Systematic evaluation and rational design of metalloporphyrin-based humidity sensors: Effects of central metal, solvent, and water coordination

Abstract

Understanding the hydration-induced modulation of structural and electronic properties in metalloporphyrin complexes is essential for the development of responsive humidity sensors. In this study, we conducted a comprehensive density functional theory (DFT) investigation on TDCPP and its first-row transition metal complexes (Co, Fe, Mn, Ni, Zn) across five solvent environments, incorporating up to three explicit water molecules. Adsorption energies of coordinated water molecules were computed to evaluate hydration thermodynamics and solvent effects. In parallel, a comprehensive set of quantum reactivity descriptors, including frontier orbital energies, global hardness and softness, and electrophilic charge transfer parameters, was computed to characterize hydration-induced electronic modulation. The calculated adsorption energies revealed that Zn-TDCPP forms stable water complexes across all solvents (e.g., −2.16 eV in water), indicating robust hydration compatibility. In contrast, Fe- and Mn-TDCPP complexes showed less favorable or even endothermic adsorption in polar solvents, especially in overhydrated states, suggesting potential instability under high humidity. Our findings reveal that Zn-TDCPP exhibits exceptional electronic tunability with hydration, maintaining a moderate HOMO–LUMO gap (∼2.78–2.82 eV), low chemical hardness (∼1.35 eV), and redox stability, making it a promising candidate for humidity sensing. Conversely, Mn- and Fe-based complexes exhibit significantly higher electrophilic character and greater capacity to accept electronic charge, indicating strong molecular reactivity in moisture-rich environments. Furthermore, hydration-driven enhancements in total dipole moment were observed across all metallated TDCPP systems, with values ranging from ∼1.9 D to over 9.7 D depending on metal center and solvent polarity, underscoring the role of polarizability as an auxiliary electronic descriptor relevant to dielectric response and sensor sensitivity. This work offers a detailed descriptor map linking hydration, electronic flexibility, and sensing potential, and provides a rational foundation for designing porphyrin-based sensors under variable humidity conditions.