Systematic tuning and switching of neutral and ionic phases in a donor-acceptor chain compound by doping with...

The temperature-induced stepwise neutral-ionic (N-I) phase transition in the covalently bonded donor-acceptor chain compound [Ru2 (2,3,5,6-F4 PhCO2 )4 DMDCNQI]⋅ 2(p-xylene) (2,3,5,6-F4 PhCO2 (-) =2,3,5,6-tetrafluorobenzoate; DMDCNQI=2,5-dimethyl-N,N'-dicyanoquinodiimine) was systematically tuned over a wide temperature range using two techniques: 1) A chemical technique based on doping with a less-active donor unit [Ru2 (II,II) (F5 PhCO2 )4 ] (F5 PhCO2 (-) =pentafluorobenzoate), thereby providing an isostructural doped series [{Ru2 (II,II) (2,3,5,6-F4 PhCO2 )4 }1-x {Ru2 (II,II) (F5 PhCO2 )4 }x DMDCNQI]⋅2(p-xylene), with x=0.06, 0.10, 0.21, and 0.24; and 2) a physical technique, which was the application of hydrostatic pressure to the doped compounds. The stepwise N-I transition observed in the original compound was systematically varied in terms of the viewpoints of both transition temperature and transition features (stepwise or monotonic) dependent on the amount of dopants x. Application of pressure efficiently tuned the N-I transitions, with the oxidation phases being dramatically modified by applying only weak pressure up to 4 kbar. Even in cases that led to N-I transitions in small domains of the chains at ambient pressure, the application of pressure caused an expansion of the domains that enabled N-I transitions, finally leading to a complete change in the oxidation state of the chains, from neutral to ionic, accompanied by a change from a paramagnetic state to a ferrimagnetically ordered state.