We report on the first direct observation of photoinduced domain formation by photocarrier doping of the stable organic radical TTTA (1,3,5-trithia-2,4,6-triazapentalenyl), which exhibits unusually large thermal hysteresis width of 75 K of the paramagnetic-to-diamagnetic phase transition around room temperature. For that purpose, a new setup for powder photodiffraction was developed, composed of a second harmonic pump (532 nm) of Nd:YAG laser coupled with powder diffractometer using synchrotron radiation as probe at the beamline BL15XU at the 8 GeV third generation synchrotron SPring-8 in Hyogo (Japan). Lowering of the lattice symmetry by thermally induced and photoinduced formation of structural domains of two different phases was observed. Both phases can be decayed by thermal treatment. The results show, for the first time, that the photoinduced and the thermally induced phases in this open-shell spin-Peierls system are structurally different, in accordance with the different proposed mechanisms for their creation.

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