Molecular Engineering of Non-Centrosymmetric Langmuir-Blodgett Films for NLO Applications : Films with Interlocking Lego-Type Structures (Special Issue on Organic Molecular Electronics for the 21st Century)

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The cationic dye, E-N-alkyl-4-[2-(4-alkyloxy-naphthyl) ethenyl] quinolinium bromide has a wide-bodied chromophore with alkyl groups at opposite ends, i.e. C_mH_<2m+1>-D-π-A^+-C_nH_<2n+1>X^-, where D and A are electron-donor and electron-acceptor groups respectively, π is a conjugated bridge and X^- is the bromide counterion. It forms non-centrosymmetric Langmuir-Blodgett (LB) structures in which the packing is dependent upon the combination and compatibility of the alkyl groups. Films fo two isomeric analogues where m+n=34, have a layer thickness of 4.3nm when the groups are significantly different (m=12 and n=22) which decreases to 2.5nm when they are similar (m=18 and n=16). The latter adopts an interdigitating layer arrangement at both the upper and lower interfaces and it is the first example of a Lego-type structure with non-centrosymmetric alignment of the optically nonlinear chromophores. This is evidenced by the second-harmonic intensity, which increases quadratically with the number of layers, i.e. as I^<2ω>_<(N)>= I^<2ω>_<(1)>N^2.
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2000-07-25