Next, we irradiated the samples with 340 nm light. As shown in the blue curves of Figure , the intensities of the characteristic Raman bands for the E isomer at 1564 and 1709 cm -1 decrease in all cases, while the intensities of the peaks at 1436, 1476, and 1675 cm -1 partially recover, which means that part of the E isomer has isomerized back to the Z state. The E/Z ratio at PSS 340 for Cn HAT (n = 3, 8, 10, 11) was calculated to be 39:61, 71:29, 75:25, and 59:41, respectively. The highest back conversion efficiency was observed in C11 HAT powder (38 % of the E-C11 HAT have isomerized), which could be attributed to the loosest packing of C11 HAT compared with other Cn HAT molecules. Notably, although the Z-to-E isomerization efficiency of C11 HAT powder after 410 nm light irradiation was comparable to that of C11 HAT in toluene, the back conversion efficiency is much lower than that of C11 HAT in toluene (75% of the E-C11 HAT has isomerized). This difference is tentatively ascribed to the different packing of the Z and E isomers of C11 HAT in powder samples. We speculate that after C11 HAT molecules switch from Z to E, the E molecules are more closely packed in bulk materials than the Z molecules, due to the smaller molecular volume.