Squaraine-Based Optical Sensors: Designer Toolbox for Exploring Ionic and Molecular Recognitions

Abstract

Small molecule-based chromogenic and fluorogenic probes play an indispensable role in many sensing applications. Ideal optical chemosensors should provide selectivity and sensitivity towards a variety of analytes. Synthetic accessibility and attractive photophysical properties have made squaraine dyes an enticing platform for the development of chemosensors. This review highlights the versatility of modular assemblies of squaraine-based chemosensors and chemodosimeters that take advantage of the availability of various structurally and functionally diverse recognition motifs, as well as utilizing additional recognition capabilities due to the unique structural features of the squaraine ring.

Steric and Electronic Effect in Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells

“…Squaraines belongs to the polymethine dye family where the squaric acid unit is flanked between two aniline or indoline donor units, which possesses a high intense peak in the far-red region with the high molar extinction coefficient (>10 5 M −1 cm −1 ). 25−28 These dyes aggregate in solution as well as in bulk, and such properties have been widely utilized in developing metal-ion sensors 29 and optoelectronic applications. 30,31 Though the aggregation of dyes on TiO 2 surface is considered to be a detrimental process for the device DSSC efficiency, it helps broadening the light harvesting efficiency (LHE), and hence, controlling the aggregation of dyes became a strategy to enhance the photocurrent generation from the aggregated structures.…”

“…High efficient DSSC devices with the performances over 14–15.2% − by the cosensitization approach have been achieved under 1 Sun condition, and 37% has been achieved under ambient condition. Squaraines belongs to the polymethine dye family where the squaric acid unit is flanked between two aniline or indoline donor units, which possesses a high intense peak in the far-red region with the high molar extinction coefficient (>10 5 M –1 cm –1 ). − These dyes aggregate in solution as well as in bulk, and such properties have been widely utilized in developing metal-ion sensors and optoelectronic applications. , Though the aggregation of dyes on TiO 2 surface is considered to be a detrimental process for the device DSSC efficiency, it helps broadening the light harvesting efficiency (LHE), and hence, controlling the aggregation of dyes became a strategy to enhance the photocurrent generation from the aggregated structures. , Such types of controlled assembly of dyes on TiO 2 can be achieved by introducing a steric feature in the sensitizer by introducing alkyl groups, which resulted in the generation of photocurrent from aggregated structures, , and the hydrophobic alkyl groups passivate the anode and avoid the charge recombination process which improves the open-circuit potential ( V OC ) of the DSSC device. ,, Such aggregated structures can be modulated by the solvents in both OPV and DSSC research . Furthermore, the electronic properties of squaraine chromophores has been modulated significantly by extending the π-conjugation inside the polymethine framework, whereas only noticeable changes have been observed while extending the conjugation outside the polymethine framework. − Furthermore, fine-tuning in energy levels by introducing the various electron-rich and -deficient substituents known for squaraine chromophores that were utilized for field effect transistor, but such type of systematic study is required for the D–A–D type unsymmetrical squaraine dyes in DSSC.…”

A Near InfraRed Emissive Chemosensor for Zn2+ and Phosphate Derivatives Based on a Di-(2-picolyl)amine-styrylflavylium Push-Pull Fluorophore

“…With respect to NIR fluorophores, some of the most noteworthy include heptamethyne cyanine [ 17 ], BODIPYs [ 18 ], squaraine [ 19 , 20 ], dicyanomethylene [ 21 , 22 ] and modified rhodamines [ 23 , 24 ] ( Figure 1 ). Although not regarded as your typical fluorescent unit, synthetic flavylium dyes can be easily modified to include moieties for binding external analytes, thereby allowing for the development of new probes [ 25 , 26 , 27 ].…”

Synthesis and Characterization of Newly Designed and Highly Solvatochromic Double Squaraine Dye for Sensitive and Selective Recognition towards Cu2+

“…This geared the momentum of research for the development of such a class of intensely colored dyes utilizing vastly available electron-donating aromatic rings and squaric acid [ 11 , 12 , 13 , 14 ]. This quest for the design and development of novel squaraine dyes started about six decades ago, which is expected to be continued in future owing to the plethora of technological applications in the area of probes for bioimaging [ 15 , 16 ], semiconducting elements for organic field-effect transistors [ 4 , 5 ], next-generation solar cells such as organic thin film, dye-sensitized and perovskite solar cells [ 17 , 18 ], huge circular dichroism [ 19 ], and fluorescence based electrofluorochromic devices [ 20 , 21 ] along with diverse applications in sensing assisted by molecular self-assembly controlled guest-host interactions [ 22 , 23 , 24 , 25 , 26 ].…”