Time-resolved infrared vibrational spectroscopy of the photoinduced phase transition of Pd(dmit)₂ salts having different orders of phase transition

To clarify the mechanism of the later process of photoinduced phase transition (PIPT) in organic charge-transfer complexes, we examined by time-resolved infrared vibrational spectroscopy two dimeric anion radical salts, Et₂Me₂Sb[Pd(dmit)₂]₂ (Et ₂Me₂Sb salt) and Cs[Pd(dmit)₂]₂ (Cs salt) (Et, Me, and dmit are C₂H₅, CH₃, and 1,3-dithiol-2-thione-4,5-dithiolate, respectively), having similar characteristics except for the order of their phase transitions at thermal equilibrium. The phase transition is first order for the Et₂Me ₂Sb salt and second order for the Cs salt at thermal equilibrium. Although both salts exhibit a high-temperature phase at later delay times (>100 ps) after the photoexcitation of the low-temperature phase, the time required for the emergence of the high-temperature phase was significantly different: 70 ps for the Et₂Me₂Sb salt and <0.1 ps for the Cs salt. The slow emergence of the high-temperature phase in the PIPT of the Et₂Me₂Sb salt presumably has an origin similar to that recognized for the first-order thermal phase transition, that is, steric effects of the Et₂Me₂Sb cation when the phase transitions occur.