Effect of Substituents on Optical Properties and Charge-Carrier Polarity of Squaraine Dyes

A series of squaraine dyes was synthesized with electron-donating (OH, C6H13) and electron-withdrawing (F, CF3) groups allowing for tuning of the optical and electrochemical properties of the dyes. The squaraines exhibited strong absorbance (epsilon = 10(4)-10(5) M-1 cm(-1)) at long wavelengths (lambda(max) = 660-690 nm), attributed to an intramolecular charge transfer (ICT). As the electron-donating character of the dyes increases, the absorbance of the dyes red shifts and the band gap decreases. Interestingly, the ICT band seemed to be strongly dependent on the nature of the solvent, providing insights into the importance of hydrogen-bonding-induced coplanarity in these molecules. The squaraines were investigated for their charge-carrier mobility in FET configuration. Dyes with fluorine functional groups were found to exhibit either ambipolar (-F) or n-type (-CF3) charge-carrier characteristics, although the molecules themselves are made of traditionally p-type triarylamines.