Thermotropic behaviour, self-assembly and photophysical properties of a series of squaraines

A series of squaraine dyes, based on 2,4-bis[4-(N,N-di-n-alkylamino)-2- hydroxyphenyl] squaraine including ethyl, propyl, butyl, pentyl, hexyl and heptyl derivatives, were synthesised by condensation of the corresponding 4-(N,N-di-n-alkylamino)-2-hydroxyphenol with squaric acid. The thermal behaviour of the series was recorded using both thermogravimetic analysis and differential scanning calorimetry while their crystalline structures were elucidated via single-crystal X-ray diffraction. The length of the alkyl chain proved to have a significant effect on both the thermotropic behaviour and the crystalline structure of the squaraine series. Two derivatives, butyl and heptyl, revealed the presence of liquid crystalline mesophases, smectic and nematic, respectively, which were confirmed and characterised via polarised light microscopy and X-ray diffraction. Several of the derivatives formed H- and/or J-aggregates upon thin film formation via spin coating before and after the thermal annealing treatment as indicated by UV-vis spectroscopy. The molecular and crystal structure, aggregation and thermal behaviour provide insight into the supramolecular assembly of this important class of materials. Photophysical measurements revealed large molar absorptivity, reasonably high fluorescence quantum yields and significant fluorescence anisotropy by making these derivatives suitable candidates for a number of electro-optic and photonics applications.