The interplay of chromophore-spacer length in light-induced gold nanocluster self-assembly

The light-induced self-assembly of chromophore-tethered precision nanoclusters (NCs) has recently received significant attention due to their facile control over structure, function, and reversibility under ambient conditions. However, the magnitude of assembly depends on the photoswitching efficiency, chemical structure, and proximity of the chromophore to the NC surface. Herein, using azobenzene alkyl monothiol (AMT)-capped gold NCs with two different spacer lengths (denoted as C₃-NC and C₉-NC), we show that reversible cis ↔ trans isomerization efficiency can be readily tuned to control the self-assembly kinetics of NCs. Irrespective of the chain length, the time required for trans-to-cis (140 s) and cis-to-trans (260 s) isomerization of individual C₃-AMT and C₉-AMT is identical in dichloromethane solution. When a similar experiment was performed using a solution of C₃-NCs and C₉-NCs, it resulted in self-assembled disc-like superstructures. Notably, the trans-to-cis photoswitching in C₃-NC could reach only 65% even after 460 seconds of irradiation. On the other hand, C₉-NC completed this process within 160 seconds of irradiation. The low photoswitching efficiency of the C₃-NC analog is due to the short and rigid spacer length of C₃-AMT ligands, which are in close proximity to the NC surface, resulting in steric hindrance experienced at the NC-chromophore interface. Importantly, the slow photoswitching in C₃-NCs helps isolate and investigate the intermediates of assembly. Using high-resolution electron microscopy, atomic force microscopy, and 3D reconstruction, we show that the discs are made up of densely packed arrays of NCs. The prolonged illumination of C₉-NCs results in a chain-like assembly due to the dipolar attraction between the previously assembled superstructures. The efficient photoisomerization of chromophores located away from the nanocluster surface has been identified as the key element to speed up the light-induced assembly in chromophore-tethered nanoclusters. Such information will be useful while developing nanoscale photoswitches for electrochemistry, biosensors, and electronic devices.