Of central interest in this work is the study of the alkyl chain length on the photoisomerization of HAT at the monolayer level. To investigate the switching behavior of hydrazone at the molecular level, Cn HAT molecules were self-assembled on a metal surface to form monolayers. A common strategy to produce self-assembled monolayers (SAMs) is through adsorption of alkanethiols on Au, Ag, and other metals due to the high affinity of the thiolate headgroup for the metal substrate. In our model molecules, alkanethiol linkers were attached to the hydrazone skeleton, and therefore the assembly of Cn HAT molecules on a metal surface could be more complicated than that of simple alkanethiols on metal. Due to the large terminal group (hydrazone skeleton), the resulting structures probably are more disordered and less dense than those formed from alkanethiols, but the coverage of Cn HATs on Au is still expected to be close to a full monolayer. To quantify the conversion efficiency of Cn HAT SAMs on Au surfaces after light irradiation, UV-vis spectra of these samples were recorded and are shown in Figure . Before any light irradiation, the maximum absorption peak in the UV spectra of the Cn HAT SAMs appears between 375 to 380 nm for C3, C8, C10, and C11 HAT (Figure , black curves). After irradiation at 415 nm, all the Cn HAT SAMs samples exhibited a ca. 40 nm hypsochromic shift. As shown in Table , the E/Z ratio at PSS 415 for C3 HAT was calculated to be 53:47 based on Gaussian fitting (Figure ), while the E/Z ratio at PSS 415 for other HAT compounds was as high as 90:10 (Table ). Compared with other HAT compounds, the C3 HAT SAM on Au thus exhibits a lower PSS after irradiation at 415 nm. A drop in conversion efficiency of C3 HAT could result from steric/packing effects, as explained above for the case of C3 HAT powder. To assess the contribution of steric effects, we investigated the isomerization of C3 HAT on Au diluted in a 10-fold excess of propanethiol using UV-Vis spectroscopy. As shown in Figure , the E/Z ratio at PSS 415 of the diluted C3 HAT SAM is also 53:47, indicating that lowering the packing density of C3 HAT on Au does not affect its conversion efficiency. Although the phase separation of binary thiol compounds is likely to occur under ambient conditions, the steric effects in the diluted C3 HAT SAM is significantly reduced. This observation leads us to conclude that the low isomerization of C3 HAT results from strong quenching of the excited C3 HAT molecules by the Au surface. The isomerization of on-surface photoswitches concurrently involves several complex processes, such as direct molecular excitation and/or substrate-mediated charge transfer processes. First, upon irradiation at 415 nm, some of the molecules are promoted to an excited state. However, these excited adsorbates are prone to relax to the ground state via a non-radiative decay because of efficient electron/energy transfer to the metal surface. On the other hand, we have proposed a substrate-mediated charge transfer mechanism for the isomerization of an on-surface hydrazone photoswitch, supported by our previous experimental results. According to this mechanism, the excitation of light first induces the formation of hot carriers in the d band of the metal surface, followed by transfer of these hot carriers to the molecules, leading to the formation of excited intermediates, which then results in isomerization. The whole process is also dependent on the distance between the switch group and the metal surface, as the tunnelling rate of these hot carriers from the saturated alkyl chain to the switching group (the C=N bond) decays exponentially as a function of the donor-acceptor distance based on Marcus theory . At the same time the further away the hydrazone is from the surface, the more direct light isomerization will take place. The interplay between these effects results in an enhancement in the E/Z ratio at PSS 415 as the linker length is increased from 1 nm to 2 nm. Ultimately, the outcome of these effects is that the E/Z ratio reaches a maximum value of 90:10 for C8 HAT and above, which is higher than that reported for C6 HAT (82:18). One can speculate that the latter is the sweet spot where mainly Marcus theory is working. The samples were also irradiated at 340 nm, and the maximum absorption peak was found to shift back by a different extent, to between 354 nm and 364 nm (Figure , blue curves), indicating that back isomerization occurs. Subsequently, Gaussian functions were again fitted to the absorption peaks (Figure ). Based on the area ratio of the two absorption peaks of both isomers, the percentage of the E-Cn HAT molecules was calculated. Table summarizes the absorption wavelength of each sample and the conversion efficiency in %. Surprisingly, unlike the back isomerization of Cn HAT in the solid state (Table ), which varies dramatically as a function of the linker length, Cn HAT SAMs on Au reached a PSS 340 of around 41% E-Cn HAT in all cases. When comparing the E/Z isomer ratio following irradiation at 415 nm (forward reaction) with that for irradiation at 340 nm (back reaction), it becomes evident that back isomerization for C3 HAT is also less efficient than for the other Cn HAT molecules. To verify this, Cn HAT compounds were dissolved in EtOH followed by irradiation with 415 nm light, which resulted in a PSS 415 of 93 % E-Cn HAT (Figure ). Subsequently, the Au substrates were immersed in these solutions to obtain a E-Cn HAT SAMs on Au. Irradiation of these samples with 340 nm light results in various E/Z ratios at PSS (Figure ). The E isomer ratio at PSS 340 for C3 HAT is only 51%, it decreases to 35% for C8 HAT and further decreases to 25% for C10 HAT and C11 HAT. These results confirm that the back isomerization is also prone to quenching by the metal surface when the alkyl chain length is short. The surface quenching effect on the back isomerization is almost negligible for C10 HAT and C11 HAT, which might be an indication of direct light isomerization. Notably, the back isomerization for all Cn HAT is higher than that through direct switching on Au (shown in Table ). We speculate that the discrepancy between two experiments may be because of the different topography of Au substrates prepared in two batches, which resulted in different molecular orientation and packing density of Cn HAT adsorbed on Au. These factors inevitably affected the conversion efficiency. To shed light on the surface quenching effect on the isomerization reaction, we carried out DFT calculations for Cn HAT molecules adsorbed on Au(111) surfaces. Based on these results we propose a one-dimensional double-well model to understand the observed phenomenon. As shown in Figure , square potentials are used in a double-well model, whose depths are given by the work function (E f -E v ) of the metal and electron affinity (E LUMO -E v ) of the Cn HAT molecules. The double-well is separated by the length of the alkyl chain linker. We approximate the width of the potentials to be W metal = 5.2 and W mol = 2.8 Å, which are taken from the average distance between molecules on a Au surface (√3 a, being the average bond length of Au-Au) and the length of the dihedral atoms (C-C=N-N, corresponding to the LUMO orbitals), as shown in Figure . We solved this double-well problem using the Schrödinger equation to approximate the energy level of the ground (red) and excited states (green). As suggested by the double-well model, with increasing the length of the linker, excitation energy decreases and levels out at n=8. When the linker is short (n=3), the excitation energy is 3.10 eV, which is slightly higher than that of the HAT molecules with a longer linker (E ex = 3.02 eV), suggesting a shorter lifetime of the excited state of C3 HAT (see details about the calculations in supporting information). As shown in Figure , photo-induced hot carriers in the excited states of C3 HAT are partially trapped in the potential well of Au and therefore excited molecules are easily quenched by the Au surface, in contrast to other HAT molecules with a longer linker (Figure and Table ). This is in agreement with our experimental results. Moreover, the excitation energy of Cn HAT on Ag was calculated. We found a similar trend as that observed for Cn HAT on Au (see Figure and Table ), which means that the isomerization of C3 HAT on Ag is probably quenched by the surface.