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, Et 2 Me 2 Sb[Pd(dmit) 2 ] 2 (Et 2 Me 2 Sb salt) and Cs[Pd(dmit) 2 ] 2 (Cs salt) (Et, Me, and dmit are C 2 H 5, CH 3, 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 Et 2 Me 2 Sb 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 Et 2 Me 2 Sb salt and <0.1 ps for the Cs salt. The slow emergence of the high-temperature phase in the PIPT of the Et 2 Me 2 Sb salt presumably has an origin similar to that recognized for the first-order thermal phase transition, that is, steric effects of the Et 2 Me 2 Sb cation when the phase transitions occur.