Fast development, spectral and optical gas sensing characteristics of NDIs-HOFs films based optical waveguides

aphthalene diimide and derivatives (NDIs) are preferential to construct stable optic gas sensitive hydrogen-bonded organic frameworks (HOFs) owing to their photo-responsive electron accepting-donating conjugated building blocks. In this study, we developed series of NDIs-HOFs films with structural, spectral diversity, and optical gas sensing specificities that vary with the shape or length of the NDIs substituent. A series of NDIs-HOFs films were constructed from benzene dicarboxylic acid (bdc), NDIs (NDI, dpNDI, H₄BIPA-TC), and phenylporphyrin sulfonate (TPPS), and were rapidly fabricated on the surface of titanium-dioxide (TiO₂) film optical waveguides (OWGs) substrate using UV-light irradiation approach. The surface of NDIs-HOFs films showed different morphologies due to the difference in shapes or length of NDIs, i.e., NDI-HOF showed a spherical shape, dpNDI-HOF showed a straw-like hexagonal shape, and BIPA-TC-HOF showed a smooth plane amorphous structure. The structural diversity of NDIs-HOFs inducing differences in spectral characteristics and gas sensing specificity. Owing to the Lewis base nature of amide bound in NDI central aromatic ring, the NDIs-HOFs films OWGs exhibited sensing response to H₂S. The H₂S detectability of NDIs-HOFs films OWGs were varied from their structure and pore-size, following the order of NDI-HOF > BIPA-TC-HOF > dpNDI-HOF. Among the series NDIs-HOFs films, NDI-HOF films OWG exhibits most sensitive response (response time: 2 s; detection range: 0.01-10 ppm; limit of detection: 0.92 ppm) to H₂S without any interference from the other acid/base gases (e.g., NH₃, SO₂, NO₂, or HCl) and ambient humidity.