Optical properties and aggregation of novel azo-dyes bearing an end-capped oligo(ethylene glycol) side chain in solution, solid state and Langmuir–Blodgett films

Optical properties and aggregation of two novel azo-dyes N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(3,6,9-trioxadecas-1-yl)amine (RED-PEGM-3) and N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl] phenyl}-N-(3,6,9,12,15,18,21,24-octaoxapentaeicos-1-yl)amine (RED-PEGM-8) were studied by UV–vis spectroscopy in solution, solid state and Langmuir–Blodgett films. The results were compared to those obtained for their precursor 1-amino-4′-nitroazobenzene (Disperse Orange 3, DO3) and 1-N-methylamino-4′-nitroazobenzene (RED-H). Increasing the polarity of methanol:water mixtures gave rise to the formation of H-aggregates for all dyes. Similar aggregates were also detected in cast films. NOESY, 2D 1H NMR experiments carried out in aqueous solutions of RED-PEGM-8 revealed the formation of atypical antiparallel H-aggregates. Only RED-PEGM-3 gave traces of J-aggregates in the solid state. RED-PEGM-3 and RED-PEGM-8 readily form J-aggregates in Y-type Langmuir–Blodgett films.