Lattice Distortion Stabilizes the Photoinduced Metallic Phase in the Charge-Ordered Organic Salts $(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_3{X}_2$ ($X\mathbf{=}{\mathrm{ReO}}_4$, ${\mathrm{ClO}}_4$)

Photoinduced effects caused by intramolecular excitation were investigated by simultaneous optical and transport measurement in two charge-ordered organic salts, $(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_3{X}_2$ ($X={\mathrm{ReO}}_4$, ${\mathrm{ClO}}_4$) [$\mathrm{BEDT}-\mathrm{TTF}=\mathrm{\text{bis}}(\mathrm{\text{ethylenedithio}})\mathrm{\text{tetrathiafulvalene}}$]. Although the two salts have the same molecular (average) charge and arrangement, they showed different photoinduced effects. A photoinduced insulator-to-metal phase transition with a metastable charge order-melting state was observed in the ${\mathrm{ReO}}_4$ salt where the charge ordered state is associated with the lattice distortion. On the other hand, no photoinduced insulator-to-metal phase transition was noted in the ${\mathrm{ClO}}_4$ salt where the charge ordered state is not accompanied by the lattice distortion. This comparative study suggested that the lattice distortion plays a key role in the stabilization of the photoinduced phase.