8% of the amorphous phase. This ability to quantify the fractions of charged and neutral molecules of piroxicam in the amorphous phase highlights the unique capability of solid-state NMR to quantify mixtures in the absence of standards. Other compounds of piroxicam, which are yellow, are known to contain zwitterionic piroxicam molecules. The present work describes a system in which proton transfer accompanies both solid-state disorder and a change in color induced by mechanical stress, a phenomenon which may be termed mechanochromism of piroxicam.

Structure of the photocolored 2-(2',4'-dinitrobenzyl)pyridine crystal: two-photon induced solid-state proton transfer with minor structural perturbation

The X-ray structure of the blue phototautomer of 2-(2',4'-dinitrobenzyl)pyridine (DNBP) produced by two-photon excitation in a single crystal is the first direct evidence of a product in the DNBP photochromic family. A nitro-assisted proton transfer mechanism is attributed to the photocoloration of solid DNBP.