ABSTRACT

This chapter provides studies on the spectroscopic and photochemical properties of squaraines in homogeneous and heterogeneous media as well as the effect of aggregation on these properties. A major part of the work on the synthesis of squaraines was carried out by the former group and by Sprenger and Ziegenbein. While the net molecular carbon–carbon bond order suggests a cross-conjugated cyanine like structure, the squaraine also possesses cyclobutadienylium dication character. MNDO and CNDO semiemperical calculations have also confirmed that the squaraines possess a distinct quinoid character. The intramolecular charge-transfer nature of the electronic transitions of squaraines makes their photophysical properties highly sensitive to the properties of the surrounding medium. Due to the large polarizability and the existence of a wide transparent window in the visible region of many squaraines, they are also ideally suited for second harmonic generation and other nonlinear optical applications.