Strategies and investigations on bridging squaraine dye units

Synthetic strategies and basic molecular principles were investigated in order to achieve chromophores absorbing light in the far red or even near infrared region. Therefore, a comprehensive series of new squaraine dyes were synthesized and characterized. Beside monomeric unsymmetrical and symmetrical squaraine dyes, also squaraine dye dimers and oligomers bridged with phenylene, fluorene, iso-octyloxy phenylene and tetrafluoro phenylene moieties or directly coupled via condensation reaction were obtained. The influence of the various molecular dye structures to their absorption properties was studied with UV–vis measurements supported by theoretical investigations applying Kiprianov's theory of coupled dyes. With regard to Meyer's theory of the limiting values, the effective number of repeating units for dye series for the highest possible absorption maximum was determined. Observed trends and limiting values for electrochemical potentials and electrical band gaps of the dyes are in agreement with the UV–vis investigations. Finally, calculated molecular structures and frontier orbitals of dimers verify the results of the optophysical and electrochemical studies.

Figure optionsDownload as PowerPoint slideHighlights
► Influence of some aromatic end groups on the squaraine dye spectroscopic properties.
► Coupling of two and three squaraine dyes to reach the very far red and the near infra-red region.
► Study of the structure property relation of squaraines with one, two and three dye units.
► Comparison of the described properties with molecular modelling.