Green synthesis, structural tailoring, and optical optimization of porphyrins for high-performance thin film resistive-capacitive sensors

Abstract

This study describes the green synthesis and investigation of linear and nonlinear optical properties of three different porphyrins: 5,10,15,20-tetrakis(p-methoxyphenyl)porphyrin (H₂MeTPP), 5,10,15,20-tetrakis(p-methoxyphenyl)porphyrinatocopper(II) (MeTPPCu), and 5,10,15,20-tetrakis(p-methoxyphenyl)porphyrinatozinc(II) (MeTPPZn). Three thin films of these porphyrins each with a thickness of 465.3 nm were deposited on quartz wafers by thermal evaporation. Surface morphology and crystallite size of as-deposited thin films were analyzed using SEM and XRD, while molecular structures were confirmed through UV–Vis, IR, NMR, and HR-MS techniques. Optical properties were assessed using the Drude and Wemple-DiDomenico models. Their optoelectronic properties showed notable differences, with MeTPPZn showing the most promising electronic polarizability characteristics. It exhibited the highest dispersion energy (37.33 eV), indicating strong electron delocalization, and the smallest bandgap (1.24 eV), making it the most polarizable porphyrin. Moreover, it had the highest charge carrier concentration (1.54×10²⁶ m⁻³), refractive index (2.82), dielectric constants (ε_∞ = 7.37, ε_L = 7.94), plasma frequency (1.93×10¹⁵ Hz), and optical mobility (1.33×10⁻¹ C.Sec.kg⁻¹), coupled with the lowest optical resistivity (3.05×10⁻⁷ kg·m³·C⁻²·s⁻¹), highlighting its exceptional potential for optoelectronic applications. Additionally, MeTPPZn showed the highest electric susceptibility, and molecular polarizability (1.90×10–⁻²² cm³.mole⁻¹), which facilitate the quicker separation of charge carriers and lower recombination losses. These attributes make MeTPPZn highly suitable for the fabrication of resistive-capacitive type thin-film sensors. Under varying illumination and humidity levels, the Ag/MeTPPZn/Ag bimodal sensor outperformed the Ag/H₂MeTPP/Ag and Ag/MeTPPCu/Ag sensors, demonstrating that at up to 55 % RH, it is highly sensitive to resistance, while beyond 55 % RH, capacitance increases sharply and resistance remains saturated.