Tuning of Stepwise Neutral–Ionic Transitions by Acceptor Site Doping in Alternating Donor/Acceptor Chains

The stepwise neutral–ionic
(N–I) phase transition found in the alternating donor/acceptor
(DA) chain [Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)]·2­(p-xylene) (0; 2,3,5,6-F4PhCO2– = 2,3,5,6-tetrafluorobenzoate;
DMDCNQI = 2,5-dimethyl-N,N′-dicyanoquinonediimine)
was tuned by partly substituting the acceptor DMDCNQI with 2,5-dimethoxy-N,N′-dicyanoquinonediimine (DMeODCNQI),
which displays a poorer electron affinity in an isostructural series.
The site-doped series comprised [Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)1–x(DMeODCNQI)x]·2­(p-xylene) for doping rates (x) = 0.05 (0.05-MeO), 0.10 (0.10-MeO), 0.15 (0.15-MeO), and
0.20 (0.20-MeO). The neutral chain [Ru2(2,3,5,6-F4PhCO2)4(DMeODCNQI)]·4­(p-xylene) (1), which only contained DMeODCNQI, was also
characterized. All site-doped compounds were isostructural to 0 except 1 despite their identical DA chain motif.
Except at an x value of 0.20, they displayed a two-step
N–I transition involving an intermediate phase. This transition
occurred at high temperatures in 0 but shifted to lower
temperatures in a parallel manner with increasing doping rate. Simultaneously,
each transition broadened with increasing doping rate, leading to
a convergence of two transitions at an x value approximating
0.2. Donor/acceptor-site-doping techniques present somewhat different
impacts in terms of interchain Coulomb effects.