Squaraine dye

Synthesis of squaraine dyes was reported at least in 1966. They are derived from squaric acid which undergoes an electrophilic aromatic substitution reaction with an aniline or another electron rich derivative to form a highly conjugated product with extensive charge distribution. For instance, squaraine dyes are also formed via reaction of squaric acid or its derivatives with so-called "methylene bases" like 2-methyl-indolenines, 2-methyl-benzthiazoles or 2-methyl-benzo-selenazoles. Indolenine-based squaraines combine good photostability including high quantum yields when bound to proteins and reactive versions of these dyes are commonly used as fluorescent probes and labels for biomedical applications. ==Fluorescence==

Squarylium dyes are soluble in dichloromethane; it is soluble in few other solvents. Their absorption peaks at ~630 nm and luminescence at ~650 nm. The luminescence is photochemically stable and its quantum yield is ~0.65. Squarylium dye molecules can be encapsulated into carbon nanotubes enhancing the optical properties of carbon nanotubes. Efficient energy transfer occurs between the encapsulated dye and nanotube — light is absorbed by the dye and without significant loss is transferred to the nanotubes. Encapsulation increases chemical and thermal stability of squarylium molecules; it also allows their isolation and individual characterization. For example, encapsulation of dye molecules inside carbon nanotubes completely quenches strong dye luminescence, thus allowing measurement and analysis of their Raman spectra. {{cite journal |author=Y. Saito|title=Vibrational Analysis of Organic Molecules Encapsulated in Carbon Nanotubes by Tip-Enhanced Raman Spectroscopy |journal=Jpn. J. Appl. Phys. |volume=45 |issue= 12 |pages=9286–9289 |year=2006 |doi=10.1143/JJAP.45.9286 |bibcode=2006JaJAP..45.9286S ==See also==