The optical sensing of iron(II) based on the chromophore, (E) 4′-(2-(4,4″-bisdecyloxy-2,2′:5′,2″-terthiophen-3′-yl)ethenyl)-2,2′:6′,2"-terpyridine, incorporated in a plasticized poly(vinyl chloride) matrix was studied. The chromophore was obtained by the chemical modification of terpyridine with a terthiophene derivative. This modification not only enhanced the application of terpyridyl iron(II) sensing as a result of reduced aqueous solubility but also enhanced the optical sensitivity of the chromophore and greatly influenced the kinetics of ligand metal interactions by decelerating the complex formation between iron(II) and the terpyridine. This latter feature allowed the exploration of novel spectroscopic methods of Fe2+ analysis based on concentration- and time-dependent complexation of iron(II) by the chromophore. Membranes containing the chromophore were deposited on glass and paper strips and studied further as disposable iron(II)-sensitive sensors. Kinetic-based methods of ion analysis were developed by employing absorbance measurements. This report presents the possibility of chemical modification of the complexing agents in order to decelerate the kinetics of their complex formation with the analyte, thus allowing a novel type of optical ion sensing.