Tetramethylbenzidine-TCNQ: Studies of the neutral-ionic transition by time-resolved non-degenerate four-wave mixing

Y. Iwasa et al. reported recently experimental evidence for the occurrence of a temperature-induced neutral-ionic transition (TINT) in a new mixed stack charge transfer (CT) compound consisting of 3,3′,5,5′ tetramethyl benzidine (TMB) as the donor and TCNQ as the acceptor (TC = 205 ± 5K) We have used femtosecond laser pulses and the technique of time resolved non degenerate four-wave mixing to measure the third order susceptibility χ(3) of thin films of TMB-TCNQ, and the corresponding temporal response as a function of temperature above and below the neutral-ionic transition. The change of the decay time of different contributions to χ(3) with T will be discussed in connection with the physical process related to this neutral-ionic phase transition. We point out the importance of the electron-phonon interactions in this phase transition.