Langmuir-Blodgett nanoarchitectonics for photoinduced redox-isomeric transformations in lanthanide bis-phthalocyaninates at air/water and air/solid interfaces

In this work, new photoinduced reversible molecular switches in planar supramolecular systems based on crown-substituted samarium and europium bis-phthalocyaninates have been discovered and investigated. The possibility to control the redox-isomeric equilibrium, established in Langmuir monolayers of the studied compounds at the air/water interface, by UV-irradiation and red light has been shown for the first time. In this work the fact of redox isomerization is experimentally detected using UV–Vis absorption and X-ray photoelectron spectroscopy. It is found that UV-irradiation of the monolayer induces the redox isomerization (intramolecular electron transfer) [Ln²⁺L^•-₂]⁰ → [L^•-Ln³⁺L²⁻]⁰ accompanied by oxidation of the metal center and reduction of the ligand. The reverse process [L^•-Ln³⁺L²⁻]⁰ → [Ln²⁺L^•-₂]⁰ occurs spontaneously in the dark but can be promoted by irradiation into the Q-band region. The possibility to implement analogous switching in Langmuir-Blodgett films (LBF) on solid substrates is also demonstrated. Moreover, during such photoisomerization in LBFs on the electrode surface, changes in the stationary electrochemical potential and in the surface plasmon resonance angle were registered, in addition to spectral switching. The above findings make it possible to state the discovery of a new class of photochromic compounds whose colour change is associated with photoinduced valence tautomerism.