Structural study of the thermally induced and photoinduced phase transitions of the l,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 (l,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 hightemperature 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.