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.