Aggregation, Chemistry, Liquid crystals, Self assembly, Squaraine


Squaraine dyes have been a subject of extensive investigations lately due to their wide applications in important technological fields such as bioimaging probes, bioconjucation, second generation photosensitizers for photodynamic therapy, second harmonic generating organic dyes, two-photon absorbing materials with large cross section values, and, finally, photoconducting materials in photovoltaic cells. While a large number of patents and papers has been produced regarding their applications limited work has been done concerning their thermotropic behavior, including their liquid crystalline properties, or correlation of the crystalline structure to both the solid state aggregation and their photophysical properties. In the first chapter of this dissertation, a series of squaraine dyes, based on 2, 4-(4-(N,N-di-n-alkylamino)-2-hydroxyphenyl)squaraine including ethyl, propyl, butyl, pentyl, hexyl, and heptyl derivatives, were synthesized by condensation of the corresponding 4-(N,N-di-n-alkylamino)-2-hydroxyphenol with squaric acid. The thermal behavior of the series was recorded using both thermogravemetic analysis (TGA) and differential scanning calorimetry (DSC) 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 behavior and the crystalline structure of the squaraine series. Two derivatives, butyl and heptyl, revealed the presence of liquid crystalline mesophases, smectic and nematic, respectively, that were confirmed and characterized via polarized light microscopy (PLM) and X-ray diffraction. In the second chapter, J- and H- aggregates were investigated in thin films by UV-vis spectroscopy; several of the derivatives formed H- and/or J-aggregates upon thin film formation via spin coating before and after thermal annealing, as indicated by UV-vis spectroscopy. The molecular structure, crystal structure, aggregation, and thermal behavior 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, making these derivatives suitable candidates for a number of electro-optic and photonics applications. The third chapter was devoted to investigate liquid crystal-directed supramolecular assembly of a squaraine dye. Thus, The squaraine (SQ) dye, 2, 4-bis (4-(N,N-di-n-hexylamino)-2-hydroxyphenyl)squaraine was used to prepare a series of SQ dye/cholesteryl pelargonate mixtures with varying dye concentrations (1%, 3%, 7.5%, 10.8%, 15%, and 20% w/w). Their phase transitions were investigated using differential scanning calorimetry, polarized light microscopy and X-ray diffraction. The squaraine dye itself exhibits no liquid crystalline behavior. The concentration of the dye in the cholesteric compound proved to have a significant effect on the dye aggregation behavior and phase transitions in cholesteryl pelargonate manifested by the appearance of new mesophases and formation of J- and H- aggregates. The texture morphology, X-ray diffraction analyses, and UV-vis absorbance spectra provide compelling evidence of the viability of the self-assembly of squaraines in the liquid crystalline mesophase. In the last chapter we will discuss possible modifications that can improve the aggregation systems.


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Graduation Date





Belfield, Kevin D.


Doctor of Philosophy (Ph.D.)


College of Sciences










Release Date

May 2011

Length of Campus-only Access


Access Status

Doctoral Dissertation (Open Access)

Included in

Chemistry Commons