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ABSTRACT: Bis(2,2-bipyridine)-5-amino-1,10-phenanthroline ruthenium(II) (Ru(bpy)2(phen-NH2)2+), an MLCT complex, has a long-lived triplet state in water (λex: 473 nm; λem: 620 nm; τ = 615 ns; Φ = 1 relative to that of Ru(bpy)32+) and a structure analogous to Ru(bpy)32+. When Ru(bpy)2(phen-NH2)2+ was subjected to diazotization in the presence of carbon nanotubes (CNTs), it formed nanodots on the CNTs, rendering the resulting tubes (Ru@CNT) capable of transducing photo stimuli (473 nm) into electricity and magnetism at ambient conditions. The increased functionality was highly reproducible, as evidenced by conductive-mode AFM, vibrating sample magnetometry (VSM), and AC susceptibility analysis. The local magnetism probing of the Ru@CNT with magnetic-mode AFM techniques (MFM) indicated that the magnetism originated from the unpaired electrons formed on the photoexcited nanodots. The resulting phase shift behaved as a function of the luminous power and the voltage (Vb) of the electrical bias applied to the Ru@CNT. The Vb dependence deviated from the expected quadratic correlation, confirming that the formation of the photoinduced charge separation state at the nanodots is responsible for the photomagnetism. The Ru@ CNT tubes showed mobility toward external magnets (65 G) when floating on water and under 473 nm illumination. The Ru@ CNT thus appears to be a multifunctional material that might be useful in spintronics. The optical control of physical properties and the development of new materials with increased functionality are subjects of intensive research in the area of materials science.1,2 Carbon nanotubes (CNTs) represent such a multifunctional material, featuring high chemical stability, excellent mechanical strength,3 and diverse electronic properties that depend on their helicity and size4 and can be manipulated by an external electromagnetic field.5 In contrast to their electronic characteristics, intrinsic CNTs are diamagnetic at room temperature.6 Although this conclusion remains the subject of some debate,7 examples of applications of intrinsic CNTs in magnetismrelated applications at ambient conditions are rare.8,9 CNTs can be magnetized by the presence of ferromagnetic nanocrystals,10 by the creation of a ferromagnetic surface,11 or through chemical modification.12 Various magnetic CNTs have been prepared as of this writing, and the products have the potential for use in many fields.13 Despite this, preparing photomagnetic CNTs continues to remain a challenge.14 Photomagnetism dates back to 1967, when silicon-doped yttrium iron garnet was reported to develop magnetocrystalline anisotropy on exposure to IR irradiation.15 Since that report, a handful of spin-crossover compounds have been characterized,16 but most show short lifetimes for the photoinduced spin states at ambient temperature. In 2002, Shimamoto et al. observed that the electronic state of Na0.68Co1.20[Fe(CN)6]· 3.7H2O could be converted from FeII(S = 0)-CN-CoIII(S = 0) to FeIII(S = 1/2)-CN-CoII(S = 3/2) under laser-pulse irradiation at room temperature.17 Liu et al. supported the observation, showing that the reversible valence tautomeric conversion in Na0.36Co1.32Fe(CN)6.5·6H2O could be induced by a single-shot laser pulse (duration of 8 ns) above 200 K in a thermal hysteresis loop due to a cooperative interaction among the local photoexcited sites.18 By monitoring the photoexcited heptanuclear complex [MoIV(CN)2(CNCuL)6]8+ (L: tris(2aminoethyl)amine) at low temperature, Herrera et al. reported on the formation of a high-spin molecule with ferromagnetic interaction between the spin carriers.19 In 2005, Kimel employed circularly polarized femtosecond laser pulses to control the spin dynamics in DyFeO3 at low temperature, offering prospects for applications of ultrafast lasers in magnetic devices.20 According to the progress of photomagnetic materials, Létard suggested a guideline for the rational design of materials with long-lived photomagnetic lifetimes at room temperature.21 In 2008, Cobo et al. isolated {FeII(pyrazine)[Pt(CN)4]} and reported that the crystals exhibited a thermal spin transition at around room temperature, which was accompanied by a 14 K wide hysteresis loop. Within the thermal hysteresis region a complete bidirectional photoconversion between the high-spin and low-spin phases occurred when the sample was irradiated with a short single laser pulse (4 ns, 532 nm).22 Bozdag et al. also observed reversiblephotoinduced magnetic phenomena for a V−Cr prussian blue analogue. When illuminated with UV light (350 nm) at low temperatures (10 K), the magnet exhibited a change in magnetization due to structural distortion, and the photoexcited magnetic state completely recovered back to the ground state via thermal relaxation or partially recovered when illuminated with green light (514 nm).23 Inspired by the room-temperature photoinduced electron transfer occurring in prussian blue analogues, researchers agree that the use of light can have future applications in the elaboration of molecular memories and switching devices.24 From theoretical aspects, charge-transfer complexes are potential candidates for use in photomagnetic applications at ambient conditions, because many metal complexes possess a relatively long lifetime that is associated with a metal-to-ligand charge transfer (3MLCT) excited state, and the ability to act as an electron acceptor or donor.25 The findings reported herein show that when CNTs are surface-bonded with bis(2,2-bipyridine)-5-amino-1,10-phenanthroline ruthenium(II) (Ru(bpy)2(phen-NH2)2+),26 a longlived 3MLCT molecule, the resulting tubes exhibit magnetism when exposed to visible light at room temperature. This photomagnetism is highly reproducible, revealing the potential for use in spintronics research.27 |
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