Z. Xu et al., ELECTRONIC RESONANCE AND LOCAL-FIELD EFFECTS ON THE NONLINEAR-OPTICALACTIVITY OF H-AGGREGATE-FORMING STILBAZOLIUM AMPHIPHILES, Journal of physical chemistry, 99(18), 1995, pp. 7154-7159
Stilbazolium dyes, which form H-aggregates upon condensation at the ai
r-water interface. typically display nonlinear optical (NLO) activity
far below what would be predicted on the basis of their large intrinsi
c molecular hyperpolarizability, beta. Three separate hypothesis have
been put forward to explain this observation: structural inversion to
produce a centrosymmetric head-to-tail structure. local field effects,
which act to reduce the effective magnitude of the nonlinear driving
field, and changes in electronic structure, which shift the electronic
resonance frequencies for the aggregates. Investigation of the linear
and nonlinear spectroscopy of the dye 4-(4-dihexadecylaminostyryl)-N-
methylpyridinium iodide were performed on a series of samples in which
the aggregation state of the structure was changed systematically, bu
t without changing the composition of the film. These experiments shaw
that, for this particular dye. changes in electronic structure upon a
ggregation comprise the most important factor in determining the reduc
tion in second harmonic generation (SHG) efficiency of the aggregate.