Spatial and temporal control the self-assembly of inorganic nanocrystals is both fundamentally interesting and technically challenging. Here, we apply photochemistry-based self-assembly to fabricate electronically coupled and locally ordered PbS quantum dots thin films at a liquid-air interface. We demonstrate the assembly process can be triggered by the activation of a photoacid generator, Ph2I-pyranine. The altered surface chemistry of the quantum dots allows for positive or negative pattern generation using quantum dots assemblies down to monolayer thickness. We show that the film structure can be tuned between six-fold and four-fold symmetry by controlling the exposure time. The influence of photo dosage on local assembly structures, optical properties and electronic conductivity is studied in details. We propose the photo-triggered strategy as a new approach to design nanocrystal assemblies for advanced fundamental studies and emerging technological applications.