Temperature-induced neutral-ionic transition in 2-chloro-5-methyl-p-phenylenediamine-2,5-dimethyl-dicyanoquinonediimine

A mixed-stack organic charge-transfer crystal of 2-chloro-5-methyl- $p$-phenylenediamine-2,5-dimethyl dicyanoquinonediimine (ClMePD-DMeDCNQI), which undergoes a temperature-induced neutral-ionic transition (TINIT), is reported. From infrared spectroscopy, the molecular ionicity (\ensuremath{\rho}) of ClMePD-DMeDCNQI was found to change continuously from \ensuremath{\rho}=0.3 at 300 K to \ensuremath{\rho}=0.6 at 100 K. The neutral-ionic interface was crossed without a discontinuity in \ensuremath{\rho} at around 200 K, accompanied by stack dimerization. These features were quite different from the discontinuous change in \ensuremath{\rho} observed for the TINIT of tetrathiafulvalene-$p$-chloranil (TTF-CA) and tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). The transition characteristics in ClMePD-DMeDCNQI are discussed by drawing a comparison with those seen in TTF-CA and TMB-TCNQ.