Spin-state dynamics of a photochromic iron(II) complex and its immobilization on oxide surfaces via phenol anchors

Abstract

This work presents a detailed study of the photo-induced spin-state dynamics of the photochromic iron(II) complex 1, where the metal ion is in the field of a tripodal hexa-imine ligand with protolysable phenol groups. The nature of the complex’s ground state has been identified as a spin singlet by ¹H NMR and steady-state UV/vis spectroscopies, and its distorted octahedral structure was analyzed via crystal structure determination. Sub-picosecond and nanosecond time-resolved laser flash photolysis experiments identify the long-lived quintet state of 1 as the selective product of photoexcitation in the UV/vis spectral region. Thermal barriers of spin-state interconversion as a function of solvent and added base are derived from temperature-dependent rates of transient decay. Ground-state recovery is found to be significantly affected by the solvent and is strongly enhanced, in particular, by base-driven solvolysis of the ligand’s phenol groups. Partial spontaneous deprotonation of the phenolic hydroxyl groups of 1 seems to prevail on metal oxide surfaces, i.e. on alumina. Composite materials, like 1 at Al₂O₃, that retain the characteristic spectral features of the parent iron(II) complex can be readily obtained by wet impregnation of hydrous alumina with solutions of 1.

Graphical abstract

Keywords:

• Spin crossover
• Laser flash photolysis
• Photophysics
• Responsive surfaces
• Phenols
• Hydrogen bonding

Acknowledgements

Support of this work by the Deutsche Forschungsgemeinschaft (SFB 658, “Elementary processes in molecular switches on surfaces”) is gratefully acknowledged. Professor Dr Thorsten Ressler (Technische Universität Berlin) is thanked for assistance with diffuse reflectance spectroscopy. G.H. thanks Professor Dr Dirk Guldi (Friedrich-Alexander-Universität Erlangen-Nürnberg) for providing access to the fs-LFP facilities and Professor Dr Bronisław Marciniak and Dr Tomasz Pedziński (Adam Mickiewicz University Poznań) for continuous support and assistance with ns-LFP photolysis.

Disclosure statement

No potential conflict of interest was reported by the authors.