De. Lynch et al., SYNTHESIS AND NONLINEAR-OPTICAL PROPERTIES OF (N-ALKYLPYRROL-2-YL)SQUARAINE DERIVATIVES - PART-2, Perkin transactions. 2, (4), 1998, pp. 779-784
Two squaraine dyes, bis(5-ethyl-1-octadecylpyrral-2-gl)squaraine (Et-P
y) and -dimethyl-4-ethyl-1-octadecylpyrrol-2-yl)squaraine (MeEt-Py) ha
ve been synthesised and studied as Langmuir-Blodgett (LB) monolayers u
sing atomic force microscopy (AFM), angle dependent X-ray photoelectro
n spectroscopy (ADXPS) and grazing incidence X-ray diffraction (GIXD).
Additional investigations include both their linear and second-order
non-linear optical properties, the latter giving d(eff) = 1.5 +/- 0.2
pm V-1 for Et-Py and 1.1 +/- 0.2 pm V-1 for MeEt-Py, Comparisons with
the previously reported dyes, bis(1-octadecylpyrrol-2-yl)squaraine (Py
) and bis(3,5-dimethyl-1-octadecylpyrrol-2-yl) (Me-Py) reveal that Me-
Py is the only one of the four which deposits as ordered, densely pack
ed LB firms which allows further studies on Me-Py so as to ascertain a
valid mechanism for the occurrence of second harmonic generation (SHG
) in these centrosymmetric molecules. The results presented demonstrat
e that the experimentally observed SHG in squaraines arises from a sur
face induced orientational effect where higher SHG is achieved if the
squaraine head-group is essentially perpendicular to the required hydr
ophilic substrate.