Cracking under Internal Pressure: Photodynamic Behavior of Vinyl Azide Crystals through N₂ Release

Abstract

When exposed to UV light, single crystals of the vinyl azides 3-azido-1-phenylpropenone (1a), 3-azido-1-(4-methoxy­phenyl)­propenone (1b), and 3-azido-1-(4-chloro­phenyl)­propenone (1c) exhibit dramatic mechanical effects by cracking or bending with the release of N2. Mechanistic studies using laser flash photolysis, supported by quantum mechanical calculations, show that each of the vinyl azides degrades through a vinylnitrene intermediate. However, despite having very similar crystal packing motifs, the three compounds exhibit distinct photomechanical responses in bulk crystals. While the crystals of 1a delaminate and release gaseous N2 indiscriminately under paraffin oil, the crystals of 1b and 1c visibly expand, bend, and fracture, mainly along specific crystallographic faces, before releasing N2. The photochemical analysis suggests that the observed expansion is due to internal pressure exerted by the gaseous product in the crystal lattices of these materials. Lattice energy calculations, supported by nanoindentation experiments, show significant differences in the respective lattice energies. The calculations identify critical features in the crystal structures of 1b and 1c where elastic energy accumulates during gas release, which correspond to the direction of the observed cracks. This study highlights the hitherto untapped potential of photochemical gas release to elicit a photomechanical response and motility of photoreactive molecular crystals.

Tribochemistry, Mechanical Alloying, Mechanochemistry: What is in a Name?

“…Any transformation that is somehow facilitated by mechanical energy, or reactions that result from thermal- or photochemically induced stress and strain in a solid [the chemomechanochemical effect ( Boldyrev, 2018 )] seem now to be denoted as “mechanochemical”. Thermal or photochemical transformations in solids which have been mechanically pre-treated are similarly denoted as being “mechanically activated”, and hence also fall within the current paradigm of mechanochemistry ( Boldyrev, 2018 ; Galwey, 2020 ; Shields et al, 2020 ).…”

“…Thermal or photochemical transformations in solids which have been mechanically pre-treated are similarly denoted as being 'mechanically activated', and hence also fall within the current paradigm of mechanochemistry. [246][247][248] With this growing diversity of the community and the phenomena being explored comes a confusion of scientific languages on the scale of the Tower of Babel. Terminologies and jargon used by experts from one discipline are often misunderstood by experts from a different background.…”

Programmable photoresponsive materials based on a single molecule via distinct topochemical reactions

“…[19][20][21] Such topochemical reactions have fascinating energy transfer, which is capable of converting light into chemical energy and kinetic energy rapidly and efficiently. 18,22 On the one hand, the spatial movements of lattice atoms will generate local stress in surrounding p-dimers, enabling the deformation of crystals. 23,24 For instance, Naumov and Vittal reported smart molecular crystals based on [2 + 2] photocycloaddition, achieving a variety of photomechanical dynamic behaviors such as bending, jumping, rolling, and photosalience.…”

Surprising Chemistry of 6-Azidotetrazolo[5,1-a]phthalazine: What a Purported Natural Product Reveals about the Polymorphism of Explosives

“…In addition, the importance of azides in many other areas of science is growing rapidly, 41 as illustrated by their recent use in solid-state click reactions and in studies of the photodynamics of crystals. 42,43 The presence of multiple short azide−tetrazole N•••N contacts in Form I and low-temperature Form II may reflect the characteristic polarization of ATPH (Figure 2a), which is expected to strengthen the association by enhancing the electrophilic character of the azido group and the electron-rich nature of the tetrazole ring. Recent computational studies have confirmed that interactions of the type suggested by the sideon structure 8 (where D is a suitable donor) can offer significant stabilization and help determine the conformations and associative preferences of organic azides.…”