Non-Covalent Interactions Directed Azobenzene Crystals: Photoisomerization-Induced Melting and Melt-Mediated Polymorphism
Year: 2026
Type: preprint
Abstract: Azobenzene molecular crystals have emerged as a compelling class of stimuli-responsive materials, yet the structural factors that determine whether a given crystal bends, melts, or remains entirely inert under UV irradiation are far from understood. Here, we investigate a series of eight structurally homologous 2,2′-dihydroxyazobenzene derivatives (Azo-0 to Azo-7) in which terminal functional groups (‒OH vs. ‒OMe) and pendant alkyl spacer length are independently varied, allowing the contributions of intramolecular conformation, intermolecular packing topology, and lattice compliance to be disentangled. Single-crystal Xray diffraction (SCXRD), UV-vis spectroscopy, differential scanning calorimetry (DSC), together with thermal-stage polarized optical microscopy, reveal two mechanistically distinct melting pathways. The central finding is that an intramolecular O-H···N hydrogen bond, present in four derivatives (Azo-0, Azo-1, Azo-3, Azo-5), forms a pseudo-six-membered chelate ring that locks the azobenzene core in its planar trans conformation, fully suppressing photoisomerization in both solution and solid state. Removal of this intramolecular constraint through ‒OMe substitution liberates the azobenzene core in Azo-2, Azo-4, Azo-6, and Azo-7, enabling efficient and fully reversible cis-trans photoisomerization with visible-light-driven recovery. Critically, irradiation of these unlocked crystals below their equilibrium melting points (Ts < Tm) triggers rapid, cooperative crystal-to-liquid transitions that are absent in the dark and under 456 nm light irradiation, establishing photoisomerization-induced lattice strain—not photothermal heating—as the operative mechanism. The sole outlier, Azo-5, melts under intense UV irradiation through a purely photothermal pathway, consistent with its locked conformation and strong 370 nm light absorption. Additionally, Azo-7 undergoes a thermally driven solid, liquid, solid polymorphic transformation: melt-recrystallization converts the high-melting polymorph Azo-7α (Tm ≈ 59 °C) into a structurally distinct, lower-melting polymorph Azo-7β (Tm ≈ 42 °C), demonstrating that thermal history is itself an independent handle for programming crystal packing. These findings translate directly into three molecular design principles for photomechanical organic crystals: phenolic OH gates the photoisomerization switch via intramolecular hydrogen bonding; alkyl chain length tunes lattice compliance and packing dimensionality; and melt-mediated crystals provide access to structurally and thermally distinct polymorphic forms.
Source: ChemRxiv
Institutions: University of Missouri, University of Warsaw
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7214992197