Researchers have recently developed an interpenetration-transformation strategy that enables topochemical polymerization in diacetylene-based metal-organic frameworks (MOFs), offering a new approach to enhancing their nonlinear optical (NLO) properties.

The study was led by Prof. ZHANG Jian and Prof. GU Zhigang from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences and was published in Angewandte Chemie International Edition.

Diacetylene-based MOFs are promising candidates for NLO materials due to their π-conjugated systems and unsaturated sites, which can undergo 1,4-addition topochemical polymerization, significantly enhancing material conjugation and functionality. However, achieving precise control over this polymerization remains challenging because most reported diacetylene MOFs lack the stringent molecular alignment and ordered stacking required for the reaction.

The researchers synthesized two interpenetrated MOFs from a diacetylene ligand and zinc nitrate: twofold-interpenetrated CAS-20 and threefold-interpenetrated CAS-22.

Through crystallographic analysis and detailed spectroscopic characterization, the researchers found that the interpenetration transformation from CAS-20 to CAS-22 proceeded through a set of non-interpenetrated semi-crystalline intermediates (CAS-20-d, CAS-21 and CAS-21-d), accompanied by guest loss. The semi-crystalline state resulted from non-synchronous transformations of the two individual frameworks in interpenetrated CAS-20.

The interpenetration transformation enabled the diacetylene groups to adopt suitable ordered and continuous stacking, allowing thermally induced topochemical polymerization in CAS-22 through a 1,4-addition reaction.

The results showed that the interpenetrated MOFs exhibited classic nonlinear optical limiting performance, while the polymerized CAS-22 showed a third-order nonlinear absorption coefficient approximately 69 times that of CAS-20.

Femtosecond Z-scan measurements further revealed ultra-broadband reverse saturable absorption from the visible to the near-infrared region, making CAS-22-heat a rare material for femtosecond optical limiting.

According to the researchers, the findings demonstrate that interpenetration transformation can provide a strategy for enabling topochemical polymerization in MOFs and may facilitate the design of high-performance optoelectronic materials for smart sensing and laser protection.

Interpenetration transformation from twofold interpenetrated CAS-20 to threefold interpenetrated CAS-22 via semi-crystalline intermediates, enabling topochemical polymerization. (Image by FJIRSM)