Vapachromism is a phenomenon in which substances change color in response to the presence of vapor of a particular organic solvent1. Vapochromic substances are used in sensors that detect volatile organic compounds (VOCs) in a variety of environments including industrial, domestic and medical areas.
Because of their use in detecting harmful organic solvents, there is an increasing interest to develop sensory materials that are capable of recognizing the presence of organic solvents. Researchers at the Hokkaido University’s Department of Chemistry have recently developed a new type of sensor material that has a potential to be used both in chemical sensors and rewritable memory devices.
Vapochromic materials that are capable of changing their spin states can be applied in rewritable optical and magnetic memory devices1. Several spin- crossover Fe(II) complexes were investigated for their vapochromic properties, along with their ability of spin transition to move from high spin and low spin states.
The downside of such Fe(II) complexes is that these systems cannot function in a wide temperature range as a result of their vapor induced spin transition that is based on shift of equilibrium between the two spin states2. Other vapochromic/ magnetic materials lack specificity in detecting VOCs. Therefore, the quest for the development of vapochromic materials capable of spin transition to function in a wide temperature range is still ongoing2.
In an effort to solve this problem, Masako Kato’s team investigated a series of nickel (II) complexes with N- substituted benzoquinonemonoimine derivatives (H2LR) for their potential to be used as temperature robust - vapochromic/magnetic materials2. H2LR ligands are known to exhibit chromic behavior when in solution due to the unique electronic properties of their doubly extended p-orbitals2.
The hydrogen bond donor and acceptor sites in the H2LR ligands facilitate the development of versatile hydrogen bonded structures2 and a change in N substituents in H2LR ligands would alter their stearic properties, therefore providing an opportunity to customize vapor selectivity based on the attached ligand moiety2.
Ni (II) complexes were known to exhibit spin state switching in solution and in a polymer film but their spin state switching abilities in solid phase is not investigated until now. Masako Kato’s team for the first time synthesized a highly stable and electronically flexible NiII complex [Ni(H2LMe)2], where the H2LR is 4-methylamino-6-methyliminio-3-oxo- cyclohexa-1,4-dien-1-olate, by reacting Nickel acetyl acetonate ((Ni(acac)2) with H2LMe in the presence of trimethylamine2. The [Ni(H2LEt)2] complex prepared in a similar way using H2LEt did not show such electronic flexibility and stability2.
An orange colored solid was obtained when [Ni(H2LMe)2] is synthesized in methanol, while a purple solid was obtained when ethanol is used as a solvent instead of methanol. No such color change was observed in the final product of [Ni(H2LEt)2] regardless of whether the solvent was methanol or ethanol2.
Further experiments utilizing elemental analysis, thermogravimetric analysis, IR spectroscopy and single crystal X ray analyses revealed that the orange colored compound is the non-solvated [Ni(H2LMe)2] form and the purple colored compound is the methanol- coordinated [Ni(HLMe)2- (MeOH)2] form2.
Solid state Ni(H2LMe)2 exhibited reversible and distinct color change depending on the vapor atmosphere. Methanol- coordinated [Ni(HLMe)2- (MeOH)2] changed its color from orange to purple upon exposure to ethanol vapor2. This reversible change of color is found to be as a result of the removal of methanol group from the Ni(HLMe)2- (MeOH)2 solvated form by Powder X-Ray Diffraction (PXRD).
The Ni(H2LMe)2 color change from orange to purple occurs within one hour upon exposure to methanol vapor, while such color change took 5 days in presence of other solvents making this process extremely selective to methanol2.
Furthermore, magnetic susceptibility measurements showed that Ni(H2LMe)2 has paramagnetic behavior while the solvated Ni(HLMe)2- (MeOH)2 form exhibited diamagnetic behavior2. This change in magnetic behavior is attributed to the change in coordination environment around the NiII center from a high-spin octahedral form to low-spin square-planar form, suggesting the potential of Ni(H2LMe)2 to develop rewritable optical and magnetic memory devices2.
The Ni(H2LMe)2 exhibited vapochromism which is highly selective to methanol and exhibited temperature robust spin transition as a result of transformation from square-planar to octahedral geometry by size selective reversible coordination of methanol to the NiII center2.
The Japan based research team is hopeful that the selective vapochrmomic properties and on-off switchable paramagnetism of Ni(II) conjugated quinonoid ligand suggests its potential use as a sensor material selective to methanol and in magnetic data storage devices1,2.
- “New Type of Sensor Material Developed.” Hokkaido University, www.global.hokudai.ac.jp/blog/new-type-of-sensor-material-developed/.
- P. Kar, M. Yoshida, Y. Shigeta, A. Usui, A. Kobayashi, T. Minamidate, N. Matsunaga, M. Kato, Angew. Chem. Int. Ed. 2017, 56, 2345.
- Image Credit: Shutterstock.com/madeira