Modulation of the spiropyran-merocyanine reversion via metal-ion selective complexation: Trapping of the "Transient" cis-merocyanine

The effects of molecular structure and metal-ion complexation on the photochromic and thermochromic properties of N-functionalized carboxyl ligands of 6 ' -nitrobenzoindolinospiropyrans (6-nitroBIPS) with varying methylene spacers (n = 2, 4, and 5) were examined. Irradiation of this series of molecules in the presence of Cu2+ and Zn2+ produced two species that were spectroscopically distinguishable based on their absorption characteristics and stable toward the thermal reversion process. The two bands were diagnostic of the cis- and trans-MC-M2+ complexes, with lambda (cis) = 410 nm and lambda (trans) = 490 nm, respectively. It was found that lengthening the N-moiety spacer produced no perturbations in the metal-ion complexation behavior. Addition of an equimolar amount of the chelating agent, EDTA, to these solutions resulted in the disappearance of the cis-MC-M2+ and trans-MC-M2+ complex absorptions and a simultaneous increase in free trans-MC which could then be observed to revert back to the SP form by a first-order rate decay process. Thus it is shown that Cu2+ and Zn2+ function as traps of the "transient" cis-merocyanine stereoisomer formed upon irradiation of spiropyrans functionalized with carboxyl ligands having two, four, and five methylene spacer units, with the potential to modulate this molecular switch by selection of the appropriate metal.