Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis

Abstract

The thermal-induced and photoinduced valence tautomerism of a series of Co(dioxolene)(2)(4-X-py)(2) complexes (dioxolene = 3,5-di-tert-butylcatecholate or 3,5-di-tert-butylsemiquinonate; 4-X-py = 4-(X)pyridine, X = H (1), OMe (2), Me (3), CN (4), Br (5), NO(2) (6)) is described. The thermal valence tautomerism (ls-Co(III)(SQ)(Cat)(4-X-py)(2) <--> hs-Co(II)(SQ)(SQ)(4-X-py)(2)) is only observed for complexes 4, 5, and 6 where each is accompanied by a hysteresis loop of ca. 5 K. When a crystalline sample of 4-6 is held at 10 K in a SQUID magnetometer and irradiated with white light (lambda = 400-850 nm), the hs-Co(II) tautomer is formed. When the light source is removed, and the sample is slowly heated, the hs-Co(II) tautomer persists until ca. 90 K, approximately 40 K higher than the thermal stability of previously reported complexes. Heating and cooling the sample while maintaining irradiation results in the appearance of a new light-induced thermal hysteresis loop below 90 K (DeltaT = ca. 25 K). Below 50 K, the hs-Co(II) tautomer displays temperature-independent relaxation to the ls-Co(III) form, and above 50 K, the relaxation is thermally activated with an activation energy E(a) > ca. 1500 cm(-1). The coordination geometry (trans-pyridines), pyridine substitution, and crystal packing forces conspire to create the comparatively thermally stable photogenerated hs-Co(II) tautomer, thus providing an excellent handle for molecular and crystal engineering studies.

Towards controlling the valance tautomer interconversion character by solvation

“…Benzene solvated crystals III are described by P1 space group, Z=2, and at 100 K. When compared with I and II, benzene solvated crystals present the shortest C ar -H…O i diox and similar C ar -H… ONO i hydrogen bonds lengths. They also present the shortest Co III -Co III distances between inlayer molecules linked by Car-H… O i diox and the longest Co III -Co III distances between Thus, as already pointed out before [6], VT in [Co III (SQ)(Cat)(4-NO 2 -py) 2 ] seems to be due to the intermolecular interactions between inlayer molecules, however the interconversion cooperativity seems to be related with the strength of the Car-H…O i diox interaction and with the correspondent Co-Coi separation.…”

“…of complex [Co III (SQ)(Cat) (4-NO 2 -py) 2 ] (I), where py= pyridine, Cat = 3,5-di-tbutylcatecholate and SQ=3,5-ditbutylsemiquinonate, were first studied with respect to low spin (LS) to high spin (HS) thermo and photoinduced conversion. It was reported that I become HS-Co III at temperatures below 110 K with non cooperative LS-Co III n↔ HS-Co II tautometic interconversion [ 6 ]. Crystals of Co III (SQ)(Cat)(4-NO 2 -py) 2 ]/Toluene (II) and Co III (SQ)(Cat)(4-NO 2 -py) 2 ]/Benzene (III) have been prepared to investigate further the solvation effects on the interconversion properties.…”

Photoswitchable stable charge-distributed states in a new cobalt complex exhibiting photo-induced valence tautomerism

“…Below a certain blocking temperature, the photoexcited metastable state is maintained for very long times, which range from minutes till several days 22 . Thermal relaxation drives the system back into a ground state, which can be described by an Arrhenius law (τ: relaxation time, τ 0 : inverse attempt frequency, ∆: energy barrier, k B : Boltzmann constant, T : temperature).…”

“…These compounds usually undergo the intramolecular ET between the coordinated metal center and the surrounding ligands, while this phenomenon is often accompanied by SCO behaviour of the metal ion, owing to the closely spaced d levels. The coordination compounds of the Co-catecholate/semiquinone families are known to display VT, and several examples have been reported, attracting considerable interest 7,10,21,22,[28][29][30][31][32] . Usually, the VT mechanisms implies the electron transfer from the ligands, leading to the interconversion between the lowspin Co III -catechole (Cat) and high-spin Co II -semiquinone (SQ) triggered by the presence of redox-active quinone ligands.…”

Magnetic anisotropy in thin films of Prussian blue analogues

“…For example, with the drive to develop new devices, applications with single crystals are being explored. 18 However, the exploitation of molecule-based magnetic films may be more attractive for industrial fabrication, and devices based on metalphthalocyanines 19 and metal-͓TCNE: tetracyanoethylene͔ x ͑Ref. 20͒ are two examples of work in this direction.…”