Interface Asymmetry Induced and Surface Pressure Controlled Valence Tautomerism in Monolayers of bis-Phthalocyaninates of Lanthanides

Abstract

Supramolecular systems based on transition metal complexes capable of reversible redox isomerization due to intramolecular electron transfer are one of the most interesting objects from the viewpoint of molecular switches’ design. In the present work, a comparative analysis of valence transformation of lanthanide complexes (Sm, Er, Tm and Yb) with donor-substituted bis-phthalocyaninates occurring during the formation and compression–extension of Langmuir monolayers was carried out using data of UV–Vis–NIR spectroscopy. It is shown that the numerical values of the Q-band positions in the absorption spectra for the extended monolayers of the complexes under study depend linearly on the ionic radius of the metal center, if the metals have an oxidation state of +2. This makes it possible to draw a direct analogy between the behavior of the studied compounds and analogous europium and cerium complexes, for which direct evidence of the valence tautomerism in such planar systems was obtained earlier. This led to the conclusion that the intramolecular electron transfer from the phthalocyanine ligand to the central metal ion [Ln3+(R4Pc2‑)(R4Pc•−)]0 → [Ln2+(R4Pc•−)2]0 occurs when solutions of donor-substituted bis-phthalocyaninates of samarium, erbium, thulium, and ytterbium are deposited onto the water subphase, and the reverse redox-isomeric transition is observed in most cases when the monolayer is compressed to high surface pressures. The first of these switches is related to the asymmetry of the air/water interface, and the second one is controlled by the lateral compression–expansion of the monolayer. It has been demonstrated that when bis-phthalocyanine monolayers of lanthanides with variable valence are transferred to solid substrates, the valence state of the metal center, and consequently, the redox-isomeric state of the complex, do not change. This means that we are able to form films with a predetermined state of the complex. Note that the redox-isomeric state of complexes should affect the entire range of physicochemical properties of such films.

Rare-earth based tetrapyrrolic sandwiches: chemistry, materials and applications

“…The phenomenon was also found to be general for most of bisphthalocyaninates, which are formed by lanthanides exhibiting di-and trivalent states -Sm, Eu, Er, Tm, and Yb. 62,63 Homonuclear trisphthalocyaninates of strongly paramagnetic lanthanides -Ln 2 [(15C5) 4 Pc] 3 , Ln = Tb, Dy, Ho, and Tm were studied by variable-temperature NMR showing the possibility of their application as thermosensors in the physiological temperature range (303-323 K) 64,65 due to the sensitivity of lanthanide-induced shifts to temperature. The Tb(III) complex was found to have the highest sensitivity of this shift among the reported paramagnetic complexes of 4f and d elements with polydentate ligands in organic media.…”

“…The phenomenon was also found to be general for most of bisphthalocyaninates, which are formed by lanthanides exhibiting di- and trivalent states – Sm, Eu, Er, Tm, and Yb. 62,63…”

Gradual solid-state redox-isomerism in the lanthanide series

“…However, the maximal χ M T value (1.53 cm 3 mol −1 K at 200 K) is noticeably lower than that in other Yb 3+ (electronic configuration [Xe] 4f 13 ) species (2.3-3.0 cm 3 mol −1 K or 4.3-4.9μ B ). 24 This can be explained by the presence of isomer 2b that contains diamagnetic Yb 2+ (electronic configuration [Xe]4f 14 ) and the dpp-bian radical-anion (S = 1 2 ). Increasing the temperature from 200 to 300 K causes a decrease of the χ M T value from 1.53 cm 3 mol −1 K to 1.45 cm 3 mol −1 K (or from 3.5 to 3.4μ B ) due to a partial conversion of isomer 2a into isomer 2b.…”

“…11 Reversible intramolecular electron transfer in the ytterbium system is accompanied by an alteration of the magnetic moment, metal-to-ligand bond distances and unit cell parameters. Later on, intramolecular electron transfer in cerium, 12 europium 13 and other lanthanide phthalocyaninates 14 has been stimulated by the compression/extension of their Langmuir monolayers. Recently, valence tautomerism was discovered in the samarium coordination polymer SmI 2 ( pyrazine) 3 .…”