Squaraines A versatile class of functional near infrared dyes

Excerpt: NIR absorber sensitiser for semi conductor: Squaraine dyes are used in the Xerox industry as photo conductors. They have very strong absorption visible to nearer to the region used.

Squaraines - A versatile class of functional near-infrared dyes*

Dr. Suresh Das Director, National Institute for Interdisciplinary Science and Technology, Trivandrum

Designing photoresponsive molecules

PHOTORESPONSIVE materials and photochromic systems are known and in a controlled manner, it has applications in digital storage of information. When you design molecules that will shine like 100% chemical transformation in a reversible manner, it has different properties, which you can measure. You can design molecules with knowledge of photochemistry, photophysics and synthetic ability. Materials is a different phenomenon and it is not molecules which are coming together and self assemble but depending upon the way they come together, photophysical and photochemical properties can change dramatically and the effects that happen.

Using the principle of self assembly molecules of liquid crystals, they have designed a variety of photoresponsive liquid crystals to develop materials which one can record images in colour in a reversible manner. Also work with molecules self assemble to form nano fibres or solvent attracts, called gelators. Using light we can disrupt the super molecular properties in a reversible manner for controlled release systems. Luminescent materials and organic light is important and luminescent materials are made in such a way you can bring about photochemical transformation in a reversible manner for recording images.

NIR absorber sensitiser for semi conductor: Squaraine dyes are used in the Xerox industry as photo conductors. They have very strong absorption visible to nearer to the region used because they are good sensitisers for semi conductors. Micro properties are tuned in such a manner that they could use them for generating singlet oxygen, which is of interest in photo biology. When the molecules come together and self assemble their properties can change. Further, these molecules have shown good properties with respect to artificial organic photovoltaics.

Conventional use of dyes impart colour to substrate while functional colorants are for special electric properties e.g. Phthalocyanine for colouring jeans (blue) and the same dyes with appropriate purity can be used as semiconductor for the material and they have absorption in the near infrared region. They are important for use in optical storage devices low cost diode rays and of interest in terms of optoelectronic devices.

Near infrared dyes for biological systems – biological molecules, nucleic acid, protein absorb in the UV region and for biological resistance, one can have systems like phthalocyanine and porphyrin, which are absorbents in this region. They can be used for photo biological applications. Penetration of light can be seen at 400 nm for blue, green and near infra red. Even in an area like photodynamic therapy, this ability to penetrate tissues to deeper length and also lack of interference from core system of these molecules makes dye absorption in this region.

Squaraine dyes have intense ability to sharp absorption, visible to near infrared which can be easily seen by modifying structure of these dyes. Optical properties are sensitive to environment. These dyes are absorbed and emitted in the visual NIR range, optical properties sensitive to environment and photo physical properties are sensitive to surrounding medium, which is of advantage for designing molecular probes.

Squaraine dyes are a class of organic dyes showing intense fluorescence, 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. Most squaraines are encumbered by nucleophilic attack of the central four membered ring, which is highly electron deficient. This encumbrance can be attenuated by the formation of a rotaxane 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. Queries and Responses: author.colourage@gmail.com

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Symmetrical squaraine dyes are used in Xerox industry as photo conductors, unsymmetrical squaraine dyes as they have photophysical properties. Squaraines are also used in photodynamic therapy. Dyes having strong absorption in the near infrared can be used for biological applications e.g. PDT in which dyes are used as measure for treating cancer. Organic materials for photovoltaic applications, Nobel Prize awarded for photo conducting polymers shows that organic molecules can behave as inorganic semi conductor; unlike inorganic semi conductors, organic semi conductors charge separation is not easy.


Infrared absorbing dyes find important applications in photon-based technologies ranging from optoelectronics to medicine. Among the various types of dyes absorbing in this region, squaraines form a special class of functional dyes, possessing intense absorption and emission in the visible and near infrared region (NIR) coupled with high thermal and photochemical stability. Their unique photochemical and photophysical properties which are very sensitive to structural as well as environmental changes, make them highly tunable for a variety of applications ranging from copiers, optical discs, sensors to photodynamic therapy. An important aspect of squaraines is their tendency to form aggregates in concentrated solutions and in films, which can bring about significant changes in their photochemical and photophysical properties, significantly affecting their performances in optoelectronic devices. Design novel squaraine dyes with improved absorption and emission in the NIR region and exploring their applications as sensors and sensitizers.

Squaraine dyes are considered an important group of photoactive materials in the field of organic photovoltaic devices. Although the ability of squaraines to sensitize large band gap semiconductors have been well known, the efficiencies of dye sensitized solar cells utilizing squaraines had been generally low. Aggregation of dyes on the TiO2 electrode is generally considered to be disadvantageous for the sensitization process in DSSCs. Their studies have shown that whereas aggregation of symmetrical dyes on the TiO2 photo electrode results in significant decrease in their sensitization efficiency, efficient sensitization was observed from both the monomeric and aggregated forms of the unsymmetrical squaraines. As a result panchromatic sensitization is observed in DSSCs utilizing unsymmetrical squaraines. This has led to a resurgence of interest in the use of squaraines as sensitizers in solar cell devices.

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