Squaraine dyes are a class of organic dyes showing intense

fluorescence Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colore ...

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

squaric acid Squaric acid or quadratic acid (so named because its four carbon atoms approximately form a square) is a diprotic organic acid with the chemical formula . The conjugate base of squaric acid is the hydrogensquarate anion ; and the conjugate base ...

. 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 a ...

of the central four membered ring, which is highly electron deficient. This encumbrance can be attenuated by the formation of a

rotaxane 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 trapped since ...

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 (SEAr) is an organic reaction in which an atom that is attached to an aromatic ring, aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitut ...

reaction with an

aniline Aniline (From , meaning ' indigo shrub', and ''-ine'' indicating a derived substance) is an organic compound with the formula . Consisting of a phenyl group () attached to an amino group (), aniline is the simplest aromatic amine. It is an in ...

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.

Fluorescence

Squarylium dyes are soluble in

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

; 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 In particle physics, the quantum yield (denoted ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system. \Phi(\lambda)=\frac Applications Fluorescence spectroscopy The fluorescence ...

is ~0.65. Squarylium dye molecules can be encapsulated into

carbon nanotubes A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range (nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized: * ''Single-walled carbon nanotubes'' (''SWC ...

enhancing the

optical properties of carbon nanotubes The optical properties of carbon nanotubes are highly relevant for materials science. The way carbon nanotubes 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 a spontaneous emission of radiation from an electronically or vibrationally excited species not in thermal equilibrium with its environment. A luminescent object emits ''cold light'' in contrast to incandescence, where an obje ...

, thus allowing measurement and analysis of their Raman spectra.

References

{{Reflist Fluorescent dyes Cyclobutenes

Synthesis

Fluorescence

See also

References