Orthogonal Photoswitching and Second-Harmonic Generation in Azobenzene-DASA Dyads: Progress toward Fully Wavelength-Controlled Multistate Nonlinear Optical Switching

The interplay between photoswitching and second-order nonlinear optical responses in azobenzene-DASA dyads linked by alkyl spacers of variable length is investigated by using a custom in situ setup. This original experiment combines wavelength-selective LED irradiation to trigger orthogonal photoswitching. The irradiation is operated in counter-propagation with the near-infrared laser, while the second-harmonic scattered signal is collected at right angle. Systematic comparison with the isolated subunits and their physical mixtures reveals how intersubunit interactions modulate both the kinetics of photoswitching and the magnitude of the nonlinear molecular polarizability. Time-dependent density functional theory calculations provide a molecular-level rationale for the observed photophysical and nonlinear optical behavior.