Computational Screening of Zeolitic Imidazolate Frameworks (ZIFs) for Optical Sensing of VOCs via Refractive Index Modulation

Abstract

Volatile Organic Compounds (VOCs) pose a serious threat to indoor air quality, contributing to both short and long-term health issues. Identifying materials capable of selectively and sensitively detecting VOCs is, therefore, essential. Among various sensing strategies, optical detection based on refractive index (RI) changes has shown great promise for detecting VOCs at low concentrations[1]. However, accurately predicting the dielectric and optical properties of crystalline materials using ab initio methods such as periodic Density Functional Theory (DFT) is computationally intensive. To efficiently identify suitable materials from a broader selection, a computational screening approach that balances accuracy and resource demands is imperative. In this study, we explore the potential of Zeolitic Imidazolate Frameworks (ZIFs), a subclass of MOFs known for their structural tuneability and chemical diversity for RI-based VOC sensing. We begin with a fragment-based method (FBM) developed by Treger et al. [2] that decomposes each ZIF into its inorganic node and organic linker components. Polarizabilities are calculated for each fragment using DFT, and the refractive index is estimated through the Clausius-Mossotti relation[3]. This method allows rapid screening while significantly reducing the computational load compared to full periodic DFT calculations. We apply this approach to ten structurally diverse ZIFs, systematically varying in metal composition, linker chemistry, and topology, and evaluate their RI response to three representative VOCs: acetone, toluene, and methanol. As a next step, we assess the validity of the FBM by performing full periodic DFT calculations on selected systems for benchmarking. Our findings highlight that ZIFs with smaller unit cell volumes, such as ZIF-7 and ZIF-9, both incorporating benzimidazole linkers, exhibit the most pronounced RI changes upon VOC adsorption, followed by ZIF-1. While unit cell size and linker chemistry contribute to this behaviour, the framework topology also plays a critical role. ZIFs with tightly packed, low-porosity structures tend to show more increased RI shifts. This subtle influence arising from the spatial arrangement of building blocks was particularly well-captured by the fragment-based method (FBM). Moreover, the consistency in the RI change trends between FBM and periodic DFT further validates the reliability of this cost-effective approach. Overall, this workflow offers an effective and scalable strategy for discovering advanced materials for optical sensing, paving the way for next-generation VOC detection technologies.