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.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films