Exploring Favorable Supramolecular Interactions of Multifluorinated Aromatics in Dendronized Push-Pull Chromophores for Electro-Optics

Multifluorinated aromatics serve as supramolecular synthons in the research of organic electro-optic (EO) materials by exploiting pi-pi stacking interaction between the aromatic hydrocarbon and multifluorinated aromatic groups for performance improvement. However, non-classical hydrogen bonding remains largely unexplored in fluorinated EO dendrimers. In this study, three Fr & eacute;chet-type generation 1 benzyl ether co-dendrons were synthesized by replacing one benzyl group with 2,3,5,6-tetrafluorobenzyl (p-HF4Bz), pentafluorobenzyl (C6F5Bz), and 2,3,4,5-tetrafluorobenzyl (o-HF4Bz) groups, to afford the benzoic acid derivatives D1, D2, and D3, which were further bonded to the donor and pi-bridge moieties to afford three co-dendronized push-pull phenyltetraene chromophores EOD1, EOD2, and EOD3, respectively. The weak C-H & sdot;& sdot;& sdot;X (X=O, F) interactions in the crystal structure of D1 cumulatively add to the benzoic acid dimers to form an extended hydrogen-bonded network, while D2 is crystallized into a centric one-dimensional chain with strong intermolecular interactions. The poled films of EOD1 with PMMA exhibited the largest and most stable EO activity with optical homogeneity among the series. The results identify the effectiveness of weak but favorable hydrogen bonds enabled by the enhanced carbon acidity of p-HF4Bz synthon in D1, over the interactions in D2 and D3, for the rational design of supramolecular EO dendrimers. A single fluorine to hydrogen substitution significantly altered the supramolecular interaction of multifluorinated aromtics. It showcased the exceptional effectiveness of weak hydrogen interactions of C-H & sdot;& sdot;& sdot;X (X=F and O) enabled by the 2,3,5,6-tetrafluorophenyl synthon, over the widely studied quadrupolar C6F5/C6H5 interactions, in improving the performance of NLO dendrimers. image