Unexpected Nonresponsive Behavior of a Flexible Metal-Organic Framework under Conformational Changes of a Photoresponsive Guest Molecule

In this article, we describe the synthesis, characterization, and optical properties of a photochromic-guest-incorporated metal-organic framework (MOF). The photochromic guest molecule, 2-phenylazopyridine (PAP), was introduced into a pre-synthesized porous crystalline host MOF, [Zn₂(1,4-bdc)₂(dabco)]<i>_n</i> (<b>1</b>). The successful embedment of PAP has been confirmed by elemental analysis, powder X-ray diffraction measurements, IR spectroscopy, etc. The number of PAP molecules per unit cell of host was 1.0, as evidenced by elemental and thermogravimetric analyses of the host–guest composite, <b>1⊃PAP</b>. The <b>1⊃PAP</b> composite did not adsorb N₂, revealed by the adsorption isotherm of <b>1⊃PAP</b>, which indicates the pore blockage by the close contact of the host framework with the guest PAP in the trans form. The light-induced trans/cis isomerization with partial reversibility of the guest molecule (PAP) in this hybrid host–guest compound (<b>1⊃PAP</b>) has been investigated by detailed IR spectroscopy and UV–vis spectroscopy. The structural transformation from tetragonal in <b>1</b> to orthorhombic in <b>1⊃PAP</b> exhibits dynamic nature of the framework upon inclusion of guest in the framework, which remarkably becomes nonresponsive with the photoirradiation of guest PAP, retaining its orthorhombic structure in the photoirradiated complex, <b>1⊃PAP­(UV)