Photochromic crowned spirobenzopyrans: Quantitative metal-ion chelation by UV, competitive selective ion-extraction and metal-ion transportation demonstration studies

Following our publication of the synthesis of the new 5-substituted crowned spirocyclic benzopyrans (3a–c), and the fact that suitably placed substituents can be used to influence their metal-ion chelating properties [Roxburgh, CJ, Sammes, PG. Dyes Pigments 1995;28:317.], we now report the results of an extensive quantitative study to demonstrate that these systems selectively and photoreversibly chelate a variety of metal-ions i.e. Li+, Na+, Ca2+, Ba2+, and Mg2+. Experiments were conducted at seven different spiropyran concentrations, for each of the three crowned spiropyrans, and with the five different metal-ions (above). The 5-trifluoromethyl-substituted system produced the most pronounced effects and was thus chosen as the best system for further selective investigations which include: (1) competitive relative selective metal-ion chelating capability – investigated by devising a ‘matrix’ series of experiments, in order to ascertain which metal-ions, under dual competition (two metal-ions simultaneously present) were relatively more chelated (a priority order for the relative selectivity was consequently deduced), and (2) a system and experiments were devised to demonstrate that metal-ions could reversibly, under photodynamic control, be transported across a barrier (an immiscible layer of 1,2-dichloroethane and water: metal-ion concentrations were measured before and after photoirradiations using inductively coupled plasma mass spectrophotometry (ICP-MS) a powerful and accurate analytical technique for the quantification of metal-ions) to establish the levels of transportation.