The synthesis and non-linear optical properties of (N-alkylpyrrol-2-yl)squaraine derivatives. Part 1

Abstract

The synthesis of a soluble poly(pyrrolylsquaraine) was achieved by N-alkylation of the pyrrole nucleus with 1-bromooctadecane and subsequent condensation with squaric acid. Bis(1-octadecylpyrrol-2-yl)- squaraine was also produced and found to exist in a variety of inseparable isomeric forms. When deposited as Langmuir–Blodgett monolayers on hydrophilic substrates, these dyes exhibit second harmonic generation (SHG) with d_eff = 0.5 ± 0.1 pm V^-1 for both. For comparison, bis(3,5-dimethyl-1-octadecylpyrrol-2-yl)squaraine was synthesised and revealed a higher SHG activity of d_eff = 1.8 ± 0.2 pm V^-1. Poly(1-octadecylpyrrolylsquaraine) is the first alternating donor–acceptor polymer to give SHG.