This study systematically investigates the interfacial assembly and photoswitching behavior of the photoresponsive molecule AZO-C1 at the air/water interface by integrating surface tension measurements, second harmonic generation (SHG), sum frequency generation (SFG) vibrational spectroscopy, and quantum chemical calculations. The results demonstrate that the AZO-C1 monolayer exhibits stable and reversible surface tension switching (between ∼57 and ∼63 mN/m) under alternating UV and visible light irradiation. SHG experiments reveal reversible changes in the second-order nonlinear optical response, primarily attributed to interfacial molecular reorganization and variations in the effective number density of responsive molecules induced by azobenzene photoisomerization, rather than uniform molecular reorientation. SFG spectra further confirm a significant enhancement of the N═N stretching vibration signal (∼1516 cm–1) upon UV exposure, characteristic of the cis-isomer, while the C–H vibration signals of the alkyl chains remain unchanged. This indicates a decoupled reorganization mechanism: the azobenzene headgroup undergoes reversible conformational and polarity changes, while the hydrophobic alkyl chains maintain a stable conformation, preserving the structural integrity of the interfacial assembly. The multiscale analysis bridges macroscopic interfacial properties with molecular-level conformational changes, providing a rational strategy for designing robust, fast-switching, and reversible photocontrolled interfacial materials based on a domain-decoupling design principle.
Qin et al. (Sat,) studied this question.