“…Squaraines are 1,3-disubstituted squaric acids existing in a zwitter-ionic form which consists of a donor–acceptor–donor (D–A–D) structure due to the strong electron withdrawing central squaryl moiety flanked between aromatic or heterocyclic donor rings on either side. Their planar rigid structure, a strong narrow absorption in the Visible/NIR region, high molar extinction coefficient, intense fluorescence spectra in the red and near-infrared region, good solubility in low polarity solvents, remarkable chemical stability, high redox reversibility, inherent photostability, and good photoconductivity are noteworthy properties of these dyes. − These unique electronic structures and optical properties render them most suitable for a wide range of technological applications such as metal ion sensors, sensitizers in DSSC, photodynamic therapy, electrophotography, long wavelength fluorescent biomedical labels and probes, nonlinear optics, optical data storage, TPA applications, biomedical imaging, xerographic organic photoreceptors, photoconductive material in photocopies, and light emitting field effect transistors …”