%0 Journal Article
%T From Ultrafast Photoinduced Small Polarons to Cooperative and Macroscopic Charge-Transfer Phase Transition.
%A Privault G
%A Hervé M
%A Godin N
%A Bertoni R
%A Akagi S
%A Kubicki J
%A Tokoro H
%A Ohkoshi SI
%A Lorenc M
%A Collet E
%J Angew Chem Int Ed Engl
%V 0
%N 0
%D 2024 Jul 9
%M 38979690
%F 16.823
%R 10.1002/anie.202408284
%X We study by femtosecond infrared spectroscopy the ultrafast and persistent photoinduced phase transition of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98∙0.2H2O material, induced at room temperature by a single laser shot. This system exhibits a charge-transfer based phase transition with a 75 K wide thermal hysteresis, centred at room temperature, from the low temperature Mn3+-N-C-Fe2+ tetragonal phase to the high temperature Mn2+-N-C-Fe3+ cubic phase. At room temperature, the photoinduced phase transition is persistent. However, the out-of-equilibrium dynamics leading to this phase is multi-scale. Femtosecond infrared spectroscopy, particularly sensitive to local reorganizations through the evolution of the frequency of the N-C vibration modes with the different characteristic electronic states, reveals that at low laser fluence and on short time scale, the photoexcitation of the Mn3+-N-C-Fe2+ phase creates small charge-transfer polarons [Mn2+-N-C-Fe3+]* within ≃ 250 fs. The local trapping of photoinduced intermetallic charge-transfer is characterized by the appearance of a polaronic infrared band, due to the surrounding Mn2+-N-C-Fe2+ species. Above a threshold fluence, when a critical fraction of small CT-polarons is reached, the macroscopic phase transition to the persistent Mn2+-N-C-Fe3+ cubic phase occurs within ≃ 100 ps. This non-linear photo-response results from elastic cooperativity, intrinsic to a switchable lattice and reminiscent of a feedback mechanism.