Synthesis, photophysical properties and applications of NIR absorbing unsymmetrical squaraines: A review

Unsymmetrical squarylium dyes (unsymmetrical squaraines) represent one of the most promising class of squaraine chemistry, due to their outstanding photophysical properties, including absorption in NIR region, good intramolecular charge transfer, high molar absorption coefficients and tunable electrochemical properties. These properties making them extremely useful for photovoltaic as well as biomedical applications. Moreover, the synthesis of unsymmetrical squarylium dyes is excellent for modifying their absorption and emission wavelength by changing suitable aromatic or heterocyclic moiety. There is no collective review for unsymmetrical squaraines in the literature therefore we are presenting this review detailing their synthetic methodologies reported for different classes of unsymmetrical squaraines and their applications comprising dye sensitized solar cells (DSSCs), small molecule organic solar cells (OSCs), photodynamic therapy (PDT), chemosensors and fluorescence probes.