Second harmonic generation of dipolar dendrons in the assembled thin films

Dendritic macromolecules, called 'dendrons', having a branching structure modified with an electron donor/acceptor functionalized azobenzene chromophore have been synthesized. The structurally different dendrons possess a different number, 1, 3, 7, and 15, of chromophore units. The second harmonic generation (SHG) of dendrons was measured in the molecular assembled thin films prepared by the Langmuir-Blodgett (LB) transfer technique. The SHG results indicated that dendrons could be highly organized in these films, where film structure and molecular orientation were analogous to those in the conventional SHG active LB films. The SHG intensity for these molecular organized thin films revealed that the individual chromophores in dendrons were oriented non-centrosymmetrically and coherently contributed to the increase in their molecular hyperpolarizability. The SHG activity became larger as the number of branching units of dendrons increased. The highest molecular hyperpolarizability was found to be 4000 × 10^-30 esu for a dendron containing 15 chromopholic units. The SHG result suggested that the structure of dendrons synthesized was uniaxally dipolar, and that there was a particular merit in this structure for the generation of SHG.