Synthesis and non-linear optical properties of (N-alkylpyrrol-2-yl)squaraine derivatives. Part 2†

Abstract

Two squaraine dyes, bis(5-ethyl-1-octadecylpyrrol-2-yl)squaraine (Et-Py) and bis(3,5-dimethyl-4-ethyl-1-octadecylpyrrol-2-yl)squaraine (MeEt-Py) have been synthesised and studied as Langmuir–Blodgett (LB) monolayers using atomic force microscopy (AFM), angle dependent X-ray photoelectron 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)squaraine (Me-Py) reveal that Me-Py is the only one of the four which deposits as ordered, densely packed LB films 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 demonstrate that the experimentally observed SHG in squaraines arises from a surface induced orientational effect where higher SHG is achieved if the squaraine head-group is essentially perpendicular to the required hydrophilic substrate.