Metal complexes of new photochromic chelator: Structure, stability and photodissociation

• A new spironaphthopyran-based photochromic chelator was synthesised.
• Merocyanine isomer was formed 1:1 and 1:2 complexes with metal ions simultaneously.
• The structure of merocyanine–ZnCl2 complex was characterised by X-ray diffraction.
• A technique of quantum yields estimation of complex photodissociation was proposed.

A new spironaphthopyran-based photochromic chelator was synthesised, and its photochromic, thermochromic and metallochromic properties were investigated. In the presence of Cd2+, Mn2+, Ni2+, Co2+, Zn2+ and Cu2+ ions in acetone solutions 1:1 and 1:2 metal:merocyanine complexes were formed simultaneously. Electron absorption spectroscopy (isomolar solution technique) and electrospray ionisation mass-spectrometry techniques were employed in order to define stoichiometry of the complexes. Additionally, single-crystal X-ray diffraction was used to determine molecular structure of merocyanine–zinc chloride 1:1 complex in the solid state. By means of multiwavelength spectrophotometric analysis the thermodynamic and spectral parameters of the 1:1 and 1:2 metal:merocyanine complexes were calculated. The obtained results demonstrate that stability of the complexes increases in the range of metal ions Cd2+, Mn2+, Ni2+, Co2+, Zn2+, Cu2+. The stability constants of 1:1 complexes are higher than those of the complexes formed at the second complexation step. Complexation with Cd2+ and Zn2+ can be reversed by irradiation of the coloured merocyanine complex with visible light which leads to dissociation towards the initial spironaphthopyran and the metal ion. A new approach to estimate the metal–merocyanine complex photodissociation quantum yields was proposed and employed for Cd2+ and Zn2+ containing complexes.

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