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.

Well-defined dinuclear silver­(I) complexes have been targeted for applications in catalysis and materials chemistry, and the effect of close silver–silver interactions on electronic structure remains an area of active inquiry. In this study, we describe the synthesis, structure, and photophysical properties of dimeric silver complexes featuring a redox-active naphthyridine diimine ligand. Unusually for silver­(I), these complexes display absorption features in the visible region due to metal–metal to ligand charge transfer (MMLCT) transitions, which arise from the combination of close silver–silver interactions and low-lying ligand π* orbitals. The complexes’ photophysical properties are explored via a combination of spectroscopic and computational studies, revealing MMLCT excited state lifetimes that exceed 1 μs. These results portend previously unforeseen applications of silver­(I) dimers in visible light absorption and excited state reactivity.

We investigate the effect of assembly on charge transfer, charge recombination, and the persistence of radical cations in halogen-substituted triphenylamine (TPA) dimers. A series of urea-tethered TPA derivatives 1 (X = H, Cl, Br, and I) are compared, which have one phenyl group modified at the para position with a halogen. Ureas direct the assembly of these derivatives while halogen substituents influence the packing of the TPA units. These modifications affect the generation and persistence of TPA radical cations as monitored by electron paramagnetic resonance (EPR) spectroscopy. The formation and degradation pathways of the radical cations in solution and gas phase were probed by ion-mobility spectrometry mass spectrometry. In contrast, supramolecular assembly enhanced the stability of these materials as well as the persistence of their photogenerated radical cations, which appear to undergo charge recombination without degradation. Greater quantities of these radical cations are observed for the bromo and non-halogenated derivatives (1Br, 1H). Time-dependent density functional theory (TD-DFT) calculations on single molecules and hydrogen-bonded dimers suggest the stability of TPA radical cations largely depends on initial photoinduced charge separation and electronic coupling between assembled TPA dimers. The latter was found to be about 7 times stronger in 1I than in 1Br dimers, which may explain faster charge recombination and shorter lifetimes of 1I radicals. Transient absorption (TA) spectroscopy and TD-DFT were able to identify the charged species for 1Br along with the kinetic traces and measured lifetime of ∼80 ns. Fluorescence quenching studies are consistent with initial charge separation and subsequent charge transfer event between nearby TPAs. Future exploration will focus on the mobility and application of these TPA assemblies as hole transport materials.

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Photolysis of 2,3-diazidonaphthalene-1,4-dione (1) in methyltetrahydrofuran matrices forms 2-(λ 1 -azaneyl)-3-azidonaphthalene-1,4-dione (vinylnitrene 3 2), as confirmed by electron paramagnetic resonance spectroscopy. The zero-field splitting (zfs) parameters for 3 2 (D/hc = 0.5338 cm −1 , and E/hc = 0.0038 cm −1 ) reveal significant 1,3biradical character. Irradiating 3 2 yields 2-(λ 1 -azaneyl)-1,3-dioxo-2,3dihydro-1H-indene-2-carbonitrile (alkylnitrene 3 3), which has zfs parameters typical of a cycloalkylnitrene (D/hc = 1.57 cm −1 , and E/hc = 0.00071 cm −1 ). Photolysis of 1 in argon matrices verifies that 3 2 forms 3 3.