Incorporation of Amphiphilic Ruthenium(II) Ammine Complexes into Langmuir–Blodgett Thin Films with Switchable Quadratic Nonlinear Optical Behavior

Nine nonlinear optical (NLO) chromophores with pyridinium
electron
acceptors have been synthesized by complexing new proligands with
{RuII(NH3)5}2+ electron-donor
centers. The presence of long alkyl/fluoroalkyl chain substituents
imparts amphiphilic properties, and these cationic complexes have
been characterized as their PF6– salts
by using various techniques including electronic absorption spectros copy
and cyclic voltammetry. Each complex shows three reversible/quasireversible
redox processes; a RuIII/II oxidation and two ligand-based
reductions. The energies of the intense visible d → π*
metal-to-ligand charge-transfer (MLCT) absorptions correlate to some
extent with the ligand reduction potentials. 1H NMR spectroscopy
also provides insights into the relative electron-withdrawing strengths
of the new ligands. Single crystal X-ray structures have been determined
for two of the proligand salts and one complex salt, [RuII(NH3)5(4-C16H33PhQ+)]­Cl3·3.25H2O (PhQ+ = N-phenyl-4,4′-bipyridinium), showing centrosymmetric
packing structures in each case. The PF6– analogue of the latter complex has been used to deposit reproducibly
high-quality, multilayered Langmuir–Blodgett (LB) thin films.
These films show a strong second harmonic generation (SHG) response
from a 1064 nm laser; their MLCT absorbance increases linearly with
the number of layers (N) and I2ω/Iω2 (I2ω = intensity at 532 nm; Iω = intensity at 1064 nm) scales quadratically with N, consistent with homogeneous deposition. LB films on indium
tin oxide (ITO)-coated glass show electrochemically induced switching
of the SHG response, with a decrease in activity of about 50% on RuII → RuIII oxidation. This effect is reversible,
but reproducible over only a few cycles before the signal from the
RuII species diminishes. This work extrapolates our original
solution studies (Coe, B. J. et al. Angew. Chem., Int. Ed. 1999, 38, 366) to the first demonstration
of redox-switching of NLO activity in a molecular material.