Amphiphilic azo-dyes (RED-PEGM). Part 2: Charge transfer complexes, preparation of Langmuir–Blodgett films and optical properties

Optical properties of three amphiphilic azo-dyes bearing end-capped oligo(ethylene glycol) segments: N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl] phenyl}-N-(3, 6, 9-trioxadecas-1-yl)amine (RED-PEGM-3), 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) and N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl] phenyl}-N-(3, 6, 9, 12, 15, 18, 21, 24, 27, 30)-decaoxauntricontas-1-yl)amine (RED-PEGM-10) were studied in Z-type LB films by absorption spectroscopy. RED-PEGM-8 and RED-PEGM-10 formed intramolecular charge transfer complexes in aqueous solution due to coiling of the end-capped poly(ethylene glycol) side-chain around the azobenzene groups. This was detected by 1H NMR, 2D NOESY spectroscopy and the optimised geometries of the complexes were estimated by DFT calculations. The formation and optical properties of CT complexes are discussed with respect to the poly(ethylene glycol) segments length. Z-type Langmuir–Blodgett (LB) films of RED-PEGM-8 exhibited χ(2)χ(2) non-linear optical properties (NLO) like second harmonic generation (SHG) depending on the number of deposited layers. The χ31(2) and χ33(2) NLO-coefficients were evaluated to be approximately 2.7 and 12.5 pm V−1, respectively for a single RED-PEGM-8 layer sample.