We first employed UV-vis spectroscopy to study the photoisomerization of the synthesized Cn HAT switches in toluene. The UV-vis spectra of C3 HAT in toluene before and after irradiation at  = 410 nm or 340 nm are shown in Figure , and the UV-vis spectra of C8-C11 HAT in toluene are shown in Figure . The absorption maxima ( max ) of all four Z-Cn HAT in toluene appears at 369 nm. Following irradiation of the Cn HAT solutions by 410 nm light,  max of these compounds shifts to 333 nm, indicating the occurrence of Z-to-E isomerization. Notably, the shift of  max in the UVvis spectra of all Cn HAT solutions is exactly the same as that observed in the UV-vis spectra of C6 HAT in toluene upon irradiation by 410 nm. The ratio of Z to E isomers was calculated by fitting two Gaussian peaks to the UV-vis spectra according to their simulated UV-vis spectra (Figure ). A 98 % E-Cn HAT content at the PSS 410 was estimated from this fit (Table ). In addition, the conversion efficiency of Cn HAT in solution following irradiation was studied by 1 H NMR spectroscopy. All studied Cn HATs showed similar photophysical properties, owing to the similarity of their electronic structures. Upon 410 nm light irradiation of Cn HAT a 98 -99 % content of the E isomer was found for PSS 410 (Figure ). The quantum yield (Φ Z→E ) of the process was determined to be 3.1 -4.0 % (Figure , S25, S27, S29). The back isomerization can be triggered by irradiating the samples with 340 nm light. As shown in the blue curve of Figure , irradiation of the samples with 340 nm light results in a redshift of  max to 364 nm with a PSS 340 consisting of 77 % Z-Cn HAT (Gaussian fit, table ), similar to that observed at the PSS 340 of C6 HAT. These results demonstrate that the efficiency of the Z-to-E and the back isomerization is independent of the carbon chain length of the linker in solution. Irradiation of the obtained samples with 340 nm light yields a PSS 340 consisting of 77 -78 % of the Z isomer using 1 H NMR spectroscopy (Figure ) with Φ E→Z of 5.6 -8.3 % (Figure , S26, S28, S30). The half-life ( 1/2 ) of the E isomer was measured to be 196-370 years at 298 K using the model compounds Cn Br (Table ).