Squaraine dyes are a class of organic dyes showing intense

fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

, typically in the red and near infrared region (absorption maxima are found between 630 and 670 nm and their emission maxima are between 650–700 nm). They are characterized by their unique aromatic four membered ring system derived from

squaric acid Squaric acid, also called quadratic acid because its four carbon atoms approximately form a square, is a diprotic organic acid with the chemical formula C4O2(OH)2. The conjugate base of squaric acid is the hydrogensquarate anion ; and the conjug ...

. Most squaraines are encumbered by

nucleophilic attack In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

of the central four membered ring, which is highly

electron deficient Electron deficiency (and electron-deficient) is jargon that is used in two contexts: species that violate the octet rule because they have too few valence electrons and species that happen to follow the octet rule but have electron-acceptor properti ...

. This encumbrance can be attenuated by the formation of a

rotaxane In chemistry, a rotaxane () is a mechanically interlocked molecular architecture consisting of a dumbbell-shaped molecule which is threaded through a macrocycle (see graphical representation). The two components of a rotaxane are kinetically t ...

around the dye to protect it from nucleophiles. They are currently used as sensors for ions and have recently, with the advent of protected squaraine derivatives, been exploited in biomedical imaging.

Synthesis

Synthesis of squaraine dyes was reported at least in 1966. They are derived from squaric acid which undergoes an

electrophilic aromatic substitution Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic ni ...

reaction with an

aniline Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine In organic chemistry, an aromatic amine is an organic compound consisting of an aroma ...

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

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.

Squarylium dye III

Squarylium dyes have poor solubility in most solvents, except for

dichloromethane Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...

and a few others. Their absorption peaks at ~630 nm and luminescence at ~650 nm. The luminescence is photochemically stable and its

quantum yield The quantum yield (Φ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system. Applications Fluorescence spectroscopy The fluorescence quantum yield is defined as the ratio of the numb ...

is ~0.65. Squarylium dye molecules can be encapsulated into

carbon nanotubes A scanning tunneling microscopy image of a single-walled carbon nanotube Rotating single-walled zigzag carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers. ''Single-wall carbon nan ...

enhancing the

optical properties of carbon nanotubes The optical properties of carbon nanotubes are highly relevant for materials science. The way those materials interact with electromagnetic radiation is unique in many respects, as evidenced by their peculiar absorption, photoluminescence (fl ...

. 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 Luminescence is spontaneous emission of light by a substance not resulting from heat; or "cold light". It is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions or stress on a cryst ...

, thus allowing measurement and analysis of their Raman spectra.

References

{{DEFAULTSORT:Squaraine Dye Fluorescent dyes Cyclobutenes

Synthesis

Squarylium dye III

See also

References