Structural study of the thermally induced and photoinduced phase transitions of the 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) radical.

We report on the first direct atomic-level photodiffraction evidence for domain formation by doping with charge photocarriers of the stable organic radical TTTA (1,3,5-trithia-2,4,6-triazapentalenyl), which exhibits unusually wide thermal hysteresis of the paramagnetic-to-diamagnetic phase transition around ambient temperature. A setup for steady-state powder photodiffraction was developed for the purpose, composed of a second harmonic pulsed visible pump (532 nm) coupled with a powder diffractometer using monochromatic synchrotron X-ray radiation as the probe. The refined structures of the photoinduced phase and the high-temperature phase are structurally identical. The result supports the model according to which photoexcitation causes suppression of the spin-Peierls instability and separation of the radical dimers, resulting in a structure that is essentially identical with the unperturbed high-temperature phase.